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Applications of Polymer, Composite, and Coating Materials

Scalable and automated fabrication of conductive tough-hydrogel microfibers with ultra-stretchability, 3D printability and stress-sensitivity Shanshan Wei, Gang Qu, Guanyi Luo, Yuxing Huang, Huisheng Zhang, Xuechang Zhou, Liqiu Wang, Zhou Liu, and Tiantian Kong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00379 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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ACS Applied Materials & Interfaces

Scalable and automated fabrication of conductive tough-hydrogel microfibers with ultra-stretchability, 3D printability and stresssensitivity Shanshan Wei1,#, Gang Qu1,#, Guanyi Luo1,#, Yuxing Huang2, Huisheng Zhang1, Xuechang Zhou2, Liqiu Wang3,4,*, Zhou Liu2,*, Tiantian Kong1,4,* 1

Guangdong Key Laboratory for Biomedical Measurements and Ultrasound Imaging, Department of Biomedical Engineering, School of Medicine, Shenzhen University, Shenzhen, China 2 College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, China 3 Department of Mechanical Engineering, University of Hong Kong, Hong Kong. 4 HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), Hangzhou, Zhejiang, China. Correspondence and requests for materials should be addressed to L.Q.W., Z. L. and T. T. K. (email: [email protected], [email protected]; [email protected]) Keywords: tough hydrogels, 3D printing, wearable electronics, ultra-stretchablity, bio-inspired fabrication Abstract Creating complex three-dimensional structures from soft yet durable materials enables advances in fields, such as flexible electronics, regenerating tissue engineering and soft robotics. The tough-hydrogels that mimic the human skin can bear enormous mechanical loads.

By employing a spider-inspired biomimetic

microfluidic-nozzle, we successfully achieve the continuous-printing of toughhydrogels into fibers, 2D networks and even 3D structures without compromising its extreme mechanical properties. The resultant thin fibers demonstrate a stretch up to 21 times of its original length at a water content of 52%, and are intrinsically transparent, biocompatible, and conductive at high stretches. Moreover, the printed robust tough-hydrogel networks can sense strain that are orders-of-magnitude lower 1 / 20

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than stretchable conductors by percolations of conductive particles. To demonstrate its potential application, we use the printed tough-hydrogel fiber-networks as wearable sensors for detecting human motions. The capability to shape tough-hydrogels into complex structures by scalable continuous printing opens opportunities for new areas of applications such as tissue scaffolds, large-area soft electronics and smart textiles.

Introduction The printing of three-dimensional architectures from biomaterials with control over both structure and functions is advanced by emerging applications, such as artificial tissue, flexible electronics and regenerating medicine. Additive 3D bioprinting creates application-specific design from bio-inks, such as photo-crosslinkable hydrogels and thermoplastic polymers, with high resolution and low cost. Despite the advances in bio-inks, the printed structures rarely achieves the high toughness of human tissue, such as our skin, limiting their practical uses1–5. The recent tough-hydrogels, such as the polyacrylamide (PAA) and alginate polymers, have demonstrated extreme toughness and open up plenty of new opportunities in bioprinting 3–21. Different from hydrogels with single networks of ionic crosslinks, for instance, alginate-based composite

hydrogels24–30,

tough-hydrogels

demonstrate

superior

mechanical

properties, such as excellent stretchability, due to the intertwined networks of both covalent and ionic crosslinks. However, the current molding approach for fabrication of tough-hydrogels only produce limited simple and flat samples, restraining their utilization in applications, such as weavable fiber-shaped supercapacitors, and tissuelike constructs, in which complex patterns and 3D structures are necessary2,20,22. 2 / 20


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Shaping tough-hydrogels, such as PAA-alginate hydrogels, into tailor-designed patterns or 3D structures using additive printing approaches is highly desired, but remains insufficiently explored. In 3D printing, the ink exiting the nozzle solidifies rapidly to maintain the printed structure, facilitating a layer-by-layer production on the working substrate20,23. Printing tough-hydrogels into complex structures following such strategy is, however, hindered by the slow and complex solidification. One strategy to address this challenge is pre-mixing the gel precursors to make the printed structure more readily solidified upon depositing on the substrate. However, to avoid clogging, the crosslinkers in precursors normally have low concentrations, which significantly compromise the mechanical properties of the resultant materials

