Stretchable and Self-Healable Conductive Hydrogels for WearableMultimodal Touch Sensors with Thermoresponsive BehaviorO. Young Kweon,†,∥ Suman Kalyan Samanta,‡,∥ Yousang Won,§ Jong Heun Yoo,†

and Joon Hak Oh*,§

†Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic ofKorea‡Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India§School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Seoul 08826, Republicof Korea

*S Supporting Information

ABSTRACT: Multifunctional hydrogels with properties including transparency, flexibility,self-healing, and high electrical conductivity have attracted great attention for their potentialapplication to soft electronic devices. The presence of an ionic species can make hydrogelsconductive in nature. However, the conductivity of hydrogels is often influenced bytemperature, due to the change of the internal nano/microscopic structure when temperaturereaches the sol−gel phase transition temperature. In this regard, by introducing a novelsurface-capacitive sensor device based on polymers with lower critical solution temperature(LCST) behavior, near-perfect stimulus discriminability of touch and temperature may berealized. Here, we demonstrate a multimodal sensor that can monitor the location of touchpoints and temperature simultaneously, using poly(N-isopropylacrylamide) (PNIPAAm) inhybrid poly(vinyl alcohol) (PVA) and sodium tetraborate decahydrate cross-linked hydrogelsdoped with poly(sodium acrylate) (SA) [w/w/w = 5:2.7:1−3]. This multimodal sensor exhibits a response time of 0.3 s and atemperature coefficient of resistance of −0.58% K−1 from 20 to 40 °C. In addition, the LCST behavior of PNIPAAm-incorporated PVA/SA gels is investigated. Incorporation of LCST polymers into high-end hydrogel systems may contribute tothe development of temperature-dependent soft electronics that can be applied in smart windows.

KEYWORDS: conductive hydrogel, stretchable electronics, self-healing, thermoresponsive gel, touch sensor

1. INTRODUCTION

The largest sensory organ of the human body is human skin,and its sense of touch deals with the temporal and spatialperception of external stimulus through a numerous number ofreceptors (e.g., mechanoreceptors for pressure/vibration,thermoreceptors for “warm” and “cold” temperature) that aredensely distributed all over an organ or tissue in our body.1,2

Human skin is composed of multilayers of muscles and fatsand a complex structure supported on a deformable system. Inparticular, the Ruffini corpuscle, a type of slow-adaptingmechanoreceptor, is present in the dermis and has a spindlelikestructure tied to the collagen matrix, making it sensitive to skinstretching and slippage. The Ruffini corpuscle is also classicallyregarded as a thermoreceptor, because there will be pain in thecase of deep burns as this receptor is burned off.3

Various artificial electronic skins (e-skin) and soft electronicdevices with multimodality, high flexibility, and human-friendliness have been intensively studied that could mimicthe sensory aptitudes of human skin for their potentialapplications in soft robotics, prosthetics, and human−machineinterfaces.4−7 Despite these efforts, precise stimulus detectionand perfect discrimination of multi-signals have not yet beenfully achieved in multimodal sensing platforms. Cho et al.8

developed a stretchable and transparent all graphene multi-functional e-skin sensor matrix that can detect humidity,temperature, and pressure. Ko et al.9 reported multimodalferroelectric sensors using microstructured hierarchicallyengineered elastic carbon nanotube fabrics, which were capableof simultaneously sensing external multistimuli, such as touchand temperature. Most researchers developed multimodalsensors by laminating or assembling each individual sensorlayer that can provide electrical signals, such as changes inresistance, capacitance, and current. Moreover, with thecontinuous development of soft electronics, efficient integra-tion of wearable, attachable, or implantable devices with thehuman body is required.10−15 Particularly, touch sensors canact as transducers that convert changes in external stimuli tomeasurable signals for elucidating a precise position where theyare touched. Although developing a pixel-by-pixel-basedsensing matrix is often employed for electronic devices orsmart phones (e.g., projected capacitive touch technology), amultimodal sensor array can also be fabricated by introducing a

Received: March 18, 2019Accepted: June 25, 2019Published: June 25, 2019

Research Article

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© XXXX American Chemical Society A DOI: 10.1021/acsami.9b04440ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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single conductive layer into a surface-capacitive array plat-form.3,16

