Antifreezing, Antidrying, and Conductive Hydrogels for Electronic Skin Applications at Ultralow Temperatures

Although conductive hydrogel-based flexible electronic devices have superb flexibility and high conductivities, they tend to malfunction in dry or frigid areas. Herein, an ultralow-temperature tolerant, antidrying, and conductive composite hydrogel is designed for electronic skin applications on the basis of the synergy of double-cross-linked polymer networks, Hofmeister effect, and electrostatic interaction and fabricated by in situ free radical polymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid and acrylic acid in the presence of poly(vinyl alcohol) and conductive MXene sheets, followed by impregnation with LiCl. Thanks to the synergy of LiCl and the charged polar terminal groups of the synthesized polymers, the composite hydrogel can not only bear an ultralow temperature of -80 °C without freezing but also maintain its original mass. Meanwhile, the resultant hydrogel possesses satisfactory self-regeneration ability benefiting from the moisturizing effect of LiCl. The conductive network of MXene sheets greatly improves the ionic conductivity of the hydrogel at low temperatures, exhibiting an ionic conductivity of 1.4 S m-1 at -80 °C. Furthermore, the electronic skin assembled by the multifunctional hydrogel is efficient in monitoring human motions at -80 °C. The antifreezing and antidrying features along with favorable ionic conductivity, high tensile strength, and outstanding flexibility make the composite hydrogel promising for applications in frigid and dry regions.

A Choline-Based Antifreezing Complexing Agent with Selective Compatibility for Zn-Br2 Flow Batteries

The high freezing point of polybromides, charging products, is a significant obstacle to the rapid development of zinc-bromine flow batteries (Zn-Br2 FBs). Here, a choline-based complexing agent (CCA) is constructed to liquefy the polybromides at low temperatures. Depending on quaternary ammonium group, choline can effectively complex with polybromide anions and form dense oil-phase that has excellent antifreezing property. Benefiting from indispensable strong ion-ion interaction, the highly selectively compatible CCA, consisting of choline and N-methyl-N-ethyl-morpholinium salts (CCA-M), can be achieved to further enhance bromine fixing ability. Interestingly, the formed polybromides with CCA-M are able to keep liquid even at -40 °C. The CCA-M endows Zn-Br2 FBs at 40 mA cm-2 with unprecedented long cycle life (over 150 cycles) and high Coulombic efficiency (CE, average ≈98.8%) at -20 °C, but also at room temperature (over 1200 cycles, average CE: ≈94.7%). The CCA shows a promising prospect of application and should be extended to other antifreezing bromine-based energy storage systems.

Digital Light Processing 4D Printing of Poloxamer Micelles for Facile Fabrication of Multifunctional Biocompatible Hydrogels as Tailored Wearable Sensors

The lack of both digital light processing (DLP) compatible and biocompatible photopolymers, along with inappropriate material properties required for wearable sensor applications, substantially hinders the employment of DLP 3D printing in the fabrication of multifunctional hydrogels. Herein, we discovered and implemented a photoreactive poloxamer derivative, Pluronic F-127 diacrylate, which overcomes these limitations and is optimized to achieve DLP 3D printed micelle-based hydrogels with high structural complexity, resolution, and precision. In addition, the dehydrated hydrogels exhibit a shape-memory effect and are conformally attached to the geometry of the detection point after rehydration, which implies the 4D printing characteristic of the fabrication process and is beneficial for the storage and application of the device. The excellent cytocompatibility and in vivo biocompatibility further strengthen the potential application of the poloxamer micelle-based hydrogels as a platform for multifunctional wearable systems. After processing them with a lithium chloride (LiCl) solution, multifunctional conductive ionic hydrogels with antifreezing and antiswelling properties along with good transparency and water retention are easily prepared. As capacitive flexible sensors, the DLP 3D printed micelle-based hydrogel devices exhibit excellent sensitivity, cycling stability, and durability in detecting multimodal deformations. Moreover, the DLP 3D printed conductive hydrogels are successfully applied as real-time human motion and tactile sensors with satisfactory sensing performances even in a -20 °C low-temperature environment.

Secondary Chemical Cross-Linking to Improve Mechanical Properties in a Multifaceted Biocompatible Strain Sensor

A new conductive and transparent organohydrogel is developed with high stretchability, excellent mechanical, self-healing, antifreezing, and adhesive properties. A simple one-pot polymerization method is used to create polyacrylamide cross-linked through N,N'-methylenebis(acrylamide) (MBAA) and divinylbenzene (DVB). The dual chemical cross-linked gel network is complemented by several physical cross-links via hydrogen bonding and π-π interaction. Multiple chemical and physical cross-links are used to construct the gel network that allows toughness (171 kPa), low modulus (≈45 kPa), excellent stretchability (>1100%), and self-healing ability. The use of appropriate proportions of the water/glycerol binary solvent system ensures efficient environment tolerance (-20 to 40 °C). Phytic acid is used as a conductive filler that provides excellent conductivity and contributes to the physical cross-linking. Dopamine is incorporated in the gel matrix, which endows excellent adhesive property of the gel. The organohydrogel-based strain sensors are developed with state-independent properties, highly linear dependence, and excellent antifatigue performance (>100 cycles). Moreover, during the practical wearable sensing tests, human motions can be detected, including speaking, smiling, and joint movement. Additionally, the sensor is biocompatible, indicating the potential applications for the next generation of epidermal sensors.

Facile Preparation of a Photo-Cross-Linked Silk Fibroin-Poly Ionic Liquid Hydrogel with Antifreezing and Ion Conductive Properties

The silk fibroin (SF)/ionic liquid (IL) based hydrogel is a kind of remarkable substrate for flexible devices because of its subzero-temperature elasticity, electrical conductivity, and water retention, although the procedure of the gelation is considered complex and time-consuming. In this work, we introduced an approximate method for the development of novel photo-cross-linked SF/IL hydrogel, that is, SF-IMA/PIL hydrogel via the modification of silk fibroin chain with 2-isocyanatoethyl methacrylate (SF-IMA) in a certain ionic liquid with an unsaturated double bond. The chemical cross-linking between methacrylated SF and IL was triggered by UV light, while the physical cross-linking of the hydrogel was attributed to the β-sheet formation of SF in SF-IMA/IL mixed solution. In addition to being a UV-induced three-dimensional (3D) printable one, the SF-IMA/PIL hydrogel performed significant ionic conductivity between room temperature and -50 °C and water retention within a wide range of relative humidity, which were the featured advantages as the ionic liquid involved. Moreover, the static and dynamic mechanical tests demonstrated that the hydrogel reserved its great elasticity at -50 °C and displayed its stiffness transition temperatures between -100 and -70 °C.

Tough Interfacial Adhesion Enabled Extremely Durable Flexible Supercapacitors

The interfacial void and delamination between the hydrogel electrolyte and flexible electrode caused by the inconformal contact and weak adhesion lead to serious performance degradation of solid-state-sandwiched supercapacitors (SCs) upon repetitive deformation. Herein, we propose a hydrogel polymer electrolyte (HPE) engineering strategy for enhancing the interfacial adhesion (Γ) to achieve extremely durable SCs via the soft, tough, and self-adhesive HPE. Using a self-cross-linked poly(N-hydroxyethyl acrylamide)/H3PO4 (PHEAA/H3PO4) HPE as the model, the interfacial adhesion between HPE and polyaniline (PANI)-modified carbon cloth (CC) electrode (CC/PANI) reaches up to 556 J/m2, leading to excellent durability of electrochemical performance under long-term repetitive deformations. The as-assembled sandwiched SC retains 94.14 and 93.62% of initial capacitance after 180° bending and twisting for 100,000 cycles, respectively. Furthermore, benefiting from the addition of H3PO4, the flexible sandwiched SC displays excellent tolerance to low temperatures and delivers a capacitance retention of 98.03% after 180° bending for 10,000 cycles at -20 °C. This work highlights the importance of interfacial adhesion engineering for the design of extremely deformation-tolerable SCs.

