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11710 | J. Mater. Chem. C, 2019, 7, 11710--11730 This journal is©The Royal Society of Chemistry 2019

Cite this: J.Mater. Chem. C, 2019,

7, 11710

An overview of stretchable strain sensors fromconductive polymer nanocomposites

Jianwen Chen,ab Qunli Yu,a Xihua Cui,b Mengyao Dong,cd Jiaoxia Zhang,ce

Chao Wang,f Jincheng Fan,g Yutian Zhu *a and Zhanhu Guo *c

There is a growing demand for stretchable strain sensors because of their potential applications in

various emerging fields, such as human motion detection, health monitoring, wearable electronics, and

soft robotic skin. Recently, strain sensors based on flexible conductive polymer composites (FCPCs)

composed of conductive materials and a stretchable elastomer have received intensive attention owing

to their high stretchability, good flexibility, excellent durability, tunable strain sensing behaviors, and ease

of processing. Here, we systematically summarize the recent progress of stretchable strain sensors

based on FCPCs. This review covers the classification and sensing mechanisms as well as the influence

of multiple factors on the sensing behaviors of FCPC based strain sensors with detailed examples.

1. Introduction

Conductive polymer composites (CPCs) consisting of an insulatingpolymer matrix and conductive materials, such as carbonaceousnanomaterials (e.g., carbon black (CB),1–3 carbon nanotubes(CNTs),4–9 carbon fiber (CF),10–12 graphite13–15 and graphene(GE)16–22), nanometals (e.g., nanowires,23–25 and nanoparticles26–31)or intrinsically conductive polymers (e.g., poly(3,4-ethylenedioxy-thiophene):polystyrene sulfonate (PEDOT:PSS),32–35 polypyrrole(PPy)36,37 and polyaniline (PANI)38–40), have received extensiveattention for several decades due to their good processability,cost-effectiveness, tunable electrical properties, and wide ranges ofapplications.41–43 The electrical conductivity of CPCs highlydepends on the concentration of the conductive materials. Abovea critical concentration, known as the conductive percolationthreshold, conductive networks can be formed throughout the

polymer matrix, which can achieve an insulator/conductortransition for CPCs.44 The conductive percolation thresholdcan be estimated by the following equation:45

s p (�rc)t for rc (1)

where s is the electrical conductivity of the CPC, r is the massfraction of the conductive materials, rc is the conductivepercolation threshold, and t is a critical exponent related tothe dimension of the conductive network in the CPC.46–49 It wasfound that the electrical resistance of CPCs varied with the evolu-tion of the conductive networks inside the polymer matrix.29,50–52

When CPCs are exposed to some external environment stimuli,such as strain,53–58 temperature,59–64 vapor,65–68 or liquid,11 theconductive networks can be varied, thus leading to a correspondingchange in the electrical resistance of CPCs. Therefore, CPCs can beused as versatile sensors to monitor the applied external stimuli.Among various sensors, CPC based strain sensors have arousedgreat interest because of their emerging applications, such ashuman motion detection,69,70 health diagnosis,71,72 wearableelectronics,73–75 and electronic skin.76

To serve as smart strain sensors for various applications,superior stretchability, excellent flexibility, a wide sensingrange, and high sensitivity are the essential requirements. Tomeet the demands of stretchability and flexibility, elastomers,including poly(dimethylsiloxane) (PDMS),10,77–79 rubber (e.g., naturalrubber (NR),80 and isoprene rubber (IR)81), thermoplastic elastomers(e.g., thermoplastic polyurethane (TPU),14,82–84 olefin blockcopolymer (OBC),85, and poly(styrene–butadiene–styrene)(SBS)4,52,86), in addition to natural polymers or products,87 areusually used as the matrix or substrate of stretchable strainsensors because of their flexibility, stretchability, and durability.Compared with strain sensors made of metals or semiconductors,

a College of Materials, Chemistry and Chemical Engineering,

Hangzhou Normal University, No. 2318 Yuhangtang Rd., Cangqian,

Yuhang District, Hangzhou, 311121, China. E-mail: [email protected] State Key Laboratory of Polymer Physics and Chemistry,

Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,

Changchun 130022, Chinac Integrated Composites Laboratory (ICL), Department of Chemical & Biomolecular

Engineering, University of Tennessee, Knoxville, TN 37996, USA.

E-mail: [email protected] Key Laboratory of Materials Processing and Mold (Zhengzhou University),

Ministry of Education, National Engineering Research Center for Advanced

Polymer Processing Technology, Zhengzhou University, Zhengzhou, Chinae School of Material Science and Engineering, Jiangsu University of Science and

Technology, Zhenjiang, Chinaf School of Materials Science and Engineering, North University of China,

Taiyuan 030051, Chinag College of Materials Science and Engineering, Changsha University of Science and

Technology, Changsha 410114, China

Received 5th July 2019,Accepted 29th August 2019

DOI: 10.1039/c9tc03655e

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stretchable strain sensors based on flexible conductive polymercomposites (FCPCs) have prominent advantages in stretchability,flexibility, environmental stability and processability. Therefore,FCPC-based stretchable strain sensors have received more and moreattention from the academy and industry.

Until now, the published review papers of CPCs mainlyfocused on their electrical properties and the applications ofCPCs. For instance, Pang et al.41b systematically summarizedthe influence of the filler type, host polymers, and dispersionmethods on the electrical properties of the resulting CPCs.Deng et al.88 comprehensively reviewed various methods totune the electrical properties of CPCs and their applicationsin various fields. Recently, Xie et al.89 reviewed the recentprogress in utilizing phase morphologies, such as the segregatedmorphology and the co-continuous morphology, to improve theelectrical properties of CPCs. Only a very limited number ofreview papers focused on strain sensing.

This review paper aims to summarize the categories, sensingmechanisms, and affecting factors of the strain sensing perfor-mance of FCPC-based stretchable strain sensors. This reviewpaper is organized as follows. First, FCPC-based strain sensorsare classified based on the composite modes of the flexiblepolymer matrix and conductive components. In this section,recent progress of each type of FCPC-based strain sensorsis systematically summarized. Second, the strain-responsivemechanisms of the various FCPC-based strain sensors areexplained. Third, a series of influence factors of the strainsensing performance of FCPC-based stretchable strain sensorsare summarized and discussed. Finally, conclusions and futureperspective of FCPC-based stretchable strain sensors are brieflydiscussed.

