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

Zwitterionic Biomaterials

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

Ultrafast, autonomous self-healable iontronic skin exhibiting piezo-ionic dynamics

The self-healing properties and ionic sensing capabilities of the human skin offer inspiring groundwork for the designs of stretchable iontronic skins. However, from electronic to ionic mechanosensitive skins, simultaneously achieving autonomously superior self-healing properties, superior elasticity, and effective control of ion dynamics in a homogeneous system is rarely feasible. Here, we report a Cl-functionalized iontronic pressure sensitive material (CLiPS), designed via the introduction of Cl-functionalized groups into a polyurethane matrix, which realizes an ultrafast, autonomous self-healing speed (4.3 µm/min), high self-healing efficiency (91% within 60 min), and mechanosensitive piezo-ionic dynamics. This strategy promotes both an excellent elastic recovery (100%) and effective control of ion dynamics because the Cl groups trap the ions in the system via ion-dipole interactions, resulting in excellent pressure sensitivity (7.36 kPa-1) for tactile sensors. The skin-like sensor responds to pressure variations, demonstrating its potential for touch modulation in future wearable electronics and human-machine interfaces.

An Elastic and Damage-Tolerant Dry Epidermal Patch with Robust Skin Adhesion for Bioelectronic Interfacing

On-skin patches that record biopotential and biomechanical signals are essential for wearable healthcare monitoring, clinical treatment, and human-machine interaction. To acquire wearing comfort and high-quality signals, patches with tissue-like softness, elastic recovery, damage tolerance, and robust bioelectronic interface are highly desired yet challenging to achieve. Here, we report a dry epidermal patch made from a supramolecular polymer (SESA) and an in situ transferred carbon nanotubes' percolation network. The polymer possesses a hybrid structure of copolymerized permanent scaffold permeated by multiple dynamic interactions, which imparts a desired mechanical response transition from elastic recoil to energy dissipation with increased elongation. Such SESA-based patches are soft (Young's modulus ∼0.1 MPa) and elastic within physiologically relevant strain levels (97% elastic recovery at 50% tensile strain), intrinsically mechanical-electrical damage-resilient (∼90% restoration from damage after 5 min), and interference-immune in dynamic signal acquisition (stretch, underwater, sweat). We demonstrate its versatile physiological sensing applications, including electrocardiogram recording under various disturbances, machine-learning-enabled hand-gesture recognition through electromyogram measurement, subtle radial artery pulse, and drastic knee kinematics sensing. This epidermal patch offers a promising noninvasive, long-duration, and ambulant bioelectronic interfacing with anti-interference robustness.

A Strand Entangled Supramolecular PANI/PAA Hydrogel Enabled Ultra-Stretchable Strain Sensor

Hydrogel electronics have attracted growing interest for emerging applications in personal healthcare management, human-machine interaction, etc. Herein, a "doping then gelling" strategy to synthesize supramolecular PANI/PAA hydrogel with a specific strand entangled network is proposed, by doping the PANI with acrylic acid (AA) monomers to avoid PANI aggregation. The high-density electrostatic interaction between PAA and PANI chains serves as a dynamic bond to initiate the strand entanglement, enabling PAA/PANI hydrogel with ultra-stretchability (2830%), high breaking strength (120 kPa), and rapid self-healing properties. Moreover, the PAA/PANI hydrogel-based sensor with a high strain sensitivity (gauge factor = 12.63), a rapid responding time (222 ms), and a robust conductivity-based sensing behavior under cyclic stretching is developed. A set of strain sensing applications to precisely monitor human movements is also demonstrated, indicating a promising application prospect as wearable devices.

