Sensors and Actuators B: Chemical - Infoscience: Scientific

13 Pages·2011·3.62 MB·English

by  Gursel Alici

SensorsandActuatorsB157 (2011) 72–84

ContentslistsavailableatScienceDirect

Sensors and Actuators B: Chemical

journal homepage: www.elsevier.com/locate/snb

Enhancement of actuation ability of ionic-type conducting polymer actuators

using metal ion implantation

GurselAlicia,b,∗,AndresPunningc,HerbertR.Sheac

aUniversityofWollongong,SchoolofMechanical,MaterialsandMechatronicEngineering,NSW,Australia
bARCCentre o fExcellencefo rElect rom aterialsSci enceUnive rsity ofWollongo ng,Intelligen tPoly merResearchInstituteInnovationCampus,NSW,Australia
cEcole Polyte ch niqueFédé rale deLausanne(EP FL),Mic rosystems for SpaceTechn ologiesLab oratory, Neuchâte l,Switzer land

a r t i c l e i n f o a b s t r a c t

Articlehistory: Inthisstudy,wepresenttheresultsandimplicationsofanexperimentalstudyintotheeffectofgold-ion
Receiv ed14December2010 im plan tation on theactua tion perfor man ceofionic-ty pe co nductingpoly merac tuat ors, repres en tedhere
AReccceepivteedd i1n3 r Mevaisrechd 2fo0r1m 1 12 February 2011 by cantilevere d t ri-l ayer polyp yrrole (PPy) a ct uators. We implant go ld ions be neath the outer surfac es of
PPy-basedconductingpolymerlayersoftheactuatorsinordertoincreasetheconductivityoftheselayers,
Available online 21 March 2011
andthereforeimprovetheoverallconductivityoftheactuators.AFilteredVacuumCathodeArc(FVCA)

ionsourcewasusedtoimplantgoldparticlesintotheconductingpolymerlayers.Electroderesistance
Keywords:
andcapacitance,surfaceresistance,currentresponse,mechanicalworkoutputoftheactuatorsamples
Electroactivepolymeractuators
weremeasuredand/orcalculatedfortheactuatorsampleswithandwithoutgoldimplantationinorder
Metalion-implantation
AElcetcutart oodrep ceornfodrumctainvciteyenhancement etole dcetrmodonesstdrautrein thget hefefierc‘te olef ctthreo cghoeldm-iommpelcahnatantiicoanl’. Tachteu cautirornenwt pasasmsienags tuhrreodugtoh dtheete cromnidnuecttihneg cphoalyrgminegr
timeconst antoft heact uators.Themechanicaldis placement outp utofthea ctu atorswasr eco rded.The

resultsdemonstratethattheconductivityoftheactuatorsincreasesnoticeably,whichhasaflowoneffect

onthecurrentresponse(i.e.,chargeinjectedintothepolymerlayers)andthemechanicalworkoutput.

Whilethegoldimplantedactuatorshadahighermechanicalstiffnessthereforeasmallerdisplacement

output,theirtimeconstantissmaller,indicatingahigherresponsespeed.Thegold-implantedactuators

generateda15%highermechanicalworkoutputdespitetheadverseeffectsonthepolymerofthevacuum

processingneededfortheionimplantation.

© 2011 Elsevier B.V. All rights reserved.
1. Introduction moredetailinSection2.Fig.1isanexampleoftri-layerstructure

consisting of polymer–electrolyte-containing substrate–polymer.

Electroactivepolymersactuatorsarecommonlyreferredtoas Whenanelectricalpotentialisappliedbetweenthepolymerson

artificialmuscles[1]inviewoftheirremarkablemuscle-likeprop- bothsidesoftheactuator,anelectrochemicalreactionoccurs;the

erties:highflexibility,highstrainandhighenergydensity.Unlike polymer layers are electrochemically charged (doped) and dis-

other ionic-type conducting polymer actuators which can only charged(undoped)throughtheexchangeofcharge-balancingions

operateinaliquidelectrolyte,theionic-typeconductingpolymer betweenthepolymerlayersandtheelectrolytestoredinapassive

actuators considered in this study can operate in air and liquids substrate,inourcasepolyvinylidenefluoride(PVDF).Thiselectro-

using a very low electrical power (1V, 15–20mA) with a high chemicalreactioncausesavolumechangeinthepolymerlayers,
speed (∼ 400H z,d ependingo ntheir siz e). Whent hey aremi ni atur- leadingto swellin gandc on traction .Thevo lu me changeis easily

izedfurthertomicro-scale,becomingMEMSandbioMEMSdevices, controlled by the electrical potential. The rate and magnitude of

theycanbesuitabletoapplicationsrangingfrombiotechnologyto the flow of charges in or out of the polymer layers depends on:

micro-robots,especiallyinthefieldoflifesciences. theelectricalconductivityofelectroactivepolymerlayers,theionic