Ionic conductive hydrogels are good candidates for softelectronics since they show excellent transparency and minimalvariation in resistance under large deformable or stretchingstates.17−19 Such devices could include, for example, a touchpanel on the surface of a printed circuit board or a flexibleelectronic device attached to human skin.19 Many hydrogelsare biocompatible, so they can be used for transplantation intothe body, healing therapy, and drug delivery. Some hydrogelsshow high transparency, allowing 99% transmission of thevisible light; hence, they are useful for transmission of opticalinformation.20 In addition, hydrogels containing a largeamount of water are capable of dissolving ions, thus enablingthe gels to serve as ionic conductors.21 Furthermore, these softelectronic devices sometimes require self-healing properties forrepeatedly recovering their mechanoelectrical performances atroom temperature, even at the point of damage, or under highdegrees of stretching. Joo et al.22 reported uniformly dispersedpoly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid)(PEDOT:PSS) within hybrid organogels composed ofpolyacrylamide (PAAm) in ethylene glycol (EG). Uponincreasing the PEDOT:PSS content to a smaller extent, theelectrical percolation in the gels could be greatly improved andthe fabricated PEDOT:PSS−PAAm organogels exhibitedexcellent electrical conductivity (0.01 S cm−1) with stretch-ability up to 50% strain. Park et al.23 reported a self-healing andconductive hydrogel made by the polymerization method ofconductive pyrrole in agarose solution. These conductivehydrogels showed long-term stability within living organismsand conductivities of 0.65−0.41 S cm−1, under both bendingand stretching tests over a range 0−35% of the original length.However, none of these hydrogel-based electronic devices canshow excellent bending properties and stable electricalperformances when stretched.In addition, the conductivity of hydrogels is often influenced

by temperature due to the internal nano/microscopic structurechanges when the temperature reaches the sol−gel phasetransition temperature. This also provides an indirect modelfor variation of the internal percolation network with changesin temperature. This means that the state of aggregation andextent of hydration of such polymers can be modulated at aspecific temperature, known as the lower critical solutiontemperature (LCST). The rational control of this propertymay lead to variations in the electrical properties of thepolymer solutions or films. The LCST is an interestingbehavior that is observed in certain polymer solutions. Atemperature lower than the LCST enables a polymer to beentirely miscible in a particular solvent in all proportions,whereas phase separation takes place above the LCST. TheLCSTs of few water-soluble polymers are particularly exciting,as they can cause phase separation from its solution uponheating. This may lead to the alteration of several keyproperties, including hydrophobicity, volume, as well as surfacephenomena.24 One of the representative temperature-respon-sive polymers featuring an LCST in water is poly(N-isopropylacrylamide) (PNIPAAm).25,26 PNIPAAm is a tem-perature-responsive polymer showing a sharp transitionbehavior and an LCST of about 33 °C. Above thistemperature, a reversible phase transition takes place from aswollen hydrated state to a shrunken dehydrated state,27 thuscausing a phase separation from water. Incorporation of suchLCST active polymers into a transparent hydrogel and the

behavior of these thermoresponsive coatings may lead totemperature-dependent transparency28 and conductivity of thehydrogel film, which has potential for application to smartwindows.Herein, we report a new type of stretchable and self-healable

touch sensor using ionic hydrogels composed of poly(vinylalcohol) (PVA), sodium tetraborate decahydrate (borax), andsodium polyacrylate (SA). The developed PVA/SA-basedmultimodal sensor shows excellent discriminability betweenthe touch position and surrounding temperature simulta-neously, with advantages of high optical transparency (ca.91%), self-healing capability (within 7.0 s), and outstandingbendability (pressure/bending-insensitive characteristics). Therapid self-healing process of the hydrogel is helpful inrecovering the electrical properties of the sensors and avoidingdegradation of performance during large deformations.Furthermore, using the surface-capacitive behavior of PNI-PAAm-incorporated PVA/SA gel, we have fabricated a large-area stretchable device (12 cm × 9 cm) that exhibits a responsetime of 0.3 s with a temperature coefficient of resistance(TCR) of −0.58% K−1 from 20 to 40 °C in real time. To thebest of our knowledge, this is the first demonstration of thefabrication of PVA-based hydrogels integrated with the LCSTbehavior of the PNIPAAm polymer and their application inmultimodal touch sensors. The developed multimodal sensorsnot only exhibit excellent stimulus discriminability of touchand temperature by monitoring the signal pattern of electricalconductivity but also show temperature-variable opticaltransparency. On the basis of these key features, our PVA/SA gels can be used for monitoring environmental stimuli andhealth conditions, as well as for energy-saving devices such assmart windows.

2. EXPERIMENTAL SECTION2.1. Preparation of PVA/SA Composite Hydrogels. PVA,

sodium borate decahydrate (borax), and sodium polyacrylate (SA)were purchased from Sigma-Aldrich. The 5 wt % PVA (MW 85 000−124 000 g mol−1) solution was prepared by dissolving PVA powder indeionized water at ∼85 °C with vigorous stirring for 3 h. Afterobtaining a viscous PVA solution, 35 wt % SA in distilled water wasinjected drop by drop into the PVA solution at ratios of 5:1, 5:2, and5:3 PVA/SA (w/w) solution. Next, PNIPAAm was added to the totalsolution at ratios of 1, 2, and 4 mg mL−1. After cooling the mixture toroom temperature, borax (10 wt % in H2O) solution (2.5 mL) wasadded with gentle stirring to form a gel. The total mixture was thenheated at 80 °C until completely dissolved. The prepared gel waspoured into a customized rectangular-shaped polyethylene plasticmold, and the mold was placed in a vacuum chamber to removeresidual bubbles and create a flat-shaped gel. After the polymerizationreaction was completed, the gel layer was dipped into an EGsolution−distilled water mixture (25:75) for 48 h.

