Development of a distance-independent wireless passive RF resonator sensor and a new telemetric measurement technique for wireless strain monitoring

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

Development

of

a

distance-independent

wireless

passive

RF

resonator

sensor

and

a

new

telemetric

measurement

technique

for

wireless

strain

monitoring

Akbar

Alipour

a

_,

_Emre

_Unal

a

_,

_Sayim

_Gokyar

a

_,

_Hilmi

_Volkan

_Demir

a,b,∗

a_{DepartmentofElectricalandElectronicsEngineering,DepartmentofPhysics,andUNAM-NationalNanotechnologyResearchCenterandInstituteof}

MaterialsScienceandTechnology,BilkentUniversity,Bilkent,06800,Ankara,Turkey

b_{LUMINOUS!CenterofExcellenceforSemiconductorLightingandDisplays,SchoolofElectricalandElectronicEngineering,SchoolofMathematicaland}

PhysicalSciences,NanyangTechnologicalUniversity,Singapore639798,Singapore

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received14September2016

Receivedinrevisedform

15December2016

Accepted9January2017

Availableonline11January2017

Keywords: Strainsensor PassiveRFresonator Inductivecoupling Wirelessmonitoring

a

b

s

t

r

a

c

t

WeproposedanddevelopedanovelwirelesspassiveRFresonatorschemethatenablestelemetricstrain sensingavoidingtheneedforcalibrationatdifferentinterrogationdistances.Thespecificarchitectureof theproposedstructureallowsforstronginductivecouplingand,thus,ahigherwirelesssignal-to-noise ratio.Here,inoperation,thefrequencyscanofthesensorimpedancewasusedtomeasuresimultaneously boththeimpedanceamplitudeandresonancefrequency.Usingthiswirelesssensor,wefurther intro-ducedanewtelemetricmonitoringmodalitythatemploysfullelectricalcharacteristicsofthesystemto achievecorrectstrainextractionatanyinterrogationdistance.Inprinciple,anydeformationofthesensor structureresultsintheresonancefrequencyshifttotrackstrain.However,changingoftheinterrogation distancealsovariestheinductivecouplingbetweenthesensoranditspick-upantennaatthe interro-gationdistance.Therefore,atvaryinginterrogationdistances,itisnotpossibletoattributeanindividual resonancefrequencyvaluesolelytoanindividualstrainlevel,consequentlyresultingindiscrepancies inthestrainextractioniftheinterrogationdistanceisnotkeptfixedordistance-specificcalibrationis notused.Inthiswork,weshowedthatbyusingboththeproposedpassivesensorstructureandwireless measurementtechnique,straincanbesuccessfullyextractedindependentoftheinterrogationdistance forthefirsttime.Theexperimentalresultsindicatehighsensitivityandlinearityfortheproposed sys-tem.Thesefindingsmayopenupnewpossibilitiesinapplicationswithvaryinginterrogationdistance formobilewirelesssensing.

©2017ElsevierB.V.Allrightsreserved.

1. Introduction

Precisestrainmonitoringisessentialinnumerousstrainsensing applicationsincludingimplants, foodqualitycontrol,and struc-turalhealthmonitoring[1–4].Overthepastfewdecades,enormous progresshasbeenachievedinwirelesspassiveandimplantable strainmonitoringtechniques.Wirelessreadability,lowcost,power source-freeoperation,andlowperturbationfromthesurroundings areamongthemainadvantagesofthewirelesspassiveapproaches

∗ Correspondingauthorat:DepartmentofElectricalandElectronicsEngineering,

DepartmentofPhysics,andUNAM-NationalNanotechnologyResearchCenterand

InstituteofMaterialsScienceandTechnology,BilkentUniversity,Bilkent,06800,

Ankara,Turkey.

E-mailaddress:volkan@bilkent.edu.tr(H.V.Demir).

overcommonactiveones[5,6].Intelemetricstrainsensing,the abilitytowirelesslytrackthepassiveresonanceisofcritical impor-tance[7].Thefundamentaloperatingprincipleofwirelesssensing resultsfrom inductive couplingbetween thepassive radio fre-quency(RF)resonatoranditsreaderantenna[8].Theconceptof suchpassiveRFresonatorsusingdifferentelectromagnetic struc-tures has beenproposed and demonstrated for the purposeof wirelessstrainmonitoring.Sincetheelectromagneticresonance frequencyofaresonatorissensitivetoitsphysicaldimensions,any deformationinitsgeometryresultsinresonancefrequencychange [9].