Ionic-typeconductingpolymeractuatorsconsistofatleastone conductivityoftheelectrolyte,thediffusiontimeofionsinthepoly-

conductivepolymerlayerandanelectrolytereservoir,discussedin merlayers,theporosityandgeometryofthepolymerlayers,and

appliedpotentialdifference.Thefundamental(orcharging)time

constantoftheseactuatorsdependsontheirtotalresistanceand

capacitance(RC).Thelowerthetotalresistance,thefastertheactu-
∗
Corresponding author at: University of Wollongong, School of Mechanical, ation process. Further, it is known that there is a direct relation
Materials and Mech atronic Eng ineering, NS W 2522, Austral ia. Tel.: + 61 242214145; betwe enthec hargingr at ea ndthe actua tions tra in orme chanical
fax:[phone].
displacementgenerated[7].
E-mail address: [email protected] (G. Alici).
0925-4005/$–seefrontmatter© 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.snb.2011.03.028
G.Alicietal./SensorsandActuatorsB157 (2011) 72–84 73

asignificantresearchproblem.Previousattemptstodepositacon-

ductingmetalliclayeronthesurfaceofconductingpolymersfilms

(which were used as linear actuators in an aqueous electrolyte)

werenotsuccessfulduetothepoorbinding(nointerpenetration

betweenthegoldandthepolymerlayers)betweenthepolymer

surfaceandthethinfilmcoatings[20–22]orexcessivestiffening.

Inthisstudy,weemployalow-energy(5keV)metalionimplan-

tationtechniquebasedonapulsedvacuumarcplasmasourceto

increasetheconductivityofthepolymerelectrodes,andtherefore

enhance the actuation of performance of this class of electroac-

tivepolymeractuators.Thision-implantationtechniquehasbeen

employedbeforetomakecompliantgoldelectrodesindielectric

elastomeractuators[2,27]andstretchableelectrodes[25]bycre-

atinga20nmthickgold–siliconnanocomposite[6,26].Forthefirst

time,weemploythistechniquetoincreasetheconductivityofthe

ionic-typeconductingpolymeractuators,hencetoenhancetheir

actuationability,whichdependsonthepolymerconductivity.We

comparetheexperimentalandnumericalresultsfrombareactua-

torsamplesandfromgold-implantedsamplestodemonstratethe

efficacyofthegoldimplantationinincreasingtheirperformance.

Electroderesistance,electrodecapacitance,surfaceresistance,cur-
Fig. 1. Structure and schematic representation of the operation principle of the rentrespo nse,timec onstant,e lectricalpow erenerg yconsump tion
cantileveredpolymeractuators.
andmechanicalworkoutputoftheactuatorsamplesweremea-

suredand/orcalculated.Thecontributionofthisstudyistopresent

With this in mind, it is the aim of this study to increase theresultsontheeffectofgoldimplantationontheactuationper-

theelectricalconductivityoftheelectroactivepolymerlayersby formance.Tothebestoftheauthors’knowledge,thisisthefirst

implantinggoldionsafewtensofnmbeneaththeoutersurface studyfocusingontheinfluenceofthemetalionsinjectedintothe

of the polymer layers, which are electrically contacted by a pair conductingpolymerlayersoftri-layerconductingpolymeractua-

ofmetalelectrodescoveringasmallfractionofthepolymer(see torsontheiractuationability.

Fig.1).Aswidelyreportedintheliterature[3–15],iftheconductiv-

ityofthecontactsandthepolymerisverygood,therewillbeasmall

ohmicpotentialdropalongtheactuatorlength,andthecharging 2. Backgroundonsynthesisandoperationofelectroactive

ratewillbehigh,whichgeneratehigherstrains.Thehigheristhe polymeractuators

strain,thehigheristhetipdisplacementoftheactuators.Thefunc-

tionoftheseadditionallayerofgoldistoincreasetheconductivity Polymers based on pyrrole, thiophene or aniline, which are

ofthepolymerelectrodes,andthustoalsominimizetheohmic(IR) known as electroactive materials, have been extensively studied

potentialdropalongtheactuatorlength. tousethemasactuatorswithnewfunctionalityandperformance

As reported recently [16], the Cl and SF plasma treatment [13–15]. Electroactive polymer actuators are classified as inter-
2 6

of the ionic-polymer metallic composite (IPMC) actuators has nalandexternalactuators,dependingontheirelectrolytestorage

improvedtheactuationperformanceofthisclassofactuatorinclud- method.Forinternalactuatorsthereisapassivelayer,whichstores

ingoperationallife,bendingdisplacement,andblockingforce.This theelectrolyte.Forexternalactuatorstheelectrolyteisstoredin

improvementisduetoanincreaseinthesurfaceconductivity.The anoutercellandthepolymeractuatormustbepartiallyorfully

higheristheconductivityofthesurface,thehigheristhebending immersedintheelectrolyte.Theactuatorsconsideredinthisstudy

displacement of the actuators. IPMC consist of an ionic polymer areinternal-typeconductingpolymeractuators.

sandwiched between two compliant electrodes. The ionic poly- Polymeractuatorsusedinthisstudyhaveamulti-layerstruc-

mer(cellseparator)Nafionisplasmacoatedinordertoincrease ture.Afullaccountoftheactuatorsynthesisprocedureispresented

the penetration of the platinum particles in order to create an in[11–13].Themulti-layeractuatorstructureshowsasimplebend-

electrodecoatingintotheNafionlayer.Withtheplatinumnanopar- ing motion like a bilayer cantilever beam, as depicted in Fig. 1.

