Recyclable, Healable, and Tough Ionogels Insensitive to Crack Propagation

Straightforward preparation of a tough and stretchable ion gel

Ion gels, polymer networks swollen by ionic liquids, are expected to be applied to wearable devices that are tolerant to repeated stretching. High strength and excellent stretchability was achieved due to the numerous physical cross-links with abundant polymer chain entanglements in addition to a small number of immobile chemical cross-links, even though the ion gel was prepared by a facile methodology.

Jellyfish-Inspired Polyurea Ionogel with Mechanical Robustness, Self-Healing, and Fluorescence Enabled by Hyperbranched Cluster Aggregates

Ionogels are promising for soft iontronics, with their network structure playing a pivotal role in determining their performance and potential applications. However, simultaneously achieving mechanical toughness, low hysteresis, self-healing, and fluorescence using existing network structures is challenging. Drawing inspiration from jellyfish, we propose a novel hierarchical crosslinking network structure design for in situ formation of hyperbranched cluster aggregates (HCA) to fabricate polyurea ionogels to overcome these challenges. Leveraging the disparate reactivity of isocyanate groups, we induce the in situ formation of HCA through competing reactions, enhancing toughness and imparting the clustering-triggered emission of ionogel. This synergy between supramolecular interactions in the network and plasticizing effect in ionic liquid leads to reduced hysteresis of the ionogel. Furthermore, the incorporation of NCO-terminated prepolymer with dynamic oxime-urethane bonds (NPU) enables self-healing and enhances stretchability. Our investigations highlight the significant influence of HCA on ionogel performance, showcasing mechanical robustness including high strength (3.5 MPa), exceptional toughness (5.5 MJ m-3), resistance to puncture, and low hysteresis, self-healing, as well as fluorescence, surpassing conventional dynamic crosslinking approaches. This network design strategy is versatile and can meet the various demands of flexible electronics applications.

Mechanical Regulation of Polymer Gels

The mechanical properties of polymer gels devote to emerging devices and machines in fields such as biomedical engineering, flexible bioelectronics, biomimetic actuators, and energy harvesters. Coupling network architectures and interactions has been explored to regulate supportive mechanical characteristics of polymer gels; however, systematic reviews correlating mechanics to interaction forces at the molecular and structural levels remain absent in the field. This review highlights the molecular engineering and structural engineering of polymer gel mechanics and a comprehensive mechanistic understanding of mechanical regulation. Molecular engineering alters molecular architecture and manipulates functional groups/moieties at the molecular level, introducing various interactions and permanent or reversible dynamic bonds as the dissipative energy. Molecular engineering usually uses monomers, cross-linkers, chains, and other additives. Structural engineering utilizes casting methods, solvent phase regulation, mechanochemistry, macromolecule chemical reactions, and biomanufacturing technology to construct and tailor the topological network structures, or heterogeneous modulus compositions. We envision that the perfect combination of molecular and structural engineering may provide a fresh view to extend exciting new perspectives of this burgeoning field. This review also summarizes recent representative applications of polymer gels with excellent mechanical properties. Conclusions and perspectives are also provided from five aspects of concise summary, mechanical mechanism, biofabrication methods, upgraded applications, and synergistic methodology.

A Universal Zwitterionic Cross-Linking Strategy for Designing Conductive Soft Electronics with Enhanced Mechanical Properties

Ionogels are emerging as promising electronics due to their exceptional ionic conductivity, stretchability, and high thermal stability. However, developing ionogels with enhanced mechanical properties without compromising conductivity and ion transport rates remains a significant challenge. Here, we report a zwitterionic cross-linker, 4-(2-(((2-(methacryloyloxy)ethyl)carbamoyl)oxy)ethyl)-4,14-dimethyl-8,13-dioxo-7,12-dioxa-4,9-diazapentadec-14-en-4-ium-1-propanesulfonate (MEPS) and utilized it to cross-link a variety of functional monomers, leading to the synthesis of conductive ionogels that exhibit both high mechanical strength and versatile applicability. Due to its abundant hydrogen bond donors/acceptors and zwitterionic moiety, MEPS exhibits several hundred times higher solubility in ionic liquids compared to conventional cross-linkers. As a proof-of-concept, the poly(acrylic acid-MEPS) ionogels demonstrate enhanced elongation, fracture toughness, and superior thermal stability, all while maintaining high conductivity due to the high affinity between ionic liquids and zwitterionic networks. Furthermore, MEPS-cross-linked poly(α-thioctic acid) electronics can be engineered as strain sensors, showing exceptional antifatigue properties and recyclability, remaining stable and functional over 300 consecutive cycles. This universal cross-linking strategy not only improves the overall performance of ionogels but also contributes to the development of next-generation soft electronics with enhanced functionality and durability.

Multi-Modal Sensing Ionogels with Tunable Mechanical Properties and Environmental Stability for Aquatic and Atmospheric Environments

Ionogels have garnered significant interest due to their great potential in flexible iontronic devices. However, their limited mechanical tunability and environmental intolerance have posed significant challenges for their integration into next-generation flexible electronics in different scenarios. Herein, the synergistic effect of cation-oxygen coordination interaction and hydrogen bonding is leveraged to construct a 3D supramolecular network, resulting in ionogels with tunable modulus, stretchability, and strength, achieving an unprecedented elongation at break of 10 800%. Moreover, the supramolecular network endows the ionogels with extremely high fracture energy, crack insensitivity, and high elasticity. Meanwhile, the high environmental stability and hydrophobic network of the ionogels further shield them from the unfavorable effects of temperature variations and water molecules, enabling them to operate within a broad temperature range and exhibit robust underwater adhesion. Then, the ionogel is assembled into a wearable sensor, demonstrating its great potential in flexible sensing (temperature, pressure, and strain) and underwater signal transmission. This work can inspire the applications of ionogels in multifunctional sensing and wearable fields.

A Recyclable Ionogel with High Mechanical Robustness Based on Covalent Adaptable Networks

Ionogels are an emerging class of soft materials for flexible electronics, with high ionic conductivity, low volatility, and mechanical stretchability. Recyclable ionogels are recently developed to address the sustainability crisis of current electronics, through the introduction of non-covalent bonds. However, this strategy sacrifices mechanical robustness and chemical stability, severely diminishing the potential for practical application. Here, covalent adaptable networks (CANs) are incorporated into ionogels, where dynamic covalent crosslinks endow high strength (11.3 MPa tensile strength), stretchability (2396% elongation at break), elasticity (energy loss coefficient of 0.055 at 100% strain), and durability (5000 cycles of 150% strain). The reversible nature of CANs allows the ionogel to be closed-loop recyclable for up to ten times. Additionally, the ionogel is toughened by physical crosslinks between conducting ions and polymer networks, breaking the common dilemma in enhancing mechanical properties and electrical conductivity. The ionogel demonstrates robust strain sensing performance under harsh mechanical treatments and is applied for reconfigurable multimodal sensing based on its recyclability. This study provides insights into improving the mechanical and electrical properties of ionogels toward functionally reliable and environmentally sustainable bioelectronics.