The most recently studied architecture for wireless passive strain monitoring includes micro-strip patch antennas, radio frequency identification(RFID),andmetamaterial-basedRF res-onators.Thefeasibilityofusingacircularmicro-strippatchantenna (CMPA)wasreportedbyDalirietal.[10].Usingtheoretical cal-http://dx.doi.org/10.1016/j.sna.2017.01.010

culationsandnumericalsimulations,theauthorsshowedalinear relationshipbetweenthestrainandtheresonancefrequency of theproposedsystem.AnychangeinthedimensionoftheCMPA structurechangestheimpedanceofthesystem,whichleadstothe shiftinresonancefrequency.AcomplementaryworkonCMPAs wasreportedin [11].It wasshownthat CMPAscanbeusedas apassivesensorforwirelessstrainmonitoringincivilstructures andaerospace.Theeffectoftheconductivityofthehostmaterial onthewirelessmeasurementefficiencywasalsoinvestigated.It wasdemonstratedthatstraincanbemonitoredwirelesslyinany desireddirectionusingalinearlypolarizedhornantenna.However, duetothesensorsignalinterferencewithbackgroundreflections, onlyalimitedinterrogationdistancewasachieved.

RFID structures have also been investigated for wireless passive strain measurements. The RFID-based wireless sen-sors are fed by interrogation electromagnetic wave radiated from a reader antenna. Xiaohua et al. [12] developed a passive wireless antenna sensor for strain and crack detec-tion. The antenna signal modulation was used to isolate the backscattered signal from the undesired environmental reflec-tions.

Fromthepreviousworksofourgroup,Meliketal.[13]proposed ahighlysensitivemetamaterial-basedstrainmonitoringtechnique totrackbonefracturehealing.Thissensorconsistsofdouble comb-shapedmultiplesplitringresonators(SRRs).Thecompactnested architecture of this SRR design with multiple gaps provides a lower operating frequency and higher sensitivity. In operation, thewireless passive metamaterial strain sensor is mountedon animplantablefracturefixationhardwaretomonitorandaccess theprogressionofbonefracturehealing.Whenanexternalload isappliedtothehardware,thestrainisrecordedremotelyusing thecoaxialprobelocatedintheproximityofthesensor.This sen-sorcanmeasurestrainbasedonthechangeinthecapacitanceof nestedSRRthatresultsinthetransmissionspectrumshiftusing distance-specificcalibration.Theinvivoresultsshowedthatthe metamaterial-basedpassivestrainsensorprovidestheabilityto determinestatisticallyimportantdifferencebetweenthefracture healingandnon-healinggroups[14].However,thesesensorsare resonantstructureswithoutagroundplane,sotheyexhibit rela-tivelylowsignal-to-noise(SNR)ratioandqualityfactor.Ourgroup alsodevelopedanothermetamaterial-basedremotestrainsensor systemusinganarrayoftheSSRspatternedoveraflexibleKapton goldcladsubstrate[15].Thisapproachwaslimitedbytheweak reflectedsignalfromthesensortothereaderantenna.

In most of these studies, misalignment and variation in interrogationdistancecanchangethetransmissionspectrum, con-sequentlycontributing someinaccuracy tostrainextraction. To overcomethis problem, in this paper, we propose a distance-independentpassive RFresonator sensor and a newtelemetric measurementmethodologyforitswirelessstrainmonitoring.Here thecomb-shapesplitringsarepatternedonbothsidesofthe flexi-bledielectrictoformadistributedcapacitanceandinductancetank circuit.Thecombinationofthecapacitanceandinductancecreates anLCresonatortooperateatacertainfrequency.Thecomb-shape splitringsarealignedby90◦rotationwithrespecttoeachotheron bothsidesofthedielectric.Thisspecificarchitectureofthesensor allowsforthepossibleexcitationofbothofthelayersbythesame incomingelectromagneticwave.Thus,bothofthelayerscontribute totheresonancefrequencyandthequalityfactorofthestructure. Wealsointroduceanewmeasurementtechniqueofwirelessstrain sensingwhich,incombinationwiththeproposedsensor,straincan bemonitoredindependentlyoftheinterrogationdistance.Based onthisapproach,possiblediscrepanciesinducedbythecoupling coefficientvariationsareeliminatedinthestrainextraction. Exper-imentalresultsindicategreatlinearityandsensitivity.

