Mechanical behavior of strain sensors based on PEDOT:PSS and silver nanoparticles inks deposited on polymer substrate by inkjet printing

(1)

ContentslistsavailableatScienceDirect

Sensors

and

Actuators

A:

Physical

jo u r n al hom e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / s n a

Mechanical

behavior

of

strain

sensors

based

on

PEDOT:PSS

and

silver

nanoparticles

inks

deposited

on

polymer

substrate

by

inkjet

printing

Michela

Borghetti

a

_,

_Mauro

_Serpelloni

a,∗

_,

_Emilio

_Sardini

a

_,

_Stefano

_Pandini

b

a_Dip._di_Ingegneria_{dell’Informazione,}_University_of_Brescia,_Brescia,_Italy b_Dip._di_Ingegneria_Meccanica_e_Industriale,_University_of_Brescia,_Brescia,_Italy

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received22February2016

Receivedinrevisedform16March2016 Accepted21March2016

Availableonline22March2016 Keywords: Strainsensor Inkjetprinting Silvernanoparticles PEDOT:PSS Tensiletests Mechanicalbehavior

a

b

s

t

r

a

c

t

Recently,inkjetprintingtechnologyhasreceivedgrowingattentionasamethodtoproducelow-cost large-areaelectronics,sensors,andantennasonpolymersubstrates.Thistechnologyreliesonprinting techniquestodepositelectricallyfunctionalmaterialsontopolymersubstratestofabricateelectronic componentsorsensingelements.Inthispaper,weappliedaninkjetprintedtechnologyforthe develop-mentandcharacterizationofﬁlmsonapolymersubstrateaimingatgivingdesignconsiderationsforthe optimizationofstrainsensorsorprintedelectronicsobtainedbyinkjetprinting.Twoinksweretested overapolyimidesubstrate,awater-basedconductivepolymer,PEDOT:PSS,andasilvernanoparticles ink.Theirsensingcapabilitieswereinvestigatedundertensileconditionsandvariousstrainhistories (strainramp;cyclicloading-unloadingtests;applicationofconstantstrainoverprolongedtime)aiming athighlightingthecorrelationbetweenelectricalresponse,appliedstrain,timeandmechanicalhistories. Furthermore,themechanicalviscoelasticresponseofthesubstratewasinvestigatedundersimilarstrain historiesinterpretingtheresultsatthelightofthesubstratedeformationalcharacteristicsandevaluating itsinﬂuence.

1. Introduction

Inrecentyears,sensorsprintedonpolymersubstratesrepresent anincreasingareaofresearchanddevelopmentduetothe grow-ingdemandforbiosensors[1],artificialskin[2],chemicalsensors [3],force[4]andstrainsensors[5–9].Inparticular,resistivestrain sensorsonpolymericsubstratesareemployed,ingeneral,forthe measurementofforces[6],movements[7]anddisplacements[8]. Therefore,theyareusedinmanydifferentfields,andnotleast,in thebiomedicalfield[9].Novelsystemshavebeenrecently devel-opedadoptingpolymersinsteadofthemoreconventionalstrain gauges,leadingtosensorsbasedonconductivepolymers[10], poly-mercomposites[11],polymerelastomers[12],thermoplastics[12], epoxies[13].Asa benefit,sensorsbasedonpolymersallowthe useofconventionalandconvenientpolymerprocessingand post-processingtechniques.

Resistive strain sensors are typically manufactured deposit-ingaspeciﬁcresistiveinkonapolymersubstrate.Thepolyimide ﬁlmsare often favored as substrate due to theirhigh durabil-ity,thewiderangeofworkingtemperaturesandthestabilityto

∗ Correspondingauthor.

E-mailaddress:mauro.serpelloni@unibs.it(M.Serpelloni).

environmentalfactors[14]. Inkdepositioncanbeeasilycarried out throughsimple and low cost printing technologiessuchas inkjetprinting.Thistechniqueisoneofthelatestmethodologies developedfortheimplementationofelectroniccomponents,and recentlyitisbecomingincreasinglypopular,asevidencedbyits adoptioninnumerousscientificpublications[15–17].This tech-niqueofferstheadvantageofbeingfastandrelativelyinexpensive, therefore suitable especially for the rapiddevelopment of pro-totypestobetested.It alsogivesthepossibilitytodeposit inks onvarious,rigidorflexible,substrates[15].Themainadvantage ofinkjetprinted straingauges,over thetraditionalones,liesin theirsimple rapidprototyping process,thewide range of elec-tricresistance,itscompatibilitywiththemechanicalpropertiesof thematerialanditsbuilt-flexibilityforthemeasurementoflarge deformations.

Two typical examples of conductive materials adopted as ink for inkjet-printing strain sensors are non-polymeric inks incorporating silver nanoparticles and the intrinsically con-ductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate)(PEDOT:PSS).Asexample,astrainsensorbasedonsilver ink,whichcouldplayastrategicrole forelectrodesandsensing structures due totheextremely low resistivityand mechanical properties, was recently proposed by Andò et al. [18]. On the otherhand,piezoresistivesensorsbasedonPEDOT:PSScanbeused http://dx.doi.org/10.1016/j.sna.2016.03.021

(2)

PEDOT:PSSaninterestingmaterial,whichalsopresentsacertain versatility,suchasthepossibilityofdepositionondifferent sub-strates.SomeexamplesofsensorsmanufacturedwithPEDOT:PSS depositedbyinkjetprinting arereportedinRefs.[21]and [22], providing a characterization of their electrical and mechanical properties.InRef.[22],asensorbasedonPEDOT:PSSisadopted tomeasurethebendingangleofkneeﬂexionandwristrotation. Theresultssuggestthatbendingsensorsfabricatedbyinkjet print-ingadopting PEDOT:PSS or silver nanoparticles ink offer many advantageswithrespecttoothersystemsforhumanmovement monitoring(insideandoutsidethehumanbody),suchas,thesmall footprint,lowcostandversatility.However,toobtainsatisfactory resultsfromtheuseofthesesensors,itisnecessaryto character-izeproperlytheirmechanicalandelectricalresponse,sotoproperly correlate,atthelightofthesubstratedeformation,sensorresponse andmovement.