2. Categories of FCPC-basedstretchable strain sensors

A variety of composite modes have been developed to combineflexible polymers and conductive components together togenerate strain sensing materials. Based on the compositemodes and the configuration of the resulting strain sensors,FCPC-based strain sensors can be roughly categorized intothree types: (1) filled-type FCPC-based strain sensors, (2)sandwich-type FCPC-based strain sensors, and (3) adsorption-type FCPC-based strain sensors. These different strain sensorspossess different microstructures, and thus exhibit distinctlydifferent sensing performances. The preparation methods,characteristics, representative studies, and advantages anddisadvantages for each type of FCPC-based strain sensors arediscussed in this section.

2.1. Stretchable strain sensors based on filled-type FCPCs

Strain sensors from filled-type FCPCs are usually fabricated bydirectly dispersing conductive fillers, especially carbonaceousconductive fillers, into a flexible polymer matrix by meltcompounding or solution mixing. Generally, melt compounding,where conductive fillers and polymer particles are dry-blended

at a certain temperature and subsequently hot-pressed orextruded to produce the samples, can be readily scaled up inproduction due to its simplicity. Solution mixing normally canproduce CPCs with a better conductive filler dispersion bydispersing the fillers and dissolving the polymers in a solventwith ultrasound or shearing, and subsequently casting thehomogenized solution and hot curing to get the conductivepolymer composites.

For the filled-type FCPCs, a relatively high loading ofconductive fillers in the insulating polymer matrix is neededto achieve the insulator/conductor transition, which leads to ahigh Young’s modulus, low flexibility, complex processing, andhigh cost of the materials.90–92 For example, Mattmann et al.93

prepared strain sensors based on a thermoplastic elastomerfilled with CB through melt blending and extrusion molding.It was found that the strain sensor could exhibit a monotonicchange of electrical resistance with strain during the stretchingprocess only when the content of CB increased to 50 wt%.However, high CB content resulted in a brittle composite whichwas unacceptable in sensing applications. Therefore, a numberof strategies have been developed to lower the percolationthreshold and enhance the electrical conductivity of CPCs. Inthe past few years, some strategies such as improving fillerdispersion,94–97 adding large aspect ratio fillers, using hybridfillers,12,51,80,98–100 or designing novel hierarchical structures(such as double percolation structures,4,85,101,102 segregatedstructures,55,103,104 and porous structures105,106) have beenapplied to lower the percolation threshold of CPCs. For example,Costa et al.95 investigated the effects of different surfactants onthe electrical conductivity and strain sensing behaviors of CNT/triblock copolymer styrene–ethylene/butylene–styrene (SEBS)composites. They found that the electrical conductivity andstrain sensitivity of the resulting CNT/SEBS composites couldbe significantly improved through adding suitable surfactantsacting as the dispersing agents of CNTs, which could decreasethe size of CNT agglomerates in the SEBS matrix. Zheng et al.107

fabricated stretchable strain sensors based on CB/CNT/PDMSFCPCs by dispersing CB and CNTs into a PDMS matrix, as shownin Fig. 1. The bridged and overlapped hybrid CNT–CB structureendowed the stretchable strain sensor with good electrical con-ductivity, a wide strain sensing range (ca. 300% strain), excellentrepeatability, and superior durability (2500 cycles at 200%strain). Moreover, Oh et al.99 designed a hybrid carbon nano-tube–graphene (CNT–GE) structure in a PDMS matrix, andprepared strain sensors only containing a small mass fractionof conductive fillers. Compared with the CNT/PDMS or GE/PDMS composites, the hybrid CNT/GE/PDMS composites pos-sessed a higher electrical conductivity. For a FCPC with a binaryblend of conductive fillers and polymer, the fillers are randomlydistributed in the polymer matrix. As a result, this type of FCPCnormally shows a small gauge factor (GF, defined as: (DR/R)/e),especially at a small strain.56,57 To solve this problem, Pu et al.85b

designed 2D end-to-end contact conductive networks of multi-walled carbon nanotubes (MWCNTs) in an OBC matrix, whichshowed a high GF of 248 even at a strain of 5% and a linearresistance response throughout the whole strain range (Fig. 2).

Review Journal of Materials Chemistry C

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In order to improve the electrical properties of FCPCs,researchers usually used the surface modification of theconductive nanomaterial to improve the dispersion of the nano-material in the polymer matrix. Paradoxically, surface modifica-tion of the conductive nanomaterial could also degrade theelectrical conductivity of the conductive nanomaterial, whichcould counteract the contribution of improvement in the dis-persion of the nanomaterial. Moreover, selecting a large aspectratio nanomaterial as the conductive filler will lead to not only acomplex synthesis process but also difficulty in the dispersion ofthe nanomaterial in the polymer matrix. As a result, some morecomplex hierarchical structures were also utilized to lower thepercolation threshold and to enhance the strain sensitivityof FCPCs. For example, a double percolation structure (firstproposed by Sumita et al.108), i.e., the conductive fillers areselectively located in one phase of a co-continuous polymerblend to build a percolated conductive network, has been usedto lower the percolation threshold of CPCs. For example, Duanet al.101 compared the strain sensing behaviors of binary andternary composites based on TPU and OBC. They found that alower percolation threshold and a higher strain sensitivity wereobtained for the ternary composites with a co-continuous phasestructure. A novel double-interconnected network composed ofcontinuous GE conductive networks was constructed to design ahighly stretchable and sensitive strain sensor.102 Specifically, abutadiene styrene rubber (SBR)/NR–GE composite with a double-interconnected structure, in which GE was evenly dispersedinside the NR phase to form the conductive networks, was

fabricated by a simple and effective assembly approach, asshown in Fig. 3a. It was observed that the percolation thresholdof the SBR/NR–GE was 12-fold lower than that of the SBR/NR/GEcomposites with a homogeneous dispersion of GE (Fig. 3b).Interestingly, the fabricated strain sensors also exhibited goodstretchability and sensitivity (Fig. 3c). Moreover, Wang et al.104

tailored the percolating conductive networks of natural rubbercomposites via a cellulose nanocrystal-assisted latex assemblyapproach, and obtained highly flexible strain sensors (topimages of Fig. 4A). The percolated 3D conductive network insidethe CNT/NR composites, segregated by NR particles, couldendow the strain sensors with a very low electrical conductivitypercolation threshold (4-fold lower than that of the conventionalCNT/NR composites), good reproducibility, and high sensitivity.Oh et al.105 designed a stretchable pressure insensitive strainsensor based on porous CNT/PDMS composites by using an allsolution-based process, as shown in Fig. 4B. It was found thatthis sensor showed high sensitivity to strain and negligibleresponse to pressure, which could be used to detect joint motionand to distinguish shear stress and normal pressure. Wanget al.106 prepared a porous fiber-shaped strain sensor based onCNT/TPU via a simple and cost-efficient wet-spinning method.This strain sensor exhibited an ultra-wide response range (320%)and a fast response time (200 ms).