2.2. Preparation of Thermoresponsive Touch Sensors. Theprepared rectangular-shape hydrogel film was placed on a 0.15 mmPET film. Copper electrodes for transmission of a current-changesignal to the board were connected at each of the four corners of thefilm. A signal-processing controller that can provide touchapplications was purchased from 3 M Corp; this controller has fiveelectronic wires. Among them, four wires except the top-roofelectrode were used to connect the prepared rectangular-shapedhydrogel film to the controller. We connected the wires clockwise.Downloaded EX II series capacitive touch system electronics was usedto detect the appropriate current level of the hydrogels.

2.3. Characterization of PVA/SA Hydrogels. The physical andmechanical characteristics of the PVA/SA hydrogels were studiedusing a force test stand (M7-10, ESM303; Mark-10), field emissionscanning electron microscopy (S-4800; Hitachi, Japan), and a UV−vis

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near infrared (NIR) spectrophotometer in the Korea Basic ScienceInstitute (Daegu, Korea). FT-IR (VERTEX 70; Bruker) in trans-mission mode was used to characterize the molecular structure of thehydrogel. Electrochemical impedance spectroscopy (EIS) measure-ments were carried out using electrochemical instrumentation(IVIUM Stat; IVIUM Technologies, Eindhoven, The Netherlands).Heavily n-doped silicon wafers (<0.004 Ω cm, 1 cm × 2 cm) and aconductive carbon tape were used as substrates and electrodes,respectively. The ionic conductivity of the PVA/SA compositehydrogels was calculated from the corresponding Nyquist plotsderived from EIS measurements. The electrical characterization andsensing tests were performed under ambient conditions using aKeithley 4200 semiconductor parameter analyzer to measure thecurrent−time characteristics, response times, and recovery times ofthe touch sensors at an applied voltage of 1 V.

3. RESULTS AND DISCUSSION

3.1. Fabrication of PVA-Based Conductive CompositeHydrogels. Schematics of the device configuration and thefabrication process of the PVA-based hydrogels are shown inFigure 1 (see the Experimental Section for details). Forfabrication, aqueous solution of PVA (MW 85 000−124 000)and borax (Na2B4O7·10H2O) were mixed, which formed cross-linked hydrogels via reaction of the borate ions, B(OH)4

− withthe hydroxyl groups (OH) of the PVA.29 The cross-linkingprocess between the tetra borate ions and −OH groups caneasily break and reform, building the hydrogel to form three-dimensional (3D) networks. This process is particularlyeffective for fabricating self-healable gels and maintains theirform factor. Moreover, such dynamic cross-links can be easilydisrupted by mechanical deformation, whereas facile bondreformation can take place due to the proximal association ofthe borate ions and −OH groups; thus, the self-healing processwas allowed at room temperature. However, the PVA−boraxhydrogel often flows under low stress and exhibits limiteddimensional stability due to its non-Newtonian behavior.30,31

This is because of the mobility of the PVA polymer chains andfree borate ions, showing the fast self-healing process withrapid cross-linking in the broken interfaces without the aid ofexternal stimuli. Although PVA gels themselves were reportedto form hydrogels with self-healing properties, the concen-tration of the introduced PVA was very high and limitedstretchability was observed in the stretching tests.32−34

Therefore, to address this issue, a range of quantities of SAand 5 wt % aqueous solution of PVA were homogeneouslymixed at 80 °C for the preparation of hydrogels. SA, a sodiumsalt of polyacrylic acid, is a superabsorbent polymer with theability to absorb as much as 100−1000 times its mass ofwater.35 The SA is an anionic polyelectrolyte with negativelycharged carboxylate groups in the main chain. During thehydrogel cross-linking process, PVA and SA solutions weresurrounded in 3D networks by the borate ions, thus formingthe conductive self-healing hydrogel (Figure 1b).With incorporation of a mere 0.59 wt % of SA [w/w = 5:1 of

