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Bioinspired ultra-stretchable and anti-freezing conductive hydrogel fibers with ordered and reversible polymer chain alignment

Bioinspired ultra-stretchable and anti-freezing conductive hydrogel fibers with ordered and... ARTICLE DOI: 10.1038/s41467-018-05904-z OPEN Bioinspired ultra-stretchable and anti-freezing conductive hydrogel fibers with ordered and reversible polymer chain alignment 1 1 1 1 1 1 Xue Zhao , Fang Chen , Yuanheng Li , Han Lu , Ning Zhang & Mingming Ma High-performance stretchable conductive fibers are desired for the development of stretchable electronic devices. Here we show a simple spinning method to prepare con- ductive hydrogel fibers with ordered polymer chain alignment that mimics the hierarchically organized structure of spider silk. The as-prepared sodium polyacrylate hydrogel fiber is further coated with a thin layer of polymethyl acrylate to form a core–shell water-resistant MAPAH fiber. Owing to the coexistence and reversible transformation of crystalline and amorphous domains in the fibers, MAPAH fibers exhibit high tensile strength, large stretchability and fast resilience from large strain. MAPAH fiber can serve as a highly stretchable wire with a conductive hydrogel core and an insulating cover. The stretchability and conductivity of the MAPAH fiber are retained at −35 °C, indicating its anti-freezing property. As a prime example of stretchable conductive fibers, MAPAH fibers will shed light on the design of next generation textile-based stretchable electronic devices. CAS Key Laboratory of Soft Matter Chemistry, iChEM (Innovation Center of Chemistry for Energy Materials), Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China. These authors contributed equally: Xue Zhao, Fang Chen. Correspondence and requests for materials should be addressed to M.M. (email: mma@ustc.edu.cn) NATURE COMMUNICATIONS | (2018) 9:3579 | DOI: 10.1038/s41467-018-05904-z | www.nature.com/naturecommunications 1 1234567890():,; ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05904-z tretchable conductive materials are essential for the emer- After a systematic exploration, we have found that long and 1,2 ging stretchable electronic devices , such as stretchable uniform filaments can be readily drawn from a gel-like PAAS 3 4–6 7,8 Spanels , energy-storage devices and sensors . Among solution in a mixture of water (as a good solvent) and dimethyl 9–11 various functional hydrogels , conductive hydrogels are very sulfoxide (DMSO, as a poor solvent). Water evaporation from the promising as stretchable conductive materials, which have been PAAS filaments in air enriches the poor solvent DMSO in the extensively studied for applications such as stretchable filaments, which triggers a quick phase transition to form PAAS 7,8,12 13,14 sensors and supercapacitors . Most of the conductive hydrogel (PAH) fibers with adjustable diameters and arbitrary hydrogels are made into two-dimensional films or three- length as wish. Just like spider silk, PAH fiber shows a unique 15,16 36 dimensional monoliths by molding methods , which result beads-on-a-string structure and good mechanical properties. To in an amorphous material with polymer chain’s random orien- achieve a good water resistance property, PAH fibers are coated tation and disordered alignment. If conductive hydrogels can be with a thin layer of polymethyl acrylate (PMA) to form core–shell made into one-dimensional fibers with ordered alignment of PMA–PAAS hydrogel (MAPAH) fibers. Remarkably, the water- polymer chains, some properties of conductive hydrogel fibers resistant MAPAH fibers exhibit a unique combination of high (e.g., mechanical properties and conductivity) would be greatly tensile strength (5.6 MPa) and stretchability (elongation at a enhanced over conventional conductive hydrogel films and break of 1200%), fast resilience (<30 s) from large stretching 17 −1 monoliths . However, the ordered chain alignment of conductive strain, high electrical conductivity (2 S m ), and great anti- polymers has been achieved in solid-state microfibers prepared by freezing property. The simultaneous achievements of these prime 17–19 20 template directed polymerization or alignment , which are properties by MAPAH fibers are attributed to the coexistence and not suitable for the preparation of conductive hydrogel fibers at reversible transformation of crystalline and amorphous domains macroscopic scale. On the other hand, spinning methods are in hydrogel fibers, which are enabled by the spinning and gelation widely used to prepare polymer fibers with ordered chain process. The multifunctional MAPAH fiber as a high- 21,22 alignment . But conductive hydrogels are rarely made into performance and low-cost stretchable conductive fiber will shed long fibers by spinning, due to the poor spinnability of current light on the design of next generation textile-based stretchable conductive hydrogels or their precursor solutions. In addition, electronic devices. conductive hydrogel films and monoliths are also limited by their slow resilience from large deformation due to the moderate reversibility of polymer chain alignment , and the loss of func- Results tions at subzero temperature . Previously reported polyelec- Fabrication of PAH and MAPAH fibers. Among many different trolyte fibers based on polycation–polyanion interfacial types of commercial available PAAS, the non-crosslinked PAAS complexation are rigid and insulating materials, which are mainly with an ultra-high molecular weight (M ~3× 10 Da) and low used for biomedical applications, such as tissue engineering and cost (~$20 per kilogram) was chosen as the starting material. The 24,25 drug delivery . Previously reported stretchable conductive preparation procedure of PAH and MAPAH fibers is summarized 6,26 27,28 fibers based on carbon materials or metal nanomaterials in Fig. 1a. PAAS powder was dissolved in a mixture of water and coating on elastomeric fibers are also limited by their sophisti- DMSO at 80 °C to form a transparent solution (Supplementary cated fabrication process and moderate stretchability . There- Fig. 1). The composition of the water/DMSO mixture and the fore, high-performance conductive hydrogel fibers with ordered concentration of PAAS in the solution are the two key factors for and reversible chain alignment are highly desired for the devel- the successful preparation of PAH fibers. The optimal solution opment of stretchable electronics, especially for the textile-based was found as 4 wt% PAAS dissolved in the water/DMSO mixture 14,30 stretchable electronic devices , but remain as a challenge. solvent with DMSO% = 20 wt%. As shown in Supplementary In nature, spiders spin silk fibers from aqueous protein Fig. 2, 4 wt% PAAS solutions in the water/DMSO mixture with solutions at ambient conditions . The hierarchically organized different water: DMSO ratios were prepared at 80 °C. Upon 32 33 structure of spider silk and its unique spinning process are cooling to room temperature, a clear phase transition process was the key factors to achieve its superb properties . For example, observed, indicating a critical concentration of DMSO in the spider dragline silk is a semi-crystalline protein polymer, where range of 20–22 wt%. With DMSO% ≤20 wt%, the PAAS solution alanine-rich crystalline regions are connected by soft glycine- remained as a homogeneous solution at room temperature. With rich amorphous regions as linkers . Inspired by the organized DMSO% ≥22 wt%, a white precipitate gradually formed during structure and the unique spinning process of spider silk, we the cooling process. A similar phase transition was observed for propose to develop a simple spinning method to prepare con- covalently crosslinked PAAS hydrogels swelled by water/DMSO ductive hydrogel fibers with ordered and reversible chain mixture . The addition of DMSO into water causes significant alignment from aqueous solution of polyelectrolytes at ambient desolvation of PAAS chains upon the counterion Na binding conditions. Aiming at the production at low cost, a mass- carboxylate groups, increasing the polymer–polymer interactions produced synthetic polyelectrolyte: sodium polyacrylate and triggering the phase transition . The optimal solution for (PAAS), was chosen as the starting material, which has been PAH fiber preparation has DMSO% = 20 wt%, which is very close widely utilized in daily supplies and in industry. In water to the critical DMSO concentration for the phase transition. solutions, the anionic carboxylate groups on a PAAS chain Therefore, when a PAAS filament was drawn out from the repel each other via double layer forces, which causes the optimal PAAS solution at room temperature, the DMSO% in the polymer