Novel Stretchable Printed Wearable Sensor for Monitoring ...

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8-2015

Novel Stretchable Printed Wearable Sensor for Monitoring Body Novel Stretchable Printed Wearable Sensor for Monitoring Body

Movement, Temperature and Electrocardiogram, along with the Movement, Temperature and Electrocardiogram, along with the

Readout Circuit Readout Circuit

Ali Eshkeiti Western Michigan University, [email protected]

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NOVEL STRETCHABLE PRINTED WEARABLE SENSOR FOR MONITORING

BODY MOVEMENT, TEMPERATURE AND ELECTROCARDIOGRAM,

ALONG WITH THE READOUT CIRCUIT

by

Ali Eshkeiti

A dissertation submitted to the Graduate College

in partial fulfillment of the requirements

for the degree of Doctor of Philosophy

Electrical and Computer Engineering Department

Western Michigan University

August 2015

Doctoral Committee:

Massood Zandi Atashbar, Ph.D., Chair

Bradley Bazuin, Ph.D.

Paul D. “Dan” Fleming, Ph.D.

Margaret Joyce, Ph.D.

NOVEL STRETCHABLE PRINTED WEARABLE SENSOR FOR MONITORING

BODY MOVEMENT, TEMPERATURE AND ELECTROCARDIOGRAM,

ALONG WITH THE READOUT CIRCUIT

Ali Eshkeiti, Ph.D.

Western Michigan University, 2015

Conformal and stretchable wearable sensors provide vital real time

information about individual’s health conditions. In this work, Traditional printing

methods are used for fabrication of stretchable and wearable sensors which can be

mounted on the human skin for the purpose of health tracking. In addition, screen

printing technology was utilized to develop printed and flexible electronic circuit

board which can be used as a readout circuit along with fabricated wearable sensors.

The dissertation is organized and pursued in three projects.

In the first project, screen printing was used to fabricate multi-layer PCBs

using printed deposited materials on three distinct substrates. The different

characteristics of PET, paper and glass as a substrate for PCBs were analyzed. A

method for populating electronic components onto the printed PCB pads was

established and demonstrated. Different analysis such as effect of the roughness of the

substrates on the electrical performance of the printed lines and effect of the bending

on the resistivity of the printed lines were performed. The capability of the printed

hybrid PCB circuit to correctly operate and drive an LCD was shown.

In the second project, a new method for fabrication of wavy lines and

structures was devised for formation of stretchable and flexible wearable sensors.

Thermoplastic polyurethane (TPU) was used as a substrate in this project. Different

wavy structure designs were printed and analyzed to determine the best design rules.

Silver (Ag) and Carbon Nano Tubes (CNTs) were used for fabrication of wavy lines.

The printability, bendability and stretchability of all printed lines were tested and

analyzed. Different design rules and parameters such as the ratio of the width of the

lines to their radius (W/r) and the length of the extended line to the diameter of the arc

(L/D) were tested and analyzed. Printed wavy structures using CNTs showed below

35 % change in the resistance when it was stretched and 50 % strain was applied on

the structure. 40 % change in the resistance of the lines printed using silver was

obtained when 10 % strain was applied. The results obtained demonstrated that CNTs

showed a promising potential to be used for fabrication of strain wearable sensors.

In the third project, a novel wearable sensing platform for the detection of

disorder in body movement, temperature and ECG was fabricated. Both wavy

structure and non-wavy structure were implemented in these devices. Silver and

CNTs inks were used for the fabrication of different sensing parts on the PDMS and

tattoo paper as substrates. The strain sensor fabricated on tattoo paper and PDMS

were tested towards bending of finger to angles of 10, 20, 30 and 40 degree. 3.66 %,

4.7 % and 4.18 % change was observed in the resistance for each step for sensor 1,

sensor 2 and sensor 3, respectively, on tattoo paper. The printed sensor on the PDMS

demonstrated average change of 2.7 %, 2.2 % and 2.8 % for sensor 1, sensor 2 and

sensor 3, respectively, for each step of bending of finger. The printed sensor on the

PDMS was successfully implemented as temperature sensors for tracking of the skin

temperature. Mixture of silver and silicone was used for fabrication of flexible

electrocardiogram sensor on PDMS. The devices’ response demonstrates the

feasibility of printed multi-functional wearable sensors for health monitoring

applications.

Copyright by

Ali Eshkeiti

2015

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ACKNOWLEDGEMENTS

I would like to begin with expressing my appreciation and sincere thanks for

the help and advice I have received from my advisor Prof. Massood Zandi Atashbar.

His guidance, support and encouragement throughout this dissertation were

invaluable for pursuing this work. His guidance also helped me to mature as an

engineer and person. My sincere thanks also go to members of my committee Dr.

Bradley Bazuin, Dr. Margaret Joyce and Dr. Paul D. "Dan" Fleming for taking their

valuable time to support my thesis with their detailed comments and advices.

Chemical and Paper Engineering department and Electrical and Computer

engineering department professors and staff also deserve thanks for their support in

completion of this dissertation. Many thanks go to Matthew Stoops (Laboratory

Technician) for his contribution in performing measurements. I would like to

especially thank Dr. Margaret Joyce for her invaluable time spent with me to clear my

doubts and for providing necessary materials.

I owe a large thanks to my fellow lab members Dr. Binu Baby Narakathu, Dr.

Sai Guruva Reddy Avuthu, Sepehr Emamian, Mohammed Ali, Michael Joyce, Zeinab

Ramshani, Morteza Rezaee, Amer Chlaihawi and Dinesh Maddipatla. Finally, I

would like to give special thanks to my uncle Hamed Shakoury and my best friends

Omar Shariff and Navid Mujaddedi for their encouragements. I would like to express

my sincere gratitude to my parents (Homa and Reza), my brother (A’ala) and my

forever girlfriend Mitra and her family for all their supports and help over past years.

Ali Eshkeiti

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TABLE OF CONTENTS

ACKNOWLEDGMENTS ...................................................................................... ii

LIST OF TABLES .................................................................................................. vii

LIST OF FIGURES ................................................................................................. viii

CHAPTER

I. INTRODUCTION ........................................................................................... 1

1.1. Motivation .................................................................................................. 1

1.2. Author’s Contribution ................................................................................ 3

1.3. Organization of the Dissertation ............................................................... 4

1.4. References .................................................................................................. 7

II. LITERATURE REVIEW ................................................................................ 13

2.1. Introduction ................................................................................................ 13

2.2. Sensors ....................................................................................................... 13

2.2.1. Introduction to Sensors .................................................................... 13

2.2.2. Types of Sensors .............................................................................. 14

2.2.2.1. Thermal Sensors .................................................................... 14

2.2.2.2. Mechanical Sensors .............................................................. 16

2.2.2.3. Acoustic Sensors ................................................................... 17

2.2.2.4. Chemical Sensors and Biosensors ........................................ 18

2.2.2.5. Optical Sensors ..................................................................... 19

2.3. Wearable Electronics and Applications ..................................................... 22

2.3.1. Introduction to Wearable Electronics ............................................... 22

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Tables of Contents—continued

2.3.2. Previous Works ................................................................................. 23

2.4. Printed Electronics ..................................................................................... 30

2.4.1. Types of Printing ............................................................................... 32

2.4.1.1. Screen Printing ......................................................................... 33

2.4.1.2. Inkjet Printing ......................................................................... 34

2.4.1.3. Flexographic Printing ............................................................... 36

2.4.1.4. Gravure Printing ....................................................................... 37

2.5. Application of PE in Printing Wearable Electronics ................................. 43

2.6. Summary .................................................................................................... 45

2.7. References .................................................................................................. 46

III. SCREEN PRINTING OF MULTY-LAYERED HYBRID PRINTED

CIRCUIT BOARDS (PCB) ON DIFFERENT SUBSTRATES ..................... 65

3.1. Introduction ................................................................................................ 65

3.2. Background ................................................................................................ 67

3.3. Objectives .................................................................................................. 69

3.3.1. Materials and Sample Preparation .................................................... 69

3.3.2. Design of the Circuit ........................................................................ 71

3.3.3. Screen Printing of PCB ..................................................................... 72

3.4. Results ........................................................................................................ 73

3.4.1. Analysis of Printed Lines .................................................................. 73

3.4.2. Electrical Analysis ............................................................................ 76

3.4.3. LCD Operation .................................................................................. 78

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3.5. Summary .................................................................................................... 79

3.6. References .................................................................................................. 81

IV. DESIGN, FABRICATION AND ANALYSIS OF PRINTED WAVY

LINES FOR STRETCHABLE ELECTRONIC DEVICES ............................ 86

4.1. Introduction ................................................................................................ 86

4.2. Experimental .............................................................................................. 87

4.2.1.Design and Fabrication of Wavy Lines .............................................. 87

4.2.2. Simulation ......................................................................................... 90

4.2.3. Chemical, Materials and Sample Preparation ................................... 95

4.2.4. Fabrication of Printed Wavy Lines ................................................... 96

4.2.5. Experiment Setup .............................................................................. 97

4.2.6. Results .............................................................................................. 97

4.3. Summary .................................................................................................... 112

4.4. References .................................................................................................. 113

V. FABRICATION OF WEARABLE SENSORS USING PRINTING

METHODS ...................................................................................................... 118

5.1. Introduction ................................................................................................ 118

5.2. Experimental .............................................................................................. 119

5.2.1. Chemical Materials and Sample Preparation .................................... 119

5.2.2. Fabrication of Printed Strain sensor on Tattoo paper ....................... 119

5.2.3. Experiment Setup .............................................................................. 121

5.2.4. Results ............................................................................................... 121

5.2.4.1. Strain Sensor on Tattoo Paper ................................................. 121

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5.2.4.2. Strain Sensor on PDMS .......................................................... 132

5.2.4.3. Strain Sensor on PVA .............................................................. 142

5.2.4.4. Temperature Sensor on PDMS ............................................... 144

5.2.4.5. Electrocardiogram Sensor (ECG) ........................................... 149

5.3. Summary .................................................................................................... 153

5.4. References .................................................................................................. 155

VI. CONCLUSION AND FUTURE WORK ........................................................ 157

6.1. Conclusion ................................................................................................. 157

6.2. Future Work ............................................................................................... 159

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LIST OF TABLES

2.1: Different types of sensors and their stimulus [18]. ......................................21

2.2: Different characteristics of printing methods [19]. ......................................34

3.1: Summary of different characteristics of substrates including surface

energy, thickness and roughness..................................................................70

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LIST OF FIGURES

2.1: Examples of sensor input/output relationship ............................................ 15

2.2: Liquid in thermometer [14]. ....................................................................... 16

2.3: Pressure sensor as a mechanical sensor [14, 15]........................................ 17

2.4: Schematic of acoustic gas sensor ............................................................... 18

2.5: Schematic of an array of chemical or biosensor [15] ................................. 19

2.6: Schematic of optical fiber guide ................................................................ 20

2.7: Basic structure of optical fiber ................................................................... 20

2.8: a) spring form of interconnections, b) and c) wavy form of

interconnections (i) chemical bonding along the ribbon and (ii)

chemical bonding in the selected areas, courtesy of [68] ............................ 25

2.9: Multifunctional epidermal electronics, courtesy of [81] ............................ 27

2.10: Epidermal wearable electronics, courtesy of [83] .................................... 28

2.11: Wearable patch courtesy of [84] .............................................................. 30

2.12: Summary of different forms of printing ................................................... 33

2.13: Screen printing ......................................................................................... 35

2.14: (a) Continuous inkjet, (b) Thermal inkjet and (c) Piezo inkjet [94] ........ 36

2.15: Flexographic printing ............................................................................... 37

2.16: Gravure printing ....................................................................................... 38

2.17: Printed cantilever structure courtesy of [133] .......................................... 40

2.18: Printed PH monitoring sensor on tattoo paper courtesy of [139] ............ 42

2.19: Market growth for printed sensors [150] ................................................. 43

2.20: Market share for wearable sensors [151] ................................................. 44

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List of Figures—continued

2.21: Predicted growth of market to different types of wearable sensors

[151] ............................................................................................................ 44

3.1: Picture of (a) Bottom layer (b) top layer (c) Final sample ......................... 68

3.2: (a) Martin dot liner 06.6 (b) Fine tech placement instrument .................... 69

3.3: The layout of PCB design created in PCB123® design software. This

design consists of the pads for a DC to DC convertor, a

microcontroller and necessary passive components. (Red Layer –

Bottom Electrodes; Green Layer – Dielectric Layer; Yellow Layer –

Top Electrodes) ........................................................................................... 71

3.4: 3D vertical scanning interferometer images of (a) silver and (b)

dielectric on paper; (c) silver and (d) dielectric on glass; and (e) silver

and (f) dielectric on PET ............................................................................. 73

3.5: (a) Photograph of three layers of printed PCB on PET and b) A

photograph of the completed PCB on paper ............................................... 74

3.6: (a) Summary of line width measurement on different substrates; (b)

3D profilometry picture and (c) optical microscope image of printed

lines for microcontroller contact pads ......................................................... 75

3.7: Effect of roughness on resistivity of printed lines ..................................... 76

3.8: Summary of the effect of the resistivity of the lines on the

performance of the circuit ........................................................................... 77

3.9: SEM images of the printed lines (a) and (b) before bending and (c)

and (d) after bending ................................................................................... 78

3.10: Photograph of powered, operating microcontroller and passive

electronics on the printed glass substrate driving an LCD display ............. 79

4.1: Design parameters of wavy lines ............................................................... 88

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4.2: Schematic of wavy structure a) two half circles connected to each

other (design 1), w=800 µ r=2000µm, b) two half circles connected to

each other (design 1), w=800 µ r=4000µm, and c) two half circles

connected through a straight line w=800 µm r=4000µm

L/D=1/2(design 2) and c) two half circles connected through a

straight line w=800 µm r=4000 µm L/D=2 (design 2) ............................... 88

4.3: Schematic of horse show (design 3) wavy lines (Ɵ=80) a) L/D=1/4,

b) L/D=1/2, c) L/D=3/4, d)L/D=1 ............................................................... 89

4.4: Schematic of horse show wavy lines (design 3) a) Ɵ=30, b) Ɵ=45, c)

Ɵ=60 ........................................................................................................... 90

4.5: Simulation results obtained for the stretching of lines (design 1) a)

10, b) 100 and c) 500 µm ............................................................................ 92

4.6: Simulation results obtained for the stretching of lines

(design 2) a) 10, b) 100 and c) 500 µm ....................................................... 93

4.7: Simulation results obtained for the stretching of lines (design 3) a)

10, b) 100 and c) 500 µm ............................................................................ 94

4.8: Concentration of stress in a) design 1, b) design 2 and c) design 3 ........... 95

4.9: 3D vertical scanning interferometer images of a) design 1, b) design

2 and c) design 3 ......................................................................................... 96

4.10: Experiment set up .................................................................................... 97

4.11: Effect of displacement on the resistance of lines with different sizes

a) W=1600 μm, r=4000 μm, b) W=1600 μm, r=8000 μm c) W=800

μm, r=2000 μm d) d) W=800 μm, r=4000 μm e) w=800 μm, r=6000

μm ............................................................................................................... 99

4.12: Change in the resistance of lines having different sizes a) W=1600

μm, r=4000 μm, b) W=1600 μm, r=8000 μm c) W=800 μm, r=2000

μm d) d) W=800 μm, r=4000 μm e) w=800 μm, r=6000 μm ..................... 100

4.13: Analysis of W/r ratio on stretchability of the lines. a) W=8000 μm

r= 2000, 4000 and 6000 μm (W/r=0.4, 0.2 and 0.13), b) W=1600 μm,

r= 4000 and 8000 μm (W/r=0.2 and 0.4) c) r= 4000 μm, W=400 and

800 μm (W/r =0.2 and 0.1) ......................................................................... 101

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4.14: Effect of the displacement on the resistance of lines having different

sizes a) W=800 μm, r=4000 μm and L/D=1/2, b) W=800 μm, r=4000

μm, L/D=2, c) Change in the resistance of lines. The line with L/D

smaller than one shoes less change in the resistance .................................. 103

4.15: Effect of the displacement on the resistance of lines having different

angles a) Θ =80 b) Θ =60 c) Θ =45 d) Θ =30 ............................................. 104

4.16: Percentage change in the resistance of lines having different angles ...... 104

4.17: Analysis of the effect of L/D on the stretchability of the structure .......... 106

4.18: Analysis of the effect of thickness on the stretchability of the

structure. The with three layered sample shows less change in the

resistance of line in compare with one layer and two layered sample ........ 107

4.19: 3D vertical scanning interferometer images of a) curved part of the

line after a) 1mm (no breakage), c) 5 mm (no breakage), e) 7 mm

(started beaking), g) 11 mm (broken), straight part of the line after b)

1mm (no breakage) , d) 5 mm (no breakage), f) 7 mm (no breakage),

h) 11 mm (started breaking) ........................................................................ 108

4.20: a) Effect of the displacement on the resistance of line (L/D= ¼), b)

Change in the resistance of the line (L/D= ¼) ............................................ 110

4.21: a) Effect of the displacement on the resistance of line (L/D= 3/4), b)

Change in the resistance of the line (L/D= 3/4) .......................................... 111

5.1: Fabrication steps of strain sensor on tattoo paper a) tattoo paper in

cleaned using air gun b) the sensor is printed on the tattoo paper

substrate c) thin layer of PDMS is screen printed on the sensor d) the

structure of skin-sensor after attaching onto the skin ................................. 120

5.2: Photograph of printed sensor in both wavy and straight form

mounted on the finger ................................................................................. 121

5.3: Experiment setup ....................................................................................... 121

5.4: Resistive response of the sensor towards bending of the finger for

different angles (Sensor 1) .......................................................................... 123

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5.5: Percentage change in the resistive response of the fully printed strain

sensor towards bending of the finger for different angles (Sensor 1) ......... 124

5.6: Resistive response of the sensor towards bending of the finger

(Sensor 2) .................................................................................................... 124



5.8: Resistive response of the sensor towards bending of the wrist (Sensor

3) ................................................................................................................. 125




different angles ............................................................................................ 127

5.11: Percentage change in the resistive response of the fully printed

strain sensor towards bending of the finger for different angles ................. 127

5.12: Comparison of the percentage change of each step for different

sensors ......................................................................................................... 128

5.13. Resistive response of the straight line sensor towards bending of the

finger for different angles............................................................................ 129

5.14: Resistive response of the straight line sensor towards bending of the





finger for different angles (Sensor 1 in a straight form) ............................. 131


strain sensor towards bending of the finger for different angles

(Sensor 1 in a straight form) ....................................................................... 131

5.18: Photograph of the printed strain sensor on the PDMS (w=800 µm,

r=2000 µm) ................................................................................................. 132

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(Sensor 1 on PDMS) ................................................................................... 133

5.20: Percentage change in the resistive response of the printed strain

sensor towards bending of the finger for different angles (Sensor 1 on

PDMS) ........................................................................................................ 133


(Sensor 2 on PDMS) ................................................................................... 134



(Sensor 2 on PDMS) ................................................................................... 134


(Sensor 3 on PDMS) ................................................................................... 135



(Sensor 3 on PDMS) ................................................................................... 135


different angles ............................................................................................ 136


strain sensor towards bending of the finger for different angles ................. 137

5.27: Comparison of the percentage change of each step for different

sensors ........................................................................................................ 137

5.28: Strain sensor printed on PDMS (w=800 µm, r=4000 µm) ...................... 138

5.29: Change in the resistive response of the fully printed strain sensor

towards bending downward and upward of the knee (Sensor 1) ................ 139





5.32: Performance of the sensor after 3, 4, 5 and 6 hours ................................. 141

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5.33: Response of the strain sensor attached on the knee to different angle

of bending ................................................................................................... 142

5.34: Screen printed stretchable sensor: (a) on PVA substrate, and (b)

after transferring onto the forearm ........................................................... 143

5.35: Profilometry scan of deposited CNT layer............................................... 144

5.36: Resistive response of the sensor towards flexion and extension

movements of the elbow .......................................................................... 144

5.37: Photograph of printed temperature sensor attached on the skin .............. 146

5.38: Resistance of the sensor versus temperature and the computed linear

response using linear regression method ................................................. 146

5.39: Resistance of the sensor versus temperature and the computed linear

response using linear regression method ................................................. 147

5.40: Resistance vs heat transferred on the skin ............................................... 147

5.41: Change in the temperature after eating spicy food measured by

resistive sensor ............................................................................................ 148

5.42: a) Photograph of printed ECG pads on the PDMS b) 3D vertical

scanning interferometer images of Pads ..................................................... 149

5.43: Experiment setup ..................................................................................... 151

5.44: ECG signals obtained for a) Left hand, right hand and right leg b)

chest close to left and right shoulders and lower-left edge of the rib

cage c) chest close to left and right shoulders and right leg ........................ 152

5.45: Picture of multi-functional wearable sensor fabricated on PDMS .......... 152

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1

CHAPTER I

INTRODUCTION

1.1. Motivation

Research on the development of flexible, stretchable, conformal and wearable sensors

which can be mounted on the human skin has been gaining increasing interest [1-5]. These

sensors are capable of providing real time information about the vital signs of an individual’s

health and physical condition. There have been reports on the use of these sensors in health

tracking devices [6-9], hydration level measurement sensors [10-13] and as an electronic nose

[14-16]. The various applications of wearable sensors range from monitoring parameters, such as

body movement, body temperature, electrocardiogram (ECG), electroencephalogram (EEG) and

blood pressure for different uses in the biomedical, medical skin care and military industries [17-

20].