Inthispaper,wepresenttheresultsoftensiletestingprotocols, properlydesignedtoevaluatetheresponseofPEDOT:PSSandsilver nanoparticlesinksdepositedbyinkjetprintingundermonotonic andcyclicstrainhistories.Theaimistoimprovetheknowledge onthedesignofstrainsensorsonpolymersubstrate orprinted electronicsmanufacturedbyinkjetprinting,withparticular atten-tionforwhatconcernstherelationshipbetweenappliedstrainand measuredelectricalresistance,andstabilityoftheresponseover timeandforprolongedcyclicemploy.

2. Preparationofsensingﬁlms

Twotypesofsensorswerepreparedandinvestigated,theywere obtaineddepositingconductiveinksonpoly(imide)ﬁlm(Kapton® HNDuPont[23])byaproposedinkjetprintingtechnique.Forthe ﬁrstsensortype, theconductivelayeris aninkbased onsilver nanoparticles.In particular,theinkis commercializedbySigma Aldrichunderthetradename736465;ithasavolumeresistivity of11␮W/cm,ananoparticlescontentof30–35%andaparticlesize lessthan50nm[24].Forthesecondtypeofsensor,theconductive layerisobtainedbythedepositionofPEDOT:PSS,OrgaconIJ-1005, marketedbyAGFA-Gevaert[25]having0.8%ofsolidpart.

Theemployedﬁlmpresentsathicknessof25␮m.Thechoice ofpolyimidewasmainlymotivatedonitshighadhesiontothe choseninksandbiocompatibility[26].Furthermore,itpresentsan enhancedthermalstability,whenexposedtorelativelyhigh tem-perature,thankstoitshighglasstransitiontemperatures(about 360◦C).Thisisanimportantfeatureforthesubstrate,sincesensors manufacturingprocessinvolvesthermaltreatmenttopromotethe evaporationofthesolvents.Thethermaltreatmenthelpstowards increasingtheconductivityoftheinkandimprovingadhesionto thesubstrate:themoderatethermalexpansionofthesubstrateon therangeoftemperatureallowsavoidingsigniﬁcantgeometrical distortionsofthesubstrate.

The sensors were made using the proposed inkjet printing method.Alow-costdesktopprinter hasbeenusedfor theﬁlm depositions.Theselectedprinterisoneofthecheapestmodelsof Epson,XP-215,havingfourseparatecartridgeswith128nozzles forblackand42nozzlesforeachcolor,andaprintresolutionupto 5760×1440dpi.

Beforetheinkprinting,thesheetofpolyimide(155×21mm2₎ wascleaned by placing it in an ultrasonic bath of acetone for 15minatroomtemperature.Aftercleaning,thestripwasdried using dry compressed air. An additional oxygen plasma treat-ment(Colibrì,byGambetti)undervacuumwascarriedoutforthe PEDOT:PSS-typesubstratesfor 180sat35WRF powerin order toimprovetheadhesionbetweeninkandpolyimide.Infact,the

Fig.1.(a)Exampleofasensingstripforthetensiletests;observationsattheoptical microscopeoftheconductivetrack(b)forthesilvernanoparticlesinkand(c)for PEDOT:PSS.

plasmamodiﬁeschemicalstateofthepolyimidesurface,which becomeshydrophilic.

Anemptyprintercartridgewasrefilledbythesilver nanopar-ticlesorPEDOT:PSSink.Inkpathswerecreatedbyrepeatingthe printingprocessforthreetimesforsilver-basedsensorandfive forthePEDOT:PSS-basedsensor,thisensuresagoodconductivity. Afterprintingeachofthelayers,thesheetwasdriedinastaticoven for1minat50◦Ctopreventthespreadofinkwiththefollowing printing.Afterdepositingallthelayers,thesheetwithdevicesin silvernanoparticleswasplacedintheovenfor30minat150◦C (6minat130◦CforPEDOT:PSS).Tocreatecontactsasilverpaste (Dupont5028)hasbeendepositedmanuallyanddriedat130◦Cfor tenfurtherminutes,afterwhichcopperwiresweresolderedonto thesilverpaste,allowingmeasuringeasilytheelectricalresistance. Atypicalspecimenpreparedforthetensiletesting, representa-tiveofthesensor,isrepresentedinFig.1a,andisobtainedbycutting rectangularstrips(155×7mm2₎_from_the_printed_sheet._This_figure showsthesensitivetrackconsistingofasinglerectangularstripeof conductiveink,alignedwiththespecimenlengthandplacedinthe mid-span.Forallthesensors,theconductivestripehasanominal lengthof30mmandanominalwidthof1mm.

Wemeasuredtheresistance(R)ofthedepositedstripeswitha 6½digitdigitalmultimeter.Theaverageresistanceis6900ofthe stripesbasedonthePEDOT:PSSinkand30oftheonesbasedon

(3)

Fig.2. Schematicrepresentationoftheexperimentalsetup;thedynamometer(Instron3366)isemployedtoapplyspeciﬁcdeformationhistoriesonthesample,measuring forceanddisplacement,whiletheelectricalresistance(R)issimultaneouslymeasuredbythemultimeter.Thetwocrocodileclampsareconnectedtothemultimeterfor resistancemeasurement.

Fig.3. Schematicrepresentationsofthethreesetsoftests:(a)amonotonicrampatconstantdisplacementrate;(b)loading–unloadingcyclesatvariousmaximumlevelsof strains;(c)maintainedstrainlevelsforprolongedtime.

thenanoparticlesilverink.Wealsomeasuredthethickness(t)of thedepositedinkswithastylusproﬁlometer(BrukerDektakXT). TheaveragethicknessofthePEDOT:PSSandofthenanoparticles silveris400nmand600nm,respectively.

