Topoarchitected polymer networks expand the space of material properties

A gradient-distributed binder with high energy dissipation for stable silicon anode

Silicon is considered as a promising alternative to traditional graphite anode for lithium-ion batteries. Due to the dramatic volume expansion of silicon anode generated from the insertion of Li+ ions, the binder which can suppress the severe volume change and repeated massive stress impact during cycling is required greatly. Herein, we design a gradient-distributed two-component binder (GE-PAA) to achieve excellent cyclic stability, and reveal the mechanism of high energy dissipative binder stabilized silicon electrodes. The inner layer of the electrode is the polyacrylic acid polymer (PAA) with high Young's modulus, which is used as the skeleton binder to stabilize the silicon particle interface and the electrode structure. The outer layer is the gel electrolyte polymer (GE) with lower Young's modulus, which releases the stress generated during the lithiation and de-lithiation process effectively, achieving the high structural stability at the molecular level and silicon particles. Due to the synergistic effect of the gradient binder design, the silicon electrode retains a reversible capacity of 1557.4 mAh g-1 after 200 cycles at the current density of 0.5 C and 1539.2 mAh g-1 at a high rate of 1.8 C. This work provides a novel binder design strategy for Si anode with long cycle stability.

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.

Mechanochemistry: Fundamental Principles and Applications

Mechanochemistry is an emerging research field at the interface of physics, mechanics, materials science, and chemistry. Complementary to traditional activation methods in chemistry, such as heat, electricity, and light, mechanochemistry focuses on the activation of chemical reactions by directly or indirectly applying mechanical forces. It has evolved as a powerful tool for controlling chemical reactions in solid state systems, sensing and responding to stresses in polymer materials, regulating interfacial adhesions, and stimulating biological processes. By combining theoretical approaches, simulations and experimental techniques, researchers have gained intricate insights into the mechanisms underlying mechanochemistry. In this review, the physical chemistry principles underpinning mechanochemistry are elucidated and a comprehensive overview of recent significant achievements in the discovery of mechanically responsive chemical processes is provided, with a particular emphasis on their applications in materials science. Additionally, The perspectives and insights into potential future directions for this exciting research field are offered.

Determining fatigue threshold of elastomers through an elastic limit strain point

Determining the fatigue threshold of elastomers is particularly important to predict their durability and lifespan. However, there has been almost no effective approach to calculate this threshold fast and accurately so far. To realize rapid fatigue performance testing, in this paper, a new method is proposed to calculate the fatigue threshold through the elastic limit strain point of elastomers that can be obtained from the continuous Mullins test. Compared with the traditional binary method to predict fatigue threshold, this method significantly reduces the time required for fatigue testing of elastomers, which can save approximately 2-3 sampling points in the fatigue test, i.e. a time savings of around 40%. Our method also proved to be effective for the elastomers at 0 °C. Additionally, a dual-network elastomer was synthesized using the interpenetrating network method, exhibiting improved elasticity (2.1 MPa and 3.1 MPa), toughness (1423 J m-2 and 2100 J m-2), and higher fatigue threshold (125 J m-2 and 385 J m-2) at 0 °C and room temperature. This material presents great application potential in marine anti-fouling.

Multifunctional Polyoxometalates-Based Ionohydrogels toward Flexible Electronics

Multifunctional flexible electronics present tremendous opportunities in the rapidly evolving digital age. One potential avenue to realize this goal is the integration of polyoxometalates (POMs) and ionic liquid-based gels (ILGs), but the challenge of macrophase separation due to poor compatibility, especially caused by repulsion between like-charged units, poses a significant hurdle. Herein, the possibilities of producing diverse and homogenous POMs-containing ionohydrogels by nanoconfining POMs and ionic liquids (ILs) within an elastomer-like polyzwitterionic hydrogel using a simple one-step random copolymerization method, are expanded vastly. The incorporation of polyzwitterions provides a nanoconfined microenvironment and effectively modulates excessive electrostatic interactions in POMs/ILs/H2O blending system, facilitating a phase transition from macrophase separation to a submillimeter scale worm-like microphase-separation system. Moreover, combining POMs-reinforced ionohydrogels with a developed integrated self-powered sensing system utilizing strain sensors and Zn-ion hybrid supercapacitors has enabled efficient energy storage and detection of external strain changes with high precision. This work not only provides guidelines for manipulating morphology within phase-separation gelation systems, but also paves the way for developing versatile POMs-based ionohydrogels for state-of-the-art smart flexible electronics.

