Skin-like mechanoresponsive self-healing ionic elastomer from supramolecular zwitterionic network

Strong and Healable Elastomers with Photothermal-Stimulus Dynamic Nanonetworks Enabled by Subnano Ultrafine MoO3-x Nanowires

One-dimensional nanomaterials have become one of the most available nanoreinforcing agents for developing next-generation high-performance functional self-healing composites owing to their unique structural characteristics and surface electron structure. However, nanoscale control, structural regulation, and crystal growth are still enormous challenges in the synthesis of specific one-dimensional nanomaterials. Here, oxygen-defective MoO3-x nanowires with abundant surface dynamic bonding were successfully synthesized as novel nanofillers and photothermal response agents combined with a polyurethane matrix to construct composite elastomers, thus achieving mechanically enhanced and self-healing properties. Benefiting from the surface plasmon resonance of the MoO3-x nanowires and interfacial multiple dynamic bonding interactions, the composite elastomers demonstrated strong mechanical performance (with a strength of 31.45 MPa and elongation of 1167.73%) and ultrafast photothermal toughness self-healing performance (20 s and an efficiency of 94.34%). The introduction of MoO3-x nanowires allows the construction of unique three-dimensional cross-linked nanonetworks that can move and regulate interfacial dynamic interactions under 808 nm infrared laser stimulation, resulting in controlled mechanical and healing performance. Therefore, such special elastomers with strong photothermal responses and mechanical properties are expected to be useful in next-generation biological antibacterial materials, wearable devices, and artificial muscles.

Transparent, mechanically robust, conductive, self-healable, and recyclable ionogels for flexible strain sensors and electroluminescent devices

A mechanically robust, self-healable, and recyclable PVP-based ionogel was achieved through a simple one-pot photoinitiated polymerization process. This ionogel exhibits a combination of excellent properties, including transparency, high mechanical strength, good ionic conductivity, self healability, and recyclability. A wearable resistive strain sensor based on the ionogel is successfully assembled and demonstrated accurate response to human motion. Moreover, a flexible electroluminescent device has been fabricated based on our ionogel, which can maintain optimal luminescence functionality even when subjected to bending. Considering the simple preparation method and excellent applications, we believe that our PVP-based ionogel has promising applications in many fields such as in wearable devices, electronic skin, implantable materials, robotics and human-machine interfaces.

Low-Hysteresis and Tough Ionogels via Low-Energy-Dissipating Cross-Linking

Low-hysteresis merits can help polymeric gel materials survive from consecutive loading cycles and promote life span in many burgeoning areas. However, it is a big challenge to design low-hysteresis and tough polymeric gel materials, especially for ionogels. This can be attributed to the fact that higher viscosities of ionic liquids (ILs) would increase chain friction of polymeric gels and eventually dissipate large amounts of energy under deformation. Herein, a chemical design of ionogels is proposed to achieve low-hysteresis characteristics in both mechanical and electric aspects via hierarchical aggregates formed by supramolecular self-assembly of quadruple H-bonds in a soft IL-rich polymeric matrix. These self-assembled nanoaggregates not only can greatly reinforce the polymeric matrix and enhance resilience, but also exhibit low-energy-dissipating features under stress conditions, simultaneously benefiting for low-hysteresis properties. These aggregates can also promote toughness and subsequent anti-fatigue properties in response to external cyclic mechanical stimuli. More importantly, these ionogels are presented as a model system to elucidate the underlying mechanism of the low hysteresis and fatigue resistance. Based on these findings, it is further demonstrated that the supramolecular low-hysteresis strategy is universal.

Flexible iontronics with super stretchability, toughness and enhanced conductivity based on collaborative design of high-entropy topology and multivalent ion-dipole interactions

All-solid-state ionic conductive elastomers (ASSICEs) are emerging as a promising alternative to hydrogels and ionogels in flexible electronics. Nevertheless, the synthesis of ASSICEs with concomitant mechanical robustness, superior ionic conductivity, and cost-effective recyclability poses a formidable challenge, primarily attributed to the inherent contradiction between mechanical strength and ionic conductivity. Herein, we present a collaborative design of high-entropy topological network and multivalent ion-dipole interaction for ASSICEs, and successfully mitigate the contradiction between mechanical robustness and ionic conductivity. Benefiting from the synergistic effect of this design, the coordination, de-coordination, and intrachain transfer of Li+ are effectively boomed. The resultant ASSICEs display exceptional mechanical robustness (breaking strength: 7.45 MPa, fracture elongation: 2621%, toughness: 107.19 MJ m-3) and impressive ionic conductivity (1.15 × 10-2 S m-1 at 25 °C). Furthermore, these ASSICEs exhibit excellent environmental stability (fracture elongation exceeding 1400% at 50 °C or -60 °C) and recyclability. Significantly, the application of these ASSICEs in a strain sensor highlights their potential in various fields, including human-interface communication, aerospace vacuum measurement, and medical balloon monitoring.

Construction of gradient ionogels by self-floatable hyperbranched organosilicon crosslinkers for multi-sensing and wirelessly monitoring physiological signals

Monitoring complex human movements requires the simultaneous detection of strain and pressure, which poses a challenge due to the difficulty in integrating high stretchability and compressive ability into a single material. Herein, a series of hyperbranched polysiloxane crosslinkers (HPSis) with self-floating abilities are designed and synthesized. Taking advantage of the self-floating capabilities of HPSis, ionogels with gradient composition distribution and conductivities are constructed by in situ one-step photopolymerization, and possess satisfactory stretchability, high compressibility and excellent resilience. The gradient-ionogel-based strain sensor exhibits extraordinary pressure sensitivity (19.33 kPa-1), high strain sensitivity (GF reaches 2.5) and temperature sensing ability, enabling the monitoring of the angles and direction of joint movements, transmitting Morse code and wirelessly detecting bioelectrical signals. This study may inspire the design of development of multi-function flexible electronics.

Screening of Optimal Konjac Glucomannan-Protein Composite Gel Formulations to Mimic the Texture and Appearance of Tripe

This study aimed to develop a product that closely replicates the texture and appearance of tripe. The effect of three different proteins (soy protein isolate (SPI), pea protein isolate (PPI), and whey protein isolate (WPI)) at different protein levels and processing conditions (heating (90 °C, 1 h) followed by cooling (4 °C, 12 h) and heating (90 °C, 1h) followed by freezing (-18 °C, 12 h)) of konjac glucomannan (KGM) was analyzed. The optimal formulations for simulating tripe were screened by examining their similarity to real tripe in terms of texture, color, and sensory experience. The screened formulations were also subjected to a preliminary mechanistic investigation. The results show that all three proteins improved the gel's textural properties to varying degrees. At the same concentration, the hardness and chewiness of the KGM/WPI composite gel were significantly higher than those of the other two KGM/protein composite gels, among which the composite gel obtained by adding 8% WPI and 5% KGM heating-frozen (FWK4) had the greatest hardness and chewiness of 4338.07 g and 2313.76, respectively, and the springiness differences in all of the composite gels were small. In addition, the addition of protein increased the whiteness of the hybrid gels, with WPI having the most significant effect on the whiteness of the composite gels (whiteness increased from 30.25 to 62.80 as the concentration of WPI increased from 0 to 10%). Freezing increased composite gel hardness and chewiness, but reduced gel springiness and whiteness. Cluster analysis showed that the composite gel obtained by heating-cooling 8% WPI and 5% KGM (WK4) was very similar to the real tripe in terms of chewiness and whiteness, and WK4 had the highest sensory scores for color, tissue morphology, tactile sensation, taste, and odor. The acceptability score in terms of tissue morphology reached 4.3. Meanwhile, the characterization results of WK4 indicate the presence of large junction areas in the gel network. Fourier transform infrared spectroscopy (FTIR) analysis, X-ray diffraction, and intermolecular force contributions indicated that the incorporation of WPI promoted integral interactions, and that hydrophobic interactions and disulfide bonding played a key role in the WK4 composite gel system. Moreover, scanning electron microscopy (SEM) also showed that the combination of WPI and konjac glucan resulted in a more compact gel structure. This study is informative for the development of the field of bionic tripe processing.

