Ultrastretchable Strain Sensors and Arrays with High Sensitivity and

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Ultra Stretchable Strain Sensors and Arrays with High Sensitivity and Linearity Based on Super Tough Conductive Hydrogels Zhenwu Wang, Jing Chen, Yang Cong, Hua Zhang, Ting Xu, Lei Nie, and Jun Fu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03999 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 1, 2018

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Chemistry of Materials

Ultra Stretchable Strain Sensors and Arrays with High Sensitivity and Linearity Based on Super Tough Conductive Hydrogels Zhenwu Wang1, Jing Chen1, Yang Cong2, Hua Zhang1, Ting Xu1, Lei Nie1, and Jun Fu*,1 1

Cixi Institute of Biomedical Engineering & Polymers and Composites Division, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Zhongguan West Road 1219, Zhenhai District, Ningbo, 315201, People’s Republic of China 2

School of Materials Chemistry and Chemical Engineering, Ningbo University of Technology, Fenghua Road 201, Jiangbei District, Ningbo, 315201, People’s Republic of China ABSTRACT: Biocompatible conductive hydrogels with intrinsic flexibility, high sensitivity, linearity, and outstanding reliability are highly demanded for wearable devices or implantable sensors. Here we report novel tough conductive hydrogels comprised of interpenetrating polyaniline (PANI) and poly(acrylamide-co-hydroxyethyl methylacrylate) (P(AAm-coHEMA)) networks. Intrinsic interactions between the conductive PANI network and the flexible P(AAm-co-HEMA) endowed hydrogels with outstanding strength and toughness to cyclic loadings. The conductive hydrogels show very high sensitivity (gauge factor 11) and outstanding linear dependence of sensitivity on strain. Strain sensors based on the conductive hydrogels demonstrate reliable detection of repeated large strains and subtle vibrations, including the movements of various human joints, pulses and voiceprints. Moreover, a prototype 2D sensor array is fabricated to sense strains or pressures in the two dimensions, which is promising for electronic skin, touchpads, biosensors, human-machine interfaces, biomedical implants, wearable electronic devices and so on.

INTRODUCTION Ultra-stretchable conductive materials are widely pursued 1, 2 for applications in electronic skin, human-machine inter3 4 5 faces, biomedical implants, human activity monitors, 6-9 wearable electronic devices and so on. Linearity and high sensitivities to force and strain, as well as outstanding con10, 11 ductivity at high strains, are desired. Most flexible con12, 13 ductors are composites of elastomers and metal particles, 14 15, 16 17 liquid metals, carbon, ionic liquids, or conductive pol18, 19 ymers. Many contain high filler contents and low fracture strain (< 100%) due to the poor filler-matrix compatibility. Flexible conductors with high extensibility and stable conductivity at large strain are highly demanded for sensors and 20 devices. Conductive hydrogels with excellent biocompatibility and intrinsic flexibility have recently emerged as promising mate21 4, 22 rials for wearable devices or implantable sensors . Hydrogel composites with functionalized single-wall carbon nano23, 24 25 tubes or conductive polymers (e.g., PEDOT: PSS, poly26 27 aniline, polypyrrole, etc) have been developed as strain/force sensors. However, many are limited by poor mechanical strength (tens of kilopascals) or delicacy to cyclic 27, 28 loadings for practical use. To compensate such shortcomings, self-healing conductive hydrogels have been prepared, but with slow healing and trade-offs between strength and 23, 24, 29 healing. Only a few tough conductive hydrogels with 30, 31 26 double network, microsphere crosslinking, or nano32 composite structures have been reported to withstand re-