PVA/SA], it was possible to obtain a transparent andstretchable PVA-based hydrogel. Even with 1.76 wt % SA[w/w = 5:3], the optical transmittance of a 1 mm thickpolymer film was ca. 91%, as shown in Figure 2a. Thetransmittance spectrum at 200−800 nm is mainly related tothe light scattering and degree of crystallinity of the PVA-basedhydrogels.36,37 The optical transparencies of PVA/SA gelsprepared on transparent Petri dishes with different ratios ofPVA/SA from 5:1 to 5:4 were compared (Figure S1,Supporting Information), and it was found that the trans-parency of the prepared PVA/SA films decreased withincreasing SA concentration from 0.59 to 2.36 wt %. Thedecrease in transparency upon addition of SA was due to theformation of aggregates and their nonuniform distributioncausing stronger light scattering, refraction, or reflection.36

Scanning electron microscopy (SEM) images (Figures 2b andS2, Supporting Information) showed a fibrillar network

Figure 1. Experimental setup and a schematic image of the PVA/SA multimodal sensor. Schematic illustration of (a) the device configuration and(b) the overall fabrication process of conductive hydrogel for multimodal touch/temperature sensor application.

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structure for the PVA−borax gel, which is usually observed forfreeze-dried gels.38,39 However, the structure of the driedPVA/SA hydrogel appeared as dense 3D networks, probablydue to cross-linking of the acrylate with some immobilizedPVA polymers. In addition, the Fourier transform infrared(FT-IR) spectra of the PVA/SA and PVA gels in the range of4000−500 cm−1 are shown in Figure S3, SupportingInformation. A wide band of the stretching vibrations of freeand hydrogen-bonded OH groups and a B−H stretching bandappeared in the range of 3700−3000 and 2900 cm−1,respectively.40 The spectral signature at 850−1050 cm−1

indicated the B−O vibration frequency upon gel formation,with borax working as a cross-linker.41 A characteristicabsorption band at 1451 cm−1 corresponds to the C−OHvibration, and two bands at 1653 and 1324 cm−1 correspond tothe typical symmetrical stretching vibrations of carboxyl anions−COO− for the anionic salts of carboxylic acids. Slightvariations in the shape of these bands suggest structural andconformation changes occurring during gel formation with SAand are only related to building-in of Na+ cations in thepolymer chain (Table S1, Supporting Information).

3.2. Mechanical Stability of PVA/SA CompositeHydrogels. By tuning the weight ratios of the PVA and SApolymers, a series of composite hydrogels showing distinctmechanical properties were obtained. As shown in therepresentative photographic images (Figure 2c), the more SAcontents increased, the higher Young’s modulus and maximumtensile stress of composite polymers were observed. Althoughall of the composite hydrogels showed good stretchability, theSA content of 0.59 wt % provided the highest values, withstrains exceeding 600%, and exhibited a creep behavior underthe influence of constant mechanical stress (Figure 2c,d). Theelastic modulus of the PVA/SA composite hydrogels increaseddramatically compared to that of the PVA gel, which resultedin values ten times higher compared with the pristine PVA gel(3.7 kPa). The toughest PVA/SA composite hydrogel [w/w =5:3] exhibited Young’s modulus of 423 kPa, which iscomparable to those of conventional soft rubbers (Figure 2eand Table S2, Supporting Information).42−50 The compositehydrogels exhibited a completely reversible behavior for strainsand immediately recovered to their original length (below50%), whereas the hydrogels were not completely reversible at

Figure 2. Physical and mechanical properties. (a) Transmittance spectrum of PVA−borax gel and PVA/SA−borax gels with a film thickness of 1mm (inset: landscape view of the PVA/SA film on a transparent Petri dish). (b) Scanning electron microscopy (SEM) image of PVA/SA [w/w =5:3] composite hydrogel film. (c) Stretchability of PVA/SA composite hydrogels with different SA concentrations. From left to right, the photoimages indicate gels prepared at ratios of PVA/SA w/w = 5:1, 5:2, and 5:3. (d) Stress−strain tests of PVA/SA hydrogels with different SA ratios.(e) The larger modulus of the PVA/SA gel is comparable to that of conventional soft rubbers (inset: photographic image of loading 300 g of PVA/SA gel).

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higher strains; however, even after stretching to 10 times theiroriginal length at a strain of 100%, the composite hydrogels cangradually recover back to their original length within ∼1 min.It is also remarkable that the composite hydrogels show agreater modulus at higher loading rates (300 g) (inset ofFigure 2e).3.3. Electrical Characteristics of PVA/SA Composite

Hydrogels. Apart from the high transparency and stretch-ability, the composite hydrogels also exhibited higher ionicconductivity due to the existence of ionic borate salts and Na+

cations on the polymer chains, indicating that the gel was anionic conductor. Figure 3a shows that the ionic conductivitiesof a series of composite hydrogels of PVA/SA [w/w = 5:1, 5:2,5:3] at 25 °C were 1.31 × 10−4, 4.66 × 10−4, and 6.64 × 10−4 Scm−1, respectively. The ionic conductivity was calculated fromthe impedance plot of the corresponding PVA/SA gels at 25°C using the following equation (Figure S4, SupportingInformation)51

