Biomimetic high performance artificial muscle built on sacrificial coordination network and mechanical training process

Wet-Spinning Carbon Nanotube/Shape Memory Polymer Composite Fibers with High Actuation Stress and Predesigned Shape Change

Actuators based on shape memory polymers and composites incorporating nanomaterial additives have been extensively studied; achieving both high output stress and precise shape change by low-cost, scalable methods is a long-term-desired yet challenging task. Here, conventional polymers (polyurea) and carbon nanotube (CNT) fillers are combined to fabricate reinforced composite fibers with exceptional actuation performance, by a wet-spinning method amenable for continuous production. It is found that a thermal-induced shrinkage step could obtain densified strong fibers, and the presence of CNTs effectively promotes the tensile orientation of polymer molecular chains, leading to much improved mechanical properties. Consequently, the CNT/ polyurea composite fibers exhibit stresses as high as 33 MPa within 0.36 s during thermal actuation, and stresses up to 22 MPa upon electrical stimulation enabled by the built-in conductive CNT networks. Utilizing the flexible thin fibers, various shape change behavior are also demonstrated including the conversion between different structures/curvatures, and recovery of predefined simple patterns. This high-performance composite fibers, capable of both thermal and electrical actuation and produced by low-cost materials and fabrication process, may find many potential applications in wearable devices, robotics, and biomedical areas.

Coiled Carbon Nanotube Fibers Sheathed by a Reinforced Liquid Crystal Elastomer for Strong and Programmable Artificial Muscles

Fibers of liquid crystal elastomers (LCEs) as promising artificial muscle show ultralarge and reversible contractile strokes. However, the contractile force is limited by the poor mechanical properties of the LCE fibers. Herein, we report high-strength LCE fibers by introducing a secondary network into the single-network LCE. The double-network LCE (DNLCE) shows considerable improvements in tensile strength (313.9%) and maximum actuation stress (342.8%) compared to pristine LCE. To facilitate the controllability and application, a coiled artificial muscle fiber consisting of DNLCE-coated carbon nanotube (CNT) fiber is prepared. When electrothermally driven, the artificial muscle fiber outputs a high actuation performance and programmable actuation. Furthermore, by knitting the artificial muscle fibers into origami structures, an intelligent gripper and crawling inchworm robot have been demonstrated. These demonstrations provide promising application scenarios for advanced intelligent systems in the future.

Rapid-Response Water-Shrink Films with High Output Work Density Based on Polyethylene Oxide and α-Cyclodextrin for Autonomous Wound Closure

Conventional wound closure methods, including sutures and tissue adhesives, present significant challenges for self-care treatment, particularly in the context of bleeding wounds. Existing stimuli-responsive contractile materials designed for autonomous wound closure frequently lack sufficient output work density to generate the force needed to bring the wound edges into proximity or necessitate stimuli that are not compatible with the human body. Here, semi-transparent, flexible, and water-responsive shrinkable films, composed of poly(ethylene oxide) and α-cyclodextrin, are reported. These films exhibit remarkable stability under ambient conditions and demonstrate significant contraction (≈50%) within 6 s upon exposure to water, generating substantial contractile stress (up to 6 MPa) and output work density (≈1028 kJ m-3), which is 100 times larger than that of conventional hydrogel and 25 times larger than that of skeletal muscles. Remarkably, upon hydration, these films are capable of lifting objects 10 000 times their own weight. Leveraging this technology, water-shrink tapes, which, upon contact with water, effectively constrict human skin and autonomously close bleeding wounds in animal models within 10 seconds, are developed further. This work offers a novel approach to skin wound management, showing significant potential for emergency and self-care scenarios.

High-performance electrically responsive artificial muscle materials for soft robot actuation

Traditional robotic devices are often bulky and rigid, making it difficult for them to adapt to the soft and complex shapes of the human body. In stark contrast, soft robots, as a burgeoning class of robotic technology, showcase exceptional flexibility and adaptability, positioning them as compelling contenders for a diverse array of applications. High-performance electrically responsive artificial muscle materials (ERAMMs), as key driving components of soft robots, can achieve efficient motion and deformation, as well as more flexible and precise robot control, attracting widespread attention. This paper reviews the latest advancements in high-performance ERAMMs and their applications in the field of soft robot actuation, using ionic polymer-metal composites and dielectric elastomers as typical cases. Firstly, the definition, characteristics, and electro-driven working principles of high-performance ERAMMs are introduced. Then, the material design and synthesis, fabrication processes and optimization, as well as characterization and testing methods of the ERAMMs are summarized. Furthermore, various applications of two typical ERAMMs in the field of soft robot actuation are discussed in detail. Finally, the challenges and future directions in current research are analyzed and anticipated. This review paper aims to provide researchers with a reference for understanding the latest research progress in high-performance ERAMMs and to guide the development and application of soft robots. STATEMENT OF SIGNIFICANCE.