Disorder in movement of the body is a common problem among the elderly as well as

injured patients. There have been studies on the fabrication of different types of strain sensors for

body movement tracking [2, 21-23]. A major challenge faced by these sensors is the ability to

achieve a better contact with the body to collect accurate data while maintaining the ease and

comfort of the patient. Also, body temperature measurements as well as ECG monitoring provide

important information regarding the health monitoring of the elderly, athletes and soldiers in the

field [24].

Wearable devices are typically fabricated using traditional CMOS based electronic

manufacturing techniques, which are often expensive and laborious [25-27]. These devices are

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not suitable to being attached to the body for a long time as they are rigid and quickly cause

discomfort. The drawbacks associated with the fabrication of wearable sensors can be overcome

by employing traditional printing techniques such as gravure, screen, flexo and inkjet for

deposition of different materials on the flexible substrates such as plastic and paper. [28-30].

Reducing the wastage of materials during fabrication, low temperature operation, fast and easy

processes as well as cost efficiency are some of the advantages of employing printing methods

for the fabrication of electronics [31, 32].

Printed electronics (PE) is a quickly growing technology with noteworthy commercial

potential that is attracting significant investments into the research and development of flexible

electronics [33, 34]. In this technology, electronic components and devices are fabricated on

different flexible substrates, such as plastic, paper, and textiles. The components and devices

consist of electrically functional materials and inks forming metallization, dielectric,

semiconductors and different sensing layers and are deposited using standard printing processes

such as screen, gravure, flexo and inkjet printing. Some examples of already fabricated electronic

devices include organic thin film transistors (OTFT) [35, 36], organic light emitting diodes

(OLED) [37, 38], photovoltaics (PV) [39, 40], radio frequency identification tags (RFID) [41,

42], and biochemical sensors [43, 44]. The main advantage of PE technology is the additive

nature of the deposition processes. All the layers having different designs are selectively printed,

which excludes the need for masking and etching, resulting in less usage of material and faster

fabrication processes. In addition, printers are generally low cost compared with conventional

silicon based fabrication equipment. All the advantages of PE makes it a promising technology

for fabrication of a variety of next generation electronic ranging from fully printed wearable

devices to touch screen display applications.

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Over the last decades, a steady and considerable effort has also been directed towards the

development of materials and strategies to enable electronic devices and sensors to be transferred

directly onto the human skin [2, 4, 22, 45]. However, most of the materials that are compatible

with human skin require time consuming methods for synthesis and preparation [46, 47]. Even

though there are flexible and stretchable wearable sensors reported in the literature, they are

usually fabricated using silicon based techniques and must often be transferred using

complicated methods onto a flexible substrate or directly on the skin [2, 22, 45]. The problem of

transferring and mounting the sensor directly on the skin can be solved by employing a

sacrificial layer which can be used as a temporary holder for the sensor during the fabrication

process. The sensors can be directly printed on the sacrificial layer and then after fabrication, the

sensor can be readily removed from the substrate by dissolving the sacrificial layer. All the

improvements in PE and the availability of compatible materials/substrates with human skin

have motivated the author to focus on design, fabrication and analysis of printed, stretchable and

flexible wearable sensing systems for human health monitoring applications.

1.2. Author’s Contributions

The author’s research work has resulted in thirty seven conference publications, two

intellectual property (IP) disclosures, one patent application and five high-quality peer-

reviewed journal publications as given in the list of publications in Appendix A. The articles

directly related to the research work presented in this dissertation have been marked with “*”, in

the Appendix A. The outcomes of this research have been published and accepted to be

published in prestigious journals such as IEEE Transactions on Components, Packaging and

Manufacturing Technology and Sensors and Actuators: B Chemical. The author has also

contributed in presenting the result of the projects at several international conferences. The

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conference papers have been published in the proceedings of the IEEE Sensors Conference

(2012, 2013, 2014); International Meeting for Chemical Sensors (IMCS) (2012, 2014); and

Eurosensors Conference (2011). The author has been also awarded the All-University Graduate

Research and Creative Scholar Award for 2011-12, Departmental Graduate Research and

Creativity Scholar at the doctoral level for 2014, the prestigious Kenneth W. Knight Award

to Graduate students for 2012 and Graduate Research Award (2011, 2012, 2013, 2014) by

Western Michigan University. The author also has achieved First Place in the Graduate

Research Poster Competition (WMU College of Engineering and Applied Sciences) for 2014

as well as a Third Place in the Student Poster Presentation, at the FlexTech conference for

2014.

1.3. Organization of the Dissertation

In this dissertation, the author provides details on the development of a wearable sensing

system that employs traditional printing methods for the fabrication of stretchable and wearable

sensors, which can be mounted on human skin for health tracking. In addition, the author

presents the capability of employing conventional screen printing technology to develop a

printed and flexible multi-layered electronic circuit board, which can be used as a readout circuit

along with the fabricated wearable sensors. This work is organized and was pursued as three

distinct projects with unique contributions that may be brought together to provide an even

significant research outcomes.

In chapter 2, a comprehensive literature review is provided. Introductions to the

fabrication of electronic devices using printing methods as well as a summary of the history of

wearable sensors are presented. A discussion covering different types of sensors and their

working principles and an introduction to wearable electronic devices and their applications is

5

included. A review of the previous work in the area of wearable flexible sensors as well as theirs

strength and weakness; including, the complexity of their fabrication process is discussed in

detail. This is followed by an introductory explanation of traditional printing methods and their

ability to be employed as alternative techniques for the manufacturing of electronic device and

sensors.

In Chapter 3, screen printing is used for the fabrication of multi-layer PCBs using PE

deposited materials on three distinct substrates. The different characteristics of PET, paper and

glass as a substrate for PCBs is analyzed. A method for attaching electronic components onto the

printed PCB pads is established and demonstrated. The capability of the printed hybrid PCB

circuit to operate correctly and to drive an LCD is shown. The results obtained showed the

potential use of printed PCBs as the readout circuit for printed sensors.

In Chapter 4, screen printing is used as a new method for the fabrication of wavy lines

and structure that allow the formation of stretchable and flexible wearable sensors.

Thermoplastic polyurethane (TPU) is used as a substrate for this project. Different wavy test

structures are printed and analyzed to determine appropriate design rules and the best design

configuration. Different materials including silver (Ag) and Carbon nano tubes (CNTs) are

deposited in this process. The printability, bendability, and stretchability of all printed lines are

tested and analyzed. This work demonstrates the capability of printing techniques to provide low

cost strategies for the fabrication of wearable sensors.

In Chapter 5, a novel wearable sensing platform to be used for the detection of disorder in

body movement, temperature, and ECG is developed using both wavy an non-wavy structures.

Different inks including Ag and CNTs are used for the fabrication of different sensing elements.

Polydimethylsiloxane (PDMS)/ tattoo paper is used as the substrate for fabrication of these

6

sensors. Different methods are developed for transferring and attaching the sensors onto human

skin. The measured responses of the system demonstrate the feasibility of printed multi-

functional wearable sensors for use in health monitoring applications.

Finally in Chapter 6, the author presents the summary of the dissertation activities and

accomplishments along with suggestions for future work.

7

1.4. References

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Mater., vol. 20, pp. 4887-4892, 2008.

[2] W. H. Yeo, Y. S. Kim, J. Lee, A. Ameen, L. Shi, M. Li, S. W. R. Ma, S. H. Jin, Z. Kang, Y.

Huang, J. A. Rogers, “Multifunctional Epidermal Electronics Printed Directly Onto the

Skin”, Adv. Mater., vol. 25, pp. 2773-2778, 2013.

[3] D. H. Kim, J. H. Ahn, W. M. Choi, H. S. Kim, T. H. Kim, J. Song, Y. Y. Huang, Z. Liu, C.

Lu, J. A. Rogers, “Silicon nanomembranes for fingertip electronics”, Science, vol. 320, pp.

507-511, 2008.

[4] W. Honda, S. Harada, T. Arie, S. Akita, K. Takei, “Wearable, Human-Interactive, Health-

Monitoring, Wireless Devices Fabricated by Macroscale Printing Techniques”, Adv. Funct.

Mater, vol. 24, pp. 3299–3304 2014.

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made of silver nanowires”, Nanoscale, vol. 6, pp. 2345–2352, 2014.

[7] S. Krishnamoorthy, “Nanostructured Sensors for Biomedical Applications — a Current

Perspective” Current Opinion in Biotechnology, 34, pp. 118–124, 2015.

[8] P. Kassal, J. Kim, R. Kumar, W. R. de Araujo, I. M. Steinberg, M. D. Steinberg, J. Wang,

“Smart Bandage with Wireless Connectivity for Uric Acid Biosensing as an Indicator of

Wound Status” Electrochemistry Communications 56, pp. 6–10, 2015.

[9] C. Wong, Z. Q. Zhang, B. Lo, G. Z. Yang, “Wearable Sensing for Solid Biomechanics: A

Review” IEEE sensors journal, Vol. 15, pp. 2747-2760, 2015.

8

[10] T. Guinovart, A. J. Bandodkar, J. R. Windmiller, F. J. Andra, J. Wang, “A potentiometric

tattoo sensor for monitoring ammonium in sweat”, Analyst, vol. 138, pp. 7031–7038, 2013.

[11] J. Kim, W. R. de Araujo, I. A. Samek, A. J. Bandodkar, W. Jia, B. Brunetti, T. R.L.C.

Paixão, J. Wang, “Wearable temporary tattoo sensor for real-time trace metal monitoring in

human sweat” Electrochemistry Communications 51, pp. 41–45, 2015.

[12] A. J. Bandodkar, V. W. S. Hung, W. Jia, G. V. Ram´ırez, J. R. Windmiller, A. G. Martinez,

J. Ram´ırez, G. Chan, K. Kerman, J. Wang “Tattoo-based potentiometric ion-selective

sensors for epidermal pH monitoring”, Analyst, Vol. 138, pp. 123–128, 2013.

[13] Y.Y.G. Hoe, B.H. Johari, Ju. Meongkeun, K. Sangho, K. Vaidyanathan, K. T. Goo, “A

microfluidic sensor for human hydration level monitoring,” Defense Science Research

Conference and Expo (DSR), pp.1-4, 2011.

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Novel Wearable Electronic Nose for Healthcare Based on Flexible Printed Chemical Sensor

Array” Sensors, Vol. 14, pp. 9700-19712, 2014.

[15] T. Kinkeldei, C. Zysset, N. Münzenrieder, G. Tröster, “An electronic nose on flexible

substrates integrated into a smart textile” Sensors and Actuators B: Chemical, Vol. 174,

Pages 81–86, 2012.

[16] T. K. Tiong, C. S. Wen, C. M. Fan, H. C. Cheng, J. M. Shyu, “A wearable Electronic Nose

SoC for healthier living,” Biomedical Circuits and Systems Conference (BioCAS), pp.293,

296, 2011.

[17] J. R. Windmiller, J. Wang, “Wearable Electrochemical Sensors and Biosensors: A Review”,

Electroanalysis, vol. 25, pp. 29–46, 2013.

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9

[18] S. Patel, H. Park, P. Bonato, L. Chan, M. Rodgers, “A review of wearable sensors and

systems with application in rehabilitation” Journal of Neuro Engineering and Rehabilitation,

pp.9-21, 2012.

[19] O.D Lara, M.A Labrador, “A Survey on Human Activity Recognition using Wearable

Sensors,” Communications surveys & tutorials, IEEE, vol.15, pp.1192-1209, 2013.

[20] P. Bonato, “Wearable Sensors and Systems,” Engineering in medicine and biology

magazine, IEEE, vol.29, pp.25-36, 2010.

[21] H. Zeng, Y. Zhao, “Sensing Movement: Microsensors for Body Motion Measurement”,

Sensors 2011, pp. 638-660, 2011.

[22] D. Kim, N. Lu, R. Ma, Y. Kim, R. Kim, S. Wang, J. Wu, S. M. Won, H. Tao, A. Islam, K. J.

Yu, T. Kim, R. Chowdhury, M. Ying, L. Xu, M. Li, H. Chung, H. Keum, M. McCormick, P.

Liu, Y. Zhang, F. G. Omenetto, Y. Huang, T. Coleman, J. A. Rogers, “Epidermal

Electronics”, Science, vol. 333, pp. 838-843. 2011.

[23] H. Nakamoto, H. Ootaka, M. Tada, I. Hirata, F. Kobayashi, F. Kojima, “Stretchable Strain

Sensor Based on Areal Change of Carbon Nanotube Electrode” Ieee sensors journal, Vol.

15, pp. 2212-2218, 2015.

[24] E. Valchinov, A. Antoniou, K. Rotas, N. Pallikarakis, “Wearable ECG System for Health

and Sports Monitoring” EAI 4th International Conference on Wireless Mobile

Communication and Healthcare (Mobihealth), pp. 63-66, 2014.

[25] Y. Mengüc, Y. Park, E. Martinez-Villalpando, P. Aubin, M. Zisook, L. Stirling, R. J. Wood,

C. J. Walsh “Soft Wearable Motion Sensing Suit for Lower Limb Biomechanics

Measurements” 2013 IEEE International Conference on Robotics and Automation (ICRA),

pp. 5309-5316, 2013.

10

[26] J. Tanaka, M. Shiozaki, F. Aita, Seki, M. Oba, “Thermopile infrared array sensor for human

detector application,” Micro Electro Mechanical Systems (MEMS), 2014 IEEE, pp. 1213-

1216, 2014.

[27] D. Son, J. Lee, S. Qiao, R. Ghaffari, J. Kim, J. E. Lee, C. Song, S. J. Kim, D. J. Lee, Samuel

W. Jun, S. Yang, M. Park, J. Shin, K. Do, M. Lee, K. Kang, C. Seong Hwang, N. Lu, T.

Hyeon, and D. Kim, “Multifunctional wearable devices for diagnosis and therapy of

movement disorders”, Nature Nanotechnology, vol. 9, 397–404, 2014

[28] A. Russo, B.Y. Ahn, J.J. Adams, E.B. Duoss, J.T. Bernhard and J.A Lewis, “Pen-on-paper

flexible electronics”, Adv. Mat., vol. 23, pp. 3426-3430, 2011.

[29] J. Lessing, A.C. Glavan, S.B. Walker, C. Keplinger, J.A. Lewis and G.M. Whitesides,

“Inkjet printing of conductive inks with high lateral resolution on omniphobic “RF paper”

for paper-based electronics and MEMS”, Adv. Mat., vol. 26, pp. 4677-4682, 2014.

[30] K. Jost, D. Stenger, C.R. Perez, J.K. McDonough, K. Lian, Y. Gogotsi and G. Dion,

“Knitted and screen printed carbon-fiber supercapacitors for applications in wearable

electronics”, Energy & Environ. Sci, vol. 6, pp. 2698-2705, 2013.

[31] K. Dongjo, J, Sunho, L. Sul, P. K. Bong, M. Jooho, “Direct writing of copper conductive

patterns by ink-jet printing” Thin Solid Films. vol. 515, pp. 7706–7711, 2007.

[32] A. S. G. Reddy, B. B. Narakathu, M. Z. Atashbar, M. Rebros, E. Hrehorova, B. J. Bazuin,

M. K. Joyce, P. D. Fleming, A. Pekarovicova, “Printed electrochemical based biosensors on

flexible substrates” Sensor Letter. vol. 9, pp. 869-871, Apr. 2011.

[33] A.C. Arias, J.D. MacKenzie, I. McCulloch, J. Rivnay and A. Salleo, “Materials and

applications for large area electronics: solution-based approaches”, Chem. Rev., vol. 110,

pp. 3-24, 2010.

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11

[34] C.C. Tsai, “Recent development in Flexible Electronics”, Proc. IEEE Optoelectron.

Commun., pp. 370-371, 2011.

[35] S. Chung, S.O. Kim, K. Soon-Ki, C. Lee and Y. Hong, “All-inkjet-printed organic thin-film

transistor inverter on flexible plastic substrate”, IEEE Electron Device Lett., vol.32, pp.

1134-1136, 2011.

[36] H. Kang, R. Kitsomboonloha, J. Jang, V. Subramanian, “High-Performance Printed

Transistors Realized Using Femtoliter Gravure-Printed Sub-10 μ m Metallic Nanoparticle

Patterns and Highly Uniform Polymer Dielectric and Semiconductor Layers ” Adv. Mater.,

vol. 24, pp. 3065–3069, 2012.

[37] S.H. Jung, J.J. Kim and H.J. Kim, “High performance inkjet printed phosphorescent organic

light emitting diodes based on small molecules commonly used in vacuum processes”, Thin

Solid Films, vol. 520, pp. 6954-6958, 2012.

[38] V. L. Calil, C. Legnani, G. F. Moreira, C. Vilani, K. C. Teixeira, W. G. Quirino,

R. Machado, C. A. Achete and M. Cremona, “Transparent thermally stable poly(etherimide)

film as flexible substrate for OLEDs” Thin Solid Film, Vol. 518, pp. 1419–1423, 2009.

[39] Y. Liu, T.T. Larsen-Olsen, X. Zhao, B. Andreasen, R.R. Sondergaard, M. Helgesen, K.

Norrman, M. Jorgensen, F.C. Krebs and X. Zhan, “All polymer photovoltaics: From small

inverted devices to large roll-to-roll coated and printed solar cells”, Sol. Energy Mater. Sol.

Cells, vol. 112, pp. 157-162, 2013.

[40] J. M. Ding, A.Vornbrock, C. Ting, V. Subramanian, “Patternable polymer bulk

heterojunction photovoltaic cells on plastic by rotogravure printing” Solar Energy Mater.

Solar Cells, Vol. 93, pp. 459–464, 2009.

12

[41] J. Chang, G. Tong and Lin Tong, “Fully-additive printed RFID on a plastic film”, IEEE

Microwave Workshop Series on RF and Wireless Technologies for Biomedical and

Healthcare Applications (IMWS-BIO), pp. 1-3, 2013.

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multilevel interconnects and inductive components on plastic for ultra-low-cost RFID

applications” MRS Proceedings, Vol. 769, 2003.

[43] V.V. Shumyantseva, T.V. Bulko, A.V. Kuzikov, R. Khan and A.I. Archakov, “Development

of methods for functionalization of screen printed electrodes with biocompatible organic-

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B Chem., vol. 8, pp. 237-242, 2014.

[44] S. Andreescu, O.A. Sadik, “Trends and challenges in biochemical sensors for clinical and

environmental monitoring” Pure Appl. Chem., Vol. 76, pp. 861–878, 2004.

[45] X. Huang, W.H. Yeo, Y. Liu, J. A. Rogers, “Epidermal Differential Impedance Sensor for

Conformal Skin Hydration Monitoring” Biointerphases 2012.

[46] D. Kim, J. Viventi, J. J. Amsden, J. Xiao, L. Vigeland, Y. Kim, J. A. Blanco, B. Panilaitis,

E. S. Frechette, D. Contreras, D. L. Kaplan, F. G. Omenetto, Y. Huang, K. Hwang, M. R.

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integrated electronics”, Nature Materials, vol. 9, pp. 511–517, 2010.

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TiO2 nanotubes on biomedical Ti Alloys”, vol. 9, pp. 1-10, 2014.

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13

CHAPTER II

LITERATURE REVIEW

2.1. Introduction

In this chapter, the author provides a comprehensive literature review on wearable and

stretchable sensors, their fabrication mechanism and PE. The background review on the history

of wearable and health monitoring sensors will provide a vision into the science behind the

design, fabrication and challenges of manufacturing of skin like sensors. The author then

presents an introduction and review of different forms of printing as alternative methods for

deposition of active material layers used in the fabrication of electronic devices. A detailed

review on the development of new materials, as well as strategies for building wearable sensors

using PE will also be presented.

2.2. Sensors

2.2.1. Introduction to Sensors

Sensors are used for detection of different parameters such as physical, chemical or

biological changes. These devices are capable of transmitting and receiving signals based on

changes in a system which makes them capable of responding to physical phenomenon such as

light, heat, pressure, magnetism or motion and send out the appropriate signal.

According to Merriam-Webster’s Dictionary, “… a sensor is a device that responds to a

physical stimulus (as heat, light, sound, pressure, magnetism, or a particular motion) and

transmits a resulting impulse (as for measurement or operating a control) …” [1]. The human

14

body is an amazing example of a sensing system that is composed of different sensors, which can

provides information. The common sensors found in a human being are the five traditional

senses; sight, hearing, smell, taste and touch [2].

Advancement in the semiconductor industry and the discovery of addition functional

materials has assisted the development of new and different sensors and sensing systems [3-13].

On a daily basis, many old and new sensing devices are being used to improve and facilitate our

life. Temperature controllers, health monitoring devices, automated car fuel alarm system, touch

screen devices, among others systems, are examples of sensors utilized on a daily basis. The role

of these sensors can be classified in detecting the information, generating an output which results

on the helping in decision making and generating an output signal. Combining multiple sensors

together of distributing the information to existing systems can improve or enhance decision

making and actions. On the simplest level, examples of input/output signals are illustrated in

Figure 2.1. For example, when the resistive temperature sensor is subjected to the heat the

impedance of the sensor will be changed.

2.2.2. Types of Sensors

The capability of sensing devices, for use in different applications, is identified by the

sensor detection modes. The different classes of detection modes include thermodynamic,

mechanical, acoustic, chemical and biological stimuli, each of which has its own unique

characteristics.

2.2.2.1. Thermal Sensors

Thermal sensors are used for detection of changes in temperature or energy (heat).