σ = dR S

ionic conductivity ( )1

s (1)

where d is the thickness of the thin film, S is the area ofelectrodes in contact with the film, and Rs is the bulkresistance. Basically, the ionic conductivity gradually increasesby increasing the salt concentration. This is because thenumber of free ions also increases, as the salt concentrationbecomes larger. This result indicates that diffusion of ionsthrough the free volume of the hydrogel increases. When SA isadded, the number of free ions also gets larger until it issaturated. These ions get closer to one another, whichdecreases the free volumes and conductivity and thus showsnonlinearity of the conductivity.52

In addition, the stretchable ionic conductors developedherein exhibited excellent self-healing capabilities. Generally,PVA-based hydrogel cross-linked by borax is known to exhibitexcellent self-healing capabilities;53 therefore, we examinedhow the self-healing ability is affected by the addition of SA.The self-healing characteristics of the PVA/SA gel were testedwith an increasing ratio of PVA/SA from 5:1 to 5:3. It wasfound that the self-healing capability was degraded when theratio of PVA/SA increased from 5:1 (within 5.0 s) to 5:3(within 7.0 s). The representative optical microscope imagesfor the healing of PVA/SA composite hydrogel after completeseparation by a razor blade are shown in Figure S5, SupportingInformation. After the razor blade was removed, the twocracked parts came into contact with each other. The separated

parts were partially healed after 5 s and completely recoveredto normal after ca. 7.0 s at ambient conditions without anyexternal stimulus (e.g., light, heat, or force). To address theself-healing nature and the recovery of conductivity of thePVA/SA hydrogels, the current changes versus time of thePVA/SA composite hydrogels during the cut and healingprocess were observed (Figure 3b). When the compositehydrogel was totally cut with a razor blade, an open circuit wasobserved. As the razor blade was removed between twobifurcated parts, the current dropped quickly, reaching aconstant value within ca. 7.0 s. The currents of the sample wererelatively stable and easily recovered for five cycles, and a highself-healing efficiency was observed in each cutting and healingprocess. The healing efficiency of the PVA/SA hydrogel [w/w= 5:3] was calculated using σr/σi, where σr is the recoveredconductivity and σi is the initial conductivity of the PVA/SAfilm, respectively. The value of σr/σi was ≈100% after healing,indicating that the PVA/SA hydrogel has significant andrepeatable electrical recovery performance. It is noteworthythat the resistance of gels is very similar to the original valuewhen the two parts come into contact due to the hydrogenbonds from the PVA/SA polymer, thus activating themechanical healing of the composite so that Na+ ions cantransfer freely in the matrix.54

3.4. Thermosensitive Behavior of PNIPAAm-Incorpo-rated PVA/SA Gel. The ionic conductive PVA/SA compositehydrogel can be incorporated with PNIPAAm, which is atypical temperature-responsive polymer with an LCST of ca.33 °C in water,27 to obtain adaptive hydrogel films forswitchable glazing that can be passively switched on by externalheat, such as sunlight.First, we tuned the properties of PVA/SA gels with the

PNIPAAm polymer incorporated by changing the solvent fromH2O to EG/H2O (w/w) (25:75). The improvement ofenvironmental durability due to the introduction of SA, aswell as solvent exchange by partially replacing H2O with EG,was addressed. EG is a solvent that is similar to H2O in termsof solubility parameters (δ) and hydrogen-bonding ability (δEG= 34.9, δH2O = 48.0).55 Since the affinity between H2O and EGis much stronger than with other solvents, EG surrounds H2Omolecules and prevents penetration of other solvents orevaporation of H2O. Moreover, the high boiling point of EG(∼197.3 °C), together with its low volatility, increases theambient stability of the hybrid hydrogels. In addition, byintroducing EG, it was possible to lower the equilibriumswelling ratio of the gel, which decreased the transparency of

Figure 3. Ionic conductivity and self-healing properties. (a) Ionic conductivity of PVA/SA composite hydrogel with different w/w ratios. (b) Timeevolution of the electrical healing process was measured under ambient conditions. Cycling of the cutting-healing processes was performed at thesame location for the PVA/SA [w/w = 5:3] gel.