ticleswithathicknessof50–70nm,thesurfaceconductivityand Oftheselayers,thegold-coatedPVDFinthecentrewithathick-
capac itance o ftheIPMC a ctuator sare inc reasedb y20timesa nd2 ne ssof1 10(cid:2)m isa non-conduct ivepo ro usm embra neus ed asan

times,respectively,comparedtoanactuatorsamplewithoutany electrochemical cell separator, and it also stores the electrolyte

plasma treatment. In another relevant study, Punning et al. [17] (LiTFSI+solvent), facilitating the operation of such conducting

reportedontheirexperimentalstudyonthesurfaceresistanceof polymeractuatorsinair.ThegoldlayeroneachsideofthePVDF

IPMCactuatorsandsensors.Theyconcludethat(i)thesurfaceresis- hasapproximately10nmthickness.Polypyrrole(PPy)layers,with
tance changesw ith theradi usof curvature ofth e actu atorsan d(ii) ath icknessof30(cid:2) m, are electroact ivecompone ntsu sedas elec-

thereisadiscernabledifferencebetweentheresistanceofthecom- trodes[14].Thetotalthicknessoftheactuatorsisapproximately
pressi ng surfaceand expanding surface oft hecantilev er ed IPMC 170(cid:2)m .

actuators.BecausetheIPMCactuatorshavetwometallicelectrodes WiththeelectrolytestoredinthePVDFlayer,themulti-layer

through which they are electrically actuated, there is a wide lit- structureformsanelectrochemicalcell.Whenapotentialdiffer-

eratureonimprovingtheelectricalcharacteristicsofthemetallic enceorcurrentispassedbetweenthePPylayersviatheelectrical

electrodes and their implications for the actuation performance contacts,thewholestructureischargedlikeacapacitor.Tomain-
[16–19]. B ut, t he ac tuators consi dere d in this stud y have non- tain char ge n eutral ity within th e PPy la yers , TFSI− anio ns move

metallic,butinherentlyconductingpolymersaselectrodeswhose fromtheelectrolyteinthemiddlelayerintothepositivelycharged

conductivityhasasignificanteffectontheiractuationperformance. polymer(PPy)electrodeandhencecauseavolumeexpansion.At
Therefore,im pro v ingthecon ductiv ity ofth eseelectr odesthrough thesame time ,inthepo sitive electr ode/a no de,the anions(TFS I−)

apermanentlayerofaconductorsuchasimplantedgoldparticlesis leave the negatively charged electrode as reduction of the PPy
74 G.Alicietal./SensorsandActuatorsB157 (2011) 72–84

Fig.2. (a)PhotographoftheplasmaexitingthemacroparticlefilteroftheFCVAandimpingingonthenegativelybiasedsubstrateholderand(b)theschematicrepresentation

oftheionimplantationsystemused.

From[26].

causesittobecomeunchargedandavolumecontractionoccurs. ators[2,25].Thesegold-implantedelectrodescanbestretchedto

The overall result is that the cantilevered structure will bend strains of up to 175% while remaining conductive, and, crucially

towardsthenegativeelectrode/cathode,asdepictedinFig.1.The for polymer actuation applications, the implanted electrodes are

volume change primarily happens due to the movement of the muchlessstiffthanifthesameamountofmetalhadbeensputtered

chargebalancinganionsinandoutofthepolymerlayers,andper- asacontinuousfilmonthepolymer.Thisisduetotheresulting

hapssomesolventmoleculesmoveinsidethepolymerlayers,due microstructure,forming2–30nmdiametergoldnanoparticleupto

toosmoticeffects,tobalancetheionicconcentration.Itistheout 100nmbelowthesurfaceofthepolymer,allowingconductionby

ofscopeofthispapertoprovideadetailedperformancecharacter- ohmiccontactbetweennanoparticlesthatarefreetomovewith

izationoftheactuators.Thereaderisreferredtoour[11–15]and respecttoeachother[26].

othergroup’s[3–7,23]previouswork. Theimplantationtechniqueusedhere,whichisknownasfil-

teredcathodicvacuumarc(FCVA),isaplasmabasedimplantation

technique.Thedetailsofthision-implantationtechniquearepre-

3. Lowenergyionimplantation sentedin[2,25–27].Itsschematicdiagramandaphotographduring

onepulsearedepictedinFig.2[26].Itisimportanttonotethat

Ion implantation is commonly used to inject dopants into intheFCVAimplantationtechnique,theenergyoftheionsvaries

semiconductors such as silicon in order to modify their electri- withineachpulse,the firstionsseeingthe full potentialdropof

calproperties.Theelectrical,mechanical,chemicalandstructural 2.5kV,whilethefinalionshaveenergyontheorderof50eV.

