Highly Flexible Wrinkled Carbon Nanotube Thin Film Strain Sensor to Monitor Human Movement

UC Irvine UC Irvine Previously Published Works

Title Highly Flexible Wrinkled Carbon Nanotube Thin Film Strain Sensor to Monitor Human Movement

Permalink https://escholarship.org/uc/item/86f9d3sx

Journal Advanced Materials Technologies, 1(2)

Authors Park, S-J Kim, J Chu, M et al.

Publication Date 2021-06-27

Supplemental Material https://escholarship.org/uc/item/86f9d3sx#supplemental

License https://creativecommons.org/licenses/by/4.0/ 4.0

Peer reviewed

eScholarship.org Powered by the California Digital Library University of California Communication

1600053 and Ryu How- [25] [22] [42] However, However, nanowires (1 of 8) [44] [35] For example, For [16] CNT thin films [27–31] www.advmattechnol.de [36] Conversely, Yan et al. Yan Conversely, [33] Li et al. investigated gra- [32] nanoparticles, [13,14] Furthermore, PDMS becomes Furthermore, [15] wileyonlinelibrary.com and graphene. [45,46] Amjadi et al. reported highly flexible, [20–26] [43,44] Zaretski et al. reported a highly sensitive Zaretski [34] nanotubes, Therefore, percolating networks of CNTs have the Therefore, percolating networks of CNTs [17–19] [37–41] Another popular functional material that is used for strain Another popular functional material that Typical wearable strain sensors consist of patterned thin wearable strain Typical Another aspect to consider for improving wearable strain (NWs), stretchable sensitive strain sensors based on silver nanowires silver on based sensors sensitive strain stretchable a GF of 14 at 70% strain. with PDMS that had et al. showed that aligned CNT fibers grown on flexible substrates 47. to up of GF having 900% to out stretch to able were phene woven fabric for strain sensing (GF ≈ 1000), but was still for strain sensing phene woven fabric limited to a maximum strain of 6%. released the strain to create wavy buckled structures. These to create wavy buckled structures. These released the strain - provided strain relief for brittle mate wavy buckled structures stretchable. rials rendering them have also shown the ability to bend repeatedly without frac- have also shown the ability to bend repeatedly turing. Lipomi et al. investigated spring like structures in the nanotube that reach 150% strain with good conductivity. sensing is carbon nanotubes (CNTs). Percolating networks of networks Percolating (CNTs). nanotubes carbon is sensing substrates have been reported to on flexible elastomeric CNTs strain due to the have electromechanical stability under high robust contact between individual CNTs. films on flexible elastomeric substrates. Recent developments substrates. Recent developments films on flexible elastomeric stretchable wearable strain sensors used in creating highly films, nanoscale metal thin potential to be used as highly stretchable strain sensing devices. potential to be used as highly stretchable CNT thin film on PDMS et al. aligned instance, Yamada For (GF < 0.82), and showed 280% strain but low sensitivity reported highly stretchable graphene-nanocellulose nanopaper reported highly stretchable graphene-nanocellulose the strain sensor only that can stretch out to 100% strain, but had a GF of 7.1. ever, this process required highly ordered alignment of CNT of alignment ordered highly required process this ever, fibers using intricate dry spinning methods. sensor is the careful selection of the stretchable elastomeric strain sensors have been fabricated using substrate. Many polydimethylsiloxane (PDMS) as the stretchable and flexible ease of fabrication, nontoxicity, substrate due to its flexibility, and biocompatibility. many PDMS substrate based strain sensors showed low stretchability with high hysteresis due to poor adhesion between the as well as increased functional material and substrate polymer, friction during the strain. stiffer, delaminates, and slips after absorbing human sweat delaminates, and slips after stiffer, strain sensor using metallic nanoislands on graphene with GF strain sensor using metallic nanoislands the dynamic range of this sensor 1335 at 1% strain. However, Additionally, strain. 10% than less of strain with limited, was 19 cycles at 1% strain. the GF dropped to 743 after Kang et al. utilized brittle platinum thin films to achieve high Kang et al. utilized brittle platinum thin able sensitivity (GF > 2000), but the strain sensor was only to withstand strains of up to 2%.