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the gel slightly but increased the durability significantly (morethan 1 month). Without EG, the ionic charges of SA would behydrated by water molecules to form a hydration spherearound the ionic charges. However, addition of EG satisfiessuch hydration partially through its polar hydroxyl groups andalso increases hydrophobicity of the system through theethylene moieties. Therefore, after solvent exchange with EG,uptake of new water molecules is expected to be lowered. Wealso found that our composite hydrogel remained stable underambient conditions for 3 weeks or more with full functionalityin e-skin or attachable sensor devices (Figure S6, SupportingInformation).The solvent substitution was conducted because the weak

mechanical properties and swelling of the hydrogels limitedtheir practical utility in electronic applications. Bian et al.reported a one-pot solvent exchange method for the gelationprocess, based on a synergistic effect of two physical cross-linkings in a binary solvent system where the polymers showedexcellent mechanical toughness; the resulting hydrogelspossessed a steady water content of more than 70%.56,57

Therefore, we chose the solvent system based on thePNIPAAm and PVA gels, which were swollen in the EG−water mixture. Typical temperature-induced volume phasetransition curves of PVA/SA/PNIPAAm gels and theircomposites, containing different EG−water ratios, are shownin Figure 4a. By comparing these curves, it is apparent that thecloud point Tc (30.1 °C) of PVA/SA/PNIPAAm gel in anEG−water binary solvent system did not change significantly.58

In addition, the swelling ratios of the PVA/SA/PNIPAAmhydrogels remained almost constant as long as the EGconcentration was higher than 50% (v/v). Below thisconcentration, the swelling ratio started to increase pro-gressively along with decreasing EG concentration. Theswelling ratio showed a sharp transition at EG concentrationsbelow 25% (v/v). Increasing the extent of hydrogen-bondingpropensity of EG could substantially improve the mechanicalstrength of the gel through the PVA chains that are denselypacked by hydrogen bonding.57 However, this could alsointroduce a loss of transparency and disable the LCSTbehavior of the PNIPAAm for applications such as smartwindows. Therefore, we fixed the composition of EG at 25% toachieve transparency and mechanical strength simultaneouslyfor the following testing.The transparent−opaque transition for the PNIPAAm

containing PVA/SA gel was investigated in detail by measuringthe temperature-dependent light transparency of the gel layersplaced between two layers of glass using ultraviolet−visible(UV−vis) spectroscopy (Figure S7, Supporting Information).PNIPAAm-incorporated PVA/SA hydrogels exhibited areversible transparent−opaque phase transition because ofthe phase separation at the LCST (Tc = 30.1 °C). Changes inthe optical transparency of the PVA layers with or without SAare shown in Figure 4b. The composite hydrogel was found tobe transparent at a temperature of 20 °C and became opaqueat 40 °C. The transparent−opaque transitions were fullyreversible and reproducible. When the same amount ofPNIPAAm (0−4 mg mL−1 in total solution) was loaded, theLCST of the PNIPAAm hydrogels containing PVA/SAexhibited nearly the same Tc value as that of the PNIPAAm-incorporated PVA gel. Both systems exhibited an abrupttransparent−opaque transition close to LCST of 30.1 °C dueto the phase separation. This result indicates that the SAconcentrations do not have a significant influence on the phase

transition temperature of the composite hydrogel. The phasetransition associated with PNIPAAm is not affected by thehydrogel network. The temperature-dependent switching forPNIPAAm/PVA/SA took place between a clear state with atransmittance of 77.5% and a cloudy state with 57.3%, whenmeasured in the analytical absorption cells (10 mm, quartz).The transparency contrast ratio of the hydrogel film isoptimum for their practical application in switchable glazingfor the screening of real-life temperature changes. We note thatthe transmittance of the ternary PNIPAAm/PVA/SA compo-site below the LCST is slightly lower than that of pure binaryPNIPAAm/PVA due to the absorption of SA but it is stillsufficient for use in clear glazing (Figures S8 and S9,Supporting Information). Below the LCST of 30.1 °C, wateremerges to be a good solvent for the PNIPAAm chains;therefore, the polymer chains are solvated in water, which leadsto a transparent solution. Near the LCST phase transition,PNIPAAm undergoes a conformational change and becomeshydrophobic in nature.58 Thus, above the LCST, water

Figure 4. Thermotropic behavior of PNIPAAm-incorporated PVA/SA composite hydrogels. (a) Equilibrium swelling ratio of PNIPAAm-incorporating PVA/SA composite hydrogel with different H2O andEG ratios. (b) Temperature−transmittance correlation for thePNIPAAm-incorporated PVA/SA gel from 20 to 40 °C by usingUV−vis spectroscopy. The temperature was raised by 2 °C for 1 minin the heating run, and samples were cooled at the same rate. (c)Temperature dependence of the conductivity of PVA/SA [5:3]-basedhydrogel depending on the amount of added PNIPAAm.