Thermal sensors are classified in two forms, contact and non-contact, based on their modes of

15

Figure 2.1: Examples of sensor input/output relationship.

detection. A good example of the contact mode is a thermometer [14]. Figure 2.2 represents a

typical structure for a thermometer, consisting of three main components; the bulb which is

usually made of glass, the temperature sensitive liquid which responds to increasing and

decreasing temperature, and a scale which is used for identifying the temperature changes. The

thermometer is used frequently due to ease of use and accuracy. The change in the temperature is

detected based on fluctuations in the air or material placed in contact with the bulb. The bulb is

loaded with temperature sensitive liquid, typically mercury or red alcohol. The difference

between the volumetric reversible thermal expansion of the liquid and glass is used to change the

liquid level and measure the relative change in temperature. For a faster response the bulb should

OUTPUT INPUT

Sensor

16

be small and thin. The scale is a representation of degrees, which are etched or printed in the

glass of the thermometer as markings once calibrated based on the liquid and the size of the glass

capillary tube used. As the temperature fluctuates, the heat sensitive liquid increases or decreases

in volume and level due to gravity and capillary action. The measurement of Infrared (IR)

radiation is an example of a non-contact thermal sensor.

Figure 2.2: Liquid in thermometer [14].

2.2.2.2. Mechanical Sensors

Mechanical deformation or deflection of sensitive material is used for generating the

output for mechanical sensors. Some of the different parameters that are detected by mechanical

sensors include; changes in position, pressure, stress, acceleration and flow rate. A schematic

representation of a mechanical pressure sensor is shown in Figure 2.3. The capacitive pressure

sensor shown is composed of two conductive plates and a spacer. When a pressure is applied on

-20

-10

50

0

10

20

30

40

17

the sensor, a deformation will be generated in one or both plates and the dielectric layer causing

the thickness or distance between the two conductive electrodes to change. As a result, the

capacitance will be increased and the change in applied pressure can be detected.

Figure 2.3: Pressure sensor as a mechanical sensor [14, 15].

The ratio of the capacitance of a non-deflected structure and the deflected structure can be used

to calculate the deflection distance and determine the applied pressure [14, 15].

2.2.2.3. Acoustic Sensors

Acoustic sensors are capable of sensing the physical phenomenon of waves propagating

throughout different materials such as lithium niobate, lithium tantalate, quartz , etc. When the

wave travels through or along the surface of well-defined classes of material, the properties of

the material might affect the characteristics of the wave such as velocity or amplitude. These

changes can be detected by measuring the frequency or phase in sensors. Based on the mode of

the wave traveling in the material, these sensors are categorized into two forms, bulk or surface

wave sensors. In bulk acoustic wave sensors, the wave travels through the volume of the

material. In the second form, which is called surface acoustic wave sensor, the wave travels on

the surface of the sensor. This class of acoustic sensors is divided into shear horizontal waves or

shear vertical waves based on the mode of propagation. Piezoelectric material is generally used

Substrate

Top conductive layer

Bottom conductive layer

Dielectric layer

https://en.wikipedia.org/wiki/Lithium_niobate

https://en.wikipedia.org/wiki/Lithium_tantalate

https://en.wikipedia.org/wiki/Quartz

18

in acoustic devices to generate acoustic waves. An acoustic gas sensor is depicted in Figure 2.4

[14].

Figure 2.4: Schematic of acoustic gas sensor.

The gas sensing film in placed on the surface of the device. When this film is exposed to

the gas, the properties of the sensing film (mechanical and electrical) will change which results

in the change in the oscillation frequency of the sensor. In the case of electrical change, the

conductivity of the sensing material can change when it is exposed to the gas which results in

change in the oscillation frequency. For the mechanical change, concentration of adsorbed gas

into the bulk of the sensing film can also cause a change in the oscillation frequency of the

sensor.

2.2.2.4. Chemical Sensors and Biosensors

Chemical sensors and biosensors are devices that transform the chemical information of a

measurand to an output electrical signal. This chemical information can be the result of a

chemical reaction of the analyte or the binding between two molecules in a sample. Usually, a

chemically sensitive layer is integrated into both chemical sensors and biosensors. Transducer

along with associated electronics or signal processing components is commonly present on

chemical sensors. A basic diagram for chemical sensor is represented in Figure 2.5 [15]. The

Input IDTs Output IDTs

Sensing layer

19

diagram shows an array of interdigitated electrodes (IDTs) that has a sensing layer coated on

them. When the sample contacts the sensing area, chemical or biological reaction will occur and

continue to be accrued. The reaction causes change in the capacitance or impedance of IDTs,

which arrives at a steady-state and can be used for detection of the test sample.

Figure 2.5: Schematic of an array of chemical or biosensor [15].

2.2.2.5. Optical Sensors

The functionality of optical sensors is based on detecting the change in the measurand

using light as a stimulus. Optical sensors consist of optical waveguides and optical fibers. Optical

fibers are made up of a core and cladding. Figure 2.6 shows a simple example of an optical

sensor, which consists of cylindrical cladding layer and a circular core Figure 2.7 illustrates the

basic structure and physical parameters of an optical fiber utilized in optical sensors. To prevent

light propagation losses into the fiber and accomplish complete internal reflection, the angle of

incident light should be higher than the critical angle. The critical angle (φ1) is obtained by the

following equation 1, where nj is the refractive index of the core in the optical fibers and np is

that for the cladding’s [16, 17].

𝜙1 = 𝑠𝑖𝑛−1𝑛𝑝

𝑛𝑗 (1)

Sensing Layer

Interdigitated Electrodes Electrical

Output

20

As the light enters the optical fiber, if the angle of introduced light into the optical fiber is

more than the critical angle it will continue traveling through the fiber, reflecting at the core

boundary, for a significant distance without propagation loss.

Figure 2.6: Schematic of optical fiber guide.

Glass and plastic are the main types of raw materials utilized to fabricate and develop

optical fibers for wavelengths in the visible light region. Material such as glass and plastic, along

with reflective and polished coated hollow tubes, are used for channeling light in the near and far

infrared regions [16, 17].

Figure 2.7: Basic structure of optical fiber.

Core

Cladding

Coating

Φ

21

Optical sensors are typically non-electric based as well as contactless sensing devices, which

provides the advantage of remote sensing. Such devices are resistant to radio frequency and

electromagnetic interference and they don’t bother the environment.

A summary of different types of sensor and their stimulus is presented in table 2.1.

Table 2.1: Different types of sensors and their stimulus [18].

In the following sections, the author presents the details of wearable sensors and their

applications as well as previous works on design, strategies for fabrication and analysis of

wearable sensors. The advantages and disadvantages of different fabrication methods and their

applications are also provided.

22

2.3. Wearable Electronics and Applications

2.3.1. Introduction to Wearable Electronics

Wearable electronic devices, which can be mounted on the skin or garment, have gained

in interest over the last few decades [19-28]. Development of the technology which made

electronic devices mobile and portable has provided new application for electronic devices such

as smart phones, small music players and different personal access devices. While the wrist

watch and small portable radios may be categorized as first generation of wearable electronics, in

1960s, one of the first recognized wearable devices was made by Edward Thorp and Claude

Shannon for prediction of numbers in roulette [29].

Advancements in computing systems, along with the miniaturizing of the size of the

devices, have enabled the users to carry sophisticated systems and have access to data and

information almost everywhere. Wearable computers are defined as miniaturized version of

desktop computers that can be carried during operation [30]. These wearable computers are

either used for a specific target, such as heart rate monitoring or for general support and

assistance in daily life. The wearable computing systems can be attached to cloth, such as a

jacket or belt, as well as a human body. The term, wearable electronics refers to that type of

electronic devices that are worn or mounted on the human body or clothing for an extended

period of time [30]. Wearable devices can be divided into two forms, 1) those which are attached

to clothing and 2) those which are mounted on the body.

Sensors and sensing systems have been become pervasive during last few decades. For

example, sensors are being inserted widely into our daily lives including in cars, cameras, cell

phones and even in clothing and buildings. There has been a serious need for the development of

23

flexible and stretchable wearable electronic systems to overcome the drawbacks of ambulatory

methods for monitoring the vital signs of individuals. These devices should be capable of

monitoring of the patient over weeks or even months. Such devices can be used in biomedical,

epidermal, military and environmental monitoring application [31-43]. Flexibility and

stretcahbility can improve the functionality of these devices while they are attached on the body.

During the last decade, the improvement of wearable electronic devices has led to an increasing

interest in the design and fabrication of health monitoring, wearable sensors [44-51].

An important challenge faced by development of wearable electronic devices is ability to

maintain ease and comfort of the [52-59]. In this chapter, some of the previous works that have

concentrated on design and fabrication of flexible wearable sensors and their challenges will be

discussed. Problems associated with the fabrication of these wearable sensors will be analyzed.

2.3.2. Previous Works

Most fabricated electronic devices are made of single-crystal, inorganic materials such

as silicon, which is rigid. In the case of flexible and stretchable sensors, the interfaces for these

devices with the human body play an important role in the sensitivity and the operation of the

devices. While most efforts during the last few decades were directed toward reducing the size

of the electronic devices and increasing the operational speed, new applications of electronics

require new form factors and focus such as flexibility and stretchability, which can make the

electronics conformal. Most of the newer devices are fabricated on non-wafer substrates such as

plastic and paper. Stretcahbility to some extend can help and plays an important role in

performance of conformal and flexible sensors [60-64].There has been a significant amount of

24

research and development of methods and materials to improve flexibility and stretchability of

sensors [65-78].

One strategy to overcome these drawbacks is to fabricate electronic devices using rigid

components that are connected using stretchable connectors such as gold and silver, which have

been frequently reported [65-68]. This form of electronics is suitable for devices that don’t

have too many active components. John Rogers and his group have reported different methods

and strategies for fabrication of stretchable and flexible electronic devices [68, 79-81]. Building

of a ribbon structure in form of a coil, or helix is one of the reported methods for fabrication of

stretchable interconnections. This ribbon acts like a spring and enables the device to stretch.

This kind of semiconductor ribbons can be fabricated on the single-crystal wafers. The strained

multilayered film is grown on these wafers. These ribbons are then cut using lithography into

nano ribbons and they will be released from their supporting substrate by dissolving the

sacrificial layer [79, 81]. After releasing from the supporting substrate, the strained layers in the

ribbons cause them to bend upward and make a tube shape [68, 79-81] which is shown in Figure

2.8 (a). The geometry of the fabricated structures depends on properties of the substrate and the

thickness of deposited material [67, 68, 79-81]. Another method for fabrication of stretchable

interconnections is based on the use of ripple configurations, which are illustrated in figure 2.8

(b, c) [67, 68, 79-81]. Fabrication steps for these nano-ribbons start by deposition of material on

a source or host wafer using silicon based technologies. Poly methyl methacrylate (PMMA), a

sacrificial layer, is cast coated on the source wafer. Then the conductors or semiconductors are

deposited on the substrate and formed in parallel arrays of ribbons. Next, the sacrificial layer is

etched. After this step, pre-strained PDMS, stretched from length of L to L+∆L which results in

pre-strain of ∆L/L, will be placed in contact with the ribbons, and, by applying pressure, the

25

ribbons will become attached to the PDMS. The ribbons are attached to the surface of PDMS

based on the van der Waals chemical bonding [67, 68, 79-81]. Releasing the strain in the PDMS

results in the formation of wavy/buckled ribbons, as shown. The ribbons are then transferred

onto a final flexible substrate. Deposition of material and patterning in the desired form can be

done using different conventional methods based on the properties of different materials [62-66,

70, 71].

Figure 2.8: a) spring form of interconnections, b) and c) wavy form of interconnections (i) chemical

bonding along the ribbon and (ii) chemical bonding in the selected areas, courtesy of [68].

Discussing this fabrication method in more depth, if the surface of PDMS is oxidized

only in selected areas, after releasing the pre-strain, the ribbons form buckled shapes, which are

completely separated from the PDMS in non-oxidized areas. PDMS is made of polymeric chain

with the repeating unit of –(CH3)2SiO2–. Low surface energy as well as low Young’s modulus

enables the PDMS to form a conformal contact with almost any substrates. Exposing PDMS to

the ozone changes the hydrophobic properties of the surface to hydrophilic by oxidizing the –

26

CH3 of the surface and generating –Si–OH. After this conversion, the surface of PDMS can

react and make a chemical bond with different materials. The wavelength and the amplitude of

the fabricated lines can be predicted using the following formulas [68, 79-81].

𝜆0 =𝜋 ∗ ℎ

√ԑ𝑐

(5)

𝐴0 = ℎ√ԑ𝑝𝑟𝑒

ԑ𝑐− 1 (6)

ԑ𝑐 = 0.52√[𝐸𝑃𝐷𝑀𝑆 ∗ (1 − 𝑣𝑥

2)

𝐸𝑥 ∗ (1 − 𝑣𝑃𝐷𝑀𝑆2 )

]

23

(7)

where ԑ𝑐 is the critical strain for buckling, ԑ𝑝𝑟𝑒 is the level of prestrain, 𝜆0 is the wavelength, and

𝐴0 is the amplitude. The Poisson ratio is v, the Young’s modulus is E, and the subscripts refer to

properties of the PDMS or deposited material (x). The thickness of the deposited material is h.

Usually, three different techniques are used for pre-straining the PDMS. In the first

method, PDMS is mechanically rolled after bringing it in contact with the deposited material on

the holder substrate (usually silicon wafer). The wavy structure made using this method does not

show uniform wavelength and amplitude. In the second method the PDMS is exposed to a 30 °C

to 180 °C temperature before bringing it in contact with the deposited material on silicon and it is

cooled down in next step. The wavy lines show uniformity in a large area with high

reproducibility. In the third method, the PDMS is stretched using mechanical stage and pre-strain

is physically released after removal from the wafer. Fabricated structures have been used for

fabrication of range of electronic components and sensors having diverse applications in health

monitoring as well as epidermal electronics [68, 79-81].

27

John Rogers and his group [81] reported the fabrication of multifunctional epidermal

electronics, which can be directly transferred onto the human skin by employing the strategies

discussed above. This device consists of multiple sensors for measuring temperature, strain, ECG

and EMG as shown in Figure 2.9 [81]. All of these sensors are fabricated using lithography to

form of wavy and buckled lines. After fabrication, the device was transferred onto polyvinyl

alcohol (PVA), a flexible substrate, or directly onto skin. Wavy lines with different width were

fabricated to determine the best interface with the human body and to overcome potential

breakage due to the roughness of skin. Challenges in transferring the sensor directly onto the skin

were analyzed and reported. The authors have provided results of different sensors, which look

promising for use as wearable devices [81].

Figure 2.9: Multifunctional epidermal electronics, courtesy of [81].

In a separate article, John Rogers’s group [82] reported fabrication of another set of

epidermal electronics, which includes sensing systems as well as transmission components, as

shown in Figure 2.10. Interconnections as well as some components are fabricated by employing

28

wavy structures. This device was transferred onto the forearm and forehead of a human test

subject and the effects of body motion on the stability of the device were analyzed. Mechanical

properties of the fabricated device were also analyzed, which showed sufficient strength and a

good tolerance for stress based on the movement of the body and the continued operation of the

device [82].

Figure 2.10: Epidermal wearable electronics, courtesy of [83].

Fabrication of differential impedance sensors for skin hydration monitoring, using

transfer printing of the device on a flexible substrate, was also reported by John Roger’s group.

These devices include different forms of sensors; circular, meander and interdigitated. For each

form of Sensor, The identical structures was used for both reference and measurement

electrodes. The fabrication of a complete device was started by spin coating of PMMA and poly

Imide (PI) on a silicon wafer. Photolithography was used for patterning the deposited material to

form interconnections. The final device was transferred onto a layer of PVA as a temporary

29

flexible substrate. The sensor was transferred onto the skin and was attached to the skin by Van

Der Waals forces. The device showed stability towards applied stretching in different directions.

The lines were connected to an external data acquisition instruments. The change in the

impedance of the sensors was obtained by changing the hydration of the skin, which showed the

capability of the stretchable and flexible sensor to monitor the hydration level of the skin [83].

In another work, the Dae-Hyeong Kim group [84] used the same principle as Rogers'

group for fabrication of wearable and stretchable devices. The device includes a data storage

unit, a diagnostic section and a drug delivery section in stretchable form as shown in Figure 2.11.

The fabricated wearable patch is capable of measuring movement disorders as well as delivering

a drug to the surface of the skin. Strain sensors were used for the detection of disorder in

movement. The monitored data were stored in the flexible memory and analyzed. Based on

defined criteria, feedback in the form of drug delivery was provided. Implementation of different

stretchable sensors and electronic parts, fabricated on a light weight and thin patch, can play an

important role in the future of flexible sensing systems. As shown, the various elements and

sensors of this device were tested and the functionality and limitation of each part has been

discussed [84].

30

Figure 2.11: Wearable patch courtesy of [84].

There are additional reports on the fabrication of flexible electronic components, such as

transistors and diodes [85-92]. Even though, all these devices showed potential to perform under

compressive and tensile stress the problem associated with fabrication of these devices is the

complicated process of fabrication. The silicon based technologies are time consuming,

expensive and require expertise used for fabrication of these devices on the rigid substrate. In the

next step, complicated processes are used to transfer the final device onto the flexible substrate.

Therefore, there is a need for an alternative method to overcome these drawbacks. Printing

technologies, which will be discussed in the next section, can be used as alternative methods for

deposition of functional materials required for the fabrication of electronic devices. Some of the

advantages of these methods are fast processing, less waste of material, and the deposition of

material at room temperature.

2.4. Printed Electronics

Current research and the growth of interests in flexible electronic devices and sensors is

due to their potential applications in different aspects of daily life and the need for an alternate,

31

simple methods for fabrication. The fabrication method of choice for flexible devices in this

research involves traditional printing methods. The capability of printing technology for the

manufacturing of electronic devices come from the additive deposition of multiple layers in the

form of thin films in a simple and cost-effective way as compared to existing electronics

production processes. Overall, PE offers the implementation of new form factors, such as

flexibility and strechability, and the fabrication of new types of electronics based on the

deposition of active materials on flexible and stretchable substrates. The advantages of PE in

manufacturing of electronic devices include: cost efficiency, flexibility, lightweight, fast

production and less waste of material. The origins of the printed electronics manufacturing come

from the membrane switch and Electroluminescence lamp (EL) which used printing as a

manufacturing technique [93]. PE can promise simple manufacturing process for flexible large-

area devices, which can be disposable. PE has been growing gradually over last 30 years and

more recently by advancement in the printing of nanomaterials; an emerging industry that has

shown the potential to revolutionize numerous products in the market.

Merging traditional printing methods with conventional electronic fabrication process has

also shown potential for high volume electronic manufacturing. In contrast with lithography

based subtractive methods, the additive properties of PE offer lower cost processes for electronic

manufacturing. In comparison with other methods PE does not require large amounts of material

and the waste of resources during fabrication is reduced due to deposition of material only on the

desired areas.

The development and improvement of different inks such as conductive, dielectric and semi-

conductive along with advancement of ink deposition methods for fabrication of electronics have

also enabled the integration of PE processing steps to be incorporated in current electronics

32

fabrication processes. Similar to all technologies, PE faces a range of difficulties and challenges.

Some of these challenges include the stability, durability and yield of the fabricated devices.

Functionality and feature sizes of the printed devices are additional limitation for this emerging

industry as compared with silicon based devices. The standards for this field as well as design

rules are some of the ongoing research areas that require further study and development.

2.4.1. Types of Printing

Printing processes are categorized into two classes: (a) impact printing and (b) non-

impact printing [101]. Non-impact printing does not require a mask or image carrier. It can be

used for printing on rigid or flexible substrates. In this method the image is mainly controlled

digitally. In impact printing, the image is transferred onto the substrate by using an image carrier.

Different forms of printing are summarized in Figure 2.12.

Printed techniques can be classified into four traditional methods which are used in the

fabrication of PE devices: screen printing, inkjet printing, flexo printing and gravure printing.

Flexography and gravure printing techniques are identified as roll-to-roll printing, due to the fact

that the printing process is typically performed on a continuous sheet of substrate delivered from

a large roll of material. Inkjet printing, in contrast, is considered for thin film printing and non-

contact nature; while screen printing is known for printing thick film layers. Further details of the

printing methods will be discussed in the following section. A summary of the characteristics of

these four printing process is shown in Table 2.2 [93, 95]. The ink viscosity, expected thickness

of the layer, the resolution and the roughness of the layer play an important role in selection of

proper printing methods for fabrication of electronic devices.

33

Figure 2.12: Summary of different forms of printing.

2.4.1.1. Screen Printing

The main components used for screen printing are the image carrier which is called

screen printing plates and the squeegee for applying the pressure on the ink. Material which is

used for fabrication of squeegees include rubber or other polymeric materials. The screen plate

has two parts 1) frame and 2) the stencil. The frame is used to hold the screen mesh and the

stencil in the image carrier. The frame is generally made of steel or aluminum for their strength

and rigidity as well as the ease of cleaning. The material used for the screen fabric and the stencil

Printing Methods

Impact

Lithography

Gravure

Flexography

Screen

Waterless Offset

Offset

Non-Impact

Photography

Thermography

Inkjet

Magnetography

Ionography

Electrophotograph

y

Drop on

Demand

Continues

34

depends on the ink solvent. Ease and comfort of the cleaning also plays a role in choice of

material for fabrication of frame.

Table 2.2: Different characteristics of printing methods [95].