A Crown-Ether-Based Elastomer Bearing Loop Structures with Dissipating Characteristics and Enhanced Mechanical Performance

Loops are prevalent topological structures in cross-linked polymer networks, resulting from the folding of polymer chains back onto themselves. Traditionally, they have been considered as defects that compromise the mechanical properties of the network, leading to extensive efforts in synthesis to prevent their formation. In this study, we introduce the inclusion of cyclic dibenzo-24-crown-8 (DB24C8) moieties within the polymer network strands to form CCNs, and surprisingly, these loops enhance the mechanical performances of the network, leading to tough elastomers. The toughening effect can be attributed to the unique cyclic structure of DB24C8. The relatively small size and the presence of rigid phenyl rings provide the loops with relatively stable conformations, allowing for substantial energy dissipation upon the application of force. Furthermore, the DB24C8 rings possess a broad range of potential conformations, imparting the materials with exceptional elasticity. The synergistic combination of these two features effectively toughens the materials, resulting in a remarkable 66-fold increase in toughness compared to the control sample of covalent networks. Moreover, the mechanical properties, particularly the recovery performance of the network, can be effectively tuned by introducing guests to bind with DB24C8, such as potassium ions and secondary ammonium salts.

A Solvent-Templated Porous Liquid Crystal Elastomer with Tactile Sensation beyond Reversible Actuation toward Versatile Artificial Muscles

Liquid crystal elastomers (LCEs), as a classical two-way shape-memory material, are good candidates for developing artificial muscles that mimic the contraction, expansion, or rotational behavior of natural muscles. However, biomimicry is currently focused more on the actuation functions of natural muscles dominated by muscle fibers, whereas the tactile sensing functions that are dominated by neuronal receptors and synapses have not been well captured. Very few studies have reported the sensing concept for LCEs, but the signals were still donated by macroscopic actuation, that is, variations in angle or length. Herein, we develop a conductive porous LCE (CPLCE) using a solvent (dimethyl sulfoxide (DMSO))-templated photo-cross-linking strategy, followed by carbon nanotube (CNT) incorporation. The CPLCE has excellent reversible contraction/elongation behavior in a manner similar to the actuation functions of skeletal muscles. Moreover, the CPLCE shows excellent pressure-sensing performance by providing real-time electrical signals and is capable of microtouch sensing, which is very similar to natural tactile sensing. Furthermore, macroscopic actuation and tactile sensation can be integrated into a single system. Proof-of-concept studies reveal that the CPLCE-based artificial muscle is sensitive to external touch while maintaining its excellent actuation performance. The CPLCE with tactile sensation beyond reversible actuation is expected to benefit the development of versatile artificial muscles and intelligent robots.

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

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

Ultra-Large Stress and Strain Polymer Nanocomposite Actuators Incorporating a Mutually-Interpenetrated, Collective-Deformation Carbon Nanotube Network

Stimulus-responsive polymer-based actuators are extensively studied, with the challenging goal of achieving comprehensive performance metrics that include large output stress and strain, fast response, and versatile actuation modes. The design and fabrication of nanocomposites offer a promising route to integrate the advantages of both polymers and nanoscale fillers, thus ensuring superior performance. Here, it is started from a three-dimensional (3D) porous sponge to fabricate a mutually interpenetrated nanocomposite, in which the embedded carbon nanotube (CNT) network undergoes collective deformation with the shape memory polymer (SMP) matrix during large-degree stretching and releasing, increases junction density with polymer chains and enhances molecular orientation. These features result in substantial improvement of the overall mechanical properties and during thermally actuated contraction, the bulk SMP/CNT composites exhibit output stresses up to 19.5 ± 0.97 MPa and strains up to 69%, accompanied by a rapid response and high energy density, exceeding the majority of recent reports. Furthermore, electrical actuation is also demonstrated via uniform Joule heating across the self-percolated CNT network. Applications such as low-temperature thermal actuated vascular stent and wound dressing are explored. These findings lay out a universal blueprint for developing robust and highly deformable SMP/CNT nanocomposite actuators with broad potential applications.

Understanding Lignin Dissolution with Urea and the Formation of a Lignin Nano-Aggregate: A Multiscale Approach

This study employs a combined computational and experimental approach to elucidate the mechanisms governing the interaction between lignin and urea, impacting lignin dissolution and subsequent aggregation behavior. Molecular dynamics (MD) simulations reveal how the urea concentration and temperature influence lignin conformation and interactions. Higher urea concentrations and temperatures promote lignin dispersion by disrupting intramolecular interactions and enhancing solvation. Density functional theory (DFT) calculations quantitatively assess the interaction energy between lignin and urea, supporting the findings from MD simulations. Anti-solvent precipitation demonstrates that increasing the urea concentration hinders the self-assembly of lignin nanoclusters. The findings provide valuable insights for optimizing lignin biorefinery processes by tailoring the urea concentration and temperature for efficient extraction and dispersion. Understanding the influence of urea on lignin behavior opens up avenues for designing novel lignin-based materials with tailored properties. This study highlights the potential for the synergetic application of MD simulations and DFT calculations to unravel complex material interactions at the atomic level.