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becomes a poor solvent for the solvation of the polymer,because the hydrogen bonding between the polymer and wateris broken, which leads to the phase separation between thepolymer chains and water. Therefore, above the LCST, anopaque state appears because of the heterogeneous scatteringdomains from the PNIPAAm chains. Thus, above the LCST, astable equilibrium cloudy state is generated by the polymerchains, leading to a relatively constant optical transmittance.These transitions for the PNIPAAm/PVA/SA composite andPNIPAAm/PVA were found to be fully reversible, along with asmall hysteresis for both samples, as usually observed for gelsamples (Figure 4b).39

Furthermore, we found that the electrical properties of ourhydrogels are greatly affected by the composition andenvironmental temperature (Figure 4c). The conductivityshowed a nearly linear increase with temperature up to ∼20−50 °C at all compositions. The linear part of log σ versus 1/Tplots obeyed the Arrhenius type equation. Interestingly, theconductivity variation of PNIPAAm-incorporated PVA/SAcomposite hydrogels as a function of temperature was largerthan that of PVA/SA hydrogel alone, which may be related tothe phase transition of added PNIPAAm that may increase theconductivity by making more available conducting pathways.59

3.5. Thermosensitive Touch Window Application. Wefurther investigated the benefits of self-healing ionic con-ductors as components in soft machines. The PVA/SAhydrogel is a promising platform for wearable sensors.Therefore, we demonstrated the performance of such trans-parent, stretchable, and self-healable ionic conductors within awearable touch sensor and a smart touch window systemcombined with surface-capacitive technology. To assess thepotential for applications in touch sensors, we fabricated one-

dimensional (1D) strips of the composite hydrogels for sensingdevices. A single strip of PNIPAAm-incorporated PVA/SAcomposite hydrogel [w/w = 5:3] was connected to copper(Cu) electrodes on both sides, and an alternating current (AC)voltage was applied at both ends. The circuit schematicdiagram of the ionic touch strip is shown in Figure 5a (1D stripsensor; see Figure S10, Supporting Information). One side ofthe electrode had a constant voltage of 1 V applied, whereasthe other side was gradually increased from 0 to 1 V, and thecurrent flowing through the strip I1 was measured. When weapplied the same voltage to both electrodes, we measured astable current with a very small flow (Figure S11).19 When the1D strip was touched by a human finger, this event was splitinto two resistive parts at the touch point because the fingerimposed a grounding. The current signals were measured atthe corner of A1 electrode, as shown in Figure 5b. Themanufactured gel pads can recognize a range of motions,including drag, single, and double clicks due to their responsetime of 0.3 s, as shown in Figure 5b. The touch location couldbe described by a normalized distance, whereas the left andright ends of the strip were defined as α = 0 and α = 1,respectively. Therefore, the current flowing through the tworesistors can be obtained by the following equations.19

α=+

= −IR

R RI I(1 )1

2

1 2t t

(2)

α=+

=IR

R RI I2

1

1 2t t

(3)

where I1 and I2 are the touching currents measured from thecorner of A1 and A2 electrodes, respectively, and It = I1 + I2.

Figure 5. Thermoresponsive touch sensor application of PNIPAAm-incorporated PVA/SA hydrogels. (a) Working devices of the touch sensorsystem using 4-point surface-capacitive touch technology. (b) Time-dependent variation of current with touch/untouch cycles and normalizeddistance L of the 1D strip touch sensor. (c) Changes in the currents of touch/untouch are shown under various bending conditions. (d) Reflectedcomputer screen when touched as red and blue rectangular shapes, as displayed in (a). (e) Temperature-dependent current variation of PNIPAAm-incorporated PVA/SA hydrogels. (f) Time-dependent variation of current with touch/untouch cycles at various temperatures on the surface of thehydrogel sensors (the left-panel graph). The average of the base current, depending on temperature, is depicted in the right-panel graph.

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Furthermore, bending tests were conducted with the 1 mmthick hydrogel films bent into cylinders with Rb of 12.5, 10.5,7.0, 3.5, or 0.5 mm to clarify the bending-insensitivecharacteristics of the devices, as shown in Figure 5c. Notethat the PVA/SA devices were stable at Rb as small as 0.5 mm.The tensile strain was 4.0% upon estimation using ε = d/(2Rb),where ε represents the strain and d is the thickness of thedevice.60 This demonstrates the effectiveness of surface-capacitive PVA/SA hydrogels in terms of flexibility. Thecurrents with or without touch (L = 0.50α) were stable, evenafter repeated and severe bent state. Under bent conditions,the total length of hydrogel slightly increased because the toplayer of hydrogel receives tensile stress. However, the dividedportion with touch showed negligible change, which is due tocharacteristics of surface-capacitive mode. This suggestsoutstanding mechanical flexibility and operational stability inthe bent state.Since the PVA/SA hydrogel is based on polymeric materials,