Although the strain sensors based on filled-type FCPCspossess obvious advantages (such as low-cost and large-scaleproduction), they also have many shortcomings. For example,the existing hysteresis arising from both the viscoelastic nature

Fig. 1 (a) Schematic illustration of the fabrication of the strain sensor based on CB/CNT/PDMS FCPCs. (b) Schematic illustration of the hybrid CNT–CBconductive network in the PDMS matrix. The black spheres and blue rods represent CB and CNTs, respectively. The red circles show the three differentcontact modes of the hybrid conductive network in the composites, i.e., CNT–CNT, CNT–CB and CB–CB contacts. (c) DR/R0 as a function of the appliedstrain at a rate of 10 mm min�1. The inset shows the tensile test fixture. (d) Response of the sensor to cyclic stretching at a strain of 200%. Reprinted withpermission from ref. 107. Copyright (2018) Elsevier.

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Fig. 2 (a) Schematic illustration of the fabrication of FCPC-based strain sensors with an end-to-end structure of MWCNT networks in OBC. (b) Responseof different samples to strain during the tension process. The sample with the end-to-end structure exhibits a linear resistance response throughout thewhole strain range. (c) GF of different samples versus strain. Reprinted with permission from ref. 85b. Copyright (2018) Royal Society of Chemistry.

Fig. 3 (a) Schematic illustration of the fabrication of SBR/NR–GE with a double-interconnected network. (b) Electrical conductivity of SBR/NRcomposites as a function of GE content. (c) DR/R0 measured as a function of applied strain. Reprinted with permission from ref. 102. Copyright(2016) Royal Society of Chemistry.


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of the polymers and the interaction between the fillers andpolymers usually leads to irreversible sensing performanceupon dynamic loading. Moreover, the high filler loading ofthis type of strain sensor may also affect the strain sensitivity,especially for the strain sensitivity under small strain. As aresult, the strain sensors based on filled-type FCPCs are moresuitable for detecting large strains like human joint motions.

2.2. Stretchable strain sensors based on sandwich-type FCPCs

Recently, a new type of strain sensors based on FCPCs withsandwich-like structures was developed, and the conductivenetwork or layer was sandwiched between two flexible polymerlayers. The most commonly used strategy to fabricate sandwich-type FCPCs is to deposit,109–112 transfer41,113–115 or print28,116 aconductive layer on a pre-fabricated flexible polymer substrate,and then coat with another flexible polymer thin film as theprotective layer. For example, Roh and coworkers32 designeda stretchable strain sensor with a sandwich structure ofPU-PEDOT:PSS/CNT/PU-PEDOT:PSS prepared by layer stacking.As shown in Fig. 5, this strain sensor had excellent opticaltransparency and ultra-high strain sensitivity, and could detecteye movement and distinguish between the emotions of laughingand crying. Yang et al.112 prepared an ultrasensitive strain sensor

based on GE/PDMS composites with a sandwich structure.The strain sensor exhibited an ultra-high GF of 1054 within awide strain range (e = 26%), and had tremendous potentialin health monitoring, mechanical control, real-time motionmonitoring, etc.

Another strategy to fabricate strain sensors based onsandwich-type FCPCs is to directly infiltrate the flexible polymerinto the prefabricated conductive layer.82,117–119 For example,Zheng et al.119 prepared dual-functional wearable strain sensorsand switches based on the infiltration of PDMS into porous GEfoam, as shown in Fig. 6. This strain sensor could well detect asubtle radial pulse and bending of the index finger. Moreover,the sensitivities and switching capabilities of GE foam filmscould be adjusted simply by controlling the thickness of theGE foam. Lu et al.110 fabricated a strain sensor based on acomposite of a thermoplastic polyurethane electrospunmembrane (TPUEM) and PDMS with a sandwiched silver nano-wire (AgNW) conductive layer between them. More specifically,AgNWs were attached on the TPUEM by filtration, followed byspin-coating of liquid PDMS. The strain sensor could detect bothstretching and bending deformations with high sensitivity,excellent reliability and remarkable stability. Recently, inspiredby the sensory system of spiders, a series of crack-based sensors

Fig. 4 (A) (a1) Schematic depiction of CNT@CNC/NR FCPCs with a 3D hierarchical conductive network fabrication process. (b1) Electrical conductivity ofCNT/NR and CNT@CNC/NR FCPCs as a function of CNT volume fraction. (c1) DR/R0 measured as a function of applied strain. (d1) Response of the sensorto cyclic stretching at a strain of 100%. Reprinted with permission from ref. 104. Copyright (2016) Royal Society of Chemistry. (B) (a2) Schematic depictionof the stretchable pressure insensitive strain sensor fabrication process. (b2) SEM image of the stretchable pressure insensitive strain sensor showing aporous structure. (c2) Optical photograph of the stretchable pressure insensitive strain sensor showing the stretchability. (d2) Curve of the relative changein resistance versus compressive or tensile strain. Reprinted with permission from ref. 105. Copyright (2018) American Chemical Society.


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have been fabricated for gaining higher sensitivity.120–124 Forexample, a strain sensor with ultra-high sensitivity under micro-strain was developed after a process of coprecipitation, reduction,vacuum filtration, and casting, as shown in Fig. 7.122 The highsensitivity of this strain sensor is attributed to the crack andoverlapped morphologies of the conductive layers of the AgNWsand reduced graphene oxide (RGO), which were successfullyformed after a prestretching process (Fig. 7b). Furthermore, itwas found that the AgNW/RGO/TPU composites with a smallerprestretching had a higher strain sensitivity (Fig. 7d). Electricalmeasurements implied that this crack-based strain sensorexhibited very high gauge factors (20 for De o 0.3%, 1000 for0.3% o De o 0.5%, and 4000 for 0.8% o De o 1%), whichwas sensitive enough to detect microstrain, such as a pulseand voice.

Sandwich-type strain sensors are usually fabricated by sand-wiching a conductive layer between two stretchable polymericlayers. Clearly, the preparation of this type of strain sensorneeds relatively high production costs and cumbersomeprocessing procedures, and they are difficult to mass produce.

The repeatability of sandwich-type strain sensors is also notgood since it is difficult to get precise control of the thicknessand microstructure of the conductive layer by spray or spincoating. In addition, this type of strain sensor could not sufferlarge strain because of the brittle conductive layer. Comparedto the strain sensors based on filled-type FCPCs, however, thestrain sensors based on FCPCs with a sandwich structurepossess higher sensitivity, and are suitable for the detectionof microstrain. Therefore, this type of strain sensor is moreappropriate for health monitoring, such as breathing, pulses,and heartbeats.

