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

Progress of Research on Conductive Hydrogels in Flexible Wearable Sensors

Conductive hydrogels, characterized by their excellent conductivity and flexibility, have attracted widespread attention and research in the field of flexible wearable sensors. This paper reviews the application progress, related challenges, and future prospects of conductive hydrogels in flexible wearable sensors. Initially, the basic properties and classifications of conductive hydrogels are introduced. Subsequently, this paper discusses in detail the specific applications of conductive hydrogels in different sensor applications, such as motion detection, medical diagnostics, electronic skin, and human-computer interactions. Finally, the application prospects and challenges are summarized. Overall, the exceptional performance and multifunctionality of conductive hydrogels make them one of the most important materials for future wearable technologies. However, further research and innovation are needed to overcome the challenges faced and to realize the wider application of conductive hydrogels in flexible sensors.

Multifunctional organohydrogel via the synergy of dialdehyde starch and glycerol for motion monitoring and sign language recognition

Conductive hydrogel which belongs to a type of soft materials has recently become promising candidate for flexible electronics application. However, it remains difficult for conductive hydrogel-based strain sensors to achieve the organic unity of large stretchability, high conductivity, self-healing, anti-freezing, anti-drying and transparency. Herein, a multifunctional conductive organohydrogel with all of the above superiorities is prepared by crosslinking polyacrylamide (PAM) with dialdehyde starch (DAS) in glycerol-water binary solvent. Attributing to the synergy of abundant hydrogen bonding and Schiff base interactions caused by introducing glycerol and dialdehyde starch, respectively, the organohydrogel achieved balanced mechanical and electrical properties. Besides, the addition of glycerol promoted the water-locking effects, making the organohydrogel retain the superior mechanical properties and conductivity even at extreme conditions. The resultant organohydrogel strain sensor exhibits desirable sensing performance with high sensitivity (GF = 6.07) over a wide strain range (0-697 %), enabling the accurate monitoring of subtle body motions even at -30 °C. On the basis, a hand gesture monitor system based on the organohydrogel sensors arrays is constructed using machine learning method, achieving a considerable sign language recognition rate of 100 %, and thus providing convenience for communications between the hearing or speaking-impaired and general person.

Engineered Living Material Bioreactors with Tunable Mechanical Properties using Vat Photopolymerization

3D-printed engineered living materials (ELM) are promising bioproduction platforms for agriculture, biotechnology, sustainable energy, and green technology applications. However, the design of these platforms faces several challenges, such as the processability of these materials into complex form factors and control over their mechanical properties. Herein, ELM are presented as 3D-printed bioreactors with arbitrary shape geometries and tunable mechanical properties (moduli and toughness). Poly(ethylene glycol) diacrylate (PEGDA) is used as the precursor to create polymer networks that encapsulate the microorganisms during the vat photopolymerization process. A major limitation of PEGDA networks is their propensity to swell and fracture when submerged in water. The authors overcame this issue by adding glycerol to the resin formulation to 3D print mechanically tough ELM hydrogels. While polymer concentration affects the modulus and reduces bioproduction, ELM bioreactors still maintain their metabolic activity regardless of polymer concentration. These ELM bioreactors have the potential to be used in different applications for sustainable architecture, food production, and biomedical devices that require different mechanical properties from soft to stiff.

Nanoarchitectonics composite hydrogels with high toughness, mechanical strength, and self-healing capability for electrical actuators with programmable shape memory properties

Hydrogel materials show promise in various fields, including flexible electronic devices, biological tissue engineering and wound dressing. Nevertheless, the inadequate mechanical properties, recovery performance, and self-healing speed still constrain the development of intelligent hydrogel materials. To tackle these challenges, we designed a composite hydrogel with high mechanical strength, rapid self-recovery and efficient self-healing ability based on multiple synergistic effects. With the synergistic effect of hydrogen bonds, metal coordination bonds and electrostatic interaction, the synthesized hydrogel could reach a maximum tensile strength of 6.2 MPa and a toughness of 50 MJ m-3. The interaction between the weak polyelectrolyte polyethyleneimine and polyacrylic acid aided in improving the elasticity of the hydrogel, thereby endowing it with prompt self-recovery attributes. The multiple reversible effects also endowed the hydrogel with excellent self-healing ability, and the fractured hydrogel could achieve 95% self-healing within 4 h at room temperature. By the addition of glycerol, the hydrogel could also cope with a variety of extreme environments in terms of moisture retention (12 h, maintaining 80% of its water content) and freeze protection (-36.8 °C) properties. In addition, the composite hydrogels applied in the field of shape memory possessed programmable and reversible shape transformation properties. The polymer chains were entangled at high temperatures to achieve shape fixation, and shape memory was eliminated at low temperatures, which allowed the hydrogels to be reprogrammed and achieve multiple shape transitions. In addition, we also assemble composite hydrogels as actuators and robotic arms for intelligent applications.

