Conductive Hydrogel-Based Electrodes and Electrolytes for Stretchable and Self-Healable Supercapacitors

Stretchable self-healing supercapacitors (SCs) can operate under extreme deformation and restore their initial properties after damage with considerably improved durability and reliability, expanding their opportunities in numerous applications, including smart wearable electronics, bioinspired devices, human–machine interactions, etc. It is challenging, however, to achieve mechanical stretchability and self-healability in energy storage technologies, wherein the key issue lies in the exploitation of ideal electrode and electrolyte materials with exceptional mechanical stretchability and self-healing ability besides conductivity. Conductive hydrogels (CHs) possess unique hierarchical porous structure, high electrical/ionic conductivity, broadly tunable physical and chemical properties through molecular design and structure regulation, holding tremendous promise for stretchable self-healing SCs. Hence, this review is innovatively constructed with a focus on stretchable and self-healing CH based electrodes and electrolytes for SCs. First, the common synthetic approaches of CHs are introduced; then the stretching and self-healing strategies involved in CHs are systematically elaborated; followed by an explanation of the conductive mechanism of CHs; then focusing on CH-based electrodes and electrolytes for stretchable self-healing SCs; subsequently, application of stretchable and self-healing SCs in wearable electronics are discussed; finally, a conclusion is drawn along with views on the challenges and future research directions regarding the field of CHs for SCs.     

Self-Healing Titanium Dioxide Nanocapsules-Graphene/Multi-Branched Polyurethane Hybrid Flexible Film with Multifunctional Properties toward Wearable Electronics

The integration of self-healing capabilities into flexible electronics arouses extensive attention. The application of self-healing electronics with multifunctional properties in a variety of exceptional environments has been identified to be significantly challenging and not yet proven to be fully viable thus far. In the present study, the self-healing octadecane loaded titanium dioxide nanocapsules (OTNs)-graphene/multi-branched polyurethane (PU) hybrid flexible multifunctional film is successfully prepared. The prepared film exhibits a novel self-repair capability that consists of disulfide bonds in the leading chains for efficient self-healing of PU damage, as well as multiple amino groups in the branches for damage between OTNs-graphene and PU. Impacted by the constructed self-healing system and well-dispersed OTNs-graphene, the prepared flexible film demonstrates a prominent performance in piezoresistive sensing and a desirable outcome of ultraviolet protection properties, which can effectively prolong its service life, especially when used outdoors. Moreover, the film exhibits thermal insulating properties, capable of offering a suitable route for thermal protection of bio-integrated wearable electronic devices system. Thus, this self-healing multifunctional film is promising in wearable electronics, human–machine interaction, artificial intelligence devices, etc.     

Dynamic Photomask‐Assisted Direct Ink Writing Multimaterial for Multilevel Triboelectric Nanogenerator

Triboelectric nanogenerator (TENG) devices are extensively studied as a mechanical energy harvester and self‐powered sensor for wearable electronics and physiological monitoring. However, the conventional TENG fabrication involving assembling steps and using the single property of matrix material suffers from simple devices shape and a single level of mechanical response for sensing and energy harvesting. Here, the printed multimaterial matrix for multilevel mechanical‐responsive TENG with on‐demand reconfiguration of shape is reported. A multimaterial 3D printing approach by using dynamic photomask‐assisted direct ink writing printing together with a two‐stage curing hybrid ink is first developed. Multimaterial structures with location‐specific properties, such as tensile modulus, failure stress, and glass transition temperature for controlled deformation, crack propagation path, and sequential shape memory, are directly printed. The printed multimaterial structure with sequential deformation behavior is used to fabricate a multilevel‐TENG (mTENG) device for multiple level mechanical energy harvesters and sensors. It is demonstrated that the mTENG can be embedded in shoe insoles to achieve both comfortable wearing and motion state monitoring. This work provides a new approach to combine multimaterial 3D printing with TENG devices for functional wearable electronics as energy harvester and sensors.     