chain to adopt an expanded, rigid-rod-like con- filament increased due to water evaporation and quickly exceeded formation. Adjusting pH, addition of salts or poor solvents tune the critical concentration. Therefore, PAAS in the filament the electrostatic interactions and alter the conformation of rapidly aggregated to form a fine PAH fiber, while the excess PAAS chains in solution, leading to the change of bulk prop- aqueous solution can automatically form liquid droplets due to erties (such as viscosity and sol–gel transition) .Based on the surface tension (Fig. 1a). These liquid droplets either flowed away 31–33 knowledge learned from spider silk , we expect that the along a tilted PAH fiber (Supplementary Movie 1) or gradually stimuli-responsive properties of PAAS in solution would enable solidified on a horizontal PAH fiber to form the beads-on-a- PAAS solution a good spinnability, where the spinning and string structure that mimics spider silk (photograph in Fig. 1a). drawing process would enable an ordered alignment of PAAS Similar to spider silk, PAH fibers are elastic and sticky, which can chains in the as-spun filaments. be used to weave a web that mimics a spider web (Supplementary 2 NATURE COMMUNICATIONS | (2018) 9:3579 | DOI: 10.1038/s41467-018-05904-z | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05904-z ARTICLE PAH fiber MAPAH fiber O Na PAAS Water – – – – – – PMA coating n O O OO OH O O O O O O O O O evaporates + + Na Na Na H H H HH CH H H O H O H C H O O H O PAAS 3 S O O H H O H H O H H H H H O + H separates out + + Na + Na O O Na Na Na O O OO H H H H H H H CH H H 3 O O O H H H O O O O O + S H + H H + PMA + Na O H Na H H Na Na CH – – – – – – O O O OO OO O O OHO O O O Solvent: H O/DMSO PAH fiber PAH fiber MAPAH fiber PMA PAAS PAH fiber MAPAH fiber Fig. 1 Preparation of PAH and MAPAH fibers. a Schematic illustration of the preparation of PAH and MAPAH fibers. A photograph of as-prepared PAH fiber shows the beads-on-a-string structure. b Photograph of a 1.1 m-long as-prepared PAH fiber. c SEM images of a PAH fiber. The scale bar is 100 μm. d SEM images of a MAPAH fiber. The scale bar in the left image is 200 μm, and the scale bar in the right image is 5 μm. A thin PMA layer on the PAAS core is clearly observed in the right image. e Photograph of a PAH web damaged by liquid water and a MAPAH web resistant to liquid water. Red circles indicate water droplets remaining on the hydrophilic PAH fiber, while no water droplet stays on the hydrophobic MAPAH fiber Fig. 3). PAH fibers with arbitrary length can be easily made MAPAH fibers. Although the thickness of PMA layer was only through continuous drawing from the optimal PAAS solution. 1–2 μm, this compact PMA layer enables MAPAH fibers a great Two examples of as-prepared PAH fibers are shown in Fig. 1b water resistance property. Observed under an optical microscope, (~1.1 m) and in Supplementary Movie 1 (~6.2 m). Since the a PAH fiber was swelled and gradually dissolved by a water preparation of PAH fibers is similar to the process of gel spin- droplet, while the MAPAH fiber remained unchanged with the ning , we envisage that the preparation could be facilely adapted water droplet (Supplementary Fig. 6). The contact angle of water onto a gel spinning equipment for the mass-production of PAH droplet on MAPAH fiber was measured as ~92° (Supplementary fibers. Fig. 7), indicating a good hydrophobicity of the PMA coating Due to the hydrophilic nature of PAAS, PAH fibers can be layer. As shown in Supplementary Movie 2, a MAPAH fiber quickly swelled and weaken in liquid water. To achieve a good retained its great water resistance property even in a stretched water resistance, we coated the PAH fibers with a thin condition (~200% elongation), while a PAH fiber was quickly hydrophobic layer of polymethyl acrylate (PMA). PMA was broken by liquid water at the same condition. As expected, the synthesized through conventional radical polymerization (M = web made from PAH fibers was not resistant to liquid water, 3.8 × 10 Da, PDI = 1.93, see SI for details). As-prepared PAH while a water-proof web can be made from MAPAH fibers, which fibers were immersed in a 5 wt% PMA solution in ethyl acetate remained stable and elastic after being treated by liquid water for several seconds and then taken out. After the quick (Fig. 1e). evaporation of ethyl acetate, a thin and uniform layer of PMA was firmly coated on the PAH fiber to form the core–shell MAPAH fiber. As judged by scanning electron microscopy (SEM, Characterization and optimization of the PAAS solutions.To Fig. 1c, d), both PAH and MAPAH fibers show a cylindrical understand the unique spinnability of PAAS solutions in water/ shape with consistent diameter. Observed under an optical DMSO mixture solvent, rheological measurements were con- microscope, both fibers show a good uniformity of their diameter ducted to study the viscoelastic properties of PAAS solutions. As along the fiber length (Supplementary Fig. 4). Fracture surfaces shown in Supplementary Fig.8, the storage moduli (G’) are from both fibers show a compact and homogenous interior core. dominant over loss moduli (G”) across the whole range of fre- As shown in Fig. 1d, a thin and compact PMA layer can be clearly quencies studied, which indicates the viscoelastic behavior of observed on the PAH fiber surface that firmly wraps the PAH PAAS solutions. The optimal PAAS solution (4 wt%) appeared as core. PAH fibers with different diameters in the range of 20–300 a physical gel at room temperature (Fig. 2a), and the high visc- μm can also be prepared by varying the preparation conditions osity of the gel is an important factor for drawing good PAH (Supplementary Fig. 5), which can be further converted to fibers. At lower concentrations (e.g., 2 wt%), the PAAS solution NATURE COMMUNICATIONS | (2018) 9:3579 | DOI: 10.1038/s41467-018-05904-z | www.nature.com/naturecommunications 3 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05904-z 2% 4% 6% 4%-G’ 4%-G” 447% 1 10 100 1000 Oscillation strain (%) 4%-H O-G’ 4%-H O-G” 4%-G’ 4%-G” 0 50 100 150 200 250 0.1 1 10 100 Time (s) Angular frequency (rad/s) Fig. 2 Preparation and rheological characterization of PAAS solutions. a Photograph of PAAS solutions with different concentrations (2%: viscous liquid; 4%: physical gel; 6%: precipitate and liquid) in H O:DMSO= 4:1 mixture at room temperature. b Strain-dependent and c step-strain oscillatory rheology of −1 the optimal PAAS solution. The applied oscillatory strain in c alternated between 1 and 1000% for 30 s periods (ω = 10 rads , 25 °C). d Frequency- dependent oscillatory rheology of 4% PAAS solution with pure water (black) or H O: DMSO= 4:1 (red) as solvent remained as viscous liquid and the filaments drawn from this effect of NaCl was observed by using rheological measurements solution were thinner and unstable (Supplementary Fig. 9). At (Supplementary Fig. 14): the gel to sol cross-over point was higher concentrations (e.g., 6 wt%), PAAS precipitated out of the increased from 447% to 680% strain simply by adding 50 mM solution upon cooling to room temperature. Strain-dependent NaCl into the optimal PAAS solution, which indicates the oscillatory rheology of the optimal PAAS solution displays a enhanced polymer–polymer interactions by addition of NaCl into broad linear gel region with a gel to sol cross-over point solution . appearing at 447% strain (Fig. 2b). In the step-strain measure- ments (Fig. 2c), the optimal PAAS solution exhibits a fast and Mechanical performance of PAH and MAPAH fibers. The complete recovery to its initial modulus, which indicates a great mechanical properties of both PAH and MAPAH fibers were reversibility due to the fast alignment and relaxation of PAAS studied based on fibers drawn from the same optimal PAAS chains in the physical gel . As shown in Fig. 2d, both G’ and G” solution, whose diameters were controlled as 200 ± 20 μmby values of a 4 wt% PAAS solution in pure water are higher than adjusting the drawn speed in a proper range. The environmental that of the optimal PAAS solution (4 wt% PAAS in water/DMSO temperature and relative humidity were maintained at 25 °C and solvent), indicating that PAAS chains are more extended in pure 42 ± 2%, respectively. The water content of PAH fibers and water than in water/DMSO mixture. However, filaments drawn MAPAH fibers measured by thermogravimetric analysis (TGA) from this 4 wt% PAAS solution in pure water were thinner and were similar, both of which were in the range of 25 ± 3% (Sup- unstable, which could not form PAH fibers (Supplementary Fig. plementary Fig. 15). Representative stress–strain curves of PAH 10). Reduction of the content of DMSO (e.g., from 20 wt% to and MAPAH fibers show a rubber-like characteristics (Fig. 3a) , 