Stretchable Electrodes with Interfacial Percolation Network

Stretchable electrodes are an essential component that determines the functionality and reliability of stretchable electronics, but face the challenge of balancing conductivity and stretchability. This work proposes a new conducting concept called the interfacial percolation network (PN) that results in stretchable electrodes with high conductivity, large stretchability, and high stability. The interfacial PN is composed of a 2D silver nanowires (AgNWs) PN and a protruding 3D AgNWs PN embedded on the surface and in the near-surface region of an elastic polymer matrix, respectively. The protruded PN is obtained by changing the arrangements of AgNWs from horizontal to quasi-vertical through introducing foreign polymer domains in the near-surface region of the polymer matrix. The resulting electrode achieves a conductivity of 13 500 S cm-1 and a stretchability of 660%. Its resistance changes under stretched conditions are orders of magnitude lower than those of conventional 2D PN and 2D + 3D PN. An interfacial PN electrode made from liquid metal remained its conductivity at 46 750 S cm-1 after the electrode underwent multiple stretch-release cycles with a deformation of >600%. The concept of interfacial PN provides fruitful implications for the design of stretchable electronics.

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

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

Effect of Cross-Link Homogeneity on the High-Strain Behavior of Elastic Polymer Networks

Cross-link heterogeneity and topological defects have been shown to affect the moduli of polymer networks in the low-strain regime. Probing their role in the high-strain regime, however, has been difficult because of premature network fracture. Here, we address this problem by using a double-network approach to investigate the high-strain behavior of both randomly and regularly cross-linked networks with the same backbone chemistry. Randomly cross-linked poly(n-butyl acrylate) networks with target molecular weights between cross-links of 5-30 kg/mol were synthesized via free-radical polymerization, while regularly cross-linked poly(n-butyl acrylate) networks with molecular weights between cross-links of 7-38 kg/mol were synthesized via cross-linking of tetrafunctional star polymers. Both types of networks were then swollen in a monomer/cross-linker mixture, polymerized to form double networks, and characterized via uniaxial tensile testing. The onset of strain stiffening was found to occur later in regular networks than in random networks with the same modulus but was well-predicted by the target molecular weight between cross-links of each sample. These results indicate that the low- and high-strain behavior of polymer networks result from different molecular-scale features of the material and suggest that controlling network architecture offers new opportunities to both further fundamental understanding of architecture-property relationships and design materials with independently controlled moduli and strain stiffening responses.

Highly Resilient Noncovalently Associated Hydrogel Adhesives for Wound Sealing Patch

The development of hydrogel adhesives with high mechanical resilience and toughness remains a challenging task. Hydrogels must exhibit high mechanical resilience to withstand the inevitable movement of the human body while simultaneously demonstrating strong wet tissue adhesion and appropriate toughness to hold and seal damaged tissues; However, tissue adhesion, toughness, and mechanical resilience are typically negatively correlated. Therefore, this paper proposes a highly resilient double-network (DN) hydrogel wound-sealing patch that exhibits a well-balanced combination of tissue adhesion, toughness, and mechanical resilience. The DN structure is formed by introducing covalently and non-covalently crosslinkable dopamine-modified crosslinkers and physically interactable linear poly(vinyl imidazole) (PVI). The resulting hydrogel adhesive exhibits high toughness and mechanical resilience due to the presence of a DN involving reversible physical intermolecular interactions such as hydrogen bonds, hydrophobic associations, cation-π interactions, π-π interactions, and chain entanglements. Moreover, the hydrogel adhesive achieves strong wet tissue adhesion through the polar hydroxyl groups of dopamine and the amine group of PVI. These mechanical attributes allow the proposed adhesive to effectively seal damaged tissues and promote wound healing by maintaining a moist environment.

Hydrogen Bonds-Pinned Entanglement Blunting the Interfacial Crack of Hydrogel-Elastomer Hybrids

Anchoring a layer of amorphous hydrogel on an antagonistic elastomer holds potential applications in surface aqueous lubrication. However, the interfacial crack propagation usually occurs under continuous loads for amorphous hydrogel, leading to the failure of hydrogel interface. This work presents a universal strategy to passivate the interfacial cracks by designing a hydrogen bonds-pinned entanglement (Hb-En) structure of amorphous hydrogel on engineering elastomers. The unique Hb-En structure is created by pinning well-tailored entanglements via covalent-like hydrogen bonds, which can amplify the delocalization of interfacial stress concentration and elevate the necessary fracture energy barrier within hydrogel interface. Therefore, the interfacial crack propagation can be suppressed under single and cyclic loads, resulting in a high interfacial toughness over 1650 J m-2 and an excellent interfacial fatigue threshold of 423 J m-2. Such a strategy universally works on blunting the interfacial crack between hydrogel coating and various elastomer materials with arbitrary shapes. The superb fatigue-crack insensitivity at the interface allows for durable aqueous lubrication of hydrogel coating with low friction.