Sea Cucumber-Inspired Polyurethane Demonstrating Record-Breaking Mechanical Properties in Room-Temperature Self-Healing Ionogels

Practical applications of existing self-healing ionogels are often hindered by the trade-off between their mechanical robustness, ionic conductivity, and temperature requirements for their self-healing ability. Herein, this challenge is addressed by drawing inspiration from sea cucumber. A polyurethane containing multiple hydrogen-bond donors and acceptors is synthesized and used to fabricate room-temperature self-healing ionogels with excellent mechanical properties, high ionic conductivity, puncture resistance, and impact resistance. The hard segments of polyurethane, driven by multiple hydrogen bonds, coalesce into hard phase regions, which can efficiently dissipate energy through the reversible disruption and reformation of multiple hydrogen bonds. Consequently, the resulting ionogels exhibit record-high tensile strength and toughness compared to other room-temperature self-healing ionogels. Furthermore, the inherent reversibility of multiple hydrogen bonds within the hard phase regions allows the ionogels to spontaneously and efficiently self-heal damaged mechanical properties and ionic conductivity multiple times at room temperature. To underscore their application potential, these ionogels are employed as electrolytes in the fabrication of electrochromic devices, which exhibit excellent and stable electrochromic performance, repeatable healing ability, and satisfactory impact resistance. This study presents a novel strategy for the fabrication of ionogels with exceptional mechanical properties and room-temperature self-healing capability.

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.

Versatile ionic liquid gels formed by dynamic covalent bonding and microphase separated structures

It is challenging for ionic liquid gels to achieve the combination of rapid self-healing with high toughness. Here, ionic liquid gels (DI-PR) were prepared from readily available materials. A dynamic covalently bonded oxime-carbamate was prepared from polycaprolactone diol, isophorone diisocyanate and dimethylethyleneglyoxime, followed by addition of the "rigid-flexible" cross-linking agent rutin to chemically cross-link the polymer chains and afford the ionic liquid gels, DI-PR. The tensile strength, elongation at break and toughness of the DI-PR gels were as high as 16.5 MPa, 1132.6%, and 52.6 MJ m-3, respectively. The toughness is similar to that of natural silkworm silk (70 MJ m-3) and wool (60 MJ m-3). After stretching, the DI-PR can rebound within 1 s, their room temperature self-healing rate is as high as 92%, they remain functional over the temperature range -50 °C to 140 °C and the interface with a steel plate has an adhesion toughness of >2000 J m-2. These properties mean that the DI-PR gels are particularly suitable for use as anticorrosion coatings for submarine and underground gas and oil pipelines. The use of rutin, which combines rigid quercetin-based structural units with flexible glycoside-based structural units, as a cross-linking agent, provides a new method for improving the toughness of soft materials through its synergistic interaction with hard and soft chain fragments of polyurethanes.

Internal catalysis significantly promotes the bond exchange of covalent adaptable polyurethane networks

Self-healing covalent adaptable networks (CANs) are not only of fundamental interest but also of practical importance for achieving carbon neutrality and sustainable development. However, there is a trade-off between the mobility and cross-linking structure of CANs, making it challenging to develop CANs with excellent mechanical properties and high self-healing efficiency. Here, we report the utilization of a highly dynamic four-arm cross-linking unit with an internally catalyzed oxime-urethane group to obtain CAN-based ionogel with both high self-healing efficiency (>92.1%) at room temperature and superior mechanical properties (tensile strength 4.55 MPa and toughness 13.49 MJ m-3). This work demonstrates the significant potential of utilizing the synergistic electronic, spatial, and topological effects as a design strategy for developing high-performance materials.

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.

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.

Self-Healing Ionogel-Enabled Self-Healing and Wide-Temperature Flexible Zinc-Air Batteries with Ultra-Long Cycling Lives

Hydrogel-based zinc-air batteries (ZABs) are promising flexible rechargeable batteries. However, the practical application of hydrogel-based ZABs is limited by their short service life, narrow operating temperature range, and repair difficulty. Herein, a self-healing ionogel is synthesized by the photopolymerization of acrylamide and poly(ethylene glycol) monomethyl ether acrylate in 1-ethyl-3-methylimidazolium dicyanamide with zinc acetate dihydrate and first used as an electrolyte to fabricate self-healing ZABs. The obtained self-healing ionogel has a wide operating temperature range, good environmental and electrochemical stability, high ionic conductivity, satisfactory mechanical strength, repeatable and efficient self-healing properties enabled by the reversibility of hydrogen bonding, and the ability to inhibit the production of dendrites and by-products. Notably, the self-healing ionogel has the highest ionic conductivity and toughness compared to other reported self-healing ionogels. The prepared self-healing ionogel is used to assemble self-healing flexible ZABs with a wide operating temperature range. These ZABs have ultra-long cycling lives and excellent stability under harsh conditions. After being damaged, the ZABs can repeatedly self-heal to recover their battery performance, providing a long-lasting and reliable power supply for wearable devices. This work opens new opportunities for the development of electrolytes for ZABs.

Ligand Dissociation of Metal-Complex Photocatalysts toward pH-Photomanipulation in Dynamic Covalent Hydrogels for Printing Reprocessable and Recyclable Devices

Dynamic covalent hydrogels are gaining attention for their potential in smart materials, soft devices, electronics, and more thanks to their impressive mechanical properties, biomimetic structures, and dynamic behavior. However, a significant challenge lies in designing precise and efficient dynamic photochemistry for their preparation, allowing for complex structures and control over the dynamic process. Herein, we propose a general and straightforward orthogonal dynamic covalent photochemistry strategy for preparing high-performance printable dynamic covalent hydrogels, thereby broadening their advanced applications. This photochemical strategy uses a bifunctional photocatalyst to initiate radical polymerization and release ligands through a rapid light-mediated dissociation mechanism. This process leads to a controlled increase in system pH from mildly acidic to alkaline conditions within one hundred seconds, which in turn triggers the pH-sensitive model reactions of boronic acid/diol complexation and Knoevenagel condensation. The orthogonal photochemistry enables the formation of interpenetrated and conjoined networks, significantly enhancing the mechanical properties of the hydrogels. The reversible bonds formed during the process, i.e., boronic ester and unsaturated ketone bonds, confer excellent self-healing, reprocessable, and recyclable properties on the hydrogels through photochemical pH variations. Furthermore, this rapid, controlled fabrication process and dynamic behavior are highly compatible with printing techniques, enabling the design of adaptive and recyclable sensors with different structures. These advancements are promising for various material science, medicine, and engineering applications.