Lateral Resolution

(µm)

Ink Film

Thickness (µm)

Viscocity

(Pa.s)

Printing speeds

(ft/min)

Flexography 15 0.5-2 0.05-0.05 300-1000

Gravure 15 0.5-8 0.05-0.2 1500-3000

Screen 15 3-60 0.5-50 300-500

Inkjet 20 0.05-0.50 0.001-0.04 500

(for 600×600 dpi)

Screen printing is considered a push through process. The ink is applied on the screen and ink

passes through the screen mesh and is transferred onto the substrate when the ink is swept by the

squeegee. Figure 2.13 illustrates the typical screen printing process [94].

2.4.1.2. Inkjet Printing

Inkjet printing is a non-impact printing process that does not use image carrier or mask.

The image is usually formed digitally. This process is divided into two types of printing

techniques: 1) continuous inkjet and 2) drop on demand inkjet. Drop on demand inkjet printing

include 1) thermal and 2) piezoelectric actuation. Figure 2.14 a, b and c illustrates the difference

between these classifications.

35

Figure 2.13: Screen printing.

In continuous inkjet printing, a continuous stream of ink droplets is generated during the

printing process. This stream is electronically controlled in a process where a high voltage,

electrostatic field is formed that can deflect ink droplets from n image area to a non-image area.

The deflected ink is directed back into the printer. In comparison, a drop on demand inkjet

printing generates ink droplets only on the desired locations to form the images. Drop on demand

inkjet printing techniques use either thermal or piezoelectric techniques to jet individual drops of

ink [94].

36

Figure 2.14: (a) Continuous inkjet, (b) Thermal inkjet and (c) Piezo inkjet [94]

2.4.1.3. Flexographic Printing

Flexographic printing is a roll-to-roll printing technique that became popular in the 20th

century. The main components of this printing process are a plate cylinder, which is made of

rubber or photopolymers, an impression cylinder, an anilox roll, and an inking unit. The imaged

areas on the plate cylinder are raised with respect to the overall surface of the plate cylinder. The

image is transferred from the plate cylinder to a substrate through the use of an inking unit. The

inking unit first transports the ink particles onto the anilox roll, which is metered with the

assistance of a doctor blade that wipes the excess ink from the surface of the anilox roll. In the

next step, the raised parts of the plate cylinder will receive the ink from the anilox roll. Because

the non-image areas are not raised with respect to the surface of the cylinder they don’t receive

(a)

(b)

(c)

37

the ink. The image on the plate cylinder is transferred to the substrate by the use of an impression

cylinder. Figure 2.15 shows the Flexographic printing process [94].

Figure 2.15: Flexographic printing

2.4.1.4. Gravure Printing

Figure 2.16 illustrates different components of gravure printing method which include an

image carrier that is called the gravure cylinder, impression cylinder for helping in transfer of the

image, an ink container which is called ink fountain and a wiping part which is called doctor

blade and. Steel having the coating of copper on the surface is a typical material for fabrication

of the image carrier. In order to create the image on the cylinder, small recessed cells that are

made using engraving onto the copper which is chrome plated and it can provide wear resistance.

38

The impression cylinder is manufactured with materials like rubber. The rotation of the gravure

cylinder in the ink fountain results in the filling of the cells on the cylinder. Doctor blade wipes

off the remaining excess ink on the surface of the image carrier. The ink in the cells is imprinted

onto the substrate with the help of the impression cylinder [95, 96].

Printing techniques have shown their ability for fabrication of electronic devices and

components, such as electrochemical sensors [97, 98], strain gauges [99, 100], pressure sensors

[101, 102], substrate for Raman spectroscopy [103], TFTs [104-107], and solar cells [108, 109],

paper based devices, [110, 111]. There has been an increasing interest in utilizing printing

methods for fabrication of wearable and stretchable electronics. Some of the major works will be

discussed below.

Figure 2.16: Gravure printing

Ongoing research on fabrication of stretchable and wearable sensors using PE can be divided

into two categories, the development of material and the development of strategies. There have

been some researches on development of stretchable and mechanically strong inks to be used in

this area [110-119].

Gravure cell

filled with ink

Substrate with

transferred ink

Ink Pan

Engraved Cylinder

Impression Roller Substrate Web

Doctor Blade

39

In most fabricated devices, silver or gold are used as a metallization layer. Both of these

two materials are not stretchable and when placed under tensile strain the structure will break. To

overcome this problem silver nano wires (AgNWs) are a good choice of material. Shanshan Yao

and Yong Zhu [120] fabricated multi-functional wearable sensor using AgNWs. This sensor is

capable of detecting strain and pressure, including a finger touch with high sensitivity and a fast

response time. Feng Xu and Yong Zhu [121] reported fabrication of stretchable silver nanowire

conductors. In this work, first AgNWs are drop-casted on substrates such as silicon (Si) wafers,

glass slides, or plastic materials prior to being dried. Next liquid PDMS is casted on the NWs

and cured at 65 ° C for 12 hours. The AgNWs/PDMS composition is peeled off of the substrate.

This mixture shows promising properties as a conductor for the fabrication of wearable and

stretchable electronic devices [121].

Carbon Nano Tube (CNTs) based inks are one the most popular materials in the field of

PE and wearable sensors. CNTs are mechanically strong and they can tolerate to some extent

applied strain on their structures; therefore, they offer promising properties for stretchable

materials. Usually CNTs are not highly conductive, which can be a drawback for some devices

where high conductivity is needed. For these applications, a mixture of CNTs with other

conductive materials is being used. For example, Kyoung-Yong Chun demonstrated printable

and stretchable hybrid composites consisting of silver flakes, multi-walled carbon nanotubes and

self-assembled silver nanoparticles [114]. They reported that the maximum conductivities of

these composites were 5,710 S cm-1

at 0% strain and 20 S cm-1

at 140% strain, at which point the

film ruptured.

Along with the development of stretchable materials, there have been a significant

amounts of work on the development of new strategies for fabrication of flexible wearable

40

sensors using printing methods [122-132]. Some of the accomplishments will be discussed in

following section.

Yang Wei and his coworkers employed screen printing for the fabrication of printed

cantilever structures used for wearable motion detection applications. Polyester cotton-fabric was

used as a substrate. To reduce the roughness of the fabric, a UV curable polymer was deposited

and used as an interface material. After this step, the electrodes and sacrificial layer were printed

on the substrate to form the cantilever structure. The picture of the sensor is shown in Figure

2.17. The fabricated sensor was placed on the arm of a human and motion was detected. While

promising, as the authors suggested, there is a need to encapsulate the sensor and to perform

additional design modifications to reduce the size of the sensor and also test the sensor at a lower

frequencies. Also future work in this paper suggests printing of circuit board directly on the

fabric to integrate the full system on the flexible substrate. [133].

Figure 2.17: Printed cantilever structure courtesy of [133].

There have been multiple attempts into the fabrication of printed strain sensors for

movement measurement. Various materials, such as silver, carbon nanotube, and PEDOT:PSS

41

have been used as metallization layers. All these printed sensors showed ability to measure

motion to some degrees. However, a weak response to applied strain on the fabricated devices

has been one of their major problems. When these devices are stretched, the lines often break.

Therefore, there is a need for alternative materials and strategies to enable printed sensor to be

functional while being stretched [134-136].

Wataru Honda and his coworkers [136] reported fabrication of multi-functional, wireless

health monitoring wearable devices using a macro scale printing methods. The devices consist of

a screen printed LC circuit, as a proof of concept for wireless communication, a temperature

sensor, and a microfluidic channel fabricated using a conventional method for drug delivery. The

performance of different parts of the device was analyzed and reported. While this device is not

fully printed and other methods were involved in the fabrication of the device, the concepts and

results are useful and interesting [137].

Hydration level sensors are an example of wearable devices that have been fabricated

using PE on different substrates, such as plastic and tattoo paper. Joseph Wang and his co-

workers [138, 139] have reported fabrication of tattoo-paper based sensors for epidermal PH

monitoring. A picture of the printed sensor is shown in Figure 2. 18. Screen printing was used as

deposition method for this sensor. The device was printed on a tattoo paper and transferred after

fabrication onto human skin. The device was mounted on lower back, neck and wrist of the

human body and tested for real-time measurement of the pH levels of perspiration during

exercise. Motion and deformation of the body during the exercise did not affect the performance

of the sensor [138, 139].

42

Figure 2.18: Printed PH monitoring sensor on tattoo paper courtesy of [139].

Different materials such as silver (Ag), gold (Au) and aluminum (Al) have been used and

analyzed for the fabrication of printed wearable sensors. The major focus of most of the reported

devices was on the flexibility and bendability of the sensors but, as mentioned before in this

chapter, stretchability plays an important role in functionality when the sensors are attached to

the human body. There are only a few reports on the development of stretchable materials or

strategies for applications in wearable devices fabricated using printing methods. Therefore,

there is a significant need for further study, characterization of new materials, and strategies for

the printing of stretchable and wearable electronics.

In this work, the screen printing was used for the fabrication of multi-functional, flexible

and stretchable sensors that can be directly transferred onto the skin. The author utilized both

stretchable materials and developed strategies for the fabrication of stretchable devices to design

and fabricate fully printed wearable sensors. The performance of these devices was studied and

analyzed. In the following section the applications of PE in wearable sensors and predicted

market for them is presented.

43

2.4.2. Applications of PE in Fabrication of Wearable Electronics

There are different commercialized wearable devices in the market ranging from google

glass to health monitoring bracelets [140-149]. PE and especially the field of printed sensors

have the potential to revolutionize fabrication of such devices. Based on an IDTechEx report,

printed sensors are emerging from R&D stages to the market [150]. IDTechEx predicts the

market for printed sensors will be above $8 billion by 2025. As an example of commercialized

printed devices, printed Glucose sensors have a market of greater than $8 million per year.

Figure 2.19 represents the expected growth of market and demand for printed sensors over the

next 10 years [150].

Figure 2.19: Market growth for printed sensors [150].

The market size for different wearable sensors by 2025 is illustrated in Figure 2.

20. It shows that chemical sensors will have the largest share of the market for wearable

sensors in 10 years. Figure 2.21 illustrates the prediction for growth of the market for

44

different types of wearable sensors. As it can be seen stretchable strain and pressure

sensors are expected to grow rapidly over the next 10 years [151].

Figure 2.20: Market share for wearable sensors [151].

Figure 2.21: Predicted growth of market to different types of wearable sensors [151].

45

2.5. Summary

In this chapter, the author presented a comprehensive literature review that includes a

detailed introduction to sensors and PE. A discussion covering different types of sensors and

their working principle is presented. An introduction and review of the types of wearable

electronic and sensors which have been designed, fabricated and tested as part of this dissertation

was also presented. The major works accomplished by other research groups on the fabrication

of wearable sensors was explained and the strengths and weaknesses of these devices were

analyzed. This was followed by an introduction to PE, the different forms of printing used, and

their characteristics. The application of PE to the fabrication of wearable sensors and stretchable

devices was introduces and some of the previous work in this area were discussed. The next

chapter discusses a project that involved the use of screen printing for the fabrication of a multi-

layered printed electronic circuit board on non-conventional substrates which can be used as a

readout circuit for fabricated sensors. The background for this work, design of the circuit,

fabrication process, and analysis of the printed lines is discussed. A method for electronic

component attachment on the printed pads is also established and presented.

46

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CHAPTER III

SCREEN PRINTING OF MULTI-LAYERED HYBRID

PRINTED CIRCUIT BOARDS (PCB) ON

DIFFERENT SUBSTRATES

3.1. Introduction

As embedded electronics have become common in an ever widening array of products

and applications, the form factors, flexibility and materials used to construct circuit boards must

evolve to support emerging applications. As a result, there has been an increasing demand for

further development and understanding of the characteristic traits pertaining to the printing of

flexible electronic circuits and components. Printed electronic (PE) devices, created using

traditional printing techniques such as; gravure [1, 2], inkjet [3, 4], flexography [5, 6] and screen

printing [7, 8], have shown the ability to produce circuits and sensors for many applications [9-

13]. Some examples of printed electronic devices are organic thin film transistors (OTFT) [14,

15], flexible displays [16, 17], flexible printed solar cells [18, 19] and substrates for surface

enhancement Raman spectroscopy [20], which have shown varying levels of applicability for use

in commercial applications.

Although the current performance of PE devices does not match the switching speed,

density and higher current handling of traditional solid-state devices, produced using

conventional manufacturing techniques, there are several advantages of using printing techniques

for the fabrication of less demanding devices. These include a lower-cost of manufacturing,

lower-processing temperatures and a reduction in resources used during fabrication. These

advantages have facilitated the need for the production and manufacturing of PE products on a

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commercial scale [21, 22]. Moreover, printing techniques, which lend themselves to roll-to-roll

(R2R) processes, enable electronic devices to be produced in high volumes at high speeds

without the intricate process requirements associated with conventional silicon manufacturing

technologies, such as complicated photolithographic patterning procedures, high temperatures

and vacuum deposition methods.

Although many researchers have shown the ability to produce electronic components and

devices on flexible substrates [23-28], there have been very few reports involving the printing of

PCBs on flexible and rigid substrates such as paper, polyethylene terephthalate (PET) and glass.

Most of the currently available flexible PCBs are fabricated using methods such as spin coating,

sputtering, and spray coating [29]. These processes are typically used for the deposition of the

conductive materials onto the substrate [29]. Some of the disadvantages associated with these

methods include increased production time and material waste, which can be overcome by

employing printing methods. To date, there are only a few reports on the printing of PCBs,

however, none of these involved the screen printing of a multi-layer hybrid PCB prototypes with

integrated electronic components on flexible substrates [30, 31].

Both plastic and paper could play an important role in the future of light-weight and

flexible PCBs. Plastic offers the advantages of high smoothness, transparency and low porosity

[32, 33]. Even though plastic is not very amenable to the use of different solvents, it has been

one of the main materials used in conventional flexible electronics [34, 35]. In comparison to

plastic, paper is more temperature resistant, stiffer, more renewable and may be less susceptible

to moisture problems [36]. Currently, paper is the material of choice for many products used on a

daily basis, and therefore, the fabrication of PCBs on paper may open new market opportunities

for PEs and paper. The ability to produce flexible PCBs on paper and PET enable them to be

67

placed on and conform to different form factors and surfaces, where spacing or shape would

prohibit the placement of a rigid conventional circuit board. Alternately, for some applications,

such as the auto industry, a rigid transparent PCB on glass would be beneficial to enable devices

to be integrated into the windshields or mirrors of cars. Moreover, glass offers the added

advantages of heat stability, transparency and smoothness [37].

In this work, the author has employed screen printing to fabricate multi-layer PCBs on

PET, paper and glass. Pick and place equipment was used to attach solid state components onto

the boards to produce a working hybrid PCB. The ability to fabricate a PCB incorporating an

embedded microcontroller to drive a 160 × 100 pixels LCD display with +3V power supply was

demonstrated.

3.2. Background

The initial objective for this study was to define and construct a proof of concept for the

fabrication of high yield, multi-layer electronic circuit boards on flexible substrate as well as

glass. This section summarizes the key activities and result of this research work. The first

generation of PCBs was printed using gravure printing methods on polyethylene terephthalate

(PET). In this work Silver nano particle ink (Inktec) was used as a metallization layer (top and

bottom layers) and UV clear from ecology was used as a dielectric layer. Figures 3.1 (a) and (b)

show the top and bottom layer printed individually on PET. The component attachment was done

using the facility of Finetech Company. A Martin dot liner 06.6 was used for dispensing of the

silver epoxy (Figure 3.2 (a)) and Fine tech placement instrument was used for attachment of the

component. (Figure 3.2 (b)).

https://www.google.com/search?es_sm=122&q=uv+dielectric+from+ecology&spell=1&sa=X&ei=i6KaVY24BoGuyATn6byQBw&ved=0CBsQBSgA

68

Figure 3.1: Picture of (a) Bottom layer (b) top layer (c) Final sample

These instruments provided high accuracy in dispensing of the silver epoxy as well as

attachment of the electronic components. The final sample after printing of three layers as well

as component attachment is illustrated in Figure 3.1 (c).

After component attachment, the PCB was tested. A few problems were observed. Due to

spreading of the ink after printing on PET, some of the lines were shorted. The DC to DC

convertors was electrically shorting and some of the component came off in this section due to

small dimension of the pads. When voltage was applied to the circuit, this voltage was seen

across the components but there was a problem in programing the microcontroller. Based on

(a) (b)

(c)

69

these problems, the author modified the design and chose another method for fabrication of the

printed PCDs which is explained in details in following sections.

Figure 3.2: (a) Martin dot liner 06.6 (b) Fine tech placement instrument

3.3. Objectives

3.3.1. Materials and Sample Preparation

Three different substrates were of interest; 130 μm thick flexible PET film

(Melinex ST 506) from DuPont Teijin Films, plate glass from Corning and paper (NB-

RC3GR120) from Mitsubishi. Some of the important characteristics of the different substrates

are summarized in Table 3.1. For example, glass demonstrated a higher surface energy when

compared with paper and PET. Adhesion of the silver ink employed for traces on all of these

substrates was good, thereby showing promise as substrates for the fabrication of PCBs. The

roughness of paper, PET and glass was measured to be 0.175 µm, 0.015 µm and 0.005 µm,

respectively, using a WYKO RST-plus optical profiler. Based on these measurements, both glass

and PET have relatively smooth surfaces, making them good substrates for printing thin

conductive traces. In comparison to the glass and PET, paper is considerably rougher, although

this paper is very smooth by paper standards. However, unlike PET and glass, paper is more

absorptive, which can reduce the amount of ink spreading after printing. Since the thickness of a

(a) (b)

70

printed ink film depends on the amount of ink spreading and absorption, if both properties are

controlled, highly conductive lines can be produced on paper. The roughness and absorptive

properties of paper could also be an added advantage for ink adhesion. Hence it is worthy of

study, since it is flexible, readily available, tunable and of low cost.

Metal and Dielectric: There are different materials such as silver (Ag), gold (Au), and

copper (Cu) that can be employed for the fabrication of electronic circuit boards. In this paper, a

Ag flake ink (Electrodag 479SS) and a UV acrylic-based ink (Electrodag PF-455B) from Henkel

were used as the metallization and dielectric layers, respectively. All layers were printed using an

AMI 485 semiautomatic screen printing press at room temperature.

After construction of the PCB, components must be attached to the contact pad location

using a conductive adhesive. A commercially available Ag conductive epoxy (H20E), was

purchased from Epoxy Technologies Inc. and was used to attach the surface mounted devices

(SMD) to the different substrates. The epoxy was screen printed onto the pads prior to placement

of the electronic components with an automated pick and place instrument (MY100sxe) from My

Data Inc.

Table 3.1: Summary of different characteristics of substrates including surface energy,

thickness and roughness

Substrate Surface energy [dynes/cm] Thickness [µm] Roughness [µm]

Paper 53.9 177±12 0.175

PET 43.8 127±1 0.015

Glass 58.21 629±1 0.005

71

3.3.2. Design of the Circuit

In order to demonstrate that the hybrid PCB would work once fabricated, a

microcontroller based circuit was designed to control an externally attached LCD and a three-

layer PCB layout pattern was produced. The circuit design consisted of a DC to DC convertor,

microcontroller and other necessary passive components. The design layout was done using

PCB123® design software and then transferred to a mutlilayer Adobe Illustrator file for screen

generation. The layout of the design that consists of two conducting layers and an insulating

layer, which prevents shorting between the top and bottom Ag layers, is shown in Figure. 3.3.

Figure 3.3: The layout of PCB design created in PCB123® design software. This design consists of the pads

for a DC to DC convertor, a microcontroller and nessesary passive components. (Red Layer – Bottom

Electrodes; Green Layer – Dielectric Layer; Yellow Layer – Top Electrodes).

72

3.3.3. Screen Printing of PCB

The conductive and dielectric materials were screen printed onto the substrates using an

AMI 485 semi-automatic screen printing press, at room temperature. A 320 mesh count screen

was used for printing the materials on the various substrates. After printing the first or bottom

metallization layer on all three substrates, the ink was cured for 20 min in a VWR 1320

temperature-controlled oven at 120 °C. The thickness of the ink film on paper, glass, and PET

was measured to be 14.8, 10, and 12.8 μm, respectively, using a Bruker vertical scanning

interferometer microscope (CounterGT) [Figure. 3.4 (a), (c), and (e)]. All PCB lines were found

to be continuous (no breaks) and conductive. Next, the dielectric layer was screen printed over

the first metal layer. The printed layer was then cured using a UV fusion drying system equipped

with a D bulb. Figure. 3.4 (b), (d), and (f) show the dielectric layer printed over the Ag layer on

paper, glass, and PET, respectively.

The thickness of the dielectric layer on paper, glass, and PET was measured as 14.00,

9.46, and 8.62 μm, respectively. As shown, the dielectric layer was printed uniformly, thereby

preventing any shorting between the top and bottom Ag layers. Finally, the top metallic layer

was printed and thermally dried in an oven. A photograph of three printed layers on PET is

provided in Figure. 3.5 (a).

The operational circuit was fabricated by attaching SMD components to defined pad

areas on the PCB. The silver epoxy, previously described, was screen printed onto the pads using

a 200-μm mesh screen and polymer squeegee of 80D hardness. The components were then

placed onto the epoxy using an automated pick and place instrument from My Data Inc. The

samples were then initially cured at 130 °C for 10 min followed by a second cure at 120 °C for 5

min. A photograph of the completed PCB on paper is shown in Figure. 3.5 (b).

73

Figure 3.4: 3D vertical scanning interferometer images of (a) silver and (b) dielectric on paper; (c) silver and

(d) dielectric on glass; and (e) silver and (f) dielectric on PET.