Water vapor assisted aramid nanofiber reinforcement for strong, tough and ionically conductive organohydrogels as high-performance strain sensors

Conductive organohydrogels have gained increasing attention in wearable sensors, flexible batteries, and soft robots due to their exceptional environment adaptability and controllable conductivity. However, it is still difficult for conductive organohydrogels to achieve simultaneous improvement in mechanical and electrical properties. Here, we propose a novel "water vapor assisted aramid nanofiber (ANF) reinforcement" strategy to prepare robust and ionically conductive organohydrogels. Water vapor diffusion can induce the pre-gelation of the polymer solution and ensure the uniform dispersion of ANFs in organohydrogels. ANF reinforced organohydrogels have remarkable mechanical properties with a tensile strength, stretchability and toughness of up to 1.88 ± 0.04 MPa, 633 ± 30%, and 6.75 ± 0.38 MJ m-3, respectively. Furthermore, the organohydrogels exhibit great crack propagation resistance with the fracture energy and fatigue threshold as high as 3793 ± 167 J m-2 and ∼328 J m-2, respectively. As strain sensors, the conductive organohydrogel demonstrates a short response time of 112 ms, a large working strain and superior cycling stability (1200 cycles at 40% strain), enabling effective monitoring of a wide range of complex human motions. This study provides a new yet effective design strategy for high performance and multi-functional nanofiller reinforced organohydrogels.

Topological MXene Network Enabled Mixed Ion-Electron Conductive Hydrogel Bioelectronics

Mixed ion-electron conductive (MIEC) bioelectronics has emerged as a state-of-the-art type of bioelectronics for bioelectrical signal monitoring. However, existing MIEC bioelectronics is limited by delamination and transmission defects in bioelectrical signals. Herein, a topological MXene network enhanced MIEC hydrogel bioelectronics that simultaneously exhibits both electrical and mechanical property enhancement while maintaining adhesion and biocompatibility, providing an ideal MIEC bioelectronics for electrophysiological signal monitoring, is introduced. Compared with nontopology hydrogel bioelectronics, the MXene topology increases the dynamic stability of bioelectronics by a factor of 8.4 and the electrical signal by a factor of 10.1 and reduces the energy dissipation by a factor of 20.2. Besides, the topology-enhanced hydrogel bioelectronics exhibits low impedance (<25 Ω) at physiologically relevant frequencies and negligible impedance fluctuation after 5000 stretch cycles. The creation of multichannel bioelectronics with high-fidelity muscle action mapping and gait recognition was made possible by achieving such performance.

Shape memory hydrogels with remodelable permanent shapes and programmable cold-induced shape recovery behavior

Most shape memory polymers apply glass transition or crystallization of domains to fix temporary shapes and shape recovery is induced by heating, which hinders their application under heat-intolerant conditions. Moreover, the permanent shapes of polymers normally cannot be altered arbitrarily after fabrication. Herein, we present a novel shape memory hydrogel with a remodelable permanent shape and programmable cold-induced shape recovery behavior. Poly(acrylic acid) (PAA) hydrogel is prepared in the presence of diethylenetriamine (DETA) and subsequently treated with calcium acetate (Ca(Ac)2). The charge-assisted hydrogen bonding between PAA and DETA imparts the hydrogel with remodelability, while the heat-induced hydrophobic aggregation of polymer chains and acetate groups results in shape fixation by heating and shape recovery by cooling. Afterwards, programmable deformable devices are obtained by assembling hydrogel blocks with different concentrations of Ca(Ac)2. This design strategy promotes the development of shape memory polymers with diverse potential applications.

