3D Printing of Highly Stretchable Strain Sensors Basedon Carbon Nanotube Nanocomposites
Mohammad Abshirini, Mohammad Charara, Yingtao Liu,* Mrinal Saha, andM. Cengiz Altan
This paper presents the additive manufacturing of a stretchable andelectrically conductive polydimethylsiloxane (PDMS) nanocomposite for strainsensing. A long, thin PDMS strip in a zig-zag pattern of five parallel lines is3D printed on elastomer substrate using an in-house modified 3D printer.Multi-walled carbon nanotubes (MWCNTs) are uniformly deposited on theuncured PDMS lines and cured in an oven. An additional layer of PDMS isthen applied on top of MWCNTs to form a thin protective coating. The 3Dprinted PDMS/MWCNT nanocomposites are characterized using a scanningelectron microscope (SEM) to validate the thickness, uniformity, andmicrostructural features of the sensor cross-section. The strain sensingcapability of the nanocomposites is investigated under tensile cyclic loadingat different strain rates and maximum strains. Long-term performance istested under cyclic tensile loads for 300 cycles. Sensing experiments indicatethat under cyclic loading, the changes in piezo resistivity mimic the changesin the applied load and the measured material strain with high fidelity. Insitu micro-mechanical testing in SEM is carried to investigate the piezoresis-tive sensing mechanism. Due to the high flexibility of PDMS, the 3D printedsensors are tested to monitor the bending of a human wrist joint as awearable sensor.
Additive manufacturing, commonly referred to 3D printing, hasattracted great attention due to its promising potential for rapidprototyping of new materials in complex 3D geometries withprecise microarchitectures. Current 3D printing technologies,including fused deposition modeling (FDM),[1,2] stereolithog-raphy (SLA),[3,4] and selective laser sintering (SLS),[5,6] have beenused for rapid prototyping of polymers. Thermoplastic polymers,such as acrylonitrile butadiene styrene (ABS),[7,8] polylactic acid(PLA),[9] polyamide (PA),[10] and polycarbonate (PC),[11] as well asthermosetting polymers, such as epoxy resins,[12] have been usedin different 3D printing technologies. Recent developments in
Prof. Y. T. Liu, M. Abshirini, M. Charara, Prof. M. Saha,Prof. M. C. AltanSchool of Aerospace and Mechanical EngineeringUniversity of Oklahoma865 Asp Ave. #218A, Norman, OK 73019, USAE-mail: [email protected]
The ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/adem.201800425.
additive manufacturing technologiesfor polymers have been reviewed inliterature.[13–15]
With the advancements in nanotechnol-ogy, carbon based nanoparticles have beenemployed to develop new nanocompositematerials with versatile functionalities,such as self-sensing composites[16] andself-assembled supercapacitors.[17] Multi-walled carbon nanotubes (MWCNTs) are ofparticular interest due to their outstandingelectrical, mechanical, and thermal proper-ties. The high electrical conductivity ofMWCNTs has been utilized in severalinnovative ways to enhance the conductiv-ity of nanocomposites, resulting in newpiezoresistance based autonomous loadsensing capabilities. These sensing capa-bilities have been shown in various matrixmaterials, such as polymers,[18] cements,[19]
and fiber reinforced composites.[20]
Polymer matrix nanocomposite materialshave been widely used to develop strainsensors that can respond to externalmechan-ical deformations by changing their electricalcharacteristics, including capacitance andresistance.[21] High performance strain sen-
sors should have high sensitivity (i.e., gauge factor), goodrepeatability, and low fabrication cost. Stretchable and sensitivestrain sensors developed using nanocomposites can be used aswearable sensors and devices in applications including rehabilita-tion, personal health monitoring, and sports performancemonitoring.[22–24] Due to the high technical requirements andbroadpotential applications of highperformance strain sensors, thedevelopment of highly flexible, skin mountable, and biologicallycompatible strain sensors is necessary and can be useful for a widevariety of medical and wearable equipment and devices.