- [10–12] Thus, [1,2] [1] [3–9] 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag © , is related to the mechanical strain mechanical the to related is , 0 ΔR/R (1) 0 / ε

RR ∆

= 2016, 1600053 Technol. Mater. Wearable strain sensor devices are designed to be mounted Wearable In order to be an effective strain sensor for monitoring In order to be an effective Micro- and nanostructures have been implemented by by the gauge factor (GF) according to Equation (1) by the gauge factor (GF) according to Equation GF Adv. www.MaterialsViews.com on the human skin to measure the strain induced by muscle on the human skin to measure the strain the mechanical deforma- sensors transduce movements. Many and releasing. This tion to a resistance change upon stretching The relative effect. phenomenon is known as the piezoresistive resistance, in change DOI: 10.1002/admt.201600053 S.-J. Park, J. Kim, Prof. M. Khine Department of Chemical Engineering and Materials Science Irvine University of California Irvine, CA 92697, USA E-mail: [email protected] M. Chu, Prof. M. Khine Department of Biomedical Engineering Irvine University of California Irvine, CA 92697, USA ε

Sensor to Monitor Human Movement Human to Monitor Sensor Michelle Khine* Joshua Kim, Michael Chu, and Sun-Jun Park, Highly Flexible Wrinkled Carbon Nanotube Thin Film Strain Film Strain Thin Nanotube Carbon Wrinkled Flexible Highly These micro/nanostructures are typically generated via buckling example, researchers have from compressive substrates. For deposited brittle materials onto pre-strained silicon rubbers and flexible electronics will fulfill the growing interest in long term flexible electronics will fulfill the growing physical rehabili- monitoring systems for personalized fitness, entertainment devices, tation, finger sensing microswitches, parameters such and continuous health monitoring for various as blood pressure, breathing and body temperature. human motion, the sensors must have high flexibility and flexibility high have must sensors the motion, human a large across sensitivity while maintaining stretchability, example, high gauge factor strain sensors dynamic range. For angle deformations are able to measure both small and large there has been a large inherent to human joint motion. Thus, and sen- stretchable amount of research in developing highly sitive wearable strain sensors by using micro/nanostructured thin films, functional materials, and elastomeric substrates. researchers to improve the elasticity of brittle materials. There is a growing need to improve the mechanical reliability to improve the mechanical reliability There is a growing need applications as the demand for ‘wearable’ of flexible electronics monitoring increases. These new wear in health and medical curvilinear to soft conform components must able electrical motion, and be light- flex with the body’s surfaces, stretch and brittle, rigid, to overcome It is critical weight and unobtrusive. electronics and sensors to develop more and largely planar monitoring. insightful investigations for human body Communication 1600053 www.advmattechnol.de in stiffness