Biosensors and Bioelectronics 26 (2011) 18251832

Contents lists available at ScienceDirect

Biosensors and Bioelectronics

journa l homepage: www.e lsev ier .co

Conductive polymer-based sensors for biomedica

Shruti NaDepartment of

a r t i c l

Article history:Received 24 JuReceived in re23 SeptemberAccepted 23 SAvailable onlin

Keywords:Intrinsically coConducting poBiosensorsElectrochemicTactile sensorsThermal senso

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18252. Representative applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1826

2.1. Electrochemical sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18262.2.2.3.

3. Challe4. Concl

Refer

1. Introdu

Polymerthing thatconstitutingcommerciatransportatetc. (CarrahcombinatioNaturally otion of cera

CorresponOntario, Canad

E-mail add(J.T.W. Yeow).

0956-5663/$ doi:10.1016/j.Tactile sensor (articial skin) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1828Thermal sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1829nges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1831usion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1831ences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1831

ction

s form an integral part of our existence and every-surrounds usfrom the basic building blocks of lifeof proteins, nucleic acids, and polysaccharides, to the

l products obtained from automobile, construction andion industries, plastic toys and tools, reading glasses,er, 2010). Most of these materials are composed of an of one ormorematerials to formpolymer composites.ccurring polymer composites such as bone (combina-mic calcium phosphate crystallites and collagen bres

ding author at: E3X-3159, 200 University Avenue West, Waterloo,a N2L 3G1. Tel.: +1 519 888 4567x32152; fax: +1 519 746 4791.resses: jyeow@engmail.uwaterloo.ca, jyeow@uwaterloo.ca

can form either the dense, strong cortical bone or the spongy,shear-resistant cancellous bone), teeth (tooth enamel, dentin, andcementum, all of which contains varying percentages of inorganichydroxyapatite crystals and organic material such as collagenousor non-collagenous proteins) andwood (a combination of celluloseand lignin), possess a unique combination of material proper-ties and a broad spectrum of applications which the constituentsalone cannot offer. In early twentieth century, plastic compositesdeveloped by mixing resin and ller material (wood our, micaor glass) with enhanced strength and stability marked the begin-ning of synthetic polymer composites. Early investigations usedconductive ller materials such as carbon black, graphite bres,metal-coated glass bre, metal particles or metal akes for prepa-ration of composite materials (Bhattacharya, 1986). Hence, a newclass of materials called the conducting polymer composites (CPC)

see front matter 2010 Elsevier B.V. All rights reserved.bios.2010.09.046mbiar, John T.W. Yeow

Systems Design Engineering, University of Waterloo, Waterloo, Ontario, Canada

e i n f o

ly 2010vised form2010eptember 2010e 1 October 2010

nducting polymerlymer composites

al sensors

rs

a b s t r a c t

A class of organic polymers, known as conducting polymers (CPs), has become increasingly popular dueto its unique electrical and optical properties. Material characteristics of CPs are similar to those of somemetals and inorganic semiconductors, while retaining polymer properties such as exibility, and easeof processing and synthesis, generally associated with conventional polymers. Owing to these charac-teristics, research efforts in CPs have gained signicant traction to produce several types of CPs since itsdiscovery four decades ago. CPs are often categorised into different types based on the type of electriccharges (e.g., delocalized pi electrons, ions, or conductive nanomaterials) responsible for conduction.Several CPs are known to interact with biological samples while maintaining good biocompatibility andhence, they qualify as interesting candidates for use in a numerous biological and medical applications.In this paper, we focus on CP-based sensor elements and the state-of-art of CP-based sensing devicesthat have potential applications as tools in clinical diagnosis and surgical interventions. Representativeapplications of CP-based sensors (electrochemical biosensor, tactile sensing skins, and thermal sensors)are briey discussed. Finally, some of the key issues related to CP-based sensors are highlighted.

2010 Elsevier B.V. All rights reserved.m/locate /b ios

l applications

1826 S. Nambiar, J.T.W. Yeow / Biosensors and Bioelectronics 26 (2011) 18251832

emerged opening up exciting new applications in several eldsincluding bioelectronics. The CPCs typically consist of a combina-tion of one ormore non-conducting polymers and conductive-llermaterials distributed throughout the polymer matrix. The conduc-tivityofCPCconductiveat a givenwcentration,with eachport necessa sudden drdrawbackstions,mechconducting

Metalliccrystalline pdants) wasin 1977 (Chdopants (re(Chiang etof organic cducting polcapableof aor reductionin ICPs. Som(PPy), polytof conductiredox polyelectrolytesThey have ltrical condusuggests, coin electrochductivity aresistance oICPs have hto acquire dICPs have tbiochemicasors in thebetween bireadouts. Tmolecules ihave attracof biomolecrial propertmeasuremeet al., 1988Gerard et aLewis et alBlends of ICimprove mand Deore,

In this pabased sensotype depentemperatureries, and thcholesterolical propertbiosensors.of sensingor vice-verBorole et aRossi et al.,Kaushik et a

2007; Schuhmann, 1995; Trojanowicz et al., 1997; Wen and Fang,2008).