Enhancing Temperature Adaptability of Aqueous Zinc Batteries via Antifreezing Electrolyte and Site-Selective ZnSe-Ag Interface Layer Design

Rechargeable aqueous zinc batteries (RAZBs) represent a sustainable, environmentally benign, cost-efficient energy storage solution for the scaled renewable power system. However, the cycling endurance and temperature adaptability of RAZBs are hindered by practical technological barriers such as the subzero freezing point of aqueous electrolyte, severe cation dissolution of the cathode, and dendrite growth on the Zn anode. Herein, we optimize the hybrid electrolyte formulation of 8 M ZnCl2 in the ethylene glycol-water mixed solvent to reconfigure the hydrogen bonding and [Zn(H2O)1.80(EG)0.23]2+ solvation sheath, which well balances the ionic conductivity and the antifreezing property until -125 °C. As monitored by operando X-ray diffraction, meanwhile, the structural dissolution of the V2O5 cathode upon the dynamic cycling and static idling storage at elevated temperature are effectively restrained. At the anode side, the thermally induced substitution between the Ag2Se overcoating and Zn foil in situ constructs the site-selective, mosaic interface layer, in which the solvophilic ZnSe facilitates the desolvation, while the Ag species provide zincophilic nucleation sites for high-throughput Zn deposition. The synergistic coupling of the antifreezing electrolyte and anode interfacial design enables the wide-temperature-range adaptability of the RAZB prototype (10 μm Zn foil and 1 mAh cm-2 V2O5 cathode), which balances the cycling endurance (92.5% capacity retention rate for 1000 cycles), 84.7% mitigation of the self-discharge rate at 55 °C, as well as the secured cyclability even at -40 °C.

Hazardous Gases-Responsive Photonic Crystals Cryogenic Sensors Based on Antifreezing and Water Retention Hydrogels

Nowadays, the sensing of hazardous gases is urgent for the consideration of public safety and human health, especially in extreme conditions of low temperatures. In this study, a photonic crystals (PhCs) sensor with water retention and antifreezing properties was developed and applied to visual hazardous gases sensing at low temperature, passively. The sensor was prepared by dip-coating with poly(methyl methacrylate) (PMMA) colloidal microspheres followed by embedding in k-carrageenan/polyacrylamide-ethylene glycol (k-CA/PAM-EG) hydrogel. The sensor responded to hazardous gases, including ammonia, toluene, xylene, acetone, methanol, ethanol, and 1-propanol, with a change in the reflection wavelength and visible structural color. At room temperature, the reflection wavelength of the sensor blue-shifted 49 nm in ammonia, and the structural color changed from red to yellow. For low temperatures, the sensor showed great water retention and antifreezing properties even at -57 °C due to the double network. The sensor still had a great response to hazardous gases after freezing at -20 °C for 12 h and testing at 0 °C, and the obtained results were similar to those at room temperature. Based on this excellent stability and visual sensing at low temperature, the sensor demonstrates the potential for detection of hazardous vapors in extreme environments.

One-Step in Situ Synthesis of Tough and Highly Conductive Ionohydrogels with Water-Retentive and Antifreezing Properties

Flexible and conductive gels are promising materials as intelligent and wearable electronics. Herein, through a facile one-step in situ free-radical polymerization, tough VSNPs-PAA-Zr⁴⁺ ionohydrogels with integrated multiple functionalities are prepared, which are dually cross-linked by multivalent vinyl-functionalized silica nanoparticles (VSNPs) and metal coordination between Zr⁴⁺ and the carboxyl groups in PAA chains. The incorporation of Zr⁴⁺ with stable valency during polymerization enables the direct formation of a large number of metal coordination cross-links for adequate energy dissipation, overcoming the inhibition of unstable metal ions on the polymerization process. Meanwhile, VSNPs serve as multivalent cross-linkers and effective stress transfer centers. The obtained VSNPs-PAA-Zr⁴⁺ ionohydrogels show high toughness of up to 25 MJ m^-3 with a high tensile strength of 3010 kPa and a large elongation at break of 1360%, along with reliable adhesive performance. Attributed to use of an IL/water binary solvent, the ionohydrogels possess excellent water-retentive and antifreezing abilities. Moreover, the existence of large quantities of mobile ions endows the VSNPs-PAA-Zr⁴⁺ ionohydrogels with a superior conductivity of 4.77 S m^-1 and a high strain sensitivity with a gauge factor (GF) of 9.04, which are promising materials as intelligent and wearable strain sensors.

Multifunctional MXene Conductive Zwitterionic Hydrogel for Flexible Wearable Sensors and Arrays

Conductive hydrogels have good prospects in the fields of flexible electronic devices and artificial intelligence due to their biocompatibility, durability, and functional diversity. However, the process of hydrogel polymerization is time-consuming and energy-consuming, and freezing at zero temperature is inevitable, which seriously hinders its applications and working life. Herein, zwitterionic conductive hydrogels with self-adhesive and antifreeze properties were prepared in one minute by introducing two-dimensional (2D) MXene nanosheets into the autocatalytically enhanced system composed of tannic acid-modified cellulose nanofibers and zinc chloride. The system has strong environmental applicability (-60 to 40 °C), good stretchability (ductility ≈ 980%), durable adhesion (even after 30 days of exposure to air), and strong electrical conductivity (20 °C, 30 mS cm^-1). By virtue of these advantages, the prepared zwitterionic hydrogels can be developed into flexible strain sensors to monitor large human movements and subtle physiological signals over a wide temperature range and to capture signals from handwriting and voice recognition. In addition, multiple flexible sensors can be assembled into a three-dimensional (3D) array, which can detect the magnitude and spatial distribution of strain or force. These results demonstrate that the prepared zwitterionic hydrogels have promising applications in the fields of medical monitoring and artificial intelligence.

Highly Stretchable, Self-Adhesive, Antidrying Ionic Conductive Organohydrogels for Strain Sensors

As flexible wearable devices, hydrogel sensors have attracted extensive attention in the field of soft electronics. However, the application or long-term stability of conventional hydrogels at extreme temperatures remains a challenge due to the presence of water. Antifreezing and antidrying ionic conductive organohydrogels were prepared using cellulose nanocrystals and gelatin as raw materials, and the hydrogels were prepared in a water/glycerol binary solvent by a one-pot method. The prepared hydrogels were characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. The mechanical properties, electrical conductivity, and sensing properties of the hydrogels were studied by means of a universal material testing machine and LCR digital bridge. The results show that the ionic conductive hydrogel exhibits high stretchability (elongation at break, 584.35%) and firmness (up to 0.16 MPa). As the binary solvent easily forms strong hydrogen bonds with water molecules, experiments show that the organohydrogels exhibit excellent freezing and drying (7 days). The organohydrogels maintain conductivity and stable sensitivity at a temperature range (-50 °C-50 °C) and after long-term storage (7 days). Moreover, the organohydrogel-based wearable sensors with a gauge factor of 6.47 (strain, 0-400%) could detect human motions. Therefore, multifunctional organohydrogel wearable sensors with antifreezing and antidrying properties have promising potential for human body monitoring under a broad range of environmental conditions.

Muscle Contraction-Inspired Tough Hydrogels

In many animals, tough skeletal muscle contraction occurs, producing a strong force through myofilaments attaching to and sliding on fibrous actin filaments. In contrast, the strength of typical synthetic hydrogels is facilitated mainly by polymeric chains. We propose a strategy for developing strong and tough hydrogels in which the side groups on polymeric chains strongly interact with dispersing medium. The hydrogels are fabricated with a polyacrylamide-alginate double network in a choline chloride saturated solution. The hydrogels are not only highly transparent, tough, fatigue-resistant, self-recovering, self-healing, and adhesive but also water-retentive, antifreezing, and conductive. The hydrogels are strengthened by hydrogen bonds in dispersing medium with a clathrate framework structure. This work may inspire the development of tough and conductive gels for applications of e-skins, soft robots, and intelligent devices.