Finally,wecalculatedtheresistivity()ofthesetwomaterials byusing

R=× L

W×t (1)

whereLandWarethelengthandthewidthofthestripe, respec-tively.Weobtainedameanvalueof92␮mforthePEDOT:PSS and 0.60␮mforthenanoparticle silver. Thelowerresistivity ofthePEDOT:PSSwithrespecttothepristinePEDOT:PSSis justi-ﬁedbythepresenceofthediethyleneglycol(DEG)intheliquid ink [27,28]. Indeed, DEGinduces tocreate a highly conducting PEDOT:PSSnetwork.Theresistivityofthenanoparticlesilverinkis usually0.30–0.40␮m,butinsomecasesitcouldbehigherthan 1␮m[29,30].Thesinteringprocesscouldbeoneofthereasons ofahigherresistivity,asconﬁrmedintheliterature[31].

Fig.1bandcrefertotheconductivetrackobtainedbysilver nanoparticlesandbyPEDOT:PSSinks,respectively,asobservedby anopticalmicroscope.

3. Experimentalsetup

Theexperimentalactivityconsistedinsimultaneously monitor-ingstrainandelectricalsignalwhenspeciﬁcdeformationhistories wereapplied.Aschematicblockdiagramoftheexperimentalsetup isdisplayedinFig.2.

The strain history was applied,under tensile configuration, bymeansofanelectromechanicaldynamometer(Instron,model 3366),equippedwitha 500Nloadcelland screw-typegrips.In recentyears,thestandardizationworkforthemechanical charac-terizationofprintedelectronicsorsensorsdepositedonpolymeric substrateshasledtothepublicationofnumerousstandards(such asIEC62047orIEC62899, etc.)alsoveryspecifictothe particu-lar fieldof application.However,inourresearch,thegoalis to provideimportantdesignconsiderationsfortheoptimizationof sensors onpolymeric substratesor printed electronics, or con-nections. Therefore, the samplesthat weremanufactured were analyzedviaprotocolsclassicallyusedforthecharacterizationof polymeric materialsin deformations. Tensiletesting conditions wereadoptedsincetheyallowprovidingthebestdescriptionof thestressandstrainconditionsofthesensingportionofthefilm, whereasbendingwouldhaveledtolocallyinhomogeneousstain levelsthroughoutthespecimenlength.

A6½digitsmultimeter(Agilent34401A)measuredthe electri-calresistanceusingfourwireconnections.Apersonalcomputer (PC)withLabVIEWvirtualInstrument(VI)controlledtheInstron

(4)

Fig.4.(a)Stressvs.straincurvesforthepolyimidespecimens;thetwodottedlines representconstructioncurvesforthedeterminationoftheyieldpoint,thecrosses correspondstothebreakpoint;(b)Constructioncurves.

andacquiredthedataofload,displacementandresistancein real-time.

The correlation between stress, strain and variation of the electricalresistancewere investigatedunder three deformation histories,schematicallyshowninFig.3.Thesetestswerecarriedout onsensors,and,separately,alsoonneatpolyimideﬁlmsamples.i.e. polyimidestripswithoutconductiveink,sotoevaluatemechanical, viscoelasticand cyclicfeatures of sensor’ssubstrate.Neat poly-imidesampleswerepreparedas160mmlongand10mmlarge rectangularstrips.Thegaugelength(120mm)oftheseneat sam-plesissigniﬁcantlylongerthanthelengthofthesensitivetrackin ordertobettercharacterizetheintrinsicpropertiesofthematerial andprovideresultsfreefromeffectsoftriaxialitytogrips.

Intheﬁrsttypeoftest,strainramptest(Fig.3a),themobile crossheadwasmovedatconstantspeed(10mm/min)upto speci-menfailure.

Thesecondtypeoftestconsistsincyclicloading-unloadingtests, carriedoutbysubjectingthesensorto60cyclesforagivenlevelof maximumstrain.Inthepresentresearch,theeffectoftheapplied

Fig.5. Cyclicbehaviorofthesubstratewhenthemaximumdeformationis(a)1.5% and(b)3%.

strainwasinvestigatedinsubsequentcyclicexperiments,increased themaximumstrainattheendofeachstep,thus,thesame speci-menwassubsequentlysubjectedto60cyclesatgivenlevelofstrain, laterincreasedforanothersetof60cycles,anduptothemaximum strain.Theexploredsetofstrainwasequalto0.5%,1%,1.5%,2% and3%.Thespeedofthecrossbeamwasadjustedsothatthetests hadthesametotaldurationofabout10min;sothestrainratewas set,respectively,5.7%/min,11.4%/min,17.1%/min,22.8%/minand 34.3%/min.Aftereachstepthespecimenwasunloadedand10min elapsedbeforethesubsequentstep.

The third type of test consistsin applying a given level of strain and maintaining it for a given time, a testing protocol thatcorrespondstothepolymertestingmethodologyknownas stress relaxation test. Similarly to the cyclic test, the effect of theappliedstrainwasinvestigatedinsubsequentrelaxationstep, increasingthelevelofdeformationappliedaftereachstep. Progres-sivelyincreasingdeformationsequalto0.5%,1%,1.5%,2%and3% wereapplied,withinloadingrampatconstantspeed(2mm/min), and maintainedfor 12minin each step,after which the speci-menwasunloadedand10minelapsedbeforethefollowingstep. In order tostart with thespecimen in puretension, a preload ofmoderateentity(0.5N)wasappliedonthesensorbeforeeach

(5)

Table1

Mechanicalpropertiesofthepolyimidesubstrateasdeterminedinstrainramp tensiletests.

Polyimidecharacteristics Value E,Youngmodulus(GPa) 3.6±0.2 ␴y,stressatyield(MPa) 66±4 ␧y,strainatyield(%) 2.3±0.4 ␴b,stressatbreak(MPa) 230±50 ␧b,strainatbreak(%) 55±27

ramp,allowingcontrollingproperlythestrainvalueappliedineach

step.

Thestressvaluewascalculatedasengineeringstress,by

nor-malizing the measured load to the initial cross-section of the

sample.Forthiscalculation,thethicknessofthepolyimide

sub-strate(25–30␮m)wasconsidered;thethicknessoftheconductive

layer,equaltofewhundredofnanometer,wasconsidered

negli-gible.Thestrainwasexpressedasengineeringstrain,whichisthe

ratiobetweentheelongationofthespecimenandtheinitiallength

betweengrips.

4. Experimentalresults

4.1. Mechanicalbehaviorofthepolyimidesubstrate

Mechanicaltestswerepreliminarilycarriedoutonfour

speci-mensofthepolyimidesubstrateinordertoevaluateitsmechanical

responsetothevariousdeformationhistories.