However, in this paper, we discuss the state-of-art of sensorsbased on two main classes of CPsCPCs and ICPs. A brief his-

CPCrinc

es, fotivelbaseed se

rese

ectro

rk an50lectrpmering-on-csenstion, nucensa seng elezym

at intdeteut toce thctingin(Cos

ctingnsduPumarticlal., 2r potrochelopNI, Pndmort pers ag (Sreubleocalie see dislymeett, 2en th2002ationion)an ilsingElecon

ucibchniqs is governedbypercolation theory,whichdescribes thephase of CPC formed by a network of the llermaterialseight percentage. Forller loadingsbelowacertain con-the ller particles no longer maintain physical contactother to provide continuous path for electron trans-ary for conduction and subsequently, the CPC exhibitsop in conductivity (percolation threshold). Some of theof CPCs include high dependence on processing condi-anical instability and an insulated surface layer over thematerial (Freund and Deore, 2007).conductivity in organic conducting polymers such asolyacetylenelms combinedwith p-type dopants (oxi-rst discovered by Shirakawa, MacDiarmid and Heegeriang et al., 1977; Shirakawa et al., 1977). Soon, n-typeducing agents) were found to depict similar effectsal., 1978). Following these discoveries, a new classonducting polymers, also known as intrinsically con-ymers (ICPs), was established. ICPs contain monomerscquiringpositiveor anegative charge throughoxidationwhich in turn contributes to the electrical conductivitye examples of ICPs are polyacetylene (PA), polypyrrolehiophene (PT) andpolyaniline (PANI). Twoother classesng polymers that emerged around the same time are:mers and ionically conducting polymer (polymer/salt). Redoxpolymers are less conductive compared to ICPs.ocalized electron redox sites that contribute to the elec-ctivity. In ionically conducting polymer, as the namenduction is achieved through ow of ions. Their useemical sensing is largely limited by the low ionic con-t room temperature and time-dependent increase inf the polymer electrolyte (Freund and Deore, 2007).ighly exible chemical structure that can be modiedesired electronic and mechanical properties. Since thehe ability to efciently transfer electrons produced byl reactions, they have been used extensively in biosen-form of transducer that form an intermediate layerological samples and the electronics used for signalhey are also known to be compatible with biologicaln neutral aqueous solution. For the same reason, ICPsted much attention as a suitable matrix for entrapmentules. Several studies have explored these uniquemate-ies of ICPs to produce a wide range of biosensors fornt of vital analytes relevant to clinical diagnosis (Bidan; Borole et al., 2006; Boyle et al., 1989; Cosnier, 1999;l., 2002; Janata and Josowicz, 2003; Kranz et al., 1998;., 1999; Schuhmann, 1995; Trojanowicz et al., 1997).Ps and CPCs have also been investigated in order to

echanical stability and processability of CPCs (Freund2007).per, we review the state-of-art of conducting polymer-rs developed for biomedical applications. The sensords on the parameters-of-interest such as skin/tissuee, force exerted by tissues/blood vessels during surg-e presence of biochemical components like glucose and. CPs possess excellent electrical, chemical andmechan-ies useful for designing efcient, real-time and versatileSeveral reviews and studies based on specic typemechanism employed by one or more classes of CP,sa can be found in the literature (Bidan et al., 1988;l., 2006; Boyle et al., 1989; Cosnier, 1999, 2007; De2005; Gerard et al., 2002; Janata and Josowicz, 2003;l., 2008; Kranz et al., 1998; Lewis et al., 1999; Mueller,

tory ofbasic panalytrespecto CP-CP-basics.

2. Rep

2.1. El

Claalmostto an edevelomonitoin labhighlyestimabodiespathogsists ofsensin(e.g., ensor) thsignalthe inp

Sinconduducerssignalscondubio-tra2005;nanopZou etrials foin elecfor devare: PAbility atransppolymsensinand doof de-limprovtors arthe poand Brbetweet al.,modulbilizatshowsfor sen2005).a commreprodthis te- and ICP-based sensors along with an overview of theiples behind sensing clinical parameters such as bio-rce and temperature are presented in Sections 2.12.3y. Section 3 highlights some of the challenges relatedd sensors. Finally, we summarize the key aspects ofnsors and its potential use in the eld of bioelectron-

ntative applications

chemical sensors

d Lyons (1962) developed the rst biosensing deviceyears ago by integrating enzyme and glucose oxidaseode. Since then, much progress has been made in thent of biosensors for use in diagnostic detection andof biological metabolites. Moreover, recent advanceship devices have stimulated demand for portable,itive and precise analytical tools for easy and real-timeof desired analytes such as glucose, cholesterol, anti-leic acids, hormones, drugs, viruses, neurotransmitters,and toxins. An electrochemical biosensor typically con-sing element and a transducer (Gerard et al., 2002). The

ment is a biorecognition layer made up of biomoleculeses act asbiorecognitionentities in anenzymatic biosen-eractswith the analyte of interest producing a chemicalctable by the transducer, which ultimately transformsgive an electrical readout.