Fig.1. Schematicoftheproposedsystem.Thesensorisinductivelycoupledtothe

pick-upreaderantenna.Toobtaintheexactvalueoftheresonancefrequency(f0)

ofthesensor,alltheresistance,capacitanceandinductancevaluesinducedbythe

cable,theconnector,andtheantennaareincluded.

2. Methods 2.1. Theory

Inoursystem,theprincipleofwirelessstrainmonitoringbased oninductivecouplingisillustratedin Fig.1.Weinclude allthe electrical components that come from the system (e.g., cable, connector,andantenna)todeterminetheaccurateresonance fre-quency of the sensor. All the impedance parameters of these componentswere calculated, and theireffects weretaken into account in theresonance frequency extraction. Zeq1, Zeq2,Zeq3, andZeq4aretheimpedancesthatareseenattheinputofcable, cable-connector, cable-connector-antenna,and cable-connector-antenna-sensor,respectively.

The inductive coupling between the sensor and the reader antennaaffectstheimpedanceattheinputoftheantenna.Since thesensorimpedanceZsreachesaminimumatitsresonance fre-quency(f0),thisprovidesmaximumimpedance,whichappearsat theinputoftheantennaatf0.Byscanningthevariationonthe equivalentimpedance,monitoredbythereaderantenna,the reso-nancefrequencycanbeextracted.Assuminganinductivecoupling betweenthesensorandreaderantenna,thesensorimpedanceZs, measuredbythereaderantennaisgivenby

V={(R1+jwL1)+



(Rc+_jwC1 c



(Ra+jwLa−jwMI_I 3 2 )]}I1 (1) whereMisthemutualcouplingcoefficientbetweenthesensorand thepick-upantenna,

(Rs+jwLs+_jwC1

Rs+jwLs+_jwC1 s=Zs (3) From(1)and(3); Zeq=_IV 1 = (R1+jwL1)+



Rc+_jwC1 c

 

Ra+jwLa+w 2_M2 Zs



(4) Zs= w2_M2

_R 1+jwL1+Rc+jwC1c−Zeq

(Zeq−R1−jwL1)

Rc+jwC1c+Ra+jwLa

−

Rc+jwC1c

(Ra+jwLa) (5) Theresonancefrequencyofthesensorisdeterminedby scan-ningthefrequencyresponseofthesensorimpedance,Zs.When thesensorstaysintheinterrogationregionofthepick-upreader antenna,anetresonancecomesalongattheoperatingfrequency ofthesensor.Thevariationofthecouplingvaluewouldleadtoa changeintheimpedanceZsand,consequently,theshiftofthe sys-temresonancefrequency.Anystrainvariationsonthesensorcan befaithfullydetectedusingtheassociatedelectricalcharacteristics (f0and|Zs|)changes,whicharepickedupbythereaderantenna. Each(f0and|Zs|)setpointisdevotedtoanindividualstrainvalue.

Thus,usingbothfrequencyandimpedanceinformationtogether straincanbecalculatedindependentofthecouplingvalue.

Here,notethattheproposedsensorhastwomainelements:(1) thesplitringcapacitorsbetweenthefingers,whichserveasthe sensingcomponent,and(2)thesplitringsinductor,whichcollects electromagneticenergyfromantenna.

2.2. Modeling

Thedesignofthestrainsensorreliesontheresonance character-istics,especiallytheresonancefrequency,f0andthequality-factor, Q,whicharefunctionsofthesensorcapacitance(Cs),inductance (Ls),andresistance(Rs).Asimplegeometryoftheproposedstrain sensorwithsevengeometricalvariablesisshowninFig.2a.The valuesofCs,Ls,andRsaredeterminedbythesedesignparameters: numberoffingersn,gapwidths,fingersspacingt,sensorlengthk, linewidthw,shortfingerslengthl1,andlongfigureslengthl2.

ThesensorismodeledtooperateasanelectricalLCresonant circuit.Fig.2bshowsa3-dimensional(3D)sketchofourproposed strain sensor. The top/frontside of the device includesa layer ofcomb-shapedSRRsmetallization.Thebottom/backsideofthe deviceisthe90◦rotatedversionofthetop/frontsidethatis pat-ternedontheotherfaceofthedielectricsubstrate.Thisarchitecture bringsus two mainadvantages: first,it allows electromagnetic wavetopenetratethroughtheupperlayerandbottomone. Accord-ingly,withproperpolarization,bothofthelayerscanbeexcited bythe sameincomingelectromagneticwave. Thus, both layers contributetotheoperatingfrequencyandQ-factorofthedevice. Second,duetosymmetricstructure ofthesensor, straincanbe measuredinanydirection.