Small Ionic-Liquid-Based Molecule Drives Strong Adhesives

Nature has inspired scientists to fabricate adhesive materials for applications in many burgeoning areas. However, it is still a significant challenge to develop small-molecule adhesives with high-strength, low-temperature and recyclable properties, although these merits are of great interest in various aspects. Herein, we report a series of strong adhesives based on low-molecular-weight molecular solids driven by the terminal modification of ionic liquids (ILs) and subsequent supramolecular self-assembly. The emergence of high strength and liquid-to-solid transitions for these supramolecular aggregates relies on modifying IL with a high melting point motif and enriching the types of noncovalent interactions in the original ILs. Using this strategy, we demonstrate that our IL-based molecular solids can efficiently obtain a high adhesion strength (up to 8.95 MPa). Importantly, we elucidate the mechanism underlying the reversible and strong adhesion enabled by monomer-to-polymer transitions. These fundamental findings provide guidance for the design of high-performance supramolecular adhesive materials.

Biomimetic Multiscale Oriented PVA/NRL Hydrogel Enabled Multistimulus Responsive and Smart Shape Memory Actuator

Shape memory hydrogels provide a worldwide scope for functional soft materials. However, most shape memory hydrogels exhibit poor mechanical properties, leading to low actuation strength, which severely limits their applications in smart biomimetic devices. Herein, a strategy for muscle-inspired shape memory-oriented polyvinyl alcohol (PVA)-natural rubber latex (NRL) hydrogel (OPNH) with multiscale oriented structure is demonstrated. The shape memory function comes from the stretch-induced crystallization of natural rubber (NR), while PVA forms strong hydrogen bonding interactions with proteins and phospholipids on the surface of NRL particles. Meanwhile, the reconfigurable interactions of PVA and NR produce a multiscale-oriented structure during stretch-drying, improving the mechanical and shape memory properties. The resultant OPNH shows excellent interfacial compatibility, exhibiting outstanding mechanical performance (3.2 MPa), high shape fixity (≈80%) and shape recovery ratio (≈92%), high actuation strength (206 kPa), working capacity (105 kJ m- 3), extremely short response time (≈2 s), low response temperature (28 °C) and smart thermal responsiveness. It can even maintain muscle-like working capacity when lifting a load equivalent to 372 times its weight, providing a new class shape memory material for the application in smart biomimetic muscles and multistimulus responsive devices.

Biocompatible Tough Ionogels with Reversible Supramolecular Adhesion

Cohesive and interfacial adhesion energies are difficult to balance to obtain reversible adhesives with both high mechanical strength and high adhesion strength, although various methods have been extensively investigated. Here, a biocompatible citric acid/L-(-)-carnitine (CAC)-based ionic liquid was developed as a solvent to prepare tough and high adhesion strength ionogels for reversible engineered and biological adhesives. The prepared ionogels exhibited good mechanical properties, including tensile strength (14.4 MPa), Young's modulus (48.1 MPa), toughness (115.2 MJ m-3), and high adhesion strength on the glass substrate (24.4 MPa). Furthermore, the ionogels can form mechanically matched tough adhesion at the interface of wet biological tissues (interfacial toughness about 191 J m-2) and can be detached by saline solution on demand, thus extending potential applications in various clinical scenarios such as wound adhesion and nondestructive transfer of organs.

Superstrong Ionogel Enabled by Coacervation-Induced Nanofibril Assembly for Sustainable Moisture Energy Harvesting

Ionogels have grabbed significant interest in various applications, from sensors and actuators to wearable electronics and energy storage devices. However, current ionogels suffer from low strength and poor ionic conductivity, limiting their performance in practical applications. Here, inspired by the mechanical reinforcement of natural biomacromolecules through noncovalent aggregates, a strategy is proposed to construct nanofibril-based ionogels through complex coacervation-induced assembly. Cellulose nanofibrils (CNFs) can bundle together with poly(ionic liquid) (PIL) to form a superstrong nanofibrous network, in which the ionic liquid (IL) can be retained to form ionogels with high liquid inclusion and ionic conductivity. The strength of the CNF-PIL-IL ionogels can be tuned by the IL content over a wide range of up to 78 MPa. The optical transparency, high strength, and hygroscopicity enabled them to be promising candidates in moist-electricity generation and applications such as energy harvesting windows and wearable power generators. In addition, the ionogels are degradable and the ionogel-based generators can be recycled through dehydration. Our strategy suggests perspectives for the fabrication of high-strength and multifunctional ionogels for sustainable applications.

Highly Stretchable Double Network Ionogels for Monitoring Physiological Signals and Detecting Sign Language

At the heart of the non-implantable electronic revolution lies ionogels, which are remarkably conductive, thermally stable, and even antimicrobial materials. Yet, their potential has been hindered by poor mechanical properties. Herein, a double network (DN) ionogel crafted from 1-Ethyl-3-methylimidazolium chloride ([Emim]Cl), acrylamide (AM), and polyvinyl alcohol (PVA) was constructed. Tensile strength, fracture elongation, and conductivity can be adjusted across a wide range, enabling researchers to fabricate the material to meet specific needs. With adjustable mechanical properties, such as tensile strength (0.06-5.30 MPa) and fracture elongation (363-1373%), this ionogel possesses both robustness and flexibility. This ionogel exhibits a bi-modal response to temperature and strain, making it an ideal candidate for strain sensor applications. It also functions as a flexible strain sensor that can detect physiological signals in real time, opening doors to personalized health monitoring and disease management. Moreover, these gels' ability to decode the intricate movements of sign language paves the way for improved communication accessibility for the deaf and hard-of-hearing community. This DN ionogel lays the foundation for a future in which e-skins and wearable sensors will seamlessly integrate into our lives, revolutionizing healthcare, human-machine interaction, and beyond.