3.4 . Results

3.4.1. Analysis of Printed Lines

The widths of the printed lines were measured with an ImageXpert (KDY Inc.) image

analyzer. The results are shown in Figure. 3.6 (a). The patterned 300 and 600 μm lines on paper

printed as 263 and 566 μm, which correlate to a −13.3% and −5.6% loss, respectively. The

negative gains are attributed to ink absorption by the paper. On PET, the line widths are

measured as 323 and 625 μm, which correlate to a gain of 7.6% and 4.16%, respectively. The

(a) (b)

(c) (d)

(e) (f)

74

increase in linewidth is attributed to the wetting and spreading of the ink on the PET film. For

glass, the line widths were measured to be 295 and 608 μm, producing a negative gain of −1.6%

and positive gain of 1.3%. Fig. 3.6 (b) shows the vertical scanning interferometer images of the

printed lines for the microcontroller contact pads. It was observed that these lines were printed

uniformly, with complete separation from one another. The picture of the pads for the

microcontroller with 500 μm spacing is shown in Fig. 3.6 (c). The separation of the pads can be

attributed to the lack of ink spreading.

Figure 3.5: (a) Photograph of three layers of printed PCB on PET and b) A photograph of the completed

PCB on paper.

(a)

(b)

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Figure 3.6: (a) Summary of line width measurement on different substrates; (b) 3D profilometry picture and

(c) optical microscope image of printed lines for microcontroller contact pads.

Figure 3.7 shows a comparison of the effect of substrate roughness on the resistance of

the printed lines. For example, it was seen that the lowest resistance (1.06 Ω) was obtained on

paper, for the 600 µm lines. This is because paper absorbs the ink, thereby keeping the particles

(a)

(b) (c)

76

in close contact with one another, and hence the lines are more conductive. The printed lines on

the glass substrate exhibited lower resistance (1.13 Ω) than the printed lines on PET (1.16 Ω) due

to the lower roughness of glass creating a more consistently larger cross-sectional area for the

line. It was observed that higher roughness affected the uniformity of printed lines and decreased

the conductivity due to variations in ink film thickness at some points. Figure 3.7 shows that the

printed lines on the PET are slightly higher in resistivity when compared to those on glass. This

is due to the higher roughness of the PET producing a less consistent and effectively smaller

cross-sectional area.

Figure 3.7: Effect of roughness on resistivity of printed lines.

3.4.2. Electrical Analysis

Line Resistivity: The effect of the resistivity of the lines on the performance of the circuit

was also investigated. The LCD was driven, with a series of conventional resistances and it was

observed that the LCD worked with line resistances of up to 3.7 kΩ. Since the resistance of all

77

the printed lines was measured to be below 10 Ω, it was determined that the performance of the

LCD would not be affected by the resistance of the printed lines. The summary of this test is

shown in Figure 3.8.

Figure 3.8: Summary of the effect of the resistivity of the lines on the performance of the circuit

Effect of Bending: The effect of bending of the substrate on the resistance of the lines

was also analyzed using a force gauge (Mark-10). The PCB was mounted on the support of the

3-point test fixture and subjected to 10000 cycles of 5-mm elongation. After 10 000 cycles of

bending, a 1.8% increase in the base resistance was observed. This is negligible for the given

above stated tolerance of the line resistance. The scanning electron microscope (SEM) images of

the printed lines were acquired using a Hitachi S-4500 model SEM, with Quantax 200 software

package before and after bending. Figure 3.9 (a)–(d) shows the SEM images of the printed lines

before and after bending, respectively. It was observed that even though there were some small

cracks, the printed lines were conductive after bending.

78

Figure 3.9: SEM images of the printed lines (a) and (b) before bending and (c) and (d) after bending.

3.4.3. LCD Operation

The printed PCB circuit was then energized and used to drive an LCD display. The codes

for the microcontroller, a low-power Texas Instruments (TI) MSP430 processor, were written

using TI’s Code Composer Studio. Figure 3.10 shows the performance of the printed PCB when

3 V was applied to it. The preloaded software executing on the microcontroller produced a

graphic message (CAPE), as shown, on the LCD. This effectively demonstrated that the

embedded microcontroller driving an attached device suggest numerous other small battery-

operated applications. This includes low cost R2R electronic circuit boards for consumer

electronics (e.g., in packaging industry) and medical wearable devices that can be attached to the

79

human skin, postal security systems (printed directly to packages and envelopes), and very-light-

weight circuit boards for aircrafts or automobiles.

Figure 3.10: Photograph of powered, operating microcontroller and passive electronics on the

printed glass substrate driving an LCD display.

3.5. Summary

In this chapter, the author presented an introduction to new substrates and fabrication

methods for electronic devices and the need for fabrication of flexible and low cost electronic

circuit boards for different applications such as biomedical and auto industry. In addition, the

background and details of previous work that have led to the current project was presented. This

includes the fabrication of first generation of printed PCBs using gravure printing methods, and

the difficulties that were seen during the process. This was followed by an improved

methodology, a detailed explanation of screen printing for PCBs an various substrates and the

analysis of results and performance of the printed PCBs.

To summarize, Screen-printed multilayered PCBs using PE-deposited materials on three

distinct substrates were successfully fabricated. The different characteristics of PET, paper, and

glass as the substrates for PCBs were analyzed. A method for populating electronic components

onto the printed PCB pads was defined and demonstrated. The capability of the printed hybrid

80

PCB circuit to operate correctly and to drive an LCD was shown. The fabrication of PCBs on

flexible substrates such as paper and PET opens numerous opportunities for the development of

low-cost and light-weight circuit boards for embedded electronic devices and applications that

may include the requirement for conformal shapes or surfaces. At the same time, the fabrication

of PCBs on flexible or rigid glass may lead to new designs for automotive and aerospace

applications.

In the following chapter, the author presents a project that includes design, simulation,

fabrication and analysis of the wavy lines for fabrication of stretchable sensors as well as

interconnections of stretchable systems. The measurement set-up, testing and results obtained are

also presented.

81

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CHAPTER IV

DESIGN, FABRICATION AND ANALYSIS OF PRINTED

WAVY LINES FOR STRETCHABLE ELECTRONIC

DEVICES

4.1. Introduction

There have been steady efforts in semiconductor and device technology working towards

increasing the operation speed of transistor and electronic devices and reducing the dimensions

of end device for almost half a century [1-4]. Over the last two decades, there has been an

increasing interest in the fabrication of electronic devises on light weight and flexible substrates,

such as paper and plastic for enabling the manufacturing of large area electronics [5-15]. The

flexibility of electronic devices improves the placement and interfacing of such devices with

their surrounding environment. Some examples of these devices include flexible sensors [16],

paper like displays [17], thin film transistors [18], radio frequency identification tags [19] and

wearable electronics [20]. Development of this new type of electronic devices requires new

forms of materials and strategies that are mechanically strong enough to tolerate the applied

strain and stress. A large number of materials, when deposited on a thin substrate or close to

neutral plane in a thin layer form, can flex properly [21-25]. For specific applications, the

amount of stress that these devices experience can remain below the fracture limit. Fabricated

stretchable electronic devices, which can be stretched to some point, have shown many

applications in unconventional electronics, especially in health monitoring devices [26- 30].

There has been numbers of reports on stretchable lines for the fabrication of flexible

electronics [21, 22, 31-33]. Most of these fabricated devices have used conventional electronic

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device manufacturing techniques that are then transferred onto a final flexible substrate. These

methods are also time consuming and require expensive instruments and complicated procedure

for fabrication. These drawbacks can be surpassed by employing printing methods for the

deposition of functional materials in stretchable forms. As explained in chapter 2 section 2.4,

these methods have advantages such as fast, low cost and low temperature fabrication

processing. There are a few reports on the fabrication of stretchable lines using carbon nanotubes

and silver nanowires deposited using printing technologies [34, 35]; however, to the best of the

author’s knowledge, there are no reports on the fabrication of wavy lines using printing methods.

Wavy lines have shown many applications in epidermal electronics. In this work screen printing

was employed for the fabrication of flexible and stretchable lines for use as health monitoring

devices that will have a better interface with the human body. The goal of this chapter is to fully

characterize the design and fabrication of wavy lines and determine the important design

parameters for the stretchability of these lines.

4.2. Experimental

4.2.1. Design and Fabrication of Wavy Lines

One method used for the fabrication of stretchable sensors is the design of interconnects

and sensors in the form of wavy or meander lines on a stretchable substrates. There have been

many reports on different challenges and parameters in the design and analysis of wavy lines.

The major designs are divided in in three categories: 1) wavy lines formed with two half circle

connected to each other; 2) wavy lines having extended lines between two half circles; and 3)

horse-shoe designs. The design parameters for wavy structure are shown in Figure 4.1. A wavy

line has parameters such as r (radius of arc), Ɵ (arc angle), L (straight section of wavy lines), W

(width of wavy lines), and D (diameter if the arc).

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Figure 4.1: Design parameters of wavy lines.

Figure 4.2: Schematic of wavy structure a) two half circles connected to each other (design 1), w=800 µ

r=2000µm, b) two half circles connected to each other (design 1), w=800 µ r=4000µm, and c) two half circles

connected through a straight line w=800 µm r=4000µm L/D=1/2(design 2) and c) two half circles connected

through a straight line w=800 µm r=4000 µm L/D=2 (design 2).

In order to study the effect of the applied strain on the resistance of wavy stretchable lines

different structures having the widths of 400 µm, 800 µm and1600 µm were designed in different

W

L

r

Ɵ

W/2+r

(a)

(b)

(c)

(d)

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forms based on the three categories described. The effects of the W/r ratio and L/D ratio,

important design parameters related to stretchability, have been studied.

Figure 4.2 a) and b) illustrate the design of wavy structure (which will be called design 1)

having two half circles connected to each other. These lines were designed having line widths of

1600 µm, 800 µm and 400 µm and with varying radii of 2000 µm, 4000 µm, 6000 µm and 8000

µm. The schematic of wavy lines having extended lines with Ɵ=90 (design 2) are shown in

Figure 4.2 c) and d).

The wavy structures with extended lines in form of horse-shoes (design 3) and with

different lengths of extended lines are shown in Figure 4.3.

Figure 4.3: Schematic of horse show (design 3) wavy lines (Ɵ=80) a) L/D=1/4, b) L/D=1/2, c) L/D=3/4, d)

L/D=1.

The schematic of horse show form with different Ɵ is presented in Figure 4.4. To analyze the

effect of the angle on the stretchability of the printed lines different horse-shoe structures having

angles of 30, 45 and 60 degree were designed.

(a)

(b)

(c)

(d)

90

Figure 4.4: Schematic of horse show wavy lines (design 3) a) Ɵ=30, b) Ɵ=45, c) Ɵ=60.

4.2.2. Simulation

Three different meander structures were simulated and the effect of applying stress to

them was analyzed using CoventorWare®. All of the designs have a line width of 400 µm and

radius of 4000 µm. In the simulation the lines were formed using copper. The effect of the

substrate on the strechability of the lines was neglected. 10 µm, 100 µm and 500 µm

displacements were applied to the meander structures. Figure 4.5 a) shows the result of applying

a 10 µm displacement to design 1. High levels of stress are observed in the crest of the wavy

structure. This stress could lead to breakage in the curves. By increasing the applied

displacement, the amount of stress applied on the structure increased. The amount of stress on

the crest was 0.54 MPa when the structure was stretched 10 µm. Figure 4.5 b) and c) show that

the stress increased to 54 MPa and 260 MPa when the line was stretched for 100 µm and 500

(a)

(b)

(c)

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µm, respectively. To avoid high concentration of stress in the crest of the waves the straight

extended lines were added to the design (design 2). Figure 4.6. a), b) and c) represent the effect

of applying 10 µm, 100 µm and 500 µm displacements to design 2. Adding the extended lines to

the design offers a better distribution of stress in the structure as compared with design 1. By

direct comparison to design 1, the applied strain on the crest on the waves for all three

displacements has been reduced to 0.18 MPa, 2 MPa and 97 MPa, respectively when the

structure was stretched for 10 µm, 100 µm and 500 µm. The straight vertical line might limit the

deformation of the wavy lines in cases that biaxial deformation is needed. To overcome the

limitation of the deformation causes by straight lines, they have been changed to the horse shoe

form for design 3. The results obtained for this simulation are illustrated in Figure 4.7 a), b) and

c). As it can be seen from the image, the stress is distributed all over the curved part as well as

the extended line. In comparison with design 1, the amount of stress applied is decreased to 0.24

MPa, 1.9 MPa and 140 MPa.

In all of the designs, failure can happen in the regions that have a higher concentration of

stress. The zoomed in pictures of those points are shown in Figure 4.8 a), b) and c). The

simulations have shown that adding extended line to the design of meanders has helped the

stretchability, reducing the induced strain in the structures. For example in case of applying 10

µm displacement to different designs the maximum strain tolerated by the structure was

decreased 66% and 56 % and for design 2 and design 3 as compared with design 1.

In the next section the author will explain the details of fabrication as well as the resulting

characteristics, in particular the mechanical and electrical performance of the printed lines.

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Figure 4.5: Simulation results obtained for the stretching of lines (design 1) a) 10, b) 100 and c) 500 µm

(a)

(b)

(c)

93


(a)

(b)

(c)

94


(a)

(b)

(c)

95

Figure 4.8: Concentration of stress in a) design 1, b) design 2 and c) design 3.

4.2.3. Chemicals, Materials and Sample Preparation

Thermoplastic polyurethane (TPU) from Bemis Associates, Inc, was used as a substrate

for the fabrication of wavy lines. Silver (Ag) ink (Electrodag 479SS from Henkel) as well as

(a)

(b)

(c)

http://www.bemisworldwide.com/

96

Carbon nano tubes (CNTs) ink (VC101 from SWENT) were used for the fabrication of wavy

lines.

4.2.4. Fabrication of Printed Wavy Lines

A semi-automatic screen printing press (AMI 485) was used for the fabrication of wavy

lines on a TPU substrate. The screens provided by Microscreen® were of a stainless steel mesh

count of 325 and MS-22 emulsion thickness of 12.7 µm. The wire diameter of the screen was

28 µm with an angle of 22.5 degrees. The printing process was performed at room temperature

and three runs of printing, depositing three layers, was used to deposit the conductive materials.

Deposited silver ink was cured in a VWR 1320 temperature-controlled oven at 120 °C for 20

min. The same structures were printed on TPU using CNTs. Isopropyl alcohol was used as a

cleaning agent. Printed CNTs were cured in the oven for 10 min at 120 °C. The thickness of

lines having 400, 800, 1600 µm line width were measured to be 24, 25, and 23 μm, respectively,

using a Bruker vertical scanning interferometer microscope (CounterGT). (Figure 4.9 a), b) and

c))

Figure 4.9: 3D vertical scanning interferometer images of a) design 1, b) design 2 and c) design 3.

(a) (b)

(c)

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4.2.5. Experiment Setup

The experiment setup is shown in Figure 4.10. The printed wavy lines were placed

between a force gauge (Mark-10 model M5-200) and vertically movable platform (Mark-10

ESM 301 motorized test stand). The lines were subjected to different displacements ranging from

1 mm to 8 mm for 100 cycles. The resistance of the lines was measured after each step and

change in the resistance was recorded. The failure point for each structure was determined.

Figure 4.10: Experiment set up.

4.2.6. Results

Figure 4.11 represents the effect of the different stretching tests on the resistance of the wavy

lines (design 1). Figure 4.12 illustrates the change in resistance of the lines versus applied strain.

The resistance of all lines increased more than 100 % in response to applied strain. For W=1600

μm, r=4000 μm by stretching the line for 1 mm, 2 mm, 3 mm, and 4 mm the resistance increased

from 1 Ω to 1.4 Ω, 1.9 Ω, 2.4 Ω and 3.4 Ω which corresponds to 34 %, 84 %, 131 % and 218 %

change in the resistance. For the line with W=1600 μm, r=8000 μm the base resistance was 2.1 Ω.

After stretching the line for 1 mm, 2 mm, 3 mm, 4 mm and 5 mm, the resistance increased to 2.5

Ω, 2.9 Ω, 3.6 Ω, 4.2 Ω and 5.2 Ω, respectively, which corresponds to 16 %, 36 %, 69 %, 95 %

and 141 % changes in the base resistance of the line. In the case of wavy structure with W=800

μm, r=2000 the resistance increased from 1 Ω to 1.6 Ω, 2.4 Ω and 3.4 Ω which is equal to 53%,

Connected

Via Alligator

Clips

98

128 % and 221 % changes in the resistance the line when it was stretched 1 mm, 2 mm, and 3

mm. The wavy line having W=800 μm, r=4000 μm with the base resistance of 2.2 Ω was

subjected to stretching for 1 mm, 2 mm and 3 mm. The resistance increased to 2.7 Ω, 4.7 Ω and

5.5 Ω. This shows 20 %, 110 % and 146 % changes in the resistance of the line. The line with

W=800 μm, r=6000 μm was also stretched for 1 mm, 2 mm, 3 mm and 4 mm. The base resistance

of 3.3 Ω was increased to 3.5 Ω, 5 Ω, 5.8 Ω and 6.5 Ω, respectively, which correspond to 5 %, 52

%, 74 % and 96% changes. The wavy line having the width of 1600 μm and radius of 8000 μm

showed the maximum tolerance toward applied strain, which was calculated to be 12 %. The

applied strain can be mathematically calculated using:

0/ LL (1)

where the ∆L is change in the length of the sensor and L0 is the initial length of the sensor.

The ratio of width to radius of waves (W/r) is one of the important parameters in

analysis and design of the wavy lines. It has been reported that the smaller this ratio the less

strain is applied on the line during stretching [36]. Figure 4.13 a) and b) presents the effect of the

W/r on the stretchability of the lines. Figure 4.13 a) shows three lines having the width of 800

µm with different radius of 2000 µm, 4000 µm and 6000 µm which correspond to W/r ratio of

0.4, 0.2 and 0.13. It can be seen that the lower W/r ratio shows less change in the resistance of

the printed lines when the stress is applied. For example when the lines are stretched 3 mm, the

line having W/r=0.13 showed 74 % change in the resistance where the lines with W/r= 0.2 and

W/r= 0.4 showed 146% and 221%. Figure 4.13 b) compares strechability of two lines having the

width of 1600 µm and radius of 4000 and 8000 µm. It is seen that the smaller W/r results in a

less strain and more stretchability. Both of the lines were stretched 1mm, 2mm, 3mm, and 4 mm

and in all steps the line having W/r= 0.2 showed less change in the resistance of the line. For

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another test the radius of the arc was kept constant (r=4000 µm) and two lines having width of

400 and 800 µm and extended line (L=4000 µm) were printed. Figure 4.13 c) shows that the

smaller W/r ratio revealed smaller change in the resistance of the line when displacement was

applied to the line.

Figure 4.11: Effect of displacement on the resistance of lines with different sizes a) W=1600 μm,

r=4000 μm, b) W=1600 μm, r=8000 μm c) W=800 μm, r=2000 μm d) d) W=800 μm, r=4000 μm e)

w=800 μm, r=6000 μm.

(a) (b)

(c) (d)

(e)

100

Figure 4.12: change in the resistance of lines having different sizes a) W=1600 μm, r=4000 μm, b)

W=1600 μm, r=8000 μm c) W=800 μm, r=2000 μm d) d) W=800 μm, r=4000 μm e) w=800 μm,

r=6000 μm.

(a) (b)

(c) (d)

(e)

101

Figure 4.13: analysis of W/r ratio on stretchability of the lines. a) W=8000 μm r= 2000, 4000 and 6000 μm

(W/r=0.4, 0.2 and 0.13), b) W=1600 μm, r= 4000 and 8000 μm (W/r=0.2 and 0.4) c) r= 4000 μm, W=400 and

800 μm (W/r =0.2 and 0.1)

(a)

(b)

(c)

102

According to the previous simulations explained in Section 4.2.3, wavy lines having

straight extended lines (design 2) show better stretchability, which is based on the distribution of

the strain through the crest and added extended line. Figure 4.14 a) and b) shows the results

obtained after stretching the lines for 1 mm, 2 mm, 3 mm,4 mm and 5mm. For wavy line with

W=800 μm, r=4000 μm , L= 4000 μm and Ɵ=90 ° the base resistance increased from 1.9 Ω to

2.1 Ω, 2.3 Ω, 2.8 Ω, 3.3 Ω and 3.6 Ω which correspond to 10 %, 22 %,48 %, 72 % and 89 %

incensement of resistance. Resistance of line with W=800 μm, r=4000 μm, L= 16000 μm and

Ɵ=90 ° was increased from 2.6 Ω (base resistance) to 3.1 Ω, 3.6 Ω, 4 Ω, 4.4 Ω and 5.4 Ω. This

corresponds to 19 %, 37 %, 52 %, 68 % and 107 % incensement of the base resistance for 1mm,

2mm, 3mm, 4mm and 5 mm displacement. Figure 4.14 c) shows that the percentage changes in

the resistance of the lines were decreased in comparison with design 1. This can be related to the

effect of adding the extended straight line to the structure of the wavy lines. It helps in

distribution of the strain through the crest as well as extended line and decreases the chance of

breakage on the curved part of the structure.

103

Figure 4.14: Effect of the displacement on the resistance of lines having different sizes a) W=800 μm,

r=4000 μm and L/D=1/2, b) W=800 μm, r=4000 μm, L/D=2, c) Change in the resistance of lines. The line

with L/D smaller than one shoes less change in the resistance.

