FEASIBILITY OF ADDITIVE MANUFACTURING METHOD FOR DEVELOPING STRETCHABLE &
FLEXIBLE EMBEDDED CIRCUITS
MSME Plan B ProjectAugust 2014
OVERVIEW
• Introduction
• Additive Manufacturing process
• Extruder design
• Material characterization
• Printing functional circuits
• Conclusion
INTRODUCTION
STRETCHABLE ELECTRONICS
Integrated circuits that can be stretched, compressed,
twisted, bent, and deformed
Favorable attributes:
• Conformity
• Lightweight structure
• Shock-resistant construction
CATEGORIES
Foldable electronicsApplications in smart packaging
& paper-based electronics
Embedded electronics that can
stretch with molding processMoldable electronics
Conforming to human anatomy,
bio-integrated devicesConformable
electronics
Consumer electronics: flexible
displays, keypads, smart textilesRollable electronics
Electroative haptic switches;
consumer electronicsMophable electronics
www.idtechex.com/ applications of stretchable electronics
FABRICATION
2 Main Approaches:
• Printed and thin film monolithic circuits
• Islands of rigid electronics with flexible polymer
substrate and interconnects accommodating
elastic strain
MATERIALS
single-walled carbon nanotubes (SWNTs) as conductive dopants
in a rubber matrix
silver nanoparticle ink for direct writing onto a flexible substrate
polyurethane containing spherical nanoparticles deposited by either layer-by-layer assembly or vacuum-assisted flocculation
ionic conductor (hydrogels, etc.)
FABRICATION METHODS
Molded Interconnect Device (MID) technology: rigid or
flexible standard components that are interconnected by
meander shaped metallic wires and embedded
by molding in a stretchable substrate polymer
the chemical vapor deposition (CVD) growth of NWs
followed by their transfer and assembly on flexible
substrates (PDMS, etc.)
conductive composite mat of silver nanoparticles with
rubber fibres (Park et al., 2012)
Aerosol Printing: print multiwall carbon nanotube solution
onto an insulating flexible substrate
(Thompson et al.)
GAP
Current methods of manufacturing:
Fabricated with planar technologies: require
complex, dedicated infrastructure
Prolonged fabrication time
Expensive
To overcome these use of an additive manufacturing
process is proposed:
• Cost-effective and accessible
• Easy-to-use
• efficient free-form fabrication of CAD designs
• Ability to print with biocompatible Silicone
ADDITIVE
MANUFACTURING
ADDITIVE MANUFACTURING
PROCESSES
additive process: successive layers of material are
laid down
Realize complex parts, molds, circuit boards without
requirement of enterprise scale facilities
Capability to print with a wide variety of materials:
plastic, silicone, ceramics, metals, etc.
Opportunity to develop freeform fabrication systems
capable of producing electronics and structures
together for stretchable electronics.
PRIOR WORK
3D printing electronics (Periard et al.):
- printing of both structural layers and
electronic circuitry.
- Fully-functional, rigid electronic
devices.
3D tissue-engineering of living
human organs: (Mironov et al.)
- Cell printer that: print gels,
single cells and cell aggregates.
Printing Bionic organs: (Mannoor et
al.)- additive manufacturing of
biological cells with structural and
nanoparticle derived electronic silicones
PRINTING WITH SILICONE
3D printing with multi-material fabrication:
Silicone based conductive material
Silicone as structural material
Compatibility between silicone and added cells.
• Advantages of silicone: Stretchable, Bio-compatible,
ideal substrate and structural material
• However there is no study on 3D printing of electronics
with silicone on stretchable substrates
GOAL
Design an extrusion method for silicone for
stretchable electronics
Study the material characteristics of conductive
silicone under stretching
Print complete integrated and fully-functional
electronic circuits that are capable of withstanding
stretching and flexion.
