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The development of multi-functional hydrogels is necessary to meet the needs of flexible sensors in different application scenarios. A conductive hydrogel with high toughness, adhesion, self-healing and anti-freezing properties was prepared. The hydrogel was synthesized by in-situ polymerization of acrylic acid in glycerol hydrate solution system of gelatin and aluminum ion, and then cryogenically refrigerated. After low temperature treatment, the triple helix structure produced by the self-assembly of the gelatin in the hydrogel increases the cross-linking density of the hydrogel, and forms a double network with polyacrylic acid to improve its mechanical properties (stress up to 85.7 kPa, strain up to 1428 %). In addition, this triple helix structure can dissociate and recombine at different temperatures, and interact with other dynamic bonds (hydrogen bonds, Al3+ coordination) in the system, so that the hydrogel has excellent self-healing ability (healing rate up to 86.8 %). Because the system contains a large number of -OH, -NH2 and -COOH groups, it can adhere to the surface of various materials through hydrogen bonding. The free Al3+ makes the hydrogel obtain good electrical conductivity and strain sensitivity (GF=3.01), and the strain sensor assembled by it has a stable and accurate monitoring effect on the fine movements of human body such as various joint movements and pronunciation. Additionally, the presence of glycerol provides anti-freezing properties, ensuring flexibility and electrical conductivity even at low temperatures (-20 ◦C). This hydrogel is a promising candidate for intelligent wearable devices in extreme environments such as snow and ice sports.

• A thermochromic hydrogel with high strength, adhesion and antifreezing functions is prepared. • Antifreezing temperature of smart window is reduced to below −15 °C. • Novel smart window combines solar modulation and heat storage to cut off building energy consumption. • Cold and warm experiments are conducted for indoor thermal regulation. Thermochromic smart windows based on hydrogels with adaptive regulation of solar radiation have attracted an increasing attention due to their potential in temperature management. However, hydrogels with extremely high water content may freeze, leading to the risk of transparency reduction and even window breakage in cold environment. In this study, co-crosslinked P(NIPAM-co-AM)@GLY@PVA hydrogel with excellent solar radiation regulation and anti-freezing performance was prepared by introducing antifreeze glycerol (GLY) and polymer polyvinyl alcohol (PVA). By adjusting the amount of GLY, PVA and hydrophilic AM, P(NIPAM-co-AM)@GLY@PVA hydrogel showed a satisfactory thermochromic performance, with a luminous transmittance (T lum) of 68.05 %, a solar modulation ability (Δ T sol) of 62.11 %, a lower critical solution temperature (LCST) of ∼24 °C and an antifreeze temperature of −15 °C. The thermal management experiments proved that the thermochromic@anti-freezing (TCA) smart window assembled based on P(NIPAM-co-AM)@GLY@PVA hydrogel had an excellent performance in managing indoor temperature and cyclic stability. This work may open a new avenue to solve problems in the application field of hydrogel-based smart windows. [ABSTRACT FROM AUTHOR]

Hydroxyethyl urea is a widely used moisturizer in cosmetics, since it can form strong hydrogen bonds with water. It could be introduced into the hydrogel to reduce the freezing point and enhance the performance of electrolyte at low temperature. Additionally, hydroxyethyl urea can change the solvation structure of Zn2+, further prevent dendrite formation and side reactions. We constructed a hydrogel electrolyte made from polyacrylamide crosslinked using hydroxyethyl urea, which can form an antifreeze gel and reduce the HER, corrosion and dendrite growth reactions. This work provides a simple strategy for enhancing low temperature performance of zinc ion batteries. [Display omitted] • The hydroxyethyl urea additive can break the hydrogen bonds of water. • The hydroxyethyl urea additive can optimizes the solvation structure of Zn2+. • At a temperature of -40 ℃, the Zn//Zn symmetry achieves steady cycles over 3000h. • The Zn//NH 4 V 4 O 10 full battery delivers a capacity of 195.14 mAh g−1 at −40 ℃. Aqueous zinc ion batteries are considered as a promising energy storage resource due to their high security, abundant resources and low price. However, the development of aqueous zinc ion batteries has been severely hindered by compulsive dendrite generation, serious side reactions and poor temperature adaptability. Herein, we used hydroxyethyl urea as a hydrogel electrolyte additive to address above-mentioned challenges. Hydroxyethyl urea can break the hydrogen bonds (HBs) of water and enhancing the freezing-tolerance ability of the electrolyte. Meanwhile, hydroxyethyl urea can prevent the corrosion issue and inhibition of Zn dendrites. Consequently, the Zn//Zn symmetric battery can sustain stable cycling for over 3000 h at 1 mA cm−2, and it achieves a high coulombic efficiency of 99.6 %. Even at −40 ℃ the batteries show excellent cycling stability, the Zn//Zn symmetry battery can achieve steadily cycles over 3000 h. The Zn//NVO battery equipped with the altered electrolyte exhibits enhanced capacity retention compared to the one without additives. It demonstrates not only excellent cycling stability at room temperature but also maintains commendable functionality down to −40 ℃, validating the method's efficacy. This work provides a simple strategy for enhancing low temperature performance of zinc ion batteries. [ABSTRACT FROM AUTHOR]