Piezoionic Elastomers by Phase and Interface Engineering for High-Performance Energy-Harvesting Ionotronics

Piezoionic materials play a pivotal role in energy-harvesting ionotronics. However, a persistent challenge lies in balancing the structural requirements for voltage generation, current conduction, and mechanical adaptability. The conventional approach of employing crystalline heterostructures for stress concentration and localized charge separation, while effective for voltage generation, often compromises the stretchability and long-range charge transport found in homogeneous quasisolid states. Herein, phase and interface engineering strategy is introduced to address this dilemma and a piezoionic elastomer is presented that seamlessly integrates ionic liquids and ionic plastic crystals, forming a finely tuned microphase-separated structure with an intermediate phase. This approach promotes charge separation via stress concentration among hard phases while leveraging the high ionic charge mobility in soft and intermediate phases. Impressively, the elastomer achieves an extraordinary piezoionic coefficient of about 6.0 mV kPa-1, a more than threefold improvement over current hydrogels and ionogels. The resulting power density of 1.3 µW cm-3 sets a new benchmark, exceeding that of state-of-the-art piezoionic gels. Notably, this elastomer combines outstanding stretchability, remarkable toughness, and rapid self-healing capability, underscoring its potential for real-world applications. This work may represent a stride toward mechanically robust energy harvesting systems and provide insights into ionotronic systems for human-machine interaction.

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.

Microphase-Separated Elastic and Ultrastretchable Ionogel for Reliable Ionic Skin with Multimodal Sensation

Bioinspired artificial skins integrated with reliable human-machine interfaces and stretchable electronic systems have attracted considerable attention. However, the current design faces difficulties in simultaneously achieving satisfactory skin-like mechanical compliance and self-powered multimodal sensing. Here, this work reports a microphase-separated bicontinuous ionogel which possesses skin-like mechanical properties and mimics the multimodal sensing ability of biological skin by ion-driven stimuli-electricity conversion. The ionogel exhibits excellent elasticity and ionic conductivity, high toughness, and ultrastretchability, as well as a Young's modulus similar to that of human skin. Leveraging the ion-polymer interactions enabled selective ion transport, the ionogel can output pulsing or continuous electrical signals in response to diverse stimuli such as strain, touch pressure, and temperature sensitively, demonstrating a unique self-powered multimodal sensing. Furthermore, the ionogel-based I-skin can concurrently sense different stimuli and decouple the variations of the stimuli from the voltage signals with the assistance of a machine-learning model. The ease of fabrication, wide tunability, self-powered multimodal sensing, and the excellent environmental tolerance of the ionogels demonstrate a new strategy in the development of next-generation soft smart mechano-transduction devices.

Bicontinuous vitrimer heterogels with wide-span switchable stiffness-gated iontronic coordination

Currently, it remains challenging to balance intrinsic stiffness with programmability in most vitrimers. Simultaneously, coordinating materials with gel-like iontronic properties for intrinsic ion transmission while maintaining vitrimer programmable features remains underexplored. Here, we introduce a phase-engineering strategy to fabricate bicontinuous vitrimer heterogel (VHG) materials. Such VHGs exhibited high mechanical strength, with an elastic modulus of up to 116 MPa, a high strain performance exceeding 1000%, and a switchable stiffness ratio surpassing 5 × 103. Moreover, highly programmable reprocessing and shape memory morphing were realized owing to the ion liquid-enhanced VHG network reconfiguration. Derived from the ion transmission pathway in the ILgel, which responded to the wide-span switchable mechanics, the VHG iontronics had a unique bidirectional stiffness-gated piezoresistivity, coordinating both positive and negative piezoresistive properties. Our findings indicate that the VHG system can act as a foundational material in various promising applications, including smart sensors, soft machines, and bioelectronics.

Designing Ultratough Single-Network Hydrogels with Centimeter-Scale Fractocohesive Lengths via Inelastic Crack Blunting

Fractocohesive length, defined as the ratio of fracture toughness to work of fracture, measures the sensitivity of materials to fracture in the presence of flaws. The larger the fractocohesive length, the more flaw-tolerant and crack-resistant the hydrogel. For synthetic soft materials, the fractocohesive length is short, often on the scale of 1 mm. Here, highly flaw-insensitive (HFI) single-network hydrogels containing an entangled inhomogeneous polymer network of widely distributed chain lengths are designed. The HFI hydrogels demonstrate a centimeter-scale fractocohesive length of 2.21 cm, which is the highest ever recorded for synthetic hydrogels, and an unprecedented fracture toughness of ≈13 300 J m-2 . The uncommon flaw insensitivity results from the inelastic crack blunting inherent to the highly inhomogeneous network. When the HFI hydrogel is stretched, a large number of short chains break while coiled long chains can disentangle, unwind, and straighten, producing large inelastic deformation that substantially blunts the crack tip in a plastic manner, thereby deconcentrating crack-tip stresses and blocking crack extension. The flaw-insensitive design strategy is applicable to various hydrogels such as polyacrylamide and poly(N,N-dimethylacrylamide) hydrogels and enables the development of HFI soft composites.

Fracture-Resistant Stretchable Materials: An Overview from Methodology to Applications

Stretchable materials, such as gels and elastomers, are attractive materials in diverse applications. Their versatile fabrication platforms enable the creation of materials with various physiochemical properties and geometries. However, the mechanical performance of traditional stretchable materials is often hindered by the deficiencies in their energy dissipation system, leading to lower fracture resistance and impeding their broader range of applications. Therefore, the synthesis of fracture-resistant stretchable materials has attracted great interest. This review comprehensively summarizes key design considerations for constructing fracture-resistant stretchable materials, examines their synthesis strategies to achieve elevated fracture energy, and highlights recent advancements in their potential applications.

High-Toughness and High-Strength Solvent-Free Linear Poly(ionic liquid) Elastomers

Solvent-free elastomers, unlike gels, do not suffer from solvent evaporation and leakage in practical applications. However, it is challenging to realize the preparation of high-toughness (with both high stress and strain) ionic elastomers. Herein, high-toughness linear poly(ionic liquid) (PIL) elastomers are constructed via supramolecular ionic networks formed by the polymerization of halometallate ionic liquid (IL) monomers, without any chemical crosslinking. The obtained linear PIL elastomers exhibit high strength (16.5 MPa), Young's modulus (157.49 MPa), toughness (130.31 MJ m-3 ), and high crack propagation insensitivity (fracture energy 243.37 kJ m-2 ), owing to the enhanced intermolecular noncovalent interactions of PIL chains. Furthermore, PIL elastomer-based strain, pressure, and touch sensors have shown high sensitivity. The linear noncovalent crosslinked network endows the PIL elastomers with self-healing and recyclable properties, and broad application prospects in the fields of flexible sensor devices, health monitoring, and human-machine interaction.

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.