2.3. Stretchable strain sensors based on adsorption-typeFCPCs

Adsorption-type FCPCs are composited using a nonconductiveflexible polymer layer coated with a layer of conductive materialsby methods such as transferring,125–127 soaking,18,77,128 ordepositing.114,129,130 For example, Wang et al.18a developed astretchable strain sensor with an interesting three dimensionalconductive network by using RGO to decorate a flexible

Fig. 5 (a) Schematic illustration of the strain sensor consisting of a stacked nanohybrid structure of PU-PEDOT:PSS/CNT/PU-PEDOT:PSS on a PDMSsubstrate. (b) Transmittance spectra of the strain sensor in the visible wavelength range from 350 to 700 nm. A photograph of the sensor is shown as aninset. (c) Strain dependent DR/R0 of the strain sensor at stretching strains ranging from 10% to 100%. (d–f) Time-dependent DR/R0 responses of the strainsensor when a subject was blinking (d), laughing (e), and crying (f). Reprinted with permission from ref. 32. Copyright (2015) American Chemical Society.


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electrospun TPU mat through ultrasonication. RGO was anchoredonto the TPU fibers with the help of ultrasonication, as shown inFig. 8a. From Fig. 8, it is clearly observed that this type of FCPCsis endowed with super-stretchability, high sensitivity, gooddurability and stability, and fast response speed. Zhou et al.114

designed a crack-based CNT/TPU strain sensor with bothsuperior sensitivity and high stretchability through spray-coatingcarbon CNT ink onto a TPU fibrous mat with prestretchingtreatment. As shown in Fig. 9, the resulting strain sensorexhibited a wide workable strain range (up to 300%), excellentdurability, and a very short response time (70 ms) becauseof the ultra-stretchable electrospun network and the crackstructure. Moreover, the GF of this strain sensor is as highas 83982.8 at a strain of 220–300%. Pan et al.126 fabricated a

sensitive and stretchable strain sensor based on the adsorptionof GE onto thin PDMS. The resulting GE/PDMS hybrid filmsdisplayed excellent strain-sensing performance including awide workable strain range (up to 187%), high sensitivity(GF of up to 1500 at strain of 187%), and remarkable durabilityin resistance owing to the 3D conducting networks. Interest-ingly, a wearable and stretchable strain sensor was rapidlyfabricated by depositing graphite and CNT ink in sequenceonto a latex piece, and could be employed to monitor plantgrowth in real time.129 Generally, the key point to prepareadsorption-type FCPCs is good adhesion between the flexiblepolymer thin film and the conductive coating material, whichis essential to ensure stability and reproducibility of the strainsensing behaviors of adsorption-type FCPC based strain sensors.

Fig. 6 (a) Flow chart for the preparation of GE foam/PDMS composites. (b and c) Resistance change (b) and GF (c) of the composites with differentthicknesses (200–1600 mm). (d and e) Relative resistance changes in the strain sensors due to a radial pulse (d) and bending of the index finger to differentangles (e). Reprinted with permission from ref. 119. Copyright (2018) Royal Society of Chemistry.


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Therefore, surface modification of the conductive materials orpolymer matrix is normally used to introduce strong surfaceinteractions, such as covalent or ionic bonds. Zhang et al.77a

reported a flexible strain sensor fabricated by diffusing CNTsinto PDMS by using a facile swelling/permeating method.The CNTs were modified by using a silane coupling agent(SCA) to improve their dispersion and their interaction withPDMS. As shown in Fig. 10, surface modification of the CNTssignificantly improved their permeation depth and dispersion,and thereby enhanced the strain sensitivity of the resultingstrain sensor.

For adsorption-type FCPCs, the conductive layer isconstructed by some types of weak interactions via soaking ordepositing. This means that it’s hard to precisely control theconductive layer in the sensor preparation, which is similar tosandwich-type strain sensors. As a result, strain sensors basedon adsorption-type FCPCs also possess similar disadvantages,

including high-cost, complex processing procedures, and inferiorrepeatability from lots of different sensors.

In addition, there are other methods to design strain sensorsbased on adsorption-type FCPCs. For example, Wang et al.131

embedded CNTs into common elastic bands (EB) through aswelling-ultrasonication treatment and subsequently coatedthe hybrid CNT/EB bands with a polydopamine layer by self-polymerization of dopamine to stabilize them. The fabricatedflexible strain sensors showed a wide sensing range (up to 920%strain), large sensitivity and excellent durability (10 000 cyclesat 100% strain). Li et al.132 used highly stretchable TPU fiberyarns to decorate with multi-walled and single-walled CNTsprepared by a method combined the electrospinning, ultra-sonication adsorption, and bobbin winder techniques. Thefabricated TPU fiber yarns possessed high conductivity andstretchability and could be embedded into PDMS to fabricatestretchable strain sensors.

Fig. 7 (a) Schematic illustration of the fabrication of AgNW/RGO/TPU composites. (b) Schematic illustration of the prestretching and releasingprocess of the as-prepared AgNW/RGO/TPU composites. (c) Change in resistance with strain for the strain sensor under a prestretch of 10% strain.Insets are the SEM images during stretching. (d) Change in resistance under increasing strain with different prestretching levels. (e and f) Resistancechange of the strain sensor in response to a pulse and voice, respectively. Reprinted with permission from ref. 122. Copyright (2016) AmericanChemical Society.


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3. Strain sensing mechanism ofFCPC-based stretchable strainsensors

The repeatable damage and repair of the electrically conductivepathway inside the stretchable polymer matrix is the theoreticalbasis of FCPC-based stretchable strain sensors. Clearly, thecontinuity of the conductive pathway is related to the tunnelingeffect between conductive nanomaterials as well as the physicalcontact of the conductive network. Based on these two criticalfactors of the conductive pathway, some mechanisms, includingthe tunneling effect, disconnection of overlapped conductivematerials, and crack propagation, have been established to explainthe strain sensing behaviors of various types of FCPC-basedstretchable strain sensors. In this section, we will systematicallydiscuss each mechanism.