2,16

. Alternatively,

we can use precursor inks with special rheological properties, such as shear-thinning fluids, for printing. However, such inks usually involve a lot of additives, such as nanoparticles, which also limit the stretchability of materials and cannot be readily removed 2,20. In nature, spiders spin silks by pulling the liquid proteins from different glands to form a gel mixture right before spinning, and the silk subsequently transforms into a solid fiber upon exiting the spider. Inspired by this mechanism, we design a biomimetic microfluidic printer-head for producing tough-hydrogels in shapes of continuous microfibers, vascular networks as well as 3D structures. The resultant tough-hydrogel microfibers are homogeneous, transparent, ultra-stretchable, and conductive by incorporating electrolytes. The printed microfibers can reach a stretch 3 / 20



up to 21 times of their original length. Moreover, the ionic tough-hydrogel microfibers are stress-sensitive, and remain conductive at a high stretch of 11, which is significantly different from the percolated networks of conductive particles. We have also demonstrated that the robust, large-area, sensitive tough-hydrogel webs can be used to monitor human motions with sensitive response. Our results offer a bioinspired, scalable strategy to shape tough hydrogels into designed structures with extreme mechanical properties, which have important implications in developing sensitive electronic skin, artificial tissues, human-robot interfaces and wearable sensors. Results To continuously fabricate the PAA-alginate tough-hydrogel microfibers at large scale, we employ a biomimetic microfluidic device as the printer-head in a 3-axis translation stage (Fig. 1a-b). The microfluidic nozzle consists two coaxially aligned channels for injecting the core and sheath liquid phases respectively. The monomer AA, its photo-initiator, crosslinker, as well as the alginate polymer are used as the sheath liquid phase; while the core liquid phase consists of the crosslinking accelerator for PAA and the ionic crosslinker for alginate. Direct mixing of the two phases leads to instant gelation and clogging of nozzle. Owing to the laminar feature of microfluidics (Reynolds number Re 70% , the decrease in 6 / 20


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elasticity makes it vulnerable for severe deformations λ < 10 (Fig. 2 b-c). Thus, the stretchability and toughness are the greatest for the mediate water content. For instance, the measured average stretch is between 15 and 21 at a water content within 50%-60% (Fig. 2c)5. The results imply that the stretchability can be easily tuned by changing the water content through moisturizing the gel fiber. The fiber diameter has negligible effect on its stretchability within the range we tested (See details at Supporting Information Note-4). The tough-hydrogel microfiber with optimized water content and flow rate ratio can be twisted into various shapes including heart, octagon, hexagonal star, and star anise, at 600% stretch (Fig. 2d). The ultra-stretchable tough-hydrogel fibers are conductive that makes them suitable candidates in wearable electronics. We propose that the porous tough-hydrogels can be dehydrated for storage (See details at Supporting Information Note-5). Afterwards, they become conductive by absorption in electrolyte solutions (Fig. 3a, see details at Supporting Information Note-6). Since the conductivity of electrolyte solutions is much higher than that of the dehydrated gel, the contribution from the former dominates the conductivity of the tough-hydrogel microfibers. Indeed, as the water content increases above 40%, the resistivity of the tough-hydrogel microfibers reaches the plateau (See details at Supporting Information Note-7). For microfibers absorbed with electrolyte solutions (0.01M KCl or 0.1M KCl aqueous solution), the initial resistivity ρ0 of un-stretched microfibers are measured to be 1.4 Ω·m and 0.67 Ω·m, respectively (Fig. 3b). These ionic microfibers remain conductive when they are stretched, as indicated by the linear relationship between the current and applied 7 / 20



voltage (Fig. 3b, and See details at Supporting Information Note-9). The changes in resistivity are negligible from stretch λ=1 to λ=11 (Fig. 3c). Thus, the resistance of the stretched ionic microfibers, R, can be approximately predicted by: R/R0~λ2, where R0 is the resistance of the un-stretched microfibers (Fig. 3 b-c). These electrical properties of these tough-hydrogel microfibers are significantly different from those doped with conductive particles, where their conductivity decrease sharply during stretching, due to the large contact resistance between conductive particles26. More importantly, the stretchability is not influenced by the ion concentration (See details at Supporting Information Note-10). The conductive and stretchable fibers can even tie together a series of light-emitting diode (LED) bulbs, and light them up as shown in Fig. 3d and Supporting Movie 1. The stretch-sensitive resistance of the tough-hydrogel fibers makes them useful for detecting body motion. For wearable sensors, its robustness against severe deformation is essential for the practical use. Owing to the printability of the toughhydrogel fibers, we print a web sensor of size ~144mm×144mm conveniently and test its robustness. Remarkably, as we drop an egg of weight ~ 60g from a height of 1 m onto the printed web, the egg bounced several times on the tough-hydrogel web and remain intact (Fig. 4a and Supporting Movie 2). We hit the printed web with eggs a few times, and the tough-hydrogel web are unbroken and still stretchable (See details at Supporting Information Note-11). The super-robust web is then used as a sensor to detect the movement of human elbow. We wrap the web around the elbow of a volunteer, and fix the edges with clamps. As the elbow bends and stretches repeatedly, 8 / 20