between the SMP and the deposited material during grated withthinfilmssuchasCNT, Ag, Au, and Si. more stressintothestrainsensor, andevenmoresowheninte- PDMS (0.4–3.5MPa) andthehumanskin(25–220kPa) induces and moisture.Finally, theYoung’s modulimismatchbetween ≈125 kPa, similartothatofhumanepidermis. lyzed elasticsiliconematerial.Ecoflexhasa Young’s modulusof human skinmoreaccuratelybyusingEcoflex,aplatinum-cata- polystyrene(PS). biaxial shrinkingonshape memory polymers(SMPs),suchas structures in metalthinfilmscan becreatedvia heatinduced viously demonstrated that self-similar hierarchal wrinkled microsized wrinkled CNT thin film in Ecoflex. Wehave pre- to fabricatewearablestrainsensorswithself-similar nano-to for wearablestrainsensors. FDA approved.Therefore,Ecoflexisamoresuitablecandidate human body. Lastly, Ecoflexisbiocompatible,nontoxic,and is highlywaterresistantallowingforlongtermwearonthe back to its initial size without tearing and distortion. Third, it stretch andwithstandstrainsgreaterthan900%rebounds and CNTthinfilmonPSsubstrate (right). CNT thinfilmonEcoflexsubstrate. h)PhotographoftheCNTthinfilmateachstepfabricationprocess: Asdeposit,shrunken,transferred(left), f) Organicsolventbathwasusedtocomplete transferprocess.g)Finalself-similarwrinkled shrinking. e)Ecoflexwascuredon shrunken PSsubstrate. to depositCNTthinfilm.c)Substrate wasrinsedwithdeionizedwaterandappliedpressurefrom a roller. d)Heatwasappliedtoinducebiaxial Figure 1. There have been recent efforts to mimic the properties of the In thispaper, weintroduceascalableandsimplemethod (2 of8) Fabrication processofself-similarwrinkles CNT-Ecoflex strain sensor. a)ShadowmaskisattachedontoPSsubstrate. b)Spraygunisused [52] wileyonlinelibrary.com Thebucklingoccursduetothemismatches © 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim [51] Second,itcan [43,47–50] the shrinking process. substrate. We useda0.5mmdiameteratomizingnozzleand used to deposit CNTs through a shadow mask and onto a PS (wCE) strainsensorscanbeseeninFigure which wereintegratedinastretchablehumanmotionsensor. onstration ofwrinkledCNTthinfilmsfabricatedusingSMPs, relief duringstretching.To ourknowledge,thisisthefirstdem- self-similar wrinkled structures provide large amounts of strain CNTs toenhancestretchability. Furthermore, thesehierarchal entangled CNTs introduce strong van der Waals forces between SMP carrierfilmintoEcoflex0030.(Smooth-On, PA).Thetight these wrinkledCNTthinfilmscanbetransferredfromtherigid nano- tomicrofeaturedwrinklesuponshrinkage.We showthat of deposited CNTs on the SMP result in densified, self-similar based strainsensorsmethods.Here wedemonstratethatmats tion processscalable,simple,andquickcomparedtootherCNT Spray gundepositionandtheuseofSMPmakesourfabrica- cess usedtouniformlydepositthinfilms,suchasCNTs. gun depositionhasalreadybeenadoptedasascalablepro- method isknowntobesimple,cost-effective andfast.Spray deposit CNTthinfilmsonSMP.Thespraygundeposition The fabrication process for self-similar wrinkled CNT-Ecoflex [53] Spray gun deposition was used to Adv. Mater. Technol. 2016,1600053 www.MaterialsViews.com 1. Aspraygunwas [54,55]