A biomimetic laminated strategy enabled strain-interference free and durable flexible thermistor electronics

The development of flexible thermistor epidermal electronics (FTEE) to satisfy high temperature resolution without strain induced signal distortion is of great significance but still challenging. Inspired by the nacre microstructure capable of restraining the stress concentration, we exemplify a versatile MXene-based thermistor elastomer sensor (TES) platform that significantly alleviates the strain interference by the biomimetic laminated strategy combining with the in-plane stress dissipation and nacre-mimetic hierarchical architecture, delivering competitive advantages of superior thermosensitivity (-1.32% °C-1), outstanding temperature resolution (~0.3 °C), and unparalleled mechanical durability (20000 folding fatigue cycles), together with considerable improvement in strain-tolerant thermosensation over commercial thermocouple in exercise scenario. By a combination of theoretical model simulation, microstructure observation, and superposed signal detection, the authors further reveal the underlying temperature and strain signal decoupling mechanism that substantiate the generality and customizability of the nacre-mimetic strategy, possessing insightful significance of fabricating FTEE for static and dynamic temperature detection.

Wireless Human-Machine Interface Based on Artificial Bionic Skin with Damage Reconfiguration and Multisensing Capabilities

Human-machine interfaces (HMIs) enable users to interact with machines, thus playing a significant role in artificial intelligence, virtual reality, and the metaverse. Conventional HMIs are based on bulky and rigid electronic devices, seriously limiting their ductility, damage reconfiguration, and multifunctionality. In terms of replacing conventional HMIs, artificial bionic skins with good ductility, self-reparation, and multisensory ability are promising candidates. Still, they in their present form require innovations in mechanical and sensory properties, especially damage recovery and environmental stability, which seriously affect the service life and result in tons of electric waste. Herein, we present a new type of artificial bionic skin with excellent mechanical performance (>13,000% strain), high environmental stability (-80 to 80 °C), and multiple sensory properties toward strain, stress, temperature, solvent, and bioelectricity. Besides, this new type of artificial bionic skin also exhibits effective reconfiguration ability after damage and recyclability. The as-prepared artificial bionic skin was used as an interactive HMI to collect and distinguish the different sensory stimuli. The electronics assembled by HMI with artificial bionic skin can adhere compliantly on the human body for wireless motion capturing and sensing via Bluetooth, Wi-Fi, and the Internet. With simple programming, complex human motions can be mimicked in real-time by robots.

Adhesion of fluid infused silicone elastomer to glass

Elastomers swollen with non-polar fluids show potential as anti-adhesive materials. We study the effect of oil fraction and contact time on the adhesion between swollen spherical probes of PDMS (polydimethylsiloxane) and flat glass surfaces. The PDMS probes are swollen with pre-determined amount of 10 cSt silicone oil to span the range where the PDMS is fluid free (via solvent extraction) up to the limit where it is oil saturated. Probe tack measurements show that adhesion decreases rapidly with an increase in oil fraction. The decrease in adhesion is attributed to excess oil present at the PDMS-air interface. Contact angle measurements and optical microscopy images support this observation. Adhesion also increases with contact time for a given oil fraction. The increase in adhesion with contact time can be interpreted through different competing mechanisms that depend on the oil fraction where the dominant mechanism changes from extracted to fully swollen PDMS. For partially swollen PDMS, we observe that adhesion initially increases because of viscoelastic relaxation and at long times increases because of contact aging. In contrast, adhesion between fully swollen PDMS and glass barely increases over time and is mainly due to capillary forces. While the relaxation of PDMS in contact is well-described by a visco-poroelastic model, we do not see evidence that poroelastic relaxation of the PDMS contributes to an increase of adhesion with glass whether it is partially or fully swollen.