A Simple and Effective Physical Ball-Milling Strategy to Prepare Super-Tough and Stretchable PVA@MXene@PPy Hydrogel for Flexible Capacitive Electronics

Biomimetic flexible electronics for E-skin have received increasing attention, due to their ability to sense various movements. However, the development of smart skin-mimic material remains a challenge. Here, a simple and effective approach is reported to fabricate super-tough, stretchable, and self-healing conductive hydrogel consisting of polyvinyl alcohol (PVA), Ti3 C2 Tx MXene nanosheets, and polypyrrole (PPy) (PMP hydrogel). The MXene nanosheets and Fe3+ serve as multifunctional cross-linkers and effective stress transfer centers, to facilitate a considerable high conductivity, super toughness, and ultra-high stretchability (elongation up to 4300%) for the PMP hydrogel with. The hydrogels also exhibit rapid self-healing and repeatable self-adhesive capacity because of the presence of dynamic borate ester bond. The flexible capacitive strain sensor made by PMP hydrogel shows a relatively broad range of strain sensing (up to 400%), with a self-healing feature. The sensor can precisely monitor various human physiological signals, including joint movements, facial expressions, and pulse waves. The PMP hydrogel-based supercapacitor is demonstrated with a high capacitance retention of ≈92.83% and a coulombic efficiency of ≈100%.

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.

Thermally-modulated shape transition at the interface of soft gel filament and hydrophobic substrate

A liquid filament may pinch off into different shapes on interacting with a soft surface, as modulated by the interplay of inertial, capillary, and viscous forces. While similar shape transitions may intuitively be realized for more complex materials such as soft gel filaments as well, their intricate controllability towards deriving precise and stable morphological features remains challenging, as attributed to the complexities stemming from the underlying interfacial interactions over the relevant length and time scales during the sol-gel transition process. Circumventing these deficits in the reported literature, here we report a new means of precisely-controlled fabrication of gel microbeads via exploiting thermally-modulated instabilities of a soft filament atop a hydrophobic substrate. Our experiments reveal that abrupt morphological transitions of the gel material set in at a threshold temperature, resulting in spontaneous capillary thinning and filament breakup. We show that this phenomenon may be precisely modulated by an alteration in the hydration state of the gel material that may be preferentially dictated by its intrinsic glycerol content. Our results demonstrate that the consequent morphological transitions give rise to topologically-selective microbeads as an exclusive signature of the interfacial interactions of the gel material with the deformable hydrophobic interface underneath. Thus, intricate control may be imposed on the spatio-temporal evolution of the deforming gel, facilitating the inception of highly ordered structures of specific shapes and dimensionalities on demand. This is likely to advance the strategies of long shelf-life analytical biomaterial encapsulations via realizing one-step physical immobilization of bio-analytes on the bead surfaces as a new route to controlled materials processing, without demanding any resourced microfabrication facility or delicate consumable materials.

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.

Stretchable Semi-Interpenetrating Carboxymethyl Guar Gum-Based Composite Hydrogel for Moisture-Proof Wearable Strain Sensor

Wearable strain sensors of conductive hydrogels have very broad application prospects in electronic skins and human-machine interfaces. However, conductive hydrogels suffer from unstable signal transmission due to environmental humidity and inherent shortcomings of their materials. Herein, we introduce a novel moisture-proof conductive hydrogel with high toughness (2.89 MJ m-3), mechanical strength (1.00 MPa), and high moisture-proof sensing performance by using dopamine-functionalized gold nanoparticles as conductive fillers into carboxymethyl guar gum and acrylamide. Moreover, the hydrogel can realize real-time monitoring of major and subtle human movements with good sensitivity and repeatability. In addition, the hydrogel-assembled strain sensor exhibits stable sensing signals after being left for 1 h, and the relative resistance change rate under different strains (25-300%) shows no obvious noise signal up to 99% relative humidity. Notably, the wearable strain sensing is suitable for wearable sensor devices with high relative humidity.