e discovery ofmetallic polymer (Shirakawa et al., 1977),polymers (CPs) have been extensively used as trans-electrochemical biosensors to measure and amplifynier, 2005). Both intrinsically conducting polymers andpolymer-nanocomposite materials have been used ascers. Carbon nanotubes (Liu et al., 2006; Perez et al.,era et al., 2006; Wang and Musameh, 2003) and metales (Park et al., 2004; Yu et al., 2003; Zeng et al., 2009;010) are some of the most commonly used ller mate-lymer composite-based functionalization of electrodesemical sensors. Some of the most commonly used ICPsment of different types of electrochemical biosensorsPy and PT. Low cost, scalability, easy processing capa-aterial properties such as large surface area, adjustableroperties, and chemical specicities makes conductingttractive candidates for applications in electrochemicaleet al., 2002). ConjugatedCPscontainalternating singlebonds in their polymer chain resulting in the formationsed electrons which act as charge carriers. In order tonsitivity and selectivity of the biosensors, redox media-persed, added as dopants or chemically conjugated intor matrix (Chen et al., 2004; Cosnier et al., 2003; Fiorito006). Conjugated CPs, thus, mediates electron transfere biorecognition layer and the nal electrode (Gerard). Moreover, the conjugated backbone of CPs allowsof its properties by enabling attachments (or immo-

to a variety of chemical moieties. For example, Fig. 1lustration of avidin-functionalized PPy nanowires usedbiotin-conjugated DNA molecules (Ramanathan et al.,trodeposition procedure based on polymerized lms istechnique used to immobilize macromolecules. Highlyle, ultrathin layers of CP coatings can be achieved byue. Furthermore, depending on the type of polymer,

S. Nambiar, J.T.W. Yeow / Biosensors and Bioelectronics 26 (2011) 18251832 1827

Fig. 1. Polymecation to biose

the electropand also, inElectrodepoing enzyme(Cosnier, 20

Initial reof enzymatsuch as gluto micromobiosensorshave also bsense biompicomolar asequently,the transduefcient af2007). Thecovalent intand afnitybetween fucovalent aninterface ha(Bakker andHabermulle2000; Schusensor instaulations (ZhimmobilizaLiu et al.,2006; Zhanet al., 2006have beenbe broadlyprising of smagnetic nsilver nanopolymermanition layerall the unittive, efcienbiomoleculof the magnet al. (2010and magneThey used anetism sepglucose (Figand a biosebased on co2010).

Based onsurement,

chemagne

ometd piperopotened bognienzysorsnoctrodevsensoroue-glusing), hasensr proonston

g of lometste a, wabstrate resulting in signicant decrease in charge-transfernce during the sensing process. Potentiometric biosensors-selective electrodes or gas sensing electrodes as physicalucers to measure electric potential due to concentration ofs-of-interest. For example, urea is detected by ureases viaduction of NH3 which, in turn, interacts with PPy to pro-

n electric potential. A number of pH-sensitive polymers (polyiaminodiphenyl ether), PANI, PPy, etc.) have been synthe-or potentiometric detection of analytes or ions-of-interestr (polypyrrole) nanowire on pre patterned electrodes and its appli-nsing was demonstrated (Source: Ramanathan et al., 2005).

olymerized lm remain stable in air, organic solventsaqueous solvents (Cosnier, 2007; Vidal et al., 2003).sition also allows precise spatial control over localiz-s or other bioactive moieties into the polymeric matrix05).search efforts on biosensors predominantly consistsic biosensors capable of detecting small moleculescose, cholesterol, lactate, and urea. from millimolarlar range. Another class of sensors known as afnity(immunosensors, DNA sensors, and receptor sensors)ecome increasingly popular due to their potential toolecules at extremely low concentrations, i.e. at thend even at the femtomolar level (Cosnier, 2005). Con-the immobilization of bioactive molecules in or oncer turns out to be a key aspect for obtaining highlynity biosensors (Geeta et al., 2006; Guimard et al.,deposition of biomolecules can be achieved by: non-eraction (physical adsorption, mechanical entrapmentbonding) and covalent linkages (chemical conjugationnctional groups of analyte and polymer). To date, bothd non-covalent modications of the bio-recognitionve been extensively explored and reviewed in detailTelting-Diaz, 2002; Cosnier, 2005; Gerard et al., 2002;r et al., 2000; Lange et al., 2008; Palmisano et al.,hmann, 2002). Some of the major concerns includebility, poor loading capability and complicated manip-ang et al., 2007). In the past decade, magnetism-basedtion of biomolecules (enzymes (Elyacoubi et al., 2006;2005; Qu et al., 2007; Rossi et al., 2004; Yu et al.,g et al., 2007; Zou et al., 2010) and antibodies (Tang)) and cancer cells (leukaemia K562) (Jia et al., 2009)reported. A typical magnetism-based biosensor candivided into 3 units: (i) A biorecognition layer com-ensing elements (biomolecules or cells) attached toanoparticles, and conducting ller materials (gold orparticles, carbon powder, etc.) all entrapped within atrix, (ii) a permanentmagnet onto which the biorecog-is mounted, and (iii) an insulator layer that packagess together. Magnetic immobilization is highly selec-t and convenient for biosensing studies. However, the

es are often covalently immobilized over the surfacesetic nanoparticles and the loading is also limited. Zou

Fig. 2. Sed Au m

amperric, anare amsuresproducbio-recin anbiosenand nafor ele(2007)sor fornano-penzymthe sen(PAMTfor biotransfeto dempositedsensinamperbon pain turnthe suresistause iontransdanalytethe produce a(4,40-dsized f) developed a magnetism-based biosensor with goldtic nanoparticles entrapped in a polymer composite.novel one-pot chemical oxidation synthesis and mag-

aration/immobilization (COSMSI) protocol for sensing. 2). They reported higher enzyme-loading capabilitynsing performance better than that of the biosensorsnventional electropolymerization protocols (Zou et al.,

the mechanism used for signal detection and mea-biosensors can be divided into several categories:

in solutionbiosensorsnisms suchand the ioncontributina change inthe polymethe dopantsummary, Cbiocompatiatic illustration of the construction of a Fe3O4AuPHDTGOx modi-tism electrode (Source: Zou et al., 2010).