The sensor has inductance Ls and capacitance Cs. Cs is thecombination of thesplit (metal-air-metal) and sandwiched (metal-substrate-metal)capacitorsthatbuildupfromthesensor architecture.TheintegrationofcapacitanceCswithinductanceLs formsaresonancecircuit.Thecorrespondingresonancefrequency issimply

f0= 1 2



LsCs

(6) TheQ-factorofthesensoraffectstheprecisewireless identi-ficationofthesensorresonancefrequency.HighQ-factorresults instronginductivecouplingandthelargerinterrogationdistance. Toincreasethepowerreflectedbythesensor,theQ-factorofthe sensorisincreasedbyimprovingthestructuralparametersofthe sensor.

Themetalthicknessisoneofthemainparametersthatcan con-troltheQ-factor,consequentlythereflectedpower.Tomaximize thereflectedpowerbacktotheantenna,themetalthicknessis settobetwotimesthickerthantheelectricalskindepth[16].The well-knownequationtocalculatetheskindepthisgivenbellow: ı=



2 2f0␮0␮R

(7) where␴istheconductivity(m−1),␮0thepermeabilityconstant (4␲×10−7H/m),and␮Rtherelativepermeability.Forthetarget operatingfrequency ofourproposedstrainsensor(∼600MHz), theskindepthofgoldiscalculatedtobe∼3␮m.Theinvestigations ofthegoldskineffectontheQ-factorattheoperatingfrequency shownthattheQ-factorisincreasedbytheincreasingmetal thick-ness,andbecomesfixedafter∼6␮mmetalthickness.Thus,inthis study,thegoldthicknessof6␮mwasusedinthesensorfabrication. Duringtheoperation,whenthesensorissubjectedtostrain_⑀ ineachdirection,thegeometricaldeformationonthesensorleads tochangeinthesplitringcapacitance(betweenfingers)values, which resultsin theresonance frequencyshift.Thevariationof capacitancevaluescontributetotheresonancefrequencyshiftand, consequentlystrainsensing.

2.3. Devicedesignandfabrication

Ourproposedarchitectureisbasedonamultilayerlaminated structure consisting of two comb-shaped SRRs, which are pat-ternedonbothsidesofthedielectricsubstrate,with90◦alignment withrespecttoeachother(Fig.2).Wefabricatedourstrainsensor using standard microfabricationtechnique. Thefabrication pro-cessesincludethefollowingsteps:(1)Thermalevaporationwas usedtodeposit a thinlayerof Auonbothsides oftheflexible dielectricsubstrate(Kapton®polyimidefilms,DuPontTM_).(2) Con-ventionallithographyandwet-etchingwereusedtopatternthe Aulayersofcomb-shapedSRRsonbothsidesof25-␮m-thick Kap-ton.Finally,a flexiblesensorwiththethicknessof∼37␮mwas obtained.AphotoofthefabricatedsensorontheflexibleKapton filmandthecorrespondingopticalimagearedisplayedinFig.2c, alongwithallphysicaldimensions.Duetotheflexibilityand ultra-thinstructureofthesensor,thesensorhasthecapabilitytoconform tovariousnon-planarsurfaces.

2.4. Experimentalsetupandmeasurements

Ourexperimentalsetupandcorrespondingblockdiagram uti-lized for the telemetric strain monitoring is shown in Fig. 3. Thefabricatedsensorissetonthehoststructuretocharacterize the sensor performance in strain measuring. The host struc-ture is a homo-polymer rod; namely, Delrin® withdimensions 1.2cm×1.2cm×10.0cm and Young’smodulus of 2.4 Gpa.The axialtensileforceof2kNwasappliedtothesysteminfour load-ingstepswith500Nloadincrementperstepusingatensiletester machine(INSTRONTM5542).Toverifytheuniformstraininduction intheareawherethesensorismounted,astandardresistivestrain gauge(TokyoSokkiKenkujoCo.,Ltd.)issettothehoststructure asshowninFig.3.Thestraingaugeispositionedontheotherside oftheDelrin,whichdoesnotaffectthesensorperformance.The teststartswith0␮␧initialloadandendsataround21,300␮␧.The impedanceofthestrainsensorismeasuredatthevarious interro-gationdistancesusingthepick-upcoil(with8mmindiameter)as thereaderantennaundereachloadingstep.