Anti-Fouling, Adhesive Polyzwitterionic Hydrogel Electrodes Toughened Using a Tannic Acid Nanoflower

Conductive polyzwitterionic hydrogels with good adhesion properties show potential prospect in implantable electrodes and electronic devices. Adhesive property of polyzwitterionic hydrogels in humid environments can be improved by the introduction of catechol groups. However, common catechol modifiers can usually quench free radicals, resulting in a contradiction between long-term tissue adhesion and hydrogel toughness. By adding tannic acid (TA) to the dispersion of clay nanosheets and nanofibers, we designed TA-coated nanoflowers and nanofibers as the reinforcing phase to prepare polyzwitterionic hydrogels with adhesion properties. The hydrogel combines the mussel-like and zwitterionic co-adhesive mechanism to maintain long-term adhesion in underwater environments. In particular, the noncovalent cross-linking provided by the nanoflower structure effectively compensates for the defects caused by free-radical quenching so that the hydrogel obtained a high stretchability of over 2900% and a toughness of 1.16 J/m³. The hydrogel also has excellent anti-biofouling property and shows resistance to bacteria and cells. In addition, the hydrogel possesses a low modulus (<10 kPa) and ionic conductivity (0.25 S/m), making it an ideal material for the preparation of implantable electrodes.

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.

Small-molecule ionic liquid-based adhesive with strong room-temperature adhesion promoted by electrostatic interaction

Low-molecular-weight adhesives (LMWAs) possess many unique features compared to polymer adhesives. However, fabricating LMWAs with adhesion strengths higher than those of polymeric materials is a significant challenge, mainly because of the relatively weak and unbalanced cohesion and interfacial adhesion. Herein, an ionic liquid (IL)-based adhesive with high adhesion strength is demonstrated by introducing an IL moiety into a Y-shaped molecule replete with hydrogen bonding (H-bonding) interactions. The IL moieties not only destroyed the rigid and ordered H-bonding networks, releasing more free groups to form hydrogen bonds (H-bonds) at the substrate/adhesive interface, but also provided electrostatic interactions that improved the cohesion energy. The synthesized IL-based adhesive, Tri-HT, could directly form thin coatings on various substrates, with high adhesion strengths of up to 12.20 MPa. Advanced adhesives with electrical conductivity, self-healing behavior, and electrically-controlled adhesion could also be fabricated by combining Tri-HT with carbon nanotubes.

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

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

Noncovalent Assembly Enabled Strong yet Tough Materials with Room-Temperature Malleability and Healability

The manufacturing of both metals and polymer materials strongly relies on melt processing at relatively high temperatures which needs complex shaping-cooling equipment, long molding time, and considerable energy consumption. Reducing the processing temperature to achieve room-temperature malleability is heavily desired for low-carbon demands but continues to be a great challenge. Here, we demonstrate a noncovalent assembly strategy to fabricate room-temperature malleable composites embedded by liquid metals with excellent toughness (105.88 MJ m^-3, higher than most traditional plastics and metallic aluminum) and strong mechanical strength (35.49 MPa). The dissociation-reconstruction of supramolecular bonding interactions between assembled nanoparticles and polymer matrix allow the malleable composite with two interchangeable supramolecular states to achieve programming at room temperature stimulated by water vapor and give it self-healing ability (self-healing efficiency of ∼100%; the healed sample can lift about 52,300 times its own weight). Furthermore, the composite also exhibits metallic luster and prospective application in thermal dissipation. This strategy might be an efficient way for the development of a method for strong and tough materials structurally designed to achieve programming at moderate conditions.

Phase-locked constructing dynamic supramolecular ionic conductive elastomers with superior toughness, autonomous self-healing and recyclability

Stretchable ionic conductors are considerable to be the most attractive candidate for next-generation flexible ionotronic devices. Nevertheless, high ionic conductivity, excellent mechanical properties, good self-healing capacity and recyclability are necessary but can be rarely satisfied in one material. Herein, we propose an ionic conductor design, dynamic supramolecular ionic conductive elastomers (DSICE), via phase-locked strategy, wherein locking soft phase polyether backbone conducts lithium-ion (Li⁺) transport and the combination of dynamic disulfide metathesis and stronger supramolecular quadruple hydrogen bonds in the hard domains contributes to the self-healing capacity and mechanical versatility. The dual-phase design performs its own functions and the conflict among ionic conductivity, self-healing capability, and mechanical compatibility can be thus defeated. The well-designed DSICE exhibits high ionic conductivity (3.77 × 10⁻³ S m⁻¹ at 30 °C), high transparency (92.3%), superior stretchability (2615.17% elongation), strength (27.83 MPa) and toughness (164.36 MJ m⁻³), excellent self-healing capability (~99% at room temperature) and favorable recyclability. This work provides an interesting strategy for designing the advanced ionic conductors and offers promise for flexible ionotronic devices or solid-state batteries.