ric, potentiometric, conductometric, optical, calorimet-ezoelectric. The most common types of transducersmetric (measures current) and potentiometric (mea-tial). Amperometric biosensors measure the currenty oxidation or reduction processes occurring in thetion layer (e.g., redox reaction of an enzyme substratematic biosensor). Investigations into amperometricbased on polymeric nanotubes (Ekanayake et al., 2007)bers (Liu et al., 2007) have opened up new frontierschemical biosensors. For example, Ekanayake et al.eloped PPy nanotube-array-based enzymatic biosen-ing glucose. PPy was deposited on a platinum plateds alumina substrate which enhanced adsorption of thecose oxidase andprovided an increased surface area forreaction. Poly-2-amino-5-mercapto-1,3,4-thiadiazoles been recently reported as a very promising ICPing applications because of its outstanding electron-perties. Kalimuthu and John (2009a,b) were the rstrate the biosensing capability of PAMT lm electrode-glassy carbon substrate. The lm was used for selective-cysteine and folic acid. He et al. (2010) enhanced theric sensing behaviour of PAMT lm by using solid car-s the substrate for electropolymerization of AMTwhich,s found to increase the amount of PAMT deposited on(Trojanowicz, 2003). Electrochemical doping of CPs inenhances electron transfer by a combination of mecha-as redox reactions of theCPs,mobility of thedopant ions-exchange properties of the polymer. One of the factorsg to an electric potential readout could be a product ofpH and the subsequent mobility of dopant ions withinr matrix triggered by an equilibration process betweens and free ions in solution (Guimard et al., 2007). InP-based biosensors are likely to address the issues ofbility for continuous monitoring of biological metabo-

1828 S. Nambiar, J.T.W. Yeow / Biosensors and Bioelectronics 26 (2011) 18251832

lites and drug dosages, and the possibility of in vivo sensing (Rajeshet al., 2009).

2.2. Tactile sensor (articial skin)

MEMS-bapplicationtelemedicinvides a rantexture, copressure-dimonitoringimally invadesigned sueries to beadvantagesblood loss,and accelerdirectly accgration of tMIS. This happroach toovercome t(Schostek eback (or hapeffectivenesvariations itexture andand to evalducts (Puan

Tactile sand manipuIn the pastdevelopinga broad speeld of medal., 1995; EKane et al.,1990; Sekit2005; Yangsensing incKane et al.,Leinewebertive or piezoKolesar, 19of using compolymers, eing elemenal., 1997; Wing devices1996; Leinetactile sensresolution athe brittlenor stretchato be packamanipulatosize-relatedsors. Polymexible, no2007).

The use(PVDF), for(1985) whiPVDF or thDargahi et a

schemt senstact foskin. (

d Solymeve toed as(PHBed anopicf a coand

f thee defe forcanthess can

o posensenschesf tac

esisti003bogicat sms on aodardneg skinyimits: aeasue of memonuncture cgh mnd red inypicaexibinfoadout. Papakostas et al. (2002) proposed a large areaensor fabricated by sandwiching semi-conductive lm inen screen-printed traces of Ag-lled thermoplastic poly-himojo et al. (2004) reported fabrication of exible tactileg array using a conductive rubber capable of sensingre-relateddeformations. Thinmetalwires that acted as sens-ctrodes were stitched on the conductive polymer matrix.a et al. (2005) developed a exible, net-shaped pressuremperature sensor fabricated by processing polyimide orthylenenaphthalate) and organic transistor-based electronicased tactile sensors have tremendous potential fors in medical robotics, interactive electronics, ande. Robust, reliable and real-time haptic feedback pro-ge of tactile information such as tissue compliance,ntact forces and torques, dynamic slip sensing, andstribution useful for identication, localization andof critical anatomical structures. The advent of min-sive surgery (MIS), which is the use of specicallyrgical instruments and visual devices that allow surg-performed through small incisions, offers distinct

to the patients in terms of reduction in intra-operativerisk of post-operative infection, less traumatic surgeryated recovery. Since the MIS operating eld cannot beessed by surgeons, there is an increasing need for inte-actile sensory devices onto the surgical tools used inas allowed researchers to adopt an interdisciplinarywards developing new techniques and technologies tohe inherent drawbacks involved in the MIS procedurest al., 2009). The key idea behind integrating tactile feed-tic interface) onto the surgical probes is to increase thes of the surgery by allowing the surgeons to measuren the supercial tissue properties such as temperature,contact force, to feel the hardness or tension of tissues,uate anatomical structures such as nerves, vessels andgmali et al., 2008).ensing provides improved dexterity, dynamic grippinglation by robots and humans (Lee and Nicholls, 1999).two decades, tremendous efforts have been towardsa human-skin-like sensor that can potentially providectrum of tactile information particularly useful in theical robotics and certain surgical procedures (Beebe etngel et al., 2003a,b; Hu et al., 2007; Jiang et al., 1997;2000; Kolesar and Dyson, 1995; Reston and Kolesar,ani et al., 2008; Shimizu et al., 2002; Someya et al.,et al., 2008). Different types ofmaterials used for tactileludes silicon-based piezoresistive (Beebe et al., 1995;2000) or capacitive sensors (Gray and Fearing, 1996;et al., 2000), and polymer-based piezoelectric, capaci-resistive sensors (Kolesar andDyson, 1995; Reston and

