A General Crosslinker Strategy to Realize Intrinsic Frozen Resistance of Hydrogels

Gel-Based Triboelectric Nanogenerators for Flexible Sensing: Principles, Properties, and Applications

The rapid development of the Internet of Things and artificial intelligence technologies has increased the need for wearable, portable, and self-powered flexible sensing devices. Triboelectric nanogenerators (TENGs) based on gel materials (with excellent conductivity, mechanical tunability, environmental adaptability, and biocompatibility) are considered an advanced approach for developing a new generation of flexible sensors. This review comprehensively summarizes the recent advances in gel-based TENGs for flexible sensors, covering their principles, properties, and applications. Based on the development requirements for flexible sensors, the working mechanism of gel-based TENGs and the characteristic advantages of gels are introduced. Design strategies for the performance optimization of hydrogel-, organogel-, and aerogel-based TENGs are systematically summarized. In addition, the applications of gel-based TENGs in human motion sensing, tactile sensing, health monitoring, environmental monitoring, human-machine interaction, and other related fields are summarized. Finally, the challenges of gel-based TENGs for flexible sensing are discussed, and feasible strategies are proposed to guide future research.

Spider-silk-inspired strong and tough hydrogel fibers with anti-freezing and water retention properties

Ideal hydrogel fibers with high toughness and environmental tolerance are indispensable for their long-term application in flexible electronics as actuating and sensing elements. However, current hydrogel fibers exhibit poor mechanical properties and environmental instability due to their intrinsically weak molecular (chain) interactions. Inspired by the multilevel adjustment of spider silk network structure by ions, bionic hydrogel fibers with elaborated ionic crosslinking and crystalline domains are constructed. Bionic hydrogel fibers show a toughness of 162.25 ± 21.99 megajoules per cubic meter, comparable to that of spider silks. The demonstrated bionic structural engineering strategy can be generalized to other polymers and inorganic salts for fabricating hydrogel fibers with broadly tunable mechanical properties. In addition, the introduction of inorganic salt/glycerol/water ternary solvent during constructing bionic structures endows hydrogel fibers with anti-freezing, water retention, and self-regeneration properties. This work provides ideas to fabricate hydrogel fibers with high mechanical properties and stability for flexible electronics.

Unlocking the Potential of CO2 Capture: A Synergistic Hybridization Strategy for Polymeric Hydrogels with Tunable Physicochemical Properties

Unlocking CO2 capture potential remains a complex and challenging endeavor. Here, a blueprint is crafted for optimizing materials through CO2 capture and developing a synergistic hybridization strategy that involves synthesizing CO2-responsive hydrogels by integrating polymeric networks interpenetrated with polyethyleneimine (PEI) chains and inorganic CaCl2. Diverging from conventional CO2 absorbents, which typically serve a singular function in CO2 capture, these hybrid PEAC hydrogels additionally harness its presence to tune their optical and mechanical properties once interacting with CO2. Such synergistic functions entail two significant steps: (i) rapid CO2-fixing through PEI chains to generate abundant carbamic acid and carbamate species and (ii) mineralization via CaCl2 to induce the formation of CaCO3 micro-crystals within the hydrogel matrix. Due to the reversible bonding, the PEAC hydrogels enable the decoupling of CO2 through an acid fumigation treatment or a heating process, achieving dynamic CO2 capture-release cycles up to 8 times. Furthermore, the polyethyleneimine-acrylamide-calcium chloride (PEAC) hydrogel exhibits varying antibacterial attributes and high interfacial adhesive strength, which can be modulated by fine-tuning the compositions of PEI and CaCl2. This versatility underscores the promising potential of PEAC hydrogels, which not only unlocks CO2 capture capabilities but also offers opportunities in diverse biological and biomedical applications.

Frost-resistant nanocellulose-based organohydrogel with high mechanical strength and transparency

Improving mechanical strength and frost-resistance is an important research direction in the field of hydrogel materials. Herein, using bacterial nanocellulose (BC) as a reinforcing agent and polyvinyl alcohol (PVA) as a polymer matrix, a frost-resistant organohydrogel was constructed via the freezing-thawing method in a new binary solvent system of N, N-dimethylformamide and water (DMF-H2O), which was designed according to the Hansen Solubility Parameter. Owing to the solvent-induced crystallization effect that led to the enhanced 3D hydrogen bonding network during the freezing-thawing process, the optimal organohydrogel achieved excellent mechanical properties with the tensile strength of 2,974 kPa and the stretchability of 277 % at room temperature, respectively. In the visiblelight range, the organohydrogel demonstrated high transmittance. Moreover, the presence of a DMF-H2O binary solvent endows it with frost-resistance, retaining the tensile strength of 508 kPa and a stretchability of 190 % even at -70 °C, respectively. This kind of transparent, frost-resistant organohydrogel has potential uses in harsh settings due to its great mechanical strength.

Skin-Inspired All-Natural Biogel for Bioadhesive Interface

Natural material-based hydrogels are considered ideal candidates for constructing robust bio-interfaces due to their environmentally sustainable nature and biocompatibility. However, these hydrogels often encounter limitations such as weak mechanical strength, low water resistance, and poor ionic conductivity. Here, inspired by the role of natural moisturizing factor (NMF) in skin, a straightforward yet versatile strategy is proposed for fabricating all-natural ionic biogels that exhibit high resilience, ionic conductivity, resistance to dehydration, and complete degradability, without necessitating any chemical modification. A well-balanced combination of gelatin and sodium pyrrolidone carboxylic acid (an NMF compound) gives rise to a significant enhancement in the mechanical strength, ionic conductivity, and water retention capacity of the biogel compared to pure gelatin hydrogel. The biogel manifests temperature-controlled reversible fluid-gel transition properties attributed to the triple-helix junctions of gelatin, which enables in situ gelation on diverse substrates, thereby ensuring conformal contact and dynamic compliance with curved surfaces. Due to its salutary properties, the biogel can serve as an effective and biocompatible interface for high-quality and long-term electrophysiological signal recording. These findings provide a general and scalable approach for designing natural material-based hydrogels with tailored functionalities to meet diverse application needs.