Thereaderantennaismountedona3Dstagemachine(VELMEX, Inc.).Thestageautomaticallyinterrogatestheregionstepbystep. We used a network analyzer(Agilent FieldFoxN9915A) as the signalacquisitioninstrumentthatwasarrangedintheoperating

Fig.2. (a).Simplesensorgeometry,thetoplayer(left)andthebottomlayer(right).Thebottomlayeris90◦_{rotatedwithrespecttothetoplayer.(b).3Dschematicsofthe}

proposedstrainsensor(notdrawntoscale).Red(onbottom/backlayer)andblue(ontop/frontlayer)dotsindicatedonthesensorgeometrycoincidewiththesamepoints

onthe3Dschematics,whichareshownwiththesamecolors.(c).Opticalimageofthefabricatedflexiblestrainsensor.(Forinterpretationofthereferencestocolourinthis

figurelegend,thereaderisreferredtothewebversionofthisarticle.)

Fig.3.Experimentalsetup.Thestrainsensorissetonthehomo-polymerrod(Delrin),apick-upantennaisusedtoreadthesensorsignal,acommercialstraingaugeis

placedattheoppositesidesoftherodtoverifythestrainvalue,anda3Dstagemachineisusedtoscantheinterrogationdistanceprecisely.Inthetoprightmostinsetwe

showablockdiagramofthepassivewirelessstrainsensingsetup.Networkanalyzerwassettoscaninthefrequencyrangefrom540to700MHzwith1601datapoints.Ata

specificinterrogationdistance,theaxialtensileforceatvariousloadswasappliedtothemechanicalsystemandtheRFdatawasrecordedusingthenetworkanalyzer.This

Fig.4.Bychangingtheinductivecoupling(byvaryingtheinterrogationdistance)

betweenthereaderantennaandsensor,theoperatingfrequencyofthesystemand

impedanceofthesystem(Zeq4)werechanged.Thestrongcoupling,medium

cou-pling,andweakcouplingcorrespondtothedistancesof0.5mm,1.3mmand2.4mm

farfromthepick-upantenna,respectively.

frequencyrangewith1601datapoints.Theexperimentwas con-ductedatteninterrogationdistancesfrom0.5to3.0mm.Thesensor withfollowingdesignparameterswasusedinthestrainmeasuring experiment:n=32,s=100␮m,t=100␮m,k=6.3mm,w=100␮m, l1=2.1mm,andl2=4.1mm.

3. Resultsanddiscussion

Beforestudying thestrain sensor performance, theeffect of interrogationdistancewasinvestigated.Asithasbeenexpected, changingtheinterrogationdistancethelevelofinductivecoupling betweenthesensorandthepick-upantennavaries,asshownin Fig.4.Duetothecouplingcoefficientvariation,thesystem oper-atingfrequency changes at variousinterrogationdistances. The variationintheoperatingfrequencycanbelargerthantheshifts causedbytheappliedload.Therefore,itbecomes impossibleto attributeanindividualresonancefrequencyvaluetoanindividual strainvalue,consequentlyresultingindiscrepanciesinthestrain extraction,unlessspecificcalibrationcurves areusedatspecific fixedinterrogationdistances.

Tosolvethisproblem,thesensorwastestedunderdifferentload valuesandvaryinginterrogationdistancestofindrelationbetween thestrain,|Zs|,andf0.Hereboththefrequencyand|Zs|information

areusedtoextracttheindividualstrainvalue.Fig.5ashowsthe resonancefrequencyvariationbychangingtheinductivecoupling coefficient(bychangingtheinterrogationdistance)undervarious strainvalues.Thisfigurealsoshowsaclearresonancefrequency increasewiththeincreasingcouplingcoefficient.

Toinvestigatethetensilestrainresponseofthesensorasthe strainincreases,theresonancefrequencyismeasuredatdifferent strainvalues.Therelationbetweentheresonancefrequencyshift ofthesensorandstrainisdisplayedinFig.5.Thisplotconfirms thatthemeasuredresonancefrequencyofthesensoratvarious loadvaluesexhibitsacharacteristiclinearresponse.Themeasured resonancefrequencyofthesensorgraduallyincreasesasthestrain increases.Theresonancefrequencyofthesensorat0␮␧isaround 591MHz,and thenitreachesapproximatelyto597MHz,asthe strainincreasesup to10,500␮␧.The experimentalresultsshow thatthesensorachievessuccessfulstrainperformancewiththe highsensitivity.Duetotheproximityofthesensorandpick-up antenna,mutualcouplingismoredominant,whichdoesnotallow forthesignificantresonancefrequencychangebythestrain vari-ation.Strongcouplingbetweenthesensorandtheantennaatthis