propertiesofothersubstratessuchaspolymerscanbesignificantly

changedbytheionsinsertedbeneaththeirsurface.Thepenetration

depthofmetallicionsdependsontheionimplantationparame- 4. Experimentalsetup

terssuchastheangleofincidence,typeofions,theenergyofthe

ions,andthecompositionofthetarget.Thehigheristheincom- The experimental setup is to measure the electrochemical

ingenergyofions,thehigheristhepenetrationdepthoftheions behaviouroftheactuatorsconsistingofapotentiostat/galvanostat

intothetargetofapolymersuchasPDMS,asshownforseveral (EG&G Princeton Applied Research Model 273A), eDAQ e-Corder

energiesin[26].Lowenergiesintherangeof50eVand5keVwere dataloggerunit,andawavegeneratorconnectedtothepotentio-

usedtoimplantgoldionsinthefirst50nmofthePDMSlayerin statunit.Thesuppliedvoltageandassociatedcurrentarerecorded

ordertomakecompliantelectrodesfordielectricelastomeractu- bytheeDAQunitinterfacewithapersonalcomputer.Theschematic

Fig.3. Schematicdiagramoftheexperimentalsetupusedtostimulatetheactuatorsamples,andtorecordtheassociatedelectricalsignals.
G.Alicietal./SensorsandActuatorsB157 (2011) 72–84 75

Fig.4. SEMSE(left)andBSE(right)imagesofagold-implantedactuatorsample.TheBSEimages(rightimages)showthedistributionofthegoldparticles(whiteareas)in

thecross-sectionbetweenthePVDFandPPylayers,andontheupperandlowersurfacesoftheactuators.

diagram of this experimental setup is shown in Fig. 3. Autolab circuitsfromonesidetotheotherthroughtheedgesofactuator.

PGSTAT12Potentiostat/GalvanostatequippedwithaGeneralPur- The top view of an actuator sample (Sample 1) under an optical

pose Electrochemical System (GPES) software was employed to microscopeisshowninFig.5,clearlyshowingthegoldimplanted

formatwoelectrodecyclicvoltammetry(CV)inordertogenerate area.

CVscans. Thesurfaceresistanceofactuatorsampleswithdifferentdimen-

The electrode resistance and capacitance of the actuators in sions is given in Table 1. These resistance data indicate that the

differentstates(normalstate,completelydryaftergoldimplanta- gold-ion implanted samples show factor of 2–3 smaller surface

tion,andre-wettedwiththeelectrolyte)weremeasuredusingan resistance.Itisthisreducedsurfaceresistanceweproposetohar-

LCRmeter(AgilentE4980APrecisionLCRMeter)connectedwith ness in order to improve the actuation ability of the conducting

a tweezers like test fixture (Agilent 16334A). The actuator sam- polymeractuators.Theelectroderesistanceandcapacitanceofthe

plesweresandwichedinthefixture.Theelectrodecapacitanceand sameactuatorsweremeasuredusingtheLCRmeterandarepro-

resistanceweremeasuredusingaparallelequivalentcircuitmode vided in Table 2. These results indicate that while the electrode

[24].Thesurfaceresistanceoftheactuatorsindifferentstateswas resistanceisdecreasing,theelectrodecapacitanceisincreasingfor

measuredbyatwoprobemethod(theactuators’surfaceareawas thegoldimplantedsamples.Thesedataweremeasuredunderan

toosmallforthefour-probemethod)usingadigitalmulti-meter AC voltage signal with amplitude of 1V and frequency of 20Hz

(Keithley2000Multimeter). (theminimumthisLCRmetercanprovide).Withreferencetothe

surfaceresistance,andelectroderesistanceandcapacitancemea-

5. Experimentalresultsanddiscussion surements,thegoldimplantationincreasestheconductivityand
thecapacita nce ofth econducting polymera ctua tors.

Anenvironmentalscanningelectronmicroscope(XL30,ESEM- Oneimplicationofincreasedcapacitanceistheincreasedblock-
FEG, P hilips) was use d to gen erate the secondary electro n (SE) ingforc eormechan ica lworkout put,whicha re lim itedforthi sclass
imag esandt heba cksca tte redelectr on( BSE)image softhec ross ofp olym er actuators.It was notpos sibleto m easuret he bloc king
section soft heac tuators.TheSE andBSE image sofago ldi mpl anted fo rce,butto estimate th eme cha nicalwor ko utputfro mm easured
actuator sa mp le(Sample 5) are sho wn inFig.4 .T h eim ageswere displa cem en tdataof sam plesloaded with 212mg atth eactuator
takenaft erallex perimen tsw ere condu cte d. tips.Thegold- impla nt edsamp lesgene rated mor em ec hani calwork
Wh enth e actuatorswer eimp lantedwithgoldions,a200(cid:2)m outp ut,a spresentedinS ection5. 4.Theelec trical powerconsu med
wide stri p on every sid e wa s masked off to prev ent a n y sh ort- bythea ctu atorswith an dwitho utg old implantat ionare calculated
76 G.Alicietal./SensorsandActuatorsB157 (2011) 72–84

Table1

Surfaceresistancemeasurementsforvariousactuatorsamples.Atleast5measurementsweretakenforeachsideoftheactuators.Theaveragevaluesarepresentedhere.

Pleasenotethattheelectrolyteevaporatesinthevacuumchamberoftheionimplantationsystem.Thatiswhythesamplesimmediatelyafterion-implantationaretermed

‘dry’,andthesesamplesarethensoakedintheelectrolytepriortotesting.