A Microphase-Separated Design toward an All-Round Ionic Hydrogel with Discriminable and Anti-Disturbance Multisensory Functions

Stretchable ionic hydrogels with superior all-round properties that can detect multimodal sensations with excellent discriminability and robustness against external disturbances are highly required for artificial electronic skinapplications. However, some critical material parameters exhibit intrinsic tradeoffs with each other for most ionic hydrogels. Here, a microphase-separated hydrogel is demonstrated by combining three strategies: (1) using of a low crosslinker/monomer ratio to obtain highly entangled polymer chains as the first network; (2) the introduction of zwitterions into the first network; (3) the synthesis of an ultrasoft polyelectrolyte as the second network. This all-round elastic ionic hydrogel exhibits a low Young's modulus (< 60 kPa), large stretchability (> 900%), high resilience (> 95%), unique strain-stiffening behavior, excellent fatigue tolerance, high ionic conductivity (> 2.0 S m⁻1), and anti-freezing capability, which have not been achieved before. These properties allow the ionic hydrogel to operate as a stretchable multimodal sensor that can detect and decouple multiple stimuli (temperature, pressure, and proximity) with excellent discriminability, high sensitivity, and strong sensing-robustness against strains or temperature perturbations. The ionic hydrogel sensor exhibits great potential for intelligent electronic skin applications such as reliable health monitoring and accurate object identification.

Super-Wide Temperature Lasers Spanning from -180 to 240 °C Based on Fully-Polymerized Blue Phase Superstructures

Blue phase liquid crystal (BPLC) lasers have potential applications in displays, sensors, and anti-counterfeiting fields owing to their outstanding optical properties. However, there remain challenges on lasing below 0 °C, which significantly limits the potential application of BPLC lasers in low-temperature environments. In this work, BPLC lasing below 0 °C is realized for the first time in a super-wide temperature range of -180-240 °C using a well-designed fully-polymerized BPLC system with a narrow line width of 0.0881 nm and a low lasing threshold of 37 nJ pulse-1. This fully-polymerized BPLC both effectively avoids low-temperature random crystallization and has excellent compatibility with dye molecules that significantly widen the lasing temperature range below 0 °C. Besides, the variations of laser peak and threshold are also revealed below 0 °C, that is, redshifted laser wavelength and increased threshold value with decreasing temperature, which contribute to a blue-shifted laser signal and a U-shaped lasing threshold in -180-240 °C. These unique laser behaviors can be ascribed to the temperature-dependent anisotropically microstructural deformation of the BP lattice. This work not only opens a door to the development of low-temperature BPLC lasers but also sets out important insights in the design of novel organic optical devices.

TEMPO bacterial cellulose and MXene nanosheets synergistically promote tough hydrogels for intelligent wearable human-machine interaction

Conductive hydrogels have received increasing attention in the field of wearable electronics, but they also face many challenges such as temperature tolerance, biocompatibility, and stability of mechanical properties. In this paper, a double network hydrogel of MXene/TEMPO bacterial cellulose (TOBC) system is proposed. Through solvent replacement, the hydrogel exhibits wide temperature tolerance (-20-60 °C) and stable mechanical properties. A large number of hydrogen bonds, MXene/TOBC dynamic three-dimensional network system, and micellar interactions endow the hydrogel with excellent mechanical properties (elongation at break ~2800 %, strength at break ~420 kPa) and self-healing ability. The introduction of tannic acid prevents the oxidation of MXene and the loss of electrical properties of the hydrogel. In addition, the sensor can also quickly (74 ms) and sensitive (gauge factor = 15.65) wirelessly monitor human motion, and the biocompatibility can well avoid the stimulation when it comes into contact with the human body. This series of research work reveals the fabrication of MXene-like flexible wearable electronic devices based on self-healing, good cell compatibility, high sensitivity, wide temperature tolerance and durability, which can be used in smart wearable, wireless monitoring, human-machine Interaction and other aspects show great application potential.

Enhanced Diabetic Rat Wound Healing by Platelet-Rich Plasma Adhesion Zwitterionic Hydrogel

BACKGROUND

The skin is the largest organ in the human body and serves as a barrier for protective, immune, and sensory functions. Continuous and permanent exposure to the external environment results in different levels of skin and extracellular matrix damage. During skin wound healing, the use of good dressings and addition of growth factors to the wound site can effectively modulate the rate of wound healing. A dressing containing bioactive substances can absorb wound exudates and reduce adhesion between the wound and dressing, whereas growth factors, cytokines, and signaling factors can promote cell motility and proliferation.

AIM AND OBJECTIVES

We prepared a functional wound dressing by combining platelet-rich plasma (PRP) and zwitterionic hydrogels. Functional wound dressings are rich in various naturally occurring growth factors that can effectively promote the healing process in various types of tissues and absorb wound exudates to reduce adhesion between wounds and dressings. Furthermore, PRP-incorporated zwitterionic hydrogels have been used to repair full-thickness wounds in Sprague-Dawley rats with diabetes (DM SD).

MATERIALS AND METHODS

Fibroblasts and keratinocytes were cultured with PRP, zwitterionic hydrogels, and PRP-incorporated zwitterionic hydrogels to assess cell proliferation and specific gene expression. Furthermore, PRP-incorporated zwitterionic hydrogels were used to repair full-thickness skin defects in DM SD rats.