Natural living systems such as wood frogs develop tissues composed of active hydrogels with cryoprotectants to survive in cold environments. Recently, hydrogels have been intensively studied to develop stretchable electronics for wearables and soft robots. However, regular hydrogels are inevitably frozen at the subzero temperature and easily dehydrated, and have weak surface adhesion. Herein, a novel hydrogel-based ionic skin (iSkin) capable of strain sensing is demonstrated with high toughness, high stretchability, excellent ambient stability, superior anti-freezing capability, and strong surface adhesion. The iSkin consists of a piece of ionically and covalently cross-linked tough hydrogel with a thin bioadhesive layer. With the addition of biocompatible cryoprotectant and electrolyte, the iSkin shows good conductivity in wide ranges of relative humidity (15-90%) and temperature (-95-25 °C). In addition, the iSkin can adhere firmly to diverse material surfaces under different conditions, including cloth fabric, skin, and elastomers, in both dry and wet conditions, at subzero temperature, and/or with dynamic movement. The iSkin is demonstrated for applications including strain sensing on both human body and winter coat, human -- machine interaction, motion/deformation sensing on a soft gripper and a soft robot at extremely cold conditions. This work provides a new paradigm for developing high-performance artificial skins for wearable sensing and soft robotics. [ABSTRACT FROM AUTHOR]

Conductive hydrogels are widely used as sensors in wearable devices. However, hydrogels cannot endure harsh low-temperature environments. Herein, a new regulatory system based on natural ice-structuring proteins (ISPs) and cellulose nanofibers (CNFs) is introduced into hydrogel network consisting of chemically crosslinked network of copolymerized acrylamide and 2-acrylamide-2-methylpropanesulfonic acid, and physically crosslinked polyvinyl alcohol chains, affording an anti-freezing hydrogel with high conductivity (2.63 S/m). These hydrogels show excellent adhesion behavior to various matrices (including aluminum, glass, pigskin, and plastic). Their mechanical properties are significantly improved with the increase in CNF content (tensile strength of 106.4 kPa, elastic modulus of 133.8 kPa). In addition, ISPs inhibit the growth of ice. This endows the hydrogels with anti-freezing property and allows them to maintain satisfactory mechanical properties, conductivity and sensing properties below zero degrees. Moreover, this hydrogel shows high sensitivity to tensile and compressive deformation (GF = 5.07 at 600–800 % strain). Therefore, it can be utilized to develop strain-type pressure sensors that can be attached directly to human skin for detecting various body motions accurately, reliably, and stably. This study proposes a simple strategy to improve the anti-freezing property of hydrogels, which provides new insights for developing flexible hydrogel electronic devices for application in winter sports. [ABSTRACT FROM AUTHOR]

Hydrogel electrolytes with anti-freezing properties are crucial for flexible quasi-solid-state supercapacitors operating at low temperatures. However, the electrolyte freezing and sluggish ion migration caused by the cold temperature inevitably damage the flexibility and electrochemical properties of supercapacitors. Herein, we introduce the concentrated electrolyte into a freeze-casted poly(vinyl alcohol) hydrogel film not only reducing the freezing point of the electrolyte (−51.14 °C) in gels for ensuring the flexibility, but also improving the ionic conductivity of the hydrogel electrolyte (5.92 mS cm−1 at −40 °C) at low temperatures. As a proof, an all-in-one supercapacitor, synthesized by the one-step polymerization method, exhibits a good specific capacitance of 278.6 mF cm−2 at −40 °C (accounting for 93.8% of the capacitance at room temperature), high rate performance (50% retention under the 100-fold increase in current densities), and long cycle life (88.9% retention after 8,000 cycles at −40 °C), representing an excellent low-temperature performance. Our results provide a fresh insight into the hydrogel electrolyte design for flexible energy storage devices operating in the wide range of temperature and open up an exciting direction for improving all-in-one supercapacitors. [ABSTRACT FROM AUTHOR]

The conductivity, high mechanical strength, long-term durability and recycle properties are vital roles for the flexible electronics. However, it's difficult to prepare a material with the balanced above properties. Here, the hydrogels are synthesized by hydroxyethyl cellulose/silver nanoparticles (HEC-Silver NPs), polyvinyl alcohol (PVA) and Glycerol-water as the binary solvent. Impressively, the HEC can improve the moisturizing property of the Glycerol-water for PVA hydrogel (Water retention after 7 days rise from 69.51% to 83.32%). In addition, the hydrogen bond between the HEC-Silver nanoparticles and PVA matrix can simultaneously improve tensile strength (from 0.65 MPa to 1.46 MPa) and elongation (from 541.91% to 680.53% at 25 °C). The conductive hydrogel was applied in the temperature sensor (low responsive time = 0.3s) and strain sensor (Gauge Factor = 2.36) for monitoring the joint movement. In short, biologically inspired hydrogels can be reshaped for reuse and used as bioelectronics to detect biological signals in harsh environments. [Display omitted] • The conductive silver NPs were synthesized by solid-state reduction method. • The anti-freezing and anti-drying property of the PVA improved by HEC. • The mechanism of the anti-drying property was explored by the DFT theory. • The hydrogel could be applied the temperature sensor of the strain sensor fields. • The hydrogel can be remolded to arbitrary shape without loss of performance. [ABSTRACT FROM AUTHOR]