14.3 wt%) would also result in thinner and fragile PAH fibers indicating the viscoelastic nature of both fibers. Remarkably, due (Supplementary Fig. 11). These results clearly demonstrate the to the good balance of strength (4.4 ± 0.5 MPa) and stretchability key effect of DMSO, which triggers the quick phase separation to (elongation at break of 740 ± 100%), PAH fibers achieved a high −3 form PAH fibers. tensile toughness of 11.1 ± 0.5 MJ m , which is the total energy Due to the polyelectrolyte nature of PAAS, pH and salts are required to break the fiber. More surprisingly, greatly enhanced also expected to affect the behavior of PAAS solutions. The tensile strength (5.6 ± 0.6 MPa) and stretchability (elongation at optimal PAAS solution is weakly basic (pH ~ 8), where the break of 1180 ± 100%) were simultaneously achieved by MAPAH −3 majority of carboxylate groups are negatively charged. In an fibers, resulting in a tensile toughness of 26.8 ± 3.1 MJ m that is 11,39–41 acidic solution, PAAS chain is protonated and the uncharged among the best of current tough hydrogels . Since the PMA linear polymer chain prefers a random coil conformation in film has a much higher stretchability (elongation-at-break solution . Therefore, when adjusting the PAAS solution to acidic ~1900%) than the PAH fiber, the very thin PMA coating layer pH (e.g., from 8 to 1), the solution viscosity was greatly reduced (thickness 2–3 μm) may reinforce the PAH fiber (diameters of and no PAH fibers can be obtained (Supplementary Fig. 12). On 200 ± 20 μm) by reducing the generation of crack on the fiber the other hand, adding salts into the PAAS solution could benefit surface. Therefore, the mechanical properties of the MAPAH the formation of PAH fibers. Using NaCl as one example, fiber is significantly better than the PAH fiber. Due to the great increasing NaCl concentration from 0 to 50 mM can significantly water-resistance and enhanced mechanical properties of MAPAH accelerate the gelation rate of PAAS filament in air, leading to the fibers, we have focused onto MAPAH fibers for further formation of thicker PAH fibers (Supplementary Fig. 13). The exploration. 4 NATURE COMMUNICATIONS | (2018) 9:3579 | DOI: 10.1038/s41467-018-05904-z | www.nature.com/naturecommunications Modulus (Pa) Modulus (Pa) Modulus (Pa) NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05904-z ARTICLE ab 0 s 4 s PAH fiber 10 mm/min PMA film 100 mm/min MAPAH fiber 1000 mm/min 10 s 6 s 0 0 0 300 600 900 1200 1500 1800 2100 0 300 600 900 1200 1500 Strain (%) Strain (%) 1.5 13.7 e Elongation 2500 f PAAS powder 0% 33% // First loading 66% 1.0 100% 200% 30 s Immediately 0.5 16.4 6.4 First unloading 17.0 0.0 5 20 10 15 0 100 200 300 5 10 15 20 Strain (%) 2 theta 2 theta PASS solution As-prepared Stretched −1 Fig. 3 Mechanical and structural characterization of hydrogel fibers. a Stress–strain profiles of a PMA film, PAH, and MAPAH fibers at 100 mm min stretching rate. b Stress–strain profiles of a MAPAH fiber under different stretching rates. c Photograph of a MAPAH fiber showing the hysteresis effect during its recovery process. d Stress–strain profiles of a MAPAH fiber subjected to a loading–unloading cycle at 300% strain (black curve). Immediately after the 1st cycle, the fiber was stretched to 300% strain (red curve). The same fiber was unloaded and stretched to 300% strain after a 30 s recovery at ambient condition (blue curve). e XRD spectra of PAAS powder, and MAPAH fibers placed parallel or perpendicular to the X-ray incidence direction. f XRD spectra of MAPAH fibers under different stretching strains. g Proposed molecular organization and orientation of PAAS chains at different conditions. PAAS is solvated and randomly oriented in solution. Coexistence of crystalline and amorphous domains in as-prepared hydrogel fibers. Mechanical stress enables PAAS chains a higher degree of crystallization and orientation along the strain direction Spider silk shows strain-rate dependent mechanical proper- studied by a cyclic loading test. A MAPAH fiber was stretched ties and hysteresis behavior upon deformation and recov- to 300% strain and unloaded. If the fiber was reloaded ery . Similar to spider silk, the mechanical performance of immediately after unloading, the reloading curve indicated a MAPAH fibers is also strain-rate dependent: the measurement moderately weakened fiber in comparison with the initial curve. at a higher stretching rate gives a higher tensile strength and a Amazingly, when the fiber was reloaded 30 s after the last lower stretchability (Fig. 3b). The measured tensile toughness of unloading, the reloading curve almost coincided with the initial MAPAH fiber increases with the increase of stretching rate, one, which indicates a complete recovery of the hydrogel fiber −3 −1 −3 from 29.2 MJ m at 10 mm min to 30.9 MJ m at 100 mm within 30 s at room temperature (Fig. 3d). Upon stretching, −1 −3 −1 min ,and furtherto45.0MJm at 1000 mm min ,which previous tough hydrogels with sacrificial non-covalent or implies that MAPAH fibers are even tougher upon fast impact dynamic covalent bonds can dissipate energy efficiently by or collision. As another feature similar to spider silk, MAPAH breaking these bonds. But the recovery of these tough hydrogels fibers show a clear hysteresis behavior during its recovery from is relatively slow (from several hours to several days), due to the 11,39,40 large deformation (Supplementary Movie 3). Firstly, a MAPAH slow chain motion in the crosslinked hydrogel matrix .In fiber was kept straight and tight between two clamps. After contrast, MAPAH fibers show both high tensile toughness and being stretched and released, the fiber was loose at the very fast recovery in several seconds, which implies a mechanism of beginning and gradually returned to the straight and tight dissipating energy and recovery that is different from previous status within 5 s (Fig. 3c). The recovery process was further tough hydrogels. NATURE COMMUNICATIONS | (2018) 9:3579 | DOI: 10.1038/s41467-018-05904-z | www.nature.com/naturecommunications 5 Stress (MPa) Stress (MPa) Stress (MPa) Relative intensity Intensity ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05904-z a b 0 100 200 300 400 500 Strain (%) c –35 °C 0.04 0.02 0.00 –0.02 –0.04 –50 –40 –30 –20 –10 0 –40 –30 –20 –10 0 10 20 Temperature (°C) Temperature (°C) Fig. 4 Electrical conductivity of stretchable and anti-freezing MAPAH fibers. a MAPAH fibers can serve as a highly stretchable wire with the PAAS hydrogel as a conductive core and the PMA coating layer as an insulating cover. b Relative resistance variation ΔR/R and relative conductivity variation Δσ/σ of a 0 0 MAPAH fiber upon stretching at 25 °C. c AMAPAH fiber under repeated weight-bearing test at −35 °C with a plastic vial ~800 times heavier than the fiber. d DSC of MAPAH fibers shows a phase transition at around −40 °C. e Conductivity of MAPAH fibers in the temperature range of −35–25 °C. The error bar for each data point in b and e is standard deviation calculated based on 6–8 parallel measurements Structural analysis of MAPAH fibers. To understand the fibers. In the PAAS solution, PAAS chains are expanded and mechanism of the unique mechanical behavior of MAPAH fibers, randomly distributed. Upon drawn from the gel-like solution, we sought to study the chemical composition and macro- orientation and crystallization of PAAS chains proceed effectively molecular alignment of MAPAH fibers by using Infrared Spec- along the fiber direction to form crystalline domains as physical troscopy (IR) and X-ray diffraction (XRD), respectively. As crosslinking points. These crystalline domains are connected by shown in Supplementary Figs.1, 6, the IR spectrum of PAH is soft amorphous domains, which could be similar to the structure identical with that of PAAS, while the IR spectrum of MAPAH is of spider dragline silk . Since MAPAH fibers contain a large just an arithmetic sum of the spectrum of PAAS and PMA, amount of water that serve as solvent and plasticizer, the amor- indicating no significant change on chemical bonding structure phous domains could be forced to form ordered alignment upon during the preparation of PAH and MAPAH fibers. On the other stretching, providing a high stretchability and high strength. After hand, the XRD spectrum of PAAS powder and PMA film shows the stretched fiber being released, the temporarily aligned PAAS only one broad peak, indicating their amorphous nature (Fig. 3e, chains would relax and adopt entropically favorable random Supplementary Fig. 17). In contrast, the XRD spectrum of both conformation and orientation, enabling the fast recovery with the PAH and MAPAH fibers clearly shows three sharp peaks at 2θ = assistance of water in fibers. Therefore, the high