3.1. Tunneling effect

The electrical conductivity of conductive polymer compositesis attributed to not only the physical contact between theconductive nanomaterials, but also tunneling or hoppingbetween adjacent conductive nanomaterials. In most previousliterature,2,4,29,52,86,106,133,134 the general point is that thetunneling effect and the variation of conductive pathwaysunder strain are the main sensing mechanisms of CPC-basedstrain sensors, especially for the strain sensors based on filled-type CPCs. It is clear that the increase of the tunnel distanceand the destruction of conductive paths lead to a significantincrease in the electrical resistance of FCPCs during thestretching process. Conversely, the conductive nanomaterialscan return to their initial places to decrease the tunnel distanceand recover the conductive pathways during the retractionprocess, thereby resulting in decreased electrical resistance of

the composites. Simmons et al. proposed a model derived fromtunneling theory,135,136 which can approximately estimate thetotal resistance R of FCPCs based on eqn (2) and (3):

R ¼ L

N

� �8phs3ga2e2

� �expðgsÞ (2)

g ¼ 4pffiffiffiffiffiffiffiffiffiffi2mjp

h(3)

where N and L are the numbers of conductive paths andnanomaterials forming a single conductive path, respectively,h is the Planck constant, s is the smallest distance betweenconductive nanomaterials, a2 and e represent the effectivecross-section and the electron change, respectively, and mand j are the electron mass and the height of the potentialbarrier between adjacent nanomaterials, respectively.

As a load is applied, the conductive nanomaterials areseparated and the distance between the nanomaterials changeslinearly and proportionally to the applied strain changing froms0 to s, as expressed as eqn (4):

s ¼ s0ð1þ CeÞ ¼ s0 1þ CDll0

� �� (4)

where e is the tensile strain of the composites, and Dl and l0

represent the deformation of the composites and the initiallength of the composites, respectively.

Owing to the high increase rate of resistivity at a largerstrain, it is assumed that the number of conductive pathschanges at a much higher rate and can be expressed as follows:

N ¼ N0

exp MeþWe2 þUe3 þ Ve4ð Þ (5)

where M, W, U, and V are constants.

Fig. 8 (a) Schematic illustration of the fabrication process of RGO/TPU strain sensors. (b) Changes in R/R0 and stress as a function of strain for the RGO/TPU strain sensors. (c) Fast response of RGO/TPU strain sensors. (d) Long-term stability of RGO/TPU strain sensors during 6000 cycles (strain from 0% to 50%).(e) Strain–time and R/R0–time curves of RGO/TPU strain sensors during step cyclic deformation with a maximum strain of 20%, 40%, 60%, 80%, and so on upto 200% in two cycles (increased by 20% for each step in a cycle). Reprinted with permission from ref. 18a. Copyright (2018) Elsevier.


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Substitution of eqn (3)–(5) into eqn (2) thus yieldseqn (6):

R ¼ 8pnhs02gN0

2a2e2ð1þ CeÞ exp gsþ ð2M þ gsCÞeþ 2We2

�

þ 2Ue2 þ 2Ve2� (6)

In some previous literature, the tunneling conduction modelcan well match the experimental data.4,29,134

3.2. Disconnection mechanism

For multidimensional conductive nanomaterials (e.g., one-dimensional (1D) CNTs, AgNWs, and two-dimensional (2D)GE), there is a large overlap of conductive nanomaterials insidethe polymer matrix or on the surface of the polymer substrate.

With the stretch of FCPCs, the occurring slippage of theconductive nanomaterials because of weak interfacial bindingand a large stiffness mismatch between the conductive nano-materials and flexible polymer leads to overlap disconnectionand reduction of the overlapping area, and thereby causesincreased electrical resistance.23,137,138 The disconnectionmechanism is often used to explain the strain sensingmechanism of CPCs consisting of flexible polymers and multi-dimensional conductive nanomaterials. For instance, Li et al.139

fabricated a highly stretchable and ultrasensitive strain sensorbased on a RGO wrapped aligned TPU fibrous mat. Theyproposed that the responsive mechanism of the strain sensorwas attributed to the changes in the overlap area between thecontacting flakes and the contact resistance in the flake–flakejunctions.

Fig. 9 (a) Schematic of the fabrication process and crack formation of CNT/TPU composites. (b) DR/R0 of the composites vs. strain. (c) Time response ofthe composite. (d) DR/R0 of the composite with 10% applied strain during 10 000 stretching–releasing cycles. Reprinted with permission from ref. 114.Copyright (2019) American Chemical Society.


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3.3. Crack propagation

For most sandwich-type FCPCs or adsorption-type FCPCs,cracks will originate and propagate in the concentrated stressareas of the brittle conductive layers coated on the flexiblepolymer matrix when the composites are stretched. Undoubtedly,the appearance and propagation of the cracks lead to a remark-able increase in the electrical resistance. During the releasingprocess, the cracks of the conductive layer can be reconnected,thus causing the drop of the electrical resistance. This reversibledisconnection and reconnection of cracks during the stretchingand releasing cycles endow the strain sensor with high sensitivityand outstanding repeatability. Based on the above sensingmechanism, intensive attention has been paid to designhierarchical crack-structures on the conductive layer to fabricatestretchable strain sensors with high strain sensitivity.69,70,123 Forinstance, Wang et al.123 successfully constructed crack-structuresof a conductive CNT-layer utilizing pre-stretching of a PDMSsubstrate, which was fabricated as a high-performance strainsensor. In that work, the working mechanism of the crack-structure was systematically investigated, as shown in Fig. 11.It was found that the increase in the electrical resistance of thecomposites was caused by the damage of the conductive pathsdue to the widening of the initial cracks as well as the creation of

new cracks during the stretching. The gap of the initial crackswas enlarged in the direction perpendicular to the stretch, whichresulted in the reduction of the contact between adjacent CNTs.Therefore, the electrical resistance of the composites wasincreased rapidly. The gap of the cracks was increased mono-tonically with a continuous crack extension during subsequentstretching, leading to a further reduction of the number ofconductive paths. Although most cracks were opened at 60%strain, there were still bridges between the islands and gaps toensure efficient electronic transport, which led to a relativelylarge workable strain range. The evolution of the crack morphologyduring the strain loading process could be vividly simulated byChinese paper cuttings (Fig. 11b). To better understand theresistance of the crack structure, a simplified resistance modelis illustrated in Fig. 11d and its electrical resistance (R) can bequantificationally calculated by eqn (7):

R ¼ R1Rc þ 2R1R2 þ R2Rc

R1 þ 2Rc þ R2(7)

where R1, R2 and Rc are the electrical resistances of the island,gap and bridges, respectively.

Although there are three different strain-responsive mechanisms,the strain sensing behavior of FCPCs is normally dominated by a

Fig. 10 (a) Diagram of the preparation process of CNT/PDMS composites. (b and c) SEM images of the surface (b) and cross-section (c) morphology ofthe PDMS/CNT samples. (d) A proposed model showing the permeation of CNTs into PDMS with surface modification. (e) DR/R0 of the compositesagainst strain (different colors of the curve represent different levels of SCA). (f) GF of the composites against SCA content. Reprinted with permissionfrom ref. 77a. Copyright (2019) Elsevier.