New insights into pure zwitterionic hydrogels with high strength and high toughness

Zwitterionic hydrogels are electrically neutral materials with both cationic and anionic groups that impart excellent anti-fouling properties and ion channel orientations. However, pure zwitterionic hydrogels generally exhibit low strength and toughness. In this study, it has been discovered that polymerizable zwitterionic monomers in aqueous solution exhibit a unique liquid-liquid phase separation phenomenon at a high monomer concentration of ≥50 wt%, resulting in pure and commercial zwitterionic hydrogels with high compressive strength (6.5 MPa) and high toughness (2.12 kJ m-2). This phase separation and the corresponding aggregations might be caused by strong dipole-dipole interactions among residual zwitterionic monomers under the lack of free-water condition. The synergistic effect of liquid-liquid phase separation and polymer entanglement enhances the mechanical strength, toughness, self-recovery, and anti-freezing properties of pure polyzwitterionic hydrogels. Moreover, the high fracture energy of highly elongated yet tough polyzwitterionic hydrogels facilitates the development of high crack propagation resistance, which supports an expanded role in tissue engineering, soft flexible devices, and electronics applications with improved durability. A wide range of applications for the proposed polyzwitterionic hydrogels is demonstrated by the development and testing of a strain sensor and a triboelectric nanogenerator device. Our findings provide novel insights into the network structure of pure polyzwitterionic hydrogels.

Transforming Commercial Polymers into Tough yet Switchable Adhesives by Trident Photoswitch Molecule Doping: Break Adhesion-Switchability Paradox

Here, a trident molecule doping strategy is introduced to overcome both cohesion-adhesion trade-off and adhesion-switchability conflict, transforming commercial polymers into tough yet photo-switchable adhesives. The strategy involves initial rational design of new trident photoswitch molecules namely TAzo-3 featuring azobenzene and hydroxy-terminated alkyl chains involved rigid-soft tri-branch structure, and subsequent doping into commercial polycaprolactone (PCL) via simple blending. Unique design enables TAzo-3 as a versatile dopant, not only regulating the internal and external supramolecular interaction to balance cohesion and interface adhesion for tough bonding, but also affording marked photothermal effect to facilitate rapid adhesive melting for great photo-switchability. Thus, the optimal TAzo-3-doped PCL (TAzo-3@P) displays markedly-improved bonding performance on diverse substrates compared to linear azobenzene-doped PCL and pure PCL. Impressively, TAzo-3@P on polymethyl methacrylate (PMMA) attains large room-temperature adhesion strength of 6.7 MPa - surpassing most reported adhesives and many commercial adhesives on PMMA, along with easy photo-induced detachment with remarkable switch ratio of 2.09 × 105. Besides, TAzo-3@P can also act as "permanent" adhesives for only adhesion, demonstrating excellent multi-reusability, anti-freezing and waterproof ability. Mechanism studies unveil that the switchable adhesion is closely linked with the dopant molecule structure while rigid-soft coupled trident structures and hydroxy-terminated alkyl chains are key factors.

Hybrid Hydrogen Bonding Strategy to Construct Instantaneous Self-Healing Highly Elastic Ionohydrogel for Multi-Functional Electronics

Gels show great promise for applications in wearable electronics, biomedical devices, and energy storage systems due to their exceptional stretchability and adjustable electrical conductivity. However, the challenge lies in integrating multiple functions like elasticity, instantaneous self-healing, and a wide operating temperature range into a single gel. To address this issue, a hybrid hydrogen bonding strategy to construct gel with these desirable properties is proposed. The intricate network of hybrid strong weak hydrogen bonds within the polymer matrix enables these ionohydrogel to exhibit remarkable instantaneous self-healing, stretching up to five times their original length within seconds. Leveraging these properties, the incorporation of ionic liquids, water, and zinc salts into hybrid hydrogen bond crosslinked network enables conductivity and redox reaction, making it a versatile ionic skin for real-time monitoring of human movements and respiratory. Moreover, the ionohydrogel can be used as electrolyte in the assembly of a zinc-ion battery, ensuring a reliable power supply for wearable electronics, even in extreme conditions (-20 °C and extreme deformations). This ionohydrogel electrolyte simplifies the diverse structural requirements of gels to meet the needs of various electronic applications, offering a new approach for multi-functional electronics.

What Exactly Can Bionic Strategies Achieve for Flexible Sensors?

Flexible sensors have attracted great attention in the field of wearable electronic devices due to their deformability, lightness, and versatility. However, property improvement remains a key challenge. Fortunately, natural organisms exhibit many unique response mechanisms to various stimuli, and the corresponding structures and compositions provide advanced design ideas for the development of flexible sensors. Therefore, this Review highlights recent advances in sensing performance and functional characteristics of flexible sensors from the perspective of bionics for the first time. First, the "twins" of bionics and flexible sensors are introduced. Second, the enhancements in electrical and mechanical performance through bionic strategies are summarized according to the prototypes of humans, plants, and animals. Third, the functional characteristics of bionic strategies for flexible sensors are discussed in detail, including self-healing, color-changing, tangential force, strain redistribution, and interfacial resistance. Finally, we summarize the challenges and development trends of bioinspired flexible sensors. This Review aims to deepen the understanding of bionic strategies and provide innovative ideas and references for the design and manufacture of next-generation flexible sensors.

Hybrid Zwitterionic Hydrogels with Encoded Differential Swelling and Programmed Deformation for Small-Scale Robotics

Stimuli-responsive shape-morphing hydrogels with self-healing and tunable physiochemical properties are excellent candidates for functional building blocks of untethered small-scale soft robots. With mechanical properties similar to soft organs and tissues, such robots enable minimally invasive medical procedures, such as cargo/cell transportation. In this work, responsive hydrogels based on zwitterionic/acrylate chemistry with self-healing and stimuli-responsiveness are synthesized. Such hydrogels are then judiciously cut and pasted to form hybrid constructs with predetermined swelling and elastic anisotropy. This method is used to program hydrogel constructs with predetermined 2D-to-3D deformation upon exposure to different environmental ionic strengths. Untethered soft robotic functionalities are demonstrated, such as actuation, magnetic locomotion, and targeted transport of soft and light cargo in flooded media. The proposed hydrogel expands the repertoire of functional materials for fabricating small-scale soft robots.

Zwitterionic Eutectogels with High Ionic Conductivity for Environmentally Tolerant and Self-Healing Triboelectric Nanogenerators

Eutectogels have garnered considerable attention for the development of wearable devices, owing to their inherent mechanical elasticity, ionic conductivity, affordability, and environmental compatibility. However, the low conductivity of existing eutectogels has impeded their progression in electronic applications. Here, we report a zwitterionic eutectogel with an impressive ionic conductivity of up to 15.7 mS cm-1. The incorporation of zwitterionic groups into the eutectogel creates ample mobile charges by dissociating the cation and anion of solvents, thereby yielding exceptional ionic conductivity. Moreover, the abundant electrostatic and hydrogen bonding interactions within the eutectogel endow it with prominent self-healing and adhesive properties. By integrating the eutectogel with a roughly patterned polydimethylsiloxane film, we have successfully constructed a triboelectric nanogenerator (TENG) with a maximum output power density of 112 mW m-2. This TENG is capable of generating stable electrical signals even in extreme temperature conditions ranging from -80 to 100 °C and effectively powering electronic devices. Furthermore, the assembled TENG displays high sensitivity as a self-powered sensor, enabling real-time and precise monitoring of signals derived from human motions. This study establishes a promising approach for the development of sustainable and multifunctional flexible electronics that are resilient in extreme environments.