Samples Surfaceresistance((cid:2)/square)

Filledwith Dry-aftergold Soakedinthe Comments

electrolyte(no. implantation electrolyte

gold aftergold

implantation) implantation
Sample1 70.4 9.28 28.04 Thesamesamplewasusedfortheexperiments
(15m m×3.2mm×0.17mm)

Sample2 55.1 2.71 19.03 Thesamesamplewasusedfortheexperiments
(18m m×3mm×0.17mm)

Sample3 47.8 3.16 19.3 Thesamesamplewasusedfortheexperiments
(12m m×4mm×0.17mm)

Sample3New 40.3 5.1 16.04 Thesamesamplewasusedfortheexperiments
(12mm ×3.8mm×0.17mm)

Sample4 41 896.5 47.4 Nogoldimplantation.But,keptinthesame
(11m m×3mm×0.17mm) vac uum chambertoge ther with th ene xtsample
Sampl e4 41 381.35 32.6 Contact area(3m m×3mm )onl yis gold implanted
(11m m×3mm×0.17mm)

Sample5 78.37 85,000 248.6 Nogoldimplantation.But,keptinthesame
(13m m×3mm×0.17mm) vac uum chambertoge ther with th ene xtsample

Sample5 68.9 6.5 31.04 Wholesurfacewasgoldion-implanted
(12m m×3mm×0.17mm)

Table2

Electroderesistanceandcapacitancemeasurementsforvariousactuatorsamples.

Samples Resistance((cid:2)),capacitance(nF)

Filledwithelectrolyte Dry-aftergold Soakedinthe Comments

(no.goldimplantation) implantation electrolyteaftergold

implantation
Sample1 1050(cid:2),110nF Nodata 40.51(cid:2),1050nF Thesamesamplewas
(15m m×3.2mm×0.17mm) used forth e

experiments
Sample2 525(cid:2),90nF Nodata 45.8(cid:2),7000nF Thesamesamplewas
(18m m×3mm×0.17mm) used forth e

experiments
Sample3 2100(cid:2),85nF Nodata 38.5(cid:2),18,615nF Thesamesamplewas
(12m m×4mmx0.17mm) used forth e

experiments
Sample3new 520(cid:2),400nF Nodata 46(cid:2),10,270nF Thesamesamplewas
(12mm ×3.8mm×0.17mm) used forth e

experiments
Sample4 85(cid:2),2680nF 3M(cid:2),267pF 150.6(cid:2),1600nF Nogoldimplantation.
(11m m×3mm×0.17mm) But ,kep tinthesame

vacuumchamber

togetherwiththenext

sample
Sample4(11×3×0.17mm) 84(cid:2),2720nF 0.5M(cid:2),1430pF 41(cid:2),14,400nF Contactarea
(3mm× 3mm)onlyis

goldimplanted

Sample5 95(cid:2),2600nF 8.0M(cid:2),90pF 191(cid:2),1050nF Nogoldimplantation.
(13m m×3mm×0.17mm) But ,kep tinthesame

vacuumchamber

togetherwiththenext

sample
Sample5 78(cid:2),3830nF 5.5M(cid:2),102pF 36.9(cid:2),20,400nF Wholesurfacewas
(13m m×3mm×0.17mm) goldio n-impla nted

Table3
Electr ochemicaltimeconstantsoftwoactuatorsamplesexposedtothesamevacuumconditions(10−5mbar).TheaveragetimeconstantsofSamples4and5are[0.465s,

0.386s]and[0.36s,0.337s],respectively.TheaverageelectricalpowerconsumedbySamples4and5arealsocalculated.

Inputvoltage Timeconstants(s)for Averagepower Timeconstants(s)for Averagepowerconsumedby

Sample4[normal,with consumedbySample4 Sample5[normal,with Sample5(mW)[normal,withgold

goldions] (mW) goldions] ions]

0.1V [0.4870,0.4160] [0.0087,0.0076] [0.3750,0.3590] [0.0139,0.0095]

0.2V [0.5610,0.4340] [0.0427,0.0391] [0.4330,0.3180] [0.0621,0.0548]

0.3V [0.5270,0.4200] [0.1013,0.0967] [0.4230,0.3470] [0.1513,0.1037]

0.4V [0.4640,0.3920] [0.2425,0.4581] [0.3390,0.2960] [0.2740,0.1972]

0.5V [0.4220,0.3960] [0.3958,0.2996] [0.3420,0.3390] [0.4611,0.3167]

0.6V [0.4010,0.3500] [0.5990,0.4536] [0.3350,0.3280] [0.6565,0.4923]

0.7V [0.4880,0.3470] [0.7676,0.6455] [0.3350,0.3700] [0.9999,0.6795]

0.8V [0.3730,0.3360] [1.1198,0.8823] [0.2940,0.3350] [1.3609,0.9601]
G.Alicietal./SensorsandActuatorsB157 (2011) 72–84 77

Fig.5. Thetopviewofanactuatorsample(Sample1)underanopticalmicroscope.Thediagramontherightshowsthesurfaceareaimplantedwithgold.Eachgridunder

theleftimageindicates1mm.

andpresentedinTable3.Theseresultsshowthatthepowercon- As given in Table 3, the time constants vary with the input

sumptionisreducedforthegoldimplantedsamplesduetotheir voltage,thoughnotsignificantly.Thisisexpectedastheoxidation

improvedconductivity. levelorthechargeinjectionisinputvoltage-dependent.Thegold-

implantedsampleshaveasmallertimeconstant,whichindicates

ahigherresponsespeed.
5.1. Measurementofactuatortimeconstant

Tofurtherdemonstratetheeffectofthegoldionimplantationon 5.2. Effectofionimplantationconditions(i.e.,vacuum)on

theactuationabilityoftheactuators,theRCtimeconstantofthe currentresponse

actuatorsamplesareidentifiedfromcurrentversustimecurves.