InFig.4a,thestressvs.strainrelationshipmeasuredalong a monotonicstrainrampatconstantdisplacementrateisreported, andthefourspecimensshowagoodrepeatability(thedeviation standardisaboutthe5%).Thistestallowedevaluatingthe mate-rialstiffnessaswellasitsyieldandbreakpoint,whosevaluesare reportedinTable1.

Atthelowerstrains,thespecimensdisplayalinearincreasing trend,onwhichtheYoungmodulus,E,wasevaluated;deviations fromthistrendarefoundathigherlevelsofstrains,andtheyare displayedastwosubsequentreductionsofthecurveslopebefore ﬁnalbreak.Theﬁrstdeviationfromthelineartrendisassumed rep-resentativeoftheyieldphenomenon.Thevaluesofyieldstress,␴y, wascalculatedasthestressattheintersectionoftwolines inter-polatingthelineartrendsdisplayedbeforeandafterthisdeviation, asschematicallysketchedinFig.4b;theyieldstrain,␧y,was eval-uatedonthestressvs.straincurvesincorrespondenceoftheyield stress.Thevaluesofstressandstrainatbreak(␴band␧b, respec-tively)wereevaluatedasthecoordinatesofthemaximumpoint ofthecurvebeforetheabruptfallofstressoccurringatbreak.The resultssuggestthatthesubstratematerialundergoesyieldingand localizedplasticdeformationalreadyatstrainlevelsequaltoabout 2.3%.Therefore,inparticularforcyclicloading-unloadingtests,the maximumlevelofadoptedstrainneverexceeded3%,whereasthe minimumvalueof0.5%wasadoptedinordertoassureareliable displacementcontrolbymeansofthedynamometer.

Cyclicloading-unloadingtestswerecarriedouttostudythe sub-strateresponseunderthisstrainhistory,andforvariouslevelsof maximumstrains,withthespeciﬁcaimtohighlighthysteresisin thestressvs.straincorrelationforrepeatedcyclesandto evalu-atetheaccumulationofresidualdeformationattheendofeach cycle.Fig.5showstheentireoutputofthewholecyclictest(up to20cycles,lastingabout15min);inparticular,Fig.5areferstoa maximumstrainequalto1.5%,whereasFig.5btoastrainequalto 3%,correspondingtovaluesrightbelowandabovetheyieldstrain, respectively.

For bothstrain levels,a hystereticresponse is evidenced,as conﬁrmedbytheareacomprisedbetweenloadingandunloading. Asthenumberofcyclesincreases,aprogressivereductionofthis

Fig.6.Stressrelaxationtestsontheneatpolyimideﬁlmundertwolevelsofstrains.

areaandanoverlappingoftheloading-unloadingcyclesoccur.The resultsshowthatundertheseconditionsthematerialundergotoa largeevolutionofstress-straincorrelationintheﬁrstcycle,tending onlyabove10cyclestoacommontrend.Further,thepresenceofa residualdeformationattheendofeachcycleisshown.Thisisnot sorelevantformaximumdeformationlevelsupto1.5%(forthis levelofstraintheresidualdeformationisabout0.05%afterthe1st cycleandapproaching0.1%afterthe20thcycle).Itbecomesmore importantforhigherlevelsofstrain(inthecaseofthehigheststrain exploredtheresidualstrainincreasesfrom0.3%to0.6%duringthe test).

Thestressrelaxationtests,carriedoutbysubjectingthe spec-imenataconstantstrainandmonitoringthestressdecreasingin timefor 30min,providedtheresultsreportedinFig.6,fortwo levelsofstrain(0.25%and1.5%,respectively),bothchosenbelow theevaluatedyieldstrain.Atalltheappliedstrains,theexpected stressreductionisfound,butwhereastheeffectismoderate(i.e.a

Fig.7.Normalizedresistancechange(R/R0)asafunctionofstrainand

simul-taneouslymeasuredstressvs.straincurveforthesensorwithsilverparticlesink (R0=31);thedottedlinesareconstructionlinestobetterevaluatethechangeof

theslopes,whilethedashedlinesdeﬁnethelevelofstrainatwhichthechange occurs.

(6)

Fig.8.SEManalysisforthemicrocrackdetectionwhenthestripesbasedonnanoparticlesilverinkare(a)unstressed,(b)elongatedat1%,(c)elongatedat5%.Nocracksare visibleintheelongatedstripes.

Fig.9. Normalizedresistancechange(R/R0)andstressevolutionofstrain,and

gaugefactors(R0=6890);thelinesrepresentthespeciﬁclevelsofstrainforthe

resistancevariationrateandthestrainatyield(fromthestresscurve).

decreaseofabout2.8%within30min)fortheloweststrain,at1.5% thedecreasebecomesmarkedlymoresigniﬁcant(i.e.about13%), fasteranditdoesnotseemtoapproachastableplateauwithinthe testtime,asaprobableconsequenceinthenonlinearviscoelasticity regime.

4.2. Sensorsresponsesintensileramptests

Sensorsresponsestoaquasi-statictensileramp(Fig.3a)are reportedinFigs.7and9,wherethespecimensaredeformedata constantstrainrateuptothespecimenfailure.

InFig.7,thesensorbehaviorwithsilvernanoparticlesinkis shown in terms of normalized resistance change, R/R0, as a functionofstrain(R0=31);alsothestressvs.straincurve, simul-taneouslymeasuredonthesensor,isreported.

Thenormalizedresistancechangeshowsanevidentincreasing dependenceonstrain,displayedasasubsequenceoftwoalmost lineartrendswithdifferentslopes.Atlevelsofstrainofabout4%, infact,theinitiallineartrendgraduallyapproachesasecondtrend withahigherslope.Interestingly,bycomparingthistrendwiththe stressvs.strainrelationship,itissuggestedthatthechangeinslope takesplaceintheregionneartoyieldoccurrence.Finally,athigher strains,butwellbeforespecimenfailure,theresistancepresentsan abruptincreaseofmanyordersofmagnitude,(conditionsof

“over-load”).Thelastdeformationvalueacquired,whichisabout8–10%, canbeadoptedtoestimatethemaximumlimitofsensitivity.