Bone-inspired stress-gaining elastomer enabled by dynamic molecular locking

The limited capacity of typical materials to resist stress loading, which affects their mechanical performance, is one of the most formidable challenges in materials science. Here, we propose a bone-inspired stress-gaining concept of converting typically destructive stress into a favorable factor to substantially enhance the mechanical properties of elastomers. The concept was realized by a molecular design of dynamic poly(oxime-urethanes) network with mesophase domains. During external loading, the mesophase domains in the condensed state were aligned into more ordered domains, and the dynamic oxime-urethane bonds served as the dynamic molecular locks disassociating and reorganizing to facilitate and fix the mesophase domains. Consequently, the tensile modulus and strength were enhanced by 1744 and 49.3 times after four cycles of mechanical training, respectively. This study creates a molecular concept with stress-gaining properties induced by repeated mechanical stress loading and will inspire a series of innovative materials for diverse applications.

Self-healing and shape-memory polymers based on cellulose acetate matrix

The creation of self-healing polymers with superior strength and stretchability from biodegradable materials is attracting increasing attention. In this study, we synthesized new biomass-derived cellulose acetate (CA) derivatives by ring-opening graft polymerization of δ-valerolactone followed by the introduction of ureidopyrimidinone (Upy) groups in the polymer side chains. Due to the semicrystalline aliphatic characteristics of the side chain poly(δ-valerolactone) (PVL) and quadruple hydrogen bonds formed by the Upy groups, the stretchability of the resulting polymers was significantly enhanced. Moreover, the shape memory ability and self-healing property (58.3% of self-healing efficiency) were successfully imparted to the polymer. This study demonstrates the great significance of using biomass sources to create self-healing polymers.

Insight into the fracture energy dissipation mechanism in elastomer composites via sacrificial bonds and fillers

Most tough elastomer composites are reinforced by introducing sacrificial structures and fillers. Understanding the contribution of fillers and sacrificial bonds in elastomer composites to the energy dissipation is critical for the design of high-toughness materials. However, the energy dissipation mechanism in elastomer composites remains elusive. In this study, using a tearing test and time-temperature superposition, we investigate the effect of fillers and sacrificial bonds on the energy dissipation of elastomer composites consisting of poly(lipoic acid)/silver-coated Al fillers. We found that the fillers and sacrificial bonds mutually enhance both the intrinsic fracture energy and the bulk energy dissipation, and moreover the sacrificial bonds play a more important role in enhancing fracture toughness than the fillers. It is unreasonable to rely solely on the loss factor for bulk energy dissipation. The addition of sacrificial bonds results in a chain segment experiencing greater binding force compared to the addition of fillers. This suggests that the chain segment consumes more energy during its movement. By calculating the length of the Kuhn chain segment and the Kuhn number, it is evident that the addition of sacrificial bonds results in a greater binding force for the chain segment than the addition of fillers, and this enhanced binding force increases the energy consumption during the motion of the chain segment.

Design principles in mechanically adaptable biomaterials for repairing annulus fibrosus rupture: A review

Annulus fibrosus (AF) plays a crucial role in the biomechanical loading of intervertebral disc (IVD). AF is difficult to self-heal when the annulus tears develop, because AF has a unique intricate structure and biologic milieu in vivo. Tissue engineering is promising for repairing AF rupture, but construction of suitable mechanical matching devices or scaffolds is still a grand challenge. To deeply know the varied forces involved in the movement of the native annulus is highly beneficial for designing biomimetic scaffolds to recreate the AF function. In this review, we overview six freedom degrees of forces and adhesion strength on AF tissue. Then, we summarize the mechanical modalities to simulate related forces on AF and to assess the characteristics of biomaterials. We finally outline some current advanced techniques to develop mechanically adaptable biomaterials for AF rupture repair.

Recent Advances of Lignin Functionalization for High-Performance and Advanced Functional Rubber Composites

The biomass lignin is the only large-volume renewable feedstock that is composed of aromatics but has been largely underutilized and is sought for valorization as a value-added material. Recent research has highlighted lignin as a promising alternative to traditional petrol-based reinforcements and functional additives for rubber composites. This review summarized the recent advances in the functionalization of lignin for a variety of rubber composites, as well as the compounding techniques for effectively dispersing lignin within the rubber matrix. Significant progress has been achieved in the development of high-performance and advanced functional rubber/lignin composites through carefully designing the structure of lignin-based additives and the optimization of interfacial morphologies. This Review discussed the effect of lignin on composite properties, including mechanical reinforcement, dynamic properties, antiaging performance, and oil resistance, and also the advanced stimuli-responsive performance in detail. A critical analysis for the future development of rubber/lignin composites is presented as concluding remarks.