It has been reported that changing the angle of the extended line might help improve the

stretchability of the wavy structure [36]. Based on the simulation this can help in distribution of

the stress through the structure. For this purpose four different structures having the extended

line with the angle of 30, 45, 60 and 80 were designed and tested. The results obtained are

illustrated in Figure 4.15.

(a) (b)

(c)

104

Figure 4.15: Effect of the displacement on the resistance of lines having different angles a) Θ =80 b)

Θ =60 c) Θ =45 d) Θ =30

Figure 4.16: Percentage change in the resistance of lines having different angles.

This result shows that comparing with design 1 the amount of strain has been reduced

and the strechability of the line increased. For example when the design 1 (W=800 μm, r=4000

(a) (b)

(c) (d)

105

μm) was stretched, the line broke after stretching 3 mm but in case of design 2 having Ɵ = 45,

60, and 80) it was possible to stretch them up to 6 mm. The percentage change in the resistance

of the structures has also decreased comparing the design 1. The change in the resistance up to

applying 8 % strain is below 50 % (Figure 4.16) where in design 1 after stretching 2 mm the

change in the resistance was 110 %.

The length of the extended line has an impact on the stretchability of the wavy structure.

In order to analyze this effect the ratio of L/D (ratio of length of the extended line to the diameter

of arc) is used as a design parameter. To analyze this parameter, horse shoe structure having the

angle of 80 degree with 4 different ratios of L/D (1/4, 1/2, 3/4 and 1) were designed. The width,

radius and angle were kept constant (w=800 µm, r=4000 µm, Ɵ=80) for all the lines. Figure 4.17

presents the effect of applying stretching tests to these lines. The change in resistance of these

lines was below 50 % until 6 mm of stretching. It has been reported that if this ratio is kept

below 1 the lines can tolerate more strain [36]. Comparing with the result presented in Figure

4.14 c) all these lines having the L/D ratio below one show less strain comparing with the L/D

ratio bigger than one. For example when the line having L/D=2 was stretched 4 mm a 40 %

change in resistance was observed (Figure 4.14 c) but for L/D less than one all of the samples

showed a change in resistance of less than 20 % when they were stretched by 4 mm. When L is

higher than D, the extended line becomes stiff and does not allow the structure to stretch more,

and, as a result, it can lead to breakage of the structure.

106

Figure 4.17: Analysis of the effect of L/D on the stretchability of the structure.

In order to analyze the effect of the thickness of the line on the stretchability, one, two

and three deposition layers for a wavy line (W=400 µm and r= 4000 µm, Ɵ=80) were printed and

then subjected to different displacements, as shown in Figure 4.18. It shows that the change in

the resistance of the three layers line was lower compared with one and two layers for all applied

strain. For example when a 5 % strain was applied on the line having three layers, a 12 % change

was obtained in the resistance. Applying the same strain on one layer and two layered lines

resulted in 19 % and 17 % changes in the resistance, respectively. It can be attributed to the fact

that stretching the lines in vertical direction results in compressive strain to the lines in the

horizontal directions. This extra strain can cause a breakage in the lines. When the line is formed

with a greater thickness, it reduces the effect of the compressive strain on the structure.

107

Figure 4.18: Analysis of the effect of thickness on the stretchability of the structure. The with three layered

sample shows less change in the resistance of line in compare with one layer and two layered sample.

3D vertical scanning interferometer images of the wavy lines were taken in order to

monitor the changes in the structure. The line with W=800 µm, r= 4000 µm and Ɵ=80 was

subjected to 100 cycles stretching for 1 to 11 mm. A profilometry picture of the line both in the

curve and in the straight section were taken after each step. Figure 4.19 shows the effect of strain

on the structure of the line after stretching 1 mm, 5mm, 7mm and 11 mm. The line did not break

until it was stretched 6 mm. After this point, first the curved part of the waves started showing

some breakage and then the straight part broke at 11mm of stretching. Based on the simulation

result presented in section 4.2.3 the high level of stress was applied on the crest part of the wavy

structure. Profilometry pictures have shown that the first part of the structure, which showed

failure during applying strain, is the curved part of the line.

108

Figure 4.19: 3D vertical scanning interferometer images of a) curved part of the line after a) 1mm (no

breakage), c) 5 mm (no breakage), e) 7 mm (started beaking), g) 11 mm (broken), straight part of the line

after b) 1mm (no breakage) , d) 5 mm (no breakage), f) 7 mm (no breakage), h) 11 mm (started breaking).

CNTs were used to print two forms of wavy line with W= 400 µm, r= 4000 µm Ɵ=80

(design 3) having L/D= 1/4 and L/D=3/4. The result of the stretching test is shown in Figures

4.20 and 4.21. In comparison with Ag, CNTS showed a higher strachability with less change in

the base resistance of the lines. Printed lines using Ag ink showed failure after stretching of 7

mm (design 3) but the lines printed using CNTs with the same design parameters were stretched

(a) (b)

(c) (d)

(e) (f)

(g) (h)

109

until 40 mm and the line was still connected and functional. On the other hand it was observed

that the change in the resistance of the line was below 10 % when the lines were stretched 10

mm. The line with w= 400 µm, r= 4000 µm Ɵ=80 (design 3) having L/D= 1/4 was stretched up

to 40 mm. When the same line was stretched 1mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8

mm, 9mm, and 10 mm the resistance of the line was increased from the base (176 kΩ) to 176.8

kΩ, 177.6 kΩ, 178.8 kΩ,180.2 kΩ, 181.6 kΩ, 184.2 kΩ, 186 kΩ, 191.1 kΩ, 192.9 kΩ and 196

kΩ, respectively. This result in 0.4 %, 0.9 %, 1.5 %, 2.4 %, 3.2 %, 4.6 %, 5.7 %, 8 %, 9.6 % and

11.3 % changes in the observed resistance. The results show, based on the stronger mechanical

properties of the CNTs, that there is less change in the resistance when the lines are subjected to

strain. Figure 4.21 illustrated the result of the stretching tests on the wavy line printed using

CNTS with w= 400 µm, r= 4000 µm Ɵ=80 (design 3) having L/D= ¾. The line was stretched to

40 mm and the percentage change in the resistance was calculated. Similar to the other line

having (L/D= ¼), this structure showed higher stretchability when compared with the printed

lines using silver. The amount of the change in resistance remained below 10 % when a 20 %

strain was applied on the line. This performance suggests that CNTs can offer better

functionality when they are subjected to strain and the chance of failure in the functionality of

the lines is less. On the other hand, for applications such as stretchable PCBs which generally are

not subjected to high amount of strain, silver can be a better candidate based on better

conductivity.

110

Figure 4.20: a) Effect of the displacement on the resistance of line (L/D= ¼), b) Change in the resistance of the

line (L/D= ¼).

(a)

(b)

111

Figure 4.21: a) Effect of the displacement on the resistance of line (L/D= 3/4), b) Change in the resistance of

the line (L/D= 3/4).

(a)

(b)

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4.3. Summary

In this chapter, the author discusses the need for a detailed study on the design and

fabrication of printed wavy lines. A detailed account of the experimental tasks involved in this

work was presented. This included design, fabrication and analysis of wavy line on the TPU as a

stretchable substrate using Ag ink and CNT ink.

In this work, different forms of wavy structures were designed and fabricated. The effect

of displacement on the stretchable lines was analyzed. Different parameters which have an

impact on the stretchability of the lines were discussed. It was shown that smaller ratios of W/r

result in more stretchability and a reduced amount of change in the resistance of the lines. The

effect of adding extended straight line segments on the performance of the patterns under applied

stress was presented. The result showed that adding the extended line helps in the distribution of

the stress in the line and reduces the chance of failure and breakage. Three different thicknesses

were also tested to determine the effect of thickness on the stretchability of the structures. It was

seen that the three layered sample showed more tolerance regarding the applied stress in

comparison with one layer and two layers printed lines. CNT ink was also used for printing wavy

lines based on their stronger mechanical properties. The printed samples showed less change in

the resistance when they were stretched.

In the following chapter, the author presents an application of printed wavy lines in

design and fabrication of stretchable wearable sensors for health monitoring devices. The

measurement set-up, testing and results obtained are presented.

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CHAPTER V

FABRICATION OF WEARABLE SENSORS USING

PRINTING METHODS

5.1. Introduction

With advancements in material science and printing technology along with the

development of prosthetics and wearable devices, much effort have been dedicated to the design

and fabrication of flexible wearable sensors on thin film substrates [1-9]. Flexibility allows a

conformal human-device interface, which improves the sensitivity of measurements. Stretchable

sensors, unlike flexible devices, have shown stronger mechanical properties, which is a very

important factor for wearable, skin mountable sensors. Among various important sensing

parameters, strain, temperature, and ECG (electrocardiogram) can provide very important

information about the vital signs of a wearer. The interface of the sensor with the skin has a

significant impact on the sensitivity of the device. Tensile and compressive strain applied to the

sensor based on the movement of the body affect the accuracy of the measurement. As an

example, the strain sensor is used for movement measurement and the strain is directly applied

on the structure of the sensor. Fabrication of wearable devices, which are both flexible to have a

conformal interface with body and stretchable to be able to tolerate the applied strain, offer a

potential for wide use in the medical devices market.

In this work, the author uses conventional screen printing techniques to fabricate a fully

printed, stretchable, multi-functional sensor for strain, temperature, and ECG measurement. A

thin layer of Polydimethylsiloxane (PDMS), as well as temporary tattoo paper, has been used as

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a substrate. CNT ink was used for printing the strain and temperature sensor, and a mixture of

silicone and silver was used for fabrication of ECG pads. The author demonstrated the capability

of the fabricated device to be used as a multi-functional wearable sensor.

5.2. Experimental

5.2.1. Chemicals, Materials and Sample Preparation

PDMS (Sylgard® 184 Silicone Elastomer from Dow Corning) was used as a substrate for

the fabrication sensors. The liquid PDMS pre-polymer was mixed thoroughly with a curing agent

at a ratio of 10:1 and degassed for 1 hour. Polyvinyl Alcohol (PVA) substrate (Watson QSA

2000) was used as a sacrificial layer for transferring the sensor directly onto the skin. Temporary

tattoo paper (Papolio) was also used as a substrate. CNT ink (VC101 from SWENT) was used

for the fabrication of resistive strain and temperature sensors. EXP-06335, a screen-printable

silicone+ Silver (Protavic America) was used for deposition of ECG pads.

5.2.2. Fabrication of Printed Strain Sensor on Tattoo Paper

Multiple sensors consisting of both wavy shape, having an 800 μm line width and overall

dimension of 3 cm × 0.4 cm, and straight line, having a width of 800 μm and length of 2 cm,

were screen printed using CNT ink on temporary tattoo paper. The CNT ink was cured in a

VWR oven at 100 °C for 10 minutes. Different steps associated with the fabrication of the strain

sensor on the tattoo paper are shown in Figure 5.1. The sensors were printed using a Semi-

automatic screen printer (AMI MSP 485) from Affiliated Manufacturers Inc. The screens, for

fabrication of the sensors, were fabricated at Microscreen® with a stainless steel mesh count of

325 and MS-22 emulsion thickness of 12.7 µm. The wire diameter of the screen was 28 µm at an

angle of 22.5.

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Initially, the tattoo paper was cleaned using an air gun (Figure. 5.1(a)) and the strain

sensor was printed on the tattoo paper (Figure. 5.1(b)). A 4 cm × 1 cm PDMS dielectric layer

was then screen printed on top of the sensor (Figure. 5.1(c)). In the next step, the sticky part of

tattoo paper was attached on the finger and the printed sensor on the tattoo paper was placed on

the sticky part. Finally, the sensor was transferred onto the skin by wetting the tattoo paper

(Figure. 5.2). The structure of the skin-device when the sensor is transferred onto the body is

shown in Figure 5.1 d).

Figure 5.1: Fabrication steps of strain sensor on tattoo paper a) tattoo paper in cleaned using air gun b) the sensor is printed on the tattoo paper substrate c) thin layer of PDMS is screen printed on the sensor d) the

structure of skin-sensor after attaching onto the skin

(a) (b)

(c)

(d)

Transparent dielectric layer

Skin

PDMS Sensor

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Figure 5.2: Photograph of printed sensor in both wavy and straight form mounted on the finger

5.2.3. Experiment Setup

The experiment setup is shown in Fig. 5.3. Printed strain sensors were mounted on the

human finger. The resistive response of the sensor was then tested by bending the finger to

different angles. The sensor was connected to an Agilent E4980A precision LCR meter with

wires attached using a mixture of silicone and Silver (Protavic America) epoxy paste.

Figure 5.3: Experiment setup

5.2.4. Results

5.2.4.1. Strain Sensor on the Tattoo Paper

The resistance of the fully printed strain sensors in both wavy and straight forms was

measured after bending the finger to different angles including 10, 20, 30 and 40 degrees. Three

Connected

via wires Sensor LCR meter

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different sensors were tested for each configuration. Each sensor was subjected to three cycles

of bending the finger to different angles and bringing it back to the original position. This test

was repeated three times for each sensor. Figure 5.4 shows the change in resistance of the strain

sensor (sensor 1) printed in a wavy form when it was subjected to the finger positions described.

The base resistance of this sensor after mounting on the finger was measured as 255 kΩ. After

bending the finger to 10, 20, 30 and 40 degrees in test 1 the resistance was increased to 265.6

kΩ, 275 kΩ, 284.6 kΩ and 295 kΩ, respectively which corresponds to 3.8 %, 7.5 %, 11.2 % and

15.3 % changes in the resistance of the sensor. In the next phase of the test the finger was moved

back to 30, 20, 10 degrees and the original position (zero degrees) of the finger. The resistance of

the sensor was decreased to 284.3 kΩ, 275.3 kΩ, 265.3 kΩ and 256 kΩ, which showed 11.6 %,

7.5 %, and 3.7 % changes in the resistance of the strain sensor. The base resistance was increased

0.2 % in comparison with the original resistance of the sensor. All Figures show the average of

three cycles of bending the finger and bringing the finger back to the original position. After 20

minutes rest, the sensor was tested again by the bending of the finger. The base resistance for test

2 was measured as 250 kΩ. The finger was bent to 10, 20, 30, and 40 degrees and the resistance

of the strain sensor was increased to 258.3 kΩ, 266.3 kΩ, 275 kΩ and 285.6 kΩ which

corresponds to 3.1 %, 6.3 %, 9.8 % and 14.1 % changes when compared to the initial base

resistance. Then it was progressively brought back to the original position, and for each step the

resistance of the sensor was decreased to 274.3 kΩ, 265.6 kΩ, 258 kΩ and 250.7 kΩ which are

equal to 9.5 %, 5.9 % and 3 % changes in the resistance of the strain sensor. The base resistance

for test 2 was increased 0.2 % when the finger was brought back to the original position. The

sensor was left in the rest position for 20 minutes and the same measurement was repeated for a

third time. When the finger was bent to 10, 20, 30 and 40 degrees, the resistance was increased

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from 256 kΩ to 265.2 kΩ, 275.3 kΩ, 285.6 kΩ, and 294.6 kΩ and when it was brought back to

the original position the resistance was measured as 285.3 kΩ, 275.3 kΩ, 265.6 kΩ and 256.5

kΩ. This measurement showed 3.6 %, 7.3 %, 11.4 %, 14.9 %, 11.3 %, 7.3 %, and 3.6 % changes

compared to the resistance of the wavy strain sensor at rest. The base resistance was increased

0.1 % after this test compared with the base resistance of the sensor. The average of each round

of measurements is shown in Figure 5.4 for sensor 1. Figure 5.5 illustrates the percentage change

in the resistance of the sensor at each degree of bending as compared with the base resistance.

Figure 5.4: Resistive response of the sensor towards bending of the finger for different angles (Sensor 1).

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b

Figure 5.5: Percentage change in the resistive response of the fully printed strain sensor towards bending of

the finger for different angles (Sensor 1).

The same measurement was repeated for sensor 2 and sensor 3. The measured resistances

as well as percentage change of the resistances are presented in Figures 5.6, 5.7, 5.8 and 5.9.

Figure 5.6: Resistive response of the sensor towards bending of the finger (Sensor 2).

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Figure 5.8: Resistive response of the sensor towards bending of the wrist (Sensor 3).

126



Figures 5.10 and 5.11 represent the average of all the measurement on each sensor. For

sensor 1, 3.5 %, 7 %, 10.8 %, 14.7 %, 10.6 %, 6.9 %, 3.4 % changes in resistance were observed

for each step of the measurement. A 0.2 % change in the base resistance of the sensor was

observed. In case of sensor 2, 4.8 %, 9.9 % 14.6 %, 19 %, 15.5 %, 10 % and 5 % changes were

measured in response to the bending of the finger. The base resistance showed a 0.4 % increase

when compared with the resistance of the sensor before bending. Finally, 3.9 %, 8.3 %, 12.4 %,

16.8 %, 12.4 %, 8.5 % and 4.3 % changes were observed in the resistance of the printed strain

sensor 3 in response to bending the finger.

The changes in the resistance of the sensors between each step of bending were

calculated and presented in Figure 5.12. Average changes of 3.66 %, 4.7 % and 4.18 % for each

step were calculated for sensor 1, sensor 2 and sensor 3, respectively. All changes in the

resistance of the sensor can be attributed to the effect of applied strain on the sensor due to the

127

Figure 5.10: Resistive response of the sensor towards bending of the finger for different angles.


the finger for different angles.

bending of the finger. When a finger is in a bent position, the sensor is subjected to

tensile strain that increases the resistance of the sensor. These changes can be used to predict the

angle of bending of the finger when the change in the resistance is measured.

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Figure 5.12: Comparison of the percentage change of each step for different sensors.

The strain sensor was printed on the tattoo paper in the form of a straight line to

compare with the wavy structure. The sensor was subjected to the same test as the wavy

sensor. Figures 5.13, 5.14 and 5.15 show the result obtained for the straight strain sensor.

Sensor 1 tested towards bending of the finger. During the third round of measurement, sensor

1 was broken and it did not respond to the test after the second cycle of the third test. Sensor

2 and sensor 3 both showed failure during the second round of testing. Figures 5.16 and 5.17

represent the performance of sensor 1 (straight line). Changes of 4.8 %, 83.5 %, 12.3 %, 16.4

%, 12.3 %, 7.9 % and 3.6 % for each step were observed. The base resistance increased 0.4

% as compared to the base resistance prior to test 1. For test 2, changes of 3.5 %, 7.5 %, 12.5

%, 16.4 %, 12.3 %, 7.9 % and 3.6 % were observed. The sensor failed during test 3.

The wavy structure showed better performance in tolerating the repeatedly applied

stress caused by bending of finger as compared with the straight line. This can be due to the

fact that when the strain is applied on the wavy structure, the amplitude and wavelength of

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the waves are changed, which results in better stretchabilty of the structure. Based on the

results presented in the chapter 4, CNT ink showed a very good tolerance toward applied

stress. On the other hand, wavy structures showed a better stretchability in comparison with

the straight line. Combination of using wavy structure and mechanically strong and

stretchable material (CNTs) has shown a promising performance for fabrication of

stretchable strain sensors for body movement measurements.

Figure 5.13: Resistive response of the straight line sensor towards bending of the finger for different angles.

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131

Figure 5.16: Resistive response of the straight line sensor towards bending of the finger for different angles

(Sensor 1 in a straight form).


the finger for different angles (Sensor 1 in a straight form).

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5.2.4.2. Strain Sensor on PDMS

The strain sensor with a wavy structure (W=800 µm, r= 2000 µm) was screen printed

onto PDMS sheet with thickness of 254 µm using CNT ink. The picture of the fabricated sensor

is shown in Figure 5.18. The sensor was tested similar to the strain sensor printed on tattoo

paper, which was explained in section 5.2.4.1. The resistive response of the strain sensor towards

bending of the finger is shown in Figure 5.19. The resistance of the sensor after mounting on the

finger was 270 kΩ. After bending of the finger to 10°, 20°, 30° and 40° the resistance was

increased to 276 kΩ, 282.6 kΩ, 292.6 kΩ and 301.6 kΩ which correspond to changes of 1.8 %,

4.2 %, 7.9 %, and 11.2 % in the resistance of the sensor compared with the base resistance.

Then, the finger was brought back to the original position in steps of 10 °. The resistance was

measured as 293 kΩ, 282.6 kΩ, 276 kΩ, and 271.6 kΩ. This showed changes of 8 %, 4.2 %, and

1.8 % in the resistance. The base resistance was increased by 0.2 % when compared with the

original resistance of the sensor. This test was repeated three times and the result are illustrated

in Figures 5.19 and 5.20. Sensor 2 and sensor 3 were subjected to the same measurement and the

results are presented in Figures 5.21, 5.22, 5.23 and 5.24.

Figure 5.18: Photograph of the printed strain sensor on the PDMS (w=800 µm, r=2000 µm).

133

Figure 5.19: Resistive response of the sensor towards bending of the finger (Sensor 1 on PDMS).

Figure 5.20: Percentage change in the resistive response of the printed strain sensor towards bending of

the finger for different angles (Sensor 1 on PDMS).

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135




136

Figures 5.25 and 5.26 represent the average of all three measurements for each sensor.