Fig.5.(a)Resonancefrequencyvariationunderdifferentstrainlevelsatdifferent

interrogationdistances.Linear fitsatvariousstrainvaluesintersectata

com-moninterpolatedreferencepoint.Duetocloseproximityofthesensorandreader

antenna(dominantcouplingeffect),measuredsignalstendtointersectatthispoint

atdifferentstrainlevels.(b).Thecalculatedslopeofanymeasuredpointwiththe

referencepointcorrespondstoanindividualstrainvalue(measuredbystraingauge).

pointallowsalllinearfitsatvariousstrainlevels(Fig.5a)tendtoa singleintersectingpointwhichwecallReferencePointhere.Fig.5b showstheslopevaluesoftheselinearfitsatvariousstrainlevels, measuredbythestraingauge.Anymeasuredsetpoint(f0and|Zs|)

createsaslopebytheReferencePoint.Thisslopegivestheextracted strainvalue.

Afterthestraincharacterizationandanalyzingtherelationship betweenthestrainandsetpoint(f0and|Zs|),similartensiletestis

conducedtoinvestigatethesensorperformanceandverification. Thesensorwassubjectedtoarandomsetofstrainlevelsand inter-rogationdistancevalues.TheexperimentalerroranalysisinFig.6a exhibitedexcellentsensorperformance.Thefittedcurveinthis fig-urereportsamaximumerrorofonlylessthat0.5%correspondingto nonlinearityerrorattheearlystagesofthestrainimplementation. Thiscouldbelargelyexplainedbythedominanteffectofthe uncor-relatedmeasuredstrainbetweenthestraingaugeandoursensor. Also,somepartofthiserrorstemsfromthestraingaugevoltage measurementerror.Byincreasingthestrain,calculatederrorwas minimized,whichcouldbeexplainedbythedominanteffectof theappliedstrain.Theresonancefrequencyand|Zs|ofthesensor

canbeinterrogatedbythepick-upantennaintherangeof3mm. Beyondthisdistance,duetotheverylowcouplingefficiency,the readerantennaisnotbeabletosensethesignalreflectedbackfrom thesensor.Therefore,thisdistanceisthelargestoperatingrange foroursystem.

Toinvestigatetheeffectofthedirectionoftheappliedloadon wireless strainmeasurement,theexperimentwasconductedat twodifferentsensororientations.Thestrainwasappliedinboth xandydirections.Duetosymmetricstructureofthesensor,the experimentsresultedinthesamebehavior.

The stability and reliability of the sensor were studied by multicyclestrainperformingonthesensor.Thesensorwasfirst subjectedtozerostrainloadingandthenfollowedbyextension loadingof5400␮␧.AsshowninFig.6b,thesestepswererepeated

Fig.6. (a)Nonlinearityerrorpercentage.Athighstrainvalueserrorwasminimized.

(b).Themulticycleoperationofthesensorbetweenthetwostrainstates(achieved

bystraingauge)showedthattheresponseofthesensorisstableandreliable.

20 times.The plot of therepeatability showedthat the sensor showedexcellent stabilityandreliability, withonlyslightdrifts (<0.01%)observedbetweenthecycles.

4. Conclusion

The present study reports a new telemetric measurement techniqueandapassiveRFsensorfordistance-independent wire-lessstrainmonitoring.Distance-independentstrainmonitoringis achievedbymeasuringtheimpedanceofthesensorinadditionto itsresonance.Inourproposedtelemetricmonitoring,weinclude theimpedanceparametersofallelectricalcomponentstomeasure preciselytheresonancefrequencyofthesensor.Theproposed sen-sorconsistsofanultra-thinandmultilayer(metal-dielectric-metal) structurewithcomb-shapedSRRsthatpatternedonbothsidesof theflexibledielectric.Topandbottomlayersofthesensorare90◦ rotatedwithrespecttoeachother.Thespecificarchitectureofthis designallowsforthebothmetallayerstocontributeinthesensor electricalcharacteristics(f0,Q-factor,andZs).Thesensoristested undervariousstrainvaluesandinterrogationdistances.Alinear relationbetweenthesensorf0and|Zs|isobtainedatdifferent lev-elsofstrain.Theslopeofthisresponsegivesthestrain.Thisstrong linearityresultsinexcellentstrainsensingperformance.By tak-ingtheadvantagesofthistelemetricmeasurementtechniqueand sensorstructure,distance-independentstrainextractionhasbeen successfullydemonstratedinthisstudy.Theresultspresentthat thesensorhasexcellentaccuracywithlessthan0.5%error,and alsoshowedgreatstabilityandreliability.