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combination of these mechanisms. For example, Yang et al.112

reported that the responsive mechanism of the strain sensor basedon GE/PDMS composites can be divided into three regions, asshown in Fig. 12. Under a strain of 5%, the electrical resistancechange is mainly attributed to the overlap disconnection of theGE sheets, thus leading to the increase of the electrical resistanceof the composites. Between a strain of 5% and 15%, the electricalresistance change is dominated by the overlap disconnection ofthe GE sheets as well as the crack origination and propagation inthe stiff GE layer. With a further increase of the strain, DR/R0

increases rapidly because the electrical resistance variation isonly determined by the crack generation and propagation.The existence of multiple conductive mechanisms in a stretchable

strain sensor leads to nonlinear changes in the electricalresistance with strain, as shown in Fig. 12d.

In summary, the sensing mechanisms of FCPC-based strainsensors are very complex, which highly depend on both thecategories of the FCPCs and the inner structures of theconductive networks inside the stretchable polymer matrix. Ingeneral, the tunneling conduction mechanism and disconnectionmechanism always coexist in filled-type FCPC-based strain sensors.However, the crack propagation mechanism may be the dominantsensing mechanism for sandwich-type FCPC-based strainsensors and adsorption-type FCPC-based strain sensors becausecracks can easily propagate in the brittle conductive layersduring stretching.

Fig. 11 Strain sensing mechanism of CNT films/PDMS strain sensors. (a) Series of optical images of the CNT films/PDMS composites beingstretched from 0% to 60%. (b) Chinese paper cuttings simulating the change of crack morphology from unstrained to strained states. (c) Average gapwidth versus strain loading. (d) The resistance model of a sensing unit. Reprinted with permission from ref. 123. Copyright (2018) Royal Society ofChemistry.

Fig. 12 The theoretical analysis of overlapping GE. (a) The randomly positioned GE scale model without strain. (b) The randomly positioned GEscale model under a strain of 10%. (c) The randomly positioned GE scale model under a strain of 25%. (d) The resistance–strain curve of this randomlypositioned GE scale model sensor built by using Matlab. Reprinted with permission from ref. 112. Copyright (2018) Royal Society of Chemistry.


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4. Factors affecting the strain sensingperformance of FCPCs

Based on the strain-responsive mechanisms of FCPCs discussedin the above section, the sensing performance of FCPCs isrelated to the factors that exert influences on the tunnelingeffect, overlapping area of the conductive nanomaterials or crackpropagation during stretching. Basically, these factors includethe shapes and dispersions of the conductive nanomaterials,interactions between the conductive nanomaterials and thepolymer matrix, inner structure of the conductive layer, andmorphologies of the polymer matrix. These multiple influencefactors have a two-blade function in the application of FCPC-based strain sensors. First of all, these numerous influencefactors make the sensing behavior more complicated. On theother hand, they provide us with more effective means tocontrol or improve the sensing performance of FCPCs. In thefollowing section, we will systematically discuss these influencefactors.

4.1. Dispersion and morphology of conductive nanomaterials

For filled-type FCPCs, it is known that the dispersion state ofthe fillers in the polymer matrix plays an important role in theelectrical properties of the composites. In general, good dispersionof fillers in the polymer matrix helps to lower the conductivepercolation threshold of the composites, and thereby affects thestrain sensing behavior of the composites.

On the other hand, the morphology or shape of conductivenanomaterials can directly affect the destruction and recon-struction of conductive paths during the stretching–releasingcycles, and thus plays an important role in the strain sensingbehaviors of FCPCs. According to the shape or dimension ofthe conductive fillers, 0D CB, 1D CNTs, AgNWs, and 2D GEare widely used to prepare FCPC-based strain sensors.

CB particles are a type of 0D nano-sized conductive materials,which tend to aggregate together to form conductive networksin a polymer matrix. In general, the conductive percolationthreshold of CB-filled FCPCs is very high. However, comparedwith the strain sensors based on FCPCs filled with 1D or 2Dconductive nanomaterials, the strain sensors based on CB-filledFCPCs possess a relatively higher strain sensitivity because theCB-network is easily destroyed and reconstructed during thestretching–releasing cycles. First of all, the interactions betweenthe CB particles are weak, which means that the CB-network iseasily destroyed during the loading process, which causes asharp increase of the electrical resistance of CB-filled FCPCs.During stretching, the CB-network is destroyed and CB particlesmove with the surrounding flexible polymer chains. On the otherhand, CB particles will return to their initial places with thesurrounding polymer chains also in the subsequent releasingprocess. Therefore, CB-filled FCPCs have higher sensitivityand better recoverability during the stretching–releasing cycles.Compared with 0D CB, 1D CNTs can form percolated conductivenetworks in the polymer matrix with a lower percolationthreshold and a higher electrical conductivity owing to theextraordinary electrical conductivity and large aspect ratio of

CNTs. However, the strain sensitivity of CNT-filled FCPCs isinferior to that of CB-filled FCPCs because of the entanglementof CNTs, which makes the CNT-networks more stable understrain. In addition, CNT-filled FCPCs often exhibit nonmonotonicstrain sensing behaviors, such as the shoulder peak phenomenon.This is attributed to the competition between the destruction andconstruction of the CNT-networks during the stretching andreleasing cycles. Moreover, AgNWs are another type of 1Dconductive nanomaterials, which have received widespreadattention for the fabrication of stretchable strain sensorsbecause of their high aspect ratio, outstanding rigidity, andsuperb conductivity. Generally, AgNWs are usually used insandwich-type FCPCs because AgNWs are more easily oxidizedin the air.

Graphene, a single layer of carbon atoms with a two dimen-sional structure, has been used to prepare FCPC-based strainsensors because of its outstanding properties.69,140,141 For GEfilled FCPCs, a low conductive percolation threshold can beachieved owing to its large specific surface area and intriguinglow electrical resistivity. However, the shoulder peak phenomenonand inferior repeatability also exist in the strain sensing behavior ofGE filled FCPCs.23,142 Liu and coworkers142 observed a ‘‘shoulderpeak’’ in the resistance–strain behavior of GE/TPU composites.Moreover, a gradual increase in the maximum DR/R0 wasobserved, which was attributed to the formed irreversibledestruction of the GE networks under cyclic loading. Sandwich-type or adsorption-type FCPCs containing GE conductive layersusually possess high strain sensitivity because the brittle GElayers easily generate high density cracks, leading to a signifi-cant increase of electrical resistance. For example, Wang et al.69

fabricated a flexible and wearable strain sensor by adhering GEwoven fabrics on a polymer and medical tape composite film.The sensors exhibited high sensitivity and high reversibility,and could be used to detect weak human motions, such asexpression change, blinking, breathing, and pulses.