Rapid Preparation of Antifreezing Conductive Hydrogels for Flexible Strain Sensors and Supercapacitors

Conductive hydrogels have shown great promise in flexible electronics, but their practical applications may be impeded by the time-consuming and energy-consuming polymerization process. We proposed a sodium lignosulfonate-Fe (SLS-Fe) strategy to address this challenge and took advantage of carboxymethyl cellulose (CMC) and poly(acrylic acid) to prepare the CMC/PAA/Fe3+/LiCl interpenetrating conductive hydrogels with good self-healing properties, antifreezing properties, and a 6-fold increase in conductivity in this study. The hydrogel-based flexible strain sensors demonstrated a broad detection range (400%), high sensitivity (GF = 6.19 at 200-400%), and human motion detection capability. The hydrogel-based supercapacitor exhibited a single-electrode specific capacitance of 122.36 F g-1 which successfully powered LEDs. Furthermore, the supercapacitor showed a single-electrode specific capacitance of 83.16 F g-1 at -23 °C (68% of the one exhibited at 25 °C). Therefore, the multifunctional performance of the CMC/PAA/Fe3+/LiCl hydrogel is anticipated to play an exemplary role in a new generation of flexible electronics.

Chestnut-Tannin-Crosslinked, Antibacterial, Antifreezing, Conductive Organohydrogel as a Strain Sensor for Motion Monitoring, Flexible Keyboards, and Velocity Monitoring

Flexible sensing devices (FSDs) fabricated using conductive hydrogels have attracted researchers' extensive enthusiasm in recent years due to their versatility. Considering the complexity of their application environments, the integration of various functional characteristics (e.g., excellent mechanical, antibacterial, and antifreezing properties) is an important guarantee for FSDs to stably perform their applications in different environments. Herein, we developed a multifunctional conductive polyvinyl alcohol (PVA) organohydrogel PVA-CT-Ag-Al-Gly (PCAAG) by using a green, natural, and cheap biomass, chestnut tannin (CT), as a crosslinking agent, nano-silver particles (AgNPs) as an antimicrobial agent, aluminum trichloride (AlCl₃) as a conducting medium, and the mixed water-glycerol as the solvent system. In this organohydrogel system, CT acted not only as the reducing and stabilizing agent for the preparation of antibacterial AgNPs but also as the crosslinking agent owing to its strong multiple hydrogen bonding interactions with PVA, realizing its multifunctional application. The PCAAG organohydrogel possessed outstanding physical and mechanical properties (350.54% of the maximum fracture strain and 1.55 MPa of the maximum tensile strength), considerable bacteriostatic effects against both Escherichia coli and Staphylococcus aureus, and excellent freeze resistance (it could function normally at -20 °C). The motion-monitoring sensor based on the PCAAG organohydrogel exhibited excellent specificity recognition for both large-amplitude (e.g., elbow bending, wrist bending, finger bending, running and walking, etc.) and small-amplitude (frowning and swallowing) human movements. The flexible keyboard constructed by using the PCAAG organohydrogel could easily achieve the transformation between digital signals and electrical signals, and the signal output had both specificity and stability. The velocity-monitoring sensor fabricated by using the PCAAG organohydrogel could accurately measure the speed of the object movement (less than 3% of relative error). In short, the present PCAAG organohydrogel solves the problems of the single application environment and a few application scenarios of traditional conductive hydrogels and possesses remarkable application potential as a multifunctional FSD in many fields such as artificial intelligence, sport management, soft robots, and human-computer interface.

Construction of Carboxymethyl Chitosan Hydrogel with Multiple Cross-linking Networks for Electronic Devices at Low Temperature

On the basis of the original hydrogen bonding interaction and physical entanglement, covalent cross-linking and ionic cross-linking were additionally introduced to construct a carboxymethyl chitosan/allyl glycidyl ether conductive hydrogel (CCH) through a one pot method by a graft reaction, an addition reaction, and simple immersion, successively. The multiple cross-linking networks significantly increased the strength of CCHs and endowed them with ionic conductivity and an antifreezing property at -40 °C, which showed stable, durable, and reversible sensitivity to finger bending activity at subzero temperature. The CCHs could even be assembled into a triboelectric nanogenerator (TENG) to provide electric energy, which demonstrated stability against temperature variation, multiple drawing, long-term storage, or large quantities of contact-separation motion cycles. CCH-TENG can also be used as a tactile sensor within the pressure range from 0.4 kPa to higher than 8000 kPa. This work provided a simple route to fabricate antifreezing conductive hydrogels based on carboxymethyl chitosan and to find potential applications in soft sensor devices under a low temperature environment.

Highly Sensitive Zwitterionic Hydrogel Sensor for Motion and Pulse Detection with Water Retention, Adhesive, Antifreezing, and Self-Healing Properties

The design and synthesis of conductive hydrogels with antifreezing, long-term stable, highly sensitive, self-healing, and reusable is a critical procedure to enable applications in flexible electronics, medical monitoring, soft robotics, etc. Herein, a novel zwitterionic composite hydrogel possessing antifreezing, fast self-healing performance, water retention, and adhesion was synthesized via a simple one-pot method. LiCl, as an electrolyte and antifreeze, was promoted to dissociate under the electrostatic interaction with zwitterions, resulting in the composite hydrogels with high electrical conductivity (7.95 S/m) and excellent antifreeze ability (-45.3 °C). Meanwhile, the composite hydrogels could maintain 97% of the initial water content after exposed to air (25 °C, 55% RH) for 1 week due to the presence of salt ions. Moreover, the active groups of zwitterions could form conformal adhesion between the composite hydrogels and skin, which was particularly crucial for the stable signal output of the sensor. The dynamic borate ester bonds, active group of zwitterions, and the hydrogen bond between different components could achieve rapid self-healing (2 h, self-healing efficiency to 97%) without any external intervention. Notably, the developed P_BAS-Li (poly(vinyl alcohol) _Borax/acrylamide/zwitterionic-LiCl) hydrogel not only succeeded in sensitively detecting human motions but also could precisely captured handwritings signals and subtle pulse waves on the neck and wrist. The above findings demonstrated the great potential of P_BAS-Li hydrogels in the field of flexible electronic devices.

A Silk Protein-Based Eutectogel as a Freeze-Resistant and Flexible Electrolyte for Zn-Ion Hybrid Supercapacitors

A eutectogel (ETG) based on immobilizing a zinc salt deep eutectic solvent (DES) in a silk protein backbone is prepared by a coagulating bath method as a solid electrolyte for Zn-ion hybrid supercapacitors (ZHSCs). The Zn salt DES is composed by ethylene glycol (EG), urea, choline chloride (ChCl), and zinc chloride (ZnCl₂) with a molar ratio of 6:10:3:3. A strong bonding of the DES liquid to the silk protein backbone is formed between protein macromolecules and the DES due to plenty of hydrogen bonds in both materials. The as-prepared ETG membrane is dense and has no obvious void defects, which possesses a fracture strength of 7.58 MPa and environmental stability. As a solid electrolyte, the ETG membrane exhibits a higher Zn²⁺ transference number of about 0.60 and a high ionic conductivity (12.31 mS cm^-1 at room temperature and 3.63 mS cm^-1 at -20 °C). A ZHSC (Zn∥ETG∥C) with the silk protein-based ETG electrolyte is assembled by Zn and active carbon as the anode and the cathode, respectively, which delivers a specific capacitance of 342.8 F g^-1 at a current density of 0.2 A g^-1 and maintains excellent cycling stability with 80% capacitance retention after 20,000 cycles at a high current rate (5 A g^-1) at room temperature. Moreover, the Zn∥ETG∥C device can safely work under a lower temperature of about -18 °C and damaging situations, such as folding states and even cutting tests. The interface evolutions between the Zn anode and the ETG electrolyte are explored, and it was found that a ZnCO₃/Zn(CH₂OCO₂)₂ solid electrolyte interphase is in situ formed on the Zn anode, which can inhibit the growth of Zn dendrites. This work provides a new way to fabricate advanced electrolytes for applications in Zn-ion hybrid supercapacitors.