Universal adhesion using mussel foot protein inspired hydrogel with dynamic interpenetration for topological entanglement

Achieving adhesion of hydrogels to universal materials with desirable strength remains a challenge despite emerging application of hydrogels. Herein we present a mussel foot protein (Mfp) inspired polyelectrolyte hydrogel of poly(ethylenimine)/poly(acrylic acid)-dopamine (PEI/PAADA) developed for universal tough adhesion. The highly-concentrated electrostatic and hydrogen-bonding interactions in PEI/PAADA hydrogel resulted in a tensile strength, strain at break, and toughness of 0.297 MPa, 2784 % and 5.440 MJ m-3, respectively. Moreover, the hydrogel can heal itself from physical damages, even can be recycled after totally dried via rehydration because of the high flexibility and reversibility of its dynamic bonds. Combining the strategies of topological stitching and direct bonding, Mfp-derived catechol and PEI/PAA backbone in PEI/PAADA corporately facilitated robust adhesion of universal materials with shear strength of up to 4.4 MPa and peeling strength of 870 J m-2, which is over 10 times greater than that of commercial fibrin gel. The adhesive also exhibited self-healing capability for at least 5 cycles, good stability in 1 M NaCl solution and characteristic debonding catalyzed by calcium. Moreover, in vitro cell behavior and in vivo wound healing assays suggested the potential of PEI/PAADA as wound dressing.

Liquid metal-hydrogel composites for flexible electronics

As an emerging functional material, liquid metal-hydrogel composites exhibit excellent biosafety, high electrical conductivity, tunable mechanical properties and good adhesion, thus providing a unique platform for a wide range of flexible electronics applications such as wearable devices, medical devices, actuators, and energy conversion devices. Through different composite methods, liquid metals can be integrated into hydrogel matrices to form multifunctional composite material systems, which further expands the application range of hydrogels. In this paper, we provide a brief overview of the two materials: hydrogels and liquid metals, and discuss the synthesis method of liquid metal-hydrogel composites, focusing on the improvement of the performance of hydrogel materials by liquid metals. In addition, we summarize the research progress of liquid metal-hydrogel composites in the field of flexible electronics, pointing out the current challenges and future prospects of this material.

Skin-Inspired Patterned Hydrogel with Strain-Stiffening Capability for Strain Sensors

Flexible materials with ionic conductivity and stretchability are indispensable in emerging fields of flexible electronic devices as sensing and protecting layers. However, designing robust sensing materials with skin-like compliance remains challenging because of the contradiction between softness and strength. Herein, inspired by the modulus-contrast hierarchical structure of biological skin, we fabricated a biomimetic hydrogel with strain-stiffening capability by embedding the stiff array of poly(acrylic acid) (PAAc) in the soft polyacrylamide (PAAm) hydrogel. The stress distribution in both stiff and soft domains can be regulated by changing the arrangement of patterns, thus improving the mechanical properties of the patterned hydrogel. As expected, the resulting patterned hydrogel showed its nonlinear mechanical properties, which afforded a high strength of 1.20 MPa while maintaining a low initial Young's modulus of 31.0 kPa. Moreover, the array of PAAc enables the patterned hydrogel to possess protonic conductivity in the absence of additional ionic salts, thus endowing the patterned hydrogel with the ability to serve as a strain sensor for monitoring human motion.

High-Performance Supramolecular Organogel Adhesives for Antimicrobial Applications in Diverse Conditions

Supramolecular organogel coatings that can disinfect the deposited microbial pathogens are emerging as an effective vehicle to prevent pathogen transmission. However, the development of anti-pathogen supramolecular adhesives with mechanical robustness and controlled oil inclusion is technically challenging. Here, we report supramolecular adhesives with mechanical integrity and robust interfacial adhesion over a wide range of biogenic antimicrobial oil. Bifunctional monomers are synthesized and assembled into linear polymers with semicrystalline stackings through hierarchical hydrogen bonds, where incorporated bioactive oil could regulate the semicrystalline stackings into nanosized crystalline domains through intermolecular hydrogen bonds. The abundant bonding motifs provided by the supramolecular cross-linked networks could accommodate oil molecules with high inclusion capability and provide more interfacial binding sites with high adhesion strength, and the nanosized crystalline domains could stabilize the organogel network and compensate for the interactions with oil molecules to enhance structural and mechanical stability. In addition, rapid healing, robust adhesion, and antimicrobial and antiviral properties of the resultant organogel coatings are demonstrated. This study paves the way for the development of high-performance antimicrobial supramolecular adhesives with controlled oil inclusion, showing potential applications in soft robotics, tissue engineering, and biomedical devices.