In our previous work,81 the dependence of the strain sensingbehaviors of isoprene rubber (IR) based conductive compositeson the filler dimensionality (0D CB, 1D CNTs and their combi-nations) was systematically investigated as shown in Fig. 13. Itwas found that CB/IR composites showed ultra-high sensitivityto strain and good repeatability in the repeated stretching–releasing cycles. This is because CB-networks have a largenumber of CB–CB contact points, which are easily brokenunder strain and quickly reconstructed after removing thestrain. However, it needs a high loading of CB for the CB/IRcomposites to achieve electrical percolation (8.01 phr), leadingto poor mechanical properties of the strain sensors. In contrast,CNT/IR composites exhibited excellent electrical conductivitybut low strain sensitivity and poor repeatability because theentangled CNTs, which made the CNT-networks relativelystable under strain. Interestingly, the combination of CB andCNTs could well overcome the drawbacks of CNT/IR and CB/IRcomposites, which possessed a low percolation threshold, highsensitivity and repeatable response to the applied strain. There-fore, the hybridization of different conductive nanomaterials isan effective strategy to improve the electrical conductivity and


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strain sensing behaviors of FCPCs, and has received intensiveattention from a large number of researchers.12,15,80,81,99,107,109,134

4.2. Interactions between the conductive nanomaterialsand the polymer matrix

It has been reported that the interactions between the conductivenanomaterials and the polymer matrix play an important role inthe sensing behavior of FCPC-based strain sensors.4,134,143 First ofall, the interactions between the conductive nanomaterials andthe polymer matrix have a great influence on the strain sensitivityof the composites. It is clear that strong interface interactionsbetween the conductive nanomaterials and the polymer matrixcan effectively transfer the load from the weak polymer matrix tothe strong conductive networks, which makes them be destroyedmore easily under strain.77,134 Therefore, a stronger filler/polymerinteraction normally results in higher sensitivity of sensors.

For instance, Narongthong et al.144 investigated the strainsensing behavior of CB-filled FCPCs via adding differentamounts of an ionic liquid (IL) to tune the interactions betweenCB and the polymer matrix. They found that the strain sensi-tivity at a small strain could be significantly improved after theintroduction of the IL due to the increased interaction betweenCB and the polymer matrix with the assistance of the IL.Meanwhile, the strain sensing range was also broadenedbecause the strong interaction facilitated the alignment ofCB-aggregates during the stretching. This phenomenon was alsoobserved in other FCPCs, especially 1D CNT-filled FCPCs.83,134,145,146

Second, interface interactions between the conductive nano-materials and the polymer matrix also affect the hysteresisbehavior of FCPCs during repeated stretching–releasing processes.Hysteresis is dominated by the viscoelastic nature of the polymersas well as the interaction between the conductive nanomaterials

Fig. 13 (a) Electrical conductivity as a function of filler content for CNT/IR, CNT/CB(1/1)/IR, CNT/CB(1/2)/IR and CB/IR composites. (b–e) DR/R0 versustime under stretching–releasing cycles for (a) 2 phr CNT/IR, (b) 10 phr CB/IR, (c) 3 phr CNT/CB(1/1)/IR and (d) 4 phr CNT/CB(1/2)/IR composites at a strainrate of 10 mm min�1 and a maximum strain of 30%. (f) Schematic illustration to illuminate the strain sensing mechanisms of FCPCs filled with CB (a0),CNTs (b0), and CB/CNTs (c0). Reprinted with permission from ref. 81. Copyright (2018) Elsevier.


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and the polymer matrix, and leads to the irreversible destruction ofthe conductive paths upon dynamic loading.23,79,137,147–149 ForFCPCs loaded with CNTs or GE nanosheets, strong interactionsbetween conductive particles and the polymer matrix can effec-tively weaken the hysteresis behavior. However, weak interactionsnormally lead to the slippage of conductive particles inside or onthe polymer matrix upon high stretching and non-timely recoveryof the conductive networks upon releasing, which results inobvious hysteresis phenomena. Duan et al.85a investigated thestrain sensing behavior of extrusion-based FCPCs filled with twodifferent CB particles. It was found that the CB possessing strongerinteractions with the polymer matrix contributed to the establish-ment of a quasi-monotonic static response and a hysteresis-lessdynamic response. Third, for adsorption-type FCPCs, strong inter-actions between the conductive layer and the polymer matrix canprevent the adsorbed conductive particles falling off from thepolymer substrate and stabilize the structure of the FCPCs, whichensures the stability and reproducibility of the strain sensors.

4.3. Morphologies and structures of FCPCs

For CPCs containing two different polymers, it is known thatthe phase morphologies of the CPCs have a great influenceon the electrical properties. In the past few decades, twophase morphologies or structures, i.e., the double percolationstructure and segregated structure, were developed to reducethe percolation threshold of the composites due to the volumeexclusion effect.5,59,150–154 Recently, this strategy was alsoused in the design of FCPC-based strain sensors. Duan andcoauthors investigated the strain sensing behavior of CB filledFCPCs, utilizing binary and ternary composites based on TPU

and OBC.101 As shown in Fig. 14a, it was found that a high CBcontent was needed for binary CB/OBC or CB/TPU composites,leading to low strain sensitivity as their conductive networkswere fully packed and therefore difficult to be truly destructed.For the CB/TPU/OBC ternary composites, much higher sensi-tivity was achieved because of the enrichment of CB in thecontinous OBC phase, which formed a less compact and‘‘brittle’’ condutive network in the OBC phase. Clearly, sucha network was more easily destroyed under strain, resultingin a rapid increase in the tunneling distance, which wascritical for high strain sensitivity. Moreover, the segregatedstructure was also utilized to improve the strain sensitivityof FCPCs, as shown in Fig. 14b.104 In that work, the authorsfabricated CNT@cellulose nanocrystal (CNC)/NR nanocompo-sites with a 3D hierarchical segregated structure, which couldbe used as highly sensitive, stretchable and reversible strainsensors.