Humidity Sensing of Stretchable and Transparent Hydrogel Films for Wireless Respiration Monitoring

Respiratory monitoring plays a pivotal role in health assessment and provides an important application prospect for flexible humidity sensors. However, traditional humidity sensors suffer from a trade-off between deformability, sensitivity, and transparency, and thus the development of high-performance, stretchable, and low-cost humidity sensors is urgently needed as wearable electronics. Here, ultrasensitive, highly deformable, and transparent humidity sensors are fabricated based on cost-effective polyacrylamide-based double network hydrogels. Concomitantly, a general method for preparing hydrogel films with controllable thickness is proposed to boost the sensitivity of hydrogel-based sensors due to the extensively increased specific surface area, which can be applied to different polymer networks and facilitate the development of flexible integrated electronics. In addition, sustainable tapioca rich in hydrophilic polar groups is introduced for the first time as a second cross-linked network, exhibiting excellent water adsorption capacity. Through the synergistic optimization of structure and composition, the obtained hydrogel film exhibits an ultrahigh sensitivity of 13,462.1%/%RH, which is unprecedented. Moreover, the hydrogel film-based sensor exhibits excellent repeatability and the ability to work normally under stretching with even enhanced sensitivity. As a proof of concept, we integrate the stretchable sensor with a specially designed wireless circuit and mask to fabricate a wireless respiratory interruption detection system with Bluetooth transmission, enabling real-time monitoring of human health status. This work provides a general strategy to construct high-performance, stretchable, and miniaturized hydrogel-based sensors as next-generation wearable devices for real-time monitoring of various physiological signals.

Hysteresis-Free Double-Network Hydrogel-Based Strain Sensor for Wearable Smart Bioelectronics

Hydrogel-based electronics have attracted substantial attention in the field of biological engineering, energy storage devices, and soft actuators due to their resemblance to living tissues, biocompatibility, tunable softness, and consolidated structures. However, combining the properties of quick resilience, hysteresis-free, and robust mechanical properties in physically cross-linked hydrogels is still a great challenge. Herein, we present a vinyl hybrid silica nanoparticle (VSNPs)/polyacrylamide (PAAm)/alginate double-network hydrogel-based strain sensor with the characteristics of quick resilience, hysteresis-free, and a low limit of detection (LOD). The physical cross-linking among PAAm chains and covalent cross-linking between PAAm, alginate, and N,N-methylenebisacrylamide chains promotes excellent mechanical properties. Moreover, the incorporation of VSNPs reinforces the mechanical strength by the dynamic cross-linking of the PAAm network to maintain the integrity of the hydrogel and works as a stress buffer to dissipate energy. The as-prepared hydrogel-based sensor exhibits a strain sensitivity (i.e., gauge factor) of 1.73 (up to 100% strain), a response time of 0.16 s, an ultra-low electrical hysteresis of 2.43%, and a low LOD of 0.4%. The outstanding properties of the hydrogel are further used to illustrate the utility of the sensor in e-skin, ranging from low-strain applications, such as carotid pulse and artificial sound detection, to large bending applications, such as sign language translations. In addition, an efficient and cost-effective synthesis of double-network hydrogel that can overcome the bottleneck of the electromechanical properties of single network hydrogel has potential prospects in soft actuators, tissue engineering, and various biomedical applications.

Advances in Ultrasound-Responsive Hydrogels for Biomedical Applications

Various intelligent hydrogels have been developed for biomedical applications because they can achieve multiple, variable, controllable and reversible changes in their shape and properties in a spatial and temporal manner,...