90). Recently researchers have explored the possibilityposite-material sensors by combining both silicon andxamples of which includes embedding of silicon sens-ts in polymer skins (Beebe and Denton, 1994; Jiang eten and Fang, 2008), packaging of silicon-based sens-in protective casing of polymer layer (Gray and Fearing,weber et al., 2000; Kane et al., 2000), etc. Silicon-basedors have proven to provide high sensitivity, high spatialnd ease of integration into electronic devices. However,ess of the silicon materials limit its use as a exibleble tactile sensor particularly when the sensors needged onto curved surfaces of surgical probes or roboticrs. Furthermore, thenite size of siliconwafers imposesdesign constraints on the dimensions of the tactile sen-er-based tactile sensors, on the other hand, are moret limited by dimensions, and chemically resistant (Liu,

of a piezoelectric polymer, polyvinylidene uoridetactile sensing was rst reported by Dario and de Rossich was soon followed by several other studies usingeir copolymers for tactile sensing (Choi et al., 2005;l., 2000; Kolesar et al., 1992, 1996; Yamada et al., 2002;

Fig. 3. A4 distincand conto form

Yuji antric posensitiwas ustiratereportendoscance oof rigidance orelativBoth thsensedtion ofsensor2007).

Twtactilebasedapproaarray opiezoret al., 2of bioltherssensormultimtive hasensinble polelementuremcapabllater dsome fon textAlthoutivity ainvolv

A tof a tactilesor reforce sbetwemer. Ssensinpressuing eleSomeyand tepoly(eatic of themultimodal tactile sensor. (a) A sensory node incorporatesors: a reference temperature sensor, a thermal conductivity sensor,rce and hardness sensors. (b) Sensor nodes are arranged in an arraySource: Engel et al., 2003a,b).

noda, 2006). De Rossi et al. (1993) reported a piezoelec-r-based, skin-like tactile sensor which was selectivelystress and shear forces. Thin polyimide lm (Kapton)a supporting structure, and PVDF and polyhydroxybu-

) were used as sensor elements. Dargahi et al. (2007)experimental design and a theoretical model of an

tooth-like tactile sensor capable of measuring compli-ntact tissue/object. The sensor set-up consisted of a paircompliant cylinders, and two PVDF lms. The compli-object in contact with the sensor was detected by theormation of the rigid and compliant sensor elements.ce applied aswell as the compliance of the tissue/objectbe measured using their sensor. The proposed applica-e sensors is in the eld of robotic surgery wherein thebe integrated onto endoscopic graspers (Dargahi et al.,

pular mechanisms employed in piezoresistive-basedsing can be broadly classied into: resistive-metaling, and conductive-polymer based sensing. Typicalfor resistive-metal based sensors involve a exibletile sensors with each sensory element consisting ofve metal capable of sensing stress/shear forces (Engel; Hwang et al., 2007). Inspired from the functionalitiesl skin, Engel et al. (2003b) reported the development ofart skinbasedon thin-lmpiezoresistive-metal-basedexible polymer substrate. They developed an array of

l tactile sensors capable of sensing contact forces, rela-ss, thermal conductivity and temperature (Fig. 3). Theconsists of an array of sensor nodes arrangedon aexi-

de (Kapton) substrate. Eachnode comprisedof 4 sensingthermal conductivity measurement unit, a tempera-

rement unit and 2membraneswithmetal strain gaugeseasuring hardness and contact forces. The same groupstrated that theirmultimodal sensorwas able tomimic

ionalities of the human skin by identifying objects basedlassicationandother sensorydata (Engel et al., 2003b).etal-based tactile sensors, in general, give good sensi-eliable response, relatively complex micromachining isfabricating the sensor elements (Cheng et al., 2010).l conductive polymer-based tactile sensor consistsle, conductive gel or elastomer capable of sensingrmation, and a set of patterned electrodes for sen-

S. Nambiar, J.T.W. Yeow / Biosensors and Bioelectronics 26 (2011) 18251832 1829

Fig. 4. (a) Thedevice under s

circuits. Crofor these kicross-talk bcomposite)Tactile devipopular becThin metaltactile senscially causeonto compltowards incHu et al. (force and ta compositin polydimeSekitani ettive materi70% withousor materiasingle-wallpresentedapolymer onmer was avariety of chexane andby windinga schematispiral electrbe twisteddamage.

2.3. Therma

Thermalturies. In 4whatever pis there to bis fever, whing pyrogennew technotic tool in athermal ima

to vascular disorders, for pre-clinical diagnosis of breast cancer,to identify neurological disorders and monitor muscular perfor-mances (Bagavathiappan et al., 2008). Thermal sensors integratedinto tactile sensing probes or catheters has the potential to be used

ical ttempn prhea

nsorwhiergyrderof bitronects:raturfrommocondufferennic mmocers (an ero, 2opuleenrame, give

2

areis thommT is tve bs inay ofd cod thaf 1therm004schematic of the proposed tactile sensing array. (b) The proposedtretching. (Source: Cheng et al., 2010).