it is well-suited for the manufacturing of low-cost and large-scale soft electronic devices (Figure 5a). We made a touchsensor device for demonstration purpose; the gels were notencapsulated, but for real-life applications, the gels can beencapsulated with suitable materials. This concept wasdemonstrated with a highly sensitive and large-area touchsensor (12 cm × 9 cm) using the surface-capacitive behavior ofan ionic conductor. For demonstration, the PVA/SAcomposite hydrogel-based sensor was directly placed on theskin to play the role of a touch panel. We demonstrated theresponse behavior of the PVA/SA composite hydrogel-basedtouch sensor during the repeated touch occurring by a finger ata frequency of 1 Hz. Upon touching the PVA/SA gel, a closedcircuit was formed and the touch point was grounded. Thisgenerates a potential between the touch location and theelectrode, resulting in a current flow. The distance between thetouch spot and the electrode determined the amount of thecurrent. By investigating the magnitude of the current at thefour corners of the PVA/SA gel, the co-ordinate of the touchlocation can be precisely obtained in real time (Figure 5d).However, the resistance slightly curved with the touch linesbecause of the nonlinearity of the surface-capacitive positionsensing mechanism so we calibrated the pad using the 25-pointmethod (3 M Micro Touch) (Figure S12, SupportingInformation). After calibration, the touch panel can workefficiently even in a bent state and can operate with isotropicdeformation. As shown in Figure 5e, for temperature detectionwith a high and reproducible TCR of −0.58% K−1 in the rangeof 20−40 °C, the PVA/SA composite hydrogel could be usedas a negative temperature coefficient thermistor. The basecurrent of our sensor increased linearly with temperature.When it was touched, the current changed dramatically inresponse to the touch on the screen and was not affected bychanges in pressure (Figure 5f, left-panel graph). The currentmeasured at the electrode was maintained even at the differenttemperatures with precise detection of the touch point.Furthermore, the device showed an ultrahigh transmittanceof 90% at room temperature and stable performance in thetemperature range from 20 to 45 °C (Figure 5f, right-panelgraph). The stimulus discrimination by our hydrogel sensorresulted from two factors: (i) the position of the touch pointcan be calculated by the current flowing through the fourresistors and (ii) the base current changes with ionicconductivity due to the temperature change of the wholehydrogel film. Furthermore, the manufactured gel pads

attached to human skin allow detection of drag, single clicks,and double clicks, which are useful for playing the mobilegame; i.e., the device has similar controllability to a mousedevice or a trackpad device (Movie S1, SupportingInformation).

4. CONCLUSIONSIn summary, we have developed extremely transparent,stretchable, and self-healable ionic conductive hydrogelscomposed of PVA/borax/SA [5:2.7:1−3]. The environmentalstability of the gel was further improved by solvent exchangewith a mixture of EG and water, as evidenced by themechanical tests. These gels showed self-healing properties incut-join tests under electrical performance. The hybridhydrogels developed herein showed high optical transparency(91% T), excellent stretchability (150% up to 600% strain),and fast self-healing capabilities (<7 s), as well as an electricalconducting behavior. In addition, when incorporated intoPNIPAAm polymer, PVA/SA composite hydrogels could beused as a stretchable, transparent, and self-healable material formultimodal sensors to discriminate touch points and temper-ature simultaneously. The thermoresponsive polymer PNI-PAAm PVA/SA sensor exhibited LCST behavior at atemperature of 30.1 °C within the PVA/SA [w/w = 5:3]composite hydrogel system. The self-healing multimodalsensor is capable of monitoring the location of the touchpoint and temperature stimuli simultaneously with a responsetime of 0.3 s and a TCR of −0.58% K−1 from 20 to 40 °C inreal time. Moreover, a large-area wearable device (12 cm × 9cm) composed of PVA/SA/PNIPAAm gel was demonstrated,with operation on human skin via the construction of Internetof Things (IoT) platforms. This method could emerge as anew route toward hydrogel-based fabrication of transparent,stretchable, and self-healable sensing materials that areresponsive to both pressure and temperature. We believethat these PVA/SA composite hydrogels can be used for thefabrication of artificial e-skin, wearable electronics, flexiblecircuits, and smart window applications.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.9b04440.

Optical images, SEM images, FT-IR data, Nyquist plots;air stability test, and thermotropic properties of PVA/SAhydrogels, and calibration data of multimodal touchsensor (PDF)

Wearable multimodal touch sensors with PVA/SAhydrogel composites (MP4)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: joonhoh@snu.ac.kr.ORCIDSuman Kalyan Samanta: 0000-0002-9587-977XJoon Hak Oh: 0000-0003-0481-6069Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.

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Author Contributions∥O.Y.K. and S.K.S. contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the National ResearchF o u n d a t i o n o f K o r e a ( N R F ) g r a n t ( N o .2017R1E1A1A01074090), the Nano Material TechnologyDevelopment Program (No. 2017M3A7B8063825), and theCenter for Advanced Soft Electronics under the GlobalFrontier Research Program (No. 2013M3A6A5073175)funded by the Ministry of Science and ICT (MSIT), Korea.

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