TheincreasingdependenceofR/R0withthestraincanbealso expressedintermsofanincreaseofgaugefactor,whichwaseasily evaluatedasslopeoftheR/R0vs.straincurve,assumingvalues ofG1=3.72intheﬁrstportionofthecurves,andbecomingalmost twice(G2=6.7)afterwards.Theincreaseinresistancewithstrain canbeascribedtoachangeingeometryoftheconductivetrack, whichincreasesitslengthandreducesitswidth,aswellastoa changeinresistivity.Inparticular,thesigniﬁcantchangeofslope occurringincorrespondencetotheyieldpoint,andtheassociated localizationofstrain,couldsuggestthatyieldingisatthebasisof theresistancevariation,asaconsequenceofdamagestothe con-ductivepathatthemicroscale.Thephenomenoncanbeconsidered increasingwithstrainuntillocalizedbreakingofthesensitivetrack ordetachmentsofthesensitivetrackwiththesilverconnection leadtotheoverload.SimilarresultswereobtainedbyValetonetal. [32],who,forasensorobtainedbydepositingasilver-basedink onaPET(poly(ethyleneterephthalate))substrate,interpretedthe changeinelectricalresistanceinducedbytensilestrainasa conse-quenceofmicrocracksontheconductivetrack,andtheasymptote athighdeformationstoexcessivemicrocracking.

In order to verify the presence of suchmicrocracks onthe stretchedsensors,themorphologyoftheconductivetrackswas investigated,usingascanningelectronmicroscope(SEM)andan optical microscope.No signiﬁcantmorphological changes were foundforstrainlevelsaboveandbelowtheyieldpointwithrespect totheundeformedspecimens.Theseresultsseemtosuggestthat suchpathalteration,ifany,couldbeascribedtoverylocalizedstrain atthesubstrate-trackinterfacebutdonotinvolveﬁlmcracking atthesurface.Asarepresentativecase,theSEManalysesonan unstressedstripe,onastripeelongatedat1%andat5%imageare showninFig.8.Clearly,furtherexperimentsarerequiredtobetter understandwhatisoccurringintheconductivepath.

InFig.9,theexperimentalresultsforPEDOT:PSS-basedsensor arereported.Thenormalizedresistancevariationshowsan increas-ingdependenceonstrain,asinthecaseofsilverink,butdiffering intermsofcurveshape,whichdisplaysasmootherand continu-ousincreaseofslope,attainingalineartrendonlyforstrainlevels aboveyielding.Interestingly,thecurveshowsanevidentcurvature, whichcloselyresemblesthestressvs.strainevolution,inparticular intheyieldregion.Furthermore,thechangeinstrainisevaluated onthewholestrainscaleuptofailure,sothatthesensing capa-bilitiesofthesensorsarelimitedonlybythesubstrateductility, probablyduetothehigherductilityofthePEDOT:PSStrackswith respecttothoseinsilvernanoparticles.

ThepronouncedcurvatureoftheR/R0vs.strainatlowstrain levelsinhibitsaproperevaluationofthegaugefactoronthesestrain levels,whereasthatathigherstrainscanbecalculatedmore

(7)

eas-Fig.10.Experimentallymeasurednormalizedresistancechange(R/R0)asa

func-tionofstrainandﬁttingwithasecondorderpolynomialcurve.

ily.Nevertheless,tocomparethesensorperformances,thevalue of gaugefactors belowand above theyield points(G1 and G2, respectively)wereattempted.ThevalueofG1,deﬁnedastheslope oftangentatthecurveforstrainsbelow3%,isseentovarybetween 0.19and1.59;thevalueofG2,evaluatedonthelastpartofthecurve (above5%,asshownintheconstructionlineinFig.9)isabout2.6. Boththevaluesofgaugefactorsoncorrespondingstrainregions areseentobelowerthanthosefoundforthesilvernanoparticles ink.

Inthiscase,incontrasttothebilineartrendfoundforthesilver nanoparticlesink-basedsensors,therelationshipbetween resis-tance variation and strain can be described by a second-order polynomial equation, as shown in Fig.10. It seems to suggest thattheresistanceresponseshouldberesearchedinachangeof theelectricalconductivitydependingontheappliedstrain;the conductivePEDOTgrains[19]changetheirinterdistanceunder dif-ferentlevelsofstrain.

ThecalibrationcurveofthesensorbasedonPEDOT:PSScould beapproximatedtoalinearcurveifitisrequiredbythespeciﬁc application.Criteriaforselectingofthetwocalibrationpoints min-imizingthe errorcouldbeadoptedsuccessfully in thiscase, as proposedbyPallàs-Arenyet.al[33].

4.3. Sensorsresponseundercyclicandstaticapplicationsofstrain Thesensor responsestoacyclic applicationof progressively increasinglevelsofstress(Fig.3b)arepresentedforthesystem printed with silver nanoparticles ink in Fig. 11. The graphs in Fig.11aandbshowthenormalizedvariationoftheelectrical resis-tancewithinthewholecyclictensiletestfortheminimum(1%)and themaximum(3%)levelofappliedstrain.Atbothlevelsofstrain, thecurvesarecharacterizedbytwoimportantdriftingphenomena: i)hystereticaleffects intheR/R0 vs.straindependence,which becomeslessimportantasthecyclesincrease;ii)averticalshiftof theR/R0vs.straincurvesasthecyclesincrease.Furthermore,the sensoratunloadingdoesnotfullyrecovertheapplied deforma-tion;although,thevalueofminimumdeformationislowerthan 0.1%,inthelastpartoftheunloadingsegmentthesensortendsto bend,presentingabucklingeffect;thiseffectbecomesmore impor-tantastheappliedstrainandthenumberofcyclesincreasing.This determinesthepresenceoftheplateaushownforlowstrainlevels inFig.11b.Thiseffectisclearlydictatedbythefactthatthesensor

Fig.11.Responseofthesilvernanoparticlesinksensorundertheapplicationof loading-unloadingcyclesatagivenmaximumstrainvalue,correspondingto(a) 1.5%and(b)3%;greycurvesrepresentthecontinuousreadingofthesensoronall cyclesandthecurvescorrespondingtothe2nd_,₃₀th_and₆₀th_cycle_are_highlighted.