RESULTS

The swelling ratio of hydrogel, hydrogel + PRP1000 (108 platelets/mL), and hydrogel + PRP1000 (109 platelets/mL) groups were similar (~07.71% ± 1.396%, 700.17% ± 1.901%, 687.48% ± 4.661%, respectively) at 144 hours. The tensile strength and Young modulus of the hydrogel and hydrogel + PRP10000 groups were not significantly different. High concentrations of PRP (approximately 108 and 109 platelets/mL) effectively promoted the proliferation of fibroblasts and keratinocytes. The zwitterionic hydrogels were not cytotoxic to any cell type. High PRP concentration-incorporated zwitterionic hydrogels increased the rate of cell proliferation and significantly increased the expression of characteristic genes such as collagen, fibronectin, involucrin, and keratin. Subsequently, zwitterionic hydrogels with high PRP concentrations were used to repair full-thickness skin defects in DM SD rats, and a wound healing rate of more than 90% was recorded on day 12.

CONCLUSIONS

PRP contains high concentrations of growth factors that promote cell viability, enhance specific gene expression, and have a high medical value in cell therapy. Zwitterionic hydrogels have a 3-dimensional interconnected microporous structure and can resist cell adhesion without causing cytotoxicity. Platelet-rich plasma-incorporated zwitterionic hydrogels further enhance the cellular properties and provide an effective therapeutic option for wound healing.

Continuous Spinning of High-Tough Hydrogel Fibers for Flexible Electronics by Using Regional Heterogeneous Polymerization

Hydrogel fibers have attracted substantial interest for application in flexible electronics due to their ionic conductivity, high specific surface area, and ease of constructing multidimensional structures. However, universal continuous spinning methods for hydrogel fibers are yet lacking. Based on the hydrophobic mold induced regional heterogeneous polymerization, a universal self-lubricating spinning (SLS) strategy for the continuous fabrication of hydrogel fibers from monomers is developed. The universality of the SLS strategy is demonstrated by the successful spinning of 10 vinyl monomer-based hydrogel fibers. Benefiting from the universality of the SLS strategy, the SLS strategy can be combined with pre-gel design and post-treatment toughening to prepare highly entangled polyacrylamide (PAM) and ionic crosslinked poly(acrylamide-co-acrylic acid)/Fe3+ (W-PAMAA/Fe3+ ) hydrogel fibers, respectively. In particular, the W-PAMAA/Fe3+ hydrogel fiber exhibited excellent mechanical properties (tensile stress > 4 MPa, tensile strain > 400%) even after 120 days of swelling in the pH of 3-9. Furthermore, owing to the excellent multi-faceted performance and one-dimensionality of W-PAMAA/Fe3+ hydrogel fibers, flexible sensors with different dimensions and functions can be constructed bottom-up, including the one-dimensional (1D) strain sensor, two-dimensional (2D) direction sensor, three-dimensional (3D) pressure sensor, and underwater communication sensor to present the great potential of hydrogel fibers in flexible electronics.

Strong and Tough Antifreezing Hydrogel Sensor via the Synergy of Coordination and Hydrogen Bonds

Hydrogel sensors are fascinating as flexible sensors and electronic skin due to their excellent biocompatibility and structure controllability. However, developing conductive hydrogels possessing both excellent mechanical and antifreezing properties for environmental-adaptive sensors remains a challenge. Herein, a strategy of combining betaine and metal ions to construct poly(acrylic acid) (PAA)-based high-conductive hydrogels has been reported. PAA-Al3+/betaine hydrogels with high toughness and antifreezing property were prepared by a one-step UV curing method. Their high toughness is attributed to the coordination of metal ions with the carboxylic groups in PAA, the interaction of betaine with PAA, and the formation of hydrogen bonds between them and water molecules. Moreover, the significant antifreezing property is due to the reduction of free water in the hydrogel. This, in turn, is attributed to the hydration of metal ions and the synergistic hydrogen bonding between betaine and water. The experiments demonstrate that the hydrogel has excellent mechanical property, high conductivity, superior transparency, antiswelling property, antipuncture as well as shape memory properties, and especially, low cytotoxicity. It can be used as a sensor for motion detection and information recognition. This work provides new insights into the application of flexible sensors and human-machine interfaces in multienvironmental conditions.

Hydrogel Electrolyte Enabled High-Performance Flexible Aqueous Zinc Ion Energy Storage Systems toward Wearable Electronics

To cater to the swift advance of flexible wearable electronics, there is growing demand for flexible energy storage system (ESS). Aqueous zinc ion energy storage systems (AZIESSs), characterizing safety and low cost, are competitive candidates for flexible energy storage. Hydrogels, as quasi-solid substances, are the appropriate and burgeoning electrolytes that enable high-performance flexible AZIESSs. However, challenges still remain in designing suitable and comprehensive hydrogel electrolyte, which provides flexible AZIESSs with high reversibility and versatility. Hence, the application of hydrogel electrolyte-based AZIESSs in wearable electronics is restricted. A thorough review is required for hydrogel electrolyte design to pave the way for high-performance flexible AZIESSs. This review delves into the engineering of desirable hydrogel electrolytes for flexible AZIESSs from the perspective of electrolyte designers. Detailed descriptions of hydrogel electrolytes in basic characteristics, Zn anode, and cathode stabilization effects as well as their functional properties are provided. Moreover, the application of hydrogel electrolyte-based flexible AZIESSs in wearable electronics is discussed, expecting to accelerate their strides toward lives. Finally, the corresponding challenges and future development trends are also presented, with the hope of inspiring readers.