Development of a novel beta-glucan supplemented hydrogel spray formulation and wound healing efficacy in a db/db diabetic mouse model

To relief the severe economic and social burdens and patient suffering caused by the increasing incidence of chronic wounds, more effective treatments are urgently needed. In this study, we focused on developing a novel sprayable wound dressing with the active ingredient β-1,3/1,6-glucan (βG). Since βG is already available as the active ingredient in a commercial wound healing product provided as a hydrogel in a tube (βG-Gel), the sprayable format should bring clinical benefit by being easily sprayed onto wounds; whilst retaining βG-Gel's physical stability, biological safety and wound healing efficacy. Potentially sprayable βG hydrogels were therefore formulated, based on an experimental design setup. One spray formulation, named βG-Spray, was selected for further investigation, as it showed favorable rheological and spraying properties. The βG-Spray was furthermore found to be stable at room temperature for more than a year, retaining its rheological properties and sprayability. The cytotoxicity of βG-Spray in keratinocytes in vitro, was shown to be promising even at the highest tested concentration of 100 μg/ml. The βG-Spray also displayed favorable fluid affinity characteristics, with a capacity to both donate and absorb close to 10% fluid relative to its own weight. Finally, the βG-Spray was proven comparably effective to the commercial product, βG-Gel, and superior to both the water and the carrier controls (NoβG-Spray), in terms of its ability to promote wound healing in healing-impaired animals. Contraction was found to be the main wound closure mechanism responsible for the improvement seen in the βG-treatment groups (βG-Spray and βG-Gel). In conclusion, the novel sprayable βG formulation, confirmed its potential to expand the clinical use of βG as wound dressing.

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

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

Antiliquid-Interfering, Antibacteria, and Adhesive Wearable Strain Sensor Based on Superhydrophobic and Conductive Composite Hydrogel

Conductive hydrogels are promising multifunctional materials for wearable sensors, but their practical applications require combined properties that are difficult to achieve. Herein, we developed a flexible wearable sensor with double-layer structure based on conductive composite hydrogel, which included the outer layer of silicone elastomer (Ecoflex)/silica microparticle composite film and the inner layer of P(AAm-co-HEMA)-MXene-AgNPs hydrogel. Through covalently cross-linking silicone elastomer on the surface of the hydrogel polymer, we bonded a thin Ecoflex film (100 μm) on the P(AAm-co-HEMA)-MXene-AgNPs hydrogel with robust interface, which can easily adhere to the Ecoflex/SiO₂ microparticle composite film by silicone glue. The Ecoflex/SiO₂ microparticle composite film endows the strain wearable sensor with superhydrophobic function that could maintain the stability under stretching or bending. Moreover, it can effectively resist the interference of water droplets and water flow. The P(AAm-co-HEMA)-MXene-AgNPs hydrogel exhibits outstanding antibacterial activity to inhibit Staphylococcus aureus, Escherichia coli, and even drug-resistant Escherichia coli. In addition, the flexible wearable sensor exhibited good self-adhesive performance by changing the reaction temperature of hydrogel and can adhere strongly onto various materials. The conductive composite hydrogel reported in this work contributes an innovative strategy for the preparation of multifunctional flexible wearable sensor.

A facile route to dual-crosslinking polymeric hydrogels with enhanced mechanical property

Polymeric hydrogels with excellent biocompatibility, high hydrophilicity, and water-holding capacity have attracted considerable concerns in widely fields. However, most hydrogels exhibit poor mechanical property, which largely limited their applications. Herein, a novel dual-crosslinking polymeric hydrogel crosslinked by covalent bonds and metal coordination interactions between Fe3+ and –COO- was fabricated through accessible method. The metal coordination interactions within the hydrogel were established through dipping in the FeCl3 solution to reinforce the backbones of the hydrogel. The obtained polymeric hydrogel exhibits enhanced tensile strength (∼4.92 MPa), stiffness (∼6.168 MPa), and toughness (∼2.835 MJ m−3).

Super Stretchable and Compressible Hydrogels Inspired by Hook-and-Loop Fasteners

Inspired by hook-and-loop fasteners, we designed a hydrogel network containing α-zirconium phosphate (ZrP) two-dimensional nanosheets with a high density of surface hydroxyl groups serving as nanopatches with numerous "hooks," while polymer chains with plentiful amine functional groups serve as "loops." Our multiscale molecular simulations confirm that both the high density of hydroxyl groups on nanosheets and the large number of amine functional groups on polymer chains are essential to achieve reversible interactions at the molecular scale, functioning as nano hook-and-loop fasteners to dissipate energy. As a result, the synthesized hydrogel possesses superior stretchability (>2100% strain), resilience to compression (>90% strain), and durability. Remarkably, the hydrogel can sustain >5000 cycles of compression with torsion in a solution mimicking synovial fluid, thus promising for potential biomedical applications such as artificial articular cartilage. This hook-and-loop model can be adopted and generalized to design a wide range of multifunctional materials with exceptional mechanical properties.