Low-Hysteresis and High-Toughness Hydrogels Regulated by Porous Cationic Polymers: the Effect of Counteranions

Service life and range of polymer materials is heavily reliant on their elasticity and mechanical stability under long-term loading. Slippage of chain segments under load leads to significant hysteresis of the hydrogels, limiting its repeatability and mechanical stability. Achieving the desired elasticity exceeding that of rubber is a great challenge for hydrogels, particularly when subjected to large deformations. Here, low-hysteresis and high-toughness hydrogels were developed through controllable interactions of porous cationic polymers (PCPs) with adjustable counteranions, including reversible bonding of PCP frameworks/polymer segments (polyacrylamide, PAAm) and counteranions/PAAm. This strategy reduces chain segment slippage under load, endowing the PCP-based hydrogels (PCP-gels) with good elasticity under large deformations (7 % hysteresis at a strain ratio of 40). Furthermore, due to the enlarged chain segments entanglement by PCP, the PCP-gels exhibit large strain (13000 %), significantly enhanced toughness (68 MJ m-3 ), high fracture energy (43.1 kJ m-2 ), and fatigue resistance. The unique properties of these elastic PCP-gels have promising applications in the field of flexible sensors.

Spider-Silk-Inspired Tough, Self-Healing, and Melt-Spinnable Ionogels

As stretchable conductive materials, ionogels have gained increasing attention. However, it still remains crucial to integrate multiple functions including mechanically robust, room temperature self-healing capacity, facile processing, and recyclability into an ionogel-based device with high potential for applications such as soft robots, electronic skins, and wearable electronics. Herein, inspired by the structure of spider silk, a multilevel hydrogen bonding strategy to effectively produce multi-functional ionogels is proposed with a combination of the desirable properties. The ionogels are synthesized based on N-isopropylacrylamide (NIPAM), N, N-dimethylacrylamide (DMA), and ionic liquids (ILs) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]). The synergistic hydrogen bonding interactions between PNIPAM chains, PDMA chains, and ILs endow the ionogels with improved mechanical strength along with fast self-healing ability at ambient conditions. Furthermore, the synthesized ionogels show great capability for the continuous fabrication of the ionogel-based fibers using the melt-spinning process. The ionogel fibers exhibit spider-silk-like features with hysteresis behavior, indicating their excellent energy dissipation performance. Moreover, an interwoven network of ionogel fibers with strain and thermal sensing performance can accurately sense the location of objects. In addition, the ionogels show great recyclability and processability into different shapes using 3D printing. This work provides a new strategy to design superior ionogels for diverse applications.

Nanoconfined polymerization limits crack propagation in hysteresis-free gels

Consecutive mechanical loading cycles cause irreversible fatigue damage and residual strain in gels, affecting their service life and application scope. Hysteresis-free hydrogels within a limited deformation range have been created by various strategies. However, large deformation and high elasticity are inherently contradictory attributes. Here we present a nanoconfined polymerization strategy for producing tough and near-zero-hysteresis gels under a large range of deformations. Gels are prepared through in situ polymerization within nanochannels of covalent organic frameworks or molecular sieves. The nanochannel confinement and strong hydrogen bonding interactions with polymer segments are crucial for achieving rapid self-reinforcement. The rigid nanostructures relieve the stress concentration at the crack tips and prevent crack propagation, enhancing the ultimate fracture strain (17,580 ± 308%), toughness (87.7 ± 2.3 MJ m-3) and crack propagation strain (5,800%) of the gels. This approach provides a general strategy for synthesizing gels that overcome the traditional trade-offs of large deformation and high elasticity.

Advancing high-performance tailored dual-crosslinking network organo-hydrogel flexible device for wireless wearable sensing

Conductive hydrogels are essential for enabling long-term and reliable signal sensing in wearable electronics due to their tunable flexibility, stimulus responsiveness, and multimodal sensing integration. However, developing durable and dependable integrated hydrogel-based flexible devices has been challenging due to mismatched mechanical properties, limited water retention capability, and reduced flexibility. This work addresses these challenges by employing a tailored physical-chemical dual-crosslinking strategy to fabricate dynamically reversible organo-hydrogels with high performance. The resultant organo-hydrogels exhibit exceptional characteristics, including high stretchability (up to ∼495% strain), remarkable toughness (with tensile and compressive strengths of ∼1350 kPa and ∼9370 kPa, respectively), and outstanding transparency (∼90.3%). Moreover, they demonstrate excellent long-term water retention ability (>2424 h, >97%). Notably, the organo-hydrogel based sensor exhibits heightened sensitivity for monitoring physiological signals and motions. Furthermore, our integrated wireless wearable sensing system efficiently captures and transmits various human physiological signals and motion information in real-time. This research advances the development of customized devices utilizing functional organo-hydrogel materials, making contributions to fulfilling the increasing demand for high-performance wireless wearable sensing.

Poly(Ionic Liquid) Double-Network Elastomers with High-Impact Resistance Enhanced by Cation-π Interactions

With the continuous development of impact protection materials, lightweight, high-impact resistance, flexibility, and controllable toughness are required. Here, tough and impact-resistant poly(ionic liquid) (PIL)/poly(hydroxyethyl acrylate) (PHEA) double-network (DN) elastomers are constructed via multiple cross-linking of polymer networks and cation-π interactions of PIL chains. Benefiting from the strong noncovalent cohesion achieved by the cation-π interactions in PIL chains, the prepared PIL DN elastomers exhibit extraordinary compressive strength (95.24 ± 2.49 MPa) and toughness (55.98 ± 0.66 MJ m-3 ) under high-velocity impact load (5000 s-1 ). The synthesized PIL DN elastomer combines strength and flexibility to protect fragile items from impact. This strategy provides a new research idea in the field of the next generation of safety and protective materials.

Healable Ionic Conductors with Extremely Low-Hysteresis and High Mechanical Strength Enabled by Hydrophobic Domain-Locked Reversible Interactions

Extremely low hysteresis, high mechanical strength, superior toughness, and excellent healability are essential for stretchable ionic conductors to enhance their reliability and meet for cutting-edge applications. However, the fabrication of stretchable ionic conductors with such mutually exclusive properties remains challenging. Herein, extremely low-hysteresis and healable ionic conductors with a tensile strength of ≈8.9 MPa and toughness of ≈23.2 MJ m-3 are fabricated through the complexation of 4-carboxybenzaldehyde (CBA) grafted poly(vinyl alcohol) (PVA) (denoted as PVA-CBA) and poly (allylamine hydrochloride) (PAH) followed by acidification and ion-loading steps. The acidification step generates the PVA-CBA/PAH ionic conductors with in situ formed dynamic hydrophobic domains that lock and stabilize noncovalent interactions. This significantly minimizes the energy dissipation of the ionic conductors during cyclic mechanical loading (≤200% strain), resulting in ionic conductors with extremely low hysteresis (≈5%). The fractured ionic conductors can be healed at 60 °C to restore their original properties. Because of the extremely low hysteresis, the PVA-CBA/PAH ionic conductors show a highly stable and reproducible electrical response over 5000 uninterrupted loading-unloading cycles at a strain of 200%. The ionic conductor based sensors exhibit a high sensitivity to a wide range of strains (1-500%).