stretchability and 6.4°, 13.7°, 17.0° (Supplementary Fig. 17), which indicates the fast resilience of MAPAH fibers can be attributed to the stress- presence of crystalline domains in both PAH and MAPAH fibers. driven alignment and entropy-driven molecular relaxation of Meanwhile, these diffraction peaks are the strongest when the PAAS chains mainly in the amorphous domains, while the fiber’s length direction and the X-ray incidence direction are crystalline domains serve as physical crosslinking points and parallel to each other, and disappear when the two directions are enable a high tensile strength . perpendicular to each other, which demonstrates that the orien- Besides the outstanding mechanical properties, a MAPAH fiber tation of PAAS chains is along the length direction of fibers. is electrically conductive due to the polyelectrolyte nature of Based on molecular mechanics simulation (Supplementary Fig. PAAS. Indeed, a MAPAH fiber can serve as a highly stretchable 18, see SI for details), PAAS chains in a crystalline bundle prefer wire in a circuit (Fig. 4a), with a conductive PAAS hydrogel core an α-helix-like conformation that has a pitch around 6.5Å, cor- and an insulating PMA cover. The MAPAH fibers can be responding to the diffraction peak at 13.7 °. The average distance repeatedly stretched, resulting in a periodical change of light between α-helix is around 5.4Å, corresponding to the peak at intensity of the LED in this circuit (Supplementary Movie 4). The −1 17.0°. When a free MAPAH fiber was stretched, the relative conductivity of MAPAH fibers was measured as ~2 S m at intensity of the peak at 13.7 ° can be increased up to 5 times room temperature, which is in the conductivity range of con- −1 29 (Fig. 3f), indicating the presence of amorphous domain in centrated PAAS solutions in water (1–4Sm ) . As shown in MAPAH fibers that can be induced by mechanical stress to form Supplementary Fig. 1,9, the PMA coating has a very good insu- crystalline domains. lation property. To probe the conductive performance of MAPAH fibers upon stretching, the electrical resistance variation ratio (ΔR/R = (R−R )/R ; R and R correspond to the resistance 0 0 0 0 Discussion without and with stretching, respectively) as a function of the strain was studied (Fig. 4a). From 0 to 300% elongation, the Based on the structural analysis, a mechanism was proposed (Fig. 3g) to explain the unique mechanical behavior of MAPAH electrical resistance of MAPAH fibers increases almost linearly 6 NATURE COMMUNICATIONS | (2018) 9:3579 | DOI: 10.1038/s41467-018-05904-z | www.nature.com/naturecommunications Heat flow (W/g) Non-conductive Conductive ΔR/R –1 Conductivity (mS cm ) Δ/ 0 NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05904-z ARTICLE with the strain (Fig. 4b). Further stretching to 500% strain leads Fabrication of PAH fibers and MAPAH fibers. The optimal PAAS solution was prepared as follows: 200 mg PAAS solid powder in 4.8 g solvent (3.84 g H O and to a faster increase of resistance with a bigger fluctuation. Even 0.96 g DMSO) was stirred and heated at 80 °C for 1 h to get a uniform, transparent stretched to 1000% strain, the MAPAH fiber remains conductive and viscous solution. As shown in Supplementary Movie 1, PAH fibers were (Supplementary Fig. 20). Interestingly, the calculated conductivity directly drawn out of the corresponding PAAS solutions at room temperature. And of MAPAH fibers also increases upon stretching. The highest it needs 1–2 min to let the fiber solidify in room temperature air. The formed PAH fibers were immersed in a 5% PMA/ethyl acetate solution for 5–10 s, and then conductivity was achieved at 300% strain, which was ~4 times of taken out to let the ethyl acetate solvent evaporate. A very thin layer of PMA was the initial conductivity of MAPAH fibers. Since the strain- coated on the PAH fibers to form the core–shell MAPAH fibers. The PAH and dependent change of electrical resistance is fully reversible, the MAPAH fibers for mechanical tests were drawn from the same optimal PAAS large increase of conductivity upon stretching could be attributed solution (4 wt% PAAS in water/DMSO mixture with 20 wt% DMSO), whose dia- meters were controlled as 200 ± 20 μm by adjusting the drawn speed. to the enhanced orientation of PAAS chains along the fiber length direction, which may facilitate the diffusion of sodium ions along Characterization of PAH fibers and MAPAH fibers. Field emission scanning the PAAS chains . electron microscope (FE-SEM, JEOL JSM-6700F) was used to image the freeze- Conventional conductive hydrogels would freeze and lose dried hydrogel fibers. For all the other characterization, freshly prepared hydrogel stretchability and conductivity at subzero temperatures. Liu has fibers were used. X-ray diffraction (XRD) was carried out on a Rigaku D X-ray reported organohydrogels based on water-ethylene glycol binary diffractometer with Cu Kα radiation (λ = 1.54178 Å). One MAPAH fiber was solution that can sustain subzero temperature due to the anti- placed parallel or perpendicular to the X-ray incidence direction. For the XRD of 23 stretched MAPAH fibers, one MAPAH fiber with certain elongation was glued on a freezing property of water-ethylene glycol mixture . Repeated silicon wafer and placed parallel to the X-ray incidence direction. Thermogravi- weight-bearing test of MAPAH fibers at low temperature metric analysis (TGA) was done by TGA Q5000IR, the temperature was from 20 °C demonstrates that the fiber retains its great stretchability and fast to 800 °C with 10 °C/min. Differential scanning calorimetric (DSC) was performed −1 resilience even at −35 °C (Fig. 4c). Indeed, the differential scan- by using TA Q2000 at identical heating and cooling rate of 2 °C min between −50 °C and 90 °C. The contact angle of water droplet on PMA film and MAPAH ning calorimetric (DSC) measurement of MAPAH fibers shows a fiber was measured by using a contact angle meter SL2008, Solon Tech. Rheological phase transition around −40 °C (Fig. 4d), which is likely corre- measurements of different PAAS solutions were conducted on a TA AR-G2 rhe- sponding to the freezing point of water in MAPAH fibers. The ometer. Mechanical properties of PAH fibers and MAPAH fibers were tested on an DSC result and the great stretchability at low temperatures Instron 3340 universal testing instrument at 25 °C and relative humidity ~42 ± 2%. The average diameter of each fiber sample was obtained from three measurements indicate that MAPAH fibers possess a remarkable anti-freezing along the fiber length using an optical microscope. The electrical resistance of property. Upon the decrease of temperature, the conductivity of MAPAH fibers was measured by using a multimeter. The electric conductivity was MAPAH fibers decreases due to the reduction of ion diffusion calculated based on the equation: σ = L/(S × R), where L, S, σ, and R are the length, −1 rate, and remains 0.1 S m at −35 °C (Fig. 4e). The molar ratio cross section area, conductivity, and electrical resistance of the MAPAH fiber. of H O:DMSO in this fiber was measured by using H NMR as ~1300:1 (Supplementary Fig. 21), which indicates water is the Data availability only solvent in the MAPAH fiber. The absence of anti-freezing The authors declare that the data supporting the findings of this study are available within the article and its Supplementary Information. organic solvents in MAPAH fiber implies a different anti-freezing mechanism from organohydrogels. Indeed, the outstanding anti- freezing property of MAPAH fibers is likely due to the high Received: 23 March 2018 Accepted: 23 July 2018 concentration of sodium ions in the PAAS hydrogel matrix, since concentrated salt solution is anti-freezing. The conductivity of MAPAH fibers is comparable to other conductive hydrogels at 13–16 and much higher than that of organo- room temperature hydrogels at low temperatures , which indicates the advantage of References MAPAH fibers as stretchable and anti-freezing conductive 1. Oh, J. Y. et al. Intrinsically stretchable and healable semiconducting polymer hydrogel fibers. for organic transistors. Nature 539, 411–415 (2016). 2. Rogers, J. A., Someya, T. & Huang, Y. 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Rammensee, S., Slotta, U., Scheibel, T. & Bausch, A. Assembly mechanism of recombinant spider silk proteins. Proc. Natl Acad. Sci. USA 105, 6590–6595 (2008). 32. Römer, L. & Scheibel, T. The elaborate structure of spider silk. Prion 2, Open Access This article is licensed under a Creative Commons 154–161 (2008). Attribution 4.0 International License, which permits use, sharing, 33. Anderson, M. et al. Biomimetic spinning of artificial spider silk from a adaptation, distribution and reproduction in any medium or format, as long as you give chimeric minispidroin. Nat. Chem. Bio. 13, 262–264 (2017). appropriate credit to the original author(s) and the source, provide a link to the Creative 34. Liu, Y., Shao, Z. & Vollrath, F. Relationships between supercontraction and Commons license, and indicate if changes were made. The images or other third party mechanical properties of spider silk. Nat. Mater. 