Thermally trainable dual network hydrogels

Inspired by biological systems, trainable responsive materials have received burgeoning research interests for future adaptive and intelligent material systems. However, the trainable materials to date typically cannot perform active work, and the training allows only one direction of functionality change. Here, we demonstrate thermally trainable hydrogel systems consisting of two thermoresponsive polymers, where the volumetric response of the system upon phase transitions enhances or decreases through a training process above certain threshold temperature. Positive or negative training of the thermally induced deformations can be achieved, depending on the network design. Importantly, softening, stiffening, or toughening of the hydrogel can be achieved by the training process. We demonstrate trainable hydrogel actuators capable of performing increased active work or implementing an initially impossible task. The reported dual network hydrogels provide a new training strategy that can be leveraged for bio-inspired soft systems such as adaptive artificial muscles or soft robotics.

Hierarchical network relaxation of a dynamic cross-linked polyolefin elastomer for advanced reversible shape memory effect

Reversible shape-memory polymers (RSMPs) are highly desired for soft actuators due to the repeatability of deformation. Herein, a polyolefin elastomer vitrimer (POEV) was prepared by constructing a dynamic cross-linked network based on boronic ester bonds. POEV showed varied network relaxation in a wide temperature range due to hierarchical network relaxation, and then the entropy decreased and the relaxation of POEV chains was facilely controlled by temperature. The controllable relaxation of POEV by programming the temperature enabled the actuation domain with a reduction in entropy and the skeleton domain with a relatively high entropy can be built in POEV, greatly affecting the reversible shape memory effects (RSMEs). The topological rearrangement resulted from the activated exchange of dynamic covalent bonds, which enables POEV with good shape reconfigurability, and allows for complicated 3D shapes and shape-shifting on demand. More interestingly, combining the decreasing entropy of POEV chains and fully topological rearrangement tailored by temperature, hybrid aligned carbon nanotubes (CNTs) can be constructed in POEV via a two-stage training. Then, the aligned CNTs can enhance the elasticity and act as a hybrid skeleton for RSMEs, avoiding the negative impact of CNTs on the reversible actuation strain. The hierarchical network relaxation facilitates combining all these unusual properties in one shape memory network synergistically, paving new avenues for realizing smart materials with advanced RSME.

Configurational Entropy Regulation in Polyolefin Elastomer/Paraffin Wax Vitrimers by Thermally Responsive Liquid-Solid Transition for Force Storage

The work output of shape memory polymers during shape shifting is desired for practical application as actuators. Herein, a polyolefin elastomer (POE) and paraffin wax (PW) are co-cross-linked by dynamic boronic ester bonds to enhance the network elasticity and the stress transfer between the two phases, endowing high force storage capacity to the prepared vitrimers. Depending on the phase of PW, one-way force storage is realized by programming at a low temperature (25 °C), owing to which solid PW can promote the locking of POE chains in a low-entropy state, while reversible force storage can be realized by programming at a high temperature (75 °C), owing to which the relaxation of chains facilitated by liquid PW can promote the construction of a stable structure. Based on one-way force storage, a weight-lifting machine with a weight of 20 mg prestrained at 25 °C can lift a 100 g weight, showing a lifting ratio of no less than 5000, with a high work output of 0.98 J/g. A high-temperature alarm can be triggered at varied temperatures (43-56 °C) through controlled force release by adjusting the PW content and programmed prestrains. Based on the reversible force storage, crawling robots and artificial muscles with a work output of 0.025 J/g are demonstrated. The dynamic cross-linking network also confers mold-free self-healing capability to POE/PW vitrimers, and the repair efficiency is enhanced compared with the POE vitrimer due to the improved POE chain motion by liquid PW. The realized one-way and reversible force storage and self-healing by POE/PW vitrimers pave the way for the application of SMPs in the fields of soft robotic actuators.

Tough and stretchable all-κ-carrageenan hydrogel based on the cooperative effects between chain conformation transition and stepwise mechanical training

The traditional κ-carrageenan (κCG)-based hydrogel obtained from hot water can rupture easily under mechanical loading. To address this vulnerability, here we presented a robust all-κCG hydrogel without employing the second synthetic network. By simply regulating the polymer chains from random coil to stiff chain conformation in NaOH/urea solvent system via the freeze-thawing process, the as-prepared hydrogel with homogeneous structure can display an enhanced stretchability from 42.1 to 156 %, while maintaining the similar fracture stress. Moreover, upon the stepwise mechanical training and subsequent incubation in KCl aqueous solution, more helical segments of κCG were aligned and involved into the association domains, thus leading to the increment in both the crystallinity and anisotropy. Consequently, a fast self-strengthening behavior occurred, and a more stretchable (fracture strain up to 396 %), strong (stress ∼ 0.55 MPa) and tough (∼1.52 MJ m^-3) κCG hydrogel was obtained. In comparison to the traditional one, the fracture strain and toughness are increased by 8.5 and 11.5 times, respectively. In addition, this κCG hydrogel can demonstrate good recovery and shape-memory behaviors under medium deformation. Hence, this tough all-κCG hydrogel is expected to be tailored into the biomaterials as the wearable device, artificial tendon, and cartilage in the future.