High-Saline-Enabled Hydrophobic Homogeneous Cross-Linking for Extremely Soft, Tough, and Stretchable Conductive Hydrogels as High-Sensitive Strain Sensors

Design of admirable conductive hydrogels combining robust toughness, soft flexibility, desirable conductivity, and freezing resistance remains daunting challenges for meeting the customized and critical demands of flexible and wearable electronics. Herein, a promising and facile strategy to prepare hydrogels tailored to these anticipated demands is proposed, which is prepared in one step by homogeneous cross-linking of acrylamide using hydrophobic divinylbenzene stabilized by micelles under saturated high-saline solutions. The influence of high-saline environments on the hydrogel topology and mechanical performance is investigated. The high-saline environments suppress the size of hydrophobic cross-linkers in micelles during hydrogel polymerization, which weaken the dynamic hydrophobic associations to soften the hydrogels. Nevertheless, the homogeneous cross-linked networks ensure antifracture during ultralarge deformations. The obtained hydrogels show special mechanical performance combining extremely soft deformability and antifracture features (Young's modulus, 5 kPa; stretchability, 10200%; toughness, 134 kJ m-2; and excellent anticrack propagation). The saturated-saline environments also endow the hydrogels with desirable ion conductivity (106 mS cm-1) and freezing resistance (<20 °C). These comprehensive properties of the obtained hydrogels are quite suitable for flexible electronic applications, which is demonstrated by the high sensitivity and durability of the derived strain sensors.

Ultralight, Heat-Insulated, and Tough PVA Hydrogel Hybridized with SiO2 @cellulose Nanoclaws Aerogel via the Synergy of Hydrophilic and Hydrophobic Interfacial Interactions

Lightweight porous hydrogels provide a worldwide scope for functional soft mateirals. However, most porous hydrogels have weak mechanical strength, high density (>1 g cm-3 ), and high heat absorption due to weak interfacial interactions and high solvent fill rates, which severely limit their application in wearable soft-electronic devices. Herein, an effective hybrid hydrogel-aerogel strategy to assemble ultralight, heat-insulated, and tough polyvinyl alcohol (PVA)/SiO2 @cellulose nanoclaws (CNCWs) hydrogels (PSCG) via strong interfacial interactions with hydrogen bonding and hydrophobic interaction is demonstrated. The resultant PSCG has an interesting hierarchical porous structure from bubble template (≈100 µm), PVA hydrogels networks introduced by ice crystals (≈10 µm), and hybrid SiO2 aerogels (<50 nm), respectively. PSCG shows unprecedented low density (0.27 g cm-3 ), high tensile strength (1.6 MPa) & compressive strength (1.5 MPa), excellent heat-insulated ability, and strain-sensitive conductivity. This lightweight porous and tough hydrogel with an ingenious design provides a new way for wearable soft-electronic devices.

Super tough and high adhesive eutectic ionogels enabled by high-density hydrogen bond network

Ionogels have attracted tremendous interest for flexible electronics due to their excellent deformability, conductivity, and environmental stability. However, most ionogels suffer from low strength and poor toughness, which limit their practical applications. This article presents a strategy for fabricating ionogels with high toughness by constructing high-density hydrogen bonds within their microstructure. The ionogels exhibit a maximum fracture strength of 11.44 MPa, and can sustain a fracture strain of 506%. They also demonstrate a fracture energy of 27.29 MJ m-3 and offer a wide range of mechanical property adjustments (fracture stress from 0.3 to 11.44 MPa, fracture strain from 506% to 1050%). Strain sensors assembled with ionogels demonstrate exceptional sensing performance and enable motion detection of human joints. This study provides a new approach for achieving strong and tough ionogel design used for high-performance flexible electronic applications.

Tough and Recyclable Phase-Separated Supramolecular Gels via a Dehydration-Hydration Cycle

Hydrogels are compelling materials for emerging applications including soft robotics and autonomous sensing. Mechanical stability over an extensive range of environmental conditions and considerations of sustainability, both environmentally benign processing and end-of-life use, are enduring challenges. To make progress on these challenges, we designed a dehydration-hydration approach to transform soft and weak hydrogels into tough and recyclable supramolecular phase-separated gels (PSGs) using water as the only solvent. The dehydration-hydration approach led to phase separation and the formation of domains consisting of strong polymer-polymer interactions that are critical for forming PSGs. The phase-separated segments acted as robust, physical cross-links to strengthen PSGs, which exhibited enhanced toughness and stretchability in its fully swollen state. PSGs are not prone to overswelling or severe shrinkage in wet conditions and show environmental tolerance in harsh conditions, e.g., solutions with pH between 1 and 14. Finally, we demonstrate the use of PSGs as strain sensors in air and aqueous environments.

Liquid-Free, Self-Repairable, Recyclable, and Highly Stretchable Colorless Solid Ionic Conductive Elastomers for Strain/Temperature Sensors

Solid-state ionic conductive elastomers (ICEs) can fundamentally overcome the disadvantages of hydrogels and ionogels (their liquid components tend to leak or evaporate), and are considered to be ideal materials for flexible ionic sensors. In this study, a liquid-free ionic polyurethane (PU) type conductive elastomer (ICE-2) was synthesized and studied. The PU type matrix with microphase separation endowed ICE-2 with excellent mechanical versatility. The disulfide bond exchange reaction in the hard phase and intermolecular hydrogen bonds contributed to damage repairing ability. ICE-2 exhibited good ionic conductivity (2.86×10-6  S/cm), high transparency (average transmittance >89 %, 400~800 nm), excellent mechanical properties (tensile strength of 3.06 MPa, elongation at break of 1760 %, and fracture energy of 14.98 kJ/m2 ), appreciable self-healing ability (healing efficiency >90 %), satisfactory environmental stability, and outstanding recyclability. The sensor constructed by ICE-2 could not only realize the perception of temperature changes, but also accurately and sensitively detect various human activities, including joint movements and micro-expression changes. This study provides a simple and effective strategy for the development of flexible and soft ionic conductors for sensors and human-machine interfaces.

