Robust organohydrogel with flexibility and conductivity across the freezing and boiling temperatures of water

Extreme condition-tolerant stretchable flexible supercapacitor and triboelectric nanogenerator based on carrageenan-enhanced gel for energy storage, energy collection and self-powered sensing

As flexible electronics devices for energy storage, mechanical energy collection and self-powered sensing, stretchable flexible supercapacitor and triboelectric nanogenerator (TENG) have attracted extensive attention. However, it is difficult to satisfy the requirements of high safety and resistance to extreme conditions. Dual roles of mechanical and electrical enhancement of inorganic salt are put forward, and a carrageenan (CG) enhanced poly (N-hydroxyethyl acrylamide)/CG/lithium chloride/glycerol (PCLG) conductive gel is prepared by designing hydrogen bonding self-crosslinking and chain entanglement. A high concentration and rapid deposition strategy is proposed to prepare a PCLG gel-based stretchable flexible all-in-one supercapacitor for energy storage, and a single electrode PCLG gel-based TENG is designed for mechanical energy collection, self-powered strain and tactile sensing. The supercapacitor has high capacitance, excellent cycling stability. The TENG possesses efficient energy harvesting with high and stable output voltage and power density, and sensitive and stable self-powered strain and tactile sensing without external power supply. Even under extreme conditions such as low temperatures, self-healing after damage, prolonged placement, deformation, post-deformation, multiple continuous work, pinprick and burning, the supercapacitor and TENG still have excellent properties. Therefore, we provide novel ideas to design flexible supercapacitor and TENG used under extreme conditions for future wearable electronics.

Towards high performance and durable soft tactile actuators

Soft actuators are gaining significant attention due to their ability to provide realistic tactile sensations in various applications. However, their soft nature makes them vulnerable to damage from external factors, limiting actuation stability and device lifespan. The susceptibility to damage becomes higher with these actuators often in direct contact with their surroundings to generate tactile feedback. Upon onset of damage, the stability or repeatability of the device will be undermined. Eventually, when complete failure occurs, these actuators are disposed of, accumulating waste and driving the consumption of natural resources. This emphasizes the need to enhance the durability of soft tactile actuators for continued operation. This review presents the principles of tactile feedback of actuators, followed by a discussion of the mechanisms, advancements, and challenges faced by soft tactile actuators to realize high actuation performance, categorized by their driving stimuli. Diverse approaches to achieve durability are evaluated, including self-healing, damage resistance, self-cleaning, and temperature stability for soft actuators. In these sections, current challenges and potential material designs are identified, paving the way for developing durable soft tactile actuators.

Recent advances in wearable iontronic sensors for healthcare applications

Iontronic sensors have garnered significant attention as wearable sensors due to their exceptional mechanical performance and the ability to maintain electrical performance under various mechanical stimuli. Iontronic sensors can respond to stimuli like mechanical stimuli, humidity, and temperature, which has led to exploration of their potential as versatile sensors. Here, a comprehensive review of the recent researches and developments on several types of iontronic sensors (e.g., pressure, strain, humidity, temperature, and multi-modal sensors), in terms of their sensing principles, constituent materials, and their healthcare-related applications is provided. The strategies for improving the sensing performance and environmental stability of iontronic sensors through various innovative ionic materials and structural designs are reviewed. This review also provides the healthcare applications of iontronic sensors that have gained increased feasibility and broader applicability due to the improved sensing performance. Lastly, outlook section discusses the current challenges and the future direction in terms of the applicability of the iontronic sensors to the healthcare.

Ultrafast fabrication of deep eutectic solvent flexible ionic gel with high-transmittance, freeze-resistant and conductivity by frontal polymerization

As a common flexible sensing device, gels are widely used in electronic skin, personalized health monitoring, and smart manufacturing. However, gel suffers from temperature sensitivity, long polymerization times or thickness limitations. Deep eutectic solvents (DESs) have abundant hydrogen bond networks and low saturated vapor pressure, which can accelerate the frontal polymerization of ionic gels, and overcome the temperature sensitivity problem. Here, we showed how choline chloride (ChCl)-glycerol (Gly) DES can be used to create ionic gels with different properties and functions by combining them with different monomers (acrylamide (AM), acrylic acid (AA) and itaconic acid (IA)). Subsequently, we revealed the rapid gelation mechanism of PAM-ChCl-Gly ionic gel from multiple perspectives by density functional theory and molecular dynamics simulation, which was then applied to flexible sensing. The experimental results showed that the PAM-ChCl-Gly ionic gel had excellent tensile properties, high transparency, self-adhesion and stability. In addition, its gelation time was only 90 s without heating. ChCl-Gly DES offered a plentiful and stable hydrogen bonding network. PAM-ChCl-Gly ionic gels can detect tiny pressure and strain changes, making them suitable for flexible sensing. This greatly enriched the theoretical research foundation of DES-based ionic gels and broadened their application areas.