ss-talk between the sensing elements poses a problemnds of sensors. Yang et al. (2008) effectively eliminatedetween adjacent sensing elements (polyimide-copperby dispensing them on a grid of copper electrodes.ces fabricated using these mechanisms became quiteause of their durability and ease to manufacture them.traces that act as sensing electrodes in polymer-basedors are mostly vulnerable to large deformations espe-d when the sensor unit are required to be integratedex surfaces. Consequently, research efforts are drivenreasing the reliability and robustness of these traces.2007) reported a exible sensor capable of sensingemperature simultaneously. The sensor comprised ofe material (multi-walled carbon nanotubes dispersedthylsiloxane (PDMS)) and liquid metal interconnects.al. (2008) proposed a stretchable, elastic conduc-

al that could be uniaxially and biaxially stretched byt causing mechanical or electrical damage. The sen-l was developed by coating PDMS-based rubber on aed carbonnanotubes composite lm. Cheng et al. (2010)highly twistablearticial skinbydispensingconductivea grid of sensing electrodes. The conductive poly-

as surgtissueablatioinvolvemal setissuesheat en

In oalitiesin elecical efftempetricityA thersemicoage diinorgaas therpolym1996; YGuerremost phave brial pacouple

Z = S

andand Smore cwhereites hachangepathwsion anshowevalue obasedet al. (2composite material which is a blend of PDMS and aonductive ller materials (copper, carbon black, cyclo-silver particles). The sensing electrodes were obtainedcopper electrodes around nylon bres. Fig. 4 shows

c of articial tactile skin and the arrangement of theodes. The authors demonstrated that the sensor couldup to 70 degrees without any structural or functional

l sensors

readings have been used in medicine for several cen-00 BC, the Greek physician Hippocrates wrote, Inart of the body excess of heat or cold is felt, the diseasee discovered. A common example of this phenomenonerein the body temperature is elevated due to circulat-s produced by our immune system. With the advent oflogies, thermal sensing has become a useful diagnos-pplications such as thermographic imaging or infraredging used for detecting small temperature changes due

and thermaorder to deconductiveticlesor ICPpolymer mductive netthe use ofmatrix of ptive photorstamping tesensor elemphotoresistarrays by dimide lmsprovide hap

Pyroelecture resultTemperaturtions of thethe materiaools that help clinicians to quantify sensitive changes inerature during surgical interventions typically in someocedures (removal or abrasion of faulty tissues) thatting by laser or RF energy. Moreover, probe-based ther-s may also provide real-time temperature proles ofch in turn may allow clinicians to precisely control the, preventing undesired tissue damage.to mimic the thermal sensing and regulation function-ological skin, temperature sensitive transducers usedic skins (E-skins) generally exploit two types of phys-thermoelectricity and pyroelectricity. Thermoelectrice sensors, also known as thermocouples, generate elec-a temperature gradient according to Seebeck effect.uple refers to the junction between two metals, or

ctors (pn couples) that is capable of producing volt-ce relative to a temperature difference. Organic andaterials or combinations of the two have been used

ouples (De Rossi et al., 2005). Intrinsically conductingFeng and Ellis, 2003; Kemp et al., 1999; Morsli et al.,t al., 2002) and carbon/polymer composites (Chung and002; Chung and Wang, 1999; Lin et al., 2006) are thear organic materials in which thermoelectric propertystudied (De Rossi et al., 2005). Thermoelectric mate-ter, proportional to its efciency of as a thermoelectricn by the gure of merit, Z, is dened as:

the electrical and thermal conductivities respectively,e Seebeck coefcient. The thermoelectric property isonly expressed as dimensionless gure of merit: ZT

he absolute temperature. Conductive polymer compos-een reported to sense temperature changes based onresistance of thematerial in a way that the conductivitythe ller elements are affected by the thermal expan-ntraction of the polymer matrix. Feng and Ellis (2003)t conjugated nanocomposite polymers can produce a ZTcomparable to commercially available semiconductor-oelectric. For applications in tactile sensing, Someya

) fabricated conformable, exible networks of pressurel sensors using active matrices of organic transistors. Invelop efcient and stable thermoresistive sensors frompolymercomposites, thellermaterial (conductivepar-s)must formaconductivenetworkwithin the insulatingatrix (Feng and Ellis, 2003). To achieve an optimal con-work of the ller material, Nocke et al. (2009) proposeddielectrophoresis for aligning tellurium nanorods in aoly(vinyl acetate) (PVAc) and a novolak-based posi-esist. Moreover, they presented photolithographic andchniques for development of resistance-based thermalents from the nanocomposite material of PVAc and

. Shih et al. (2010) fabricated thermoresistive sensorispensing a graphite-PDMS composite on exible poly-(Fig. 5). The sensor was designed for use as an E-skin totic interface to robots.tric materials respond to changes in tempera-ing in a spontaneous polarization of the material.e-dependent polarization slightly modies the posi-atoms within the crystal and produces a voltage acrossl. This change in polarization with respect to tempera-

1830 S. Nambiar, J.T.W. Yeow / Biosensors and Bioelectronics 26 (2011) 18251832

Fig. 5. Flexibltrode and com

ture can be

pi =PS,iT

where pi (Although thfor severalare relativediscoveredmer, shortlin the sameproperties,the rst tostudies havP(VDF-TrFEand Lang, 1Setiadi et acients of ababout 10 loone of the mtitanate (Pbhave, therefrials to takeperformanc2001, 2002