4, 901–905 (2005). material in this article are included in the article’s Creative Commons license, unless 35. Bordi, F., Colby, R. H., Cametti, C., De Lorenzo, L. & Gili, T. Electrical indicated otherwise in a credit line to the material. If material is not included in the conductivity of polyelectrolyte solutions in the semidilute and concentrated article’s Creative Commons license and your intended use is not permitted by statutory regime: the role of counterion condensation. J. Phys. Chem. B 106, 6887–6893 regulation or exceeds the permitted use, you will need to obtain permission directly from (2002). the copyright holder. To view a copy of this license, visit http://creativecommons.org/ 36. Sahni, V., Labhasetwar, D. V. & Dhinojwala, A. Spider silk inspired functional licenses/by/4.0/. microthreads. Langmuir 28, 2206–2210 (2011). 37. Nishiyama, Y. & Satoh, M. Solvent-and counterion-specific swelling behavior of poly (acrylic acid) gels. J. Polym. Sci. B Polym. 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Bioinspired ultra-stretchable and anti-freezing conductive hydrogel fibers with ordered and reversible polymer chain alignment

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Science, Humanities and Social Sciences, multidisciplinary; Science, Humanities and Social Sciences, multidisciplinary; Science, multidisciplinary
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Abstract

ARTICLE DOI: 10.1038/s41467-018-05904-z OPEN Bioinspired ultra-stretchable and anti-freezing conductive hydrogel fibers with ordered and reversible polymer chain alignment 1 1 1 1 1 1 Xue Zhao , Fang Chen , Yuanheng Li , Han Lu , Ning Zhang & Mingming Ma High-performance stretchable conductive fibers are desired for the development of stretchable electronic devices. Here we show a simple spinning method to prepare con- ductive hydrogel fibers with ordered polymer chain alignment that mimics the hierarchically organized structure of spider silk. The as-prepared sodium polyacrylate hydrogel fiber is further coated with a thin layer of polymethyl acrylate to form a core–shell water-resistant MAPAH fiber. Owing to the coexistence and reversible transformation of crystalline and amorphous domains in the fibers, MAPAH fibers exhibit high tensile strength, large stretchability and fast resilience from large strain. MAPAH fiber can serve as a highly stretchable wire with a conductive hydrogel core and an insulating cover. The stretchability and conductivity of the MAPAH fiber are retained at −35 °C, indicating its anti-freezing property. As a prime example of stretchable conductive fibers, MAPAH fibers will shed light on the design of next generation textile-based stretchable electronic devices. CAS Key Laboratory of Soft Matter Chemistry, iChEM (Innovation Center of Chemistry for Energy Materials), Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China. These authors contributed equally: Xue Zhao, Fang Chen. Correspondence and requests for materials should be addressed to M.M. (email: mma@ustc.edu.cn) NATURE COMMUNICATIONS | (2018) 9:3579 | DOI: 10.1038/s41467-018-05904-z | www.nature.com/naturecommunications 1 1234567890():,; ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05904-z tretchable conductive materials are essential for the emer- After a systematic exploration, we have found that long and 1,2 ging stretchable electronic devices , such as stretchable uniform filaments can be readily drawn from a gel-like PAAS 3 4–6 7,8 Spanels , energy-storage devices and sensors . Among solution in a mixture of water (as a good solvent) and dimethyl 9–11 various functional hydrogels , conductive hydrogels are very sulfoxide (DMSO, as a poor solvent). Water evaporation from the promising as stretchable conductive materials, which have been PAAS filaments in air enriches the poor solvent DMSO in the extensively studied for applications such as stretchable filaments, which triggers a quick phase transition to form PAAS 7,8,12 13,14 sensors and supercapacitors . Most of the conductive hydrogel (PAH) fibers with adjustable diameters and arbitrary hydrogels are made into two-dimensional films or three- length as wish. Just like spider silk, PAH fiber shows a unique 15,16 36 dimensional monoliths by molding methods , which result beads-on-a-string structure and good mechanical properties. To in an amorphous material with polymer chain’s random orien- achieve a good water resistance property, PAH fibers are coated tation and disordered alignment. If conductive hydrogels can be with a thin layer of polymethyl acrylate (PMA) to form core–shell made into one-dimensional fibers with ordered alignment of PMA–PAAS hydrogel (MAPAH) fibers. Remarkably, the water- polymer chains, some properties of conductive hydrogel fibers resistant MAPAH fibers exhibit a unique combination of high (e.g., mechanical properties and conductivity) would be greatly tensile strength (5.6 MPa) and stretchability (elongation at a enhanced over conventional conductive hydrogel films and break of 1200%), fast resilience (<30 s) from large stretching 17 −1 monoliths . However, the ordered chain alignment of conductive strain, high electrical conductivity (2 S m ), and great anti- polymers has been achieved in solid-state microfibers prepared by freezing property. The simultaneous achievements of these prime 17–19 20 template directed polymerization or alignment , which are properties by MAPAH fibers are attributed to the coexistence and not suitable for the preparation of conductive hydrogel fibers at reversible transformation of crystalline and amorphous domains macroscopic scale. On the other hand, spinning methods are in hydrogel fibers, which are enabled by the spinning and gelation widely used to prepare polymer fibers with ordered chain process. The multifunctional MAPAH fiber as a high- 21,22 alignment . But conductive hydrogels are rarely made into performance and low-cost stretchable conductive fiber will shed long fibers by spinning, due to the poor spinnability of current light on the design of next generation textile-based stretchable conductive hydrogels or their precursor solutions. In addition, electronic devices. conductive hydrogel films and monoliths are also limited by their slow resilience from large deformation due to the moderate reversibility of polymer chain alignment , and the loss of func- Results tions at subzero temperature . Previously reported polyelec- Fabrication of PAH and MAPAH fibers. Among many different trolyte fibers based on polycation–polyanion interfacial types of commercial available PAAS, the non-crosslinked PAAS complexation are rigid and insulating materials, which are mainly with an ultra-high molecular weight (M ~3× 10 Da) and low used for biomedical applications, such as tissue engineering and cost (~$20 per kilogram) was chosen as the starting material. The 24,25 drug delivery . Previously reported stretchable conductive preparation procedure of PAH and MAPAH fibers is summarized 6,26 27,28 fibers based on carbon materials or metal nanomaterials in Fig. 1a. PAAS powder was dissolved in a mixture of water and coating on elastomeric fibers are also limited by their sophisti- DMSO at 80 °C to form a transparent solution (Supplementary cated fabrication process and moderate stretchability . There- Fig. 1). The composition of the water/DMSO mixture and the fore, high-performance conductive hydrogel fibers with ordered concentration of PAAS in the solution are the two key factors for and reversible chain alignment are highly desired for the devel- the successful preparation of PAH fibers. The optimal solution opment of stretchable electronics, especially for the textile-based was found as 4 wt% PAAS dissolved in the water/DMSO mixture 14,30 stretchable electronic devices , but remain as a challenge. solvent with DMSO% = 20 wt%. As shown in Supplementary In nature, spiders spin silk fibers from aqueous protein Fig. 2, 4 wt% PAAS solutions in the water/DMSO mixture with solutions at ambient conditions . The hierarchically organized different water: DMSO ratios were prepared at 80 °C. Upon 32 33 structure of spider silk and its unique spinning process are cooling to room temperature, a clear phase transition process was the key factors to achieve its superb properties . For example, observed, indicating a critical concentration of DMSO in the spider dragline silk is a semi-crystalline protein polymer, where