Photonic Vitrimer Elastomer with Self-Healing,High Toughness,Mechanochromism, and Excellent Durability based on Dynamic Covalent Bond

Vitrimers, with their unique dynamic covalent bonds, possess attractive self-healability and mechanical robustness, providing an intriguing opportunity to construct functional soft materials. However, their potential for function recovery, especially optical function, is underexplored. Harnessing the synergistic effect of photonic crystals and vitrimers, a novel photonic vitrimer with light regulating and self-healing capabilities is presented. The resulting photonic vitrimer exhibits a large tensile strain (>1000%), high toughness (21.2 kJ m⁻³), bright structural color, and mechanochromism. Notably, the structural color remains constant even after 10 000 stretching/releasing cycles, showing superior mechanical stability, creep-resistance, and excellent durability. More importantly, the exchange of dynamic covalent bonds imparts the photonic vitrimer with a self-healing ability (>95% efficiency), enabling the recovery of its optical function. Benefiting from the above merits, the photonic vitrimer has been successfully used as a sensor for human motion detection, which demonstrates visualized interactive sensibility even after self-repairing. This material design provides a general strategy for optical functionalization of vitrimers. The photonic vitrimer elastomers present great potential as resilient functional soft materials for diverse flexible devices and a novel optical platform for soft robotics, smart wearable devices, and human-machine interaction.     

Self-Healing,Robust, and Stretchable Electrode by Direct Printing on Dynamic Polyurea Surface at Slightly Elevated Temperature

Printed electronics on elastomer substrates have found wide applications in wearable devices and soft robotics. For everyday usage, additional requirements exist for the robustness of the printed flexible electrodes, such as the ability to resist scratching and damage. Therefore, highly robust electrodes with self-healing, and good mechanical strength and stretchability are highly required and challenging. In this paper, a cross-linking polyurea using polydimethylsiloxane as the soft segment and dynamic urea bonds is prepared and serves as a self-healing elastomer substrate for coating and printing of silver nanowires (AgNWs). Due to the dynamic exchangeable urea bond at 60 °C, the elastomer exhibits dynamic exchange of the cross-linking network while retaining the macroscopic shape. As a result, the AgNWs are partially embedded in the surface of the elastomer substrate when coated or printed at 60 °C, forming strong interfacial adhesion. As a result, the obtained stretchable electrode exhibits high mechanical strength and stretchability, the ability to resist scratching and sonication, and self-healing. This strategy can be applied to a variety of different conducting electrode materials including AgNWs, silver particles, and liquid metal, which provides a new way to prepare robust and self-healing printed electronics.     

Self-Healing,Photothermal-Responsive,and Shape Memory Polyurethanes for Enhanced Mechanical Properties of 3D/4D Printed Objects

Additive manufacturing is a promising technology that can directly fabricate structures with complex internal geometries, which is barely achieved by traditional manufacturing. However, the mechanical properties of fused deposition modeling (FDM)-printed objects are inferior to those of conventionally manufactured products. To improve the mechanical properties of the printed products, a series of novel thermoplastic polyurethanes with self-healing properties, intrinsic photothermal effects, and excellent printability are designed and synthesized by introducing dynamic oxime–carbamate bonds and hydrogen bonds into the polymer chains. On-demand introduction of near-infrared (NIR) irradiation, direct heating, and sunlight irradiation enhances interfacial bonding strength and thus improve the mechanical properties of the printed product. Additionally, mechanical anisotropy of the printed products can be sophistically manipulated by regulating the self-healing conditions. Support-free printing and healing of damaged printed products are also achieved owing to the self-healing properties of the material. Moreover, the as-prepared materials exhibit shape-memory properties NIR irradiation or direct heating effectively triggers shape-memory recovery and demonstrates their potential in 4D printing by printing a man-like robot. This study not only provides a facile strategy for obtaining high-performance printed products but also broadens the potential applications of FDM technology in intelligent devices.     

Mechanochromic Photonic Vitrimer Thermal Management Device Based on Dynamic Covalent Bond