Polymerpared to minfrared rapresent in tcal changesand/or rearin addition(1999a,b) dThe sensorabsorber lalm) and a nand rear elebinations obetween frobased pyroepolymer (Pto the sensmeasured Icommerciatrodes.

free standing, self-absorbing 28m PVDF pyroelectric sensor (Source:t al., 1999a,b).

eral studies have investigated thermomechanical transduc-polymers in view of mimicking thermal IR sensors foundre. The ability of a jewel beetle (Melanophila acuminata)

ect forest res from a distance of 60100 miles has beenuted to the alternating hard and compliant nanolayers ofhermal sensors found in bilateral thoracic pit organs ofetle (Campbell et al., 2002; LeMieux et al., 2006). Basedprinciple of thermomechanical transduction, LeMieux et

06) developed a bimaterial polymer-silicon microcantilevertemperature resolution of 0.2mK and thermal sensitivity/mK. The high thermal sensitivity was attributed to thethermal stress induced by the plasma-enhanced polymericyers adhered onto the silicon substrate of the microcan-. Inspired from the biological IR sensors, Mueller (2007)gated the effect of changes in temperature on chitosan, alated form of a biomaterial (chitin) found in the sensoryof the jewel beetle. As an initial prototype, the author fab-bimlysiltherre oicrol exayermpoAN)articlnce

icrocd bee temperature sensor array. The insets show the interdigitated elec-posites on the electrode, respectively. (Source: Shih et al., 2010).

described as:

Cm2 K1) is dened as the pyroelectric coefcient.e pyroelectric effects had been observed and studiedcenturies, investigations of such effects in polymersly new (Bauer and Lang, 1996). Bergman et al. (1971)a strong pyroelectric effect in PVDF, an organic poly-y after the discovery of strong piezoelectric propertymaterial by Kawai (1969). Based on the pyroelectricGlass et al. (1971) and Yamaka (1972) were amonguse PVDF for infrared thermal sensing. To date, severale investigated the pyroelectric properties of PVDF and) for applications in thermal radiation sensing (Bauer996; Hammes and Regtien, 1992; Navid et al., 2010;l., 1999a,b). They typically exhibit pyroelectric coef-out 25C/m2 K and 40C/m2 K respectively which iswer than the coefcient (380C/m2 K) measured forost commonly used class of ceramics: lead zirconate

(Zr,Ti)O3) (PZT) (Bauer and Lang, 1996). Researchersore, explored the possibilities of using compositemate-advantage of the high exibility of polymers and highe of ceramics (Malmonge et al., 2003; Sakamoto et al.,).s have high coefcient of thermal expansion as com-etals and semiconductors. Most polymers absorb

diation because of the vibrational resonance modes

Fig. 6. ASetiadi e

Sevtion inin natuto detcontribmicrotthe beon theal. (20with aof 2nmstrongnanolatileverinvestideactyorgansricatedand pooped astructuered mthermamost lnanocotrile (PnanoptoenhaThe minduceheir organic bonds. Temperature-induced morphologi-such as conformational changes of polymer moleculesrangements in their crystal structure may also resultal energy transduction (Mueller, 2007). Setiadi et al.eveloped a polymer-based pyroelectric infrared sensor.comprised of a conductive polymer (PEDOT:PSS) as anyer and front electrode, a pyroelectric material (PVDFickelaluminium (NiAl)metal lm as a reector layerctrode (Fig. 6). The practical use ofmetal-polymer com-f pyroelectric sensors is hindered by the poor adhesionnt electrode (metal lm) and sensing layer (polymer-lectric lm). Setiadi et al. (1999a,b) used a conductive

EDOT:PSS) for effective adhesion of the front electrodeing material (pyroelectric PVDF) lm beneath it. TheR response was shown to be 10 higher than that oflly available PVDF lms with Ni-Al front and back elec- Fig. 7. Schem

ing applicationorph microcantilever from novolak-resin photoresisticon for IR thermal sensing. Lin et al. (2006) devel-mally sensitive microcantilever using a layer-by-layerf conductive polymer, metal and ceramic. The trilay-beam comprised of silicon nitride, a ceramic with lowpansion, an ultrathin lm of gold followed by a top-of chemically grafted polymer brushes (Fig. 7). Thesite polymer structures of functionalized polyacryloni-and polystyrene (PS) were reinforced with silver (Ag)es and single-walled carbon nanotube bundles in orderthemechanical strengthof thecompositepolymer layer.antilever design uses the phenomenon of temperature-nding of layered beams composed of one or moreatic of trilayeredMCs coatedwith polymer nanocomposite for IR sens-(Source: Lin et al., 2006).

S. Nambiar, J.T.W. Yeow / Biosensors and Bioelectronics 26 (2011) 18251832 1831

materials with very different thermal-expansion coefcients. Thethermally induced stresses resulted in reversible bending of themicrocantilevers and optical readout of the cantilever deectionwas measured. With this set-up the authors showed a four-foldincrease inmetalcera

Owing tsingle-wallindividual Sber of studal., 2006; Lal., 2005). Ppolymer nainto an elecate. They cpredominatphoto-effec

3. Challeng

The eldfore, faces athe major cchemical sttric eld (Lisensor applionic polymelastomersity subsequproperties.tion andadv2006).