Theformofthecurrent–timecurveissimilartothechargingcurve Samples1–3wereprepareddifferentlythansamples4and5.

of a capacitor. The time constant to be identified from the cur- ForSample1,Sample2,Sample3,Sample3-New,eachsamplewas
re nt curveindi cates how fasttheac tu ator ischarged unde rag iven test ed unde r a range o f input vo ltages (± 0.1–0.8 V w ith step s of
pote ntiald ifference. The curr ent curveisd e scribedb y: ±0.1V anda pe riodo f8 0s),a ndwasth engold-io n impl anted in

avacuumchamber.Thatistosay,theactuatorsampleswithout
I=Imaxe−t/(cid:3) (1) g oldimpla ntationw ereno te xp osed toh ighvacu um,whic hcauses

thesolventfromtheelectrolytetoevaporate.Aspresentedinthe
where(cid:3)isthechargingtime(RC)constantoftheactuator.Itisthe
nextparagraph,thevacuumintheionimplantaterleadstoa8–9%
timecorrespondingto36.8%ofthepeakcurrent.Usingthisdefini-
reductionoftheactuatorlength.Whentheactuatorsexposedto
tionofthetimeconstantinFig.6,thetimeconstantoftheactuator
thevacuumwerere-soakedintheelectrolyte,theyrecoveredtheir
samplesunderarangeofstepinputvoltagesareestimatedfrom
initialdimensions.Thesurfaceresistance,electroderesistance,and
theexperimentalcurrentversustimecurves,andaredepictedin
electrodecapacitanceofactuatorsexposedtovacuum(completely
Table3.Typicalcurrentresponses,forexample,fromSample5with
dry)weremeasuredtobeontheorderof2500(cid:2),4.0G(cid:2),and70fF,
andwithoutgoldionimplantationareshowninFig.7.Thecurrent
respectively. On the other hand, the actuator samples with gold
flowduringtheelectrochemicalactuationisduetothechargetaken
implantation,stilldry,havemorefavorableelectricalproperties,
offorplacedintheprimarychainofthepolymertohavecharge
asgiveninTables1and2.
neutrality.
To evaluate the effect of the vacuum on the time con-

stant of the actuators, a new sample with the dimensions of
12mm × 3.8m m×0.17m m wa s tested before it w as kept in t he
vac uum f ort hed u ratio n(∼ 20m inat10 −5mba r) used toim p lant

gold in the actuator samples. Not only the time constant of the

actuatorwasincreased,butalsothepeakcurrentwasdecreased,

asshowninTable4,andinFig.8.Thisexperimentwasrepeated4

timestominimizemeasurementerrors.

Table4

Electrochemicaltimeconstantsandpeakcurrentsofanactuatorsampleexposedto

avacuum,andnotexposedtothevacuumunderstepinputswithaperiodof80s.

Inputvoltage Timeconstants(s)[no Maximumcurrents

vacuum,aftervacuum (mA)[novacuum,after

(%)increase] vacuum,(%)decrease]

0.1V [0.5480,0.7090,29.38] [7.3371,6.1708,15.90]

0.3V [0.5430,0.6670,22.84] [14,19,23]

0.5V [0.5900,0.7150,21.19] [36.0031,31.7028,

11.94]
0.8V [0.6340,0.7410,16.88] [53.4027,47.3846,
Fig.6. Definitionofelectrochemicaltimeconstantbasedonthecurrentresponse
11.27]
oftheactuators.
78 G.Alicietal./SensorsandActuatorsB157 (2011) 72–84

Fig.7. Typicalcurrentresponses(topplots)forsample5withandwithoutgoldionimplantation–directlytakenfromthedataacquisitionsystem,underasquarewave
inpu to f±0.6V (bottom plots)wit hap eriod of8 0s.

5.3. Effectofionimplantationonworkoutputofactuators From the top left image of Fig. 9, ı =2.8mm, and from
i
the top right image, ı =3.2mm. The mechanical stiffness of
f
Toquantifytheworkoutputoftheactuatorsamples,4cylindri- this sample (Sample 4) with a gold coating in the surface
calpe rmanent mag nets withth ed ime nsionso f3mmin d iameter, area of 3m m×3mm (th e are a corres ponding to the area of

1mm in thickness, and a total mass of 212mg were attached to the electrical contacts) is 0.743N/m. The total work output
th etip of eachactua tors a mple witho rw itho utg oldim plantatio n. is c alculated from Eq. (2) as Wo =25 .85(cid:2) J. From the bottom