peated loadings. Moreover, high sensitivity and linearity are 11 pivotal for the feasibility and accuracy of hydrogel sensors, which rely on reliable interactions between the conductive network and the flexible matrix so that no damages in con33 ductive network occur during deformations. However, most conductive hydrogels usually show a non-linear dependence of sensitivity on strain, presumably due to the damage of 34 conductive network during stretching. It remains a challenge to obtain both linearity and high sensitivity for conductive hydrogels. Here, we report novel tough conductive hydrogels for applications as flexible strain sensors with very high sensitivity and linearity. The hydrogels are comprised of interpenetrating conductive polyaniline (PANI) network and biocompatible flexible network of acrylamide and hydroxyethyl methylacrylate copolymer, P(AAm-co-HEMA) (Figure 1a). The networks are closely associated through hydrogen bonds between the hydroxyl, amide, and aniline groups. Upon stretching, both networks deform uniformly, resulting in linear changes in resistance. The hydrogels show excellent 35 fatigue resistance against cyclic loadings, very high sensitivity to strain, and linearity over large strains. The hydrogels are used to detect very weak vibrations and human acitivities. Moreover, prototype 2D hydrogel arrays are fabricated for 2D strain/force sensing, which is promising for electronic skin and touchpads.

RESULTS AND DISCUSSION

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Figure 1. a) Schematic illustration to the synthesis of interpenetrating PANI/P(AAm-co-HEMA) hydrogels. Scale bars, 2 cm. SEM images of freeze fractured surface of b) the P(AAm-co-HEMA and c) PANI/P(AAm-co-HEMA) hydrogels. d) CLSM image of the PANI/P(AAm-co-HEMA) hydrogels. Figure 1a illustrates the hydrogel synthesis. First, the P(AAmco-HEMA) hydrogel was synthesized by free radical copolymerization of AAm and HEMA monomers with methylene bisacrylamide as crosslinker, with the presence of aniline (ANI) monomers. Subsequent in situ oxidation of the ANI by ammonium persulfate generated PANI network in the hydrogel, converting the transparent hydrogel into dark green. The formation of PANI was confirmed by XRD (Figure S1a, Supporting Information). The obtained hydrogels could be knotted and stretched to high strains (Figure 1a and Movie S1, Supporting Information). The PANI particles form a continuous network within the P(AAm-co-HEMA) matrix (Figure 1b,1c). Confocal laser scanning microscopy (CLSM) further demonstrated an interconnected green photoluminescent PANI network (Figure 1d). Therein, the PANI chains form hydrogen bonds with the P(AAm-co-HEMA) network, –1 as confirmed by the shift of the N-H bands at 33150-3350 cm –1 and C=O bands at 1650-1730 cm according to FTIR measurements (Figure S1b, Supporting Information). Besides, the hydrogels become swollen and release PANI when the hydrogen bonds are broken in urea solution (Figure S2, Supporting Information). The interpenetrating structures are critical for the outstanding mechanical properties and the conductivity of the gels. Figure 2a shows representative tensile stress-strain curves of hydrogels with 0.5 wt/vol% ANI at various oxidation times. The ultimate tensile strength (UTS) increases from 0.25 MPa before oxidation to 1.03 MPa (24 h oxidation), 1.52 MPa (48 h) and 7.27 MPa (72 h), while the fracture strain (εf) decreases from 530% to 440% (24 h), 310% (48 h) and 220% (72 h).

Meanwhile, the moduli and UTS of hydrogels increase with aniline content (CANI), while the εf decreases (Figure S2, Supporting Information). The corresponding toughness in Figure 2b demonstrates that PANI significantly enhance the hydro3 gels, in which the largest toughness (9.19 MJ/m ) of the PANI/P(AAm-co-HEMA) hydrogel is 11-fold higher than pris3 tine P(AAm-co-HEMA) hydrogels (0.77 MJ/m ). The compression strength is remarkably improved from 6.05 MPa before oxidation to 14.47 MPa (24 h), 17.00 MPa (48 h) and 17.46 MPa (72 h) (Figure 2c). The hydrogels sustain repeated loadings, and recover rapidly after load release (Movies S1, S2, Supporting Information). Figure 2d shows representative stress-strain curves of cyclic tensile loading-unloading tests with gradual increase in strain. The pronounced hysteresis loops demonstrate the energy dissipation during loadingunloading cycles. The areas, or the tensile toughness of the P(AAm-co-HEMA) hydrogel, as well as those under the unloading curves, increases with the strain (Figure S4a, Supporting Information). Interestingly, the second, third, and fourth loading curves overlap the previous ones, indicating an immediate recovery of the network despite the energy dissipation during loading. Such rapid recovery is likely attributed to the reversible and dynamic hydrogen bonding between the PAAm chains and the PANI moieties that tempoarily ruptures to dissipate energy upon loading and rapidly reforms during unloading. At high strains, the network may be damaged, resulting in less recovery upon unloadings. The hydrogels show high conductivity with the formation of the PANI network. With 0.5 wt/vol% ANI, for example, the