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a clear increase in resistivity is detected, which is completely conform to the real-time physiological behaviors (Fig. 4 b). The change in resistance is owing to the motion of bicep, where the peaks and the valleys are associated with the stretching and relaxing of the muscles. The resistance variation is collected while the elbow is repeatedly move from 0° to 45°, 90°, 135°, and it is shown that larger stretches of the muscles is reflected by a shaper variation in resistance (Fig. 4b). The results show that the web sensor can efficiently monitor the minor and major bodily movements with a good accuracy. Due to the large-scalability and the robustness of the printed tough-hydrogel webs, we propose it could potentially be incorporated with textile fibers for wearable functional vests. Conclusion The ionic tough-hydrogels are intrinsically transparent, biocompatible, ultrastretchable, and they response with changes in resistance to strain that is orders-ofmagnitude lower than percolated networks of conductive nanoparticles. These attributes make them appealing for applications such as wearable sensors, smart artificial skin, and next-generation human-interfaced devices. The prospective of tough hydrogels is hindered by the challenge in creating complex structures and patterns in scalable manner by 3D printing. Inspired by the silk-spinning spider, a biomimetic microfluidic nozzle is designed to enable the versatile and automated fabrication of the 3D tough-hydrogel microfibers and complex microstructures. The printed tough-hydrogel fibers exhibit extreme mechanical properties at a mediate water content. By incorporating soluble electrolytes, the tough-hydrogel fibers are 9 / 20



conductive at large deformations. The stress-sensitive electrical conductivity of the fibers is exploited in a wearable sensor, where the human motion is detected by using a printed tough-hydrogel web. The capability to shape tough-hydrogel into continuous fibers, vascular networks and cellular structures with excellent mechanical and electrical properties pave the way for their applications in flexible electronics and tissue engineering.

Figure 1. The microfluidic nozzle-head for continuous fabrication and printing 10 / 20


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tough-hydrogel fibers (a) Schematic of the printing system. (inset: microscopic image of the flow-focusing microfluidics; both liquid phases are aqueous and the blue bye is for visualization); (b) Schematic of the tough hydrogel, where the PAA and alginate polymers are covalently and ionically crosslinked by UV exposure and Ca 2+, respectively; (c) Schematic showing the 3D printing of tough-hydrogel patterns by microfluidic nozzle-head, layer by layer; (d) Optical images showing the printed tough-hydrogel fibers in the shapes of spider web, and tissue scaffolds in 2D and 3D, respectively. (e) Schematic and optical image showing the scalable printing of toughhydrogel fibers using an automated rotator.

Figure 2. Mechanical properties of the continuously printed tough-hydrogel fibers (a) Optical images of fiber sample under different stretches: 0%, 500%, 1000% and 2100%. The original length and diameter of fiber are 10 mm, 1.2 mm respectively. The water content Cw% of the fiber sample is 52%; (b) Dependence of stretchablity λ on the water content Cw% of tough-hydrogel fiber; (c) The stress-strain curves of tough-hydrogel fibers at different water contents; (d) The fabricated toughhydrogel fiber can be easily stretched or twisted into various shapes or patterns, such as heart (strain rate of 200%), octagon (strain rate of 400%), hexagonal star (strain rate of 600%) and star anise (strain rate of 400%). 11 / 20



Figure 3. Electrical properties of the continuously printed tough-hydrogel fibers (a) Schematic of the tough-hydrogel network adopted with electrolytes; (b) Sheet 12 / 20


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resistance and (c) resistivity of the tough-hydrogel fiber with different concentration of electrolytes upon stretching; The solid lines correspond to R/R0~λ2, where R0 is the resistance of the un-stretched microfibers. (d) The highly stretched tough-hydrogel fiber can be used as the electric conductor to light up a series of LED bulbs assembled to resemble the letters “SZU”.