DESIGN
SILICONE SELECTIONKey criteria:
Curing method: RTV/ moisture-accelerated/UV
Curing speed (skin-over time, tack-free time, etc.)
Extrusion rate and viscosity
Conductivity
Material consistency and appearance
Mechanical strength
SS-26S: Moisture accelerated RTV cure, thixotropic paste, non-
corrosive
Silicone Mfg.Stretch
ability*
Chemical
typeAppearance
Viscosity
(cP)
Material
typeCure
Tack-free
time
(min)
Working
time
(min)
at deg. CShore Hardness
(Durometer A)
Conductivity
(S/cm)
SS 261Silicone
Solutions
Non-
corrosive
Silver-Tan (Custom
colors available
upon request)
400000Thixotropic
paste
RTV
(Accelerated
moisture cure)
45 > 24 hrs 115 70 200
SS-26Silicone
Solutions
Non-
corrosive
Silver-Tan (Custom
colors available
upon request)
30000
to
80000
Thixotropic
paste
RTV
(Accelerated
moisture cure)
30 15Room
temp70 200
SS 26SSilicone
Solutions200
Non-
corrosive
Silver-Tan (Custom
colors available
upon request)
500000Thixotropic
paste
RTV
(Accelerated
moisture cure)
30 15Room
temp50 200
* > % Elongation @ break
Material properties (uncured) Cured materialCuring
SYRINGE-BASED EXTRUSION
Syringe and Nozzle selection:
• Inexpensive plastic syringe
• Luer-lok: Easy change of needle
• Substitute material: corn syrup
• Require uniform bead deposition – circular c/s, blunt
tip, shaft size, etc.
EXTRUSION VELOCITY & FORCE
• Desired extrusion velocity: ~5-20 mm/s
For 21 gauge needle (ID=0.5mm) with ½” shaft:
Plunger velocity, vb = 0.1mm/s
Volume flow rate, V = 1.8e-9 m3/s
• From Reynold’s no. Re = 9.79e-7, Flow is laminar
• Calculating pressure drops across all cross-sections of the
syringe using Pouiselle’s law, push force required at the
plunger of a 1ml syringe is:
F = 73.82 N
EXTRUSION HEADMotor selection:
- High torque, High gear ratio, Low speeds
• Linear actuator
• Rotary motor
Syringe tool V1: Motor: L16, Firgelli
peak power point: 175N @4mm/s
gear reduction: 150:1
maximum speed: 8mm/s
Box-type design for syringe
housing
syringe extrusion axis parallel to
the axis of the linear motor:
compact size
Drawback: Hinders loading
unloading of syringe/ material
Syringe tool V2:
Motor force along same direction as
extrusion axis
Mounting point at syringe housing
Easy loading for syringe
Drawbacks:
• Increased length due to co-axial mounting
• Cantilevered weight of motor unit on syringe housing unit
Syringe tool V3:
Phidgets 3257E
12V DC motor with optical encoder
Internal 100:1 gear ratio
3:1 gear reduction using spur gears
Torque: 1.63 Kg-cm @23 RPM
Encoder resolution: 360 CPR
Key criteria for extrusion
control:
Regulating the steady-
state extrusion speed for
the extruder.
Starting and stopping
extrusion-on-demand in-
trajectory.
Constant plunger velocity
control using PID control.