Lignin-Based Materials for Additive Manufacturing: Chemistry, Processing, Structures, Properties, and Applications

The utilization of lignin, the most abundant aromatic biomass component, is at the forefront of sustainable engineering, energy, and environment research, where its abundance and low-cost features enable widespread application. Constructing lignin into material parts with controlled and desired macro- and microstructures and properties via additive manufacturing has been recognized as a promising technology and paves the way to the practical application of lignin. Considering the rapid development and significant progress recently achieved in this field, a comprehensive and critical review and outlook on three-dimensional (3D) printing of lignin is highly desirable. This article fulfils this demand with an overview on the structure of lignin and presents the state-of-the-art of 3D printing of pristine lignin and lignin-based composites, and highlights the key challenges. It is attempted to deliver better fundamental understanding of the impacts of morphology, microstructure, physical, chemical, and biological modifications, and composition/hybrids on the rheological behavior of lignin/polymer blends, as well as, on the mechanical, physical, and chemical performance of the 3D printed lignin-based materials. The main points toward future developments involve hybrid manufacturing, in situ polymerization, and surface tension or energy driven molecular segregation are also elaborated and discussed to promote the high-value utilization of lignin.

Synthesis of cellulose nanofiber/polysiloxane-polyurea composite materials with self-healing and reprocessing properties

Inspired by the self-recoverability ability of organisms, various self-healing materials have been developed. However, most reinforced fillers are faced with the problem that mechanical strength and self-healing efficiency of materials cannot be improved simultaneously. Here we first prepared new polysiloxane-polyurea (PDMS-PU) and used it as matrix resin to prepare cellulose nanofiber (CNF)/PDMS-PU composite materials with high mechanical properties. CNFs increased the tensile strength of PDMS-PU by 38.87 % and CNF/PDMS-PU composite materials maintained the great bending resistance, transparency and reprocessing properties of PDMS-PU. Moreover, the introduction of CNFs did not reduce the self-healing efficiency of PDMS-PU, and PDMS-PU containing disulfide bonds with CNF content of 1 % (CNF/PDMS-IPDI-S-1 %) with healing efficiency of 95.58 %, and the tensile strength after three recycling processing was still as high as 92.55 % of the original. CNFs reinforced PDMS-PU composite materials are expected to replace PDMS materials in advanced engineering fields that require high strength durability and good formability.

A facile method to fabricate high performance PVA/PAA-AS hydrogel via the synergy of multiple hydrogen bonding and Hofmeister effect

Hydrogels are widely used in biomedical engineering, which often require matched mechanical properties to meet specific demands. Recently, numerous research studies have contributed to tissue engineering hydrogels by soaking strategies to obtain designed properties. Herein, a strategy to fabricate poly(vinyl alcohol)/poly(acrylic acid)-ammonium sulfate (PVA/PAA-AS) hydrogel by successively soaking an aqueous PAA solution and (NH₄)₂SO₄ solution based on the synergy of multiple hydrogen bonding and Hofmeister effect is reported, which exhibits remarkable comprehensive mechanical properties: rigidity (elastic modulus: 0.7-3.6 MPa), strength at break (tensile stress: 3.2-12.0 MPa; strain 320-650%), and toughness (fracture energy: 4.5-30.0 MJ m-3). Besides, PVA/PAA-AS hydrogel with unique spring-like microstructure exhibited super-resilience in 30% strain range by energy-transforming mechanism. Compared with pure PVA hydrogel, PVA/PAA-AS hydrogel has the equal excellent cytocompatibility. Therefore, PVA/PAA-AS hydrogel with high strength, modulus, toughness, super-resilience and excellent biocompatibility has potential applications in the soft tissue engineering field such as muscles, tendons, and ligaments.

Nanoenabled Trainable Systems: From Biointerfaces to Biomimetics

In the dynamic biological system, cells and tissues adapt to diverse environmental conditions and form memories, an essential aspect of training for survival and evolution. An understanding of the biological training principles will inform the design of biomimetic materials whose properties evolve with the environment and offer routes to programmable soft materials, neuromorphic computing, living materials, and biohybrid robotics. In this perspective, we examine the mechanisms by which cells are trained by environmental cues. We outline the artificial platforms that enable biological training and examine the relationship between biological training and biomimetic materials design. We place emphasis on nanoscale material platforms which, given their applicability to chemical, mechanical and electrical stimulation, are critical to bridging natural and synthetic systems.