An Ultrahighly Stretchable and Recyclable Starch-Based Gel with Multiple Functions

With the development of various gel-based flexible sensors, novel gels with multiple integrated and efficient properties, particularly recyclability, have been developed. Herein, a starch-based ADM (amylopectin (AP)-poly(3-[dimethyl-[2-(2-methylprop-2- enoyloxy)ethyl]azaniumyl]propane-1-sulfonate) (PDMAPS)-MXene) gel is prepared by a facile "cooking" strategy accompanying the gelatinization of AP and polymerization reaction of zwitterionic monomers. Reversible crosslinking in the gel occurs through hydrogen bonding and electrostatic interactions. The ADM gel exhibits high stretchability (≈2700%, after one month), swift self-healing performance, self-adhesive properties, favorable freezing resistance, and satisfactory moisturizing properties (≥30 days). Interestingly, the ADM gel can be recycled and reused by a "kneading" method and "dissolution-dialysis" process, respectively. Furthermore, the ADM gel can be assembled as a strain sensor with a broad working strain range (≈800%) and quick response time (response time 211 ms and recovery time 253 ms, under 10% strain) to detect various macro- and micro-human-motions, even under harsh conditions such as pronunciation and handwriting. The ADM gel can also be used as a humidity sensor to investigate humidity and human respiratory status, suggesting its practical application in personal health management. This study provides a novel strategy for the preparation of high-performance recycled gels and flexible sensors.

An Ultrastretchable Gradient Ionogel Induced by a Self-Floating Strategy for Strain Sensing

The fabrication of gradient ionogels for flexible strain sensors remains challenging because of the complex preparation procedures, and it is still difficult to prepare highly stretchable ionogels (strain > 10000%). In this study, a strategy is proposed to successfully fabricate gradient ionogels and apply them to flexible strain sensors by utilizing the self-floating character of the polysiloxane cross-linker. A gradient ionogel with ultrahigh stretchability (>14000%) is prepared via a one-step in situ photopolymerization process of the precursor with long-chain poly(dimethylsiloxane) bis(2-methyl acrylate) (PDMSMA). PDMSMA, which has a self-floating ability and excellent flexibility, induces a gradient composition distribution in the ionogel, thereby endowing the ionogel with superior stretchability and gradient changes in conductivity and adhesivity from the top to the bottom layer. Because of multiple molecular interactions, the bottom surface of the ionogel possesses good resilience and self-adhesion, whereas the top surface, which has a high PDMSMA content, shows a nonsticky performance. As a result, a singular gradient ionogel having both a sticky bottom surface and a nonsticky top surface is achieved. Furthermore, the flexible strain sensor that is created based on these gradient ionogels exhibits high sensitivity (its gauge factor reaching 5.08), a wide detection range (1-1500%), fast response times, and good linearity. Notably, the detection signal remains repeatable over 1000 uninterrupted strain cycles. The fabricated strain sensor was further utilized to monitor joint movements and physiological signals. This work provides a facile strategy for fabricating gradient ionogels and shows their application potential in the field of flexible electronics.

Degradable Supramolecular Eutectogel-Based Ionic Skin with Antibacterial, Adhesive, and Self-Healable Capabilities

The development of degradable, cost-effective, and eco-friendly ionic conductive gels is highly required to reduce electronic waste originating from flexible electronic devices. However, biocompatible, degradable, tough, and durable conductive gels are challenging to achieve. Herein, we develop a facile strategy for the design and synthesis of degradable tough eutectogels by integrating an electrostatically driven supramolecular network composed of branched polyacrylic acid (PAA) and monoethanolamine (MEA) into a green deep eutectic solvent with chitosan quaternary ammonium salt (CQS). The specially designed PAA/MEA/CQS eutectogels present multiple desired properties, including high transparency, widely adjustable mechanical properties, high resilience, reliable adhesiveness, excellent self-healing ability, good conductivity, remarkable anti-freezing performance, and antibacterial properties. The dynamic and reversible supramolecular interactions not only significantly enhance the mechanical properties of the PAA/MEA/CQS eutectogels but also enable fast degradation, addressing the dilemma between mechanical strength and degradability. More importantly, a biocompatible and degradable multifunctional ionic skin is successfully fabricated based on the PAA/MEA/CQS eutectogel, exhibiting high sensitivity, a wide sensing range, and a rapid response speed toward strain, pressure, and temperature. Thus, this study offers a promising strategy for fabricating degradable tough eutectogels, which show great potential as high-performance ionic skins for next-generation flexible wearable electronic devices.

Adaptive Ion-Gel: Stimuli-Responsive, and Self-Healing Ion Gels

Ion gels are an emerging class of polymer gels in which a three-dimensional polymer network swells with an ionic liquid. Ion gels have drawn considerable attention in various fields such as energy and biotechnology owing to their excellent properties including nonvolatility, nonflammability, high ionic conductivity, and high thermal and electrochemical stability. Since the first report on ion gels (published ∼30 years ago), diverse functional ion gels exhibiting impressive physicochemical properties have been reported. In this review, recent developments in functional ion gels that can modulate their physical properties in response to environmental conditions are outlined. Stimuli-responsive ion gels that can adaptively undergo phase transitions in response to thermal and light stimuli are initially discussed, followed by an evaluation of diverse self-healing ion gels that can spontaneously mend mechanical damage through judiciously designed ion-gel networks.

Multifunctional Ionic Conductive Anisotropic Elastomers with Self-Wrinkling Microstructures by In Situ Phase Separation

Multifunctional flexible sensors are the development trend of wearable electronic devices in the future. As the core of flexible sensors, the key is to construct a stable multifunctional integrated conductive elastomer. Here, ionic conductive elastomers (ICEs) with self-wrinkling microstructures are designed and prepared by in situ phase separation induced by a one-step polymerization reaction. The ICEs are composed of ionic liquids as ionic conductors doped into liquid crystal elastomers. The doped ionic liquids cluster into small droplets and in situ induce the formation of wrinkle structures on the upper surface of the films. The prepared ICEs exhibit mechanochromism, conductivity, large tensile strain, low hysteresis, high cycle stability, and sensitivity during the tension-release process, which achieve dual-mode outputs of optical and electrical signals for information transmission and sensors.

Integrated Moist-Thermoelectric Generator for Efficient Waste Steam Energy Utilization

Industrial waste steam is one of the major sources of global energy losses. Therefore, the collection and conversion of waste steam energy into electricity have aroused great interest. Here, a "two-in-one" strategy is reported that combines thermoelectric and moist-electric generation mechanisms for a highly efficient flexible moist-thermoelectric generator (MTEG). The spontaneous adsorption of water molecules and heat in the polyelectrolyte membrane induces the fast dissociation and diffusion of Na⁺ and H⁺ , resulting in the high electricity generation. Thus, the assembled flexible MTEG generates power with a high open-circuit voltage (V_oc ) of 1.81 V (effective area = 1cm² ) and a power density of up to 4.75±0.4 µW cm^-2 . With efficient integration, a 12-unit MTEG can produce a V_oc of 15.97 V, which is superior to most known TEGs and MEGs. The integrated and flexible MTEGs reported herein provide new insights for harvesting energy from industrial waste steam.