Biomass and Transparent Supramolecular Elastomers for Green Electronics Enabled by the Controlled Growth and Self-Assembly of Dynamic Polymer Networks

Determining the optimal method for preparing supramolecular materials remains a profound challenge. This process requires a combination of renewable raw materials to create supramolecular materials with multiple functions and properties, including simple fabrication, sustainability, a dynamic nature, good toughness, and transparency. In this work, a strategy is presented for toughening supramolecular networks based on solid-phase chain extension. This toughening strategy is simple and environmentally friendly. In addition, a series of biobased elastomers are designed and prepared with adjustable performance characteristics. This strategy can significantly improve the transparency, tensile strength, and toughness of the synthesized elastomer. The synthesized biobased elastomers have great ductility, repairability, and recyclability, and they show good adhesion and dielectric properties. A biobased ionic skin is assembled from these biobased elastomers. Assembled ionic skin can sensitively detect external stimuli (such as stretching, bending, compression, or temperature changes) and monitor human movement. The conductive and dielectric layers of the biobased ionic skin are both obtained from renewable raw materials. This research provides novel molecular design approaches and material selection methods for promoting the development of green electronic devices and biobased elastomers.

A Biodegradable, Stretchable, Healable, and Self-Powered Optoelectronic Synapse Based on Ionic Gelatins for Neuromorphic Vision System

Optoelectronic synapses have gained increasing attentions as a fundamental building block in the development of neuromorphic visual systems. However, it remains a challenge to integrate multiple functions into a single optoelectronic synapse that can be widely applied in wearable artificial intelligence and implantable neuromorphic vision systems. In this study, a stretchable optoelectronic synapse based on biodegradable ionic gelatin heterojunction is successfully developed. This device exhibits self-powered synaptic plasticity behavior with broad spectral response and excellent elastic properties, yet it degrades rapidly upon disposal. After complete cleavage, the device can be fully repaired within 1 min, which is mainly attributed to the non-covalent interactions between different molecular chains. Moreover, the recovery and reprocessing of the ionic gelatins result in optoelectronic properties that are virtually indistinguishable from their original state, showcasing the resilience and durability of ionic gelatins. The combination of biodegradability, stretchability, self-healing, zero-power consumption, ease of large-scale preparation, and low cost makes the work a major step forward in the development of biodegradable and stretchable optoelectronic synapses.

Patterning Techniques Based on Metallized Electrospun Nanofibers for Advanced Stretchable Electronics

Stretchable electronics have experienced remarkable progress, especially in sensors and wireless communication systems, attributed to their ability to conformably contact with rough or uneven surfaces. However, the development of complex, multifunctional, and high-precision stretchable electronics faces substantial challenges, including instability at rigid-soft interfaces and incompatibility with traditional high-precision patterning technologies. Metallized electrospun nanofibers emerge as a promising conductive filler, offering exceptional stretchability, electrical conductivity, transparency, and compatibility with existing patterning technologies. Here, this review focuses on the fundamental properties, preparation processes, patterning technologies, and application scenarios of conductive stretchable composites based on metallized nanofibers. Initially, it introduces the fabrication processes of metallized electrospun nanofibers and their advantages over alternative materials. It then highlights recent progress in patterning technologies, including collector collection, vapor deposition with masks, and lithography, emphasizing their role in enhancing precision and integration. Furthermore, the review shows the broad applicability and potential influence of metallized electrospun nanofibers in various fields through their use in sensors, wireless systems, semiconductor devices, and intelligent healthcare solutions. Ultimately, this review seeks to spark further innovation and address the prevailing challenges in stretchable electronics, paving the way for future breakthroughs in this dynamic field.

A Strong and Highly Transparent Ionogel Electrolyte Enabled by In Situ Polymerization-Induced Microphase Separation for High-Performance Electrochromic Devices

Electrochromic devices built with ionogel electrolytes are seen as a pivotal step toward the future of quasi-solid electrochromic devices, due to their striking properties like exceptional safety and high ionic conductivity. Yet, the poor mechanical strength of electrolyte of these devices remains a constraint that hampers their advancement. As a resolution, this research explores the use of a robust, transparent ionogel electrolyte, which is designed using an in situ microphase separation strategy. The ionogels are highly transparent and robust and exhibit excellent physicochemical stability, including a wide electrochemical window and high temperature tolerance. Benefitting from these properties, a high-performance electrochromic device is fabricated through in situ polymerization with the ionogels, PPRODOT as the electrochromic layer, and PEDOT: PSS as the ion storage layer, achieving high transmittance contrast (43.1%), fast response (1/1.7 s), high coloring efficiency (1296.4 cm2 C-1), and excellent cycling endurance (>99.9% retention after 2000 cycles). In addition, using ITO-poly(ethylene terephthalate) as flexible substrates, a deformable electrochromic device displaying high stability is realized, highlighting the potential use in functional wearables.

3D printable elastomers with exceptional strength and toughness

Three-dimensional (3D) printing has emerged as an attractive manufacturing technique because of its exceptional freedom in accessing geometrically complex customizable products. Its potential for mass manufacturing, however, is hampered by its low manufacturing efficiency (print speed) and insufficient product quality (mechanical properties). Recent progresses in ultra-fast 3D printing of photo-polymers1-5 have alleviated the issue of manufacturing efficiency, but the mechanical performance of typical printed polymers still falls far behind what is achievable with conventional processing techniques. This is because of the printing requirements that restrict the molecular design towards achieving high mechanical performance. Here we report a 3D photo-printable resin chemistry that yields an elastomer with tensile strength of 94.6 MPa and toughness of 310.4 MJ m-3, both of which far exceed that of any 3D printed elastomer6-10. Mechanistically, this is achieved by the dynamic covalent bonds in the printed polymer that allow network topological reconfiguration. This facilitates the formation of hierarchical hydrogen bonds (in particular, amide hydrogen bonds), micro-phase separation and interpenetration architecture, which contribute synergistically to superior mechanical performance. Our work suggests a brighter future for mass manufacturing using 3D printing.

Multiperformance PAM/PVA/CaCO3 Hydrogel for Flexible Sensing and Information Encryption

Currently, the development of hydrogels with excellent mechanical properties (elasticity, fatigue resistance, etc.) and conductive properties can better meet their needs in the field of flexible sensor device applications. Generally, hydrogels with a denser cross-linking density tend to have better mechanical properties, but the improvement in mechanical properties comes at the expense of reduced electrical conductivity. Directly generating CaCO3 in the hydrogel prepolymer can not only increase the cross-linking density of its network but also introduce additional ions to enhance its internal ionic strength, which is beneficial to improving the conductivity of the hydrogel. It is still a big challenge to directly generate CaCO3 in the static prepolymer solution and ensure its uniform dispersion in the hydrogel. Herein, we adopted an improved preparation method to ensure that the directly generated CaCO3 particles can be evenly dispersed in the static prepolymer solution until the polymerization is completed. Finally, a PAM/PVA/CaCO3 hydrogel with supertensile, compressive, toughness, and fatigue resistance properties was prepared. In addition, the presence of free Na+ and Cl- gives the hydrogel excellent conductivity and sensing performance to monitor daily human activities. On the basis of the application of hydrogels in information communication, we have further deepened this application by combining the characteristics of hydrogels themselves. Combined with ASCII code, the hydrogel can also be applied in information exchange and information encryption and decryption, achieving the antitheft function in smart locks. A variety of excellent performance integrated PAM/PVA/CaCO3 hydrogels have broad application prospects for flexible sensors, highlighting great potential in human-computer interaction and intelligent information protection.