Recently, innovative integration of additive manufacturingand nanocomposite materials has received considerable inter-est.[25–30] Additive manufacturing of polymer nanocompositesoffers new opportunities, allowing the manipulation of localmaterial formulation and properties to obtain optimal productperformance, multi-functionality, and reduced fabrication time.In addition, 3D printing allows for fabrication of complexnanocomposite structures without the typical material wasteinherent in conventional fabrication techniques, such as casting,molding, and machining. Thus, 3D printing of nanocompositematerials, particularly for the strain sensing applications, is an
excellent way to utilize process flexibility to achieve configurable,high performance sensors.[27,31] Nanocomposite strain sensorshave been 3D printed and reported in literature.[32–34] Elastic andthermoplastic polymers, such as polyurethane (PU) and poly-(dimethylsiloxane) (PDMS) were employed as the matrix andMWCNTs were used as the main nanoparticles to tailor thepiezoresistance. Although the reported strain sensors have greatelastic and sensing properties, several issues, such as sensorrepeatability under high fatigue cycles and the sensingmechanism have not been fully addressed.
This paper presents a 3D printing approach to fabricate highlyflexible strain sensors using PDMS elastomer and MWCNTsbased nanocomposites, with a sub-millimeter resolution. Themicrostructure of the sensor was characterized using a scanningelectron microscope (SEM). The strain sensing function wastested under cyclic tensile loading at different maximum strainsand strain rates. The repeatability of the sensing function wastested via a long durability test of 300 cycles. The sensingmechanism was investigated using an SEM equipped with an insitu micro-testing stage. The realignment of MWCNTs undertensile load conditions were studied. Finally, the 3D printedstrain sensor was tested as a type of wearable sensor and used tomonitor the bending of a human wrist joint.
2. Experimental Section
2.1. Materials
Unless otherwise stated, all materials and reagents were used asreceived. SYLGARD 184 PDMS was purchased from DowCorning. MWCNTs were purchased from Nanostructured andAmorphous Materials Inc., and had an average diameter andlength of 10–20 nm and 5–15 μm, respectively.
Figure 1. Schematic illusion illustration of the sensor fabrication process u
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2.2. Materials’ Preparation and 3D Printing of Strain Sensors
Prior to 3D printing, the PDMS prepolymer was prepared bymixing the base elastomer and curing agent at a 10:1 weight ratio,as recommended by the manufacturer. The PDMS mixture wasdegassed in a vacuum desiccator at room temperature for 30min.Then, the mixed materials were pre-cured in the oven at 80 �C for5min to increase their viscosity. Pre-curing temperature and timewere determined after several trial experiments. The preparedPDMSpolymerswere used for sensor fabrication after pre-curing.
The schematic illustration of the 3D printing and sensorfabrication procedure is shown in Figure 1. A pristine PDMSsubstrate was first prepared using a customized aluminummold.The PDMS mixture was poured into the mold and cured at130 �C for 30min. Once cured, a zig-zag pattern of five parallellines, resembling a conventional strain gauge, was printed on thesurface of the PDMS substrate using a modified 3D printer. ATronxy X5S commercial 3D printer was modified by replacing thethermoplastic extruder with a geared syringe pump. A computeraideddesign(CAD)modelof the linewascreatedusingSolidworks.A combination of Repetier and Slic3r open source software wasused to convert the Solidworks model stereolithography (STL) fileinto G-code, a file that the 3D printer could use as instructions todeposit the PDMS. A 3ml plastic syringe capped with a 25-gaugeneedle was used to print the PDMS lines as thin as possible tominimize the wetting and spreading of the printed lines over thePDMS substrate. The printing speed used was 2mms�1.Figure 2a shows the PDMS pattern being printed using the in-house modified 3D printer. Black dye was mixed with the pristinePDMS to improve the visibility in this figure. To improve electricalconductivityofPDMS,MWCNTsweredepositedontheprintedwetPDMS lines until they are completely covered. The wet PDMS andMWCNTs were placed in an oven at 130 �C for 30min to allow forcuring of the PDMS line. As the PDMS lines were curing, the
Figure 2. a) Strain sensor during the 3D printing process; b) manufactured strain sensor; c) experimental setup for cyclic tests using the 3D printedstrain sensor.
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MWCNTs were sealed within the PDMS, creating a conductingpathway throughout the cured PDMS/MWCNT line. Any extraMWCNT powders that were not sealed or bonded in the curedPDMS line were removed from the surface by compressed air.Finally, an additional layer of pristine PDMS was poured on theconductive line and substrate to seal and protect the PDMS/MWCNT lines. The top layer of PDMS was cured at 130 �C for30min to form the resulting polymer nanocomposite. The 3Dprinted strain sensor is shown in Figure 2b. The printed strainsensor linewas 30mm long, 2mmwide, with 2.5mmbetween thetwo parallel strain sensor lines.