One of tsor designelectrode suthe insufcmethods coraphy anddue to poterial propertsuch as sofprinting hasubstrates.Jang, 2009)explored thsurface (Do

Failure mnal stressesBenabdi anOhring, 199the issues amicrostructbehavioursnous, highlexible subsuch failurefully exami(Wang et al

4. Conclus

In summrelevant toOwing to thonto desire

ing elements have immense potential to integrate into micro/nanoscale devices for in vivo sensing and monitoring of bioanalytes. CP-based nanomaterials can be easily coupled with various chemicaland/or biological species to obtain highly sensitive and selective

ses. Aat orgs mic-chipmediotocoas d

itablewerpolymal prndus hybeldsck sy

nces

hiappacker, TE., Tel., Lang, B., St.J., De.J., Hsi, M., R, J.B.,arya,

ker, In., EhuiE., Cha.D., K

m. 9, 1., Genill, A.L.ll, D.Sgy. Mr Jr., Cor & FToo, CM.Y.,10.101C.K., FMacDC.K., Gl. PhysChoi,.D.L.,.D.L.,

C., LyoS., 19S., 20S., 20S., Da, J., Pa335.

, J., Sed, de Ro,D., CaEng. C, D., C, Kruts, Zhonke, E.M071i, A.,350.

, Chenors, A, ChenSoc. SEllis, T.A., BrL.F., M491thermal sensitivity when compared to conventionalmic microcantilever.o the outstanding electronic and optical properties ofed carbonnanotubes (SWNTs), IRphotoresponseof bothWNTs and SWNT lms have been reported in a num-ies (Freitag et al., 2003; Fujiwara et al., 2004; Itkis etevitsky and Euler, 2003; Pradhan et al., 2008; Qiu etradhan et al. (2008) fabricated an IR-sensitive SWNT-nocomposite sensor. They embedded 5 wt.% SWNTtrically and thermally insulated matrix of polycarbon-oncluded that for pure SWNT lm, the thermal effectes and for SWNT-polycarbonate nanocomposites, thet predominates in the IR photo-response.

es

of polymer-based sensors is still nascent and there-dynamic set of challenges as the eld evolves. Some ofoncerns of polymer materials include temperature andability, long term stability, and tolerance to high elec-u, 2007). Crosstalk is a primary issue in polymer-basedications. For example, electroactive polymers such asermetal composites and other nanomaterial-basedrespond to changes in stress/strain, heat and humid-ently affecting their thermal, mechanical and chemicalChanges in material properties can complicate calibra-ersely affect the sensor performance (Biddiss andChau,

he major challenges in CP-based electrochemical sen-is to immobilize the transducer (CP matrix) onto anbstrate for effective signal transduction. In addition toient adhesion of CPs onto the substrate, conventionalmmonly used in the sensor fabrication: photolithog-e-beam lithography may not be compatible with CPsntial adverse affects of the techniques on the mate-ies of the polymer composites. Various other methodst lithography, electrochemical deposition, and ink-jetve been proposed for patterning CP onto the electrodeHowever, theyhave limited spatial resolution (Yoonand. In order to overcome these limitations, studies havee possibility of chemical conjugating CP to the substrateng et al., 2005a,b).echanisms of conducting polymer lms due to inter-have been extensively studied (Baumert et al., 2004;d Roche, 1997; Campbell, 1969; Francis et al., 2002;2; Wang et al., 2002; Wang and Feng, 2002). Some ofddressed in the literature are: effects of thickness andures on the mechanical properties of CP lms, failureon thermal and mechanical loading, and the heteroge-y localized stress and strain distributions, typical ofstrates, found in elastomers. It is important to note thatmodels of CP-based lms and coatings need to be care-ned in order to use them for commercial applications., 2009).

ion

ary, three different types of CPC- and ICP-based sensorsclinical applications have been discussed in this paper.e exibility, biocompatibility, and ability to deposit CPsd geometrical surfaces and structures, CP-based sens-

responrize thvarioulab-onsmarttion prissues,ing suwhichin thematerisemicoused aious feedba

Refere

BagavatPani

Bakker,Bauer, SBaumertBeebe, DBeebe, DBenabdiBergmanBhattach

DekBidan, GBiddiss,Borole, D

PolyBoyle, ACampbeCampbe

noloCarrahe

TaylChen, J.,Cheng,

doi:Chiang,

S.C.,Chiang,

AppChoi, B.,Chung, DChung, DClark, L.Cosnier,Cosnier,Cosnier,Cosnier,Dargahi

329DargahiDario, P.DeRossi

Sci.De RossiDong, B.Dong, B.Ekanaya

23, 1Elyacoub

345Engel, J.

SensEngel, J.

Res.Feng, J.,Fiorito, PFrancis,

4897s discussed in Sections 2.12.3,we can further summa-anic conducting polymers can be easily integrated intoroanalytical systems such as miniaturized microuidicdevices or lab-on-tube devices (smart catheters) for ascal diagnostic tools and surgical aids. However, fabrica-ls and challenges related to the processing and stabilityiscussed in Section 3, can be improved by employ-surface functionalization of nanomaterials, some of

e presented in Section 2.1. Moreover, advancementser-processing technologies and improvements in the

operties of CPs will eventually allow integration of CPs,ctors, and the underlying electronics collectively to berid sensors for broad commercial applications in var-of biomedical engineering (prostheses, implants withstems, biochips for personalized medicine, etc.).

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Conductive polymer-based sensors for biomedical applicationsIntroductionRepresentative applicationsElectrochemical sensorsTactile sensor (artificial skin)Thermal sensors

ChallengesConclusionReferences