Thermomechanically Resilient Polyionic Elastomers with Enhanced Anti-Icing Performances

Anti-icing gels inhibit ice formation and accretion; however, current iterations face prevalent drawbacks such as poor strength, weak substrate adhesion, and limited anti-icing properties. Herein, we propose a novel approach to address these challenges by developing a thermomechanical robust polyionic elastomer (PIE) with enhanced anti-icing properties. The PIE surface exhibits an icing delay time up to 5400 s and remains frost-free after exposure to -10 °C for 3.5 h, attributed to the inhibitory effect on ice formation by ions from ILs and the polyelectrolyte network. Moreover, the PIE exhibits remarkable anti-icing durability, with ice adhesion strengths below 35 kPa after undergoing 30 icing/deicing cycle tests at -20 °C. Following sandpaper abrasion (300 cycles), scratching, and heat treatment (100 °C, 16 h), the adhesion strength remains ca. 20 kPa, highlighting its resilience under various thermal and mechanical conditions. This exceptional durability is attributed to the low volatility of the IL and the robust ionic interactions within the PIE network. Furthermore, the PIE demonstrates favorable self-healing properties and strong substrate adhesion in both low-temperature and ambient environments, facilitated by the abundance of hydrogen bonds and electrostatic forces within PIE. This work presents an innovative approach to developing high-performance, durable, and robust anti-icing materials with potential implications across various fields.

Polyelectrolyte-Mediated Modulation of Spatial Internal Stresses of Hydrogels for Complex 3D Actuators

Hydrogel actuators with complex 3D initial shapes show numerous important applications, but it remains challenging to fabricate such actuators. This article describes a polyelectrolyte-based strategy for modulating small-scale internal stresses within hydrogels to construct complex actuators with tailored 3D initial shapes. Introducing polyelectrolytes into precursor solutions significantly enhances the volume shrinkage of hydrogel networks during polymerization, allowing us to modulate internal stresses. Photopolymerization of these polyelectrolyte-containing solutions through a mask produces mechanically strong hydrogel sheets with large patterned internal stresses. Consequently, these hydrogel sheets attain complex 3D initial shapes at equilibrium, in contrast to the planar initial configuration of 2D actuators. We demonstrate that these 3D actuators can reversibly transform into other 3D shapes (i.e., 3D-to-3D shape transformations) in response to external stimuli. Additionally, we develop a predictive model based on the Flory-Rehner theory to analyze the polyelectrolyte-mediated shrinking behaviors of hydrogel networks during polymerization, allowing precise modulation of shrinkage and internal stress. This polyelectrolyte-boosted shrinking mechanism paves a route to the fabrication of high-performance 3D hydrogel actuators.

A Solid-Liquid Bicontinuous Fiber with Strain-Insensitive Ionic Conduction

Stretchable ionic conductors are crucial for enabling advanced iontronic devices to operate under diverse deformation conditions. However, when employed as interconnects, existing ionic conductors struggle to maintain stable ionic conduction under strain, hindering high-fidelity signal transmission. Here, it is shown that strain-insensitive ionic conduction can be achieved by creating a solid-liquid bicontinuous microstructure. A bicontinuous fiber from polymerization-induced phase separation, which contains a solid elastomer phase interpenetrated by a liquid ion-conducting phase, is fabricated. The spontaneous partitioning of dissolved salts leads to the formation of a robust self-wrinkled interface, fostering the development of highly tortuous ionic channels. Upon stretch, these meandering ionic channels are straightened, effectively enhancing ionic conductivity to counteract the strain effect. Remarkably, the fiber retains highly stable ionic conduction till fracture, with only 7% resistance increase at 200% strain. This approach presents a promising avenue for designing durable ionic cables capable of signal transmission with minimal strain-induced distortion.

Bioinspired Mechanically Robust and Recyclable Hydrogel Microfibers Based on Hydrogen-Bond Nanoclusters

Mechanically robust hydrogel fibers have demonstrated great potential in energy dissipation and shock-absorbing applications. However, developing such materials that are recyclable, energy-efficient, and environmentally friendly remains an enormous challenge. Herein, inspired by spider silk, a continuous and scalable method is introduced for spinning a polyacrylamide hydrogel microfiber with a hierarchical sheath-core structure under ambient conditions. Applying pre-stretch and twist in the as-spun hydrogel microfibers results in a tensile strength of 525 MPa, a toughness of 385 MJ m-3, and a damping capacity of 99%, which is attributed to the reinforcement of hydrogen-bond nanoclusters within the microfiber matrix. Moreover, it maintains both structural and mechanical stability for several days, and can be directly dissolved in water, providing a sustainable spinning dope for re-spinning into new microfibers. This work provides a new strategy for the spinning of robust and recyclable hydrogel-based fibrous materials.

Cephalopod-Inspired Chemical-Gated Hydrogel Actuation Systems for Information 3D-Encoding Display

Cephalopods evolve the acetylcholine-gated actuation control function of their skin muscles, which enables their dynamic/static multimode display capacities for achieving perfectly spatial control over the colors/patterns on every inch of skin. Reproduction of artificial analogs that exhibit similar multimodal display is essential to reach advanced information three-dimensional (3D) encoding with higher security than the classic 2D-encoding strategy, but remains underdeveloped. The core difficulty is how to replicate such chemical-gated actuation control function into artificial soft actuating systems. Herein, this work proposes to develop azobenzene-functionalized poly(acrylamide) (PAAm) hydrogel systems, whose upper critical solution temperature (UCST) type actuation responsiveness can be intelligently programmed or even gated by the addition of hydrophilic α-cyclodextrin (α-CD) molecules for reversible association with pendant azobenzene moieties via supramolecular host-guest interactions. By employing such α-CD-gated hydrogel actuator as an analogue of cephalopods' skin muscle, biomimetic mechanically modulated multicolor fluorescent display systems are designed, which demonstrate a conceptually new α-CD-gated "thermal stimulation-hydrogel actuation-fluorescence output" display mechanism. Consequently, high-security 3D-encoding information carriers with an unprecedented combination of single-input multiple-output, dynamic/static dual-mode and spatially controlled display capacities are achieved. This bioinspired strategy brings functional-integrated features for artificial display systems and opens previously unidentified avenues for information security.

Structuring and Shaping of Mechanically Robust and Functional Hydrogels toward Wearable and Implantable Applications

Hydrogels possess unique features such as softness, wetness, responsiveness, and biocompatibility, making them highly suitable for biointegrated applications that have close interactions with living organisms. However, conventional man-made hydrogels are usually soft and brittle, making them inferior to the mechanically robust biological hydrogels. To ensure reliable and durable operation of biointegrated wearable and implantable devices, mechanical matching and shape adaptivity of hydrogels to tissues and organs are essential. Recent advances in polymer science and processing technologies have enabled mechanical engineering and shaping of hydrogels for various biointegrated applications. In this review, polymer network structuring strategies at micro/nanoscales for toughening hydrogels are summarized, and representative mechanical functionalities that exist in biological materials but are not easily achieved in synthetic hydrogels are further discussed. Three categories of processing technologies, namely, 3D printing, spinning, and coating for fabrication of tough hydrogel constructs with complex shapes are reviewed, and the corresponding hydrogel toughening strategies are also highlighted. These developments enable adaptive fabrication of mechanically robust and functional hydrogel devices, and promote application of hydrogels in the fields of biomedical engineering, bioelectronics, and soft robotics.