Flexible Electrodes for Brain-Computer Interface System

Brain-computer interface (BCI) has been the subject of extensive research recently. Governments and companies have substantially invested in relevant research and applications. The restoration of communication and motor function, the treatment of psychological disorders, gaming, and other daily and therapeutic applications all benefit from BCI. The electrodes hold the key to the essential, fundamental BCI precondition of electrical brain activity detection and delivery. However, the traditional rigid electrodes are limited due to their mismatch in Young's modulus, potential damages to the human body, and a decline in signal quality with time. These factors make the development of flexible electrodes vital and urgent. Flexible electrodes made of soft materials have grown in popularity in recent years as an alternative to conventional rigid electrodes because they offer greater conformance, the potential for higher signal-to-noise ratio (SNR) signals, and a wider range of applications. Therefore, the latest classifications and future developmental directions of fabricating these flexible electrodes are explored in this paper to further encourage the speedy advent of flexible electrodes for BCI. In summary, the perspectives and future outlook for this developing discipline are provided.

Intrinsic Anti-Freezing and Unique Phosphorescence of Glassy Hydrogels with Ultrahigh Stiffness and Toughness at Low Temperatures

Most hydrogels become frozen at subzero temperatures, leading to degraded properties and limited applications. Cryoprotectants are massively employed to improve anti-freezing property of hydrogels; however, there are accompanied disadvantages, such as varied networks, reduced mechanical properties, and the risk of cryoprotectant leakage in aqueous conditions. Reported here is the glassy hydrogel having intrinsic anti-freezing capacity and excellent optical and mechanical properties at ultra-low temperatures. Supramolecular hydrogel of poly(acrylamide-co-methacrylic acid) with moderate water content (≈50 wt.%) and dense hydrogen-bond associations is in a glassy state at room temperature. Since hydrogen bonds become strengthened as the temperature decreases, this gel becomes stronger and stiffer, yet still ductile, with Young's modulus of 900 MPa, tensile strength of 30 MPa, and breaking strain of 35% at -45 °C. This gel retains high transparency even in liquid nitrogen. It also exhibits unique phosphorescence due to presence of carbonyl clusters, which is further enhanced at subzero temperatures. Further investigations elucidate that the intrinsic anti-freezing property is related to a fact that most water molecules are tightly bound and confined in the glassy matrix and become non-freezable. This correlation, as validated in several systems, provides a roadmap to develop intrinsic anti-freezing hydrogels for widespread applications at extreme conditions.

Wetting-Enabled Three-Dimensional Interfacial Polymerization (WET-DIP) for Bioinspired Anti-Dehydration Hydrogels

Anti-dehydration hydrogels have attracted considerable attention due to their promising applications in stretchable sensors, flexible electronics, and soft robots. However, anti-dehydration hydrogels prepared by conventional strategies inevitably depend on additional chemicals or suffer from cumbersome preparation processes. Here, inspired by the succulent Fenestraria aurantiaca a one-step wetting-enabled three-dimensional interfacial polymerization (WET-DIP) strategy for constructing organogel-sealed anti-dehydration hydrogels is developed. By virtue of the preferential wetting on the hydrophobic-oleophilic substrate surfaces, the organogel precursor solution can spread on the three-dimensional (3D) surface and encapsulate the hydrogel precursor solution, forming anti-dehydration hydrogel with 3D shape after in situ interfacial polymerization. The WET-DIP strategy is simple and ingenious, and accessible to discretionary 3D-shaped anti-dehydration hydrogels with a controllable thickness of the organogel outer layer. Strain sensors based on this anti-dehydration hydrogel also exhibit long-term stability in signal monitoring. This WET-DIP strategy shows great potentialities for constructing hydrogel-based devices with long-term stability.

Carboxymethylcellulose reinforced, double-network hydrogel-based strain sensor with superior sensing stability for long-term monitoring

Hydrogel-based strain sensors have garnered significant attention for their potential for human health monitoring. However, its practical application has been hindered by water loss, freezing, and structural impairment during long-term motion monitoring. Here, a strain sensor based on double-network (DN) hydrogel of polyacrylamide (PAAm)/carboxymethylcellulose (CMC) was developed in a ternary solvent system of lithium chloride (LiCl)/ethylene glycol (EG)/H₂O through a facile one-pot radical polymerization strategy. The incorporation of EG effectively mitigated the hydration of lithium salts by generating stable ion clusters with Li⁺ and stronger hydrogen bonds within the polymer matrix. The sensor demonstrated excellent mechanical properties, including a stretchability of 1858 %, toughness of 1.80 MJ/m³, and recoverability of 102 %. Furthermore, the LiCl/EG/H₂O ternary system resulted in high conductivity, excellent anti-freezing performance, and superior sensing stability. In addition, the sensor exhibited remarkable sensitivity, enabling the monitoring of human movements ranging from subtle to significant deformations, including throat motion and bending of the elbow, wrist, finger, and lower limb. This study presents a viable approach for constructing hydrogel-based strain sensors with exceptional sensing stability for long-term tracking of human motions.