Ourfuturestudywillfocusontheapplicationareasofthisstrain sensor.The proposedsensorhasgreat potentialfor performing faithfulwireless strainmeasurementinvivo whilethepatient’s bodypartsmaybemoving.Thiscanbeusedtoregisterstrain vari-ationpreciselyduringbonefracturehealingevenwhenthepatient

fracturesitecannotbepreciselylocatedexternally.Furthermore, futureresearchneedstobeconductedtoimprovesensing perfor-manceatlongerinterrogationdistances.Thesefindingsgivenfor telemetricstrainmonitoringmaybeapromisingwirelessstrain measurementmethodinthesystemswithvaryinginterrogation distance.Studiesperformedinpassivewirelessmonitoringshowed thatchangingtheinterrogationdistancemayresultininaccurate measurement.Theproposedapproachhereresolvestheproblem ofinterrogationdistancevariation.

References

[1]P.Zhao,N.Deng,X.Li,C.Ren,Z.Wang,Developmentofhighly-sensitiveand

ultra-thinsiliconstresssensorchipsforwearablebiomedicalapplications,

Sens.ActuatorsAPhys.216(2014)158–166,http://dx.doi.org/10.1016/j.sna.

2014.05.018.

[2]P.J.A,S.F.Pichorim,Biotelemetricpassivesensorinjectedwithintendonfor

strainandelasticitymeasurement,IEEETrans.Biomed.Eng.53(2006)

921–925,http://dx.doi.org/10.1109/TBME.22006.872157.

[3]E.L.Tan,W.N.Ng,R.Shao,B.D.Pereles,K.G.Ong,Awireless,passivesensorfor

quantifyingpackagedfoodquality,Sensors7(2007)1747–1756,http://dx.

doi.org/10.3390/s7091747.

[4]JohnC.Butler,JAnthonyJ.Vigliotti,FredW.Verdi,ShawnMWalsh,Wireless,

passive,resonant-circuit,inductivelycoupled,inductivestrainsensor,Sensor

Actuat.A:Phys.102(2002)61–66,

http://dx.doi.org/10.1016/S0924-4247(02)00342-4.

[5]P.Broutas,H.Contopanagos,E.D.Kyriakis-Bitzaros,D.Tsoukalas,S.

Chatzandroulis,AlowpowerRFharvesterforasmartpassivesensortagwith

integratedantenna,SensorActuat.A:Phys.176(2012)34–45,http://dx.doi.

org/10.1016/j.sna.2011.12.053.

[6]Y.Jia,K.Sun,F.J.Agosto,M.T.Qui ˜nones,Designandcharacterizationofa

passivewirelessstrainsensor,Meas.Sci.Technol.17(2006)2869,http://dx.

doi.org/10.1088/0957-0233/17/11/002.

[7]Hee-JoLee,Jong-GwanYook,Recentresearchtrendsofradio-frequency

biosensorsforbiomoleculardetection,Biosens.Bioelectron.61(2014)

448–459,http://dx.doi.org/10.1016/j.bios.2014.05.025.

[8]WilliamB.SpillmanJr.,S.Durkee,Noncontactpower/interrogationsystemfor

smartstructures,SPIE2191(1994)362–372,http://dx.doi.org/10.1117/12.

173966.

[9]X.Yi,C.Cho,J.Cooper,Y.Wang,M.M.Tentzeris,R.T.Leon,Passivewireless

antennasensorforstrainandcracksensing—electromagneticmodeling,

simulation,andtesting,SmartMater.Struct.22(2013)085009,http://dx.doi.

org/10.1088/0964-1726/22/8/085009.

[10]A.Daliri,A.Galehdar,W.S.T.Rowe,K.Ghorbani,S.John,Utilisingmicrostrip

patchantennastrainsensorsforstructuralhealthmonitoring,Intell.Mater.

Syst.Struct.23(2011)169–182,http://dx.doi.org/10.1177/

1045389x11432655.

[11]A.Daliri,A.Galehdar,S.John,C.H.Wang,W.S.T.Rowe,K.Ghorbani,Wireless

strainmeasurementusingcircularmicrostrippatchantennas,SensorActuat.

