Large-Scale Spinning Approach to Engineering Knittable Hydrogel Fiber for Soft Robots

Electrothermally powered synergistic fluorescence-colour/3D-shape changeable polymer gel systems for rewritable and programmable information display

Intelligent luminescent materials for rewritable and programmable information display have long been expected to be used to address potential environmental concerns stemming from the extensive use of disposable displays. However, most reported luminescence-colour changeable examples are chemically responsive and not well programmed to sequentially deliver different information within a single system. Additionally, they may suffer from residual chemical accumulation caused by the repeated addition of chemical inks and usually have poor rewritability. Herein, we draw inspiration from the bioelectricity-triggered information display mechanism of chameleon skin to report a robust electrothermally powered polymer gel actuator consisting of one soft conductive graphene/PDMS film and one humidity-responsive fluorescence-colour changeable CD-functionalized polymer (PAHCDs) gel layer. Owing to the good electrocaloric effect of the bottom graphene film and excellent hygroscopicity of the top PAHCDs gel layer, the as-designed actuator could be facilely controlled to exhibit reversible and synergistic 3D-shape/fluorescence-colour changeable behaviours in response to alternating electricity and humidity stimuli. On this basis, robust rewritable information display systems are fabricated, which enable not only on-demand delivery of written information, but also facile rewriting of lots of different information by the synergization of electroheat/humidity-triggered local 3D-deformation and fluorescence-colour changes. This work opens new avenues of research into rewritable information display and potentially inspires the future development of intelligent luminescent materials.

Recent Development of Fibrous Hydrogels: Properties, Applications and Perspectives

Fibrous hydrogels (FGs), characterized by a 3D network structure made from prefabricated fibers, fibrils and polymeric materials, have emerged as significant materials in numerous fields. However, the challenge of balancing mechanical properties and functions hinders their further development. This article reviews the main advantages of FGs, including enhanced mechanical properties, high conductivity, high antimicrobial and anti-inflammatory properties, stimulus responsiveness, and an extracellular matrix (ECM)-like structure. It also discusses the influence of assembly methods, such as fiber cross-linking, interfacial treatments of fibers with hydrogel matrices, and supramolecular assembly, on the diverse functionalities of FGs. Furthermore, the mechanisms for improving the performance of the above five aspects are discussed, such as creating ion carrier channels for conductivity, in situ gelation of drugs to enhance antibacterial and anti-inflammatory properties, and entanglement and hydrophobic interactions between fibers, resulting in ECM-like structured FGs. In addition, this review addresses the application of FGs in sensors, dressings, and tissue scaffolds based on the synergistic effects of optimizing the performance. Finally, challenges and future applications of FGs are discussed, providing a theoretical foundation and new insights for the design and application of cutting-edge FGs.

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.

High-strength, anti-fatigue, cellulose nanofiber reinforced polyvinyl alcohol based ionic conductive hydrogels for flexible strain/pressure sensors and triboelectric nanogenerators

Ionic conductive hydrogels (ICHs) have attracted great attention because of their excellent biocompatibility and structural similarity with biological tissues. However, it is still a huge challenge to prepare a high strength, conductivity and durability hydrogel-based flexible sensor with dual network structure through a simple and environmentally friendly method. In this work, a simple one-pot cycle freezing thawing method was proposed to prepare ICHs by dissolving polyvinyl alcohol (PVA) and ferric chloride (FeCl3) in cellulose nanofiber (CNF) aqueous dispersion. A dual cross-linked network was established in hydrogel through the hydrogen bonds and coordination bonds among PVA, CNF, and FeCl3. This structure endows the as-prepared hydrogel with high sensitivity (pressure sensitivity coefficient (S) = 5.326 in the pressure range of 0-5 kPa), wide response range (4511 kPa), excellent durability (over 3000 cycles), short response time (83 ms) and recovery time (117 ms), which can accurately detect various human activities in real time. Furthermore, the triboelectric nano-generator (TENG) made from PVA@CNF-FeCl3 hydrogel can not only supply power for commercial capacitors and LED lamps, but also be used as a self-powered sensor to detect human motion. This work provides a new approach for the development of the next generation of flexible wearable electronic devices.

Deformable Joule heating electrode based on hybrid layers of silver nanowires and carbon nanotubes and its application in a refreshable multi-cell Braille display

Stretchable electrodes are an essential component in soft actuator systems. In particular, Joule heating electrodes (JHEs) are required for thermal actuation systems. A highly stretchable, patternable, and low-voltage operating JHE based on hybrid layers of silver nanowires (AgNWs) and carbon nanotubes (CNTs) is reported. The conductive layers were applied on a locally pre-strained bistable electroactive polymer (BSEP) membrane to form a wrinkled conductive surface with a low resistance of 300 Ω/sq, and subsequently patterned to a serpentine trace by laser engraving. The resistance of the resulting electrode remains nearly unchanged up to ~80-90% area strain. By applying a voltage of 7 - 9 V to the electrode, the temperature of the BSEP membrane increased to more than 60 °C, well above the polymer's phase transition temperature of 46 °C, thereby lowering its modulus by a factor of 103. An electronic Braille device based on the JHEs on a BSEP membrane was assembled with a diaphragm chamber. The electrode was patterned into 3 × 2 individually addressable pixels according to the standard U.S. Braille cell format. Through Joule heating of the pixels and local expansion of the BSEP membrane using a small pneumatic pressure, the pixels deformed out of the plane by over 0.5 mm to display specific Braille letters. The Braille content can be refreshed for 20,000 cycles at the same operating voltage.