Tremella polysaccharide-based conductive hydrogel with anti-freezing and self-healing ability for motion monitoring and intelligent interaction

The application of conductive hydrogels in flexible wearable devices has garnered significant attention. In this study, a self-healing, anti-freezing, and fire-resistant hydrogel strain sensor is successfully synthesized by incorporating sustainable natural biological materials, viz. Tremella polysaccharide and silk fiber, into a polyvinyl alcohol matrix with borax cross-linking. The resulting hydrogel exhibits excellent transparency, thermoplasticity, and remarkable mechanical properties, including a notable elongation (1107.3 %) and high self-healing rate (91.11 %) within 5 min, attributed to the dynamic cross-linking effect of hydrogen bonds and borax. A strain sensor based on the prepared hydrogel sensor can be used to accurately monitor diverse human movements, while maintaining exceptional sensing stability and durability under repeated strain cycles. Additionally, a hydrogel touch component is designed that can successfully interact with intelligent electronic devices, encompassing functions like clicking, writing, and drawing. These inherent advantages make the prepared hydrogel a promising candidate for applications in human health monitoring and intelligent electronic device interaction.

Sustainable zinc-air battery chemistry: advances, challenges and prospects

Sustainable zinc-air batteries (ZABs) are considered promising energy storage devices owing to their inherent safety, high energy density, wide operating temperature window, environmental friendliness, etc., showing great prospect for future large-scale applications. Thus, tremendous efforts have been devoted to addressing the critical challenges associated with sustainable ZABs, aiming to significantly improve their energy efficiency and prolong their operation lifespan. The growing interest in sustainable ZABs requires in-depth research on oxygen electrocatalysts, electrolytes, and Zn anodes, which have not been systematically reviewed to date. In this review, the fundamentals of ZABs, oxygen electrocatalysts for air cathodes, physicochemical properties of ZAB electrolytes, and issues and strategies for the stabilization of Zn anodes are systematically summarized from the perspective of fundamental characteristics and design principles. Meanwhile, significant advances in the in situ/operando characterization of ZABs are highlighted to provide insights into the reaction mechanism and dynamic evolution of the electrolyte|electrode interface. Finally, several critical thoughts and perspectives are provided regarding the challenges and opportunities for sustainable ZABs. Therefore, this review provides a thorough understanding of the advanced sustainable ZAB chemistry, hoping that this timely and comprehensive review can shed light on the upcoming research horizons of this prosperous area.

Tough, adhesive biomimetic hyaluronic acid methacryloyl hydrogels for effective wound healing

The development of cost-effective, biocompatible soft wound dressings is highly desirable; however, conventional dressings are only designed for flat wounds, which creates difficulty with promising healing efficiency in complex practical conditions. Herein, we developed a tough, adhesive biomimetic hyaluronic acid methacryloyl hydrogels composed of chemically crosslinked hyaluronic acid methacryloyl (HAMA) network and poly(N-hydroxyethyl acrylamide) (PHEAA) network rich in multiple hydrogen bonding. Due to the multiple chemical crosslinking sites (acrylamide groups) of HAMA; the bulk HEMA/PHEAA hydrogels presented significant enhancements in mechanical properties (∼0.45 MPa) than common hyaluronic acid hydrogels (<0.1 MPa). The abundant hydrogen bonding also endowed the resultant hydrogels with extremely high adhesiveness on many nonporous substrates, including glass and biological tissues (e.g., heart, liver, lung, kidney, stomach, and muscle), with a considerable interfacial toughness of ∼1432 J m-2. Accordingly, since both natural hyaluronic acid derivative polymers and hydrophilic PHEAA networks are highly biocompatible, the hydrogel matrix possesses good blood compatibility (<5% of hemolysis ratio) and satisfies the general dressing requirements (>99% of cell viability). Based on these physicochemical features, we have demonstrated that this adhesive hydrogel, administered in the form of a designed patch, could be applied to wound tissue healing by promoting epithelialization, angiogenesis, and collagen deposition. We believe that our proposed biomimetic hydrogel design holds great potential for wound repair and our developed HAMA/PHEAA hydrogels are extremely promising for the next-generation tissue healings in emergency situations.

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.

Environmentally adaptive polysaccharide-based hydrogels and their applications in extreme conditions: A review

Polysaccharide hydrogels are one of the most promising hydrogel materials due to their inherent characteristics, including biocompatibility, biodegradability, renewability, and easy modification, and their structure and functional designs have been widely researched to adapt to different application scenarios as well as to broaden their application fields. As typical wet-soft materials, the high water content and water-absorbing ability of polysaccharide-based hydrogels (PHs) are conducive to their wide biomedical applications, such as wound healing, tissue repair, and drug delivery. In addition, along with technological progress, PHs have shown potential application prospects in some high-tech fields, including human-computer interaction, intelligent driving, smart dressing, flexible sensors, etc. However, in practical applications, due to the poor ability of PHs to resist freezing below zero, dehydration at high temperature, and acid-base/swelling-induced deformation in a solution environment, they are prone to lose their wet-soft peculiarities, including structural integrity, injectability, flexibility, transparency, conductivity and other inherent characteristics, which greatly limit their high-tech applications. Hence, reducing their freezing point, enhancing their high-temperature dehydration resistance, and improving their extreme solution tolerance are powerful approaches to endow PHs with multienvironmental adaptability, broadening their application areas. This report systematically reviews the study advances of environmentally adaptive polysaccharide-based hydrogels (EAPHs), comprising anti-icing hydrogels, high temperature/dehydration resistant hydrogels, and acid/base/swelling deformation resistant hydrogels in recent years. First, the construction methods of EAPHs are presented, and the mechanisms and properties of freeze-resistant, high temperature/dehydration-resistant, and acid/base/swelling deformation-resistant adaptations are simply demonstrated. Meanwhile, the features of different strategies to prepare EAPHs as well as the strategies of simultaneously attaining multienvironmental adaptability are reviewed. Then, the applications of extreme EAPHs are summarized, and some meaningful works are well introduced. Finally, the issues and future outlooks of PH environment adaptation research are elucidated.