Stretchable ionic conductors are attractive candidates for flexible ionotronics but combining high conductivity with excellent mechanical properties is challenging. Herein, the authors combine these properties in a dynamic supramolecular ionic conductive elastomer enabling lithium-ion transport in the soft phase and dynamic disulfide and supramolecular hydrogen bonding in the hard segments.

Supramolecular Adhesive Materials with Antimicrobial Activity for Emerging Biomedical Applications

Traditional adhesives or glues such as cyanoacrylates, fibrin glue, polyethylene glycol, and their derivatives have been widely used in biomedical fields. However, they still suffer from numerous limitations, including the mechanical mismatch with biological tissues, weak adhesion on wet surfaces, biological incompatibility, and incapability of integrating desired multifunction. In addition to adaptive mechanical and adhesion properties, adhesive biomaterials should be able to integrate multiple functions such as stimuli-responsiveness, control-releasing of small or macromolecular therapeutic molecules, hosting of various cells, and programmable degradation to fulfill the requirements in the specific biological systems. Therefore, rational molecular engineering and structural designs are required to facilitate the development of functional adhesive materials. This review summarizes and analyzes the current supramolecular design strategies of representative adhesive materials, serving as a general guide for researchers seeking to develop novel adhesive materials for biomedical applications.

Fatigue-free artificial ionic skin toughened by self-healable elastic nanomesh

Robust ionic sensing materials that are both fatigue-resistant and self-healable like human skin are essential for soft electronics and robotics with extended service life. However, most existing self-healable artificial ionic skins produced on the basis of network reconfiguration suffer from a low fatigue threshold due to the easy fracture of low-energy amorphous polymer chains with susceptible crack propagation. Here we engineer a fatigue-free yet fully healable hybrid ionic skin toughened by a high-energy, self-healable elastic nanomesh, resembling the repairable nanofibrous interwoven structure of human skin. Such a design affords a superhigh fatigue threshold of 2950 J m⁻² while maintaining skin-like compliance, stretchability, and strain-adaptive stiffening response. Moreover, nanofiber tension-induced moisture breathing of ionic matrix leads to a record-high strain-sensing gauge factor of 66.8, far exceeding previous intrinsically stretchable ionic conductors. This concept creates opportunities for designing durable ion-conducting materials that replicate the unparalleled combinatory properties of natural skins more precisely.

Developing robust skin-like sensing materials is essential for soft electronics and robotics with extended service life. Here, inspired by the repairable nanofibrous structure of human skin, the authors engineer a fatigue-resistant artificial ionic skin toughened by self-healable elastic nanomesh.

Dual-Driven Mechanically and Tribologically Adaptive Hydrogels Solely Constituted of Graphene Oxide and Water

Although mythologies and fictions have recorded living creatures fully composed of inorganics, it is however hard to turn inorganic constituents into lifelike materials in reality as they usually do not possess characteristics required for constructing a living organism. Here, we report to our knowledge the first biomimetic hydrogel in response to both pH and temperature variations that solely comprises graphene oxide and water. The hydrogel is capable of abruptly and reversibly switching its mechanical and tribological properties by more than 10-fold and 5-fold magnitudes, respectively, as a result of pH- and/or thermal-induced topological reconfiguration of its internal microstructure and ordering. Such behavior closely mimics some natural living organisms such as muscles and sea cucumbers. The hydrogel also shows a low coefficient of friction at pH 2 and room temperature, indicating it a potent smart lubricant free of any flammable and toxic organic base oils and additives.