Formulation of Ascorbic Acid and Betaine-based Therapeutic Deep Eutectic System for Enhanced Transdermal Delivery of Ascorbic Acid

L-ascorbic acid (AA), a potent antioxidant, is commonly used topically in the pharmaceutical and cosmetic fields. However, the incorporation of AA into topical formulations is difficult because of its highly unstable nature and relatively poor skin permeability. In this study, we propose an alternative strategy for improving the solubility and topical delivery of AA through its conversion to a therapeutic deep eutectic system (THEDES). AA and betaine (Bet)-based THEDESs were prepared at certain molar ratios and characterized using polarized optical microscopy, Fourier transform infrared spectroscopy, and differential scanning calorimetry. Solubility tests showed that AA in the form of THEDES was readily soluble in various polyols (glycerin, 1,3-butylene glycol, dipropylene glycol, and 1,3-propanediol) at a high concentration (approximately 40%). Furthermore, compared to AA alone or the physical mixture of AA and Bet, AA-based THEDES significantly enhanced AA delivery through porcine skin. In an in vivo human study, THEDES-containing serum reduced the markers of aging and induced an even skin tone. These findings indicate the utility of AA and Bet-based THEDES as novel transdermal delivery systems for AA. Furthermore, our approach also showed good extension to developing gluconolactone, a well-known natural antioxidant, and Bet-based THEDES, showing potential application in transdermal delivery systems.

Concentration-induced spontaneous polymerization of protic ionic liquids for efficient in situ adhesion

The advancement of contemporary adhesives is often limited by the balancing act between cohesion and interfacial adhesion strength. This study explores an approach to overcome this trade-off by utilizing the spontaneous polymerization of a protic ionic liquid-based monomer obtained through the neutralization of 2-acrylamide-2-methyl propane sulfonic acid and hydroxylamine. The initiator-free polymerization process is carried out through a gradual increase in monomer concentration in aqueous solutions caused by solvent evaporation upon heating, which results in the in-situ formation of a tough and thin adhesive layer with a highly entangled polymeric network and an intimate interface contact between the adhesive and substrate. The abundance of internal and external non-covalent interactions also contributes to both cohesion and interfacial adhesion. Consequently, the produced protic poly(ionic liquid)s exhibit considerable adhesion strength on a variety of substrates. This method also allows for the creation of advanced adhesive composites with electrical conductivity or visualized sensing functionality by incorporating commercially available fillers into the ionic liquid adhesive. This study provides a strategy for creating high-performance ionic liquid-based adhesives and highlights the importance of in-situ polymerization for constructing adhesive composites.

Bioinspired H-Bonding Connected Gradient Nanostructure Actuators Based on Cellulose Nanofibrils and Graphene

The construction of flexible actuators with ultra-fast actuation and robust mechanical properties is crucial for soft robotics and smart devices, but still remains a challenge. Inspired by the unique mechanism of pinecones dispersing seeds in nature, a hygroscopic actuator with interlayer network-bonding connected gradient structure is fabricated. Unlike most conventional bilayer actuator designs, the strategy leverages biobased polyphenols to construct strong interfacial H-bonding networks between 1D cellulose nanofibers and 2D graphene oxide, endowing the materials with high tensile strength (172 MPa) and excellent toughness (6.64 MJ m-3). Furthermore, the significant difference in hydrophilicity between GO and rGO, along with the dense interlayer H-bonding, enables ultra-fast water exchange during water absorption and desorption processes. The resulted actuator exhibits ultra-fast driving speed (154° s-1), excellent pressure-resistant and cyclic stability. Taking advantages of these benefits, the actuator can be fabricated into smart devices (such as smart grippers, humidity control switches) with significant potential for practical applications. The presented approach to constructing interlayer H-bonding in gradient structures is instructive for achieving high performance and functionalization of biomass nanomaterials and the complex of 1D/2D nanomaterials.

Hydrogen Bonding Competition Mediated Phase Separation with Abnormal Moisture-Induced Stiffness Boosting

Moisture usually deteriorates polymers' mechanical performance owing to its plasticizing effect, causing side effects in their practical load-bearing applications. Herein, a simple binary ionogel consisting of an amphiphilic polymer network and a hydrophobic ionic liquid (IL) is developed with remarkable stiffening effect after moisture absorption, demonstrating a complete contrast to water-induced softening effect of most polymer materials. Such a moisture-induced stiffening behavior is induced by phase separation after hydration of this binary ionogel. Specifically, it is revealed that hydrogen (H)-bonding structures play a dominant role in the humidity-responsive behavior of the ionogel, where water will preferentially interact with polymer chains through H-bonding and break the polymer-IL H-bonds, thus leading to phase separation structures with modulus boosting. This work may provide a facile and effective molecular engineering route to construct mechanically adaptive polymers with water-induced dramatic stiffening for diverse applications.

Self-Healing Hydrogel Bioelectronics

Hydrogels have emerged as powerful building blocks to develop various soft bioelectronics because of their tissue-like mechanical properties, superior bio-compatibility, the ability to conduct both electrons and ions, and multiple stimuli-responsiveness. However, hydrogels are vulnerable to mechanical damage, which limits their usage in developing durable hydrogel-based bioelectronics. Self-healing hydrogels aim to endow bioelectronics with the property of repairing specific functions after mechanical failure, thus improving their durability, reliability, and longevity. This review discusses recent advances in self-healing hydrogels, from the self-healing mechanisms, material chemistry, and strategies for multiple properties improvement of hydrogel materials, to the design, fabrication, and applications of various hydrogel-based bioelectronics, including wearable physical and biochemical sensors, supercapacitors, flexible display devices, triboelectric nanogenerators (TENGs), implantable bioelectronics, etc. Furthermore, the persisting challenges hampering the development of self-healing hydrogel bioelectronics and their prospects are proposed. This review is expected to expedite the research and applications of self-healing hydrogels for various self-healing bioelectronics.

Engineering Dual CO2- and Photothermal-Responsive Membranes for Switchable Double Emulsion Separation

Stimulus-responsive membranes demonstrate promising applications in switchable oil/water emulsion separations. However, they are unsuitable for the treatment of double emulsions like oil-in-water-in-oil (O/W/O) and water-in-oil-in-water (W/O/W) emulsions. For efficient separation of these complicated emulsions, fine control over the wettability, response time, and aperture structure of the membrane is required. Herein, dual-coated fibers consisting of primary photothermal-responsive and secondary CO2-responsive coatings are prepared by two steps. Automated weaving of these fibers produces membranes with photothermal- and CO2-responsive characteristics and narrow pore size distributions. These membranes exhibit fast switching wettability between superhydrophilicity (under CO2 stimulation) and high hydrophobicity (under near-infrared stimulation), achieving on-demand separation of various O/W/O and W/O/W emulsions with separation efficiencies exceeding 99.6%. Two-dimensional low-field nuclear magnetic resonance and correlated spectra technique are used to clarify the underlying mechanism of switchable double emulsion separation. The approach can effectively address the challenges associated with the use of stimulus-responsive membranes for double emulsion separation and facilitate the industrial application of these membranes.

Skin-Inspired, Highly Sensitive, Broad-Range-Response and Ultra-Strong Gradient Ionogels Prepared by Electron Beam Irradiation

Skin, characterized by its distinctive gradient structure and interwoven fibers, possesses remarkable mechanical properties and highly sensitive attributes, enabling it to detect an extensive range of stimuli. Inspired by these inherent qualities, a pioneering approach involving the crosslinking of macromolecules through in situ electron beam irradiation (EBI) is proposed to fabricate gradient ionogels. Such a design offers remarkable mechanical properties, including excellent tensile properties (>1000%), exceptional toughness (100 MJ m-3), fatigue resistance, a broad temperature range (-65-200°C), and a distinctive gradient modulus change. Moreover, the ionogel sensor exhibits an ultra-fast response time (60 ms) comparable to skin, an incredibly low detection limit (1 kPa), and an exceptionally wide detection range (1 kPa-1 MPa). The exceptional gradient ionogel material holds tremendous promise for applications in the field of smart sensors, presenting a distinct strategy for fabricating flexible gradient materials.