Bilayer Hydrogels by Reactive-Induced Macrophase Separation

Bilayer hydrogels encoded with smart functions have emerged as promising soft materials for engineered biological tissues and human-machine interfaces, due to the versatility and flexibility in designing their mechanical and chemical properties. However, conventional fabrication strategies often require multiple complicated steps to create an anisotropic bilayer structure with poor interfaces, which significantly limit the scope of bilayer hydrogel applications. Here, we reported a general, one-pot, macrophase separation strategy to fabricate a family of bilayer hydrogels made of vinyl and styryl monomers with a seamless interface and a controllable layer separation efficiency (20-99%). The working principle of a macrophase separation strategy allows for the decoupling of the two gelation processes to form distinct vinyl- and styryl-enriched layers by manipulating competitive polymerization reactions between vinyl and styryl monomers. This work presents a straightforward approach and a diverse range of radical monomers, which can be utilized to create next-generation bilayer hydrogels, beyond a few available today.

Massive Fabrication of Flexible, Form-Stable, and Self-Repairing Brine Phase Change Material Gels toward Smart Cold Chain Logistics

Cold chain logistics plays an extremely important role in the storage and transportation of perishable products. Nowadays, phase change materials (PCMs) have been applied in emerging cold chain logistics to overcome the problems of low stability, high energy consumption, and high cost in mechanical refrigeration-based cold chain logistics. Mass production of high-performance phase change cold storage materials toward cold chain logistics is still a major challenge. Herein, self-repairing brine phase change gels (BPCMGs) massively fabricated by ionic cross-linking, covalent cross-linking, and hydrogen bond cross-linking are proposed. Brine containing 23.3% sodium chloride (NaCl) is selected as the phase change component because its phase change temperature is suitable for the cold storage demand of aquatic products. The proposed BPCMGs demonstrate superior thermophysical properties in terms of no phase separation, no supercooling, high form stability, high latent heat, high thermal conductivity, high cyclic stability, and high self-repairing rate. Meanwhile, the BPCMGs present high cost-effectiveness. Given these advantages, BPCMGs are utilized to assemble smart cold storage equipment for the storage and transportation of aquatic products. The cold storage time reaches 36.73 h for aquatic products when the stored cold energy is 364078 J. The location and temperature of the refrigerated products are monitored in real-time. The state-of-the-art BPCMGs provide diversified possibilities for the advanced smart cold chain.

Strong, conductive, and freezing-tolerant polyacrylamide/PEDOT:PSS/cellulose nanofibrils hydrogels for wearable strain sensors

Hydrogels with prominent flexibility, versatility, and high sensitivity play an important role in the design and fabrication of wearable sensors. In particular, these flexible conductive hydrogels exhibit elastic modulus that is highly compatible with human skin, demonstrating the great potential for flexible sensing. However, the preparation of high-performance hydrogel-based sensors that can restrain extreme cold conditions is still challenging. Herein, a novel anti-freezing composite hydrogel with superior conductivity based on polyacrylamide (PAM), LiCl, and PEDOT:PSS coated cellulose nanofibrils (PAM/PEDOT:PSS/CNF) is constructed. The addition of CNF increased the hydrogen bonding sites of the molecular chains in the micro, thus improving the mechanical strength and the conductivity of the hydrogel in the macro. The hydrogels achieve a high tensile strength of 0.19 MPa, compressive strength of 0.92 MPa, and dissipation energy of 41.9 kJ/m³. Otherwise, LiCl increases the interactions between the colloidal phase and water molecules, endowing the hydrogels with excellent freezing tolerance. Specifically, the optimized hydrogel of 45 % LiCl exhibited stable mechanical properties at -40 °C. Finally, the composite hydrogel was used to assemble flexible sensors with high sensitivity of 10.3 MPa^-1, which can detect a wide range of human movements and physiological activities.

Recent Development of Self-Powered Tactile Sensors Based on Ionic Hydrogels

Hydrogels are three-dimensional polymer networks with excellent flexibility. In recent years, ionic hydrogels have attracted extensive attention in the development of tactile sensors owing to their unique properties, such as ionic conductivity and mechanical properties. These features enable ionic hydrogel-based tactile sensors with exceptional performance in detecting human body movement and identifying external stimuli. Currently, there is a pressing demand for the development of self-powered tactile sensors that integrate ionic conductors and portable power sources into a single device for practical applications. In this paper, we introduce the basic properties of ionic hydrogels and highlight their application in self-powered sensors working in triboelectric, piezoionic, ionic diode, battery, and thermoelectric modes. We also summarize the current difficulty and prospect the future development of ionic hydrogel self-powered sensors.

Highly conductive and anti-freezing cellulose hydrogel for flexible sensors

Ionic conducting hydrogels (ICHs) are emerging materials for multi-functional sensors in the fields of healthcare monitoring and flexible electronics. However, there is a long-standing dilemma between ionic conductivity and mechanical properties of the ICHs. In this work, ionic conductive, flexible, transparent, and anti-freezing hydrogels are fabricated by dissolving cotton linter pulp in ZnCl2/CaCl2 solution and cross-linking with epichlorohydrin (ECH). The presence of inorganic salt imparts the hydrogel with high ionic conductivity and low-temperature tolerance. While the introduction of ECH as the second network gives the hydrogel with desirable mechanical performance. By tailoring the ECH addition, the tensile strength, compressive strength, elongation at break, and conductivity of the hydrogel could reach 0.82 MPa, 2.80 MPa, 260 %, and 5.48 S m-1, respectively. The prepared ICHs are fabricated into sensors for detecting full-range human body motions, and they demonstrate fast response and durable sensitivity to both tensile strain and compressive deformation. Moreover, flexible sensors can work at subzero temperatures. This work provides a new idea for the preparation of cellulose-based hydrogels with good ionic conductivity and mechanical properties under extreme conditions.