Communication 1600053 (3 of 8) www.advmattechnol.de The CNT thin films wrinkle The CNT [56] wileyonlinelibrary.com shows the stretching apparatus for straining and 3 shows the stretching Figure because each individual CNT is entangled and weakly adhered entangled and weakly individual CNT is because each in one thin film forces resulting via van der Waal to each other Scanning of individual CNTs. as opposed to an assortment (SEM) images in the right-hand column electron microscopy structures are comprised 2 show that these wrinkled of Figure 2c, the wrinkled struc- As seen in Figure of individual CNTs. silicone into the soft transferring after tures were maintained slightly wrinkles the of shape the Ecoflex. However, elastomer, and shrinking of the Ecoflex during changed due to swelling transfer process. The overall shape of the the organic solvent self- structures was maintained. These wrinkled self-similar - elas silicone by a soft supported similar wrinkled structures thin film to become highly elastic. tomer allow the CNT wrinkled CNT thin the strain-resistance behavior of self-similar Figure 3, wCE strain sensors were able strain films. As seen in After heat induced shrinkage, the CNT thin film buckled and the CNT thin film induced shrinkage, heat After to those previ- structures similar wrinkled created self-similar in metal thin films. ously shown 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag © shows scanning electron microscope images of the 2 shows scanning electron Scanning electron microscope images of the wrinkled CNTs. a) As deposited CNT thin film on PS substrate. b) Wrinkled CNT thin film on thin film CNT Wrinkled b) substrate. on PS film CNT thin deposited a) As CNTs. wrinkled images of the microscope electron Scanning 2016, 1600053 Technol. Mater. Figure Adv. www.MaterialsViews.com Figure 2. Figure bar is 50 um and the inset’s is 100 nm. wrinkled CNT thin film on Ecoflex substrate. Left-hand column scale shrunken PS substrate. c) Transferred pressures below 0.5 bar for the deposition of the CNT thin film. deposition of the CNT below 0.5 bar for the pressures by the film was easily controlled of the CNT thin The thickness spray the After substrate. the on depositions spray of number with deion- thin film was rinsed the patterned CNT deposition, shadow removing the patterned and dried. After ized water used to press against the CNT thin film mask, a paint roller was pre- This step was performed to denser. to make the thin film heat during PS the into embedding from network CNT the vent - sample was then placed into a convec induced shrinking. The shrinking and wrin- °C to induce biaxial tion oven set at 150 the sensor is about shrinking, film. After kling of the CNT thin Ecoflex was shrinking, size. After 33%–40% of the original - and cured overnight at room tempera spun on top the sample was then placed into an acetone bath ture. The cured sample bath to transfer the wrinkled CNT thin followed by a toluene film onto Ecoflex. As stages of the fabrication process. CNT thin film at different 2a. Figure in shown is PS on film thin CNT planar deposited Communication 1600053 www.advmattechnol.de that ofmetalthinfilms. to formameshlikepatternof conductivepathways,similarto interface. Throughouttheentire film,thesefracturesexpand fractures willpropagatedownward towardtheCNT-Ecoflex likely wherethefracturesnucleate.Within eachvalley, the As themodelsuggests,theselocalizedstresspointsaremost sensitivity inthesecondregion. ways resultsinhighresistance changes andsubsequent high CNT-Ecoflex interface(Figure S4, Supporting Information). top of theCNT thin film, CNT-Air interface, and lowest at the more, withineachvalley, thenormalstressishighestat which willincreasestheresistanceoftubes. may alsobedeformationoftheindividualCNTs themselves, CNTs promotesanincreaseinthetunnelingresistance.There separation distance.Thisdistancechangebetweenneighboring and aseparationoftheindividualCNTs byavanderWaals also adecreaseintheeffective cross-sectionareabetweenCNTs of thewrinkle’s geometrywhenstretched.Additionally, thereis wrinkled CNTstructuresandfromtheincreaseinlength dominantly, itisduetotheincreasingseparationofneighboring in electricalresistanceisacombinationofdifferent factors.Pre- a factorof2.5andtheGFwas0.65.For thisregion,thechange 0%–400% strain, the relative change in resistance increased by strain: 0%–400%and400%–700%.Inthefirstregionfrom in Figure S2(Supporting Information). The reproducibilityforthestrainresistancebehaviorisshown kled structuresprovideda>60foldincreaseinstretchability. ≈12% (Supporting InformationFigure S1).Therefore,thewrin- CNT thinfilms(thecontrol)wereonlyabletostretchout out to≈750%andremainconductive.Onthecontrary, planar Ecoflex substrate(theinsetplotisanexpandedviewof0–400%strain) Figure 3. within thevalleysofwrinklesduringstrain. of a2Dcrosssectionshowthatthenormalstressislocalized change inresistanceverylarge.Finite ElementAnalysis(FEA) This reducesthenumberofconductivepathwaysorelative gaps withinthethinfilm(Figure S3, Supporting Information). greater than400%as evidenced fromthediscontinuities and In thisregion,theCNTthinfilmstartedtofractureatstrains in resistanceincreasedbyafactorof162and had aGFof48. second regionfromstrains400%–700%,therelativechange The strain-resistance plot shows two distinct regions of (4 of8) a) Imageofastrainsensorattheinitiallengthandstretchedto700%strain.b)RelativechangeinresistancevsonwrinkledCNTs on wileyonlinelibrary.com [63] Thereductionofconductive path- © 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim [61,62] [10,57–60] Further The [48] -