range of 20–22 wt%. With DMSO% ≤20 wt%, the PAAS solution alanine-rich crystalline regions are connected by soft glycine- remained as a homogeneous solution at room temperature. With rich amorphous regions as linkers . Inspired by the organized DMSO% ≥22 wt%, a white precipitate gradually formed during structure and the unique spinning process of spider silk, we the cooling process. A similar phase transition was observed for propose to develop a simple spinning method to prepare con- covalently crosslinked PAAS hydrogels swelled by water/DMSO ductive hydrogel fibers with ordered and reversible chain mixture . The addition of DMSO into water causes significant alignment from aqueous solution of polyelectrolytes at ambient desolvation of PAAS chains upon the counterion Na binding conditions. Aiming at the production at low cost, a mass- carboxylate groups, increasing the polymer–polymer interactions produced synthetic polyelectrolyte: sodium polyacrylate and triggering the phase transition . The optimal solution for (PAAS), was chosen as the starting material, which has been PAH fiber preparation has DMSO% = 20 wt%, which is very close widely utilized in daily supplies and in industry. In water to the critical DMSO concentration for the phase transition. solutions, the anionic carboxylate groups on a PAAS chain Therefore, when a PAAS filament was drawn out from the repel each other via double layer forces, which causes the optimal PAAS solution at room temperature, the DMSO% in the polymer chain to adopt an expanded, rigid-rod-like con- filament increased due to water evaporation and quickly exceeded formation. Adjusting pH, addition of salts or poor solvents tune the critical concentration. Therefore, PAAS in the filament the electrostatic interactions and alter the conformation of rapidly aggregated to form a fine PAH fiber, while the excess PAAS chains in solution, leading to the change of bulk prop- aqueous solution can automatically form liquid droplets due to erties (such as viscosity and sol–gel transition) .Based on the surface tension (Fig. 1a). These liquid droplets either flowed away 31–33 knowledge learned from spider silk , we expect that the along a tilted PAH fiber (Supplementary Movie 1) or gradually stimuli-responsive properties of PAAS in solution would enable solidified on a horizontal PAH fiber to form the beads-on-a- PAAS solution a good spinnability, where the spinning and string structure that mimics spider silk (photograph in Fig. 1a). drawing process would enable an ordered alignment of PAAS Similar to spider silk, PAH fibers are elastic and sticky, which can chains in the as-spun filaments. be used to weave a web that mimics a spider web (Supplementary 2 NATURE COMMUNICATIONS | (2018) 9:3579 | DOI: 10.1038/s41467-018-05904-z | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05904-z ARTICLE PAH fiber MAPAH fiber O Na PAAS Water – – – – – – PMA coating n O O OO OH O O O O O O O O O evaporates + + Na Na Na H H H HH CH H H O H O H C H O O H O PAAS 3 S O O H H O H H O H H H H H O + H separates out + + Na + Na O O Na Na Na O O OO H H H H H H H CH H H 3 O O O H H H O O O O O + S H + H H + PMA + Na O H Na H H Na Na CH – – – – – – O O O OO OO O O OHO O O O Solvent: H O/DMSO PAH fiber PAH fiber MAPAH fiber PMA PAAS PAH fiber MAPAH fiber Fig. 1 Preparation of PAH and MAPAH fibers. a Schematic illustration of the preparation of PAH and MAPAH fibers. A photograph of as-prepared PAH fiber shows the beads-on-a-string structure. b Photograph of a 1.1 m-long as-prepared PAH fiber. c SEM images of a PAH fiber. The scale bar is 100 μm. d SEM images of a MAPAH fiber. The scale bar in the left image is 200 μm, and the scale bar in the right image is 5 μm. A thin PMA layer on the PAAS core is clearly observed in the right image. e Photograph of a PAH web damaged by liquid water and a MAPAH web resistant to liquid water. Red circles indicate water droplets remaining on the hydrophilic PAH fiber, while no water droplet stays on the hydrophobic MAPAH fiber Fig. 3). PAH fibers with arbitrary length can be easily made MAPAH fibers. Although the thickness of PMA layer was only through continuous drawing from the optimal PAAS solution. 1–2 μm, this compact PMA layer enables MAPAH fibers a great Two examples of as-prepared PAH fibers are shown in Fig. 1b water resistance property. Observed under an optical microscope, (~1.1 m) and in Supplementary Movie 1 (~6.2 m). Since the a PAH fiber was swelled and gradually dissolved by a water preparation of PAH fibers is similar to the process of gel spin- droplet, while the MAPAH fiber remained unchanged with the ning , we envisage that the preparation could be facilely adapted water droplet (Supplementary Fig. 6). The contact angle of water onto a gel spinning equipment for the mass-production of PAH droplet on MAPAH fiber was measured as ~92° (Supplementary fibers. Fig. 7), indicating a good hydrophobicity of the PMA coating Due to the hydrophilic nature of PAAS, PAH fibers can be layer. As shown in Supplementary Movie 2, a MAPAH fiber quickly swelled and weaken in liquid water. To achieve a good retained its great water resistance property even in a stretched water resistance, we coated the PAH fibers with a thin condition (~200% elongation), while a PAH fiber was quickly hydrophobic layer of polymethyl acrylate (PMA). PMA was broken by liquid water at the same condition. As expected, the synthesized through conventional radical polymerization (M = web made from PAH fibers was not resistant to liquid water, 3.8 × 10 Da, PDI = 1.93, see SI for details). As-prepared PAH while a water-proof web can be made from MAPAH fibers, which fibers were immersed in a 5 wt% PMA solution in ethyl acetate remained stable and elastic after being treated by liquid water for several seconds and then taken out. After the quick (Fig. 1e). evaporation of ethyl acetate, a thin and uniform layer of PMA was firmly coated on the PAH fiber to form the core–shell MAPAH fiber. As judged by scanning electron microscopy (SEM, Characterization and optimization of the PAAS solutions.To Fig. 1c, d), both PAH and MAPAH fibers show a cylindrical understand the unique spinnability of PAAS solutions in water/ shape with consistent diameter. Observed under an optical DMSO mixture solvent, rheological measurements were con- microscope, both fibers show a good uniformity of their diameter ducted to study the viscoelastic properties of PAAS solutions. As along the fiber length (Supplementary Fig. 4). Fracture surfaces shown in Supplementary Fig.8, the storage moduli (G’) are from both fibers show a compact and homogenous interior core. dominant over loss moduli (G”) across the whole range of fre- As shown in Fig. 1d, a thin and compact PMA layer can be clearly quencies studied, which indicates the viscoelastic behavior of observed on the PAH fiber surface that firmly wraps the PAH PAAS solutions. The optimal PAAS solution (4 wt%) appeared as core. PAH fibers with different diameters in the range of 20–300 a physical gel at room temperature (Fig. 2a), and the high visc- μm can also be prepared by varying the preparation conditions osity of the gel is an important factor for drawing good PAH (Supplementary Fig. 5), which can be further converted to fibers. At lower concentrations (e.g., 2 wt%), the PAAS solution NATURE COMMUNICATIONS | (2018) 9:3579 | DOI: 10.1038/s41467-018-05904-z | www.nature.com/naturecommunications 3 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05904-z 2% 4% 6% 4%-G’ 4%-G” 447% 1 10 100 1000 Oscillation strain (%) 4%-H O-G’ 4%-H O-G” 4%-G’ 4%-G” 0 50 100 150 200 250 0.1 1 10 100 Time (s) Angular frequency (rad/s) Fig. 2 Preparation and rheological characterization of PAAS solutions. a Photograph of PAAS solutions with different concentrations (2%: viscous liquid; 4%: physical gel; 6%: precipitate and liquid) in H O:DMSO= 4:1 mixture at room temperature. b Strain-dependent and c step-strain oscillatory rheology of −1 the optimal PAAS solution. The applied oscillatory strain in c alternated between 1 and 1000% for 30 s periods (ω = 10 rads , 25 °C). d Frequency- dependent oscillatory rheology of 4% PAAS solution with pure water (black) or H O: DMSO= 4:1 (red) as solvent remained as viscous liquid and the filaments drawn from this effect of NaCl was observed by using rheological measurements solution were thinner and unstable (Supplementary Fig. 9). At (Supplementary Fig. 14): the gel to sol cross-over point was higher concentrations (e.g., 6 wt%), PAAS precipitated out of the increased from 447% to 680% strain simply by adding 50 mM solution upon cooling to room temperature. Strain-dependent NaCl into the optimal PAAS solution, which indicates the oscillatory rheology of the optimal PAAS solution displays a enhanced polymer–polymer