Figure 4. The mechanical and electrical properties of the printed tough-hydrogel web and 3D structures (a) A series of optical images showing that an egg of 60 g impacting onto the printed web from a height of 1 m remains intact; (b) The printed web can be used as sensor to detect human motion, as the elbow is subjected to repeated bending and relaxing from 00 to 450, 900 and 1350, respectively. The toughhydrogel-web sensor is highly sensitive to the frequency as well as the amplitude of the human motions.

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Methods The fabrication of microfluidic nozzle-head: A microfluidic flow-focusing device was used as the printer-head in 3D printing stage. The device was assembled by inserting a round capillary with an outer diameter of 1.5 mm into a square one with an inner diameter of 1.75 mm. The round capillary was pulled by a micropipette puller (Sutter P-97) and then tapered to size around 0.5 mm. Two syringe needles were used to inject liquid phases were injected by syringe pumps (Longer Pump) and soft microtubings (Scientific Commodities). The core phase flowed through the round capillary (Fig. 1a), and the sheath liquid phase flowed via the gap between the round and square capillary. To investigate the influence of the flow rate ratio, the flow rate of core phase, Qin, was varied from 2 ml/hr to 18 ml/hr, while that of sheath phases, Qout, was kept at 20 ml/hr.

The fabrication of Polyacrylamide (PAA)-alginate hydrogel in bulk: The bulk pieces of PAA-alginate tough-hydrogel were fabricated according to published protocols1. The acrylamide (AA) and sodium alginate were dissolved in water with a mass ratio of 8:1 or 20:1 to form a pre-hydrogel solution. The ammonium persulphate (APS) (1.5 wt%) and N,N’’-methylenebisacrylamide (MBAA) (0.09 wt%), which were the photoinitiator and crosslinker for polyacrylamide, were added into the pre-hydrogel solution. After degassing in a vacuum chamber, N,N,N’,N’tetramethylethylenediamine (TEMED) ( 4.9 wt%), and calcium sulphate slurry (1 M CaSO4·2H2O) were added as the crosslinking accelerator and the ionic crosslinker for polyacrylamide and alginate, respectively. The mixture was cured under ultraviolet light (254 nm wavelength) with 60 degrees for 30 minutes. The PAA chains form covalent crosslinks through MBAA after UV exposure and heating, which can endure a large deformation; while the alginate was crosslinked by Ca 2+, enhancing the stiffness of the network (Fig. 1b). The resultant hydrogel was left in a humid box for one day to stabilize the reactions. All chemicals were purchased from Sigma-Aldrich. The 3D printing and continuous fabrication of PAA-alginate hydrogel fiber and 3D structures: To print PAA-alginate tough-hydrogel microfibers, the dissolved AA, alginate with a mass ratio of 8:1 or 20:1 to AA, MBAA (0.09 wt%) and APS (1.5 wt%) in aqueous solution was used as the sheath phase; the TEMED (4.9 wt%) and calcium sulphate slurry in aqueous solution was used as the core phase (Fig. 1a). The core phase was dyed with methylene blue for visualization as shown in inset of Fig. 14 / 20


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1a. The microfluidic printer-head was placed on the printing carriage of a customized 3D printer. The typical deposition speed was 2 mm/s. During printing, the core and sheath liquids mixed slowly in the square capillary, preventing gelation inside the printer-head and thus facilitating the extrusion process. Upon exiting, the deposited mixture reacted under UV, and the printed structures were maintained without supporting materials. Although the crosslinking between AA and MBAA can be triggered by the initiator APS without UV32, we used UV light source to expedite the solidification of deposited structures. The printed microfibers and structures on a poly(methyl methacrylate) (PMMA) substrate were placed in a heating UV chamber to complete the covalent crosslinking. Thus, the printed tough-hydrogel 2D patterns and 3D structures were chemically united. We emphasized that no additive to increase viscosity was employed to print 3D shapes.