High-Performance Branched Polymer Elastomer Based on a Topological Network Structure and Dynamic Bonding

High performance has always been the research focus of elastomers. However, there are inherent conflicts among properties of elastomers, such as strength and toughness, strength and damping performance, strength and self-healing ability, etc. Herein, first, we synthesized a unique structure of the dangling chain containing proton donors and receptors. Then, we design and fabricate a kind of high-performance elastomer with a gradient distribution of a dangling chain and a dynamic bond structure. The dangling chains of different lengths intertwine with each other and self-assemble to form a "dense accumulation" structure driven by hydrogen bonds, and the elastomer exhibits special micro/nano scale aggregated states and microphase separation. The "dense accumulation" structure plays a vital role in the increase of mechanical properties. Meanwhile, under the joint action of a dangling chain and a dynamic bond, the damping performance and self-healing performance of the elastomer are greatly enhanced. High strength (27.5 MPa), toughness (121.9 MJ·m-3), 94.8% healing efficiency and outstanding damping performance (tan δ ≥ 0.4, high damping temperature range up to 144 °C) are simultaneously achieved beyond the current state-of-the-art. This topoarchitected polymer with a gradient distribution of dangling chains successfully solves the defects of conventional branched polymers in deteriorating their mechanical properties. This material design provides a new strategy for the development of high-performance structural and functional integrated elastomers.

Pathways toward wearable and high-performance sensors based on hydrogels: toughening networks and conductive networks

Wearable hydrogel sensors provide a user-friendly option for wearable electronics and align well with the existing manufacturing strategy for connecting and communicating with large numbers of Internet of Things devices. This is attributed to their components and structures, which exhibit exceptional adaptability, scalability, bio-compatibility, and self-healing properties, reminiscent of human skin. This review focuses on the recent research on principal structural elements of wearable hydrogels: toughening networks and conductive networks, highlighting the strategies for enhancing mechanical and electrical properties. Wearable hydrogel sensors are categorized for an extensive exploration of their composition, mechanism, and design approach. This review provides a comprehensive understanding of wearable hydrogels and offers guidance for the design of components and structures in order to develop high-performance wearable hydrogel sensors.

Hydrogels of arrested phase separation simultaneously achieve high strength and low hysteresis

Hydrogels are being developed to bear loads. Applications include artificial tendons and muscles, which require high strength to bear loads and low hysteresis to reduce energy loss. However, simultaneously achieving high strength and low hysteresis has been challenging. This challenge is met here by synthesizing hydrogels of arrested phase separation. Such a hydrogel has interpenetrating hydrophilic and hydrophobic networks, which separate into a water-rich phase and a water-poor phase. The two phases arrest at the microscale. The soft hydrophilic phase deconcentrates stress in the strong hydrophobic phase, leading to high strength. The two phases are elastic and adhere through topological entanglements, leading to low hysteresis. For example, a hydrogel of 76 weight % water, made of poly(ethyl acrylate) and poly(acrylic acid), achieves a tensile strength of 6.9 megapascals and a hysteresis of 16.6%. This combination of properties has not been realized among previously existing hydrogels.

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.

Simultaneous Polymerization Acceleration and Mechanical Enhancement for Printing a Biomimetic PEDOT Adhesive by Coordinative and Orthogonal Ruthenium Photochemistry

Conductive hydrogels are promising material candidates in fields ranging from flexible sensors and electronic skin applications to personalized medical monitoring. However, developing intrinsically conductive polymer hydrogels (ICPHs) with high mechanical properties and excellent printability is still challenging. Here, we introduce a simultaneous polymerization acceleration and mechanical enhancement (SPAME) strategy to construct PEDOT-based ICPHs via the rational design of coordinative and orthogonal ruthenium photochemistry (CORP). This orthogonal photochemistry triggers the oxidative polymerization of EDOT and the coupling of phenols within seconds under blue light irradiation. Benefiting from the bifunctional EDTA-Fe design, the photoreleased Fe(III) accelerated the EDOT polymerization and shortened the preparation time of ICPHs to a few seconds. At the same time, the addition of EDTA-Fe enhanced their mechanical properties, and both the critical strains and maximum stresses of the hydrogel doubled. Furthermore, the introduction of phenol residues in PAA-Ph significantly shortened the gelation time from several minutes to about 7 s. Thus, this fast and controllable CORP chemistry is compatible with standard printing techniques for engineering hydrogels for complex multifunctional structures for multifunctional bioelectronics and devices.