Serial input to set target
speed (with Trajectory-
Based Start and Stop
Method )
EXTRUSION VELOCITY CONTROL
2 3 4 5 6 7 8 9 10 11
x 104
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Time [ms]
Velo
city [
mm
/s]
Extruder velocity control
Target Velocity
Actual Velocity
EXPERIMENTAL SETUP
Trajectory drawing speed: 1.7 mm/s
Pen-up and pen-down speed: 0.4 mm/s
Needle pen-up and pen-down height
(from work surface): 1.0 mm
Corvus arm free travel (non-drawing
trajectory) speed: 30.0 mm/s @ 20.0mm
Motor start time (prior to drawing
trajectory): 0.500 seconds
Motor stop time (in drawing trajectory):
0.100 seconds
PRINTING PARAMETERS
MATERIAL
CHARACTERIZATION
INTERFACING WITH SILICONE TRACES
3 methods:
1. Header pin connection
2. custom-prepared electrodes/components with
steel mesh contact interfaces
3. Direct contact/insertion into silicone bead: The traces are printed first, the
component placed onto
printed pads (at required
location) on the substrate
Connectivity of conductive silicone traces to smt components:
v/s
DMA TESTING
Objectives:
study conductive behavior of SS-26S
material in an embedded silicone
environment observe performance characteristics
of the material under a uniaxial strain
Experiment conducted at 7-SIGMA
• Q800 DMA: non-contact linear drive
• Max. force 18N
• Force resolution of 0.00001N
• Strain resolution of 1nm• Force ramp: 2N/min
• Sampling rate: 10Hz
TA Instruments Q800 DMA
Keithley 6487 Picoammeter
RESULTS
0 5 10 15 20 25 30 35 40 452
3
4
5
6
7
8
9
10
Strain [%]
Resist
anc
e [O
hms]
Resistance v/s Strain
Unstretched test sample
Repeat elongation 1
Repeat elongation 2Sample:Total trace length = 37.6mmTrace width = 1mmTrace height = 1mm
Slope = 0.58 Ω/%strain
0 5 10 15 20 25 30 35 40 450.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.11
Strain [%]
Cond
uctivi
ty [S/cm
]
Conductivity v/s Strain
Δ = 0.01 S/cm
PRINTING CIRCUITS
TEST CIRCUIT
Voltage divider circuit
+5V regulated power
supply
R1 =10 ohms (fixed
resistance)
R2 = RSi (Resistance of
the Silicone trace)
8-pin ATtiny85
programmed using
Arduino as ISP
Serial communication
via TX pin out to
Bluetooth module
(Sparkfun HC-06)
SOME EXAMPLES
TESTING RIG1) Camera housing
2) HD camera for
tracking elongation of
sample
3) Cable out to
computer USB 3.0
4) Tripod for mounting
the camera housing
5) linear translation stage
with a mounting
platform
6) Rotary control for
linear slide
7) mounting plate on
linear slide platform
8) slender clamps to
constrain and grip the
test sample
9) Stretchable circuit
sample
ELONGATION TRACKING
• Computer vision algorithm: HSV color
detection of markers on sample using Slider GUI
• Separation in pixels determined
• Data integrated to serial output from
circuit with timestamp
CIRCUIT UNDER ELONGATION
Limit test:
Direct contact interface: smt LED and 1206 package resistor with
conductive silicone trace
LED worked till 60% strain. Contact lost due to delamination from
component lead
At original length @60% strain
CIRCUIT UNDER ELONGATION
RESULTSResistance computed from output
voltage of voltage divider
0 1 2 3 4 5 6 7 8 90
2
4
6
8
10
12
14
16
18
20
Strain [%]
Re
sis
tan
ce
(O
hm
s)
Resistance v/s strain
0 0.5 1 1.5 2 2.5
x 105
50
100
150
Volta
ge [
mV
]
Voltage and strain vs. time
0 0.5 1 1.5 2 2.5
x 105
0
5
10
stra