Ultrafast Self-Healing, Highly Stretchable, Adhesive, and Transparent Hydrogel by Polymer Cluster Enhanced Double Networks for Both Strain Sensors and Environmental Remediation Application

Stretchable, healable, biocompatible, and conductive hydrogels are one of the promising candidates for both wearable electronics and environmental remediation applications. To date, the design of hydrogels that integrate ultrafast self-healing with high efficiency (seconds), high stretchability, and biocompatibility and reversibility into one system is not an easy task. Herein, we demonstrate a general oxidation approach to accelerate the hydrogelation of hPEI-based double network gels via the generation of fluorescent polymer clusters at room temperature or triggered by the heating-cooling process. The resulting ohPEI hydrogel has the merit of biocompatibility over most reported hPEI hydrogels for strain sensors. It shows a high conductivity (1.3 S/m), an ultrafast self-healing ability (<3 s, 98% healing efficiency within 60 s), a high stretchability (∼1850 and ∼7000% in deformation), and reversible adhesivity on various material surfaces. The excellent performance of the hydrogel is ascribed to the cooperative and hierarchical interactions of four types of dynamic combinations, including the reversible borate bond, hydrogen bonding, electrostatic interaction, and polymer cluster interactions. The reversible fabrication process by the one-spot method (just by simple mixing of the components) and superior properties of the hydrogel make it an ideal candidate for a wearable strain sensor to monitor human motions and physiological activities. Moreover, it is also a good hydrogel absorbent for phase separation absorption of volatile organic compounds with a high capacity (for acetone: 4.75 g g^-1), reusability, and an easy handling process.

A Review on Thermal Properties of Hydrogels for Electronic Devices Applications

Hydrogels, as a series of three-dimensional, crosslinked, hydrophilic network polymers, exhibit extraordinary properties in softness, mechanical robustness and biocompatibility, which have been extensively utilized in various fields, especially for electronic devices. However, since hydrogels contain plenty of water, the mechanical and electrochemical properties are susceptible to temperature. The thermal characteristics of hydrogels can significantly affect the performance of flexible electronic devices. In this review, recent research on the thermal characteristics of hydrogels and their applications in electronic devices is summarized. The focus of future work is also proposed. The thermal stability, thermoresponsiveness and thermal conductivity of hydrogels are discussed in detail. Anti-freezing and anti-drying properties are the critical points for the thermal stability of hydrogels. Methods such as introducing soluble ions and organic solvents into hydrogels, forming ionogels, modifying polymer chains and incorporating nanomaterials can improve the thermal stability of hydrogels under extreme environments. In addition, the critical solution temperature is crucial for thermoresponsive hydrogels. The thermoresponsive capacity of hydrogels is usually affected by the composition, concentration, crosslinking degree and hydrophilic/hydrophobic characteristics of copolymers. In addition, the thermal conductivity of hydrogels plays a vital role in the electronics applications. Adding nanocomposites into hydrogels is an effective way to enhance the thermal conductivity of hydrogels.

Enhanced Copolymer Gel Modified by Dual Surfactants for Flexible Zinc-Air Batteries

Zinc-air batteries using gels as carriers for electrolyte absorption have attracted extensive attention due to their flexibility, deformability, and high specific capacity. However, traditional mono-polymer gel electrolytes display poor mechanical properties and low ionic conductivity at wide-window temperatures. Here, the enhanced gel polymer (PAM-F/G) modified by dual surfactants is present by way of pluronic F127 and layered graphene oxide introduced into the polyacrylamide (PAM) matrix. The gel electrolyte procured by absorbing 6 M KOH exhibits improved mechanical characteristics, temperature adaptability, and a satisfactory ionic conductivity (276 mS cm^-1). The results demonstrate that a flexible zinc-air battery assembled by PAM-F/G electrolyte outputs a high power density (155 mW cm^-2) and can even operate reliably (>40 h) at -20 °C. These findings are available for promoting the research and popularization of flexible zinc-air batteries with high performance.