Tough Adhesive, Antifreezing, and Antidrying Natural Globulin-Based Organohydrogels for Strain Sensors

Hydrogels are often used to fabricate strain sensors; however, they also suffer from freezing at low temperatures and become dry during long-time storage. Encapsulation of hydrogels with elastomers is one of the methods to solve these problems although the adhesion between hydrogels and elastomers is usually low. In this work, using bovine serum protein (BSA) as the natural globulin model and glycerol/H₂O as the mixture solvent, BSA/polyacrylamide organohydrogels (BSA/PAAm OHGs) were prepared by a facile photopolymerization approach. At the optimal condition, BSA/PAAm OHG demonstrated not only high toughness but also tough adhesion properties, which could strongly adhere to various substrates, such as glass, metals, rigid polymeric materials (even poly(tetrafluoroethylene), i.e., PTFE), and soft elastomers. Moreover, BSA/PAAm OHG was flexible and showed tough adhesion at -20 °C. The toughening mechanism and the adhesive mechanism were proposed. On being encapsulated by poly(dimethylsiloxane) (PDMS), it illustrated good antidrying performance. After introducing a conductive filler, the encapsulated BSA/PAAm OHG could be used as a strain sensor to detect human motions. This work provides a better understanding of the adhesive mechanism of natural protein-based organohydrogels.

Highly Adhesive, Stretchable, and Antifreezing Hydrogel with Excellent Mechanical Properties for Sensitive Motion Sensors and Temperature-/Humidity-Driven Actuators

Conductive hydrogels as flexible wearable devices have attracted considerable attention due to their mechanical flexibility and intelligent sensing. How to endow more and better performance, such as high self-adhesion, stretchability, and wide application temperature range for traditional hydrogels and flexible sensors is a challenge. Herein, a stretchable, self-adhesive, and antifreezing conductive hydrogel with multiple networks and excellent mechanical properties was prepared by a two-step method for its application in sensitive motion sensors and temperature-/humidity-driven actuators. First, quaternary chitosan (QCS) was introduced into the network of an acrylamide (AM) and 1-vinyl imidazole (VI) copolymer initiated by UV-photoinitiated radical polymerization. Then, the double-network hydrogel was immersed in a FeCl₃ solution to fabricate the P(AAm-co-VI)/QCS-Fe³⁺ ionic hydrogel with multiple physical networks. The properties of the hydrogel were controllable and adjustable. The toughness of the ionic hydrogel could reach up to 654.4 kJ/m³, the fracture strength could reach 253.1 kPa, and the compressive strength reached 8.4 MPa at an 80% compression strain. The multiple physical networks improved the mechanical properties and the quick resilience of the hydrogel. A large amount of FeCl₃ in the network greatly enhanced the ionic conductivity. Meanwhile, hydrogen bonds with water molecules inhibit the formation of ice crystals between zero water molecules and enhance the freezing resistance of P(Aam-co-VI)/QCS hydrogels. The active group on the QCS chain provided adhesiveness to various substrates for hydrogels. The P(AAm-co-VI)/QCS-Fe³⁺ hydrogel-based sensor showed high sensitivity, which can detect human movement and pulse, with a gauge factor of 2.37. Finally, due to the different dehydration rates of the P(AAm-co-VI)/QCS-Fe³⁺ and P(AAm-co-VI)/QCS hydrogel, a double-layer temperature/humidity-driven actuator was fabricated, expanding the application of conductive hydrogels.

High-Sensitivity and Extreme Environment-Resistant Sensors Based on PEDOT:PSS@PVA Hydrogel Fibers for Physiological Monitoring

The rapid development of flexible electronic devices has caused a boom in researching flexible sensors based on hydrogels, but most of the flexible sensors can only work at room temperature, and they are difficult to adapt to extremely cold or dry environments. Here, the flexible hydrogel fibers (PEDOT:PSS@PVA) with excellent resistance to extreme environments have been prepared by adding glycerin (GL) to the mixture of poly(vinyl alcohol) (PVA) and poly 3,4-dioxyethylene thiophene:polystyrene sulfonic acid (PEDOT:PSS) because GL molecules can form dynamic hydrogen bonds with an elastic matrix of PVA molecules. It is found that the prepared sensor exhibits very good flexibility and mechanical strength, and the ultimate tensile strength can reach up to 13.76 MPa when the elongation at break is 519.9%. Furthermore, the hydrogel fibers possess excellent water retention performance and low-temperature resistance. After being placed in the atmospheric environment for 1 year, the sensor still shows good flexibility. At a low temperature of -60 °C, the sensor can stably endure 1000 repeated stretches and shrinks (10% elongation). In addition to the response to a large strain, this fiber sensor can also detect extremely small strains as low as 0.01%. It is proved that complex human movements such as knuckle bending, vocalization, pulse, and others can be monitored perfectly by this fiber sensor. The above results mean that the PEDOT:PSS@PVA fiber sensor has great application prospects in physiological monitoring.

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.

Antifreezing and Nondrying Sensors of Ionic Hydrogels with a Double-Layer Structure for Highly Sensitive Motion Monitoring

Freezing and dehydration together with interfacial failure are capable of causing the functional reduction of hydrogels for sensing applications. Herein, we develop a multifunctional bilayer that consists of a mussel-inspired adhesive layer and a functionally ionic layer that is composed of sodium p-styrene sulfonate (SSS) and an ionic liquid of [BMIM]Cl. The adhesive layer enables the strong adhesion of the bilayer to the surface of the skin. The introduction of ionic elements of SSS-[BMIM]Cl not only provides the bilayer with sensing adaptability in a wide temperature range of -25 to 75 °C, but also endows it with elastic, stretchable, self-healing, and conductive features. These mechanical properties are utilized to assemble a wearable sensor that has unprecedented sensitivity and reusability in monitoring human motions, including stretching, pulsing, frowning, and speaking. It is thus expected that the concept in this work would provide a promising route to design soft sensing devices that can work in a wide temperature range.

Supramolecular Polymeric Pressure-Sensitive Adhesive That Can Be Directly Operated at Low Temperatures

Low-temperature adhesion is ubiquitous in daily life and industry. However, most supramolecular adhesives are thermoplastic materials that require heating during the adhesion. Herein, a supramolecular approach is used to construct unique pressure-sensitive adhesives (PSAs) that can be directly operated at low temperatures (-60 °C). Supramolecular polymerization between phytic acid (PA) and water (H) endows poly(PA-H)s with excellent mechanical properties and low temperature adhesion capacity. Poly(PA-H)s can easily be processed into PSA tapes, pastes, and particles. Poly(PA-H)s were directly adhered to various surfaces by pressing at low temperatures (0 to -60 °C). No heating or high-temperature-induced solid-liquid transition was required for the low-temperature adhesion of poly(PA-H)s. With the help of structural water units in supramolecular polymers, poly(PA-H)s showed strong, stable, and organic solvent resistant adhesion performances at low temperatures, with adhesion strength of up to 3.61 MPa at -60 °C.

Environment-Resistant Organohydrogel-Based Sensor Enables Highly Sensitive Strain, Temperature, and Humidity Responses

Conductive hydrogels have been extensively used in wearable skin sensors owing to their outstanding flexibility, tissuelike compliance, and biocompatibility. However, the dehydration and embrittlement of hydrogels can result in sensitivity loss or even invalidation, restraining their wearable applications in external environments, especially at low temperatures and in arid environments. Herein, an environment-resistant organohydrogel is developed for multifunctional sensors. A double-network organohydrogel based on hyaluronic acid and poly(acrylic acid-co-acrylamide) is developed, and glycerol is introduced into the organohydrogel network via a solvent displacement strategy. Owing to the water-locking effects of glycerol and tough polymeric backbone, the resultant organohydrogel not only exhibits stable tensibility but also maintains excellent flexibility and stable conductivity with the environment-resistant properties, including freezing resistance against -30 °C and moisture retention at 4% relative humidity in a high temperature of 60 °C. Moreover, a series of organohydrogel-based sensors and an array device are developed to achieve highly sensitive strain, temperature, and humidity responses and exhibit a high gauge factor of 10.79 in the strain-sensitive test. This work develops a universal ionic skin based on organohydrogels to be applied to wearable sensors for health monitoring.