Flexible self-healing thermal management devices are increasingly in demand due to their high flexibility, low driving voltage, and excellent stability of thermal property. In this paper, the design of mechanochromic self-healing thermal management devices is reported based on photonic vitrimer through self-healing dynamic covalent bond. A series of new photonic vitrimers i first prepared by dynamic disulfide covalent bond and PS@SiO₂ photonic crystals. The resulting photonic vitrimer exhibits bright structural colors, large tensile strain (>1000%), high mechanical strength (>10 MPa) and self-healing ability (>95% efficiency). More importantly, the structural color remains constant after 10000 stretching/releasing cycles, demonstrating excellent mechanical stability, creep-resistance, and durability. Taking advantage of the above features, a novel mechanochromic flexible wireless thermal management (MFW) device is developed by semi-embedding the photonic vitrimer in a thermally conductive carbon nanotube film and then integrating it with a Bluetooth module and a control chip. Interestingly, the MFW device exhibits mechanochromic property, fast thermal response, low driving voltage (103 °C, at 3 V), and precise temperature control. Notably, the device even remains electrothermal performance (105 °C) after self-healing. This work provides new insight into the self-healing photonic materials, and the device shows promising applications in wearable electronics, vitro physiotherapy, and personal heating.     

Bioinspired Self-healing Soft Electronics

Inspired by nature, various self-healing materials that can recover their physical properties after external damage have been developed. Recently, self-healing materials have been widely used in electronic devices for improving durability and protecting the devices from failure during operation. Moreover, self-healing materials can integrate many other intriguing properties of biological systems, such as stretchability, mechanical toughness, adhesion, and structural coloration, providing additional fascinating experiences. All of these inspirations have attracted extensive research on bioinspired self-healing soft electronics. This review presents a detailed discussion on bioinspired self-healing soft electronics. Firstly, two main healing mechanisms are introduced. Then, four categories of self-healing materials in soft electronics, including insulators, semiconductors, electronic conductors, and ionic conductors, are reviewed, and their functions, working principles, and applications are summarized. Finally, human-inspired self-healing materials and animal-inspired self-healing materials as well as their applications, such as organic field-effect transistors (OFETs), pressure sensors, strain sensors, chemical sensors, triboelectric nanogenerators (TENGs), and soft actuators, are introduced. This cutting-edge and promising field is believed to stimulate more excellent cross-discipline works in material science, flexible electronics, and novel sensors, accelerating the development of applications in human motion monitoring, environmental sensing, information transmission, etc.     

Highly Stretchable,Resilient, Adhesive,and Self-Healing Ionic Hydrogels for Thermoelectric Application

Harvesting low-grade waste heat from the natural environment with thermoelectric materials is considered as a promising solution for the sustainable energy supply for wearable electronic devices. For practical applications, it is desirable to endow the thermoelectric materials with excellent mechanical and self-healing properties, which remains a great challenge. Herein, the design and characterization of a series of high-performance ionic hydrogels for soft thermoelectric generator applications are reported. Composed of a physically cross-linked network of polyacrylic acid (PAA) and polyethylene glycol (PEO) doped with sodium chloride, the resulting PAA-PEO-NaCl ionic hydrogels demonstrates impressive mechanical strength (breaking stress >1.3 MPa), stretchability (>1100%), and toughness (up to 7.34 MJ m⁻³). Moreover, the reversible hydrogen bonding interaction and chain entanglement render the ionic hydrogels with excellent mechanical resilience, adhesion properties, and self-healing properties. At ambient conditions, the electrochemical and thermoelectric performance of the ionic hydrogels can be restored immediately from physical damage such as cutting, and the mechanical healing can be completely restored within 24 h. At the optimized composition, the Seebeck coefficient of the ionic hydrogels can reach 3.26 mV K⁻¹ with a low thermal conductivity of 0.321 W m⁻¹ K⁻¹. Considering the excellent mechanical properties and thermoelectric performance, it is believed that the ionic hydrogels are widely applicable in ionic thermoelectric capacitors to convert low-grade heat into electricity for soft electronic devices.     

Self‐Healing and Stretchable 3D‐Printed Organic Thermoelectrics

With the advent of flexible and wearable electronics and sensors, there is an urgent need to develop energy‐harvesting solutions that are compatible with such wearables. However, many of the proposed energy‐harvesting solutions lack the necessary mechanical properties, which make them susceptible to damage by repetitive and continuous mechanical stresses, leading to serious degradation in device performance. Developing new energy materials that possess high deformability and self‐healability is essential to realize self‐powered devices. Herein, a thermoelectric ternary composite is demonstrated that possesses both self‐healing and stretchable properties produced via 3D‐printing method. The ternary composite films provide stable thermoelectric performance during viscoelastic deformation, up to 35% tensile strain. Importantly, after being completely severed by cutting, the composite films autonomously recover their thermoelectric properties with a rapid response time of around one second. Using this self‐healable and solution‐processable composite, 3D‐printed thermoelectric generators are fabricated, which retain above 85% of their initial power output, even after repetitive cutting and self‐healing. This approach represents a significant step in achieving damage‐free and truly wearable 3D‐printed organic thermoelectrics.     