Theactuatorsmovementswererecordedwithadigitalcamerato left image, ı =5.636mm, and from the bottom right image
i
determinetheverticalmovementofthetipload.Theimagesfor ı =1mm.Usingthesedata,themechanicalstiffnessofthesample
f
theactuatorsamplesareprovidedinFig.9.Thetotalworkoutput with no gold implantation is 0.369N/m. The total work out-

WoisthesumoftheelasticpotentialenergyVetobendtheloaded put for an actuator with no gold (Sample 4) is calculated as
cantil ever eda ctu ato rfrom thestart position toth efina lpo sition, Wo =21 .93(cid:2) J, showi ng w ith a 15% decreas e in t he work o ut-

andthegravitationalpotentialenergyVgtoliftthetiploadfromthe put.

startpositiontothefinalpositionunderthegivenvoltageinput: For the work output of Sample 5 under 0.5V, from the top

left image of Fig. 10, ı =2.67mm, and from the right image
i
1 mg ı =2.89mm.ThemechanicalstiffnessofSample5withfullgold
Wwhoe=re Vıe+is Vtghe=d2ekflıe2ct+io mngoıf, thek a=ctuıaitorunderthetiploadwh(e2n) aWcofnoad t=i fn2rg2o .m 5is1 t(cid:2)0h.Je 7. F0rr8iog5 mh Nt t/ihmme. abgToehtteıo m t=o t−leafl2 t. 8wim2oamr gkme oo.ufU tFpsiigun.t g1 i0st,h ıceias=le c5u .d6laa4tt7ae ,dm tmhaes,
i f
theactuatorsareinactivated.Themechanicalstiffnessconstantof mechanical stiffness of Sample 5 with no gold implantation is
the actuators is ca lculated fro(cid:2)(cid:2)m (cid:2)(cid:2) Ho oke’s law—t he tip loa d is simp ly 0.335 N/m. T he total w or k output is calcu late d as W o= 8.22 (cid:2)J wi th
dividedbythetipdeflection ı underthetiploadoffourmagnets. a63.5%decreaseintheworkoutput.Thisisasignificantworkout-
i
Fig.8. Theeffectoftheion-implantationconditionsonthecurrentresponseoftheactuators.
G.Alicietal./SensorsandActuatorsB157 (2011) 72–84 79

Fig.9. Theactuator(Sample4)configurationunderatiploadat0V(left)and0.8V(right).Thetop2imagesareforthegoldimplantedsample,thebottomimagesare

non-implanted.
putdifferencebetweenagoldimplantedandnogoldimplanted ThedisplacementresponsesofSample5withandwithoutgold

samples. implantationweregeneratedunder0.5Vtoevaluatethetimecon-

ThisanalysisoftheexperimentaldataforSamples4and5shows stantofthiselectrochemomechanicalresponse.Theyareshownin

thatthesamplesimplantedwithgoldionsgeneratemoremechan- Fig.12thatthegoldimplantedsampleshowedafasterresponse

icalworkoutputthanthesamesampleswithoutgoldimplantation with a time constant of 1.58s, compared to the time constant

under the same potential difference. Further, with reference to (2.20s)ofthebareSample5withoutgoldimplantation.Thisalso

theelectricalpowerconsumedbythegoldimplantedsamplesin supports the conclusion that the gold implanted samples have a

Table 3, this mechanical work output is generated with a lower higher response speed compared to the actuators without gold

electricalpower.Thisfollowsthatthegoldionimplantationcon- implantation.

tributestotheefficiencyoftheseactuators,whichiswellbelow As stated above, the gold implantation increases the stiffness

1%. of the actuators. This can be quantified by calculating the flexu-

Tofurtherevaluatetheeffectofionimplantationconditionson ral rigidity of the actuators, which is a function of the modulus

theworkoutputoftheactuators,theworkoutputoftheactuator ofelasticityandtheareamomentofinertia.Theareamomentof

samplewiththecurrentresponsedatapresentedinTable4iscal- inertiaiscalculatedusingtheequivalentwidthapproach,inwhich

culatedunder0.8V.FromthetopleftimageofFig.11,ı =7mm, thewidthsofthelayerswiththehighermodulusofelasticityare
i
andfromthetoprightimageı =3mm.Themechanicalstiffness expanded to bring the whole structure into a single material of
f
ofthisuntreated(notexposedtothegoldimplantationcondition) thelowermodulusofelasticity[28].Tomaintainthesameflexural

actuatorsampleiscalculatedas0.297N/m.Thetotalworkoutputis rigidity,thewidthofthelayerwiththehighermodulusofelastic-
calculate dfromE q .(2)asWo= 3 5.65(cid:2) J.Fro mth ebot tomle ftimag e ityisinc rea sedby n= E2/ E1,E2 >E1. Ith asbeen reported [2 9]that

ofFig.11,ı =6.76mm,andfromtherightimageı =1.96mm.Using themoduliofelasticityofPPyandPVDFlayersforthebareactuator
i f
thesedata,themechanicalstiffnessofthesamesampleexposedto usedinthisstudyareapproximately190MPaand117MPa,respec-

thegoldimplantationconditionsis0.3075N/m.Thetotalworkout- tively.Thisfollowsthatn=E /E =1.624,andthewidthofthe
PPy PVDF
put isca lculatedasWo =29.83(cid:2)J ,w hichis 16.3% less than thew ork PPylay ers, forexam ple, in S ample5,isi ncreas edby ‘n’ to4.87 2m m

outputoftheuntreatedactuatorsample.Thisfollowsthatwhile (the original width is 3mm) for an actuator without any gold

theionimplantationenhancestheworkoutputoftheactuators,it implantation.Thenewcross-sectionconsistingofasinglemate-

stiffensthedevices,limitingactuationrangeforagivenvoltage. rialPVDFispresentedinFig.13.Similarly,fortheactuator(Sample
80 G.Alicietal./SensorsandActuatorsB157 (2011) 72–84