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conductivity increases from 7.02×10 ±2.95×10 S/m before oxidation to 8.14±0.50 S/m after oxidation for 72 h (Figure 3a). It sharply increases at 48 h, followed by a steady increase, suggesting the formation of conductive PANI network at around 48 h and subsequent network perfection upon prolonged oxidation. On the other hand, the conductivity monotonically increases to a plateau value (8.24±0.57 S/m) at CANI of 0.5 wt/vol% (Figure 3b). This conductivity value is 36 close to that of pure PANI hydrogel (11 S/m). The threshold CANI of 0.5 wt/vol%, much lower than those reported in liter37 ature , indicates the formation of a continuous conductive network. Such a low threshold is critical for outstanding strength, toughness and high conductivity.

The conductive hydrogels show very high sensitivity to loadings. The resistance change ratio upon loadings is defined as the resistance change |R0-R| with respect to the initial resistance R0, i.e., ΔR/R0 = |R0-R|/R0. Figure 4a shows an unprecedented linear increase in ΔR/R0 with strain from 0% to 300%, which offers a broad work region of the hydrogel as a strain sensor. The sensitivity or gauge factor (GF) is defined as GF = (ΔR/R0)/ε, where ε is the strain. The gel shows a very high sensitivity of 5.7 at very low strain (0.3%), which rapidly decreases to a plateau value (1.48) from 40% to 300%. The sensitivity is much higher than those of hydrogels in litera38 ture (0.478).



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Figure 4. The dependence of resistance change ratio and sensitivity of PANI/P(AAm-co-HEMA) hydrogels with 0.5 wt/vol% aniline on (a) tensile and (c) compression strain. Resistance changes of hydrogels upon cyclic (b) tensile and (d) compression loading/unloading tests, and the corresponding cyclic tensile results obtained at (e) 0.25 Hz and (f) 1 Hz.

Figure 5. Representative SEM images of freeze-fractured hydrogel surface with PANI content of (a) 0.1 wt/vol%, (b) 0.5 wt/vol%, and (c) 0.6 wt/vol%. The corresponding SEM images of the hydrogels under 100% tensile strain are shown in (d, e, f). Red arrows indicate the tensile direction. Moreover, an even higher sensitivity of 11 is observed upon compression at low strain (Figure 4c), close to that of sliver nanoparticle/elastomer composites with high nanoparticle 39 content (15 wt%). The hydrogel was assembled into a circuit to lighten a bulb at 18 V. During compression from 0% to 70%, the current increased from 0.58 A to 3.15 A, while the bulb dimmed upon unloading. Cyclic loading and unloading resulted in repeated enlightening and dimming of the bulb, while the current values cyclically changed (Movie S3, Supporting Information). These results indicate sensitive, relia-

ble and reversible changes in the conductive network upon loadings. Such outstanding reliable and repeatable sensing has been further demonstrated upon cyclic loadings. The hydrogels show reversible changes in ΔR/R0 during cyclic tensile (0.25 Hz, 0-200%, Figure 4b) and compression tests (0.04 Hz, 070%, Figure 4d) for 100 cycles. The ΔR/R0 remains constant at about 220% for 200% strain, and about 80% at 70% compression strain. The repeated curve shapes for each cycle