2.3. Cyclic Tensile Tests for Strain Sensing
Cyclic tensile tests were carried out on the prepared rectangularPDMS/MWCNT sensors to determine their strain sensingcapability. The samples were tested using a single-columnInstron mechanical testing system. The experimental setup isshown in Figure 2c. Each sample was tested under cyclic tensileload at max strains of 5%, 10%, 15%, 20%, 25%, and 30% at aconstant strain rate of 3% min�1. In addition, similar cyclictensile tests were conducted using six different strain rates of
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0.3% min�1, 3% min�1, 15% min�1, 30% min�1, 150% min�1,and 300%min�1 to the constant max strain of 10%. All the cyclicexperiments were conducted under displacement control.Piezoresistance data of the sensor was continuously recordedusing the two-probe method for all tests. In each test, the load-unload process was repeated six times to validate therepeatability of the sensing function. In addition, a durabilitytest was conducted to validate the long-term performance of the3D printed sensors. Fatigue tests with the load rate of3mmmin�1 and the maximum strain of 5% strain were carriedout up to 300 cycles, while recording the piezoresistance sensingdata throughout the entire experiments.
2.4. In Situ Tensile Test under SEM
The piezoresistive sensing mechanism was investigated usingan in situ mechanical testing and imaging system. Themanufactured PDMS/MWCNTstrain sensor was sputter coated,to improve the electrical conductivity and image quality on thesample surface, clamped on the micro-mechanical testing stage,and placed in an SEM (Figure 3). SEM images of the samplesurface were taken at multiple locations, at tensile strains
between 0% and 28%, to visualize the re-alignment of MWCNTsunder different load conditions. The sensing mechanism of thereported approach was explained using the SEM images.
2.5. Human Joint Motion Sensing
The 3D printed strain sensors were applied to wearable sensorapplications. A new sensor was manufactured and attached on aglove to monitor the bending motion of a human wrist joint.Repeatable piezoresistance changes were observed at differentbending speed and hold time.
Figure 3. In situ micro-mechancial testing of manufactured strainsensors in SEM.
3. Result and Discussion
3.1. Microscale Visualization of MWCNT Distribution in 3DPrinted Sensors
SEM images showed the shape and geometry of thePDMS/MWCNT line in the 3D printed strain sensor. As showninFigure 4a, the width and height of the PDMS/MWCNT linewas2mmand302 μm,respectively.Thecross-sectionof thesensorwasbell shaped due to the effects of surfacewetting and gravity duringthe 3D printing process. In addition, the SEM image also revealedthat the MWCNTs penetrated the wet, uncured pristine PDMSduring curing. AlthoughMWCNTs were deposited on the surfaceof wet PDMS before curing, the low surface energy of PDMSallowed MWCNTs to penetrate the uncured PDMS surface andbecome fully submerged within the printed PDMS line. Thedispersion of MWCNTs within the PDMS was also validated by ahigh magnification SEM image shown in Figure 4b.
3.2. Piezoresistance Based Strain Sensing under CyclicTensile Loads
The sensing capability of the 3D printed strain sensors wasstudied under cyclic tensile loads. As shown in Figure 5, sixdifferent max strains in the range of 5–30% and with the loadrate of 3% min�1 were applied. Each plot shows the measuredpiezoresistance change and the correlated strain and stressduring the cyclic tests. Highly repeatable sensing capability wasobserved. During the loading process, the sensor’s responsealmost linearly increased with the applied strain. However, a
Figure 4. SEM image of cross-section of the strain sensor.
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non-linear response was observed during the unloading process,especially when the applied strain approached close to zero. Thishysteresis effect was caused by multiple factors, including abreakdown of the conductive network, reconfiguration of theelectro-conductive percolating channels, and the relaxation andelongation of the PDMS base material. Similar hysteresis hasbeen reported in other piezoresistance based nanocompositesensors that utilize carbon black and graphene nanoparticles toform the conductive network.[35–37] In addition, as the max strainincreased, the change of piezoresistance also increased linearly.This sensing feature indicates that the developed strain sensorhas potential applications in monitoring large deformation andtensile strain in flexible engineering structures.