Multi-Healable,Mechanically Durable Double Cross-Linked Polyacrylamide Electrolyte Incorporating Hydrophobic Interactions for Dendrite-Free Flexible Zinc-Ion Batteries

Flexible aqueous zinc-ion batteries (ZIBs) are considered as one of the most promising energy storage candidates for wearable electronics, owing to their environmental friendliness, low cost, high safety, and high theoretical capacity. However, the practical application of flexible ZIBs is significantly impeded by the Zn dendrite growth and the poor mechanical endurability at the electrolyte–electrode interface. Mechanically durable hydrogel electrolyte with dendrite growth restriction and self-healing ability is highly desirable to improve the durability and extend the lifetime of the flexible ZIBs. However, it is still a big challenge to simultaneously endow hydrogel electrolytes with all necessary properties. Herein, multi-healable and mechanically durable hydrogels are fabricated by the synergy effect of strong chemical cross-linking and dynamic physical hydrophobic associations. The obtained double cross-linked polyacrylamide electrolyte (PAAm-O-B) has good tensile strain, strength, and stable electrochemical performance, with tensile strength up to 75 kPa, high self-healing efficiency up to 50 broken-healed cycles and lifespan up to 900 h dendrite inhibition behavior. The flexible Zn//MnO₂ batteries using PAAm-O-B hydrogel electrolytes exhibit good mechanical durability with high specific capacity and long cycle lifespan even under 1000 bending deformations and superior self-healing ability.     

Liquid Metal Based Island‐Bridge Architectures for All Printed Stretchable Electrochemical Devices

The adoption of epidermal electronics into everyday life requires new design and fabrication paradigms, transitioning away from traditional rigid, bulky electronics towards soft devices that adapt with high intimacy to the human body. Here, a new strategy is reported for fabricating achieving highly stretchable “island‐bridge” (IB) electrochemical devices based on thick‐film printing process involving merging the deterministic IB architecture with stress‐enduring composite silver (Ag) inks based on eutectic gallium‐indium particles (EGaInPs) as dynamic electrical anchors within the inside the percolated network. The fabrication of free‐standing soft Ag‐EGaInPs‐based serpentine “bridges” enables the printed microstructures to maintain mechanical and electrical properties under an extreme (≈800%) strain. Coupling these highly stretchable “bridges” with rigid multifunctional “island” electrodes allows the realization of electrochemical devices that can sustain high mechanical deformation while displaying an extremely attractive and stable electrochemical performance. The advantages and practical utility of the new printed Ag‐liquid metal‐based island‐bridge designs are discussed and illustrated using a wearable biofuel cell. Such new scalable and tunable fabrication strategy will allow to incorporate a wide range of materials into a single device towards a wide range of applications in wearable electronics.     

Self-Healing and Recyclable Polymer Electrolyte Enabled with Boronic Ester Transesterification for Stabilizing Ion Deposition

Damage to solid polymer electrolytes can lead to mechanical degradation, short circuits, or functional failures. Therefore, introducing a self-healing function to solid polymer electrolytes is an ideal strategy to improve the safety and reliability of electrolyte systems. Herein, dynamic boronic ester-based self-healing polymer electrolytes (DB-SHPEs) with excellent mechanical properties and interfacial stability are developed via a thermally initiated ring-opening reaction between thiol and epoxy groups. The DB-SHPEs containing boronic ester bonds can not only alter the topologies via boronic ester transesterification and exhibit good self-healing capability but also enable homogeneous deposition of Li ions on the Li metal through the Lewis acid–base interactions between boron atoms and salt anions. Furthermore, the boronic ester bonds can endow the DB-SHPE with reprocessability and recyclability taking advantage of associative transesterification reaction. More significantly, the Li/DB-SHPE/Li symmetric cells exhibit a stable voltage plateau after cycling for 1200 h and the LiFePO₄/DB-SHPE/Li batteries present excellent cycling performance, suggesting that high-performance self-healing polymer electrolytes with multiple functions are promising materials for the next-generation lithium metal batteries.     