Coacervation-Based Method for Constructing a Multifunctional Strain-Stiffening Crystalline Polyvinylamine Hydrogel

Strain-stiffening hydrogels are essential in the development of ionic skin, as human skin possesses a strain-stiffening property for self-protection. Semicrystalline polymers such as poly(vinyl alcohol) (PVA) have been widely investigated to fabricate strain-stiffening hydrogels via freeze-thaw cycling or chemical cross-linking but with limited adjustable properties. Compared with PVA, polyvinylamine (PVAm) has a higher reactive activity, making it easier to achieve multifunctionalities including strain-stiffening in a PVAm hydrogel. However, the amine moieties in the backbone tend to be ionized and form strong ionic hydrogen bonds with water, resulting in difficulties in forming crystalline hydrogels by conventional methods. Herein, a one-pot method to induce crystallinity and achieve multifunctional hydrogel is devised via coacervation of PVAm. Different from a published coacervation method to fabricate hydrogels with various properties via noncovalent interactions between different chemicals, coacervation occurs between PVAm to form aggregated and loose PVAm in our devised system. Such a strategy lowers the amine-water binding energy in the polymer-dense phase to achieve crystallinity and subsequently the strain-stiffening property; meanwhile, self-healability, self-adhesion, and ionic conductivity can be realized in the polymer-loose phase. The obtained hydrogel integrates stretchability (∼1300% elongation), toughness (227 kPa), the strain-stiffening property (∼10 times increase), self-adhesion (90 J m^-2), self-healability (∼80% healing efficiency in toughness), and ionic conductivity (0.22 mS m^-1). This convenient strategy will open a new horizon to design multifunctional skin-mimic materials.

Hydrogen-Bonding Affords Sustainable Plastics with Ultrahigh Robustness and Water-Assisted Arbitrarily Shape Engineering

Herein, the supramolecular plastic-like hydrogel (SPH) is introduced as a platform to fabricate sustainable plastics with ultrahigh stiffness and strength as well as water-assisted arbitrarily shapeable capability. The transparent plastics are constructed from SPHs of cellulose ether/polycarboxylic acid complexes and demonstrate mechanical robustness with Young's modulus up to 3.4 GPa and tensile strength up to 124.0 MPa, superior or comparable to most common plastics. Meanwhile, the shape of the plastics can be reversibly engineered by air drying of the SPHs with diverse 2D/3D shapes and structures, which are generated conveniently via origami, kirigami, embossing, etc., in virtue of plastic deformation and shape memory effect of SPHs. On the basis of multi-dimensional infrared-spectral analysis, it is revealed that the dense acid-acid and acid-ether hydrogen (H)-bonding network in the plastic is responsible for the mechanical robustness while the evolution of water-polymer H-bonds into polymer-polymer H-bonds during air drying contributes to the shape fixing. This work provides a novel method of manufacturing sustainable plastics with simultaneous strong mechanical performance and convenient processibility from hydrogels with plastic-like mechanical behavior.

Highly Stretchable Hydrogels as Wearable and Implantable Sensors for Recording Physiological and Brain Neural Signals

Recording electrophysiological information such as brain neural signals is of great importance in health monitoring and disease diagnosis. However, foreign body response and performance loss over time are major challenges stemming from the chemomechanical mismatch between sensors and tissues. Herein, microgels are utilized as large crosslinking centers in hydrogel networks to modulate the tradeoff between modulus and fatigue resistance/stretchability for producing hydrogels that closely match chemomechanical properties of neural tissues. The hydrogels exhibit notably different characteristics compared to nanoparticles reinforced hydrogels. The hydrogels exhibit relatively low modulus, good stretchability, and outstanding fatigue resistance. It is demonstrated that the hydrogels are well suited for fashioning into wearable and implantable sensors that can obtain physiological pressure signals, record the local field potentials in rat brains, and transmit signals through the injured peripheral nerves of rats. The hydrogels exhibit good chemomechanical match to tissues, negligible foreign body response, and minimal signal attenuation over an extended time, and as such is successfully demonstrated for use as long-term implantable sensory devices. This work facilitates a deeper understanding of biohybrid interfaces, while also advancing the technical design concepts for implantable neural probes that efficiently obtain physiological information.