Engineering viscoelastic mismatch for temporal morphing of tough supramolecular hydrogels

Viscoelasticity is a generic characteristic of soft biotissues and polymeric materials, endowing them with unique time- and rate-dependent properties. Here, by spatiotemporally tailoring the viscoelasticity in tough supramolecular hydrogels, we demonstrate reprogrammable morphing of the gels based on differential viscoelastic recovery processes that lead to internal strain mismatch. The spatial heterogeneity of viscoelasticity is encoded through integrating dissimilar hydrogels or by site-specific treatment of a singular hydrogel. The temporal morphing behavior of tough gels, including a fast deformation process and then a slow shape-recovery process, is related to the kinetics of associative interactions and the entropic elasticity of supramolecular networks after pre-stretching and release, which takes place spontaneously in the absence of external stimuli. Such a kinetically driven morphing mechanism resolves the trade-off between the mechanical robustness and shape-changing speed in tough hydrogels with dense entanglements and physical associations, and should be applicable to other viscoelastic materials. A numerical theory for the temporal morphing of tough supramolecular gels has been formulated by dynamic coupling of viscoelastic recovery and mechanics of deformations, which is further implemented to predict the sophisticated morphed structures. Furthermore, magnetic particles are incorporated into the morphed tough hydrogels to devise versatile soft actuators and robots for specific applications.

Recent Progress of Energy-Storage-Device-Integrated Sensing Systems

With the rapid prosperity of the Internet of things, intelligent human-machine interaction and health monitoring are becoming the focus of attention. Wireless sensing systems, especially self-powered sensing systems that can work continuously and sustainably for a long time without an external power supply have been successfully explored and developed. Yet, the system integrated by energy-harvester needs to be exposed to a specific energy source to drive the work, which provides limited application scenarios, low stability, and poor continuity. Integrating the energy storage unit and sensing unit into a single system may provide efficient ways to solve these above problems, promoting potential applications in portable and wearable electronics. In this review, we focus on recent advances in energy-storage-device-integrated sensing systems for wearable electronics, including tactile sensors, temperature sensors, chemical and biological sensors, and multifunctional sensing systems, because of their universal utilization in the next generation of smart personal electronics. Finally, the future perspectives of energy-storage-device-integrated sensing systems are discussed.

Zwitterionic Biomaterials

The term "zwitterionic polymers" refers to polymers that bear a pair of oppositely charged groups in their repeating units. When these oppositely charged groups are equally distributed at the molecular level, the molecules exhibit an overall neutral charge with a strong hydration effect via ionic solvation. The strong hydration effect constitutes the foundation of a series of exceptional properties of zwitterionic materials, including resistance to protein adsorption, lubrication at interfaces, promotion of protein stabilities, antifreezing in solutions, etc. As a result, zwitterionic materials have drawn great attention in biomedical and engineering applications in recent years. In this review, we give a comprehensive and panoramic overview of zwitterionic materials, covering the fundamentals of hydration and nonfouling behaviors, different types of zwitterionic surfaces and polymers, and their biomedical applications.

Significant Roles of Ions in Enhancing and Functionalizing Anisotropic Hydrogels

Salt ions are multifunctional in living beings, in contrast to their limited efficiency in abiotic materials. Achieving the versatility of salt ions in synthetic materials is promising yet demanding. Here, we report that multivalent metallic ions can act multiple crucial roles in a polyacrylamide/sodium alginate (PAAm/SA) composite hydrogel system, inducing a quadruple effect that toughens and functionalizes the originally weak gel. Fixation of anisotropic structures (effect I), mechanical enhancement (effect II), conductivity improvement (effect III), as well as antifreezing and moisture retention properties (effect IV) simultaneously emerge in the gel, all of which are enabled by the ion effect. The resulting tough hydrogels exhibit excellent comprehensive properties that rival existing state-of-the-art hydrogels, promising a wide range of potential applications. As proof-of-concept demonstrations, extremely durable hydrogel-based soft electronic devices are developed, which operate stably even in harsh environments. We also prove that the ion effect can be induced by other multivalent metallic ions. This work highlights the versatility of salt ions in nonliving materials, providing a simple but enlightening idea for the development of all-around soft materials.

Hydrogels Enable Future Smart Batteries

The growing trend of intelligent devices ranging from wearables and soft robots to artificial intelligence has set a high demand for smart batteries. Hydrogels provide opportunities for smart batteries to self-adjust their functions according to the operation conditions. Despite the progress in hydrogel-based smart batteries, a gap remains between the designable functions of diverse hydrogels and the expected performance of batteries. In this Perspective, we first briefly introduce the fundamentals of hydrogels, including formation, structure, and characteristics of the internal water and ions. Batteries that operate under unusual mechanical and temperature conditions enabled by hydrogels are highlighted. Challenges and opportunities for further development of hydrogels are outlined to propose future research in smart batteries toward all-climate power sources and intelligent wearables.

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.