Flaw-insensitive fatigue resistance of chemically fixed collagenous soft tissues

Bovine pericardium (BP) has been used as leaflets of prosthetic heart valves. The leaflets are sutured on metallic stents and can survive 400 million flaps (~10-year life span), unaffected by the suture holes. This flaw-insensitive fatigue resistance is unmatched by synthetic leaflets. We show that the endurance strength of BP under cyclic stretch is insensitive to cuts as long as 1 centimeter, about two orders of magnitude longer than that of a thermoplastic polyurethane (TPU). The flaw-insensitive fatigue resistance of BP results from the high strength of collagen fibers and soft matrix between them. When BP is stretched, the soft matrix enables a collagen fiber to transmit tension over a long length. The energy in the long length dissipates when the fiber breaks. We demonstrate that a BP leaflet greatly outperforms a TPU leaflet. It is hoped that these findings will aid the development of soft materials for flaw-insensitive fatigue resistance.

Bio-inspired ionic skins for smart medicine

Ionic skins are developed to mimic the mechanical properties and functions of natural skins. They have demonstrated substantial advantages to serve as the crucial interface to bridge the gap between humans and machines. The first-generation ionic skin is a stretchable capacitor comprising hydrogels as the ionic conductors and elastomers as the dielectrics, and realizes pressure and strain sensing through the measurement of the capacitance. Subsequent advances have been made to improve the mechanical properties of ionic skins and import diverse functions. For example, ultrahigh stretchability, strong interfacial adhesion, self-healing, moisturizing ability, and various sensing capabilities have been achieved separately or simultaneously. Most ionic skins are attached to natural skins to monitor bio-electrical signals continuously. Ionic skins have also been found with significant potential to serve as a smart drug-containing reservoir, which can release drugs spatially, temporally, and in a controllable way. Herein, this review focuses on the design and fabrication of ionic skins, and their applications related to smart medicine. Moreover, challenges and opportunities are also discussed. It is hoped that the development of bio-inspired ionic skins will provide a paradigm shift for self-diagnosis and healthcare.

Balanced Coexistence of Reversible and Irreversible Covalent Bonds in a Conductive Triple Polymeric Network Enables Stretchable Hydrogels with High Toughness and Adhesiveness

The application of soft hydrogels to stretchable devices has attracted increasing attention in deformable bioelectronics owing to their unique characteristic, "modulus matching between materials and organs". Despite considerable progress, their low toughness, low conductivity, and absence of tissue adhesiveness remain substantial challenges associated with unstable skin-interfacing, where body movements undesirably disturb electrical signal acquisitions. Herein, we report a material design of a highly tough strain-dissipative and skin-adhesive conducting hydrogel fabricated through a facile one-step sol-gel transition and its application to an interactive human-machine interface. The hydrogel comprises a triple polymeric network where irreversible amide linkage of polyacrylamide with alginate and dynamic covalent bonds entailing conjugated polymer chains of poly(3,4-ethylenedioxythiophene)-co-(3-thienylboronic acid) are simultaneously capable of high stretchability (1300% strain), efficient strain dissipation (36,209 J/m²), low electrical resistance (590 Ω), and even robust skin adhesiveness (35.0 ± 5.6 kPa). Based on such decent characteristics, the hydrogel was utilized as a multifunctional layer for successfully performing either electrophysiological cardiac/muscular on-skin sensors or an interactive stretchable human-machine interface.

Enhance Fracture Toughness and Fatigue Resistance of Hydrogels by Reversible Alignment of Nanofibers

Biological tissues, such as heart valve, tendon, etc., possess excellent mechanical properties, which arises from their inherent anisotropic arrangement of soft and hard phases. Inspired by the anisotropic structures, many methods have been developed to synthesize hydrogels that can achieve mechanical properties comparable to biological tissues. Here, we describe a new method to enhance fracture toughness and fatigue resistance of hydrogels by introducing nanofibers which can reversibly align with elastic deformation to form an anisotropic structure. As a demonstration, we introduce stiff, rod-like cellulose nanocrystals (CNCs) into a polyacrylamide (PAAm) network. CNCs aggregate into clusters to form hard phases and entangle with the PAAm network. The CNC/PAAm composite hydrogel is initially isotropic, becomes anisotropic upon loading, and recovers to be isotropic upon unloading. During the deformation, the aligned CNC clusters at the crack tip can transmit the stress over the size of the cluster, effectively resisting crack growth. We use photoelasticity and small-angle X-ray scattering (SAXS) tests to observe the change of microstructures associated with deformation. The fracture toughness of CNC/PAAm hydrogels with different sizes of CNCs can reach 1000 J/m². The fatigue threshold is about 100 J/m², an order of magnitude higher than that of PAAm hydrogel. This work provides a simple and general method to strengthen hydrogels under both monotonic and cyclic loads.

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

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