Establishing superfine nanofibrils for robust polyelectrolyte artificial spider silk and powerful artificial muscles

Spider silk exhibits an excellent combination of high strength and toughness, which originates from the hierarchical self-assembled structure of spidroin during fiber spinning. In this work, superfine nanofibrils are established in polyelectrolyte artificial spider silk by optimizing the flexibility of polymer chains, which exhibits combination of breaking strength and toughness ranging from 1.83 GPa and 238 MJ m-3 to 0.53 GPa and 700 MJ m-3, respectively. This is achieved by introducing ions to control the dissociation of polymer chains and evaporation-induced self-assembly under external stress. In addition, the artificial spider silk possesses thermally-driven supercontraction ability. This work provides inspiration for the design of high-performance fiber materials.

Thermo-growing ion clusters enabled healing strengthening and tough adhesion for highly reliable skin electronics

Self-healing and self-adhesion capacities are essential for many modern applications such as skin-interfaced electronics for improving longevity and reliability. However, the self-healing efficiency and adhesive toughness of most synthetic polymers are limited to their original network, making reliability under dynamic deformation still challenging. Herein, inspired by the growth of living organisms, a highly stretchable supramolecular elastomer based on thermo-responsive ion clusters and a dynamic polysulfide backbone was developed. Attributed to the synergic growth of ion clusters and dynamic exchange of disulfide bonds, the elastomer exhibited unique healing strengthening (healing efficiency >200%) and thermo-enhanced tough adhesion (interfacial toughness >500 J m-2) performances. To prove its practical application in highly reliable skin electronics, we further composited the elastomer with a zwitterion to prepare a highly conductive ionic elastomer and applied it in wearable strain sensing and long-term electrophysiological detection. This work provides a new avenue to realize high reliability in skin interfaced electronics.

Highly Stretchable, Self-Healing, and Sensitive E-Skins at -78 °C for Polar Exploration

Ultralow temperature-tolerant electronic skins (e-skins) can endow polar robots with tactile feedback for exploring in extremely cold polar environments. However, it remains a challenge to develop e-skins that enable sensitive touch sensation and self-healing at ultralow temperatures. Herein, we describe the development of a sensitive robotic hand e-skin that can stretch, self-heal, and sense at temperatures as low as -78 °C. The elastomeric substrate of this e-skin is based on poly(dimethylsiloxane) supramolecular polymers and multistrength dynamic H-bonds, in particular with quadruple H-bonding motifs (UPy). The structure-performance relationship of the elastomer at ultralow temperatures is investigated. The results show that elastomers with side-chain UPy units exhibit higher stretchability (∼3257%) and self-healing efficiency compared to those with main-chain UPy units. This is attributed to the lower binding energy variation and lower potential well. Based on the elastomer with side-chain UPy and man-made electric ink, a sensitive robotic hand e-skin for usage at -78 °C is constructed to precisely sense the shape of objects and specific symbols, and its sensation can completely self-recover after being damaged. The findings of this study contribute to the concept of using robotic hands with e-skins in polar environments that make human involvement limited, dangerous, or impossible.

Mechanically Controlled Enzymatic Polymerization and Remodeling

In nature, proteins possess the remarkable ability to sense and respond to mechanical forces, thereby triggering various biological events, such as bone remodeling and muscle regeneration. However, in synthetic systems, harnessing the mechanical force to induce material growth still remains a challenge. In this study, we aimed to utilize low-frequency ultrasound (US) to activate horseradish peroxidase (HRP) and catalyze free radical polymerization. Our findings demonstrate the efficacy of this mechano-enzymatic chemistry in rapidly remodeling the properties of materials through cross-linking polymerization and surface coating. The resulting samples exhibited a significant enhancement in tensile strength, elongation, and Young's modulus. Moreover, the hydrophobicity of the surface could be completely switched within just 30 min of US treatment. This work presents a novel approach for incorporating mechanical sensing and rapid remodeling capabilities into materials.

Polymerized and Colloidal Ionic Liquids─Syntheses and Applications

The breadth and importance of polymerized ionic liquids (PILs) are steadily expanding, and this review updates advances and trends in syntheses, properties, and applications over the past five to six years. We begin with an historical overview of the genesis and growth of the PIL field as a subset of materials science. The genesis of ionic liquids (ILs) over nano to meso length-scales exhibiting 0D, 1D, 2D, and 3D topologies defines colloidal ionic liquids, CILs, which compose a subclass of PILs and provide a synthetic bridge between IL monomers (ILMs) and micro to macro-scale PIL materials. The second focus of this review addresses design and syntheses of ILMs and their polymerization reactions to yield PILs and PIL-based materials. A burgeoning diversity of ILMs reflects increasing use of nonimidazolium nuclei and an expanding use of step-growth chemistries in synthesizing PIL materials. Radical chain polymerization remains a primary method of making PILs and reflects an increasing use of controlled polymerization methods. Step-growth chemistries used in creating some CILs utilize extensive cross-linking. This cross-linking is enabled by incorporating reactive functionalities in CILs and PILs, and some of these CILs and PILs may be viewed as exotic cross-linking agents. The third part of this update focuses upon some advances in key properties, including molecular weight, thermal properties, rheology, ion transport, self-healing, and stimuli-responsiveness. Glass transitions, critical solution temperatures, and liquidity are key thermal properties that tie to PIL rheology and viscoelasticity. These properties in turn modulate mechanical properties and ion transport, which are foundational in increasing applications of PILs. Cross-linking in gelation and ionogels and reversible step-growth chemistries are essential for self-healing PILs. Stimuli-responsiveness distinguishes PILs from many other classes of polymers, and it emphasizes the importance of segmentally controlling and tuning solvation in CILs and PILs. The fourth part of this review addresses development of applications, and the diverse scope of such applications supports the increasing importance of PILs in materials science. Adhesion applications are supported by ionogel properties, especially cross-linking and solvation tunable interactions with adjacent phases. Antimicrobial and antifouling applications are consequences of the cationic nature of PILs. Similarly, emulsion and dispersion applications rely on tunable solvation of functional groups and on how such groups interact with continuous phases and substrates. Catalysis is another significant application, and this is an historical tie between ILs and PILs. This component also provides a connection to diverse and porous carbon phases templated by PILs that are catalysts or serve as supports for catalysts. Devices, including sensors and actuators, also rely on solvation tuning and stimuli-responsiveness that include photo and electrochemical stimuli. We conclude our view of applications with 3D printing. The largest components of these applications are energy related and include developments for supercapacitors, batteries, fuel cells, and solar cells. We conclude with our vision of how PIL development will evolve over the next decade.

Practical Applications of Self-Healing Polymers Beyond Mechanical and Electrical Recovery

Self-healing polymeric materials, which can repair physical damage, offer promising prospects for protective applications across various industries. Although prolonged durability and resource conservation are key advantages, focusing solely on mechanical recovery may limit the market potential of these materials. The unique physical properties of self-healing polymers, such as interfacial reduction, seamless connection lines, temperature/pressure responses, and phase transitions, enable a multitude of innovative applications. In this perspective, the diverse applications of self-healing polymers beyond their traditional mechanical strength are emphasized and their potential in various sectors such as food packaging, damage-reporting, radiation shielding, acoustic conservation, biomedical monitoring, and tissue regeneration is explored. With regards to the commercialization challenges, including scalability, robustness, and performance degradation under extreme conditions, strategies to overcome these limitations and promote successful industrialization are discussed. Furthermore, the potential impacts of self-healing materials on future research directions, encompassing environmental sustainability, advanced computational techniques, integration with emerging technologies, and tailoring materials for specific applications are examined. This perspective aims to inspire interdisciplinary approaches and foster the adoption of self-healing materials in various real-life settings, ultimately contributing to the development of next-generation materials.