(c)Evolutionofthegaugefactorwiththecyclesandthemaximumstrainapplied.

responseisimportantunderstretching,butnotundercompression orﬂexure.Asaconsequence,thereisareductionofsensor sensitiv-itytosmalllevelsofdeformation,theextensionofthisplateauof poorsensitivityincreaseswiththenumberofcyclesandalsowith themaximumdeformationappliedduringthetest.Infact,this

(8)

phe-Fig.12.ResponseofthePEDOT:PSSinksensorundertheapplicationof loading-unloadingcyclesatagivenmaximumstrainvalue,correspondingto(a)1%and(b) 3%;greycurvesrepresentthecontinuousreadingofthesensoronallcycles,while incolorsarehighlightedthecurvescorrespondingtothe2nd_,₃₀th_and₆₀th_cycle.

Table2

Averagegaugefactorforvariouslevelsofmaximumappliedstrainandvarious cycles.

GaugeFactor

Strain(%) 2nd_cycle ₃₀th_cycle ₆₀th_cycle

0.5 1.26 1.21 1.21

1.0 1.51 1.56 1.56

1.5 1.73 1.75 1.81

2.0 2.02 2.12 2.19

3.0 2.81 3.14 3.37

nomenoniswellevidencedforthehighestlevelsofstrains,butit

startstobecomerelevantatmaximumdeformationlevelsof2%,

i.e.inproximityoftheyieldstrain.

Toprovideanevaluationoftheoverallsensorcapabilities,we

alsotried toestablishanaveragevalueofthegaugefactor,

cal-culatedasaverageslopeofthesinglecycle,andtheresultsare

reportedinTable2andinFig.11c,withparticularreferencetothe

2nd_,₃₀th_and₆₀th_cycle._The_gauge_factor_value_is_{representative}_of theaveragesensitivityonthewholestrainintervals;forthisreason,

Fig.13.Responseofthesensorswithsilverparticles-basedink(a)and PEDOT:PSS-basedink(b),whenaconstantstrainvalueisappliedandmaintainedconstantin time.

asthemaximumstrainincreases,highervaluesofgaugefactorare found,duetoinﬂuenceofthehighstrainresponse.Inanycase,it isworthwhiletonotethatforadeformationupto1.5%,thereisno speciﬁcdependenceofthegaugefactorfromthenumberofcycles. Suchadependenceonthenumberofcyclesbecomesappreciable formaximumstrainsequalto2%,anditbecomesevidentinthe testsat3%.Thiseffect,havingplaceincorrespondencetotheyield strain,canalsobeafeasibleindexforamarkedmaterialdamageof thesubstrateortheconductivepath,asunderlinedforthetensile tests.

ThePEDOT:PSSink-basedsensorsweresubjectedtocyclictests underthesameconditionsandtheresultsarereportedinFig.12a andb,formaximumdeformationlevelsof1%and3%,respectively. Thesensorsresponseundertheapplicationofaconstantstrain (Fig.3c)wasinvestigated,inordertounderstandwhetherthe mate-rialpresentsadriftintheresponsewithtime,andwhethersuch driftisaffectedbyvariousstrainregimes.InFig.13,theresults for thesensorsprinted withsilver nanoparticles-basedink and PEDOT:PSS-basedink,respectively,arereported.Thecurves rep-resenttheevolutionofR/R0 overtime;R0 istheresistanceof theundeformedsensor.Bothsensorsundergoareductionin resis-tancewithtime;furthermore,thiseffectismoreevidentforhigher appliedstrains,anditseemsthatthePEDOT:PSS-basedsensorsare morepronetosuchaneffect.Thereasonsbehindsucheffectare

(9)

notclear,andwesupposeavariationinshapeoftheconductive trackduringtime(forexampleduetothevariationovertimeof Poisson’sratioorwitheffectsinmicro-localizeddeformation)orto analterationofconductivecapacity,forexampleduetoexposure totheexternalenvironmentandatmosphericmoisture.Further,it isinterestingunderlinethatthereductioninresistance accompa-niesthestressdecreaseshowingaveryclosesimilaritywithit.In anycase,theresistancedecreaseisrelativelysmallforthe silver-basedink,whichshowsanoverallreductionof0.3%atthehighest appliedstrain;forthePEDOT:PSS-basedsensorssuchreductionis moreimportant,anditattainsvaluesofabout4%.

Thisiscomparedwithsensorsatrestfor30minindicatingthat theresistancedecreasemeasuredinthetestswithconstant defor-mation,eventhoughlimited,iscausedbythebehaviorofthesensor duringthetestsanditisnotexclusivelytheresultofthedriftthat thesensorwouldevenhaveifunloaded.

5. Conclusions

Thispaperprovidesamechanicalcharacterizationofstrain sen-sorsrealizedbyinkjetprintingonapolymersubstrateconductive tracksbasedonaPEDOT:PSS-basedinkoronaninkcontaining sil-vernanoparticles.Inallthetests,itwaspossibletohighlightthe closecorrelationofproportionalitythatexistsbetweenthe defor-mationandresistancechange.Inparticular,fromtheramptests obtainedwithsilver-inksensors,thecorrelationcanberepresented astheseriesoftwosubsequentlineartrends,whereasthis behav-iorisdifferentwithrespecttosensorsrealizedwithPEDOT:PSS,in whichthecorrelationcanberepresentedasasecondorder polyno-mialfunction.Thegaugefactorforsilver-inksensorsisabout3.7, whereasforPEDOT:PSS-inksensorsthegaugefactorislessthan 1.Theincreaseofresistancewiththedeformationisnotascribed solelytothedimensionalvariation oftheconductive track,but alsotoachangeoftheresistivitywithdeformation.Furthermore, theapplicationofaconstantdeformationovertimehasrevealed aprogressivedecreaseoftheresistance.Suchdriftismodest,the resistancevariation,calculatedwithadeformationmaintainedat 3%,isabout0.3%ifcomparedtotheinitialresistancevalue.Finally, theapplicationofloading-unloadingcyclesshowed:a)a hystere-sisinthesensorresponse,b)aprogressivedriftoftheresistance towardsgreatervalueswithincreasingnumber ofcyclesand c) theformation,withlowdeformationvalues,ofaplateauofpoor sensitivity.Thesephenomenaoccurinalimitedmodeupto defor-mationsof1.5–3%andtheseareevidentinthetestsinwhichthe sensorisdeformedupto3%.