The incorporation of ions into PAA-alginate tough-hydrogel microfibers: To incorporate ions, we immersed the printed hydrogels in potassium chloride solution with different molar concentration from 10-3 mol/L to 1 mol/L. After several minutes, the hydrogels soaked with ions were lifted and water on the surface was removed by using nitrogen. In practice, the printed tough-hydrogel microfibers were dried completely for storage, and absorbed in electrolyte solutions before usage (See details at Supporting Information Note-6). The characterization of morphology, mechanical and conductive properties: The microstructure and morphology of the printed tough-hydrogels were characterized by scanning electron microscopy (SEM, Hitachi SU-70). The rheology of the printed PAA-alginate tough-hydrogels with alginate-to-AA mass ratio of 8:1 and 20:1 were measured by a rheometer (TA Instruments Inc., AR 1000) (See details at Supporting Information Note-12). The mechanical property of the printed PAA-alginate tough-hydrogels (alginate-toAA mass ratio of 8:1 and 20:1) are characterized as following (See details at Supporting Information Note-13): we cut a printed fiber into several pieces with uniform length for mechanical tests. The typical sample dimension were 10 mm in length, 1.8 mm in width, 0.8 mm in thickness. Our customized mechanical-test system consisted two linear motion-stage (Zolix, PSA-300-11-X), a motion controller (Zolix, MC600-4B) and a force gauge with precision ~ 10-3 N and maximum loading ~2 N. The relative speed of the two motion stages was kept constant at 1 mm/s. We emphasized that the tough-hydrogel samples were under tight control of their water content. To evaluate the water content, we first weighed one segment of the test sample, evaporated all water in that segment, and then weighted the dried segment. Thus, the water content was estimated as the mass of water loss divided by that of the dried segment from the tested sample. For microfibers with different water content, the average stretchablity was calculated based on at least 20 samples. The sheet resistance (R) values were measured with a digital source meter 15 / 20



(Keithley instruments, 2612B) coupled with a motion controller (Zolix, SC300-2A). All samples for conductivity measurements were prepared with the same length, 10 mm, and cross-section area, 1.44 mm2.

Tensile tests for printed hydrogel webs: The printed tough-hydrogel webs of area~ 144 mm×144 mm and line intervals ~8mm were used in robustness test. We dropped an egg of weight ~ 60 g, from a height of 1 m, onto the hydrogel web that was fixed onto a stage by multiple clamps (Fig. S6-7 and supplementary video 2). The egg impacting and bouncing on the web was recorded by a high-speed camera (Phantom, M110) coupled with a commercial lens (Nikon, 35mm) at 400 fps. Monitoring of bodily motion using printed hydrogel web-sensors: The printed toughhydrogel web was wrapped around the elbow and fixed by clamps. The elbow movement was reflected by the morphology changes and then the electric resistance of the tough-gel web-sensor. We input a constant voltage to the web-sensor and measured the corresponding current by a digital source meter (Keithley instruments, 2612B). The electric resistance is calculated following: R=U/I using an in-house MATLAB code. The resistance was collected while the elbows were repeatedly bent in 00, 450, 900 and 1350. Supporting information Continuous printing of Polyacrylamide (PAA)-graphene oxide (GO) tough-hydrogel microfibers, effect of water content on the toughness of tough-hydrogel microfibers, influence of flow rate ratio Qin/Qout on the stretchability of the resultant PAA-alginate tough-hydrogel microfibers, effect of cross-section area on the stretchability of toughhydrogel microfibers, porous structures of the printed tough-hydrogel microfibers, the stretchability of the re-hydrated tough-hydrogel microfibers in aqueous solutions, effect of water content on electrical resistivity of tough-hydrogel microfibers, schematic of the measurement of electrical resistance, linear current-voltage relationships of the tough-hydrogel microfibers, effect of electrolyte concentrations on the stretchability of re-hydrated tough-hydrogel microfibers, the rheology characterization of PAA-alginate tough-hydrogels, stretchability of the PAA-alginate hybrid microfibers with a alginate-to-AA mass ratio of 20:1, supporting movie showing the lightening up of a series of LED bulbs connected by tough-hydrogel fibers, supporting movie showing the close-up of the egg impacting onto the printed web. Acknowledgement This research was supported by Young Scholar’s Program (NSFC 11504238, 21706161) from the National Natural Science Foundation of China, the Science and Technology Department of Guangdong Province (2016A050503048), the Natural Science Foundation of Guangdong (2017A030310444), the Fundamental Research Program of Shenzhen City (JCYJ20160308092144035), and the Natural Science Foundation of Shenzhen University (grant no. 2017030). The financial support from 16 / 20