to form. that occurs,minimizingthechanceforalargediscontinuity help increasestrainbyuniformlydistributingthefracture tudinal direction of the sensor. The uniform strain should sensor becauseitmaintainsuniformstrainalongthelongi- tion. Therefore,therectangulargeometrywaschosenfor fracturing ofthesensorinareashigherstrainconcentra- strain inthecenter. Thisnonuniformitymayresultinearlier bell andhourglassshapedgeometriesshowedmorelocalized showed themostuniformstraindistribution,whiledumb- The FEAoftherectangularshapedelastomericsubstrates flex duringstretching.(Figure S5, Supporting Information) the shapeaffects thestraindistributionthroughoutEco- geometries (rectangular, dumbbell,andhourglass)showsthat performance ofthewCEstrainsensors.FEAthreedifferent showing a1%changeofresistance. Releasingat300%strain, After releasingfrom100%strain, therewasaslighthysteresis sensors at100%and300%strain weremeasured(Figure 5b). the hysteresisresponsesfor wrinkledCNT-Ecoflex strain and S7(Supporting Information).Inadditiontocyclicstrain, 300% and500%strainfor1000 cyclesareshowninFigures S6 wCE sensorwererobustafter cyclicstrain.Cyclic strainsof ance similartothatofthecyclicstrain.Thisshows Figure 3b.At 300%straintherewas 145% changeinresist- ance wasapproximately175%whichcorrespondswellwith strain canbeseeninFigure trace resultinginsensorfailure. At 750%strain,thefracturespropagatedcompletelyacross tional strain relief came from the percolating network of CNTs. gation. Althoughthewrinkleswerecompletelyaligned,addi - more strain relief resulting in fracture nucleation and propa- this point,the wrinkled structures were notabletoprovide any pletely aligneduniaxiallyatstrainsgreaterthan400%.After follows apositivePoisson ratio.Thewrinklestructurescom- This isduetothecompressivestrainsfromEcoflex,which the CNTwrinklesstartedtoalignindirectionofload. 750%, affect theCNTwrinkledstructures.At 100%ofstrain, Figure The geometryoftheelastomericsubstratesalsoaffects the The response of the wCE strain sensor during a 300% cyclic 4. illustrateshowincrementaltensilestrains,upto 5. Thenormalizedchangeinresist- Adv. Mater. Technol. 2016,1600053 www.MaterialsViews.com Communication 1600053 (5 of 8) www.advmattechnol.de wileyonlinelibrary.com The wCE sensor can measure up [54,55,64] 750% strain with high sensitivity. Finally, we demonstrate Finally, ≈750% strain with high sensitivity. In conclusion, we have introduced a new route to develop In conclusion, we have introduced a new to of the index finger at successive bending of 30°, 60°, and 90° of the index finger at successive bending with 6c). The relatively change in resistance increased (Figure measured relative increased bending and stretching. The changes in resistance were ≈17%, 24%, and 33%, respectively. the potential use The abovementioned experiments highlight sensors as an epi- of CNT–Ecoflex nanocomposite based strain dermal device for human skin motion detection. and sensitive strain ultra-stretchable, human skin mountable, procedure by sensor with low cost and simple fabrication have also We forming stretchable wrinkled CNT networks. can be transferred demonstrated that wrinkled CNT networks stretchable elastomeric substrates such as Ecoflex, onto soft microstructures and nano- self-similar the maintaining while sizes and wavelengths of wrinkles Different and conductivity. the thicknesses of different can be produced by depositing wrinkle increases CNT solution. The feature size of the CNT Supporting Information). S8, with the film thickness (Figure shrinking, the CNT wrinkle waves by uniaxial Furthermore, - S9, Sup (Figure can alternatively align into one direction is compatible porting Information). Our fabrication method methods as with conventional and scalable manufacturing previously reported.