Exploration and Application of Self-Healing Strategies in Lithium Batteries

Lithium batteries (LBs) are developed tremendously owing to their excellent energy density as well as cyclic persistence, exhibiting promising applications from portable devices to e-transportation and grid fields. However, with the ever-increasing demand for intelligent wearable electronics, more requests are focused on high safety, good durability, and satisfied reliability of LBs. The self-healing route, which can simulate the ability of organic organisms to repair damage and recover initial function through its intrinsic vitality, is believed to be an efficient strategy to alleviate the unavoidable physical or chemical fatigue and damage issues of LBs, beneficial for the realization of the above mentioned high requests. In this review, the applicability and development of self-healing materials are summarized in electrodes, electrolytes, and interfacial layers in recent years, focusing on exploring the feasibility of different self-healing strategies in LBs, discussing the advantages and disadvantages of existing strategies in different parts of batteries, and indicating the possible research directions for beginners who are interested in this field. Finally, the critical challenges and the future research directions as well as opportunities are prospected.     

A Puncture-Resistant and Self-Healing Conductive Gel for Multifunctional Electronic Skin

Flexible electronic skins (e-skins) play a very important role in the development of human–machine interaction and wearable devices. To fully mimic the functions of human skin, e-skins should be able to perceive multiple external stimuli (such as temperature, touch, and friction) and be resistant to injury. However, both objectives are highly challenging. The fabrication of multifunctional e-skins is difficult because of the complex lamination scheme and the integration of different sensors. The design of skin-like materials is hindered by the trade-off problem between flexibility, toughness, and self-healing ability. Herein, flexible sodium methallyl sulfonate functionalized poly(thioctic acid) polymer chains are combined with rigid conductive polyaniline rods through ionic bonds to obtain a solvent-free polymer conductive gel. The conductive gel has a modulus similar to that of skin, and shows good flexibility, puncture-resistance, notch-insensitivity, and fast self-healing ability. Moreover, this conductive gel can convert changes in temperature and strain into electrical signal changes, thus leading to multifunctional sensing performance. Based on these superior properties, a flexible e-skin sensor is prepared, demonstrating its great potential in the wearable field and physiological signal detection.     

Water-Insensitive Self-Healing Materials: From Network Structure Design to Advanced Soft Electronics

Polymeric materials capable of spontaneously healing physical damages and restoring various functions have been attracting growing interest. Among these, the category of water-insensitive self-healing materials emerges as a promising research focus due to their reliable self-healing and stable mechanical properties in high-humidity environments and even underwater. In this review, an update on the significant advancements in the design of water-insensitive self-healing polymers is presented, which are based on various unique chains. Their advantages and limitations are discussed. Additionally, a series of typical dynamic interactions that are used to enable autonomous self-healing in underwater environments is highlighted. Moving beyond these fundamental designs, the diverse opportunities to leverage recent synthetic advancements in water-insensitive self-healing materials for the progression of soft electronic applications are systematically discussed. Ultimately, the significant challenges and remaining opportunities to present a comprehensive view of the future development of water-insensitive self-healing materials are highlighted. This review aims to stimulate further innovation in this burgeoning and emerging field of intrinsic healable materials, interfacing with dynamic chemistry and soft electronics.     

Permanently Mechanically Adjustable Photothermal Catalytic Spontaneous Double Cross-Linking Network Enables Durable and Stretchable Plant Skin-Like Materials

In cellulose-based plastics, as a type of thermoplastic and thermosetting materials, the excellent balance of mechanical strength and ductility poses a large challenge. To tackle this problem, a novel approach is devised to introduce reversible non-covalent ester cross-linking into dynamic covalent hydrogen-bonded polymer networks. However, the formation of ester bonds typically requires excess reactants and dehydrating agents, which is energy-intensive, environmentally harmful, and costly. To address these concerns, inspired by polyester-rich plant bark, a supramolecular composite material is developed. It can be dissolved and regenerated using a binary solvent system (choline hexanoate/choline chloride-oxalic acid). In water, this supramolecular composite material underwent self-healing and ester exchange reactions to form double-cross-linked networks, interfaced with photo-thermal catalysis promoting the reaction due to its high photo-thermal conversion efficiency (86.7%) and water evaporation rate (1.38 kg m⁻² h⁻¹). This enables the rapid and repeatable construction of durable and stretchable biomaterials. The mechanical properties of the supramolecular plastic can be adjusted by solar photo-thermal conditions of the synthesis environment. These materials exhibit high performance in solar water evaporation and have self-healing properties and are degradable, recyclable, and capable of eliminating their own adhesions.     