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

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

Bioinspired Supramolecular Hydrogel from Design to Applications

Nature offers a wealth of opportunities to solve scientific and technological issues based on its unique structures and function. The dynamic non-covalent interaction is considered to be the main base of living functions of creatures including humans, animals, and plants. Supramolecular hydrogels formed by non-covalent bonding interactions has become a unique platform for constructing promising materials for medicine, energy, electronic, and biological substitute. In this review, the self-assemble principle of supramolecular hydrogels is summarized. Next, the stimulation of external environment that triggers the assembly or disassembly of supramolecular hydrogels are recapitulated, including temperature, mechanics, light, pH, ions, etc. The main applications of bioinspired supramolecular hydrogels in terms of bionic objects including humans, animals, and plants are also described. Although so many efforts are done for revealing the synergized mechanism of the function and non-covalent interactions on the supramolecular hydrogel, the complexity and variability between stimulus and non-covalent bonding in the supramolecular system still require impeccable theories. As an outlook, the bioinspired supramolecular hydrogel is just beginning to exhibit its great potential in human life, offering significant opportunities in drug delivery and screening, implantable devices and substitutions, tissue engineering, micro-fluidic devices, and biosensors.

Entropy-Driven Design of Highly Impact-Stiffening Supramolecular Polymer Networks with Salt-Bridge Hydrogen Bonds

Impact-stiffening materials that undergo a strain rate-induced soft-to-rigid transition hold great promise as soft armors in the protection of the human body and equipment. However, current impact-stiffening materials, such as polyborosiloxanes and shear-thickening fluids, often exhibit a limited impact-stiffening response. Herein, we propose a design strategy for fabricating highly impact-stiffening supramolecular polymer networks by leveraging high-entropy-penalty physical interactions. We synthesized a fully biobased supramolecular polymer comprising poly(α-thioctic acid) and arginine clusters, whose chain dynamics are governed by highly specific guanidinium-carboxylate salt-bridge hydrogen bonds. The resulting material exhibits an exceptional impact-stiffening response of ∼2100 times, transitioning from a soft dissipating state (21 kPa, 0.1 Hz) to a highly stiffened glassy state (45.3 MPa, 100 Hz) with increasing strain rates. Moreover, the material's high energy-dissipating and hot-melting properties bring excellent damping performance and easy hybridization with other scaffolds. This entropy-driven approach paves the way for the development of next-generation soft, sustainable, and impact-resistant materials.

Solid-state, liquid-free ion-conducting elastomers: rising-star platforms for flexible intelligent devices

Soft ionic conductors have emerged as a powerful toolkit to engineer transparent flexible intelligent devices that go beyond their conventional counterparts. Particularly, due to their superior capacities of eliminating the evaporation, freezing and leakage issues of the liquid phase encountered with hydrogels, organohydrogels and ionogels, the emerging solid-state, liquid-free ion-conducting elastomers have been largely recognized as ideal candidates for intelligent flexible devices. However, despite their extensive development, a comprehensive and timely review in this emerging field is lacking, particularly from the perspective of design principles, advanced manufacturing, and distinctive applications. Herein, we present (1) the design principles and intriguing merits of solid-state, liquid-free ion-conducting elastomers; (2) the methods to manufacture solid-state, liquid-free ion-conducting elastomers with preferential architectures and functions using advanced technologies such as 3D printing; (3) how to leverage solid-state, liquid-free ion-conducting elastomers in exploiting advanced applications, especially in the fields of flexible wearable sensors, bioelectronics and energy harvesting; (4) what are the unsolved scientific and technical challenges and future opportunities in this multidisciplinary field. We envision that this review will provide a paradigm shift to trigger insightful thinking and innovation in the development of intelligent flexible devices and beyond.

Ultrasensitive stretchable bimodal sensor based on novel elastomer and ionic liquid for temperature and humidity detection

In this work, we present a novel stretchable bimodal sensor that can simultaneously detect temperature and humidity changes based on poly-hydroxyethyl acrylate (PHEA) elastomer infused with 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) ionic liquid. The sensor exhibits high transparency, stability, and biocompatibility, as well as excellent mechanical and sensing properties. The sensor can achieve a maximum strain of 761%, a sensitivity of 4.5%/°C at room temperature, a detection range from -35 to 120 °C, and a response time of 10 ms. The sensor is able to provide acute response to movement of human hand at close range and can detect temperature changes as small as 0.004 °C in the range of 20-30 °C. The sensor also responds to humidity change, showing a high sensitivity to humidity change of 4.4%/RH% under the temperature of 30 °C. The sensor can be used for various applications in wearable electronics, human-machine interfaces, and soft robotics.

Gradiently Foaming Ultrasoft Hydrogel with Stop Holes for Highly Deformable, Crack-Resistant and Sensitive Conformal Human-Machine Interfaces

Hydrogels are considered as promising materials for human-machine interfaces (HMIs) owing to their merits of tailorable mechanical and electrical properties; nevertheless, it remains challenging to simultaneously achieve ultrasoftness, good mechanical robustness and high sensitivity, which are the pre-requisite requirements for wearable sensing applications. Herein, for the first time, this work proposes a universal phase-transition-induced bubbling strategy to fabricate ultrasoft gradient foam-shaped hydrogels (FSHs) with stop holes for high deformability, crack-resistance and sensitive conformal HMIs. As a typical system, the FSH based on polyacrylamide/sodium alginate system shows an ultralow Young's modulus (1.68 kPa), increased sustainable strain (1411%), enhanced fracture toughness (915.6 J m-2 ), improved tensile sensitivity (21.77), and compressive sensitivity (65.23 kPa-1 ). The FSHs are used for precisely acquiring and identifying gesture commands of the operator to remotely control a surgical robot for endoscopy and an electric ship in a first-person perspective for cruising, feeding crabs and monitoring the environmental change in real-time.

Rapid Self-Healing Hydrogel with Ultralow Electrical Hysteresis for Wearable Sensing

Self-healing hydrogels are in high demand for wearable sensing applications due to their remarkable deformability, high ionic and electrical conductivity, self-adhesiveness to human skin, as well as resilience to both mechanical and electrical damage. However, these hydrogels face challenges such as delayed healing times and unavoidable electrical hysteresis, which limit their practical effectiveness. Here, we introduce a self-healing hydrogel that exhibits exceptionally rapid healing with a recovery time of less than 0.12 s and an ultralow electrical hysteresis of less than 0.64% under cyclic strains of up to 500%. This hydrogel strikes an ideal balance, without notable trade-offs, between properties such as softness, deformability, ionic and electrical conductivity, self-adhesiveness, response and recovery times, durability, overshoot behavior, and resistance to nonaxial deformations such as twisting, bending, and pressing. Owing to this unique combination of features, the hydrogel is highly suitable for long-term, durable use in wearable sensing applications, including monitoring body movements and electrophysiological activities on the skin.