Protein Crystallization-Mediated Self-Strengthening of High-Performance Printable Conducting Organohydrogels

Conductive polymers have many advanced applications, but there is still an important target in developing a general and straightforward strategy for printable, mechanically stable, and durable organohydrogels with typical conducting polymers of, for example, polypyrrole, polyaniline, or poly(3,4-ethylenedioxythiophene). Here we report a protein crystallization-mediated self-strengthening strategy to fabricate printable conducting organohydrogels with the combination of rational photochemistry design. Such organohydrogels are one-step prepared via rapidly and orthogonally controllable photopolymerizations of pyrroles and gelatin protein in tens of seconds. As-prepared conducting organohydrogels are patterned and printed to complicated structures via shadow-mask lithography and 3D extrusion technology. The mild photocatalytic system gives the transition metal carbide/nitride (MXene) component high stability during the oxidative preparation process and storage. Controlling water evaporation promotes gelatin crystallization in the as-prepared organohydrogels that significantly self-strengthens their mechanical property and stability in a broad temperature range and durability against continuous friction treatment without introducing guest functional materials. Also, these organohydrogels have commercially electromagnetic shielding, thermal conducting properties, and temperature- and light-responsibility. To further demonstrate the merits of this simple strategy and as-prepared organohydrogels, prism arrays, as proofs-of-concept, are printed and applied to make wearable triboelectric nanogenerators. This self-strengthening process and 3D-printability can greatly improve their voltage, charge, and current output performances compared to the undried and flat samples.

High-Sensitivity Flexible Sensor Based on Biomimetic Strain-Stiffening Hydrogel

Recently, flexible wearable and implantable electronic devices have attracted enormous interest in biomedical applications. However, current bioelectronic systems have not solved the problem of mechanical mismatch of tissue-electrode interfaces. Therefore, the biomimetic hydrogel with tissue-like mechanical properties is highly desirable for flexible electronic devices. Herein, we propose a strategy to fabricate a biomimetic hydrogel with strain-stiffening property via regional chain entanglements. The strain-stiffening property of the biomimetic hydrogel is realized by embedding highly swollen poly(acrylate sodium) microgels to act as the microregions of dense entanglement in the soft polyacrylamide matrix. In addition, poly(acrylate sodium) microgels can release Na⁺ ions, endowing hydrogel with electrical signals to serve as strain sensors for detecting different human movements. The resultant sensors own a low Young's modulus (22.61-112.45 kPa), high nominal tensile strength (0.99 MPa), and high sensitivity with a gauge factor up to 6.77 at strain of 300%. Based on its simple manufacture process, well mechanical matching suitability, and high sensitivity, the as-prepared sensor might have great potential for a wide range of large-scale applications such as wearable and implantable electronic devices.

Synthesis and Characterization of a Liquid Crystal-Modified Polydimethylsiloxane Rubber with Mechanical Adaptability Based on Chain Extension in the Process of Crosslinking

The mechanical adaptive material is a kind of functional material that can effectively dissipate energy and suppress the increase of its stress under continuous strain in a large deformation area, which are vital in artificial muscles, connection devices, soft artificial intelligence robots, and other areas. Scientists have been working to broaden the platform of the material's mechanical adaptive platform and improve its mechanical strength by specific structure design. Based on it, we expect to introduce a mechanism of energy dissipation from the molecular chain scale to further improve mechanical adaptability. We developed a liquid crystal-modified polydimethylsiloxane rubber with mechanical adaptability based on chain extension in the process of crosslinking. Results showed that liquid crystal (0.7 mol %)-modified silicone rubber can obviously dissipate energy to achieve mechanical adaptive function, and the energy dissipation ratio of polydimethylsiloxane rubber (MQ), 4-propyl-4'-vinyl-1,1'-bi(cyclohexane)-modified polydimethylsiloxane rubber (3CCV-MQ), and 4-methoxyphenyl-4-(3-butenyloxy) benzoate-modified polydimethylsiloxane rubber (MBB-MQ) gradually decreases from 30 to 24%. Excessive thiol groups of liquid crystal-modified polydimethylsiloxane react with its vinyl group to achieve the chain extension, which significantly improves the mechanical strength from 2.74 to 5.83 MPa and elongation at break from 733 to 1096%. This research offers some new insights into improving the mechanical strength of silicone rubber and is of great significance for the application of the mechanical adaptive material.

A fast-response biomimetic phototropic material built by a coordination-assisted photothermal domino strategy

Fast-response artificial phototropic materials are a promising tool for solar energy utilisation, yet their preparation remains challenging. Herein, we report the so-called photothermal domino strategy for constructing fast-response artificial phototropic materials. In this strategy, photothermal generation, heat conduction and thermal actuation are sequentially optimised by a coordination effect. For the first time, lignin-based organic radicals boosted by this coordination effect are used to significantly enhance photothermal conversion. Interfacial coordination bonds between lignin and an elastomer matrix promote interfacial heat conduction. Light-stimulated thermal actuation is significantly improved by coordination-assisted mechanical training. The prepared biomimetic phototropic material exhibits excellent phototropic ability, with a 2.5 s light-tracking process, showing great application potential for efficient solar energy utilisation. This strategy shows great significance for fabricating high-performance intelligent phototropic materials using widely available, green raw materials.