The slope of the curve shown in Figure 6a is defined as thesensor’s piezoresistance strain coefficient, also known as thegauge factor, and is usually referred to represent the sensitivity ofthe sensor under applied strain. The equation for calculating asensor’s gauge factor is:
GF ¼R�R0R0
� �,L�L0L0
� � ¼4RR0
� �,2
ð1Þ
where GF stands for the gauge factor of strain sensor, R is theelectrical resistance measured at the maximum strain, R0 is the
Figure 5. Relative piezoresistance based sensing using the 3D printed sensor, a) six cycles with 5% max strain; b) six cycles with 10% max strain; c) sixcycles with 15% max strain; d) six cycles with 20% max strain; e) six cycles with 25% max strain; f) six cycles with 30% max strain.
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initial electrical resistance measured at zero strain, 4RR0
is therelative piezoresistance change, L is the total sensor length at themaximum strain, L0 is the inital length of the sensor at zerostrain, and ϵ is the applied strain. Since the slope of the changedresistance percentage plot (Figure 6a) stayed nearly constant, the
Figure 6. a) Relative piezoresistance change under different max strain; b)
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gauge factor of the 3D printed sensor was almost constantduring these experiments. Using Equation (1), the average gaugefactor of this 3D printed sensor was calculated to be 4.3. Forcomparison, the gauge factor of a conventional metallic straingauge is around 2.0.[38] Thus, the strain sensitivity of the 3D
relative piezoresistance change under different max stress.
Figure 7. Effects of applied strain rate during cyclic tests on sensor sensitivity, a) the relative piezoresistance change under a single load-unload cycle; b)the relative piezoresistance change under the applied strain rates between 0.3% min�1 and 300% min�1.
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printed PDMS/MWCNT sensor reported in this study is abouttwice of that of a metallic strain gauge.
Besides gauge factor, the piezoresistance stress coefficient isanother parameter usually used to represent the stress sensingsensitivity, as defined in Equation (2).
π ¼R�R0R0
� �,σ
¼4RR0
� �,σ
ð2Þ
where π is defined as the piezoresistance stress coefficient, 4RR0
stands for the changed percentage of piezoresistance, and σ isthe applied stress. The slope of Figure 6b shows thepiezoresistance stress coefficient of the 3D printed sensorsreported in this paper. It is noted that a good linear response wasreceived in the stress range of 0–0.37MPa, which is meanlycaused by the elastic behavior of the PDMS. The averagepiezoresistance stress coefficient obtained from these 3D printedsensors was 312% MPa�1. Thus, these 3D printed sensors canalso be potentially used as stress sensors due to their linearsensing response.
The effect of various strain rates on the sensor’s performancewas also investigated in this study. Six different strain rates in therange of 0.3–300% min�1 were applied on the sensors, whilekeeping themax applied strain at 10%for all the tests. As shown inFigure 7, the sensor’s piezoresistive response decreased with
Figure 8. Durability tests up to 300 cycles using the 3D printed sensors, a) rethe sensing amplitude and the relative change of piezoresistance during en
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increasing strain rate. The resistance change percentage wasreduced to about 44%when the applied strain rate increased from0.3% min�1 up to 300% min�1. These results indicated that thedeveloped PDMS/MWCNT based strain sensor is more appropri-ate for quasi-static or lower strain-rate sensing applications.
Long-term performance is critical for successful developmentof strain sensors. The strain sensing capability of the 3D printedsensors was experimentally validated under cyclic loading for upto 300 cycles. As shown in Figure 8a, the relative piezoresistancechanges gradually shifted down and the value of the maximumrelative piezoresistance changes reduced during the entire cyclictests, mainly due to the material relaxation. However, the peak tovalley value of the relative piezoresistance stayed nearly constant.As shown in Figure 8b, the relative changes in the peak to valleyvalue reduced only about 2.6% during the 300 cycle tests, but therelative piezoresistance change reduced by 7.7%. Therefore, therelative change of the peak to valley value should be employed forthe long-term strain sensing.
3.3. Strain Sensing Mechanism Using In Situ MechanicalTesting and SEM Images
The piezoresistive sensing function of nanocomposites is relatedto the formation of MWCNTnetworks. External forces applied to
lative piezoresistance change at different load cycles; b) relative change oftire cyclic test.
the nanocomposites can alter the arrangement of conductivenetworks in the polymer matrix. Due to the tunneling effect oftwo adjacent MWCNTs, the electrical resistance responds to thematerial’s deformation. Although certain sensing mechanismshave been discussed and modeled in literature,[39–41] there is alack of experimental validation to show the repositioning of theMWCNTs and the rearrangement of conductive networks duringthe loading process.