Mxene reinforced supramolecular hydrogels with high strength, stretchability and reliable conductivity for sensitive strain sensors

Conductive hydrogels used as electronics have received much attention due to their great flexibility and stretchability. However, the fabrication of ideal conductive hydrogels fulfilling with excellent mechanical properties and outstanding sensitivity remains a great challenge until now. Moreover, high sensitivity and broad linearity range are pivotal for the feasibility and accuracy of hydrogel sensors. In this study, a conductive supramolecular hydrogel was engineered by directly mixing the aqueous dispersion of MXene with the precursor of N-acryloyl glycinamide (NAGA) monomer and then rapidly photo cross-linked by UV irradiation. The resultant PNAGA/MXene hydrogel-sensors exhibited high mechanical strength (4.8 MPa), great stretchability (630%), and excellent durability. The conductive hydrogel-based sensor presented excellent conductivity (17.3 S·m^-1 ) and a wide scope of linear dependence of sensitivity on strain (0-125%, gauge factor = 2.05). It displayed reliable detection of various motions, including repeated subtle movements and large strain. It was also showed good degradation in vitro and antifouling capability. This work may provide a simple and promising platform for engineering conductive supramolecular hydrogels with integrated high performance aiming for smart wearable electronics, electronic skin, soft robots, and human-machine interfacing. This article is protected by copyright. All rights reserved.

Visible-Light-Mediated Nano-biomineralization of Customizable Tough Hydrogels for Biomimetic Tissue Engineering

Biomineralized tough hydrogels (BTHs) have advanced applications in the fields of soft bioelectronics and biomimetic tissue engineering. But the development of rapid and general photomineralization strategies for one-step fabrication of customizable BTHs is still a challenging task. Here we report a straightforward, low-cost visible-light-mediated nano-biomineralization (VLMNB) strategy via a rational design of a phosphate source and efficient ruthenium photochemistry. Multinetwork tough hydrogels are simultaneously constructed under the same condition. Therefore, BTHs are rapidly prepared in a short time as low as ∼60 s under visible light irradiation. The in situ formation of calcium phosphate particles can improve BTHs' mechanical and biological properties and reduce the friction coefficient with bones. Furthermore, fast biomineralization and solidification processes in these BTHs benefit their injectable and highly flexible customizable design, showing applications of promoting customizable skin repair and bone regeneration.

Recent Advances of Self-Healing Polymer Materials via Supramolecular Forces for Biomedical Applications

Noncovalent interactions can maintain the three-dimensional structures of biomacromolecules (e.g., polysaccharides and proteins) and control specific recognition in biological systems. Supramolecular chemistry was gradually developed as a result, and this led to design and application of self-healing materials. Self-healing materials have attracted attention in many fields, such as coatings, bionic materials, elastomers, and flexible electronic devices. Nevertheless, self-healing materials for biomedical applications have not been comprehensively summarized, even though many reports have been focused on specific areas. In this Review, we first introduce the different categories of supramolecular forces used in preparing self-healing materials and then describe biological applications developed in the last 5 years, including antibiofouling, smart drug/protein delivery, wound healing, electronic skin, cartilage lubrication protection, and tissue engineering scaffolds. Finally, the limitations of current biomedical applications are indicated, key design points are offered for new biological self-healing materials, and potential directions for biological applications are highlighted.