Masking Strategy Constructed Metal Coordination Hydrogels with Improved Mechanical Properties for Flexible Electronic Sensors

Metal coordination hydrogels (MC-HGs) that introduce dynamically coordinate bonds together with metal ionic conduction have attracted considerable attention in flexible electronics. However, the traditional soaking method alleged to have technical scalability faces the challenge of forming MC-HGs with a "core-shell" structure, which undoubtedly reduces the whole mechanical properties and ionic stimulation responsiveness required for flexible electronics materials. Herein, a novel strategy referred to as "masking" has been proposed based on the theory of the valence bond and coordination chemistry. By regulating the masking agents and their concentrations as well as pairing mode with the metal ions, the whole mechanical properties of the resulting composites (MC-HGsMasking) show nearly double the values of their traditional soaking samples (MC-HGsSoaking). For example, the fracture stress and toughness of Fe-HGsMasking(SA, 5.0 g/L) are 1.55 MPa and 2.14 MJ/m3, almost twice those of Fe-HGsSoaking (0.83 MPa and 0.93 MJ/m3, respectively). Microstructure characterization combined with finite element analysis, molecular dynamics, and first-principles simulations demonstrates that the masking strategy first facilitating interfacial permeation of metal complexes and then effective coordination with functional ligands (carboxylates) of the hydrogels is the mechanism to strengthen the mechanical properties of composites MC-HGsMasking, which has the potential to break through the limitations of current MC-HGs in flexible electronic sensor applications.

Self-compliant ionic skin by leveraging hierarchical hydrogen bond association

Robust interfacial compliance is essential for long-term physiological monitoring via skin-mountable ionic materials. Unfortunately, existing epidermal ionic skins are not compliant and durable enough to accommodate the time-varying deformations of convoluted skin surface, due to an imbalance in viscosity and elasticity. Here we introduce a self-compliant ionic skin that consistently works at the critical gel point state with almost equal viscosity and elasticity over a super-wide frequency range. The material is designed by leveraging hierarchical hydrogen bond association, allowing for the continuous release of polymer strands to create topological entanglements as complementary crosslinks. By embodying properties of rapid stress relaxation, softness, ionic conductivity, self-healability, flaw-insensitivity, self-adhesion, and water-resistance, this ionic skin fosters excellent interfacial compliance with cyclically deforming substrates, and facilitates the acquisition of high-fidelity electrophysiological signals with alleviated motion artifacts. The presented strategy is generalizable and could expand the applicability of epidermal ionic skins to more complex service conditions.

Optimization of Deep Eutectic Solvents Enables Green and Efficient Cryopreservation

Cryopreservation presents significant opportunities for biomedical applications including cell therapy, tissue engineering, and assisted reproduction. Dimethyl sulfoxide (DMSO), the most commonly used cryoprotectant (CPA), can be added to cells to prevent cryogenic damage. However, the toxicity of cryoprotectants restrains its further development in many areas with safety concerns such as clinical treatment. Therefore, the development of low-toxicity cryoprotectants is essential for medical research. This work reports deep eutectic solvents (DES) as naturally biocompatible osmoprotectants for green and efficient cryopreservation of human umbilical cord mesenchymal stem cells (HuMSC), which may be an ideal alternative to DMSO. The six types of DESs were explored for thermal properties, toxicity, and permeability in cells. Raman spectroscopy and viscosity studies showed that DES exhibited an improved hydrogen-bonding system as the temperature decreased. By optimizing the freezing process (cooling rate, incubation time, and loading procedure) of DES, the viability of mouse embryonic fibroblast cells (NIH-3T3) after thawing was significantly improved. The HuMSC were successfully preserved with no significant difference (p > 0.05) in cell viability (94.65%) after thawing compared with DMSO, which preserved the cell differentiation function and improved the cell proliferation rate. The mechanism of DES in cryopreservation was investigated, and it was found that DES could bind water molecules and effectively inhibit the growth of ice crystals during ice recrystallization, reducing mechanical damage to cells. This study highlights the excellent performance of DES as a low-toxicity CPA for stem cell preservation, which may be a significant advance for future clinical cell therapy.

Fabrication of a tough, long-lasting adhesive hydrogel patch via the synergy of interfacial entanglement and adhesion group densification

Adhesive hydrogels (AHs) are considered ideal materials for flexible sensors. However, the lack of effective energy dissipation networks and sparse surface polar groups in AHs lead to poor mechanical properties and interfacial adhesion, which limit their practical application. Herein, a tough, long-lasting adhesive and highly conductive nanocomposite hydrogel (PACPH) was fabricated via the synergy of interfacial entanglement and adhesion group densification. PACPH was obtained by the in situ polymerization of highly carboxylated cellulose nanocrystals (SCNCPA, surface pre-grafted polyacrylic acid chains, C-COOH = 11.5 mmol g-1) with the acrylic acid precursor. The unique tacticity of SCNCPA provides strong interface entanglement and multiple hydrogen bonds with the PACPH network, which further increases the energy dissipated during SCNCPA displacements, and enhances the mechanical properties of PACPH (tensile strength = 1.45 MPa, modulus = 332 kPa, and fracture toughness = 13.2 MJ m-3). Meanwhile, SCNCPA increases the density of surface polar groups in PAPCH and also acts as an anchor point to improve the adhesion strength (>2-3 times) of PACPH on various substrates. The combination of excellent mechanical, adhesive, and conductive properties of the PAPCH-integrated patches enables long-term monitoring of human daily activities and electrocardiogram (ECG) signals, verifying that PAPCH is a promising material platform for the further development of flexible sensors and other health management devices.

Mechanochromic and ionic conductive cholesteric liquid crystal elastomers for biomechanical monitoring and human-machine interaction

Cholesteric liquid crystal elastomers (CLCEs) that combine rubbery elasticity with structural colour from self-assembled helical nanostructures are of paramount importance for diverse applications such as biomimetic skins, adaptive optics and soft robotics. Despite great advances, it is challenging to integrate electrical sensing and colour-changing characteristics in a single CLCE system. Here, we report the design and synthesis of an ionic conductive cholesteric liquid crystal elastomer (iCLCE) through in situ Michael addition and free-radical photopolymerization of CLCE precursors on silane-functionalized polymer ionic liquid networks, in which robust covalent chemical bonding was formed at the interface. Thanks to superior mechanochromism and ionic conductivity, the resulting iCLCEs exhibit dynamic colour-changing and electrical sensing functions in a wide range upon mechanical stretching, and can be used for biomechanical monitoring during joint bending. Importantly, a capacitive elastomeric sensor can be constructed through facilely stacking iCLCEs, where the optical and electrical dual-signal reporting performance allows intuitive visual localization of pressure intensity and distribution. Moreover, proof-of-concept application of the iCLCEs has been demonstrated with human-interactive systems. The research disclosed herein can provide new insights into the development of bioinspired somatosensory materials for emerging applications in diverse fields such as human-machine interaction, prostheses and intelligent robots.

Oxidized sodium alginate/polyacrylamide hydrogels adhesive for promoting wheat growth

Chemical modification of sodium alginate (SA) polymer chains can increase its functional group species. Sodium periodate (SP) was usually used to oxidize the hydroxyl groups on the chain of SA to aldehyde groups, the preparation of oxidized sodium alginate (OSA) using SP is not only complicated, also limits the variety of functional groups on the chain of OSA. By contrast, we have developed an innovative strategy for OSA, in which ammonium persulfate (APS) was used to oxidize SA, providing a clear elucidation of the oxidizing process and mechanism. OSA/PAM hydrogels were synthesized using OSA, the hydrogels possess excellent adhesion properties to various non-metallic and metallic substrates. Tensile and compression tests show that the cross-linked OSA/PAM hydrogels have superior mechanical properties. We exploit OSA/PAM hydrogels as soil adhesive and water-retaining agents for wheat growth. OSA/PAM hydrogels significantly improve the survival time of wheat grown in brown loam soil under a water-shortage environment, and slow down the wilting of wheat in a water-shortage environment and prolong the survival time of wheat in sandy soils. Our trials should make hydrogels important for wheat cultivation in brown loam soils and the development of desert areas.