Bioinspired Freeze-Tolerant Soft Materials: Design, Properties, and Applications

In nature, many biological organisms have developed the exceptional antifreezing ability to survive in extremely cold environments. Inspired by the freeze resistance of these organisms, researchers have devoted extensive efforts to develop advanced freeze-tolerant soft materials and explore their potential applications in diverse areas such as electronic skin, soft robotics, flexible energy, and biological science. Herein, a comprehensive overview on the recent advancement of freeze-tolerant soft materials and their emerging applications from the perspective of bioinspiration and advanced material engineering is provided. First, the mechanisms underlying the freeze tolerance of cold-enduring biological organisms are introduced. Then, engineering strategies for developing antifreezing soft materials are summarized. Thereafter, recent advances in freeze-tolerant soft materials for different technological applications such as smart sensors and actuators, energy harvesting and storage, and cryogenic medical applications are presented. Finally, future challenges and opportunities for the rapid development of bioinspired freeze-tolerant soft materials are discussed.

Green Conductive Hydrogel Electrolyte with Self-Healing Ability and Temperature Adaptability for Flexible Supercapacitors

Conductive hydrogels (CHs) are ideal electrolyte materials for the preparation of flexible supercapacitors (FSCs) due to their excellent electrochemical properties, mechanical properties, and deformation restorability. However, most of the reported CHs are prepared by the chemical crosslinking of synthetic polymers and thus usually display the disadvantages of poor self-healing abilities and nonadaptability at environmental temperatures, which greatly limits their application. To overcome these problems, in the present work, we constructed a sodium alginate-borax/gelatin double-network conductive hydrogel (CH) by a dynamic crosslinking between sodium alginate (SA) and borax via borate bonds and hydrogen bonding between amino acids in gelatin and SA chains. The CH displays an excellent elongation of 305.7% and fast self-healing behavior in 60 s. Furthermore, a phase-change material (PCM), Na₂SO₄·10H₂O, was introduced into the CH, which, combined with the nucleation effect of borax, improved the ionic conductivity and temperature adaptability of the CH. The flexible supercapacitor (FSC) assembled with the obtained CH as the electrolyte exhibits a high specific capacitance of 185.3 F·g^-1 at a current density of 0.25 A·g^-1 and good stability with 84% capacitance retention after 10 000 cycles and excellent temperature tolerance with a resistance variation of 2.11 Ω in the temperature range of -20-60 °C. This green CH shows great application potential as an electrolyte for FSCs, and the preparation method can be potentially expanded to the fabrication of self-repairing FSCs with good temperature adaptabilities.

Autonomous Self-Healing of Highly Stretchable Supercapacitors at All Climates

Realizing autonomous self-healing and high stretchability of flexible supercapacitors over a wide temperature range remains a big challenge because of simultaneous incorporation of self-healing, stretchable and temperature-tolerant elements into a device as well as unfavorable electrochemical kinetics in harsh conditions. Here, we demonstrate for the first time an autonomous self-healing and intrinsically stretchable supercapacitor that can work at all-climate environments assembled by universally self-healing and highly stretchable organohydrogel electrodes with record-high temperature-invariant conductivity of ∼965 S/cm. Benefiting from multiple hydrogen bonding and dynamic metal coordination combined with electrochemistry-favorable components and integrated device configuration, the supercapacitor exhibits outstanding long-term stability, high stretchability, instantaneous and complete capacitive self-healability, and real-time mechanical healing at harsh temperatures from -35 to 80 °C. The superiorities in stretchability, self-healability, and all-climate tolerance enable the supercapacitor presented here as the best performer among the flexible supercapacitors reported to date.

Antifreezing, Ionically Conductive, Transparent, and Antidrying Carboxymethyl Chitosan Self-Healing Hydrogels as Multifunctional Sensors

Through a simple strategy of immersion in a mixed solution of water/ethylene glycol (EG)/lithium chloride (LiCl), self-healing carboxymethyl chitosan (CA) hydrogels, that is, CA/N-vinylpyrrolidone-EG-Li⁺ hydrogels (CEH) with an ultra-low-temperature freezing resistance below -70 °C were fabricated. The introduction of electrolyte ions and small-molecule polyol also made these hydrogels highly conductive (0.8 S m^-1) and imparted antidrying property to them, showing stable and reversible sensitivity to finger-wrist bending as well as 150 cycles of stretching. Such hydrogels also presented highly efficient self-healing ability, with a stress-strain healing efficiency of over 90%. Furthermore, the CEH-based sensors maintained a stable sensing performance over a wide range of temperatures below the freezing point (from -10 to -70 °C) and exhibited stable sensitivity to temperatures with fast response and no significant hysteresis. The present work is expected to provide a simple and sustainable route for the preparation of multifunctional antifreezing conductive hydrogels based on CA, leading to a wide range of potential applications in soft sensor devices.