Hierarchical Response Network Boosts Solvent-Free Ionic Conductive Elastomers with Extreme Stretchability, Healability, and Recyclability for Ionic Sensors

The construction of solvent-free ionic conductive elastomers with high mechanical stretchability and large dynamic reversibility of chain segments is highly desired yet challenging. Here, a hierarchical response network strategy is presented for preparing highly stretchable yet mechanical robust ionic conductive elastomer composites (ICECs), among which poly(ethylene oxide) (PEO) microcrystalline serves as a physical cross-linking site providing high mechanical strength and elasticity, while dense hydrogen bonds endow superior mechanical toughness and dynamic reversibility. Due to the formation of the hierarchical response network, the resultant ICECs exhibit intrinsically high stretchability (>1500%), large tensile strength (∼2.1 MPa), and high fracture toughness (∼28 MJ m^-3). Intriguingly, due to the high reversibility of hydrogen-bonded networks, the ICECs after being crushed are capable of healing and recycling by simple hot-pressing for multiple cycles. Moreover, the ICECs are dissolvable under an alkaline condition and easily regenerated in an acid solution for manifold cycles. Importantly, the healed, recycled, and regenerated ICECs are capable of maintaining their initial mechanical elasticity and ionic conducting performance. Due to the integration of high stretchability, fatigue resistance, and ionic conductivity, the ICECs can readily work as a stretchable ionic conductor for skin-inspired ionic sensors for real-time and accurately sensing complex human motions. This study thus provides a promising strategy for the development of healable and renewable ionic sensing materials with high stretchability and mechanical robustness, demonstrating great potential in soft ionotronics.

Heat- and freeze-tolerant organohydrogel with enhanced ionic conductivity over a wide temperature range for highly mechanoresponsive smart paint

Binary solvent-based fabrication permits the conductive organohydrogel to function well at low-temperature environments. However, the deep cryogenic and high temperatures are still threatening the performance of conductive organohydrogels in the application of stretchable electronics, biosensors, and intelligent coatings. Here, a radically new method is developed to introduce propylene and carbonate cellulose nanofibrils into freeze tolerance polymer matrix, and fabricate an antifreezing/antiheating organohydrogel integrated a high mechanical strength (1.6 MPa) and high level of ionic conductivity (4.2 S cm^-1) over a wide temperature range (-40 to 100 °C). In this designed system, the propylene carbonate with low freezing point and high boiling point was shown to enhance antifreezing (-40 °C) and antiheating (100 °C) performance of organohydrogel. Furthermore, negative charge-rich cellulose nanofibrils (CNFs) were served as an ion transport channel and nanoreinforcements to boost the conductive and mechanical properties of the organohydrogel. In particular, Molecular Dynamics (MD) simulations reveal that propylene carbonate with high dielectric constant is capable of generating ion migration-facilitated effects, enabling the high ionic conductivity of organohydrogel. Tapping into these attributes, potential applications in mechanoresponsive smart coating have been demonstrated utilizing the appealing organohydrogel as a paint, rendering unprecedented protection and monitoring performance.

Ultrastretchable, Adhesive, Fast Self-Healable, and Three-Dimensional Printable Photoluminescent Ionic Skin Based on Hybrid Network Ionogels

Developing multifunctional stretchable ionic skin (I-Skin) to mimic the sensations of the human skin is of great interest and shows promising potential in wearable sensors and human-machine interfaces (HMIs). However, common ionogels prepared with small-molecule cross-linkers and single networks can hardly satisfy the requirements of adjustable mechanical properties, strong adhesion, fast self-healability, and good stability in extreme environments. Herein, an ultrastretchable (>10,000%), ultrastrong adhesive (>6.8 MPa), ultrafast self-healable (10 s), high thermally stable (-60 to 250 °C), and three-dimensional (3D)-printable photoluminescent ionogel with shape memory properties has been designed. The ionogel consists of hyperbranched polymer covalent-cross-linked poly(zwitterionic ionic liquid)-co-poly(acrylic acid) and multiple dynamic bonding cross-linked networks. The excellent performance of the ionogel-based high-stretchable strain sensor and the triboelectric nanogenerator (TENG)-based self-powered touch sensor is further demonstrated over a wide temperature range (-40 to 150 °C). More importantly, ionogel-based I-Skin can work as an HMI for human gesture recognition and real-time wireless control of robots under extreme vacuum conditions and can also self-heal immediately along with function recovery after mechanical damage.