Room-Temperature Self-Healing Soft Composite Network with Unprecedented Crack Propagation Resistance Enabled by a Supramolecular Assembled Lamellar Structure

Soft self-healing materials are compelling candidates for stretchable devices because of their excellent compliance, extensibility, and self-restorability. However, most existing soft self-healing polymers suffer from crack propagation and irreversible fatigue failure due to easy breakage of their dynamic amorphous, low-energy polymer networks. Herein, inspired by distinct structure-property relationship of biological tissues, a supramolecular interfacial assembly strategy of preparing soft self-healing composites with unprecedented crack propagation resistance is proposed by structurally engineering preferentially aligned lamellar structures within a dynamic and superstretchable poly(urea-ureathane) matrix (which is elongated to 24 750× its original length). Such a design affords a world-record fracture energy (501.6 kJ m^-2 ), ultrahigh fatigue threshold (4064.1 J m^-2 ), and outstanding elastic restorability (dimensional recovery from 13 times elongation), and preserving low modulus (1.2 MPa), high stretchability (3200%), and high room-temperature self-healing efficiency (97%). Thereby, the resultant composite represents the best of its kind and even surpasses most biological tissues. The lamellar 2D transition-metal carbide/carbonitride (MXene) structure also leads to a relatively high in-plane thermal conductivity, enabling composites as stretchable thermoconductive skins applied in joints of robotics to thermal dissipation. The present work illustrates a viable approach how autonomous self-healing, crack tolerance, and fatigue resistance can be merged in future material design.

Polysaccharide hydrogel electrolytes with robust interfacial contact to electrodes for quasi-solid state flexible aqueous zinc ion batteries with efficient suppressing of dendrite growth

Flexible aqueous zinc-ion batteries (AZIBs) require high conductive and adhesive hydrogel electrolytes. However, high adhesion tends to hinder ion conduction rate. Herein, we designed a water/glycerol binary solvent coordinating the hydrophilic polymers to reconstruct the water molecules' environment in the hydrogel. As a consequence, the interface adhesion strength between Zn and the hydrogel reached 3.0 kPa and the ionic conductivity was up to 16.8 mS cm^-1. In addition, inspired by the slurry electrode preparation method, we developed a simple blade coating technique using a non-Newtonian polysaccharide liquid solution to construct an ultra-thin hydrogel electrolyte in situ on the cathode. The thickness of the obtained hydrogel reached 70 μm, and the ultrathin flexible AZIBs were easily constructed by pasting a Zn anode directly on the adhesive hydrogel, showing the potential of flexible AZIBs scalable assembly. In addition, the Zn//Zn symmetrical cells with the hydrogel electrolyte provided stable cycling performance for over 400 h at 0.1 mA cm^-2 with suppressed dendrite growth. The assembled Zn//Polyaniline battery and Zn//V₂O₅ battery also exhibited excellent capacity retention after cycles. This work has realized the hydrogel electrolyte with high adhesion and conductivity, which has good adaptability to metal electrodes and opened up a new practical way for large-scale assembly of flexible energy storage devices.

Highly Damping and Self-Healable Ionic Elastomer from Dynamic Phase Separation of Sticky Fluorinated Polymers

Shock-induced low-frequency vibration damage is extremely harmful to bionic soft robots and machines that may incur the malfunction of fragile electronic elements. However, current skin-like self-healable ionic elastomers as the artificial sensing and protecting layer still lack the ability to dampen vibrations, due to their almost opposite design for molecular frictions to material's elasticity. Inspired by the two-phase structure of adipose tissue (the natural damping skin layer), here, a highly damping ionic elastomer with energy-dissipating nanophases embedded in an elastic matrix is introduced, which is formed by polymerization-induced dynamic phase separation of sticky fluorinated copolymers in the presence of lithium salts. Such a supramolecular design decouples the elastic and damping functions into two distinct phases, and thus reconciles a few intriguing properties including ionic conductivity, high stretchability, softness, strain-stiffening, elastic recovery, room-temperature self-healability, recyclability, and most importantly, record-high damping capacity at the human motion frequency range (loss factor tan δ > 1 at 0.1-50 Hz). This study opens the door for the artificial syntheses of high-performance damping ionic skins with robust sensing and protective applications in soft electronics and robotics.

Carbon Quantum Dot/Chitosan-Derived Hydrogels with Photo-stress-pH Multiresponsiveness for Wearable Sensors

In recent years, hydrogels have attracted extensive attention in smart sensing owing to their biocompatibility and high elasticity. However, it is still a challenge to develop hydrogels with excellent multiple responsiveness for smart wearable sensors. In this paper, a facile synthesis of carbon quantum dots (CQDs)-doped cross-linked chitosan quaternary/carboxymethylcellulose hydrogels (CCCDs) is presented. Designing of dual network hydrogels decorated with CQDs provides abundant crosslinking and improves the mechanical properties of the hydrogels. The hydrogel-based strain sensor exhibits excellent sensitivity (gauge factor: 9.88), linearity (R² : 0.97), stretchable ability (stress: 0.67 MPa; strain: 404%), good cyclicity, and durability. The luminescent properties are endowed by the CQDs further broaden the application of hydrogels for realizing flexible electronics. More interestingly, the strain sensor based on CCCDs hydrogel demonstrates photo responsiveness (ΔR/R₀ ≈20%) and pH responsiveness (pH range ≈4-7) performance. CCCDs hydrogels can be used for gesture recognition and light sensing switch. As a proof-of-concept, a smart wearable sensor is designed for monitoring human activities and detecting pH variation in human sweat during exercise. This study reveals new possibilities for further applications in wearable health monitoring.

A Full-Device Autonomous Self-Healing Stretchable Soft Battery from Self-Bonded Eutectogels

Next-generation energy storage devices should be soft, stretchable, and self-healable. Previously reported self-healable batteries mostly possess limited stretchability and rely on healable electrodes or electrolytes rather than achieving full-device self-healability. Herein, an all-component self-bonding strategy is reported to obtain an all-eutectogel soft battery (AESB) that simultaneously achieves full-cell autonomous self-healability and omnidirectional intrinsic stretchability (>1000% areal strain) over a broad temperature range (-20~60 °C). Without requiring any external stimulus, the five-layered soft battery can efficiently recover both its mechanical and electrochemical performance at full-cell level. The developed AESB can be easily configured into various 3D architectures with highly interfacial compatible eutectogel electrodes, electrolyte, and substrate, presenting an excellent opportunity for the development of embodied energy technologies. The present work provides a general and user-friendly soft electronic material platform for fabricating a variety of intrinsic self-healing stretchable multi-layered electronics, which are promising beyond the field of energy storage, such as displays, sensors, circuits, and soft robots.