Skin-inspired nanofibrillated cellulose-reinforced hydrogels with high mechanical strength, long-term antibacterial, and self-recovery ability for wearable strain/pressure sensors

The advent of electric skins (E-skin) with tactile sensation, flexibility, and human affinity characteristics have attracted considerable attention in extensive research fields, including intelligent robots and health monitoring, etc. To improve the intrinsic brittleness of hydrogels, a multifunctional E-skin was fabricated involving a TEMPO-NFC and a covalently cross-linked polyacrylamide (PAM) network. In this work, silver nanoparticles (AgNPs) as long-term antibacterial agent and conductive fillers were coated onto NFC nanofibers. Subsequently, this nanocomposite hydrogel was synthesized by free radical copolymerization of AM monomers with PNAg fibers as interpenetrating fibers network. Importantly with NFC present, the nanocomposite hydrogel exhibited superior mechanical performance and excellent self-recovery ability. The obtained sensor with excellent mechanical stability and sensing performance could detect mechanotransduction signal of human movements. This work provides a practicable method to prepare high antibacterial efficiency, excellent mechanical performance, and dual-modal nanocellulose-based hydrogel sensor for the broad-range application in human-motion detection and intelligence skins.

Highly Stretchable, Adhesive, and Self-Healing Silk Fibroin-Dopted Hydrogels for Wearable Sensors

In recent years, the preparations of flexible electronic devices have attracted great attention. Here, a simple one-pot method of thermal polymerization is introduced to fabricate silk fibroin-dopted hydrogels (SFHs), which are both chemically and physically cross-linked by acrylamide (AM), acrylic acid (AA), and silk fibroin (SF). The addition of SF can effectively enhance the mechanical property of the SFH_12% by 59% compared with SFH_0% . Taking the advantage of its wide working range of stress (about 0.455-568.9 kPa), the SFH can work as a resistance-type pressure sensor to monitor different human motions. What is more, the excellent adhesion, about 75.17 N m^-1 of SFH_46% enables it to fit tightly to other objects during the testing, which significantly reduces the loss of small signals due to poor fit. In addition, the SFH demonstrates excellent self-healing property without requiring external excitation and a sensitive temperature response in the range of -10 to 60 °C. The SFH is expected to be applied in the field of electronic skin, soft robots, and other flexible electronic products as well as speech recognition.

Biomimetic anti-freezing polymeric hydrogels: keeping soft-wet materials active in cold environments

As one of the most outstanding materials, the analysis of the structure and function of hydrogels has been extensively carried out to tailor and adapt them to various fields of application. The high water content, which is beneficial for plenty of applications in the biomedical setting, prevents the adoption of hydrogels in flexible electronics and sensors in real life applications, because hydrogels lose their excellent properties, including conductivity, transparency, flexibility, etc., upon freezing at sub-zero temperatures. Therefore, depressing the liquid-solid phase transition temperature is a powerful means to expand the application scope of hydrogels, and will benefit the chemical engineering and materials science communities. This review summarizes the recent research progress of anti-freezing hydrogels. At first, approaches for the generation of anti-freezing (hydro)gels are introduced and their anti-freezing mechanisms and performances are briefly discussed. These approaches are either based on addition of salts, alcohols (cryoprotectants and organohydrogels), and ionic liquids (ionogels), modification of the polymer network or a combination of several techniques. Then, a concise overview of applications leveraged by the widened temperature resistance is provided and future research areas and developments are envisaged.