interactions by addition of NaCl into broad linear gel region with a gel to sol cross-over point solution . appearing at 447% strain (Fig. 2b). In the step-strain measure- ments (Fig. 2c), the optimal PAAS solution exhibits a fast and Mechanical performance of PAH and MAPAH fibers. The complete recovery to its initial modulus, which indicates a great mechanical properties of both PAH and MAPAH fibers were reversibility due to the fast alignment and relaxation of PAAS studied based on fibers drawn from the same optimal PAAS chains in the physical gel . As shown in Fig. 2d, both G’ and G” solution, whose diameters were controlled as 200 ± 20 μmby values of a 4 wt% PAAS solution in pure water are higher than adjusting the drawn speed in a proper range. The environmental that of the optimal PAAS solution (4 wt% PAAS in water/DMSO temperature and relative humidity were maintained at 25 °C and solvent), indicating that PAAS chains are more extended in pure 42 ± 2%, respectively. The water content of PAH fibers and water than in water/DMSO mixture. However, filaments drawn MAPAH fibers measured by thermogravimetric analysis (TGA) from this 4 wt% PAAS solution in pure water were thinner and were similar, both of which were in the range of 25 ± 3% (Sup- unstable, which could not form PAH fibers (Supplementary Fig. plementary Fig. 15). Representative stress–strain curves of PAH 10). Reduction of the content of DMSO (e.g., from 20 wt% to and MAPAH fibers show a rubber-like characteristics (Fig. 3a) , 14.3 wt%) would also result in thinner and fragile PAH fibers indicating the viscoelastic nature of both fibers. Remarkably, due (Supplementary Fig. 11). These results clearly demonstrate the to the good balance of strength (4.4 ± 0.5 MPa) and stretchability key effect of DMSO, which triggers the quick phase separation to (elongation at break of 740 ± 100%), PAH fibers achieved a high −3 form PAH fibers. tensile toughness of 11.1 ± 0.5 MJ m , which is the total energy Due to the polyelectrolyte nature of PAAS, pH and salts are required to break the fiber. More surprisingly, greatly enhanced also expected to affect the behavior of PAAS solutions. The tensile strength (5.6 ± 0.6 MPa) and stretchability (elongation at optimal PAAS solution is weakly basic (pH ~ 8), where the break of 1180 ± 100%) were simultaneously achieved by MAPAH −3 majority of carboxylate groups are negatively charged. In an fibers, resulting in a tensile toughness of 26.8 ± 3.1 MJ m that is 11,39–41 acidic solution, PAAS chain is protonated and the uncharged among the best of current tough hydrogels . Since the PMA linear polymer chain prefers a random coil conformation in film has a much higher stretchability (elongation-at-break solution . Therefore, when adjusting the PAAS solution to acidic ~1900%) than the PAH fiber, the very thin PMA coating layer pH (e.g., from 8 to 1), the solution viscosity was greatly reduced (thickness 2–3 μm) may reinforce the PAH fiber (diameters of and no PAH fibers can be obtained (Supplementary Fig. 12). On 200 ± 20 μm) by reducing the generation of crack on the fiber the other hand, adding salts into the PAAS solution could benefit surface. Therefore, the mechanical properties of the MAPAH the formation of PAH fibers. Using NaCl as one example, fiber is significantly better than the PAH fiber. Due to the great increasing NaCl concentration from 0 to 50 mM can significantly water-resistance and enhanced mechanical properties of MAPAH accelerate the gelation rate of PAAS filament in air, leading to the fibers, we have focused onto MAPAH fibers for further formation of thicker PAH fibers (Supplementary Fig. 13). The exploration. 4 NATURE COMMUNICATIONS | (2018) 9:3579 | DOI: 10.1038/s41467-018-05904-z | www.nature.com/naturecommunications Modulus (Pa) Modulus (Pa) Modulus (Pa) NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05904-z ARTICLE ab 0 s 4 s PAH fiber 10 mm/min PMA film 100 mm/min MAPAH fiber 1000 mm/min 10 s 6 s 0 0 0 300 600 900 1200 1500 1800 2100 0 300 600 900 1200 1500 Strain (%) Strain (%) 1.5 13.7 e Elongation 2500 f PAAS powder 0% 33% // First loading 66% 1.0 100% 200% 30 s Immediately 0.5 16.4 6.4 First unloading 17.0 0.0 5 20 10 15 0 100 200 300 5 10 15 20 Strain (%) 2 theta 2 theta PASS solution As-prepared Stretched −1 Fig. 3 Mechanical and structural characterization of hydrogel fibers. a Stress–strain profiles of a PMA film, PAH, and MAPAH fibers at 100 mm min stretching rate. b Stress–strain profiles of a MAPAH fiber under different stretching rates. c Photograph of a MAPAH fiber showing the hysteresis effect during its recovery process. d Stress–strain profiles of a MAPAH fiber subjected to a loading–unloading cycle at 300% strain (black curve). Immediately after the 1st cycle, the fiber was stretched to 300% strain (red curve). The same fiber was unloaded and stretched to 300% strain after a 30 s recovery at ambient condition (blue curve). e XRD spectra of PAAS powder, and MAPAH fibers placed parallel or perpendicular to the X-ray incidence direction. f XRD spectra of MAPAH fibers under different stretching strains. g Proposed molecular organization and orientation of PAAS chains at different conditions. PAAS is solvated and randomly oriented in solution. Coexistence of crystalline and amorphous domains in as-prepared hydrogel fibers. Mechanical stress enables PAAS chains a higher degree of crystallization and orientation along the strain direction Spider silk shows strain-rate dependent mechanical proper- studied by a cyclic loading test. A MAPAH fiber was stretched ties and hysteresis behavior upon deformation and recov- to 300% strain and unloaded. If the fiber was reloaded ery . Similar to spider silk, the mechanical performance of immediately after unloading, the reloading curve indicated a MAPAH fibers is also strain-rate dependent: the measurement moderately weakened fiber in comparison with the initial curve. at a higher stretching rate gives a higher tensile strength and a Amazingly, when the fiber was reloaded 30 s after the last lower stretchability (Fig. 3b). The measured tensile toughness of unloading, the reloading curve almost coincided with the initial MAPAH fiber increases with the increase of stretching rate, one, which indicates a complete recovery of the hydrogel fiber −3 −1 −3 from 29.2 MJ m at 10 mm min to 30.9 MJ m at 100 mm within 30 s at room temperature (Fig. 3d). Upon stretching, −1 −3 −1 min ,and furtherto45.0MJm at 1000 mm min ,which previous tough hydrogels with sacrificial non-covalent or implies that MAPAH fibers are even tougher upon fast impact dynamic covalent bonds can dissipate energy efficiently by or collision. As another feature similar to spider silk, MAPAH breaking these bonds. But the recovery of these tough hydrogels fibers show a clear hysteresis behavior during its recovery from is relatively slow (from several hours to several days), due to the 11,39,40 large deformation (Supplementary Movie 3). Firstly, a MAPAH slow chain motion in the crosslinked hydrogel matrix .In fiber was kept straight and tight between two clamps. After contrast, MAPAH fibers show both high tensile toughness and being stretched and released, the fiber was loose at the very fast recovery in several seconds, which implies a mechanism of beginning and gradually returned to the straight and tight dissipating energy and recovery that is different from previous status within 5 s (Fig. 3c). The recovery process was further tough hydrogels. NATURE COMMUNICATIONS | (2018) 9:3579 | DOI: 10.1038/s41467-018-05904-z | www.nature.com/naturecommunications 5 Stress (MPa) Stress (MPa) Stress (MPa) Relative intensity Intensity ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05904-z a b 0 100 200 300 400 500 Strain (%) c –35 °C 0.04 0.02 0.00 –0.02 –0.04 –50 –40 –30 –20 –10 0 –40 –30 –20 –10 0 10 20 Temperature (°C) Temperature (°C) Fig. 4 Electrical conductivity of stretchable and anti-freezing MAPAH fibers. a MAPAH fibers can serve as a highly stretchable wire with the PAAS hydrogel as a conductive core and the PMA coating layer as an insulating cover. b Relative resistance variation ΔR/R and relative conductivity variation Δσ/σ of a 0 0 MAPAH fiber upon stretching at 25 °C. c AMAPAH fiber under repeated weight-bearing test at −35 °C with a plastic vial ~800 times heavier than the fiber. d DSC of MAPAH fibers shows a phase transition at around −40 °C. e Conductivity of MAPAH fibers in the temperature range of −35–25 °C. The error bar for each data point in b and e is standard deviation calculated based on 6–8 parallel measurements Structural analysis of MAPAH fibers. To understand the fibers. In the PAAS solution, PAAS chains are expanded and mechanism of the unique mechanical behavior of MAPAH fibers, randomly distributed. Upon