Smart Hydrogel Sensors with Antifreezing, Antifouling Properties for Wound Healing

Flexible electronic devices with biological therapeutic and sensing properties are one of the current research directions. Here, a multifunctional hydrogel for stress sensing and wound healing was prepared by a simple one-pot method and a solution replacement method. Among them, zwitterionic polymers promote wound healing by promoting the polarization of M2 macrophages, collagen deposition, and blood vessel formation. Glycerin can significantly improve the resilience and frost resistance of the hydrogel, ensuring that a sensor made using the hydrogel can work normally in a cold environment. In addition, zwitterionic polymers are highly biocompatible, providing excellent antibacterial adhesion to aid the wound healing process, and good electrical conductivity enhances sensing sensitivity and stability. Based on these properties, multifunctional hydrogels could detect human vital activities while promoting wound healing, providing new ideas for the fields of diagnosis and wound dressing.

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.

Highly Conductive and Mechanically Robust Cellulose Nanocomposite Hydrogels with Antifreezing and Antidehydration Performances for Flexible Humidity Sensors

Conductive hydrogels are emerging as an appealing material platform for flexible electronic devices owing to their attractive mechanical flexibility and conductive properties. However, the conventional water-based conductive hydrogels tend to inevitably freeze at subzero temperature and suffer from continuous water evaporation under ambient conditions, leading to a decrease in their electrical conductivities and mechanical properties. Thus, it is extremely necessary, but generally challenging, to create an antifreezing and antidehydration conductive gel for maintaining high and stable performances in terms of electrical conductivity and mechanical properties. Herein, we fabricated a cellulose nanofibril (CNF)-reinforced and highly ion-conductive organogel featuring excellent antifreezing and antidehydration performances by immersing it in the CaCl₂/sorbitol solution for solvent displacement. The incorporation of a rigid CNF serving as a dynamic connected bridge provided a hierarchical honeycomb-like cellular structure for the obtained CS-nanocomposite (NC) organogel networks, facilitating significant mechanical reinforcement. The synergy effects of sorbitol and CaCl₂ allowed high-performance integration with excellent antifreezing tolerance, antidehydration ability, and ionic conductivity. Strong hydrogen bonds were formed between water molecules and sorbitol molecules to impede the formation of ice crystals and water evaporation, thereby imparting the CS-NC organogels with extreme-temperature tolerance as low as -50 °C and pre-eminent antidehydration performance with over 90% weight retention. Furthermore, this CS-NC organogel exhibited high humidity sensitivity in a wide humidity detection range (23∼97% relative humidity) because of the ready formation of hydrogen bonds between water molecules and numerous hydrophilic groups in the binary solvent and elaborated polymer chains, which can be assembled as a stretchable humidity sensor to monitor human respiration with a fast response. This work provides a new prospect for fabricating intrinsically stretchable and high-performance humidity sensors using cellulose-based humidity-responsive materials for the emerging wearable applications.

Engineering Self-Adhesive Polyzwitterionic Hydrogel Electrolytes for Flexible Zinc-Ion Hybrid Capacitors with Superior Low-Temperature Adaptability

Flexible zinc-ion hybrid capacitors (ZIHCs) based on hydrogel electrolytes are an up-and-coming and highly promising candidate for potential large-scale energy storage due to their combined complementary advantages of zinc batteries and capacitors. However, the freezing induces a sharp drop in conductivity and mechanical property with tremendous compromise of the interfacial adhesion, thereby severely impeding the low-temperature application of such flexible ZIHCs. To achieve the flexible ZIHCs with excellent low-temperature adaptability, an antifreezing and self-adhesive polyzwitterionic hydrogel electrolyte (PZHE) is engineered via a self-catalytic nano-reinforced strategy, affording unparalleled conductivity and robust interfacial adhesion, together with superhigh mechanical strength over a broad temperature ranging from 25 to -60 °C. Meanwhile, the water-in-salt-type PZHE filled with ZnCl₂ can provide ion migration channels to enhance the reversibility of Zn metal electrodes, thus greatly reducing side reactions and extending the cycling life. With distinctive integrated merits of the water-in-salt type PZHE, the as-built ZIHCs deliver a high-level energy density of 80.5 Wh kg^-1, a desired specific capacity of 81.5 mAh g^-1, along with a long-duration cycling lifespan (100 000 cycles) with 84.6% capacity retention at -40 °C, even outperforming the state-of-the-art ZIHCs at room temperature. More encouragingly, the extraordinary temperature-adaptability for both electrochemical and mechanical performance under severe mechanical challenges is achieved for the flexible ZIHCs at extremely low temperature. Noticeably, the ZIHC is also capable of operating in an ice-water bath and vacuum. It is believed that this strategy makes contributions to inspire the design and application of high-performance PZHEs in fields of flexible and wearable electronics that can work in extremely cold environments.

Superelastic, Antifreezing, Antidrying, and Conductive Organohydrogels for Wearable Strain Sensors

Sensors based on conductive hydrogels have received extensive attention in various fields, such as artificial intelligence, electronic skin, and health monitoring. However, the poor resilience and fatigue resistance, icing, and water loss of traditional hydrogels greatly limit their application. Herein, an ionic conductive organohydrogel (PAC-Zn) was prepared for the first time by copolymerization of cardanol and acrylic acid in water/1,3-butanediol as a binary solvent system. A very small amount of cardanol (1% cardanol of total monomers) could not only significantly improve the tensile strength (∼4 times) and toughness (∼3 times) of PAA but also improve its extensibility. Due to the presence of 1,3-butanediol, PAC-Zn showed outstanding tolerance for freezing (-45 °C) and drying (over 85% moisture retention after 15 days of storage in a 37 °C oven). Compared with ethylene glycol and glycerol as antifreeze agents used in organohydrogels, the addition of 1,3-butanediol endowed the organohydrogel with not only similar frost resistance but also better mechanical performance. Besides, PAC-Zn exhibited fast resilience (almost no hysteresis loop) and excellent antifatigue ability. More importantly, a PAC-Zn organohydrogel-based sensor could detect human motion in real time (wrist, elbow, finger, and knee joints), revealing its fast response, good sensitivity, and stable electromechanical repeatability. In conclusion, the multifunctional PAC-Zn organohydrogel is expected to become a potential and promising candidate in the field of strain sensors under a broad range of environmental temperatures.

A General Crosslinker Strategy to Realize Intrinsic Frozen Resistance of Hydrogels

Development and understanding of antifreezing materials are fundamentally and practically important for materials design and delivery. However, almost all of antifreezing materials are either organic/icephobic materials containing no water or hydrophilic hydrogels containing antifreezing additives. Here, a general crosslinking strategy to fabricate a family of EGINA-crosslinked double-network hydrogels with intrinsic, built-in antifreezing and mechanical properties, but without any antifreezing additives is proposed and demonstrated. The resultant hydrogels, despite large structural and compositional variations of hydrophilies, electrolytes, zwitterions, and macromolecules of polymer chains, achieved strong antifreezing and mechanical properties in different environments including solution state, gel state, and hydrogel/solid interfaces. Such general antifreezing property of EGINA-crosslinked hydrogels, regardless network compositions, is likely stemmed from their highly hydrophilic and tightly crosslinked DN structures for inducing strong water-network bindings to prevent ice crystal formation from free waters in hydrogel networks. EGINA-crosslinked hydrogels can also serve as a key component to be fabricated into smart windows with high optical transmittance and supercapacitors with excellent electrochemical stability at subzero temperatures. This work provides a simple, blueprint antifreezing design concept and a family of antifreezing hydrogels for the better understanding of the composite-structure-property relationship of antifreezing materials and the fundamentals of confined water in wet soft materials.