in [
%]
Time [ms]
Reference Voltage
Output Voltage
Strain
0 0.5 1 1.5 2 2.5
x 105
0
10
20
Re
sist
an
ce [
Oh
ms]
Resistance and strain vs. time
0 0.5 1 1.5 2 2.5
x 105
0
5
10
stra
in [
%]
Time [ms]
Resistance
Strain
RESULTS3 cycles of stretching
to 8.7% elongation
0 1 2 3 4 5 6 7
x 105
0
5
10
15
20
25
28
Res
ista
nce
[Ohm
s]Resistance and strain vs. time
0 1 2 3 4 5 6 7
x 105
0
5
10
stra
in [%
]
Time [ms]
Resistance
Strain
0 1 2 3 4 5 6 7 8
x 105
0
50
100
145
Vol
tage
[mV
]
Voltage and strain vs. time
0 1 2 3 4 5 6 7 8
x 105
0
5
10
stra
in [%
]
Time [ms]
Reference Voltage
Output Voltage
Strain
OBSERVATIONS
SOD-323F packageZD contacts: 0.4 mm x 0.25 mm, (height 0.25mm)
1206 packageResistor contacts: 1.6 mm x 0.4 mm, (height 0.55 mm)
Contact Separation
at electrode when
relaxing from high
strain
• Sufficient contact for
conductive pathway
for 1206 package
• Delamination from
diode electrode
upon relaxation from
high strain: Loss of
electrical signal due
to lack of contact
• Isolate component
from strain to
maintain electrical
contact under elongation
CONCLUSION
Additive manufacturing process
Designed and tested custom extrusion module for
printing circuits with conductive silicone on
flexible substrates
Constant velocity controlled printing
implemented
Testing and Performance characteristics
Tested conductive properties of SS-26S under
uniaxial strain
Studied interfacing methods
Printed integrated fully-functional circuits and
tested performance under strain
FUTURE WORK
Improve interfacing with
components:
a) Use of non-conductive
mesh for strain relief
b) Use of multi-durometer skin with higher durometer
islands for optoelectronic components
Design of integrated curing module with dual-cure
(UV + moisture) for increased speed.
build and testing of a complete ‘skin-like’ device with
embedded devices custom fit to anatomical
geometry using Corvus.
AKNOWLEDGEMENTS
Dr. Tim Kowalewski, Advisor
John O’Neill: Corvus programming
Rod Dockter II: Computer vision guru
Tim Zalusky: PID implementation
Special thanks to Wade Eichhorn, Dave Winters and
Jim Norris of 7-SIGMA Inc., Minneapolis
Thank You!
The University of Minnesota is an equal opportunity educator and employer.
Syringe: 3mlNeedle size: 25GShaft length: 5/8”Force: HIGH
Syringe: 1mlNeedle size: 21GShaft: 1/2” (blunt)Force: LOW
Syringe: 1mlNeedle size: 20GShaft: 1-1/2”Force: LOW
Syringe: 12mlNeedle size: 20GShaft: 1-1/2”Force: VERY HIGH
Syringe: 1mlNeedle: 30G, 1/2”Force:NEEDLE BEND 10mm
10mm
0 5 10 15 20 25 30 35 40 450.04
0.06
0.08
0.1
Strain [%]
Cond
uctivi
ty
[S/cm
]
Conductivity v/s Strain
0 5 10 15 20 25 30 35 40 452
4
6
8
10
Strain [%]
Resist
anc
e
[Ohm
s]
Resistance v/s Strain
Unstretched test sample
Repeat elongation 1
Repeat elongation 2
Δ = 0.01 S/cm
Total trace length = 37.6mmTrace c/s = 1mm x 1mm
Slope = 0.58 Ω/%strain
0 1 2 3 4 5 6 7 8 90
5
10
15
20
Strain [%]
Res
ista
nce
[Ohm
s]
Resistance v/s strain
Elongation
Constant strain
Relaxation
0 0.5 1 1.5 2 2.5
x 105
50
100
150
Volta
ge [m
V]
Voltage and strain vs. time
0 0.5 1 1.5 2 2.5
x 105
0
5
10
str
ain
[%
]
Time [ms]
Reference Voltage
Output Voltage
Strain
0 0.5 1 1.5 2 2.5
x 105
0
10
20
Resi
stance
[O
hm
s]
Resistance and strain vs. time
0 0.5 1 1.5 2 2.5
x 105
0
5
10
stra
in [%
]
Time [ms]
Resistance
Strain