Formulating Zwitterionic, Responsive Polymers for Designing Smart Soils

The design of new remediation strategies and materials for treating saline-alkaline soils is of fundamental and practical importantance for many applications. Conventional soil remediation strategies mainly focus on the development of fertilizers or additives for water, nutrient, and heavy metal managements in soils, but they often overlook a soil sensing function for early detection of salinization/alkalization levels toward optimal and timely soil remediation. Here, new smart soils, structurally consisting of the upper signal soil and the bottom hygroscopic bed and chemically including zwitterionic, thermo-responsive poly(NIPAM-co-VPES) and poly(NIPAM-co-SBAA) aerogels in each soil layer are formulated. Upon salinization, the resultant smart soils exhibit multiple superior capacities for reducing the soil salinity and alkalinity through ion exchange, controlling the water cycling, modulating the degradation of pyridine-base ligands into water-soluble, nitrogenous salts-rich ingredients for soil fertility, and real-time monitoring salinized soils via pH-induced allochroic color changes. Further studies of plant growth in smart soils with or without salinization treatments confirm a synergy effect of soil remediation and soil sensing on facilitating the growth of plants and increasing the saline-alkaline tolerance of plants. The esign concept of smart soils can be further expanded for soil remediation and assessment.

Semi-Immobilized Ionic Liquid Regulator with Fast Kinetics toward Highly Stable Zinc Anode under -35 to 60 °C

Aqueous zinc ion batteries (ZIBs) have been extensively investigated as a next-generation energy storage system due to their high safety and low cost. However, the critical issues of irregular dendrite growth and intricate side reactions severely restrict the further industrialization of ZIBs. Here, a strategy to fabricate a semi-immobilized ionic liquid interface layer is proposed to protect the Zn anode over a wide temperature range from -35 to 60 °C. The immobilized SiO₂ @cation can form high conjugate racks that can regulate the Zn²⁺ concentration gradient and self-polarizing electric field to guarantee uniform nucleation and planar deposition; the free anions of the ILs can weaken the hydrogen bonds of the water to promote rapid Zn²⁺ desolvation and accelerate ion-transport kinetics simultaneously. Because of these unique advantages, the cycling performance of the symmetric Zn batteries is greatly enhanced, evidenced by a cycling life of 1800 h at 20 mA cm^-2 , and a cycle lifespan of 2000 h under a wide temperature window from -35 to 60 °C. The efficiency of this semi-immobilizing strategy is well demonstrated in various full cells including pouch cells, showing high performance at large current (20 A g^-1 ) and wide temperatures with extra-long cycles up to 80 000 cycles.

Digital Light Processing 3D Printing of Tough Supramolecular Hydrogels with Sophisticated Architectures as Impact-Absorption Elements

Processing tough hydrogels into sophisticated architectures is crucial for their applications as structural elements. However, Digital Light Processing (DLP) printing of tough hydrogels is challenging because of the low-speed gelation and toughening process. Described here is a simple yet versatile system suitable for DLP printing to form tough hydrogel architectures. The aqueous precursor consists of commercial photoinitiator, acrylic acid, and zirconium ion (Zr⁴⁺ ), readily forming tough metallo-supramolecular hydrogel under digital light because of in situ formation of carboxyl-Zr⁴⁺ coordination complexes. The high-stiffness and antiswelling properties of as-printed gel enable high-efficiency printing to form high-fidelity constructs. Furthermore, swelling-induced morphing of the gel is also achieved by encoding structure gradients during the printing with grayscale digital light. Mechanical properties of the printed hydrogels are further improved after incubation in water due to the variation of local pH and rearrangement of coordination complex. The swelling-enhanced stiffness affords the printed hydrogel with shape fixation ability after manual deformations, and thereby provides an additional avenue to form more complex configurations. These printed hydrogels are used to devise an impact-absorption element or a high-sensitivity pressure sensor as proof-of-concept examples. This work should merit engineering of other tough gels and extend their scope of applications in diverse fields.

Ionic interaction-driven switchable bactericidal surfaces

Bacteria in the external environment inevitably invade the wound and subsequently colonize the wound surface during surgery and biomedical operations, which slows down the process of wound healing and tissue repair; this poses a significant threat to human health. Therefore, the development of an intelligent antibacterial surface has become the focus of research in the field of antimicrobial strategies, which has important social and economic significance. Here, we present a simple approach of producing an ionic interaction-driven anionic activation substratum which is then functionalized with cationic molecules through coulombic interactional immobilization. The switchable multifunctional antibacterial surface can decrease bacterial attachment and inactivate the attached microorganisms, thus overcoming the conventional challenge for antibacterial surfaces. Briefly, poly (3-sulfopropyl methacrylate potassium salt) (PSPMA) brushes were constructed by surface-initiated atom transfer radical polymerization on silicon or cotton fabric substrates, and a positive-charged component, namely lysozyme (LYZ), hexadecyl trimethyl ammonium bromide (CTAB) or chitosan (CS), was loaded on negative-charged sulfonate groups through electrostatic interactions. The resultant brush-grafted surfaces exhibited more than ∼95.5% bactericidal efficacy and ∼92.8% release rate after the introduction of an adequate amount of contra-ions (1.0 M; Na⁺ & Cl^-) against both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus, thus achieving a regenerated surface through the cyclic process of "assembly-dissociation". Smart cotton fabric (Fabric-PSPMA/LYZ and Fabric-PSPMA/CS) surfaces were constructed, which were found to promote wound epidermal tissue regeneration with a higher efficiency after 7-day in vivo studies. This ionic interaction-driven method used in the present work is simple and can reversibly renew antibacterial surfaces, which will help in the wider utilization of switchable antibacterial materials with a more ecologic and economic significance. STATEMENT OF SIGNIFICANCE: Smart antibacterial surfaces with renewable characteristics have attracted considerable interests over the past few years. Here, we used ionic interaction-driven force to manipulate dynamic conformational changes in PSPMA surface brushes, accompanied by highly switchable bacteria killing and bacteria releasing behaviors. Different cationic molecules were also designed for assembly/dissociation on the PSPMA-modified surfaces, and the essential parameters, including chemical structures, molecular weight, and cationic charge density, were investigated. With the refined structural combinations and the balance of bacteria killing/bacteria releasing behaviors, smart cotton fabrics (e.g., Fabric-PSPMA/lysozyme and Fabric-PSPMA/chitosan) were designed that could promote wound healing and tissue repair. These results contribute to the fundamental understanding of a switchable cationic-anionic pair design and the corresponding practical, renewable, highly antibacterial fabric.