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Figure 6. The signals from hydrogel sensors (a) on the wrist cyclically bending (b) as a function of bending angle, (c,d) on the wrist pulse, and (e) on the throat of a volunteer speaking “Hello”, “Hydrogel”, and “Nice to meet you”, in comparison to (f) the same voice wave recorded by using a smartphone. (Figure 4d) indicate reliable and reversible changes in conductive network at high strains. Similar cyclic loading/unloading and sensing results to 5000 cycles were reported by Zhang et al, who conducted loading/unloading tests at much lower pressure (0~1 kPa) and strain (0~40%) on 26 conductive gels crosslinked by microspheres. The strain sensing shows some delay, particularly at relatively high frequency. Figure 4e,4f compare the cyclic electronic output with cyclic strains. The resistance change ratio waves are almost synchronous with the applied strain under a strain of 100% at 0.25 and 0.5 Hz (Figure S5, Supporting Information), while a 0.2s delay was found for measurements at 1 Hz. This frequency depedence may root from the viscoelasticity of hydrogel networks during loading–unloading pro9 cess. The sensitivity and linearity are dependent on the PANI content in the PANI/P(AAm-co-HEMA) hydrogels. With 0.1wt/ vol% PANI, the hydrogels show a sensitivity lower than 0.3 at strain < 200%, which decreases to a plateau value at high strains. Meanwhile, the gels show a low resistance non-linear dependence of the resistance change ratio on strain (Figure S5a, Supporting Information). On the other hand, with 0.6 wt/vol% PANI, the sensitivity is less than 0.4, while the resistance change ratio is much lower ( 60% (Figure S5b, Supporting Information). SEM images show that the PANI chains with 0.1wt/vol% form a loose network that is prone to damage at large stretching (Figure 5a, 5d), which may account for the low and plateau sensitivity. In contrast, a very dense network was found for gels with

0.6wt/vol% PANI (Figure 5c). The overcrowded PANI network undergoes less deformation during stretching (Figure 5f), which may explain the low sensitivity and resistance change ratios (Figure S5b, Supporting Information). In contrast, the 0.5wt/vol% PANI content appears as an optimal and threshold value for the PANI/P(AAm-co-HEMA) hydrogels to achieve high conductivity, sensitivity and linearity. Figure 5b shows the representative network structure and its deformation upon stretching for the gel with 0.5wt/vol% PANI. In contrast to those with 0.1 and 0.6wt/vol% PANI, the network deformation is adaptive to the strain, which may account for the sensitivity and linearity to strain. Upon stretching, the deformation of the continuous PANI network at 0.5wt/vol% close to the threshold PANI content may lead to an off-threshold state and large changes in conductivity or resistance. In contrast, the deformation of loose or dense PANI networks results in much less changes in the conductive network. These results explain the high strain sensitivity of the 0.5wt/vol P(AAm-co-HEMA) hydrogels as sensors in comparison to others. More evidences are yet to explore by combining in situ morphological and spectral studies during stretching. The hydrogels are used as wearable sensors directly contacted on the skin to monitor diverse macro-scale human activities. The changes in resistance upon stimulus are monitored in real time. As a hydrogel sensor was mounted on the wrist of a volunteer, the wrist bending to 30°, 60°, and 90° stretched the gel and generated repeatable ΔR/R0 of 97%, 124%, and 156% (Figure 6a). The outputs are quantitatively correlated to bending angles (Figure 6b). Similar quantitative



Figure 7. (a) A flexible 6 × 8 array of (b) conductive hydrogel cubes connected with copper tapes with (c) each cube as a single pixel. (d) A prism map of the array pressed on B3, E3, B6 and E6. (e) An electronic “CNiTECH” logo produced by pressing on the array. Scale bars: 1 cm. movement monitoring is applicable for many other joints including finger, elbow, and knee (Figure S6, Supporting Information). The encouraging results indicate great potentials of the conductive hydrogels for flexible wearable devices. The very high sensitivity at very low strain has enabled unprecedented sensitive, reliable and precise detection of weak vibrations. A hydrogel sensor mounted on the wrist sensitively captured the weak strains induced by periodic pulses (Figure 6c). The pulse rate monitored by the sensors in excellent consistence with that by electrocardiograph of the same volunteer (Figure S7, Supporting Information). Two distinctive diacritical peaks and a late systolic augmentation shoul9, 40 der are captured by the hydrogel sensor (Figure 6d). This feature is caused by the blood pressure from the left ventricle shrinkage and the repercussive waveform the lower body. The specific value of P1 and P2 (or augmentation index AIr = 41 P2/P1) is a diagnostic indicator for arterial stiffness. These results suggest potential applications of the conductive hydrogel sensor for wearable diagnostic devices to real-time monitor physiological signals. Moreover, the gel sensors enable high-resolution detections on the vibrations from vocal cords. The sensor attached on the volunteer’s throat produces very clear signals with high signal-to-noise (SN) ratio when he says “Hello”, “Hydrogel” and “Nice to meet you” normally (Figure 6e). The phonetic symbol and pitch of these words, including the very gentle pronunciation of [s], are precisely captured, and converted into electric signals, which are onsistent with the corresponding sound wave simultaneously recorded with a smartphone (Figure 6f). The results strongly suggests the outstanding immediate, sensitive and precise response of hydrogel sensors to subtle vibrations.