Biomimetic Boroxine-Based Multifunctional Thermosets via One-Pot Synthesis

Boroxine-based thermosets with remarkable mechanical tunability, self-healing ability, recyclability, and adhesive strength are of significant importance in various applications. However, complex multistep reactions are often required to prepare such thermosets. Herein, a facile one-pot approach to synthesize boroxine-based malleable thermosets is proposed. Random copolymers with pendant boronic acid groups were synthesized from alkenyl monomers containing boronic acids [4-vinylphenylboronic acid (4-VPBA), 3-vinylphenylboronic acid, or 3-acrylamidophenylboronic acid] and octadecanoxy polyethylene glycol methacrylate. Then, the as-prepared copolymers were cured to form thermosets with boroxine bonds. The tensile strengths of the thermosets were tailored to range from 9.3 to 27.5 MPa by increasing the concentration of 4-VPBA. Moreover, because of the reversible nature of dynamic boroxine bonds (transformation between boroxines and boronic acids) induced by water, the thermosets exhibit remarkable self-healing efficiency (up to 99%), tunable mechanical properties, and excellent recyclability. Additionally, the thermosets also demonstrate superior adhesive strength (as high as 73.9 MPa) on different substrates.

Hollow Au/Polypyrrole Capsules to Form Porous and Neural Network-Like Nanofibrous Film for Wearable, Super-Rapid, and Ultrasensitive NH3 Sensor at Room Temperature

Wearable conducting polymer-based NH₃ sensors are highly desirable in real-time environmental monitoring and human health protection but still a challenge for their relatively long response/recovery time and moderate sensitivity at room temperature. Herein, we present an effective route to fulfill this challenge by constructing porous and neural network-like Au/polypyrrole (Au/PPy) electrospun nanofibrous film with hollow capsular units for NH₃ sensor. Taking the unique architecture and synergistic effect between Au and PPy, our sensor exhibits not only super-rapid response/recovery time (both ∼7 s), faster than all reported sensors, but also stable and ultrahigh sensitivity (response reaches ∼2.3 for 1 ppm NH₃) at room temperature even during repeated deformation. Furthermore, good selectivity has been also achieved. These outstanding properties make our sensor hold great potential in real-time NH₃-related disease diagnosis and environmental monitoring at room temperature.

Microneedle-based bioassays

Disease diagnosis and therapeutic efficacy can be monitored via a number of established bioassays that sample body fluids to assess and monitor health conditions. Traditional bioassays generally include several steps and start with invasive body fluid extraction procedures. These steps are painful and often require specialized techniques and tailored equipment, as well as the supervision of professional medical personnel. Innovations in engineering alternative bioassays to address these shortcomings are thus desired. Microneedles (MNs) represent promising tools to sample body fluids, in view of their minimal invasiveness, painlessness, and uncomplicated implementation. Recent progress in microfabrication and materials engineering, including the development of hollow and solid MNs with uniquely optimized architectures and multi-functional materials, has positioned MN-based platforms as prime candidates for bioassay solutions. In this minireview, we summarize the studies of MN-based platforms for detection and diagnosis. We categorize the platforms based on three different mechanisms: MNs as body fluid reservoirs, MNs integrated with electrochemical assays, and MNs engineered with colorimetric analyses. A discussion of design principles for MN-based bioassay platforms is presented. We also discuss the challenges and opportunities associated with MN-based bioassays in future clinical applications.

Recent innovations in artificial skin

The skin is a "smart", multifunctional organ that is protective, self-healing and capable of sensing and many forms of artificial skins have been developed with properties and functionalities approximating those of natural skin. Starting from specific commercial products for the treatment of burns, progress in two fields of research has since allowed these remarkable materials to be viable skin replacements for a wide range of dermatological conditions. This review maps out the development of bioengineered skin replacements and synthetic skin substitutes, including electronic skins. The specific behaviors of these skins are highlighted, and the performances of both types of artificial skins are evaluated against this. Moving beyond mere replication, highly advanced artificial skin materials are also identified as potential augmented skins that can be used as flexible electronics for health-care monitoring and other applications.

Single network double cross-linker (SNDCL) hydrogels with excellent stretchability, self-recovery, adhesion strength, and conductivity for human motion monitoring

Hydrogels, as a kind of soft materials, are good candidates for smart skin-like materials. A double network is usually fabricated to improve the mechanical properties of hydrogels, and involves two different kinds of networks. In this work, a novel strategy for preparing single network double cross-linker (SNDCL) hydrogels was proposed and the prepared hydrogels exhibited excellent mechanical properties, including stretchability, compressibility, self-recovery, adhesion, shape memory and mechanical strength. N,N'-Methylenebisacrylamide forms covalent bonds with the network, while citric acid can form multiple weak interactions due to the polycarboxylic structure. This improves the tensile properties (6564%) and compressive properties of the hydrogel, and the hydrogels also exhibit long-lasting self-adhesion ability on various substrates. In addition, the hydrogels with multiple properties can be used as flexible strain sensors, allowing the monitoring of body movements. The hydrogels can hopefully be used in wearable electronic sensor devices and for healthcare monitoring.