Thesubstrateinfluenceonthesensorbehaviorhasbeen exper-imentallyverified.Thehighrigidityofthepolyimidecausesalow slopeofthefirstportionofthecurveresistance-stressobtainedin rampdeformation.Inthefuture,aimingatobtainingmore versa-tilestraingaugesensors,whichworkforexampleonalargerscale ofdeformation,elastomericmaterialscanbestudiedassubstrate. Thesesubstratescaneliminatetheproblemoftheyield,reduce thestiffness(andthusincreasethesensitivitytothestress)and mitigatethephenomenonofstressrelaxation,althoughitis neces-sarytofindtheconditionsforagoodwettabilitytotheconductive ink.Fortheelastomericsubstrate,thePEDOT:PSSinkcouldbea betteralternativethanthenanoparticlesilverinkforfabricating sensors,evenifthegaugefactorislowerandtheresistancedriftis moreevident.Indeed,whereasthelastdeformationofthestripes basedonsilveris8–10%(beforethespecimenfailure),theoneofthe PEDOT:PSSstripesacquiredbythetensileramptestis17%which correspondstothestrainatbreak.

References

[1]Y.Sun,K.He,Z.Zhang,A.Zhou,H.Duan,Real-timeelectrochemicaldetection ofhydrogenperoxidesecretioninlivecellsbyPtnanoparticlesdecorated

graphene–carbonnanotubehybridpaperelectrode,Biosens.Bioelectron.68 (2015)358–364.

[2]Y.Huang,W.Dong,T.Huang,Y.Wang,L.Xiao,Y.Su,Z.Yin,Self-similardesign forstretchablewirelessLCstrainsensors,Sens.ActuatorsA224(2015)36–42. [3]R.Ghosh,A.Singh,S.Santra,S.K.Ray,A.Chandra,P.K.Guha,Highlysensitive

large-areamulti-layeredgraphene-basedﬂexibleammoniasensor,Sens. ActuatorsA205(2014)67–73.

[4]R.D.P.Wong,J.D.Posner,V.J.Santos,Flexiblemicroﬂuidicnormalforcesensor skinfortactilefeedback,Sens.ActuatorsA179(2012)62–69.

[5]J.-H.Kim,Y.Liang,S.Seo,Patchablethin-ﬁlmstraingaugesbasedon pentacenetransistors,Org.Electron.26(2015)355–358.

[6]C.-W.Ma,L.-S.Hsu,J.-C.Kuo,Y.-J.Yang,Aﬂexibletactileandshearsensing arrayfabricatedusinganovelbuckypaperpatterningtechnique,Sens. ActuatorsA231(2015)21–27.

[7]S.-H.Bae,Y.Lee,B.K.Sharma,H.-J.Lee,J.-H.Kim,J.-H.Ahn,Graphene-based transparentstrainsensor,Carbon51(2013)236–242.

[8]A.Bessonov,M.Kirikova,S.Haque,I.Gartseev,M.J.A.Bailey,Highly reproducibleprintablegraphitestraingaugesforﬂexibledevices,Sens. ActuatorsA206(2014)75–80.

[9]G.Saggio,Electricalresistanceproﬁlingofbendsensorsadoptedtomeasure spatialarrangmentofthehumanbody,Proceedingsofthe4thInternational SymposiumonAppliedSciencesinBiomedicalandCommunication Technologies(2011),articleNo.5.

[10]J.N.Pereira,P.Vieira,A.Ferreira,A.J.Paleo,J.G.Rocha,S.Lanceros-Mendez, Piezoresistiveeffectinspin-coatedpolyanilinethinﬁlms,J.Polym.Res.19 (2012)0–6.

[11]A.Ferreira,J.G.Rocha,A.Anson-Casaos,M.T.Martınez,F.Vaz,S. Lanceros-Mendez,Electromechanicalperformanceofpoly(vinylidene ﬂuoride)/carbonnanotubecompositesforstrainsensorapplications,Sens. ActuatorsA178(2012)10–16.

[12]P.Costa,J.Silva,V.Sencadas,R.Simoes,J.Viana,S.Lanceros-Mendez, Mechanical:electricalandelectro-mechanicalpropertiesofthermoplastic elastomerstyrene–butadiene–styrene/multiwallcarbonnanotubes composites,J.Mater.Sci.48(2013)1172–1179.

[13]A.Ferreira,P.Cardoso,D.Klosterman,J.A.Covas,F.W.J.vanHattum,F.Vaz,S. Lanceros-Mendez,Effectofﬁllerdispersionontheelectromechanical responseofepoxy/vapor-growncarbonnanoﬁbercomposites,SmartMater. Struct.21(2013)075008.

[14]H.Ohya,V.V.Kudryavsev,S.I.Semenova,Polyimidemembranes:applications, fabricationsandproperties,CRCPress,2016,pp.1997.

[15]B.Andò,S.Baglio,All-Inkjetprintedstrainsensors,IEEESens.J.13(2013) 4874–4879.

[16]N.Komuro,S.Takaki,K.Suzuki,D.Citterio,Inkjetprinted(bio)chemical sensingdevices(2013)anal,Bioanal.Chem.405(2013)5785–5805. [17]G.Cummins,M.P.Y.Desmulliez,Inkjetprintingofconductivematerials:a

review,CircuitWorld38(2012)193–213.

[18]B.Andò,S.Baglio,S.LaMalfa,G.L’Episcopo,Allinkjetprintedsystemfor strainmeasurement,IEEESens.Conf.2011(2011)215–217.

[19]M.Yang,Y.Zhang,H.Zhang,Z.Li,CharacterizationofPEDOT:PSSasa biocompatibleconductivematerial,in:Proceedingsofthe10thIEEE InternationalConferenceonNano/MicroEngineeredandMolecularSystems (NEMS),Xi’an,China,2015,pp.149–151.