drawn from the gel-like solution, we sought to study the chemical composition and macro- orientation and crystallization of PAAS chains proceed effectively molecular alignment of MAPAH fibers by using Infrared Spec- along the fiber direction to form crystalline domains as physical troscopy (IR) and X-ray diffraction (XRD), respectively. As crosslinking points. These crystalline domains are connected by shown in Supplementary Figs.1, 6, the IR spectrum of PAH is soft amorphous domains, which could be similar to the structure identical with that of PAAS, while the IR spectrum of MAPAH is of spider dragline silk . Since MAPAH fibers contain a large just an arithmetic sum of the spectrum of PAAS and PMA, amount of water that serve as solvent and plasticizer, the amor- indicating no significant change on chemical bonding structure phous domains could be forced to form ordered alignment upon during the preparation of PAH and MAPAH fibers. On the other stretching, providing a high stretchability and high strength. After hand, the XRD spectrum of PAAS powder and PMA film shows the stretched fiber being released, the temporarily aligned PAAS only one broad peak, indicating their amorphous nature (Fig. 3e, chains would relax and adopt entropically favorable random Supplementary Fig. 17). In contrast, the XRD spectrum of both conformation and orientation, enabling the fast recovery with the PAH and MAPAH fibers clearly shows three sharp peaks at 2θ = assistance of water in fibers. Therefore, the high stretchability and 6.4°, 13.7°, 17.0° (Supplementary Fig. 17), which indicates the fast resilience of MAPAH fibers can be attributed to the stress- presence of crystalline domains in both PAH and MAPAH fibers. driven alignment and entropy-driven molecular relaxation of Meanwhile, these diffraction peaks are the strongest when the PAAS chains mainly in the amorphous domains, while the fiber’s length direction and the X-ray incidence direction are crystalline domains serve as physical crosslinking points and parallel to each other, and disappear when the two directions are enable a high tensile strength . perpendicular to each other, which demonstrates that the orien- Besides the outstanding mechanical properties, a MAPAH fiber tation of PAAS chains is along the length direction of fibers. is electrically conductive due to the polyelectrolyte nature of Based on molecular mechanics simulation (Supplementary Fig. PAAS. Indeed, a MAPAH fiber can serve as a highly stretchable 18, see SI for details), PAAS chains in a crystalline bundle prefer wire in a circuit (Fig. 4a), with a conductive PAAS hydrogel core an α-helix-like conformation that has a pitch around 6.5Å, cor- and an insulating PMA cover. The MAPAH fibers can be responding to the diffraction peak at 13.7 °. The average distance repeatedly stretched, resulting in a periodical change of light between α-helix is around 5.4Å, corresponding to the peak at intensity of the LED in this circuit (Supplementary Movie 4). The −1 17.0°. When a free MAPAH fiber was stretched, the relative conductivity of MAPAH fibers was measured as ~2 S m at intensity of the peak at 13.7 ° can be increased up to 5 times room temperature, which is in the conductivity range of con- −1 29 (Fig. 3f), indicating the presence of amorphous domain in centrated PAAS solutions in water (1–4Sm ) . As shown in MAPAH fibers that can be induced by mechanical stress to form Supplementary Fig. 1,9, the PMA coating has a very good insu- crystalline domains. lation property. To probe the conductive performance of MAPAH fibers upon stretching, the electrical resistance variation ratio (ΔR/R = (R−R )/R ; R and R correspond to the resistance 0 0 0 0 Discussion without and with stretching, respectively) as a function of the strain was studied (Fig. 4a). From 0 to 300% elongation, the Based on the structural analysis, a mechanism was proposed (Fig. 3g) to explain the unique mechanical behavior of MAPAH electrical resistance of MAPAH fibers increases almost linearly 6 NATURE COMMUNICATIONS | (2018) 9:3579 | DOI: 10.1038/s41467-018-05904-z | www.nature.com/naturecommunications Heat flow (W/g) Non-conductive Conductive ΔR/R –1 Conductivity (mS cm ) Δ/ 0 NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05904-z ARTICLE with the strain (Fig. 4b). Further stretching to 500% strain leads Fabrication of PAH fibers and MAPAH fibers. The optimal PAAS solution was prepared as follows: 200 mg PAAS solid powder in 4.8 g solvent (3.84 g H O and to a faster increase of resistance with a bigger fluctuation. Even 0.96 g DMSO) was stirred and heated at 80 °C for 1 h to get a uniform, transparent stretched to 1000% strain, the MAPAH fiber remains conductive and viscous solution. As shown in Supplementary Movie 1, PAH fibers were (Supplementary Fig. 20). Interestingly, the calculated conductivity directly drawn out of the corresponding PAAS solutions at room temperature. And of MAPAH fibers also increases upon stretching. The highest it needs 1–2 min to let the fiber solidify in room temperature air. The formed PAH fibers were immersed in a 5% PMA/ethyl acetate solution for 5–10 s, and then conductivity was achieved at 300% strain, which was ~4 times of taken out to let the ethyl acetate solvent evaporate. A very thin layer of PMA was the initial conductivity of MAPAH fibers. Since the strain- coated on the PAH fibers to form the core–shell MAPAH fibers. The PAH and dependent change of electrical resistance is fully reversible, the MAPAH fibers for mechanical tests were drawn from the same optimal PAAS large increase of conductivity upon stretching could be attributed solution (4 wt% PAAS in water/DMSO mixture with 20 wt% DMSO), whose dia- meters were controlled as 200 ± 20 μm by adjusting the drawn speed. to the enhanced orientation of PAAS chains along the fiber length direction, which may facilitate the diffusion of sodium ions along Characterization of PAH fibers and MAPAH fibers. Field emission scanning the PAAS chains . electron microscope (FE-SEM, JEOL JSM-6700F) was used to image the freeze- Conventional conductive hydrogels would freeze and lose dried hydrogel fibers. For all the other characterization, freshly prepared hydrogel stretchability and conductivity at subzero temperatures. Liu has fibers were used. X-ray diffraction (XRD) was carried out on a Rigaku D X-ray reported organohydrogels based on water-ethylene glycol binary diffractometer with Cu Kα radiation (λ = 1.54178 Å). One MAPAH fiber was solution that can sustain subzero temperature due to the anti- placed parallel or perpendicular to the X-ray incidence direction. For the XRD of 23 stretched MAPAH fibers, one MAPAH fiber with certain elongation was glued on a freezing property of water-ethylene glycol mixture . Repeated silicon wafer and placed parallel to the X-ray incidence direction. Thermogravi- weight-bearing test of MAPAH fibers at low temperature metric analysis (TGA) was done by TGA Q5000IR, the temperature was from 20 °C demonstrates that the fiber retains its great stretchability and fast to 800 °C with 10 °C/min. Differential scanning calorimetric (DSC) was performed −1 resilience even at −35 °C (Fig. 4c). Indeed, the differential scan- by using TA Q2000 at identical heating and cooling rate of 2 °C min between −50 °C and 90 °C. The contact angle of water droplet on PMA film and MAPAH ning calorimetric (DSC) measurement of MAPAH fibers shows a fiber was measured by using a contact angle meter SL2008, Solon Tech. Rheological phase transition around −40 °C (Fig. 4d), which is likely corre- measurements of different PAAS solutions were conducted on a TA AR-G2 rhe- sponding to the freezing point of water in MAPAH fibers. The ometer. Mechanical properties of PAH fibers and MAPAH fibers were tested on an DSC result and the great stretchability at low temperatures Instron 3340 universal testing instrument at 25 °C and relative humidity ~42 ± 2%. The average diameter of each fiber sample was obtained from three measurements indicate that MAPAH fibers possess a remarkable anti-freezing along the fiber length using an optical microscope. The electrical resistance of property. Upon the decrease of temperature, the conductivity of MAPAH fibers was measured by using a multimeter. The electric conductivity was MAPAH fibers decreases due to the reduction of ion diffusion calculated based on the equation: σ = L/(S × R), where L, S, σ, and R are the length, −1 rate, and remains 0.1 S m at −35 °C (Fig. 4e). The molar ratio cross section area, conductivity, and electrical resistance of the MAPAH fiber. of H O:DMSO in this fiber was measured by using H NMR as ~1300:1 (Supplementary Fig. 21), which indicates water is the Data availability only solvent in the MAPAH fiber. 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