For example for sensor 3, the average of base resistance was measured as 272 kΩ. After bending

of the finger 10, 20, 30 and 40 degree this resistance increased to 279.6 kΩ, 287.7 kΩ, 294.4 kΩ

and 303.1 kΩ which correspond to changes of 2.6 %, 5.6 5, 8.2 5 and 11.2 % in the resistance of

the sensor. Then the finger was brought back in steps of 30, 20, and 10 degrees and the first

position and the resistance was measured as 294.7 kΩ, 287.7 kΩ, 279.5 kΩ and 272.6 kΩ. This

showed changes of 8.1 %, 5.6 %, 2.5 % and 0.1 % in the resistance of the sensor. The average

change of 2.7 %, 2.2 % and 2.8 % between each step of bending was calculated for sensor 1,

sensor 2 and sensor 3, respectively (Figure 5.27).

Figure 5.25: Resistive response of the sensor towards bending of the finger for different angles.

137


the finger for different angles.

Figure 5.27: Comparison of the percentage change of each step for different sensors.

138

Another group of Strain sensor (w=400 µm, r=4000 µm) was printed on PDMS as

shown in Figure 5.28 using CNT ink. The sensor was mounted on the knee by help of the

sticky part of tattoo paper. The strain sensor was subjected to movement of the knee

(downward for 20 degrees and upward to original position) and the resistive response of the

sensor was obtained. For this purpose, three different sensors were fabricated and tested.

The results are presented in Figures 5.28, 5.29 and 5.30. The base resistance of sensor 1

after multiple downward and upward movements increased from 7.09 MΩ to 7.3 MΩ

corresponding to a 2.9 % change in the resistance. The base resistance of the sensor

showed a 0.1 % change in comparison with the original resistance (Figure 5.29). It was

observed that the average resistance of sensor 2 increased from 8.34 MΩ to 8.50 MΩ, for

multiple downward and upward movements, which corresponds to a 1.19 % change in the

sensor response. The base resistance of the sensor increased for 0.2 % after multiple

movements (Figure 5.30). The base resistance of sensor 3 when mounted on the knee was

measured as 7.32 MΩ. After subjecting the sensor to downward movement of the knee this

resistance was measure as 7.52 MΩ. The base resistance was increased to 7.36 MΩ which

is equal to 0.5 % change in the base resistance of the sensor.

Figure 5.28: Strain sensor printed on PDMS (w=800 µm, r=4000 µm).

139

Figure 5.29: change in the resistive response of the fully printed strain sensor towards bending downward

and upward of the knee (Sensor 1).

Figure 5.30: change in the resistive response of the fully printed strain sensor towards bending

downward and upward of the knee (Sensor 2).

140

Figure 5.31: change in the resistive response of the fully printed strain sensor towards bending downward

and upward of the knee (Sensor 3).

Sensor three remained attached for 6 hours on the knee. The response of the sensor

towards bending of the knee was measured after 3, 4, 5 and 6 hours. The base resistance of

the sensor increased to 9.2 MΩ, 9.4 MΩ, 10.6 MΩ and 11.4 MΩ after 3, 4.5 and 6 hours,

respectively. For each test, again the sensor was subjected to downward and upward

movement of the knee and the resistive response of the sensor was measured. An average

change of 2.2 % in the resistance of the sensors was obtained due to downward and upward

movement of the knee when the sensor was tested after 3 hours for each bending. Changes

of 1.8%, 6.4 % and 10.1 % in the resistance compared to the base resistance were obtained

after subjecting the sensor to downward and upward movement of the knee after 4, 5, and 6

hours, respectively. It was observed that even though the base resistance of the sensors

changed after remaining attached on the knee for a few hours, the sensor was able to

respond to the movement of the knee (Figure 5.32).

141

The strain sensor (horse-shoe design shown in Figure 5.28) attached on the knee

was tested towards bending of the knee for different angles. The knee was bent 20, 40, 60

and 80 degree. The resistance increased further for different angle of bending applied on

the sensor. Changes of 3.1 %, 5.2 %, 10.1 %, and 14.14 %in resistance were obtained in the

first measurement. The based resistance decreased when the knee was brought back the

original position. The same test was repeated again and changes of 3.3 %, 5.7 %, 10.9 %,

and 14.6 % were observed (Figure 5.33).

Figure 5.32: Performance of the sensor after 3, 4, 5 and 6 hours.

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Figure 5.33: Response of the strain sensor attached on the knee to different angle of bending.

There is a report on fabrication of capacitive strain sensor using silver nano wires which was able

to detect the bending of the finger (up to 50 % strain) [10,].The results presented in this section

show the capability of printed strain sensor towards detection of the bending of finger and knee

for multiple movement and bending. The silver nano wires show better conductivity compared

with CNT, but for the applications such as strain sensor, the printed resistive sensors showed a

very good performance and high tolerance towards applied strain.

5.2.4.3. Strain Sensor on PVA

The sensor, in a wavy shape having 800 μm line width and overall dimension of

3 cm × 0.4 cm, was screen printed using CNT ink (SWeNT CV100) on a water-soluble polymer

based PVA substrate (Watson QSA 2000) (Figure. 5.34(a)). The CNT ink was cured in a VWR

oven at 100 °C for 10 minutes. The skin was wetted and the printed sensor was mounted on the

left forearm (Fig. 5.34(b)). The sacrificial PVA layer was then washed away using water and the

(20°)

(40°)

(60°)

(80°)

(20°)

(40°)

(60°)

(80°)

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sensor transfer onto the skin was complete. The average thickness of the printed CNT layer was

measured as 5.6 μm using a Bruker Contour GTL EN 61010 profilometer (Figure. 5.35). An

Agilent E4980A precision LCR meter was used for recording the change in the resistance of the

sensor due to flexion and extension movements of the elbow. It was observed that the average

resistance of the sensor increased from 32.8 kΩ to 36 kΩ for multiple flexion and extension

movements of the elbow, which corresponds to a 10 % change in the sensor response (Figure

5.36). In addition, a 2 % change in the base resistance of the sensor was observed, when the

elbow was brought back to the original position after 10 cycles. A high possibility of failure in

performance of the sensors was observed after mounting directly on the skin. The thickness of

the printed sensor is 5.6 μm and when it is transferred on the human body, a high chance of

breakage was observed due to the roughness of the skin. The yield of functional sensors that

were directly mounted on the skin was below 10 %. As the author explained before in this

chapter, as an alternative method for fabrication of printed body movement sensors, it seems

promising to fabricate the sensor on the thin layer of substrate and then transfer it onto the skin.

Figure 5.34: Screen printed stretchable sensor: (a) on PVA substrate, and (b) after transferring onto the

forearm.

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Figure 5.35: Profilometry scan of deposited CNT layer

Figure 5.36: Resistive response of the sensor towards flexion and extension movements of the elbow.

5.2.4.4. Temperature Sensor on PDMS

The resistive temperature sensor in a wavy form with W=800 µm and r=2000 µm was printed

on the PDMS using CNT ink. The printed CNTs were then cured in the oven for 10 minutes at

100 °C. Interconnects were printed in wavy form using Ag ink on the TPU substrates and were

attached to the sensor using a mixture of silicone and Silver (Protavic America). Printed Ag ink

showed a resistance of 0.8 Ω. Due to the much smaller resistance of interconnects, no change was

145

observed in the resistance of the sensor after attaching it to the interconnect lines. The picture of

the fabricated sensor along with interconnects is shown in Figure 5.37. The effect of temperature

on the printed temperature sensor was investigated. The change in resistance was obtained by

increasing and decreasing of the temperature from 25 °C to 50 °C in steps of 1 °C and vice versa

using a hot plate, which is shown in Figure 5.38. The initial resistance of the sensor was measured

as 474 kΩ. Results of this test yield a linear relationship between resistance and change in

temperature. It was observed that the resistance of the sensor decreased about 0.36 % when the

temperature was increased by 1 °C. In a separate test the effect of increasing and decreasing the

temperature on another sensor (R0=575 kΩ) was investigated. The average of the change in the

resistance of the sensor by changing the temperature is shown in Figure 5.39, which is equal to

0.33 %. The temperature sensor showed an approximate sensitivity of 2 kΩ per °C.

After stand-alone testing, the sensor was transferred onto skin and was exposed to hot air

using a hair dryer. Three sensors were exposed to hot air for 10 seconds and the resistance of the

sensors was recorded 3 seconds after removal of the hot air. Figure 5.40 presents the results

obtained for these sensors. The results show averages decreases in resistance of 1.68 %, 1.44 %,

1.49 % for sensor 1, 2 and 3 respectively, which is indicated by average 3.7 °C, 4 °C and 4.5 °C

increases in the temperature. Before and after exposing the sensor to hot air, the skin temperature

was measured using an infrared thermometer which showed a temperature increase of 4 °C, 3.83

°C and 3.77 °C.

The temperature coefficient of the material is defined as the amount of change in the

resistance as a function of change in the temperature, which can be calculated using the formula

below

Rnew = Rold (1 + α.ΔT)

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Where the Rold is the original resistance, Rnew is the new resistance after a change in the

temperature; ΔT is the change in the temperature, and α is the thermal coefficient of the resistor.

In this work the thermal coefficient of the sensor was measured to be 0.36 and 0.33 °C-1

.

Figure 5.37: Photograph of printed temperature sensor attached on the shin

Figure 5.38: Resistance of the sensor versus temperature and the computed linear response using linear

regression method.

147

Figure 5.39: Resistance of the sensor versus temperature and the computed linear response using linear

regression method.

.

Figure 5.40: Resistance vs heat transferred on the skin.

148

As a final test, the sensor was attached onto human skin and the change in the

temperature after eating spicy food by the wearer was analyzed. The food was covered with

blazin sauce which contains habanero peppers. This pepper has a scoville heat unit of 100K-

350K which is categorized as a hot and spicy food. After eating any type of food, for

maintaining the body temperature some amount of heat will be released which causes a change

in the temperature of skin [11]. The body temperature was measured using a thermometer before

and after eating the food as 29.2 °C and 31.4 °C, respectively. The resistance of the sensor

decreased from 480 kΩ to 475.4 kΩ (Figure 5.41) which shows a change of 0.95 % in the

resistance. This change indicates a 2.6 ° increase of the temperature. This change can be

compared to the 2.2 °C increase when measured with the thermometer. One potential difference

in measured temperatures using the printed sensor and a thermometer may be due to the lack of

proper attachment of the sensor to the skin. Another factor might be changes in the resistance of

the sensor resulting from moisture on the skin due to sweating after eating spicy food. This

moisture can affect the properties of CNTs and make slight changes in the resistance of the

printed sensor.

Figure 5.41: Change in the temperature after eating spicy food measured by resistive sensor.

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5.2.4.5. Electrocardiogram Sensor (ECG)

A mixture of silicone and Silver (EXP-06335 from Protavic America) was prepared and

screen printed on a PDMS substrate to fabricate ECG measurement pads. The picture of printed

pads is shown in Figure 5.42 a) . After deposition of the mixture it was cured in the oven for 40

minutes at 100 ° C. It was obsoreved that the printed ECG pads showed a very good adhesion on

the PDMS. The thickness of the printed pads was measured using a Bruker vertical scanning

interferometer microscope (CounterGT) as 32 µm as shown in Figure 5.42 b).

Figure 5.42: a) Photograph of printed ECG pads on the PDMS b) 3D vertical scanning interferometer images

of Pads.

(a)

(b)

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In order to obtain an ECG signal, three identical sensors were placed on the left hand, right

hand and right leg of a healthy volunteer (35 years old). In the next step, the printed pads were

connected to the data acquisition device, which had been previously developed. The active

electrode, AC coupler, instrumentation amplifier, driven right leg amplifier (DRL), and active

filter were used for recording the ECG signal. The active electrodes include two input buffers and

two resistors for providing the high impedance input for the instrumentation amplifier. An AC

coupler including bio potential amplifier and an Analog Devices INA 2128 chip was employed as

the instrumentation amplifier. The feedback to achieve a high common mode rejection ratio

(CMRR) was provided to the human body by the DRL amplifier, which consisted of a two stage

amplifier. In order to record the ECG signals the ECG pads were connected to a Tektronix TDS

5104B Digital Phosphor Oscilloscope. A notch filter was used to eliminate 60 Hz power line

interference. A schematic of the setup is presented in Figure 5.43. The obtained ECG signal is

shown in Figure 5.44 a). Next, the ECG pads were connected close to the left and right shoulders

on the chest and the third electrode was connected to lower-left edge of the rib cage. The ECG

measurement obtained is shown in Figure 5.44 b). For the third measurement, the ECG electrode

from the lower-left edge of the rib cage was removed and attached onto the right leg. The ECG

signal recorded in shown in Figure 5.44 c).

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Figure 5.43: Experiment setup

The heart cells are polarized .The inside of heart muscle cells are negatively charged and

the exterior of the cells are positively charged. Depolarization and repolarization of these cells

lead to muscle contractions that circulate blood throughout the body. When some of the positive

ions move through the membrane, this decreases the potential difference between exterior and

interior of the heart muscle cells which results in depolarization. After depolarization, positive

ions move back to their first position and increase the potential difference between exterior and

interior of the heart muscle cells, which causes repolarization. In ECG signals, P-wave illustrates

the depolarization of the two atrium chambers of the heart. The Q, R and S waves are used to

characterize the depolarization of the two ventricle chambers of the heart. The T wave shows the

repolarization of the two ventricle chambers. In the measured ECG signals obtaining, it was

possible to identify the typical ECG characteristic components, which include the QRS complex

and the T-wave (Figure 5.44) [12].

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Figure 5.44: ECG signals obtained for a) Left hand, right hand and right leg b) chest close to left and right

shoulders and lower-left edge of the rib cage c) chest close to left and right shoulders and right leg.

All three types of sensors developed; strain, temperature and ECG; can be printed onto

PDMS to be used as a multi-functional wearable sensor to detect the movement of the body,

temperature of the skin, and ECG signal. A picture of the resulting device is presented in Figure

5.45

Figure 5.45: Picture of multi-functional wearable sensor fabricated on PDMS

(a)

(b)

(c)

Q S

R T

R T

Q S

R

S

T

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5.3. Summary

In this chapter, the author presented an introduction that discusses the importance of

developing a fully printed, flexible and stretchable sensing system that can be mounted onto the

skin. A detailed account of the experimental tasks involved in this work including chemical,

materials, and different sensors fabricated and tested was presented.

To summarize, screen printing techniques were employed to fabricate fully printed strain

sensors. Different methods were tested for transferring the device onto the skin including

printing the sensors on tattoo paper, PDMS, and PVA as well. CNT ink was used for fabrication

of sensors while silver ink was employed for the printing of interconnects. The printed sensors

on the tattoo paper were attached on the finger and were subjected to finger bending at angles of

10°, 20°, 30° and 40°. For sensor 1, changes in resistance of 3.5 %, 7 %, 10.8 %, 14.7 %, 10.6 %,

6.9 %, 3.4 % were obtained for each step of the measurement. A 0.2 % change in the base

resistance of the sensor was recorded. For sensor 2, 4.8 %, 9.9 % 14.6 %, 19 %, 15.5 %, 10 %

and 5 % changes were observed in response to the bending of the finger. The base resistance

showed a 0.4 % change when compared with the base resistance of the sensor. Finally, 3.9 %,

8.3 %, 12.4 %, 16.8 %, 12.4 %, 8.5 % and 4.3 % changes were measured in the resistance of the

printed strain sensor 3 in response to bending the finger. Average changes of 3.66 %, 4.7 % and

4.18 % for each step were calculated for sensor 1, sensor 2 and sensor 3, respectively. Also, the

strain sensor printed on PDMS was mounted on the skin and tested by bending the finger. The

average change of 2.7 %, 2.2 % and 2.8 % between each step of bending was calculated for

sensor 1, sensor 2 and sensor 3, respectively. The printed strain sensor on the PMDS was place

on a knee and the response of the sensor towards downward and upward movement of the knee

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was recorded. Changes of 2.9 %, 1.19 % and 2.7% in the resistance of the sensor were observed

after each movement of the knee.

A resistive temperature sensor was printed using CNT ink on PDMS substrates. A

mixture of silver and silicone was used as a metallization layer for fabrication of ECG sensors.

One of the most important challenges that the author was able to overcome in this chapter was

the deposition of conductive material on the PDMS. The mixture of silver and silicone as well as

CNT ink showed proper adhesion on the surface of PDMS. The effect of exposing the

temperature sensor to the hot air was tested. The sensitivity of the temperature sensor was

measured as 2 kΩ °C-1. A strain sensor was also printed on PVA as a sacrificial layer. After

printing it was transferred onto the skin and the PVA was washed away using water. A high

chance of breakage in the sensor was observed due to roughness of the skin. The yield of

functional devices transferred onto the skin was below 10 %.The mixture of silver and silicone

was used as a metallization layer for fabrication of ECG pads. These ECG pads were placed on

the human skin and the ECG signals were successfully obtained.

In the following chapter, the author concludes this dissertation with a summary of the

projects performed and also provides some suggestions for possible future work.

155

5.4. References

[1] Proximity Sensor Fully Fabricated by Inkjet Printing” Sensors, Vol. 10, pp. 5054-5062, 2010.

[2] G. Mattana, T. Kinkeldei, D. Leuenberger, C. Ataman, J. J. Ruan, , F. M. Lopez, A. V.

Quintero, G. Nisato, G. Tröster, D. Briand, N. F. de Rooij, “Woven Temperature and

Humidity Sensors on Flexible Plastic Substrates for E-Textile Applications” IEEE

SENSORS JOURNAL, Vol. 13, pp. 39.1-3909, 2013.

[3] G. M. Paul, F. Cao, R. Torah, K. Yang, S. Beeby, J. Tudor, “A Smart Textile Based Facial

EMG and EOG Computer Interface” IEEE SENSORS JOURNAL, Vol. 14,pp. 393-400,

2014.

[4] J. Yeo, G. Kim, S. Hong, M. S. Kim , D. Kim, J. Lee, H. B. Lee, J. Kwon, Y. D. Suh, H. W.

Kang, H. J. Sung, J. H. Choi, W. H. Hong, J. M. Ko, S. H. Lee, S. H. Choa, S. H. Ko,

“Flexible supercapacitor fabrication by room temperature rapid laser processing of roll-to-

roll printed metal nanoparticle ink for wearable electronics application” Journal of Power

Sources, Vol. 246, pp. 562-568, 2014.

[5] L. Buechley, M. Eisenberg, “Fabric PCBs, electronic sequins, and socket buttons: techniques

for e-textile craft” Pers Ubiquit Comput, Vol. 13, pp. 133–150, 2009.

[6] M. Vatani, Y. Lu, K. S. Lee, H. C. Kim, J.W. Choi, “Direct-Write Stretchable Sensors Using

Single-Walled Carbon Nanotube/Polymer Matrix” Journal of Electronic Packaging, Vol.135,

2013.

[7] L. Yang, R. Vyas, A. Rida, J. Pan, M. M. Tentzeris, “Wearable RFID-Enabled Sensor Nodes

for Biomedical Applications” Electronic Components and Technology Conference, pp.256-

2159, 2008.

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silver conductive tracks on textiles” Textile Research Journal, pp. 1-9, 2013.

[9] S. M. Lee, H. J. Byeon, J. H. Lee, D. H. Baek, K. H. Lee, J. S. Hong, S. H. Lee, “Self-

adhesive epidermal carbon nano tube electronics for tether-free long-term continuous

recording of biosignals” Sci Rep. Vol. 4, 2014.




Monitoring, Wireless Devices Fabricated by Macroscale Printing Techniques” Adv. Funct.

Mater. Vol. 24, pp. 3299-3304. 2014.

[12] Y. Sun, K. L. Chan, S. M. Krishnan, “Characteristic wave detection in ECG signal using

morphological transform” BMC Cardiovascular Disorders, Vol. 5, 2005.

http://www.ncbi.nlm.nih.gov/pubmed/25123356

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CHAPTER VI

CONCLUSION AND FUTURE WORK

6.1. Conclusion

Through this dissertation work, the author has successfully described and demonstrated

the fabrication of printed electronic circuit boards and established a method for the attachment of

electronic components on the printed lines. The author has demonstrated designs and design

techniques for the fabrication of flexible, stretchable conductive lines and sensing devices. And,

extended this work, the ability of conventional printing technologies to be utilized for the

fabrication of multi-functional stretchable and wearable sensing systems for health monitoring

applications was demonstrated. The accomplishments of each of the three research projects are

listed below:

In the first project, the author successfully fabricated multi-layer electronic circuit board

on paper, glass and PET using the screen printing methodologies. The design of the board

consists of an LCD, microcontroller and other necessary components. Silver ink was used as a

metallization layer and a dielectric ink was printed between top and bottom layer to prevent

electrical shorting. The widths of the printed lines were measured with an ImageXpert (KDY

Inc.) image analyzer. The maximum gain was measured on PET as 7.6% and the maximum loss

was measured on paper as −13.3%. The effect of the substrate roughness on the resistance of the

printed lines was analyzed. The lowest resistance of 1.06 Ω was obtained for the 600 µm lines

printed on paper. The effect of the resistance of the lines on the performance of the circuit was

determined. It was observed that the resistance of all lines was below 10 Ω which would not

affect the functionality of the circuit. The effect of bending of the substrate on the resistance of

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the printed lines was also tested. After 10,000 cycles of 5-mm elongation a negligible change in

the resistance was obtained. A method for attachment of electronic components onto the printed

PCB pads was established and demonstrated. The author also tested the capability of the printed

PCBs toward driving an LCD display. The preloaded software executing on the microcontroller

generated a graphic message (CAPE) on the LCD. Based on the results obtained, the author

showed the capability of the printing methods for manufacturing of multi-layered hybrid

electronic circuit boards and electronic systems.