Mechanically Robust, Self-Repairable, Shape Memory and Recyclable Ionomeric Elastomer Composites with Renewable Lignin via Interfacial Metal-Ligand Interactions

Lignin, the most abundant aromatic polymer in nature, is one of the most promising renewable feedstocks for value-added polymer products. However, it is challenging to prepare high-performance and multifunctional polymer materials with renewable lignin because of its poor compatibility with the elastomer matrix. In fact, lignin often requires solvent fractionation, chemical modification, or prohibitively expensive additives. This work develops a cost-effective strategy to prepare ionomeric elastomer composites based on a commercial carboxyl elastomer and a high content of lignin without purification or chemical modification. The compatibility between the elastomer and lignin is improved by the incorporation of zinc oxide which creates metal-ligand coordination at the interfaces between the carboxyl groups of the elastomer and the oxygen-bearing groups of the lignin. This results in fine dispersion of the lignin in the elastomer matrix, even when its content reaches 50 wt %. The lignin/elastomer composites show excellent mechanical properties, which are attributed to the reinforcing effect of the lignin domains and the presence of abundant sacrificial coordination bonds. Moreover, ionic bonds and ionic aggregates created by the neutralization of the zinc ions with the carboxyl groups of the elastomer behave as physical crosslinks which endow the composites with excellent recyclability; namely, their mechanical properties are retained or even improved after multiple reprocessing cycles. They also show good self-repairability and shape memory. Hence, this work may open up new avenues to utilize lignin as a renewable alternative to petroleum derivatives for designing and fabricating high-performance and multifunctional elastomer materials.

Recent Advances in Microgels: From Biomolecules to Functionality

The emerging applications of hydrogel materials at different length scales, in areas ranging from sustainability to health, have driven the progress in the design and manufacturing of microgels. Microgels can provide miniaturized, monodisperse, and regulatable compartments, which can be spatially separated or interconnected. These microscopic materials provide novel opportunities for generating biomimetic cell culture environments and are thus key to the advances of modern biomedical research. The evolution of the physical and chemical properties has, furthermore, highlighted the potentials of microgels in the context of materials science and bioengineering. This review describes the recent research progress in the fabrication, characterization, and applications of microgels generated from biomolecular building blocks. A key enabling technology allowing the tailoring of the properties of microgels is their synthesis through microfluidic technologies, and this paper highlights recent advances in these areas and their impact on expanding the physicochemical parameter space accessible using microgels. This review finally discusses the emerging roles that microgels play in liquid-liquid phase separation, micromechanics, biosensors, and regenerative medicine.

Interfacial Coordination Interaction Enables Soft Elastomer Composites High Thermal Conductivity and High Toughness

Soft elastomers have attracted wide applications, such as soft electronic devices and soft robotics, due to their ability to undergo large deformation with a small external force. Most elastomers suffer from poor toughness and thermal conductivity, which limits their use. The addition of inorganic fillers can enhance the thermal conductivity and toughness, but it deteriorates the softness (low Young's modulus and high stretchability). Integrating thermal conductivity, toughness, and softness into one elastomer is still a challenge. Here, we report a strategy of interfacial coordination interaction to achieve soft elastomer composites with high thermal conductivity and high toughness. We demonstrate the strategy by using poly(lipoic acid) elastomer and silver-coated aluminum filler as model, where silver-sulfur coordination cross-links are formed at the interface. The resultant elastomer composite shows high streachability (450%), high thermal conductivity (2.35 W m-1 K-1), low modulus (321 kPa), and high toughness (3496 J m-2), which cannot be achieve in existing elastomers. The time domain thermoreflectance technique demonstrates that the silver-sulfur coordination interaction lowers the interfacial thermal resistance, resulting in enhanced thermal conductivity of the elastomer composites. The excellent softness stems from lower bonding energy of the silver-sulfur coordination cross-links compared with covalent chemical cross-links. The high toughness also benefits from the interfacial silver-sulfur coordination interaction that can dissipate more energy upon deformation. We further demonstrate the potential application of the thermally conductive, tough, and soft elastomer composites for thermal management of chip and soft electronic devices.

Biorobotics: An Overview of Recent Innovations in Artificial Muscles

In this overview of recent developments in the field of biorobotics we cover the developments in materials such as the use of polyester fabric being used as artificial skin and the start of whole new ways to actuate artificial muscles as a whole. In this, we discuss all of the relevant innovations from the fields of nano and microtechnology, as well as in the field of soft robotics to summarize what has been over the last 4 years and what could be improved for artificial muscles in the future. The goal of this paper will be to gain a better understanding of where the current field of biorobotics is at and what its current trends in manufacturing and its techniques are within the last several years.