Ultra-Sensitive and Ultra-Stretchable Strain Sensors Based on Emulsion Gels with Broad Operating Temperature

Hydrogels with mechanical elasticity and conductivity are ideal materials in wearable devices. However, traditional hydrogels are fragile upon mechanical loading and lose functions in climate change because the internal water undergoes freeze and dehydration. Herein, we synthesize stable emulsions at high and low temperatures by introducing glycerol into the W/W emulsions. Then the high-stable emulsions are used as templates to produce the freestanding emulsion gels with enhanced mechanical strength and conductivity. The introduction of glycerol endows emulsions and emulsion gels with high and low temperature resistance (-20 to 90 °C). The fabricated strain sensors based on emulsion gels show high sensitivity (gauge factor=6.240), high stretchability (1081 %), fatigue resistance, self-healing and adhesion properties, realizing the repeatable and accurate detection of various human motions. These high-performance and eco-friendly emulsion gels can be promising candidates for next-generation artificial skin and human-machine interface.

Environmentally Compatible Wearable Electronics Based on Ionically Conductive Organohydrogels for Health Monitoring with Thermal Compatibility, Anti-Dehydration, and Underwater Adhesion

Hydrogel-based electronics have found widespread applications in soft sensing and health monitoring because of their remarkable biocompatibility and mechanical features similar to human skin. However, they are subjected to potential challenges like structural failure, functional degradation, and device delamination in practical applications, especially facing extreme environmental conditions (e.g., abnormal temperature and humidity). To address these, ionically conductive organohydrogel-based soft electronics are developed, which can perform at subzero and elevated temperatures (thermal compatibility) as well as at dehydrated and hydrated environments (hydration compatibility) for extended applications. More specifically, gelatin/poly(acrylic acid-N-hydrosuccinimide ester) (PAA-NHS ester)-based ionic-conductive organohydrogel is synthesized. By introducing a glycerol-water binary solvent system, the gel can maintain mechanical softness in a wide temperature range (from -80 to 60 °C). Besides, excellent conductivity is achieved under various conditions by soaking the gel into lithium chloride anhydrous (LiCl) solution. Strong adhesion with skin, even under water, can be realized by covalent bonds between NHS ester from gel and amino groups from human skin. The excellent performances of LiCl-loaded PAA-based organohydrogel (L-PAA-OH)-based electronics are further demonstrated under freezing and high temperatures as well as underwater conditions, unveiling their promising prospects in wearable health monitoring in various conditions.

Adhesive, Stretchable, and Transparent Organohydrogels for Antifreezing, Antidrying, and Sensitive Ionic Skins

As a flexible wearable device, hydrogel-based sensors have attracted widespread attention in soft electronics. However, the application of traditional hydrogels at extreme temperatures or for a long-term stability still remain a challenge because of the existence of water. Herein, we reported an antifreezing and antidrying organohydrogel with high transparency (over 85% transmittance), high stretchability (up to 1200%), and robust adhesiveness to various substrates, which consist of polyacrylic acid, gelatin, AlCl³⁺, and tannic acid in a water/glycerin binary solvent as the dispersion medium. As the binary solvent easily forms strong hydrogen bonds with water molecules, organohydrogels exhibited excellent tolerance for drying and freezing. The organohydrogels maintained conductivity, adhesion, and stable sensitivity after a long-term storage or at subzero temperature (-14 °C). Moreover, the organohydrogel-based wearable sensors with a gauge factor of 2.5 (strain, 0-100%) could detect both large-scale movements and subtle motions. Therefore, the multifunctional organohydrogel-wearable sensors with antifreezing and antidrying properties have promising potential for human-machine interfaces and healthcare monitoring under a broad range of environmental conditions.

Extreme Temperature-Tolerant Conductive Gel with Antibacterial Activity for Flexible Dual-Response Sensors

Flexible sensors based on conductive hydrogel show great potential in electronic skin and human-machine interface. However, pure water in hydrogel inevitably freezes or rapidly evaporates under extreme temperatures, leading to inadequate fulfillment of sensor performances. Herein, a well-designed strategy is reported for fabricating extreme temperature-tolerant gel-based sensors. By immersing a gelatin/polyacrylamide (PAAm)-clay composite (GC) hydrogel into a ZnCl₂/water/glycerol system, a phase-transition-tunable gel (PTTGC gel) is obtained with outstanding antifreezing (-82 °C) and long-lasting moisture (70 °C, more than 40 days) properties. Meanwhile, the gel also presents good antibacterial activity and biocompatibility attributing to Zn²⁺ and gelatin, respectively. Then, a dual-response sensor with a wide operating temperature (-60 to 60 °C) is proposed, presenting high stress and temperature sensitivities and long-term stability. The sensor will meet the needs of the human-machine interface for scientific investigation and data monitoring in polar, desert, etc.

Bioinspired Tough Organohydrogel Dynamic Interfaces Enabled Subzero Temperature Antifrosting, Deicing, and Antiadhesion

Icing of water (moisture) at subzero temperatures with different length scales is harmful to a variety of applications spanning from large-scale aircraft to small camera lens. Existing strategies relying on controlling the surface structure and material are encumbered with shortcomings of short frost delay time, poor durability, and difficulty in large-scale production. Inspired from the mucus secretion of mollusks, we introduce organohydrogel dynamic interfaces that can perform dynamic and reversible exchange of the cryoprotectant and water at the interface, resulting in exceptional antifrosting, antiadhesion, and deicing properties with long-term durability. The replenishable coating shows superlubrication to the surface ice with a sliding angle up to 1.9 ± 0.4^o and a frost delay time up to 970 ± 31 min in the presence of water spray (99% relative humidity) at subzero temperatures. The strategy offers a reliable and scalable solution to prevent engineering surfaces, i.e., aircraft, pavement, bridge, and other public facilities, from icing/frosting and ice adhesion, even under extreme cold environments.

Ultrastretchable, Tough, Antifreezing, and Conductive Cellulose Hydrogel for Wearable Strain Sensor

Conductive hydrogels have shown great potential in the field of flexible strain sensors. However, their application is greatly limited due to the low conductivity and poor mechanical properties at subzero temperatures. Herein, an ultrastretchable, tough, antifreezing, and conductive cellulose hydrogel was fabricated by grafting acrylonitrile and acrylamide copolymers onto the cellulose chains in the presence of zinc chloride using ceric ammonium nitrate as the initiator. The resulting hydrogel exhibited ultrastretchability (1730%), excellent tensile strength (160 kPa), high elasticity (90%), good toughness (1074.7 kJ/m³), and fatigue resistance property due to the existence of dipole-dipole and multiple hydrogen-bonding interactions on the hydrogel network. In addition, the introduced zinc chloride endowed the cellulose-based hydrogel with remarkable electric conductivity (1.54 S/m) and excellent antifreezing performance (-33 °C). Finally, the hydrogel showed high sensitivity and stability to monitor human activities. In summary, this work presented a facile strategy to construct conductive hydrogel with excellent antifreezing and mechanical properties simultaneously, which showed great potential for wearable strain sensors.

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.