Transparent and Flexible Electronics Assembled with Metallic Nanowire-Layered Nondrying Glycerogel

There has been increasing demand for transparent and mechanically durable electrical conductors for their uses in wearable electronic devices. It is common to layer metallic nanowires on transparent but stiff poly(dimethylsiloxane) (PDMS) or stretchable but opaque Ecoflex-based substrates. Here, we hypothesized that layering metallic nanowires on a stretchable and hygroscopic gel would allow us to assemble a transparent, stretchable, and durable conductor. The hygroscopic property of the gel was attained by partially replacing water in the preformed polyacrylamide hydrogel with glycerol. The resulting gel, denoted as a glycerogel, could remain hydrated for over 6 months in air by taking up water molecules from the air. The glycerogel was tailored to be stretchable up to 8 times its original length by tuning the amount of the cross-linker and acrylamide. The resulting glycerogel allowed for deposition of wavy silver nanowires using the prestrain method up to 400% prestrain, without causing kinks and interfacial cracks often found with nanowires layered onto PDMS. With a prestrain of 100%, the resulting nanowire-gel conductor exhibited optical transparency (85%) and electrical conductivity (17.1 ohm/sq) even after 5000 cycles of deformation. The results of this study would broadly be useful to improve the performance of the next generation of flexible electronic devices.

Stretchable, Biocompatible, and Multifunctional Silk Fibroin-Based Hydrogels toward Wearable Strain/Pressure Sensors and Triboelectric Nanogenerators

Nowadays, great effort has been devoted to establishing wearable electronics with excellent stretchability, high sensitivity, good mechanical strength, and multifunctional characteristics. Herein, a soft conductive hydrogel is rationally designed by proportionally mixing silk fibroin, polyacrylamide, graphene oxide, and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate). The resultant hydrogel has considerable stretchability and compressibility, which enables it to be assembled into a strain/pressure sensor with a wide sensing range (strain, 2%-600%; pressure, 0.5-119.4 kPa) and reliable stability. Then, the corresponding sensor is capable of monitoring a series of physical signals of the human body (e.g., joint movement, facial gesture, pulse, breathing, etc.). In particular, the hydrogel-based sensor is biocompatible, with no anaphylactic reaction on human skin. More interestingly, this conductive hydrogel exhibits a positive response when it works in a triboelectric nanogenerator; consequently, it lights up 20 commericial green light-emitting diodes. Thus, this silk fibroin-based hydrogel is a kind of multifunctional material toward wearable electronics with versatile applications in health and exercise monitors, soft robots, and power sources.

Conductive Self-Healing Nanocomposite Hydrogel Skin Sensors with Antifreezing and Thermoresponsive Properties

With growing interest in flexible and wearable devices, the demand for nature-inspired soft smart materials, especially intelligent hydrogels with multiple perceptions toward external strain and temperatures to mimic the human skin, is on the rise. However, simultaneous achievement of intelligent hydrogels with skin-compatible performances, including good transparency, appropriate mechanical properties, autonomous self-healing ability, multiple mechanical/thermoresponsiveness, and retaining flexibility at subzero temperatures, is still challenging and thus limits their application as skinlike devices. Here, conductive nanocomposite hydrogels (NC gels) were delicately designed and prepared via gelation of oligo(ethylene glycol) methacrylate (OEGMA)-based monomers in a glycerol-water cosolvent, where inorganic clay served as the physical cross-linker and provided conductive ions. The resultant NC gels exhibited good conductivity (∼3.32 × 10^-4 S cm^-1, akin to biological muscle tissue) and an autonomously self-healing capacity (healing efficiency reached 84.8%). Additionally, such NC gels displayed excellent flexibility and responded well to multiple strain/temperature external stimuli and subtle human motions in a wide temperature range (from -20 to 45 °C). These distinguished properties would endow such NC gels significant applications in fields of biosensors, human-machine interfaces, and soft robotics.