2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag © c inset)). The wCE sensor was attached on 6c inset)). The wCE sensor was attached on a) Relative change in resistance for cyclic strains between 0 and 300%. b) Hysteresis at 100% (blue) and 300% (orange) strains. a) Relative change in resistance for cyclic strains between 0 and 300%. b) Hysteresis at 100% Scanning electron microscope images of the wrinkled CNT thin film on Ecoflex at different strains. images of the wrinkled CNT thin film Scanning electron microscope 2016, 1600053 Technol. Mater. To demonstrate the potential and applicability of the wCE the of applicability and potential the demonstrate To Adv. www.MaterialsViews.com Figure 5. To measure the movement of the elbow, the initial elbow posi- the initial the movement of the elbow, measure To were taken while the tion was held flat and measurements back to the initial elbow was bent toward the body and returned wCE sensor stretched position. During the bending motion, the resist- in change relative the causing joint the with bent and position, the original to back returned increase. When to ance The baseline. back to returned resistance of relative change the measure 180%. To maximum strain for the elbow bending was was attached on the bending motion of the knee, the sensor taken for a squatting the knee cap and measurements were and then bent during motion. The knee was initially straight bended a squat position, the sensor the squatting motion. With resistance increased and stretched, and its relative change of to the original posi- up to 230% strain. When returning back tion, relative resistance again decreased with the releasing of measurements were made on the middle joint Lastly, sensor. there was a 7% change of resistance due to the viscoelastic there was a 7% change of resistance due properties of the Ecoflex substrate. was mounted on sensor as a wearable device, the strain sensor knee, and finger to monitor their motion. The wCE the elbow, Tape fitness tape, KT sensor was assembled on an elastic sports (Figure (KT Health 6). (Figure finger. and knee, the elbow, for joints respective the Figure 4. Communication 1600053 www.advmattechnol.de method isusedfordepositCNT solution (Aldrich)tothemaskedPS polymer film, as a shadow mask, on top of a PS substrate. Spray gun (VLS2.30, UniversalLaserSystems, AZ)andplacedpatternedadhesive Film, GrafixArts).Thepatternwas subsequentlycutwithlasercutter (Autodesk, Inc., CA) and patterned on adhesivepolymer film (Frisket by deionizedwater. Thesensor pattern wasdesigned with CADsoftware prestressed PS (KFS50-C, Grafix Arts)was rinsedwith ethanolfollowed Experimental Section are requiredforphysiologicalmonitoring. wide applicationrangewheremeasurementsoflargestrains study suggeststhatthesewCEsensorshaveapplicabilityin a and knee.Themaximumstrainmeasuredwas≈300%.This mounting thesensorsontojointareasoffinger, elbow the applicationofsensorsfordetectinghumanmotion by at jointareaofa)elbow, b)knee,andc)finger. Figure 6. Fabrication ofSelf-SimilarWrinkled CNT-EcoflexStrainSensor:Initially, (6 of8) Wrinkled CNT-Ecoflexstrainsensorwasusedtomonitorhumanmovement.Relativechangeinresistancewhilebendingthe wileyonlinelibrary.com © 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim Corporation) at150rpmfor40 s.Afterspincoating,shrunksamplewas Smooth-On, PA)waspouredand spincoated(LaurellTechnologies CNT thinfilm,aplatinum-catalyzed siliconeelastomer(Ecoflex0030, was shrunkto33%ofinitialsize.After formingaself-similarwrinkleon biaxial shrinkingandwrinklinginto theCNTthinfilm.Thesensordesign placed intoaconvection oven forheatapproximatelyto150 °Ctoinduce the deposited CNT thin film waspressed by the roller. The sample was and lightheating.After removing ofthepatternedshadow mask film, remove surfactant and organic residue and dried completely with N patterned CNT thin film was rinsed with deionized water for 10 min to size andwavelengthofwrinkleswhichwillaffectitsstretchability. The gun andtheworkingsubstrate.ThethicknessofCNTwillaffectto of thespraygunnedCNTsolutionanddistancebetween the CNTthinfilmis115nmandcouldbecontrolledbyamounts heating on the hotplate (≈65 film deposition.ThedispersedfluidofCNTsolution,wasevaporated by spray gunandmaskedPSsubstrateis25cmforuniformityofCNTthin is used,thepressurewaskeptbelow0.5baranddistancebetween substrate. For thespraygun, theatomizenozzleand0.5mmtipsize °C) while spray gunned. The thickness of Adv. Mater. Technol. 2016,1600053 www.MaterialsViews.com 2 gun Communication