Finally, we demonstrate a hydrogel array to detect 2D distribution of force or strain, which is widely pursued for 2D 5 sensing or electronic skin. A prototype 6 × 8 array of hydrogels connected by copper tapes was fabricated and encapsulated in the P(AAm-co-HEMA) hydrogel matrix (Figure 7a, 7b). Each hydrogel cube (5 mm × 5 mm) acts as a sensor unit/pixel (Figure 7c). As fingers pressed on the B3, E3, B6 and E6 units, outstanding resistance change ratios from these pixels were recorded and reproduced in computer (Figure 7d). No crosstalks occurred. Encouraged by this result, we “wrote” an electronic CNiTECH logo by sliding fingers on the array (Figure 7e). For example, successive writing on the array surface from B2 to C2, D2, E3, E4, E5, E6, D7, C7 and B7 generated electric signals as “C”. Similarly, it is convenient to “write” other letters on the array to accomplish an electronic “CNiTECH” logo (Figure 7e). This protocol can be extended to fabricate highly integrated arrays of very small gel pixels for applications as touchpad and E-skin, as recently reported by Bao et al who introduced a sophisticated FET array sen42 sor. The obtained PANI/P(AAm-co-HEMA) hydrogels show great potential for wearable sensors with excellent mechanical strength, linearity and high sensitivity. The conductivity relies on the formation of a continuous network of the conductive polymer in the hydrogel matrix. A low threshold PANI content around 0.5wt/vol% is found in this study, which is 37 much lower than those reported in literature but provides a high conductivity (8.24 S/m) close to pure PANI hydrogels (11 36 S/m), and much higher than most conductive hydrogels 23, 24 reported in literature. The continuous PANI network is closely associated with the P(AAm-co-HEMA) network skeleton through hydrogen bonds. Such network structures warrant not only high conductivity, but also outstanding strength and toughness. The near-threshold network struc-


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Chemistry of Materials ture likely offers linearity over a large strain range and high strain sensitivity (11), which is higher than the best value 43 (7.8) previously reported on conductive hydrogels , and even close to that of sliver nanoparticle/elastomer compo39 sites (12) with high nanoparticle content. In fact, such linearity has been rarely reported for conductive hydrogels and has been widely pursued for wearable sensors or electronic 11 skins. The demonstration to fabricate conductive hydrogel arrays indicates a promising feasibility to create sophisticated devices. To practical applications for large scale fabrication of wearable and implant sensors, high density integration or small hydrogel sensor units are needed, which requires a facile synthesis of tough conductive hydrogels. Besides, an ingenious process to prevent conductive hydrogels from dehydration is demanded for long-term stability and durability 44 of preformance. Moreover, adhesive conductive hydrogels are preferred for conformable integration of sensors and sub43 strate. Last but not least, novel manufacture methods are required to produce complicated circuits and matrices for 27 electronic devices or integrated bioelectronics.

CONCLUSION In summary, we have prepared novel tough and fatigue resistant conductive hydrogels comprised of interpenetrating networks of polyaniline and P(AAm-co-HEMA). The hydrogels show high sensitivity (GF=11) at low strain and outstanding linearity at high strains. These unprecedented merits provide precise and reliable detections of subtle vital signals and human movements. An array of conductive hydrogel sensors are fabricated for two-dimension detection of tiny forces. These tough and conductive hydrogels may find broad applications in electronic skin, biomedical implants, human movement monitors, and so on.

ASSOCIATED CONTENT Supporting Information Additional figures and information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51873224), Natural Science Foundation of Zhejiang Province (LY17E030011, LQ16E030002), and Natural Science Foundation of Ningbo (2017A610232, 2017A610057).

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Ultrastretchable Strain Sensors and Arrays with High Sensitivity and

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