Ultra-Sensitive, Deformable, and Transparent Triboelectric Tactile Sensor Based on Micro-Pyramid Patterned Ionic Hydrogel for Interactive Human-Machine Interfaces

Rapid advances in wearable electronics and mechno-sensational human-machine interfaces impose great challenges in developing flexible and deformable tactile sensors with high efficiency, ultra-sensitivity, environment-tolerance, and self-sustainability. Herein, a tactile hydrogel sensor (THS) based on micro-pyramid-patterned double-network (DN) ionic organohydrogels to detect subtle pressure changes by measuring the variations of triboelectric output signal without an external power supply is reported. By the first time of pyramidal-patterned hydrogel fabrication method and laminated polydimethylsiloxane (PDMS) encapsulation process, the self-powered THS shows the advantages of remarkable flexibility, good transparency (≈85%), and excellent sensing performance, including extraordinary sensitivity (45.97 mV Pa^-1 ), fast response (≈20 ms), very low limit of detection (50 Pa) as well as good stability (36 000 cycles). Moreover, with the LiBr immersion treatment method, the THS possesses excellent long-term hyper anti-freezing and anti-dehydrating properties, broad environmental tolerance (-20 to 60 °C), and instantaneous peak power density of 20 µW cm^-2 , providing reliable contact outputs with different materials and detecting very slight human motions. By integrating the signal acquisition/process circuit, the THS with excellent self-power sensing ability is utilized as a switching button to control electric appliances and robotic hands by simulating human finger gestures, offering its great potentials for wearable and multi-functional electronic applications.

Electrically Programmable Interfacial Adhesion for Ultrastrong Hydrogel Bonding

Adjustable interfacial adhesion is of great significance in smart-hydrogel-related engineering fields. This study presents an electroadhesion strategy for universal and ultrastrong hydrogel bonding with electrically programmable strength. An ionic hydrogel containing lithium ions is designed to achieve hydrated-ion-diffusion-mediated interfacial adhesion, where external electric fields are employed to precisely control spatiotemporal dynamics of the ion diffusion across ionic adhesion region (IAR). The hydrogel can realize a universal, ultrastrong, efficient, tough, reversible, and environmentally tolerant electroadhesion to diverse hydrogels, whose peak adhesion strength and interfacial adhesion toughness are as high as 1.2 MPa and 3750 J m^-2 , respectively. With a mechanoelectric coupling model, the dominant role of the hydrated ions in IAR played in the interfacial electroadhesion is further quantitatively revealed. The proposed strategy opens a door for developing high-performance adhesion hydrogels with electrically programmable functions, which are indispensable for various emerging fields like flexible electronics and soft robotics.

From room temperature to harsh temperature applications: Fundamentals and perspectives on electrolytes in zinc metal batteries

As one of the most competitive candidates for the next-generation energy storage systems, the emerging rechargeable zinc metal battery (ZMB) is inevitably influenced by beyond-room-temperature conditions, resulting in inferior performances. Although much attention has been paid to evaluating the performance of ZMBs under extreme temperatures in recent years, most academic electrolyte research has not provided adequate information about physical properties or practical testing protocols of their electrolytes, making it difficult to assess their true performance. The growing interest in ZMBs is calling for in-depth research on electrolyte behavior under harsh practical conditions, which has not been systematically reviewed yet. Hence, in this review, we first showcase the fundamentals behind the failure of ZMBs in terms of temperature influence and then present a comprehensive understanding of the current electrolyte strategies to improve battery performance at harsh temperatures. Last, we offer perspectives on the advance of ZMB electrolytes toward industrial application.

Development of Conductive Hydrogels for Fabricating Flexible Strain Sensors

Conductive hydrogels can be prepared by incorporating various conductive materials into polymeric network hydrogels. In recent years, conductive hydrogels have been developed and applied in the field of strain sensors owing to their unique properties, such as electrical conductivity, mechanical properties, self-healing, and anti-freezing properties. These remarkable properties allow conductive hydrogel-based strain sensors to show excellent performance for identifying external stimuli and detecting human body movement, even at subzero temperatures. This review summarizes the properties of conductive hydrogels and their application in the fabrication of strain sensors working in different modes. Finally, a brief prospectus for the development of conductive hydrogels in the future is provided.

Bioinspired ionic hydrogel materials with excellent antifouling properties and high conductivity in dry and cold environments

A bioinspired ionic hydrogel-based antifouling material with excellent adaptability has been constructed, featured with ultralow adhesion to various solid/viscous liquid deposition, high ionic conductivity, and excellent mechanical properties.