Antifreezing Hydrogel Electrolyte with Ternary Hydrogen Bonding for High-Performance Zinc-Ion Batteries

The new-generation flexible aqueous zinc-ion batteries require enhanced mechanical properties and ionic conductivities at low temperature for practical applications. This fundamentally means that it is desired that the hydrogel electrolyte possesses antifreezing merits to resist flexibility loss and performance decrease at subzero temperatures. Herein, a highly flexible polysaccharide hydrogel is realized in situ and is regulated in zinc-ion batteries through the Hofmeister effect with low-concentration Zn(ClO₄ )₂ salts to satisfy the abovementioned requirements. The chaotropic ClO₄ ^- anions, water, and polymer chains can form ternary and weak hydrogen bonding (HB), which enables the polymer chains to have improved mechanical properties, breaks the HB of water to remarkably decrease the electrolyte freezing point, and reduces the amounts of free water for effective side reactions and dendrite inhibition. Consequently, even at -30 °C, the Zn(ClO₄ )₂ in situ optimized hydrogel electrolyte features a high ionic conductivity of 7.8 mS cm^-1 and excellent flexibility, which enables a Zn/polyaniline (PANI) battery with a reversible capacity of 70 mA h g^-1 under 5 A g^-1 for 2500 cycles, and renderd the flexible full battery with excellent cycling performances under different bending angles. This work provides a new pathway for designing high-performance antifreezing flexible batteries via the Hofmeister effect.

Conductive Adhesive and Antibacterial Zwitterionic Hydrogel Dressing for Therapy of Full-Thickness Skin Wounds

Any sort of wound injury leads to the destruction of skin integrity and wound formation, causing millions of deaths every year and accounting for 10% of death rate insight into various diseases. The ideal biological wound dressings are expected to possess extraordinary mechanical characterization, cytocompatibility, adhesive properties, antibacterial properties, and conductivity of endogenous electric current to enhance the wound healing process. Recent studies have demonstrated that biomedical hydrogels can be used as typical wound dressings to accelerate the whole healing process due to them having a similar composition structure to skin, but they are also limited by ideal biocompatibility and stable mechanical properties. To extend the number of practical candidates in the field of wound healing, we designed a new structural zwitterion poly[3-(dimethyl(4-vinylbenzyl) ammonium) propyl sulfonate] (SVBA) into a poly-acrylamide network, with remarkable mechanical properties, stable rheological property, effective antibacterial properties, strong adsorption, high penetrability, and good electroactive properties. Both in vivo and in vitro evidence indicates biocompatibility, and strong healing efficiency, indicating that poly (AAm-co-SVBA) (PAS) hydrogels as new wound healing candidates with biomedical applications.

An Overview on Recent Progress of the Hydrogels: From Material Resources, Properties to Functional Applications

Hydrogels, as the most typical elastomer materials with three-dimensional network structures, have attracted wide attention owing to their outstanding features in fields of sensitive stimulus response, low surface friction coefficient, good flexibility and bio-compatibility. Because of numerous fresh polymer materials (or polymerization monomers), hydrogels with various structure diversities and excellent properties are emerging, and the development of hydrogels is very vigorous over the past decade. This review focuses on state-of-the-art advances, systematically reviews the recent progress on construction of novel hydrogels utilized several kinds of typical polymerization monomers, and explores the main chemical and physical cross-linking methods to develop the diversity of hydrogels. Following the aspects mentioned above, the classification and emerging applications of hydrogels, such as pH response, ionic response, electrical response, thermal response, biomolecular response, and gas response, are extensively summarized. Finally, we have done this review with the promises and challenges for the future evolution of hydrogels and their biological applications. cross-linking methods; functional applications; hydrogels; material resources This article is protected by copyright. All rights reserved.

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.

Zwitterionic Nanocapsules with Salt- and Thermo-Responsiveness for Controlled Encapsulation and Release

Intelligent polymer nanocapsules that can not only encapsulate substances efficiently but also release them in a controllable manner hold great potential in many applications. To date, although intensive efforts have been made to develop intelligent polymer nanocapsules, how to construct the well-defined core/shell structure with high stability via a straightforward method remains a considerable challenge. In this work, the target novel zwitterionic nanocapsules (ZN_Cs) with a stable hollow structure were synthesized by inverse reversible addition fragmentation transfer (RAFT) miniemulsion interfacial polymerization. The shell gradually grew from the water/oil interface due to the interfacial polymerization, accompanied by the cross-linking of the polyzwitterionic networks, where the core/shell structure could be well-tuned by adjusting the precursor compositions. The resultant ZN_Cs exhibited a salt-/thermo-induced swelling behavior through the phase transition of the external zwitterionic polymers. To further investigate the functions of ZN_Cs, different substances, such as methyl orange and bovine serum albumin (BSA), were encapsulated into the ZN_Cs with a high encapsulation efficiency of 89.3 and 93.6%, respectively. Interestingly, the loaded substances can be controllably released in aqueous solution triggered by salt or temperature variations, and such responsiveness also can be utilized to bounce off the bacteria adhered on target surfaces. We believe that these designed salt- and thermo-responsive intelligent polymer nanocapsules with well-defined core/shell structures and antifouling surfaces should be a promising platform for biomedical and saline related applications.