Beside the phase morphologies, some specific structures,such as 3D interlaced18,110 or aligned conductive fiber139 networkstructures, were also utilized to design wearable strain sensors.For example, Wang et al.18a generated a special interlacedconductive network based on RGO-decorated flexible TPUelectrospun fibrous mats, as shown in Fig. 14c. It was foundthat the RGO was dispersed on the surface of the interlacedTPU fibers, and formed an excellent three dimensional con-ductive network. This special hierarchical conductive networkendowed the RGO/TPU composites with desirable integrationof good stretchability and high sensitivity. Moreover, it was alsoreported that the strain sensors based on aligned RGO/TPUfibrous mats exhibited anisotropic strain sensing behaviors

Fig. 14 (a) Graphical representation of the network morphology evolution for binary CB/TPU composites, TPU-CB/OBC ternary composites, andOBC-CB/TPU ternary composites. Reprinted with permission from ref. 101. Copyright (2018) American Chemical Society. (b) Schematic of the resistanceincrease in a 3D conductive network nanocomposite when it is deformed by uniaxial stress in tension. NR latex particles are represented by circularparticles, the 3D conductive network by the black background and electrons by e�. Reprinted with permission from ref. 104. Copyright (2016) RoyalSociety of Chemistry. (c) Illustration of a RGO anchored TPU fibrous network. Reprinted with permission from ref. 18a. Copyright (2018) Elsevier.(d) Schematics of the microstructural development of aligned RGO/TPU/PDMS flexible conductive composites and nonaligned RGO/TPU/PDMS flexibleconductive composites in tensile tests. Reprinted with permission from ref. 139. Copyright (2018) Royal Society of Chemistry. (e) SEM image of a GEconductive layer with cracks. Reprinted with permission from ref. 112. Copyright (2018) Royal Society of Chemistry. (f) SEM image of wrinkled Pt thin films.Reprinted with permission from ref. 155. Copyright (2018) Royal Society of Chemistry. (g) SEM image of an aligned CNT film based conductive layer.Reprinted with permission from ref. 159. Copyright (2018) Elsevier.


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because of different evolution of the conductive networks indifferent directions during stretching (Fig. 14d).139

Furthermore, the morphology of the conductive layerof sandwich-type or adsorption-type FCPCs also plays animportant role in the strain sensing behavior of sensors.Different structrues of the conductive layers, including a crackstructure,112,114,120,122,123 wrinkle structure111,130,155–158 andaligned structure,78,159 have been developed in the designof strain sensors based on FCPCs. Generally, cracks in theconductive layer (Fig. 14e) endow the sensors with ultra-highstrain sensitivity and good repeatability through reversibledisconnection and reconnection of cracks during the stretching–releasing cyclic process.112 Nevertheless, FCPCs with a crackstructure usually exhibited a relatively narrow strain sensingrange due to the instability of the crack structure.69,122 Besidesthe crack structure, a wrinkle structure was also utilized todesign FCPC-based stretchable strain sensors. For instance, astretchable strain sensor containg a wrinkled platinum (wPt)film adhered on an elastomer (Fig. 14f) was fabricated by Peganet al.155 This strain sensor demonstrated a wide workable strainrange (up to 185%) owing to the wrinkle structure. In addition,the sensitivity and sensing range were tunable by adjusting thewPt film thickness. Recently, aligned conductive networks of aCNT film were designed to prepare highly stretchable multi-dimensional strain sensors. These CNT/PDMS based strainsensors showed high sensitivity (a GF value of 461 at a strainof 260%) along the CNT alignment direction but poor sensitivity(a GF value of 3.28 at a strain of 400%) along the CNT perpendi-cular direction, as shown in Fig. 14g.159

4.4. Others

Besides the above factors, some other influence factors, such asthe tunneling barrier height and properties of the flexiblepolymer matrix,160 preparation methods,52 filler content,161

and strain rate,81 may also affect the strain sensing behaviorof FCPCs. Based on the tunneling effect, a higher tunnelingbarrier height of the polymer matrix normally endows FCPCswith a higher strain sensitivity. Moreover, the viscoelasticity ofthe polymer matrix affects the hysteresis behavior as well as theresponse time during the stretching–releasing cycles. Gener-ally, a polymer matrix with a higher elasticity results in betterrepeatability of the strain sensors based on FCPCs. It was foundthat FCPCs based on an ultra-soft polymer matrix usually havea long response time because the soft polymer matrix canonly provide a low recovery force for the reestablishment ofconductive networks.162 Duan et al.52 prepared CNT-filled SBScomposites by solution blending and melt blending, respec-tively. During the dynamic loading and unloading processes,pronounced shoulder peaks were observed for the compositesfrom melt mixing, while linear relationships and reversibleresistivity were observed for the composites from solutionmixing. The difference in the strain sensing behaviors of theCNT/SBS composites prepared in different ways was attributedto different morphologies of the CNT-networks inside the SBSmatrix. Moreover, the strain rate and the conductive nanomaterialcontent also have a significant influence on the strain sensing

behaviors of FCPCs. It was reported that the electrical responseof the composite can be enhanced as the strain rate isincreased. This is because fast stretching can cause immensedestruction of the conductive networks, thus resulting in asharp increase in electrical resistance. Furthermore, it seemsthat composites with a lower conductive nanomaterial contentpossess higher strain sensitivity because the low-densityconductive network is vulnerable to damage under the appliedstrain. Whereas, a high conductive nanomaterial content canendow FCPCs with a wide strain response range because of therobust conductive network inside.

5. Conclusions and outlook

Flexible conductive polymer composites can serve as smartsensors in various emerging fields, which stimulates continuouslygrowing interest in this research field. In this review, recentadvances were summarized in different types of FCPC-basedstrain sensors as well as their sensing mechanisms and influencefactors. Basically, three types of FCPCs, i.e., filled-type, sandwich-type and adsorption-type FCPCs, were used as the sensingmaterials to design stretchable strain sensors. Based on thesensing mechanism, the sensing behavior of FCPC-basedstrain sensors was dominated by multiple factors, includingthe conductive particle shape and dispersion, the interactionsbetween the conductive particles and the polymer matrix, thepreparation method, and so on. Up to now, some high-performance FCPC-based strain sensors have been prepared,and exhibited a wide workable strain range, high sensitivity,and good repeatability.

Despite great progress in recent years, there is still a gap forFCPC-based strain sensors to meet practical needs. For example,the hysteresis and nonlinear behaviors of FCPC-based strainsensors always hinder their practical applications. A possibleroute to solve these two problems may rely on more precisedesign of the conductive pathways inside the polymer matrix. 3Dprinting technology may be one possible solution to preciselyand artificially print conductive pathways in stretchable strainsensors if this technology can construct polymeric structureswith a micron order of accuracy. When the challenges of thehysteresis and nonlinear behaviors are overcome, we believethat FCPC-based strain sensors can show great potential inhuman motion detection,69,70 health monitoring,112 wearableelectronics,73–75 and soft robotic skin.76

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National NaturalScience Foundation of China for General Program (21774126)and the start-up fund from Hangzhou Normal University.


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