Multifunctional Poly(vinyl alcohol) Nanocomposite Organohydrogel for Flexible Strain and Temperature Sensor

Hydrogels are important for stretchable and wearable multifunctional sensors, but their application is limited by their low mechanical strength and poor long-term stability. Herein, a conductive organohydrogel with a 3D honeycomb structure was prepared by integrating carbon nanotubes (CNTs) and carbon black (CB) into a poly(vinyl alcohol)/glycerol (PVA/Gly) organohydrogel. Such a nanocomposite organohydrogel is built on a physical cross-linking network formed by the hydrogen bonds among PVA, glycerol, and water. CNTs and CB had an add-in synergistic impact on the mechanical and electrical performances of the PVA/Gly organohydrogel because of the distinct aspect ratios and geometric shapes. The prepared organohydrogel integrated with a tensile strength of 4.8 MPa, a toughness of 15.93 MJ m^-3, and flexibility with an elongation at break up to 640%. The organohydrogels also showed good antifreezing feature, long-term moisture retention, self-healing, and thermoplasticity. Sensors designed from these organohydrogels displayed high stretching sensitivity to tensile strain and temperature, with a gauge factor of 2.1 within a relatively broad strain range (up to ∼600% strain), a temperature coefficient of resistance of -0.935%·°C^-1, and long-term durability. The sensors could detect full-range human physiological signals and respond to the change in temperature, which are highly desired for multifunctional wearable electronic devices.

Self-Deicing Electrolyte Hydrogel Surfaces with Pa-level Ice Adhesion and Durable Antifreezing/Antifrost Performance

Despite the remarkable advances in mitigating ice formation and accretion, however, no engineered anti-icing surfaces today can durably prevent frost formation, droplet freezing, and ice accretion in an economical and ecofriendly way. Herein, sustainable and low-cost electrolyte hydrogel (EH) surfaces are developed by infusing salted water into a hydrogel matrix for avoiding icing. The EH surfaces can both prevent ice/frost formation for an extremely long time and reduce ice adhesion strength to ultralow value (Pa-level) at a tunable temperature window down to -48.4 °C. Furthermore, ice can self-remove from the tilted EH surface within 10 s at -10 °C by self-gravity. As demonstrated by both molecular dynamic simulations and experiments, these extreme performances are attributed to the diffusion of ions to the interface between EH and ice. The sustainable anti-icing properties of EH can be maintained by replenishing in real-time with available ion sources, indicating the promising applications in offshore platforms and ships.

An Antifreezing/Antiheating Hydrogel Containing Catechol Derivative Urushiol for Strong Wet Adhesion to Various Substrates

Tough adhesive hydrogels that can tightly bond to wet tissue/polymer/ceramic/metal surfaces have great potentials in various fields. However, conventional adhesive hydrogels usually show short-term and nonreversible adhesion ability, as the water component in a hydrogel readily transforms to vapor or ice in response to fluctuation of environment temperature, hindering their applications in extreme conditions such as in freezing Arctic and roasting Africa. For the first time, urushiol (UH), a natural catechol derivative with a long alkyl side chain, is used as a starting material to copolymerize with acrylamide for fabricating adhesive hydrogels, which contain hydrophobic/hydrophilic moieties, antifreezing agent, and adhesive catechol groups. The antifreezer/moisturizer glycerol/water binary solvent dispersed in the hydrogel endows it with antifreezing/antiheating property. The hydrophobic association and π-π interaction from UH moieties of the copolymer greatly improve its mechanical strength (tensile stress: ∼0.12 MPa with strain of ∼1100%, toughness: ∼72 kJ/m³, compression stress: ∼6.72 MPa at strain of 90%). The hydrogel can strongly adhere to various dry/wet biological/polymeric/ceramic/metallic substrates at temperatures ranging from -45 to 50 °C. Under ambient conditions, its adhesion force to porcine skin, glass, and tinplate may reach up to 160, 425, and 275 N/m, respectively. Even stored at -45 or 50 °C for 30 d, the hydrogel still maintains good flexibility and robust adhesion force. It also shows repeatable underwater adhesion to biological tissue, glass, ceramic, plastic, and rubber. This novel antifreezing/antiheating adhesive hydrogel may be applied in extremely cold or hot environments and in underwater conditions.

Physically Cross-Linked Silk Fibroin-Based Tough Hydrogel Electrolyte with Exceptional Water Retention and Freezing Tolerance

Flexible ionic conductive hydrogel is attracting significant interest as it could be one of the crucial components for multifunctional ionotronic devices. However, their features of inevitably drying out without package and freezing at subzero temperatures may greatly limit the applications of conventional hydrogels in specific situations. Here, we present an ionic conductive hydrogel with water retention and freezing tolerance that consists of silk fibroin, ionic liquid, water, and inorganic salt. It is discovered that the ionic liquid serves multiple purposes to prevent water evaporation, decrease the freezing point, provide the essential conductivity of the hydrogel, etc. As a binary mixed solvent, the ionic liquid/water mixture enhances both water retention and freezing tolerance of the hydrogel electrolyte. Based on the silk fibroin (SF)/1-ethyl-3-methylimidazolium acetate (EMImAc)/H₂O/KCl hydrogel electrolyte, the flexible fiberlike supercapacitor could still function well at a temperature as low as -50 °C and after being stored in the open air for a long time. It is anticipated that this hydrogel will prove useful in developing new applications operating under harsh environments.

Alkaline Double-Network Hydrogels with High Conductivities, Superior Mechanical Performances, and Antifreezing Properties for Solid-State Zinc-Air Batteries

For the development of advanced flexible and wearable electronic devices, functional electrolytes with excellent conductivity, temperature tolerance, and desirable mechanical properties need to be engineered. Herein, an alkaline double-network hydrogel with high conductivity and superior mechanical and antifreezing properties is designed and promisingly utilized as the flexible electrolyte in all-solid-state zinc-air batteries. The conductive hydrogel is comprised of covalently cross-linked polyelectrolyte poly(2-acrylamido-2-methylpropanesulfonic acid potassium salt) (PAMPS-K) and interpenetrating methyl cellulose (MC) in the presence of concentrated alkaline solutions. The covalently cross-linked PAMPS-K skeleton and interpenetrating MC chains endow the hydrogel with good mechanical strength, toughness, an extremely rapid self-recovery capability, and an outstanding antifatigue property. Gratifyingly, the entrapment of a concentrated alkaline solution in the hydrogel matrix yields an extremely high ionic conductivity (105 mS cm^-1 at 25 °C) and an excellent antifreezing capacity. The hydrogel retains comparable conductivity and eligible strength to withstand various mechanical deformations at -20 °C. The all-solid-state zinc-air batteries using PAMPS-K/MC hydrogels as flexible alkaline electrolytes exhibit comparable values of specific capacity (764.7 mAh g^-1), energy capacity (850.2 mWh g^-1), cycling stability, and mechanical flexibility. The batteries still possess competitive electrochemical performances even when the operating temperature drops to -20 °C.

Wearable, Antifreezing, and Healable Epidermal Sensor Assembled from Long-Lasting Moist Conductive Nanocomposite Organohydrogel

Flexible wearable soft epidermal sensors assembled from conductive hydrogels have recently attracted tremendous research attention because of their extensive and significant applications in body-attachable healthcare monitoring, ultrasensitive electronic skins, and personal healthcare diagnosis. However, traditional conductive hydrogels inevitably face the challenge of long-term usage under room temperature and cold conditions, due to the lost water, elasticity, and conductivity at room temperature, and freezing at the water icing temperatures. It severely limits the applications in flexible electronics at room temperature or cold environment. Herein, we report a flexible, wearable, antifreezing, and healable epidermal sensor assembled from an antifreezing, long-lasting moist, and conductive organohydrogel. The nanocomposite organohydrogel is prepared from the conformal coating of functionalized reduced graphene oxide network by the hydrogel polymer networks consisting of poly(vinyl alcohol), phenylboronic acid grafted alginate, and polyacrylamide in the binary ethylene glycol (EG)/H₂O solvent system. The obtained organohydrogel exhibits excellent temperature tolerance (-40 °C), long-lasting moisture (20 days), reliable self-healing ability, and can be assembled as wearable sensor for an accurate detection of both large and tiny human activities under extreme environment. Thus, it paves the way for the design of highly sensitive wearable epidermal sensors with reliable long-lasting moisture and excellent temperature tolerance for potential versatile applications in electronic skins, wearable healthcare monitoring, and human-machine interaction.