Flexible Zinc-Air Battery with High Energy Efficiency and Freezing Tolerance Enabled by DMSO-Based Organohydrogel Electrolyte

With the emergence of various flexible electronics, the flexible zinc-air battery (ZAB) is considered a promising energy source with low cost, high energy density, and safety. However, gel electrolytes that improve the freezing tolerance and energy efficiency of ZABs are rarely explored. Herein, an organohydrogel electrolyte (OHE) is fabricated by soaking poly(2-acrylamido-2-methylpropanesulfonic acid)/polyacrylamide (PAMPS/PAAm) double-network hydrogel in aqueous KOH electrolyte with dimethyl sulfoxide (DMSO) additive. The prepared OHE exhibits high mechanical strength and excellent ionic conductivity. In addition, the introduction of DMSO effectively improves freezing tolerance and electrochemical performance especially in energy efficiency of ZABs due to that DMSO can break the hydrogen bonds between water molecules and alter the path of the conventional oxygen evolution reaction in ZAB simultaneously. Compared with the control hydrogel electrolyte, the optimized OHE enables flexible ZABs to not only exhibit an exceptionally low charge voltage of 1.63 V, high energy efficiency of 74.2%, and long cycling life of 177 cycles, but also to operate with an excellent specific capacity of 562 mAh g^-1 and energy density of 523.4 Wh kg^-1 at -40 °C. Moreover, the obtained flexible ZABs keep a stable output under deformations and extreme low temperature, manifesting a great potential for functional wearable devices.

Conductive and anti-freezing hydrogels constructed by pseudo-slide-ring networks

Stretchable, tough, and anti-freezing hydrogels were prepared using partially carboxymethylated polyrotaxanes and polyacrylamides. The carboxylic acid groups of α-cyclodextrins in the polyrotaxane and the amide groups in polyacrylamide are hydrogen-bonded, affording a pseudo-slide-ring network, greatly enhancing the hydrogels' macroscale mechanical properties, anti-freezing features, and electrical conductivity for the fabrication of a cold-temperature strain sensor.

An Electret/Hydrogel-Based Tactile Sensor Boosted by Micro-Patterned and Electrostatic Promoting Methods with Flexibility and Wide-Temperature Tolerance

With the rising demand for wearable, multifunctional, and flexible electronics, plenty of efforts aiming at wearable devices have been devoted to designing sensors with greater efficiency, wide environment tolerance, and good sustainability. Herein, a thin film of double-network ionic hydrogel with a solution replacement treatment method is fabricated, which not only possesses excellent stretchability (>1100%) and good transparency (>80%), but also maintains a wide application temperature range (-10~40 °C). Moreover, the hydrogel membrane further acts as both the flexible electrode and a triboelectric layer, with a larger friction area achieved through a micro-structure pattern method. Combining this with a corona-charged fluorinated ethylene propylene (FEP) film, an electret/hydrogel-based tactile sensor (EHTS) is designed and fabricated. The output performance of the EHTS is effectively boosted by 156.3% through the hybrid of triboelectric and electrostatic effects, which achieves the open-circuit peak voltage of 12.5 V, short-circuit current of 0.5 μA, and considerable power of 4.3 μW respectively, with a mentionable size of 10 mm × 10 mm × 0.9 mm. The EHTS also demonstrates a stable output characteristic within a wide range of temperature tolerance from -10 to approximately 40 °C and can be further integrated into a mask for human breath monitoring, which could provide for a reliable healthcare service during the COVID-19 pandemic. In general, the EHTS shows excellent potential in the fields of healthcare devices and wearable electronics.

Ultra-Stretchable and Self-Healing Anti-Freezing Strain Sensors Based on Hydrophobic Associated Polyacrylic Acid Hydrogels

Water-rich conductive hydrogels with excellent stretchability are promising in strain sensors due to their potential application in flexible electronics. However, the features of being water-rich also limit their working environment. Hydrogels must be frozen at subzero temperatures and dried out under ambient conditions, leading to a loss of mechanical and electric properties. Herein, we prepare HAGx hydrogels (a polyacrylic acid (HAPAA) hydrogel in a binary water-glycerol solution, where x is the mass ratio of water to glycerol), in which the water is replaced with water-glycerol mixed solutions. The as-prepared HAGx hydrogels show great anti-freezing properties at a range of -70 to 25 °C, as well as excellent moisture stability (the weight retention rate was as high as 93% after 14 days). With the increase of glycerol, HAGx hydrogels demonstrate a superior stretchable and self-healing ability, which could even be stretched to more than 6000% without breaking, and had a 100% self-healing efficiency. The HAGx hydrogels had good self-healing ability at subzero temperatures. In addition, HAGx hydrogels also had eye-catching adhesive properties and transparency, which is helpful when used as strain sensors.

3D architected temperature-tolerant organohydrogels with ultra-tunable energy absorption

The properties of mechanical metamaterials such as strength and energy absorption are often “locked” upon being manufactured. While there have been attempts to achieve tunable mechanical properties, state-of-the-art approaches still cannot achieve high strength/energy absorption with versatile tunability simultaneously. Herein, we fabricate for the first time, 3D architected organohydrogels with specific energy absorption that is readily tunable in an unprecedented range up to 5 × 10³ (from 0.0035 to 18.5 J g⁻¹) by leveraging on the energy dissipation induced by the synergistic combination of hydrogen bonding and metal coordination. The 3D architected organohydrogels also possess anti-freezing and non-drying properties facilitated by the hydrogen bonding between ethylene glycol and water. In a broader perspective, this work demonstrates a new type of architected metamaterials with the ability to produce a large range of mechanical properties using only a single material system, pushing forward the applications of mechanical metamaterials to broader possibilities.