In the second project, the author designed and fabricated different forms of wavy lines

using screen printing to analyze the effect of stretching test on them and determine the best

configuration. The design of wavy lines were divided into three groups 1) wavy lines with two

half circle connected to each other, 2) wavy lines having extended lines between two half circles

and 3) a horse-shoe design. Silver and CNTs were used for printing of the wavy lines on TPU as

a stretchable substrate. Different configurations were subjected to multiple displacements and

changes in the resistance of the lines were recorded. Different dimensionless design parameters

such as the ratio of the width of the lines to their radius (W/r) and the length of the extended line

to the diameter of the arc (L/D) were analyzed. The lower W/r ratio lines show better

stretchabilty and less change in the resistance when they were subjected to the strain. It was

observed that if the ratio of L/D is below one the lines show better stretchability. The effect of

the thickness of the lines on the stretchability was also analyzed. The printed lines having three

layers of silver ink showed better performance compared with one layer and two layers. The

change in the resistance of the lines (L/D=3/4) printed using CNTs were below 35 % when 50 %

strain was applied on the lines, while the silver printed lines having the same design parameters

showed 40 % change in the resistance when 10 % strain was applied. The results obtained

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demonstrated that wavy lines having extended line segments between two semi-circles have

better strechability. In addition, CNTs demonstrated a better capability in comparison with silver

to be used for the fabrication of stretchable electronic devices.

In the third project, the author successfully fabricated a novel, fully printed, stretchable

sensor for strain, temperature, and ECG measurements using screen printing. The sensor was

successfully printed on a PDMS, as well as tattoo paper, using CNTs for strain and temperature

and mixture of silver and silicone for ECG electrodes. The sensor printed on tattoo paper and

PDMS was subjected to bending on a human finger to angles of 10, 20, 30 and 40 degree. The

resistive response of the sensor demonstrated average changes in resistance of 3.66 %, 4.7 % and

4.18 % for each step for sensor 1, sensor 2 and sensor 3, respectively, on tattoo paper. For the

sensor printed on PDMS the average changes observed were 2.7 %, 2.2 % and 2.8 % between

each step of bending for sensor 1, sensor 2 and sensor 3, respectively. PVA was used as a

sacrificial layer to help transfer the sensor directly onto skin. The yield of the fabricated devices

using the sacrificial layer was below 10 %. The ECG pads were placed on the skin and ECG

signals were successfully observed and recorded. The author thus demonstrated the feasibility of

employing traditional printing techniques for the fabrication of flexible, stretchable, multi-

functional wearable sensors.

6.2. Future Work

The author believes that there are several possibilities and opportunities to improve upon

the current projects. Some suggestions for future work are now discussed.

Screen Printing of Multi-Layered Hybrid Printed Circuit Board (PCB) on Different

Substrates: Alternate conductive materials, such as copper, can be used for printing PCB

traces and tested for lower resistance and better performance. This may also aid in lowering

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the cost of printed PCB fabrication. The effect of encapsulation on the performance of

printed PCBs can be analyzed. Alternative substrates such as poly imide can be used for

manufacturing the PCBs. Finally, the printed PCB can be fabricated using stretchable lines

and the effect of the applying strain on the performance of the PCBs, including component

attachment, can be tested. Combining a flexible, stretchable PCB with wearable sensors

would allow a complete sensor system to be attached to a human body.

Design, Fabrication and Analysis of Printed Wavy Lines: The material used in this work

for printing wavy lines could be replaced by other functional inks such as copper, gold,

platinum, or nickel and tested for performance. Different line widths, especially thinner lines,

could be printed and tested, allowing optimization for material cost, resistance, and other

performance characteristics. Both gravure and flexo printing techniques could be employed

to fabricate the wavy structures. The wavy line using CNTs can be printed on PDMS with

different thicknesses to analyze the effect of the substrate on the stretchability of the lines.

Encapsulation of the wavy lines could help reduce the applied stress on the line. A thin layer

of spin coated PDMS or poly imide could be used for encapsulation of the wavy structures.

Fabrication of Wearable Sensors Using Priting Methodes: The effect of encapsulation on

the performance of the strain sensor should be investigated. As an alternate configuration, an

array of sensor could be fabricated for strain mapping on the body. The use of alternate

materials such as platinum could be used for printing the temperature sensor. An array of

sensors could be printed for temperature mapping on the skin. ECG signals could be obtained

during physical activates to investigate the effect of body motion on the performance of the

sensors. A thicker layer of CNT may be printed on the PVA to analyze the effect of the line

thickness on the functionality of the sensor after transferring directly onto skin.

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Wearable Sensors Systems: Combining printed, flexible and stretchable PCBs with

appropriate stretchable line interconnects to wearable sensors would allow a complete

monitoring system to be mounted on an individual and wirelessly transmit vital signs and

condition to nearby receivers. This system can be consisted of strain, temperature and ECG

sensors. Such systems could readily support both military and medical applications, where

untethered monitoring can provide continuous measurements during physical activity or

patient transfers.

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APPENDIX A

LIST OF PUBLICATIONS

A1. Inventions

[1] M.Z. Atashbar, M.K. Joyce, B.B. Narakathu, A.S.G. Reddy, A. Eshkeiti and M. Joyce,

“Smartphone based Fully Printed and Flexible Impact Sensing System” Western Michigan

University, Intellectual Property Disclosure, WMU CASE# 2014-012.

[2] M.Z. Atashbar, M.K. Joyce, B.B. Narakathu, A.S.G. Reddy, A. Eshkeiti and M. Joyce,

“Helmet Impact Monitoring System” U.S. Patent Application, (Patent Pending).

A2. Journal Papers

[1] *A. Eshkeiti, A. S. G. Reddy, S. Emamian, B. B. Narakathu, M. Joyce, M. Joyce, P. D.

Fleming, B. J. Bazuin, M. Z. Atashbar, “Screen printing of muli-layered hybrid printed

circuit board (PCB) on different substrates” IEEE Transactions on Components, Packaging

and Manufacturing Technology, 2015.

[2] M. Joyce, S. G. R. Avuthu, S. Emamian, A. Eshkeiti, M. Atashbar, P. D. Fleming, T.

Donato, “Contribution of Flexo Process Variables to Fine Line Ag Electrode Performance”,

International Journal of Engineering Research & Technology (IJERT), vol. 3, 2014.

[3] S. Emamian, A. Eshkeiti, A.S.G. Reddy, B.B. Narakathu, M.Z. Atashbar, “Gravure printed

flexible surface enhanced Raman spectroscopy (SERS) substrate for detection of 2,4-

dinitrotoluene (DNT) vapor”, Sensors and Actuators B: Chemical, (In Press).

[4] B.B. Narakathu, M.S. Devadas, A.S.G. Reddy, A. Eshkeiti, A. Moorthi, I.R. Fernando,

B. Miller, G. Ramakrishna, E. Sinn, M.K. Joyce, M. Rebros, E. Rebrosova, G. Mezei,

M.Z. Atashbar, “Novel fully screen printed flexible electrochemical sensor for the

163

investigation of electron transfer between thiol functionalized viologen and gold clusters”,

Sensors and Actuators B: Chemical, vol. 176, pp. 768-774, 2013.

[5] A. Eshkeiti, B.B. Narakathu, A.S.G. Reddy, A. Moorthi, M.Z. Atashbar, “Detection of toxic

heavy metals using a novel inkjet printed surface enhanced Raman spectroscopy (SERS)

Substrate”, Sensors and Actuators B: Chemical, vol. 171-172, pp. 705-711, 2012.

A3. Conference Papers

[1] A. Eshkeiti, Z. Ramshani, S. Emamian, B.B. Narakathu, S.G.R. Avathu, M.M. Ali, A.

Chlaihawi, M.K. Joyce, M.Z. Atashbar, “A Stretchable and Wearable Printed Sensor for

Human Body Motion Monitoring”, 14th

IEEE Sensors Conference, November 1-4, Busan,

South Korea, 2015. (Accepted)

[2] S. Emamian, S.G.R. Avuthu, B.B. Narakathu, A. Eshkeiti, A.A. Chlaihawi, M.Z. Atashbar,

“Fully Printed and Flexible Piezoelectric Based Touch Sensitive Skin”, 14th

IEEE Sensors

Conference, November 1-4, Busan, South Korea, 2015. (Accepted)

[3] B.B. Narakathu, S.G.R. Avuthu, D. Maddipatla, S. Emamian, A. Eshkeiti, A.A. Chlaihawi,

B.J. Bazuin , M.Z. Atashbar, “Rapid Prototyping of a Flexible Microfluidic Sensing System

Using Inkjet and Screen Printing Processes”, 14th

IEEE Sensors Conference, November 1-4,

Busan, South Korea, 2015. (Accepted)

[4] S.G.R. Avuthu, J. T. Wabeke, B.B. Narakathu, D. Maddipatla, S. Emamian, A. Eshkeiti,

A.A. Chlaihawi, M. K. Joyce, S. O. Obare , M.Z. Atashbar, “Development of Screen Printed

Electrochemical Sensors for Selective Detection of Heavy Metals”, 14th

IEEE Sensors

Conference, November 1-4, Busan, South Korea, 2015. (Accepted)

164

[5] S.G.R. Avuthu, J. T. Wabeke, B.B. Narakathu, D. Maddipatla, S. Emamian, A. Eshkeiti,

A.A. Chlaihawi, B.J. Bazuin, S. O. Obare , M.Z. Atashbar, “Detection of Heavy Metal Ions

using Screen Printed Wireless LC Sensor”, 14th

IEEE Sensors Conference, November 1-4,

Busan, South Korea, 2015. (Accepted)

[6] D. Maddipatla, B.B. Narakathu, S.G.R. Avuthu, S. Emamian, A. Eshkeiti, A.A. Chlaihawi,

B.J. Bazuin, M. K. Joyce, C. W. Barrett, M.Z. Atashbar, “A Novel Flexographic Printed

Strain Gauge on Paper platform”, 14th

IEEE Sensors Conference, November 1-4, Busan,

South Korea, 2015. (Accepted)

[7] A. A. Chlaihawi, B. B. Narakathu, A. Eshkeiti, S. Emamian, S. Reddy , M. Atashbar, “Novel

MWCNT/PDMS composite based screen printed dry electrode sensor for electrocardiogram

(ECG) measurement signals”, 2015 IEEE International Conference on Electro/Information

Technology, May 21-23, DeKalb, IL, USA , 2015. (Accepted)

[8] A. Eshkeiti, B.B. Narakathu, A.S.G. Reddy, S. Emamian, M.K. Joyce, B.J. Bazuin,

M.Z. Atashbar, “Screen printed flexible capacitive pressure sensor”, 13th

IEEE Sensors

Conference, November 2-5, Valencia, Spain, pp. 1192-1195, 2014.

[9] A. Eshkeiti, M. Joyce, B.B. Narakathu, A.S.G. Reddy, S. Emamian, M.K. Joyce,

M.Z. Atashbar, “A novel self-supported printed flexible strain sensor for monitoring body

movement”, 13th

IEEE Sensors Conference, November 2-5, Valencia, Spain, pp. 1615-1618,

2014.

[10] B.B. Narakathu, A.S.G. Reddy, A. Eshkeiti, S. Emamian, M.Z. Atashbar, “Development of

a novel printed flexible microfluidic sensing platform based on PCB technology”, 13th

IEEE

Sensors Conference, November 2-5, Valencia, Spain, pp. 665-668, 2014.

http://www.cityofdekalb.com/

165

[11] S. Emamian, A. Eshkeiti, B.B. Narakathu, A.S G. Reddy, M.Z. Atashbar, “Gravure printed

flexible SERS substrate for rapid and sensitive detection of 2,4-dinitrotoluene (DNT)”, 13th

IEEE Sensors Conference, November 2-5, Valencia, Spain, pp. 1069-1072, 2014.

[12] A.S.G. Reddy, B.B. Narakathu, A. Eshkeiti, S. Emamian, B.J. Bazuin, M. Joyce,

M.Z. Atashbar, “Detection of heavy metal compounds using fully printed three electrode

electrochemical sensor”, 13th

IEEE Sensors Conference, November 2-5, Valencia, Spain, pp.

669-672, 2014.

[13] B.B. Narakathu, A.S.G. Reddy, A. Eshkeiti, S. Emamian, M.Z. Atashbar, “A novel flexible

microfluidic platform: Integration of conventional printed circuit board technology and inkjet

printing”, 24th

Anniversary World Congress on Biosensors (BIOSENSORS), May 27-30,

Melbourne, Australia, pp. 120-121, 2014.

[14] B.B. Narakathu, A.S.G. Reddy, A. Eshkeiti, B.J. Bazuin, M.Z. Atashbar, “A novel flow cell

for opto-electrochemical based dual sensing of heavy metal compounds”, 15th

International

Meeting on Chemical Sensors (IMCS), March 16-19, Beunos Aires, Argentina, pp. 48, 2014.

[15] A.S.G. Reddy, B.B. Narakathu, S. Emamian, A. Eshkeiti, B.J. Bazuin, M.K. Joyce,

M.Z. Atashbar, “All printed pentacene organic thin film transistors for humidity sensing”,

15th

International Meeting on Chemical Sensors (IMCS), March 16-19, Beunos Aires,

Argentina, pp. 107, 2014.

[16] A.S.G. Reddy, B.B. Narakathu, A. Eshkeiti, S. Emamian, B.J. Bazuin, M.K. Joyce,

M.Z. Atashbar, “All screen printed circular electrodes as electrochemical sensors”, 15th

International Meeting on Chemical Sensors (IMCS), March 16-19, Beunos Aires, Argentina,

pp. 98, 2014.

166

[17] S. Emamian, A. Eshkeiti, B.B. Narakathu, M.Z. Atashbar, “Detection of 2,4-dinitrotoluene

(DNT) using gravure printed surface enhanced Raman spectroscopy (SERS) flexible

substrate”, 15th

International Meeting on Chemical Sensors (IMCS), March 16-19, Beunos

Aires, Argentina, pp. 86, 2014.

[18] M. Rezaei, A. Eshkeiti, P. Aminayi, B.B. Narakathu, M.Z. Atashbar, “Detection of heavy

metal compounds using a novel inkjet printed surface enhanced Raman spectroscopy

(SERS) substrate based on metallic triangular nano structures”, 15th

International Meeting

on Chemical Sensors (IMCS), March 16-19, Beunos Aires, Argentina, pp. 86, 2014.

[19] M. Rezaei, A. Eshkeiti, P. Aminayi, Z. Ramshani, B.B. Narakathu, M.Z. Atashbar, “A

novel inkjet printed surface enhanced Raman spectroscopy (SERS) substrate based on

Marangoni effect for the detection of heavy metal compounds”, 15th

International Meeting

on Chemical Sensors (IMCS), March 16-19, Beunos Aires, Argentina, pp. 120, 2014.

[20] A. Eshkeiti, A.S.G. Reddy, B.B. Narakathu, S. Emamian, M. Rezaei, M.K. Joyce,

P.D. Fleming, B.J. Bazuin, M.Z. Atashbar, “Screen printed capacitive pressure sensor”,

Flexible and Printed Electronics Conference (FlexTech), February 4-6, Phoenix, Arizona,

USA, pp. 45, 2014.

[21] B.B. Narakathu, M.S. Devadas, A.S.G. Reddy, A. Eshkeiti, G. Ramakrishna, E. Sinn, M.K.

Joyce, G. Mezei, M.Z. Atashbar, “Investigation of Electron Transfer between Gold Clusters

and (Pseudo) Rotaxanes Using a Novel Screen Printed Flexible Electrochemical Sensor”,

Flexible and Printed Electronics Conference (FlexTech), February 4-6, Phoenix, Arizona,

USA, 2014.

167

[22] B.B. Narakathu, A. Eshkeiti, A.S.G. Reddy, S. Emamian, M.K. Joyce, P.D. Fleming,

B.J. Bazuin, M.Z. Atashbar, “PDMS based flexible capacitive pressure sensor fabricated

using printing technology”, Flexible and Printed Electronics Conference (FlexTech),

February 4-6, Phoenix, Arizona, USA, pp. 47, 2014.

[23] A.S.G. Reddy, B.B. Narakathu, A. Eshkeiti, S. Emamian, B.J. Bazuin, M.K. Joyce,

M.Z. Atashbar, “All screen printed circular electrodes as electrochemical sensors”, Flexible

and Printed Electronics Conference (FlexTech), February 4-6, Phoenix, Arizona, USA, pp.

45, 2014.

[24] A.S.G. Reddy, B.B. Narakathu, S. Emamian, A. Eshkeiti, M.K. Joyce, B.J. Bazuin,

M.Z. Atashbar, “Fully printed electrochemical sensor on flexible substrates”, Flexible and

Printed Electronics Conference (FlexTech), February 4-6, Phoenix, Arizona, USA, pp. 45,

2014.

[25] A. Eshkeiti, M. Rezaei, B.B. Narakathu, A.S.G. Reddy, M.Z. Atashbar, “Gravure printed

paper based substrate for detection of heavy metals using surface enhanced Raman

spectroscopy (SERS)”, 12th

IEEE Sensors Conference, November 3-6, Baltimore, Maryland,

USA, pp. 1-4, 2013.

[26] B.B. Narakathu, A.S.G. Reddy, A. Eshkeiti, B.J. Bazuin, M.Z. Atashbar, “Opto-

electrochemical based dual detection of heavy metal compounds using a novel flow cell”,

12th

IEEE Sensors Conference, November 3-6, Baltimore, Maryland, USA, pp. 1-4, 2013.

[27] A.S.G. Reddy, A. Eshkeiti, B.B. Narakathu, B. J. Bazuin, M.K. Joyce, M.Z. Atashbar,

“Fully printed OTFT based flexible humidity sensors”, 12th

IEEE Sensors Conference,

November 3-6, Baltimore, Maryland, USA, pp. 1-4, 2013.

168

[28] A. Eshkeiti, A.S.G. Reddy, B.B. Narakathu, M.K. Joyce, B.J. Bazuin, M.Z. Atashbar,

“Detection of toxic heavy metals using a flexible printed surface enhanced Raman

spectroscopy (SERS) substrate”, Flexible and Printed Electronics Conference (FlexTech),

January 29-February 1, Phoenix, Arizona, USA, pp. 17-18, 2013.

[29] B.B. Narakathu, A.S.G. Reddy, A. Eshkeiti, B.J. Bazuin, M.K. Joyce, M.Z. Atashbar,

“Detection of biochemicals using a gravure printed flexible electrochemical sensor”,

Flexible and Printed Electronics Conference (FlexTech), January 29-February 1, Phoenix,

Arizona, USA, pp. 18, 2013.

[30] A.S.G. Reddy, B.B. Narakathu, A. Eshkeiti, M.K. Joyce, B.J. Bazuin, M.Z. Atashbar,

“Heavy metal detection using a flexible wireless printed LC sensor”, Flexible and Printed

Electronics Conference (FlexTech), January 29-February 1, Phoenix, Arizona, USA, pp. 17,

2013.

[31] A. Moorthi, B.B. Narakathu, A.S.G. Reddy, A. Eshkeiti, M.Z. Atashbar, “A novel flexible

strain gauge sensor fabricated using screen printing”, 6th

International Conference on

Sensing Technology: ICST 2012, December 18-21, Kolkata, India, pp. 765-768, 2012.

[32] A. Eshkeiti, B.B. Narakathu, A.S.G. Reddy, A. Moorthi, M.Z. Atashbar, “Gravure printed

surface enhanced Raman spectroscopy (SERS) substrate for detection of toxic heavy

metals”, 11th

IEEE Sensors Conference, October 28-31, Taipei, Taiwan, pp. 434-437, 2012.

[33] B.B. Narakathu, A. Eshkeiti, A.S.G. Reddy, A. Moorthi, M.Z. Atashbar, “A novel fully

printed and flexible capacitive pressure sensor”, 11th

IEEE Sensors Conference, October 28-

31, Taipei, Taiwan, pp. 1935-1938, 2012.

169

[34] A. Eshkeiti, B.B. Narakathu, A.S.G. Reddy, A. Moorthi, M.Z. Atashbar, E. Rebrosova, M.

Rebros, M. Joyce, “Detection of Toxic Heavy Metals Using A Novel Flexible Gravure

Printed Surface Enhanced Raman Spectroscopy (SERS) Based Substrate” 6th Asia-Pacific

Conference of Transducers and Micro/Nano Technologies (APCOT) 2012, July 8-11,

Nanjing, China, (2012).

[35] A. Eshkeiti, B.B. Narakathu, A.S.G. Reddy, A. Moorthi, M.Z. Atashbar, E. Rebrosova,

M. Rebros, M.K. Joyce, “A novel flexible gravure printed surface enhanced Raman

spectroscopy (SERS) sensor for the detection of toxic heavy metals”, 14th

International

Meeting on Chemical Sensors (IMCS), May 20-23, Nuremberg, Germany, pp. 1479-1482,

2012

[36] A.S.G. Reddy, A. Eshkeiti, B.B. Narakathu, A. Moorthi, M. Z. Atashbar, M. Rebros,

E. Rebrosova, M.K. Joyce, “Fully printed wireless LC sensor for toxic heavy metal

detection”, 14th

International Meeting on Chemical Sensors (IMCS), May 20-23,

Nuremberg, Germany, pp. 1191-1194, 2012.

[37] A. Eshkeiti, B.B. Narakathu, A.S.G. Reddy, A. Moorthi, M.Z. Atashbar, “A novel inkjet

printed surface enhanced Raman spectroscopy (SERS) substrate for the detection of toxic

heavy metals”, 25th

Eurosensors Conference, September 4-7, Athens, Greece, vol. 25,

pp. 338-341, 2011.

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