Tailoring intermolecular interactions in ion gels with rationally designed phosphonic acid polymers

Design strategies of phosphonic acid polymers established advanced ion gels with high ionic conductivity, mechanical strength, and self-healing ability via a configurable balance of ionic and hydrogen bonding interactions at the molecular level.

A Selective-Response Bioinspired Strain Sensor Using Viscoelastic Material as Middle Layer

Flexible strain sensors have an irreplaceable role in critical and emerging fields, such as electronic skins, flexible robots, and prosthetics. Although numerous efforts have been made to improve sensor sensitivity to meet specific application scenarios, the signal-to-noise ratio (SNR) is an extremely critical and non-negligible indicator, which takes into account higher sensitivity, meaning that they can also detect the noise signals with high sensitivity. Coincidentally, scorpions with ultrasensitive vibration sensilla also face such a dilemma. Here, it is found that the scorpion ingeniously uses the viscoelastic material in front of its slit sensilla to realize efficient preprocessing of the signal. Its mechanism is that the loss factor of materials changes with frequency, affecting energy storage and transmission. Inspired by this ingenious strategy, a bioinspired strain sensor insensitive to a low strain rate was designed using a two-step template transfer method. As a result, its relative change in resistance reached 110% under the same strain (0.3197%) but with different strain rates (0.1 Hz and ∼20 Hz). The noncontact vibration experiments also show different responses to low-frequency vibration and high-frequency impact. Moreover, it can also be used as a typical flexible strain sensor. Under the tensile state, it has a gauge factor (GF) as high as 4596 upon 0.6% strain, and the response time is 140 ms. Therefore, it is expected that this strain sensor will be used in many important ultraprecision measurement fields, especially when the measured signal is small.

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

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

A Highly Robust Ionotronic Fiber with Unprecedented Mechanomodulation of Ionic Conduction

Stretchable ionic conductors are appealing for tissue-like soft electronics, yet suffer from a tardy mechanoelectric response due to their poor modulation of ionic conduction arising from intrinsic homogeneous soft chain network. Here, a highly robust ionotronic fiber is designed by synergizing ionic liquid and liquid crystal elastomer with alternate rigid mesogen units and soft chain spacers, which shows an unprecedented strain-induced ionic conductivity boost (≈10³ times enhanced as stretched to 2000% strain). Such a surprisingly high enhancement is attributed to the formation of microphase-separated low-tortuosity ion-conducting nanochannels guided by strain-induced emergence of aligned smectic mesophases, thus allowing for ultrafast ion transport that resembles the role of "swimming lanes." Intriguingly, the boosting conductivity even reverses Pouillet's Law-dictated resistance increase at certain strains, leading to unique waveform-discernible strain sensing. Moreover, the fiber retains thermal actuation properties with a maximum of 70% strain changes upon heating, and enables integrated self-perception and actuation. The findings offer a promising molecular engineering route to mechanically modulate the ion transport behavior of ionic conductors toward advanced ionotronic applications.

Recent Progress in Bionic Skin Based on Conductive Polymer Gels

Bionic skin sensors based on conductive polymer gels have garnered interest for their potential applications in human-computer interaction, soft robotics, biomedical systems, sports, and healthcare, because of their intrinsic flexibility and stretchability embedded at the material level, and other such as self-healing, adhesion, high, and low temperature tolerance properties that can be tuned through macromolecular design. Here, important advances in polymer gel-based flexible sensors over recent years are summarized, from material design, sensor fabrication to system-level applications. This review focuses on the representative strategies of design and preparing of conductive polymer gels, and adjusting their conductivity, mechanics, and other properties such as self-healing and adhesiveness by controlling the macromolecular network structures. The state-of-art of present flexible pressure and strain sensors, temperature sensors, position sensors, and multifunctional sensors based on capacitance, voltage, and resistance sensing technologies, are also systematically reviewed. Finally, perspectives on issues regarding further advances and challenges are provided.