Hofmeister Effect Mediated Strong PHEMA-Gelatin Hydrogel Actuator

Hydrogels have become popular in biomedical applications, but their applications in muscle and tendon-like bioactuators have been hindered by low toughness and elastic modulus. Recently, a significant toughness enhancement of a single hydrogel network has been successfully achieved by the Hofmeister effect. However, little has been conducted for the Hofmeister effect on the hybrid hydrogels, although they have a special network structure consisting of two types of polymer components. Herein we fabricated hybrid poly(2-hydroxyethyl methacrylate) (PHEMA)-gelatin hydrogels with high mechanical performance and stimuli response. An ideal bicontinuous phase separation structure of the PHEMA (rigid) and gelatin (ductile) was observed with embedded microdisc-like gelatin in the three-dimensional polymeric network of PHEMA. A significant enhancement of mechanical performance by the Hofmeister effect was attributed to the salting-out-induced stronger and closer interphase interaction between PHEMA and gelatin. A superior comprehensive mechanical performance with fracture elongation over 650%, tensile strength of 5.2 MPa, toughness of 13.5 MJ/m3, and modulus of 45.6 MPa was achieved with the salting-out effect. More specifically, the synergy of phase separation and Hofmeister effect enable the hydrogel to contract with an enhanced modulus in high-concentration salt solutions, while the same hydrogel swells and relaxes in dilute solutions, exhibiting an ionic stimulus response and excellent shape-memory properties like those of most artificial muscle. This is manifested in highly stretched, twisted, and knotted hydrogel strips that can rapidly recover their original shape in a dilute salt solution. The high strength and modulus, ionic stimuli response, and shape memory property make the hybrid hydrogel a promising material for bioactuators in various biomedical applications.

Biobased Composites with High Lignin Content and Excellent Mechanical Properties toward the Ingenious Photoresponsive Actuator

The fabrication of biobased smart materials from renewable biomasses is of great importance for sustainable development. Although lignin possesses photothermal conversion potential, the development of lignin-based actuators with large contraction and fast photoresponse has various hurdles. Herein, simply by blending with castor oil-derived polyamide elastomers, a lignin-based photoresponsive actuator can be obtained, which accomplishes up to 18% light-driven contraction under loading within 3 s. The crystals in polymer matrix serve as switch segments, firmly locking the stress-induced strain energy, which is swiftly released due to photothermal processes and induced a huge contraction. The composite, LP₄-50, can contract and induce dynamic bending in multiple directions when irradiated locally with a near-infrared 808 nm laser. Furthermore, at standard 1 sun irradiation (100 mW/cm²), LP₄-50 was successfully employed to power a thermoelectric generator. This strategy establishes the groundwork for further research into the photothermal characteristics of lignin and encourages new applications in stimulus-responsive actuators.

A Shift from Efficiency to Adaptability: Recent Progress in Biomimetic Interactive Soft Robotics in Wet Environments

Research field of soft robotics develops exponentially since it opens up many imaginations, such as human-interactive robot, wearable robots, and transformable robots in unpredictable environments. Wet environments such as sea and in vivo represent dynamic and unstructured environments that adaptive soft robots can reach their potentials. Recent progresses in soft hybridized robotics performing tasks underwater herald a diversity of interactive soft robotics in wet environments. Here, the development of soft robots in wet environments is reviewed. The authors recapitulate biomimetic inspirations, recent advances in soft matter materials, representative fabrication techniques, system integration, and exemplary functions for underwater soft robots. The authors consider the key challenges the field faces in engineering material, software, and hardware that can bring highly intelligent soft robots into real world.

High-energy-density shape memory materials with ultrahigh strain for reconfigurable artificial muscles

Programmable and reconfigurable artificial muscles are highly promising and desirable for applications in various fields, including soft robotics, flexible devices, and biomedical devices. However, the combination of considerable strain and high energy density remains a dilemma to overcome. As stimulus-responsive polymers, shape memory polymers (SMPs) with enhanced mechanical properties and programmability have the potential to solve this problem. However, the purest shape memory polymer lacks reconfigurability, and it is challenging to achieve precise control due to its intrinsic thermal stimulation. Herein, we report a novel reconfigurable multi-block shape memory polymer and its composite with a granular micro-phase separated structure. The polymer demonstrated outstanding artificial muscle performance through the energy storing and releasing method, whose energy density and strain reached 717.8 kJ m^-3 and 947%, respectively (17.9 and 23.7 times higher than that of mammalian skeletal muscles). Moreover, by incorporating Fe₃O₄ nanoparticle (Fe₃O₄ NP) photothermal transducers, the composite could achieve contraction, reversible muscle-like bending and complex programmable and reconfigurable deformation under near-infrared laser irradiation, including deformation into a specific letter shape, precise origami and bending with loads. Therefore, the prepared materials have excellent prospects as artificial muscles and high-power actuators for broad applications.