As the soaring demand for energy storage continues to grow, batteries that can cope with extreme conditions are highly desired. Yet, existing battery materials are limited by weak mechanical properties and freeze‐vulnerability, prohibiting safe energy storage in devices that are exposed to low temperature and unusual mechanical impacts. Herein, a fabrication method harnessing the synergistic effect of co‐nonsolvency and “salting‐out” that can produce poly(vinyl alcohol) hydrogel electrolytes with unique open‐cell porous structures, composed of strongly aggregated polymer chains, and containing disrupted hydrogen bonds among free water molecules, is introduced. The hydrogel electrolyte simultaneously combines high strength (tensile strength 15.6 MPa), freeze‐tolerance (< −77 °C), high mass transport (10× lower overpotential), and dendrite and parasitic reactions suppression for stable performance (30 000 cycles). The high generality of this method is further demonstrated with poly(N‐isopropylacrylamide) and poly(N‐tertbutylacrylamide‐co‐acrylamide) hydrogels. This work takes a further step toward flexible battery development for harsh environments.

With rapid advances in soft electronic devices, the demand for soft conductive materials, including hydrogels, with superior mechanical properties, high conductivity and functionality under extreme environmental conditions are increasing at an unprecedented rate. Although hydrogels have favorable properties such as softness and broad tunability ranges, they freeze at subzero temperatures, leading to materials failure and device malfunctions, and the introduction of antifreezing agents into hydrogels often severely compromises their conductive or mechanical properties. The tradeoff between simultaneously endowing antifreezing hydrogels with excellent mechanical properties and high conductivity severely limits their practical applicability over a broad range of conditions. Herein, we discovered that potassium acetate (KAc) induces a salting-out effect on polyvinyl alcohol (PVA), promoting aggregation of the polymer chains and significantly improving the mechanical properties of the hydrogels. Moreover, concentrated KAc exhibits excellent anti-freezing capacity and high conductivity. The hydrogels produced by soaking frozen PVA in KAc solutions show superior mechanical properties, with a tensile strength of 8.2 MPa, conductivity of 8.0 S/m and outstanding freeze tolerance to a temperature of −60 °C. This strategy also works for other polymers, such as poly(acrylamide) and poly(2-hydroxyethyl acrylate). Additionally, the as-prepared hydrogels possess excellent anti-dehydration capacity, which is another important feature that is desirable for further enhancing the applicability and durability of hydrogel-based devices.

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Summary Hydrogels have gained tremendous attention due to their versatility in soft electronics, actuators, biomedical sensors, etc. Due to the high water content, hydrogels are usually soft, weak, and freeze below 0°C, which brings severe limitations to applications such as soft robotics and flexible electronics in harsh environments. Most existing anti-freezing gels suffer from poor mechanical properties and urgently need further improvements. Here, we took inspirations from tendon and coniferous trees and provided an effective method to strengthen polyvinyl alcohol (PVA) hydrogel while making it freeze resistant. The salting-out effect was utilized to create a hierarchically structured polymer network, which induced superior mechanical properties (Young's modulus: 10.1 MPa, tensile strength: 13.5 MPa, and toughness: 127.9 MJ/m 3 ). Meanwhile, the cononsolvency effect was employed to preserve the structure and suppress the freezing point to −60°C. Moreover, we have demonstrated the broad applicability of our material by fabricating PVA hydrogel-based hydraulic actuators and ionic conductors.

In nearly all cases of underwater adhesion, water molecules typically act as a destroyer. Thus, removing interfacial water from the substrate surfaces is essential for forming super-strong underwater adhesion. However, current methods mainly rely on physical means to dislodge interfacial water, such as absorption, hydrophobic repulsion, or extrusion, which are inefficient in removing obstinate hydrated water at contact interface, resulting in poor adhesion. Herein, we present a unique means of reversing the role of water to assist in realizing a self-strengthening liquid underwater adhesive (SLU-adhesive) that can effectively remove water at contact interface. This is achieved through multiscale physical–chemical coupling methods across millimeter to molecular levels and self-adaptive strengthening of the cohesion during underwater operations. As a result, strong adhesion over 1,600 kPa (compared to ~100 to 1,000 kPa in current state of the art) can be achieved on various materials, including inorganic metal and organic plastic materials, without preloading in different environments such as pure water, a wide range of pH solutions (pH = 3 to 11), and seawater. Intriguingly, SLU-adhesive/photothermal nanoparticles (carbon nanotubes) hybrid materials can significantly reduce the time required for complete curing from 24 h to 40 min using near-infrared laser radiation due to unique thermal-response of the chemical reaction rate. The excellent adhesion property and self-adaptive adhesion procedure allow SLU-adhesive materials to demonstrate great potential for broad applications in underwater sand stabilization, underwater repair, and even adhesion failure detection as a self-reporting adhesive. This concept of “water helper” has potential to advance underwater adhesion and manufacturing strategies.

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A seamless and tough interface to integrate incompatible/immiscible soft materials is highly desired for flexible/wearable electronics and many soft devices with multi-layer structures. Here, a surfactant-mediated interfacial chemistry is introduced to achieve seamless and tough interfaces in soft multi-layer structures, with an ultra-high interfacial toughness up to ≈1300 J m −2 for the architectural gel hybrid (AGH). The reversible noncovalent interfacial interactions efficiently dissipate energy at the interface, thereby providing excellent durability. The interfacial toughness only decreases by ≈6.9% after 10 000 tensile cycles. This strategy can be universally applied to hybrid systems with various interfaces between an interior hydrogel (PAA, PVA, PAAm, and gelatin) and an exterior hydrophobic soft matter (ionogel, lipogel and elastomer). The AGH-based mechano-sensor presents high robustness and stability in a wide range of conditions, including open air, underwater, and various solvents and temperatures. Epidermal bio-monitoring, tactile trajectory, and facial expression recognition are demonstrated using the AGH sensors in various environments. A rich set of electrophysiological signals of high quality are acquired.

Electron-ion transduction is the cornerstone for electronic devices interfacing with biological organisms and promoting emerging ionotronic devices, ranging from basic electronic elements to wearable electronics and energy harvesting devices. However, with commonly used metal electrodes, the electron-ion transduction suffers from high impedance, signal distortion and poor voltage tolerance. Surface modification with conductive porous materials could partially remedy these issues but is inevitably accompanied by weak interfacial adhesion and mechanical weakness. Therefore, freestanding electrodes integrating high mechanical and electrical properties are highly demanded. Herein, a general strategy is discovered to ameliorate these issues by introducing a conducting polymer (CP) hydrogel electrode of ultrahigh strength (~30 MPa) and conductivity (up to ~1200 S cm−1) with a capacitive behavior. These features are derived from the conductive nanoporous matrix with π-π interactions as both crosslinking sites and electron-transfer pathways, through surface gelation coupled with secondary-doping and densification. This strategy significantly decreased the low-frequency impedance and improved the signal fidelity, without affecting its high-frequency response. Furthermore, excellent biocompatibility, multifunctionality, and heart pacing upon ultra-low voltage (60% reduction) have also been demonstrated, showing the great potential of this CP material for bioelectronic applications and various human-machine interfaces.