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C) 10 min to remove bath (65 °C) 10 min to followed by a toluene °C) for 30 min : SEM (FEI Magellan of Strain: SEM (FEI and Sensing Structure Characterization The wrinkled and Knee): Elbow, Motion (Finger, the Human Detecting Finite Element Analysis: FEA was performed to examine the strain [1] [7] [6] [2] [3] [4] [5] Adv. www.MaterialsViews.com Acknowledgements Award New Innovator Director’s of Health (NIH) Institute The National Program (1DP2OD007283). Supporting Information is available from the Wiley Online Library or Supporting Information is available from the from the author. Supporting Information any residual PS then dry in air for 12 h. PS then dry in air for 12 any residual The samples wrinkles, and CNTs. for imaging the 400 XHR) was performed of Ir prior to SEM imaging. Strain sensitivity were sputter coated with was measured by a custom-made semi-staticwrinkled CNT-Ecoflex sensors apparatus. The apparatus was a leadscrew basedtensile strain stretching avoid mechanical a stepper motor (Figure 3a). To linear actuator driven by deformations while maintaining high electricalfailure and to withstand large such as eutectic gallium-indium (EGaIn)conductivities, liquid metals and 0% strain at fixed is Sensor materials. solid than rather used were cycling failure. For until increments strain 30% by measured is resistance saw- 300% using a from 0% to the sensors where strained measurement, controlled linear strain testing apparatus.tooth wave form on a computer connected to amultimeter at 2 Hz with a The resistance was sampled times at 2 Hz. The samples were cycled over 1000 computer. to tape fitness sports elastic an on assembled was sensor CNT-Ecoflex were Measurements inset). 6c (Figure together components the all hold previously mentioned. taken using the same data acquisition system as The and CNT film respectively. in the Ecoflex and stress distribution was and analysis models were created using Ansys DesignModeler, 16.0. The Ecoflex was modeled as a performed with Ansys Workbench two parameter Mooney-Rivlin hyperleastic material, placed under vacuum for 10 min to remove bubbles and cured overnight remove bubbles and cured vacuum for 10 min to placed under acetone bath was placed into an Cured sample at room temperature. (75 within the carbon nanotube film, a simplified 2D model was created within the carbon nanotube film, a simplified of the transferred carbon based off of SEM cross-section images of the assess The model was strained to 300% strain. To nanotube sensor. Ecoflex different three Ecoflex, the within distribution strain normal the dumbbell, and hourglass) were modeled; each geometries (rectangular, geometry was taken to 425% strain. nanotube film was modeled as an isotropic elastic material. The stiffness nanotube film was modeled as an isotropic elastic the wrinkle wavelength of the carbon nanotube film was calculated using et al. equation used by Fu Communication 1600053 www.advmattechnol.de [53] [52] [50] [49] [48] [47] [46] [45] [44] [43] [42] [51] [41] S. Lin, E.K.Lee,N.Nguyen,M.Khine,LabChip 4205. J. D. Pegan,A.Y. Ho, M.Bachman,Khine,LabChip USA Robotics andSystems,(Ed:OussamaKhatib),IEEE,Piscatawy, NJ, J. K. Paik,R.Kramer, R.J. Wood, IEEEInt.Conf. Intelligent F. Xu, Y. Zhu, Adv. Mater. Nano S. Ryu, P.Lee,J.B.Chou,R.Xu,Zhao,A.Hart,Kim,ACS J. Micromech.Microeng. X. Liu, Y. Zhu, M.W. Nomani, X.Wen, T.-Y. Hsia, G.Koley, Science Y. Zhang, F.G.Omenetto,Y. Huang, T. Coleman, J.A. Rogers, M. Ying, L.Xu,Li,H.-J.Chung,H.Keum,McCormick, P. 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