Fig.10. Theactuator(Sample5)configurationunderatiploadat0V(left)and0.5V(right).Thetop2imagesareforthegoldimplantedsample,thebottomimagesare

non-implanted.
5) with the gold implantation, n¯ =E /E =675.214, where 5.4. Electrochemicalcharacterizationofgoldimplantedsamples
Au PVDF

E =79GPa.ThewidthoftheimplantedAulayersisincreasedby‘n¯’
Au

to2025.6mm(theoriginalwidthis3mm)forthegoldimplanted Autolab PGSTAT12 Potentiostat/Galvanostat equipped with A

actuators. The new cross section consisting of a single material, General Purpose Electrochemical System (GPES) software was

whichisthePVDF,ispresentedintherightmostcross-sectionin employedtoformatwoelectrodecyclicvoltammetry,whichisa

Fig.13. commonlyusedmeasurementtocharacterizetheelectrochemical

For the bare actuator (Sample 5) with the dimensions of behaviour of the actuators. It is normally measured in a three-
t =30 (cid:2)m, and t =110(cid:2)m and b¯ = b×n= 3× 1.624mm, the electrode set up. H owever, i n t hi s study, it is generat ed in a two
1 2 1
area m ome ntof iner tiai sca lculatedas I =9 0 4.0 2×10 −6m m4. electrode setup to look at th e ele ctroch em ic al behavio ur of the
bare
For the actuator with the gold implantation, the other dimen- actuators—thecounterandreferenceelectrodesareshort-circuited
sion s a re the same exc ept t =29.9(cid:2)m a nd t = 100nm andconnected toonee lect rodeofthe actuators ,the workingelec-
1 gold
(assumed value). The area moment of inertia is calculated as trodetotheotherelectrodeoftheactuators,likeshowninFig.3.
I =382 7.6×10 −6mm 4.I >I ,w hi chexpla in sthestiffen ing TheC Vs cans ofsom eofthea ct uato rsamples inTa ble1ar es how n
egfofledct ofthe go ldimplanta tgiooldn. Tbhairsei ncreas edaream om entiner- inFi g.1 4.The s cansw er ege nerated under−0 .8 Vto+ 0 .8V witha

tiawillcreatesmallerbendingstress,whichfurtherdemonstrates rangeofscanrates(10,20,40,60,100mV/s).

thestiffeningeffectofthegoldimplantation,asexpressedbythe WhentheshapeofaCVscanisclosetoarectangle,itindicatesa

bendingstress(cid:4)calculatedfrom: capacitive behaviour. A rectangular-shaped CV is a typical char-

acteristic of an ideal double-layer capacitor, which our actuator

Mc structureandoperationprincipleresemble.AsshowninFig.14,the
(cid:4)= (3)
I ion-implantedactuatorsbehavelikeacapacitoroveralargevolt-

agerange,especiallywhenthescanratesarelow.Thisindicates

where the area moment of inertia I is for the new cross section, thattheactuatorscapacitivelystoremostoftheelectricalenergy.

‘c’isthehalfofthetotalthickness,andMistheinternalbending When the scan rates are higher >60mV/s, they show a resistive

momentgeneratedduetotheelectrochemicalprocess.Becausethe behaviour—mostoftheelectricalenergyisdissipatedduetothe

activelayersofbothactuatorsarethesame,theyareexpectedto electrolyteresistance,andthemovementofionsinandoutofthe

generatethesameinternalbendingmomentunderthesameinput polymerlayers.Eachscanwasrepeatedatleast10times.Thegold

voltage. implantedactuatorshaveshownrepeatablescans,almostnodevi-
G.Alicietal./SensorsandActuatorsB157 (2011) 72–84 81

Fig.11. Configurationsofanactuatorsample(Sample4)withthedimensionsof12mm×3.8mm×0.17mmtoevaluatetheeffectofion-implantationconditionsonthe

mechanicalworkoutput.Thetoptwoimagesareforthesamesamplewithnoexpositiontotheion-implantationconditions.Thebottomtwoareforthesamesample

exposedtotheion-implantationconditionandthenre-wettedwiththeelectrolytebeforetesting.Whiletheleftimagesareinitialconfigurations,therightimagesarefor

theactivatedconfigurations.

Fig.12. DisplacementresponseofSample5with(leftplot)andwithout(rightplot)goldimplantationunder0.5V.

Fig.13. Compositestructureofapolymeractuatoranditsequivalentcrosssectionusingequivalentwidthtechnique.Themechanicalpropertiesoftheequivalentsection

withasinglematerialareequivalenttothatoftheoriginalmulti-materialstructure.