Transparent, Antifreezing, Ionic Conductive Cellulose Hydrogel with Stable Sensitivity at Subzero Temperature

Cellulose was dissolved in benzyltrimethyl ammonium hydroxide (BzMe₃NOH) aqueous solution, and ionic conductive cellulose hydrogels (CCHs) with antifreezing property were directly fabricated by chemical cross-linking, without further treatment. The concentrated BzMe₃NOH solution in the CCH matrix endowed the hydrogel with transparency, ionic conductivity, and freeze tolerance. CCH maintained transparency over 90% and stable mechanical properties in the temperatures from -27.8 to 62.1 °C. Furthermore, the CCH-based sensors exhibited stable sensitivity to tensile strain, compressive pressure, and temperature, in a wide temperature range including the subzero temperature, with a fast responding time and no obvious hysteresis. The present work is expected to provide a simple and sustainable pathway to prepare an antifreezing soft conductor with multifunctions based on cellulose, which enables a broad range of potential applications.

Skin-Inspired Gels with Toughness, Antifreezing, Conductivity, and Remoldability

In recent years, nature-inspired conductive hydrogels have become ideal materials for the design of bioactuators, healthcare monitoring sensors, and flexible wearable devices. However, conductive hydrogels are often hindered by problems such as the poor mechanical property, nonreusability, and narrow operating temperature range. Here, a novel skin-inspired gel is prepared via one step of blending polyvinyl alcohol, gelatin, and glycerin. Due to their dermis-mimicking structure, the obtained gels possess high mechanical properties (fracture stress of 1044 kPa, fracture strain of 715%, Young's modulus of 157 kPa, and toughness of 3605 kJ m^-3). Especially, the gels exhibit outstanding strain-sensitive electric behavior as biosensors to monitor routine movement signals of the human body. Moreover, the gels with low temperature tolerance can maintain good conductivity and flexibility at -20 °C. Interestingly, the gels are capable of being recovered and reused by heating injection, cooling molding, and freezing-thawing cycles. Thus, as bionic materials, the gels have fascinating potential applications in various fields, such as human-machine interfaces, biosensors, and wearable devices.

Freezing-Tolerant Supramolecular Organohydrogel with High Toughness, Thermoplasticity, and Healable and Adhesive Properties

Hydrogels based on supramolecular noncovalent interactions have attracted great research interest but are still limited by relatively low mechanical strength and performance deterioration at subzero temperatures because of the formation of ice crystallization. In this study, an antifreezing and mechanically strong gelatin supramolecular organohydrogel is prepared via a simple strategy of immersing a gelatin pre-hydrogel in the citrate (Cit) water/glycerol mixture solution. In the organohydrogel, a part of water molecules are replaced by glycerol, which inhibits the formation of ice crystallization even at extremely low temperature. In addition, the formation of noncovalent interactions such as the hydrophobic aggregation induced by the salting-out effect, ionic interactions between the -NH₃⁺ of gelatin and Cit^3- anions, and hydrogen bonding between gelatin chains and glycerol endows the organohydrogels with high mechanical strength and toughness. The supramolecular organohydrogel can maintain its mechanical flexibility even at -80 °C or be stored for a long time. Moreover, the nature of noncovalent interactions endows the organohydrogel with intriguing thermoplasticity, good healable ability, and excellent adhesive behavior at various substrate surfaces.

Multifunctional Glycerol-Water Hydrogel for Biomimetic Human Skin with Resistance Memory Function

Biomimetic human skinlike materials with preferably self-healing ability, high sensitivity for external stimuli, and good adhesiveness against diverse substrates under a wide range of temperatures are of great importance in various applications such as wearable devices, human-motion devices, and soft robotics. However, most of the reported biomimetic human skinlike materials lack memory function, i.e., they cannot memorize the external stimuli once the stimuli disappear. This drawback hinders their applications in mimicking the human skin in real world. Here, a polyacrylamide/Au@polydopamine glycerol-water (GW) hydrogel has been designed to address this challenge. The as-prepared GW hydrogel exhibits a fast self-healing efficiency and good adhesiveness against diverse substrates under a wide range of temperatures (from -15 to 37 °C). Additionally, our GW hydrogel also possesses good perceived ability for external stimuli and subtle/large human motions. Most importantly, resistance memory function has been realized based on our GW hydrogel. These outstanding properties make it potentially significant in mimicking the human skin in real world.

Antifreezing Heat-Resistant Hollow Hydrogel Tubes

Hollow hydrogel tubes that are capable of maintaining their flexibility and structural stability in extreme temperature conditions have potential for use in biomedical scaffolds, carriers, and soft robotics over a wide temperature range. However, the preparation of hollow hydrogel tubes still remains challenging because it normally requires templates or complex devices and it is hard to endow the hollow tubes with antifreezing heat-resistant capabilities. We report a protocol that does not require a template or complex devices, in which sodium alginate film strips are immersed in an aqueous mixture of CaCO₃, CaCl₂, NaHCO₃, and HCl, which results in the manufacture of hollow tubes in 30 min. These hollow tubes are functionalized by glycerol and poly(ethylene glycol), which provides the tubes with antifreezing heat-resistant performances and enables them to keep their flexibility and hollow structures from -70 to 120 °C. This is the first report on antifreezing heat-resistant hollow hydrogel tubes, to the best of our knowledge. Such hollow tubes as carriers can control the sublimation of a mothball at a rate of 1.1 mg/h, which is one-tenth of the sublimating rate of an unloaded mothball. This sublimating rate reduces the hazard to environments along with maintaining the repellent effects. As the tube is a honey carrier, it enables the sustainable release of the honey over 800 min with a high efficacy for tricking and capturing ants. The simple applications demonstrate that the antifreezing heat-resistant hollow tubes might be feasible as carriers for the controlled release in extremely cold/hot environments.

Conductive Organohydrogels with Ultrastretchability, Antifreezing, Self-Healing, and Adhesive Properties for Motion Detection and Signal Transmission

Conductive hydrogels had demonstrated significant prospect in the field of wearable devices. However, hydrogels suffer from a huge limitation of freezing when the temperature falls below zero. Here, a novel conductive organohydrogel was developed by introducing polyelectrolytes and glycerol into hydrogels. The gel exhibited excellent elongation, self-healing, and self-adhesive performance for various materials. Moreover, the gel could withstand a low temperature of -20 °C for 24 h without freezing and still maintain good conductivity and self-healing properties. As a result, the sample could be applied for motion detection and signal transmission. For example, it can respond to finger movements and transmit network signals like network cables. Therefore, it was envisioned that the effective design strategy for conductive organohydrogels with antifreezing, toughness, self-healing, and self-adhesive properties would provide wide applications of flexible wearable devices.

Super Antiwetting Surfaces for Mitigating Drag-Out of Deep Eutectic Solvents

Deep eutectic solvents (DESs) are, at room temperature, about dozens to hundreds of times more viscous than water, which brings pretty thick residues on solid surfaces, for example, causing drag-out and weight loss in the transfer process. Unfortunately, until now little work had been done for solving this knotty problem. In this study, the super antiwetting surface, i.e., super-DES-phobic surface (defined as DES contact angle > 150°) is proposed and fabricated successfully by a facile coating technique. Hierarchical silver dendrites on copper foam substrate provide effective dual-roughness surfaces showing stable superDESphobicity. The superDESphobic surface can repel the DESs and their derived solutions even under elevated temperature of about 120 °C and the impact attack of drops. It is also found that the superDESphobic surface can significantly delay the DESs freezing and reduce the adhesion strength of the frozen DESs. Interestingly, the superDESphobic surface can be applied as an effective tool for gauging the density of DES using an ∼2 μL droplet in virtue of its super antiwetting property. The super antiwetting surfaces show promise for potential applications in DES self-cleaning and antifreezing.