[20]M.Hokazono,H.Anno,N.Toshima,Thermoelectricpropertiesandthermal stabilityofPEDOT:PSSﬁlmsonapolyimidesubstrateandapplicationin ﬂexibleenergyconversiondevices,J.Electron.Mater.43(2014)2196–2201. [21]Y.Seekaew,S.Lokavee,D.Phokharatkul,A.Wisitsoraat,T.Kerdcharoen,C.

Wongchoosuk,Low-costandﬂexibleprintedgraphene–PEDOT:PSSgas sensorforammoniadetection,Org.Electron.15(2011)2971–2981. [22]P.Calvert,D.Duggal,P.Patra,A.Agrawal,A.Sawhney,Conductingpolymer

andconductingcompositestrainsensorsontextiles,Mol.Cryst.Liq.Cryst. 484(2008).

[23]DuPont,DuPontkaptonHNpolyimideﬁlm,KaptonHNdatasheet(2011). [24]AdvancedNanoProductsSilverJetInk,2015.Availablefrom:URL:http://

anapro.com/eng/product/silverinkjetink.html.

[25]AfgaSpecialitiesProductsORGACONTM_Transparent_Conductive_Inkjet_Ink:

IJ-1005,2013.Availablefrom:URL:http://www.agfa.com/sp/global/en/ binaries/IJ1005-2013v1.1tcm611-86919.pdf.

[26]Y.Sun,S.P.Lacour,R.A.Brooks,N.Rushton,J.Fawcett,R.E.Cameron, Assessmentofthebiocompatibilityofphotosensitivepolyimidefor implantablemedicaldeviceuse,J.Biomed.Mater.Res.90A(2009)648–655. [27]X.Crispin,F.L.E.Jakobsson,A.Crispin,P.C.M.Grim,P.Andersson,aVolodin,C.

vanHaesendonck,M.VanderAuweraer,W.R.Salaneck,M.Berggren,The originofthehighconductivityofpoly(3,

4-ethylenedioxythiophene)-poly(styrenesulfonate)(PEDOT-PSS)plastic electrodes,Chem.Mater.18(2006)4354–4360.

[28]Y.Liu,B.Weng,J.M.Razal,Q.Xu,C.Zhao,Y.Hou,S.Seyedin,R.Jalili,G.G. Wallace,J.Chen,High-performanceﬂexibleall-solid-statesupercapacitor fromlargefree-standinggraphene–PEDOT/PSSﬁlms,Sci.Rep.5(2015)17045. [29]T.Öhlund,J.Örtegren,S.Forsberg,H.E.Nilsson,Papersurfacesformetal

nanoparticleinkjetprinting,Appl.Surf.Sci.259(2012)731–739.

[30]F.Molina-Lopez,D.Briand,N.F.DeRooij,Alladditiveinkjetprintedhumidity sensorsonplasticsubstrate,Sens.ActuatorsB:Chem.66–167(2012) 212–222.

[31]M.L.Allen,M.Aronniemi,T.Mattila,A.Alastalo,K.Ojanperä,M.Suhonen,H. Seppä,Electricalsinteringofnanoparticlestructures,Nanotechnology19 (2008)175201.

(10)

[33]R.Pallàs-Areny,J.Jordana,Ó.Casas,Optimaltwo-pointstaticcalibrationof measurementsystemswithquadraticresponse,Rev.Sci.Instrum.75(2004) 5106–5111.

Biographies

Michela Borghettireceived herMaster’sdegree, cum laude,inElectronicEngineeringfromtheUniversityof Bresciain2012.In2015,shewasvisitingPh.D.studentat UniversitatPolitècnicadeCatalunya.In2016,Shereceived thePh.D.inTechnologyforHealthfromtheUniversity of Brescia.Nowsheis Postdoctoral Reasearcherwith theDepartmentofInformationEngineering,University ofBrescia.She isworking onthedesignand fabrica-tionofsensorsforhealthcareusinglow-costtechnologies. Furthermore,sheisdevelopingelectronic systemsfor measuringandmonitoringlimbmovements.

MauroSerpelloniisassistantprofessorofmeasurement attheInformationEngineeringDepartment.Hereceived thePh.D. inelectronicinstrumentationfrom the Uni-versity of Bresciain 2006. From 2006–2010, he was PostdoctoralResearcherwiththeDepartmentof Infor-mation Engineering,University of Brescia. Nowhe is Assistant Professor and Aggregate Professor with the Department of InformationEngineering, University of Brescia.Hehasworkedonseveralprojectsrelatingto thedesign,modelling,andfabricationofmeasurement systemsforindustrialapplications.Hisresearchinterests includeelectronicinstrumentation,sensors,contactless transmissionsbetweensensorsandelectronicsandsignal processingformicroelectromechanicalsystems.

ElectronicsforAutomation,UniversityofBrescia.Since 2006heisfullprofessorofElectricalandElectronic Mea-surement.Hehasdoneintensiveresearchintheﬁeldof electronicinstrumentation,sensors,andsignal condition-ingelectronics.Recently,researchhasbeenaddressedto thedevelopmentofautonomoussensorsforbiomedical applicationswithsomespeciﬁcinteresttowarddevices implantableinsidethehumanbody.Heisauthoror coau-thor of morethan onehundred paperspublished on internationaljournal.

Pandini,bornin1975inTrento(Italy),studied Mate-rialsEngineeringattheUniversityofTrento,wherehe graduated.InhisPh.D.thesishedealtwiththetimeand temperatureontheviscoelasticbehavior of semicrys-tallinepolymersatsmallandlargedeformations.Since 2005isAssistantProfessorinMaterialsScienceand Tech-nologyattheUniversityofBrescia.Hehasanexpertise inthestructural,mechanicalandthermal characteriza-tionofpolymericandcompositematerialsforengineering andbiomedicalapplications.Hisﬁeldsofresearchmainly concernsthethermo-mechanicalresponseof polymer-basedfunctionalmaterials(micro-andnano-structured polymer-based systems; nano-ﬁlled rubbers), andthe shapememorybehavioroftailoredpolymericsystems.