APhys.184(2012)86–92,http://dx.doi.org/10.1088/0964-1726/22/8/085009.

[12]X.Yi,T.Wu,Y.Wang,R.T.Leon,M.M.Tentzeris,G.Lantz,Passivewireless

smart-skinsensorusingRFID-basedfoldedpatchantennas,Int.J.SmartNano

Mater.2(2011)22–38,http://dx.doi.org/10.1080/19475411.2010.545450.

[13]E.U.R.Melik,N.K.Perkgoz,B.Santoni,D.Kamstock,C.Puttlitz,H.V.Demir,

Nestedmetamaterialsforwirelessstrainsensing,IEEEJ.Sel.Top.Quantum

Electron.16(2010)450–458,http://dx.doi.org/10.1109/JSTQE.2009.2033391.

[14]K.C.McGilvray,E.Unal,K.L.Troyer,B.G.Santoni,R.H.Palmer,J.T.Easley,H.V.

Demir,C.M.Puttlitz,Implantablemicroelectromechanicalsensorsfor

diagnosticmonitoringandpost-surgicalpredictionofbonefracturehealing,J.

Orthop.Res.33(2015)1439–1446,http://dx.doi.org/10.1002/jor.22918.

[15]I.Jones,L.Ricciardi1,L.Hall,H.Hansen,V.Varadan,C.Bertram,S.Maddocks,

S.Enderling,D.Saint,S.Al-Saraw,WirelessRFcommunicationinbiomedical

applications,SmartMater.Struct.17(2008)015050,http://dx.doi.org/10.

1063/1.3250175.

[16]D.A.Sanz,C.Mitrosbaras,E.A.Unigarro,F.Segura-Quijanod,Passive

resonatorsforwirelesspassivesensorreadoutenhancement,Appl.Phys.Lett.

103(2013)133502,http://dx.doi.org/10.1088/0964-1726/17/1/015050.

Biographies

AkbarAlipourreceivedtheB.Sc.inElectricalandElectronicsEngineeringfrom

UrmiaUniversity,Urmia,Iranin2007.HecompletedhisMasterdegreein

bio-medicalscienceatMETUandGaziUniversity,Ankara,Turkeyin2011.Since2012,

heisworkingtowardsthePh.D.degreewiththedepartmentofElectricaland

Elec-tronicsEngineering,BilkentUniversity.Hiscurrentresearchinterestsincludethe

implantsensorsforbiomedicalapplications.

EmreUnalreceivedhisB.S.degreeinelectricalandelectronicsengineeringfrom

HacettepeUniversity,Ankara,Turkey,in2005.Heisafull-timeResearchEngineer

andNanotechnology,BilkentUniversity,Ankara,whereheisworkingonthe

devel-opmentofmicrowaveandoptoelectronicdevices.

SayimGokyarreceivedhisB.S.degreeinelectricalandelectronicsengineeringfrom

FatihUniversity,Istanbul,Turkey,in2009.HecompletedhisM.S.degreeinelectrical

andelectronicsengineeringfromBilkentUniversity,Ankara,Turkey,in2011.He

iscurrentlyaPhDcandidateatelectricalandelectronicsengineeringdepartment,

BilkentUniversity,Ankara,Turkey.Hisresearchareaofinterestincludeswireless

sensors.

HilmiVolkanDemir(M’04-SM’11)receivedtheB.S.degreeinelectricaland

elec-tronicsengineeringfromBilkentUniversity,Ankara,Turkey,in1998andtheM.S.

andPh.D.degreesinelectricalengineeringfromStanfordUniversity,Stanford,CA,

in2000and2004,respectively.InSeptember2004,hejoinedBilkentUniversity,

whereheiscurrentlyaprofessorwithjointappointmentsattheDepartmentof

Elec-tricalandElectronicsEngineeringandtheDepartmentofPhysicsandisalsowith

theInstituteofMaterialsScienceandNanotechnology.Concurrently,heisafellow

ofNationalResearchFoundationinSingaporeandaprofessorofNanyang

Tech-nologicalUniversity.Hisresearchinterestsincludethedevelopmentofinnovative

optoelectronicandRFdevices.Dr.DemirwastherecipientoftheEuropeanUnion

MarieCurieFellowship,theTurkishNationalAcademyofSciencesDistinguished

YoungScientistAward(TUBA-GEBIP),theEuropeanScienceFoundation-European

YoungInvestigatorAward(ESF-EURYI),andNanyangAwardforResearch