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The first fabrication of 3D architected organohydrogels by Digital Light Processing

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Two-step toughening effect of organohydrogels by metal coordination and hydrogen bonding

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3D structures achieved ultra-tunable range of specific energy absorption up to 5000 x

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3D architected organohydrogels were demonstrated as tunable impact attenuators

Bioinspired organohydrogels with heterostructures: Fabrications, performances, and applications

Since emerging in 1960, the artificial hydrogels have garnered enormous attentions in scientific community due to their high level of similarities to biological soft tissues in both structures and properties. With the proceeding of research, the concern of hydrogels is gradually shifted from fundamental investigation to abundant functionalization. In contrast to the natural soft tissues, the current artificial hydrogels still possess relatively simple structures and unsatisfactory environmental adaptability, extremely limiting their practical applications in complex environments. Enlightened by the prominent adaptability of biological organisms, the binary cooperative complementary principle is utilized to develop bioinspired organohydrogels by combining two components with opposite but cooperative physiochemical features. The present review provides the advanced progresses of bioinspired organohydrogels with sophisticated heterogeneous networks and desirably environmental adaptabilities. We clearly summarize the synthesizing strategies in regard to both corresponding mechanisms and typical examples, including macroscopic organohydrogels, organohydrogels with binary solvent, organohydrogels with heteronetworks, and emulsion-based organohydrogels. Meanwhile, the intriguing features of the reported organohydrogels, such as temperature resistance, switchable mechanics, adaptive wettability, and opposite components compatibility, are also clearly highlighted with a short overview of their promising applications. Ultimately, the current challenges and perspectives on the future development of bioinspired organohydrogels are also discussed.

3D Antidrying Antifreezing Artificial Skin Device with Self-Healing and Touch Sensing Capability

Hydrogels are attractive, active materials for various e-skin devices based on their unique functionalities such as flexibility and biocompatibility. Still, e-skin devices are generally limited to simple structures, and the realization of optimal-shaped 3D e-skin devices for target applications is an intriguing issue of interest. Furthermore, hydrogels intrinsically suffer from drying and freezing issues in operational capability for practical applications. Herein, 3D artificial skin devices are demonstrated with highly improved device stability. The devices are fabricated in a target-oriented 3D structure by extrusion-based 3D printing, spontaneously heal mechanical damage, and enable stable device operation over time and under freezing conditions. Based on the material design to improve drying and freezing resistance, an organohydrogel, prepared by solvent displacement of hydrogel with ethylene glycol for 3 h, exhibits excellent drying resistance over 1000 h and improved freezing resistance by showing no phase transition down to -60 °C while maintaining its self-healing functionality. Based on the improved drying and freezing resistance, artificial skin devices in target-oriented optimal 3D structures are presented, which enable accurate positioning of touchpoints even on a complicated 3D structure stably over time and excellent operation at temperatures below 0 °C without losing their flexibility.

3D Printed Ultrastretchable, Hyper-Antifreezing Conductive Hydrogel for Sensitive Motion and Electrophysiological Signal Monitoring

Conductive hydrogels with high stretchability can extend their applications as a flexible electrode in electronics, biomedicine, human-machine interfaces, and sensors. However, their time-consuming fabrication and narrow ranges of working temperature and working voltage severely limit their further potential applications. Herein, a conductive nanocomposite network hydrogel fabricated by projection microstereolithography (PμSL) based 3D printing is proposed, enabling fast fabrication ability with high precision. The 3D printed hydrogels exhibit ultra-stretchability (2500%), hyper-antifreezing (-125°C), extremely low working voltage (<100 μV), and super cyclic tensile stability (1 million cycles). The hydrogel-based strain sensor can probe both large-scale and tiny human motions, even with ultralow voltage of 100 μV at extremely low temperature around −115°C. It is demonstrated that the present hydrogels can be used as a flexible electrode for capturing human electrophysiological signals (EOG and EEG), where the alpha and beta waves from the brain can be recorded precisely. Therefore, the present hydrogels will pave the way for the development of next-generation intelligent electronics, especially for those working under extremely low-temperature environments.