High-Throughput Screening of Self-Healable Polysulfobetaine Hydrogels and their Applications in Flexible Electronics

Hydrogels are promising materials in the applications of wound adhesives, wearable electronics, tissue engineering, implantable electronics, etc. The properties of a hydrogel rely strongly on its composition. However, the optimization of hydrogel properties has been a big challenge as increasing numbers of components are added to enhance and synergize its mechanical, biomedical, electrical, and self-healable properties. Here in this work, it is shown that high-throughput screening can efficiently and systematically explore the effects of multiple components (at least eight) on the properties of polysulfobetaine hydrogels, as well as provide a useful database for diverse applications. The optimized polysulfobetaine hydrogels that exhibit outstanding self-healing and mechanical properties, have been obtained by high-throughput screening. By compositing with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), intrinsically self-healable and stretchable conductors are achieved. It is further demonstrated that a polysulfobetaine hydrogel-based electronic skin, which exhibits exceptionally fast self-healing capability of the whole device at ambient conditions. This work successfully extends high-throughput synthetic methodology to the field of hydrogel electronics, as well as demonstrates new directions of healable flexible electronic devices in terms of material development and device design.     

A Fully Self-Healing and Highly Stretchable Liquid-Free Ionic Conductive Elastomer for Soft Ionotronics

Soft ionic conductors hold great potential for soft ionotronics, such as ionic skin, human–machine interface and soft luminescent device. However, most hydrogel and ionogel-based soft ionic conductors suffer from freezing, evaporation and liquid leakage problems, which limit their use in complex environments. Herein, a class of liquid-free ionic conductive elastomers (ICEs) is reported as an alternative soft ionic conductor in soft ionotronics. These liquid-free ICEs offer a combination of desirable properties, including extraordinary stretchability (up to 1913%), toughness (up to 1.08 MJ cm⁻³), Young's modulus (up to 0.67 MPa), rapid fully self-healing capability at room temperature, and good conductivity (up to 1.01 × 10⁻⁵ S cm⁻¹). The application of these ICEs is demonstrated by creating a wearable sensor that can detect and discriminate minimal deformations and human body movements, such as finger or elbow joint flexion, walking, running, etc. In addition, self-healing soft ionotronic devices are demonstrated to confront mechanical breakdown, such as an ionic skin and an alternating-current electroluminescent device that can reuse from damage. It is believed that these liquid-free ICEs hold great promises for applications in wearable devices and soft ionotronics.     

Self‐Healing,Flexible, and Tailorable Triboelectric Nanogenerators for Self‐Powered Sensors based on Thermal Effect of Infrared Radiation

Self‐healing triboelectric nanogenerators (TENGs) with flexibility, robustness, and conformability are highly desirable for promising flexible and wearable devices, which can serve as a durable, stable, and renewable power supply, as well as a self‐powered sensor. Herein, an entirely self‐healing, flexible, and tailorable TENG is designed as a wearable sensor to monitor human motion, with infrared radiation from skin to promote self‐healing after being broken based on thermal effect of infrared radiation. Human skin is a natural infrared radiation emitter, providing favorable conditions for the device to function efficiently. The reversible imine bonds and quadruple hydrogen bonding (UPy) moieties are introduced into polymer networks to construct self‐healable electrification layer. UPy‐functionalized multiwalled carbon nanotubes are further incorporated into healable polymer to obtain conductive nanocomposite. Driven by the dynamic bonds, the designed and synthesized materials show excellent intrinsic self‐healing and shape‐tailorable features. Moreover, there is a robust interface bonding in the TENG devices due to the similar healable networks between electrification layer and electrode. The output electric performances of the self‐healable TENG devices can almost restore their original state when the damage of the devices occurs. This work presents a novel strategy for flexible devices, contributing to future sustainable energy and wearable electronics.