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the Research Grants Council of Hong Kong (GRF17207914 and GRF HKU717613E) is also gratefully acknowledged # Equal contribution Reference (1) Sun, J.-Y.; Zhao, X.; Illeperuma, W. R. K.; Chaudhuri, O.; Oh, K. H.; Mooney, D. J.; Vlassak, J. J.; Suo, Z. Highly Stretchable and Tough Hydrogels. Nature 2012, 489 (7414), 133–136. (2) Hong, S.; Sycks, D.; Chan, H. F. ai; Lin, S.; Lopez, G. P.; Guilak, F.; Leong, K. W.; Zhao, X. 3D Printing: 3D Printing of Highly Stretchable and Tough Hydrogels into Complex, Cellularized Structures. Adv. Mater. 2015, 27 (27), 4035-4040. (3) Lin, S.; Yuk, H.; Zhang, T.; Parada, G. A.; Koo, H.; Yu, C.; Zhao, X. Stretchable Hydrogel Electronics and Devices. Adv. Mater. 2016, 28 (22), 4497–4505. (4) Yuk, H.; Zhang, T.; Parada, G. A.; Liu, X.; Zhao, X. Skin-Inspired Hydrogel– elastomer Hybrids with Robust Interfaces and Functional Microstructures. Nat. Commun. 2016, 7 (May), 12028. (5) Le Floch, P.; Yao, X.; Liu, Q.; Wang, Z.; Nian, G.; Sun, Y.; Jia, L.; Suo, Z. Wearable and Washable Conductors for Active Textiles. ACS Appl. Mater. Interfaces 2017, acsami.7b07361. (6) Ren, J.; Bai, W.; Guan, G.; Zhang, Y.; Peng, H. Flexible and Weaveable Capacitor Wire Based on a Carbon Nanocomposite Fiber. Adv. Mater. 2013, 25 (41), 5965–5970. (7) Chen, X.; Lin, H.; Deng, J.; Zhang, Y.; Sun, X.; Chen, P.; Fang, X.; Zhang, Z.; Guan, G.; Peng, H. Electrochromic Fiber-Shaped Supercapacitors. Adv. Mater. 2014, 26 (48), 8126–8132. (8) Liu, X.; Wu, D.; Wang, H.; Wang, Q. Self-Recovering Tough Gel Electrolyte with Adjustable Supercapacitor Performance. Adv. Mater. 2014, 26 (25), 4370– 4375. (9) Cheng, X.; Fang, X.; Chen, P.; Doo, S.-G.; Son, I. H.; Huang, X.; Zhang, Y.; Weng, W.; Zhang, Z.; Deng, J.; Sun, X.; Peng, H. Designing One-Dimensional Supercapacitors in a Strip Shape for High Performance Energy Storage Fabrics. J. Mater. Chem. A 2015, 3 (38), 19304–19309. (10) Hong, S.; Lee, H.; Lee, J.; Kwon, J.; Han, S.; Suh, Y. D.; Cho, H.; Shin, J.; Yeo, J.; Ko, S. H. Highly Stretchable and Transparent Metal Nanowire Heater for Wearable Electronics Applications. Adv. Mater. 2015, 27 (32), 4744–4751. (11) Moon, W. G.; Kim, G. P.; Lee, M.; Song, H. D.; Yi, J. A Biodegradable Gel Electrolyte for Use in High-Performance Flexible Supercapacitors. ACS Appl. Mater. Interfaces 2015, 7 (6), 3503–3511. (12) Huang, Y.; Zhong, M.; Huang, Y.; Zhu, M.; Pei, Z.; Wang, Z.; Xue, Q.; Xie, X.; Zhi, C. A Self-Healable and Highly Stretchable Supercapacitor Based on a Dual Crosslinked Polyelectrolyte. Nat. Commun. 2015, 6, 10310. (13) Shan, S.; Kang, S. H.; Raney, J. R.; Wang, P.; Fang, L.; Candido, F.; Lewis, J. 17 / 20



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Author Contributions T. T. Kong and Z. Liu conceived, designed and supervised the project. S. S. Wei, G. Qu, and G.Y. Luo performed experiments. T. T. Kong, Z. Liu, S. S. Wei, G. Qu, G.Y. Luo and Y. X. Huang analyzed the data. X. C. Zhou, L. Q. Wang and H. S. Zhang provide important insights for analyzing data. T. T. Kong and Z. Liu wrote the manuscript. All authors commented on the paper. Additional Information Supplementary Information accompanies this paper at [URL will be inserted by Publisher] Competing financial interests: The authors declare no competing financial interests.

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