Synthesis Antifreezing and Antidehydration Organohydrogels: One-Step In-Situ Gelling versus Two-Step Solvent Displacement

Organohydrogels with distinct antifreezing and antidehydration properties have aroused great interest among researchers, and various organohydrogels and organohydrogel-based applications have emerged recently. There are two popular synthesis strategies to prepare these antifreezing and antidehydration organohydrogels: the in-situ gelling and the solvent displacement strategies. Although both strategies have been widely applied, there is a lack of comparative study of these two strategies. In this work, to elucidate the comparative advantages of the two synthesis strategies, we studied and compared the mechanical and environmental tolerant properties of the organohydrogels synthesized from both strategies. The glycerol-based and ethylene glycol-based chemical polyacrylamide (PAAm) organohydrogel and the glycerol-based physical gelatin organohydrogel were synthesized and studied. Through the comparative study, we have found that the organohydrogels from different strategies with the same dispersion medium showed similar antifreezing and antidehydration properties but different mechanical properties. The mechanical properties of these organohydrogels are influenced by two opposite factors for each strategy: the enhanced physical interactions induced strengthening and solvent effect or swelling induced weakening. We hope this study may provide a better understanding of the synthesis strategies of organohydrogels and provide a valuable guide to choose the suitable synthesis strategy for each application.

A Solvent Co-cross-linked Organogel with Fast Self-Healing Capability and Reversible Adhesiveness at Extreme Temperatures

Antifreezing gels are promising in diverse engineering applications such as structural soft matters, sensors, and wearable devices. However, the capability of fast self-healing and reversible adhesiveness still remain a huge challenge for gels at extreme temperatures. Here, we proposed a solvent-involved cross-linking system composed of polyacrylic acid, polyvinyl alcohol, borax, ethylene glycol, and water, capable of antifreezing below -90 °C. It was not only antifreezing, anticrystalline, and abundant in dynamic bonds but also highly transparent, stretchable (over 800%), and conductive over the scope of temperature from -60 to 60 °C. Moreover, this gel could self-heal within 1 min and repeatedly adhere to multiple substrates including glass, metal, and rubber with an adhesive strength greater than 18 kPa. These key functions of the gel could be mostly preserved after 5 days of storage at 70% relative humidity. It is anticipated that our research opens a new scope for high-performance extreme environment-tolerant adhesives or wearable devices.

Physical Organohydrogels With Extreme Strength and Temperature Tolerance

Tough gel with extreme temperature tolerance is a class of soft materials having potential applications in the specific fields that require excellent integrated properties under subzero temperature. Herein, physically crosslinked Europium (Eu)-alginate/polyvinyl alcohol (PVA) organohydrogels that do not freeze at far below 0°C, while retention of high stress and stretchability is demonstrated. These organohydrogels are synthesized through displacement of water swollen in polymer networks of hydrogel to cryoprotectants (e.g., ethylene glycol, glycerol, and d-sorbitol). The organohydrogels swollen water-cryoprotectant binary systems can be recovered to their original shapes when be bent, folded and even twisted after being cooled down to a temperature as low as -20 and -45°C, due to lower vapor pressure and ice-inhibition of cryoprotectants. The physical organohydrogels exhibit the maximum stress (5.62 ± 0.41 MPa) and strain (7.63 ± 0.02), which is about 10 and 2 times of their original hydrogel, due to the synergistic effect of multiple hydrogen bonds, coordination bonds and dense polymer networks. Based on these features, such physically crosslinked organohydrogels with extreme toughness and wide temperature tolerance is a promising soft material expanding the applications of gels in more specific and harsh conditions.

Novel Elastically Stretchable Metal-Organic Framework Laden Hydrogel with Pearl-Net Microstructure and Freezing Resistance through Post-Synthetic Polymerization

Nanocomposite hydrogels (NCs) with mechanical properties suitable for a diverse range of applications can be made by combining polymer hydrogel networks with various inorganic nanoparticles. However, the mechanical properties and functions of conventional NCs are seriously limited by the poor structural or functional tunability of common nanofillers and by the low amounts of such fillers that can be added. Here, the fabrication of novel elastically stretchable and compressible nanocomposite hydrogels (MIL-101-MAAm/PAAm) with a distinctive pearl-net microstructure and a metal-organic framework (MOF) content in the range of 20-60 wt% through post-synthetic polymerization (PSP) is reported. The MOFs, which are compatible with polymers and have a high degree of modifiability in structure and functions, are used as nanofillers. Such MOF-laden hydrogels can withstand 500% tensile strain or 90% compressive strain without fracture and recover quickly upon unloading. They are also resistant to freezing at -25 °C. In addition, the problems associated with poor flexibility and processability of MOFs are overcome by the hybridization of hydrogel polymer matrices with MOFs. The results of this work not only provide a new perspective on preparing NCs but also indicate a promising path for applying MOFs in flexible devices.

Engineering hydrogels by soaking: from mechanical strengthening to environmental adaptation

The soaking strategy could not only strengthen hydrogels with superior mechanical properties but also provide the hydrogels with environmentally adapting properties.