The mechanism of piezoresistive strain sensing wasinvestigated using in situ micro-mechanical testing andimaging. The fabricated sensor was tested under tensile loadwhile SEM images were taken to validate the MWCNT networkrearrangement at multiple locations and magnifications.Figure 9 shows realignment of both PDMS polymer andMWCNTs. In Figure 9a, the alignment of PDMS polymer wasobserved. Strips of PDMS with an average width of 2 μm wereformed in the tensile load direction. Figure 9b shows theMWCNT network was also altered during the tensile tests.Distances between three pairs ofMWCNTsweremeasured whileapplied tensile strain increased from 0% to 28%. Figure 9c and dshow the distances and local strains increased continuouslythroughout the loading test. Due to stress concentration, localstrains between the measured MWCNT network features werelower than the applied overall strain. The local MWCNTnetworkrearrangement is shown two areas, labeled A and B, in Figure 9b.Area A focuses on a local MWCNT network which hasundergone complete rearrangement, and area B shows two
Figure 9. SEM images of the 3D printed strain sensors under tensile loads; a)MWCNT networks; c) measured distances from three locations at different
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MWCNTs disconnected after the loading. Thus, increases inelectrical resistance during loading can be attributed to networkelongation (area A) and complete network breakdown (area B).The cumulative change of all MWCNT networks in the PDMSpolymer resulted in the piezoresistive sensing function of themanufactured strain sensors.
3.4. Strain Sensor as Wearable Sensors for Human WristMotion Recognition
The 3D printed strain sensor technology was further applied towearable sensors for in situ human motion detection. To provethe concept, a simplified two-line sensor was attached on thewrist area of a glove and used to record the bendingmotion of thejoint by the relative piezoresistance change (Figure 10e). Threetypes of wrist bending at fast, median, and slow bending speedswere monitored by the strain sensors. The total bending andrecovery of the wrist took between 3.5 and 7 s, and a relativepiezoresistance change of 6–11% was recorded, as shown inFigure 10a–c. In addition, when a 15 second holding time wasintroduced after the bending, the strain sensor was able toidentify the holding time and responded in their sensing data byshowing a plateau area in each bending cycle, as shown inFigure 10d. These experiments demonstrate the potentialcapability of the 3D printed sensor in wearable deviceapplications. More complicated circuits and control system
polymer alignment under tensile load; b) deformation and realignment oftensile strain; d) calculated local tensile strain at different tensile strain.
Figure 10. Piezoresistance based sensing of the wrist motion: a) fast bending of the wrist, b) median speed bending of the wrist, c) slow bending of thewrist, d) mixed bending and holding of the wrist, e) photos of the 3D printed wearable strain sensor.
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can be integrated to improve the feature extraction and datatransmission in future research.
4. Conclusions
In this paper, a PDMS/MWCNT based strain sensor for in situstrain sensing applications was manufactured and character-ized. The developed sensor was first 3D printed using an in-house modified extrusion-based 3D printing system. Thedistribution of MWCNTs in the host PDMS matrix wascharacterized using SEM. Detailed strain sensing capabilitywas experimentally characterized under cyclic tensile loading atsix different max strains. Good linear piezoresistant responseswere characterized until 30% max strain were applied. Theaverage gauge factor of this 3D printed sensor was 4.3, indicatingthat the strain sensitivity of the 3D printed nanocompositesensors is about twice of that of a metallic strain gauge.Additional sensing characterization was tested by holding themax strain constant at six different strain rates. In addition, thesensing repeatability was tested under cyclic tensile loads for 300cycles. Experimental results showed that the strain sensordisplays high sensitivity with a large gauge factor and linearresponse under low strain rate. The strain sensing mechanism
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was investigated using an SEM equipped with an in situmicro-mechanical testing stage. MWCNT realignment andrepositioning was observed at different tensile strains, indicatingthe change of piezoresistivity was due to tunneling effectswithin theMWCNTnetworks. The strain sensors were applied tomonitor the bending of a human wrist at different speeds. Theresults show that the 3D printed sensor is appropriate for real-time load sensing in highly flexible structures, and has potentialapplications in wearable sensors for human motion recognition.
Conflict of InterestThe authors declare no conflict of interest.
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