Cellulose nanocrystals-based optical organohydrogel fiber with customizable iridescent colors for strain and humidity response

Stimuli-responsive optical hydrogels are widely used in various fields including environmental sensing, optical encryption, and intelligent display manufacturing. However, these hydrogels are susceptible to water losses when exposed to air, leading to structural damage, significantly shortened service lives, and compromised durability. This study presents mechanically robust, environmentally stable, and multi-stimuli responsive optical organohydrogel fibers with customizable iridescent colors. These fibers are fabricated by incorporating tunicate cellulose nanocrystals, alginate, and acrylamide in a glycerol-water binary system. The synthesized fibers exhibit high strength (1.38 MPa), moisture retention capabilities, and elastic properties. Furthermore, a sensor based on these fibers demonstrates high- and low-temperature resistance along with stimuli-responsive characteristics, effectively detecting changes in environmental humidity and strains. Moreover, the fiber sensor demonstrates continuous, repeatable, and quantitatively predictable moisture discoloration responses across a humidity range of 11 % and 98 %. During strain sensing, the optical-organohydrogel-based sensor demonstrates a large working strain (50 %) and excellent cycling stability, underscoring its potential for effectively monitoring a wide range of intricate human motions. Overall, the synthesized fibers and their simple fabrication method can elicit new avenues for numerous related applications including the large-scale implementation of advanced wearable technology.

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.

Intelligent perceptual textiles based on ionic-conductive and strong silk fibers

Endowing textiles with perceptual function, similar to human skin, is crucial for the development of next-generation smart wearables. To date, the creation of perceptual textiles capable of sensing potential dangers and accurately pinpointing finger touch remains elusive. In this study, we present the design and fabrication of intelligent perceptual textiles capable of electrically responding to external dangers and precisely detecting human touch, based on conductive silk fibroin-based ionic hydrogel (SIH) fibers. These fibers possess excellent fracture strength (55 MPa), extensibility (530%), stable and good conductivity (0.45 S·m-1) due to oriented structures and ionic incorporation. We fabricated SIH fiber-based protective textiles that can respond to fire, water, and sharp objects, protecting robots from potential injuries. Additionally, we designed perceptual textiles that can specifically pinpoint finger touch, serving as convenient human-machine interfaces. Our work sheds new light on the design of next-generation smart wearables and the reshaping of human-machine interfaces.

Anisotropy in magnetic materials for sensors and actuators in soft robotic systems

The field of soft intelligent robots has rapidly developed, revealing extensive potential of these robots for real-world applications. By mimicking the dexterities of organisms, robots can handle delicate objects, access remote areas, and provide valuable feedback on their interactions with different environments. For autonomous manipulation of soft robots, which exhibit nonlinear behaviors and infinite degrees of freedom in transformation, innovative control systems integrating flexible and highly compliant sensors should be developed. Accordingly, sensor-actuator feedback systems are a key strategy for precisely controlling robotic motions. The introduction of material magnetism into soft robotics offers significant advantages in the remote manipulation of robotic operations, including touch or touchless detection of dynamically changing shapes and positions resulting from the actuations of robots. Notably, the anisotropies in the magnetic nanomaterials facilitate the perception and response with highly selective, directional, and efficient ways used for both sensors and actuators. Accordingly, this review provides a comprehensive understanding of the origins of magnetic anisotropy from both intrinsic and extrinsic factors and summarizes diverse magnetic materials with enhanced anisotropy. Recent developments in the design of flexible sensors and soft actuators based on the principle of magnetic anisotropy are outlined, specifically focusing on their applicabilities in soft robotic systems. Finally, this review addresses current challenges in the integration of sensors and actuators into soft robots and offers promising solutions that will enable the advancement of intelligent soft robots capable of efficiently executing complex tasks relevant to our daily lives.

Fully Reversible and Super-Fast Photo-Induced Morphological Transformation of Nanofilms for High-Performance UV Detection and Light-Driven Actuators

Flexible and highly ultraviolet (UV) sensitive materials garner considerable attention in wearable devices, adaptive sensors, and light-driven actuators. Herein, a type of nanofilms with unprecedented fully reversible UV responsiveness are successfully constructed. Building upon this discovery, a new system for ultra-fast, sensitive, and reliable UV detection is developed. The system operates by monitoring the displacement of photoinduced macroscopic motions of the nanofilms based composite membranes. The system exhibits exceptional responsiveness to UV light at 375 nm, achieving remarkable response and recovery times of < 0.3 s. Furthermore, it boasts a wide detection range from 2.85 µW cm-2 to 8.30 mW cm-2, along with robust durability. Qualitative UV sensing is accomplished by observing the shape changes of the composite membranes. Moreover, the composite membrane can serve as sunlight-responsive actuators for artificial flowers and smart switches in practical scenarios. The photo-induced motion is ascribed to the cis-trans isomerization of the acylhydrazone bonds, and the rapid and fully reversible shape transformation is supposed to be a synergistic result of the instability of the cis-isomers acylhydrazone bonds and the rebounding property of the networked nanofilms. These findings present a novel strategy for both quantitative and qualitative UV detection.

Porous Conductive Textiles for Wearable Electronics

Over the years, researchers have made significant strides in the development of novel flexible/stretchable and conductive materials, enabling the creation of cutting-edge electronic devices for wearable applications. Among these, porous conductive textiles (PCTs) have emerged as an ideal material platform for wearable electronics, owing to their light weight, flexibility, permeability, and wearing comfort. This Review aims to present a comprehensive overview of the progress and state of the art of utilizing PCTs for the design and fabrication of a wide variety of wearable electronic devices and their integrated wearable systems. To begin with, we elucidate how PCTs revolutionize the form factors of wearable electronics. We then discuss the preparation strategies of PCTs, in terms of the raw materials, fabrication processes, and key properties. Afterward, we provide detailed illustrations of how PCTs are used as basic building blocks to design and fabricate a wide variety of intrinsically flexible or stretchable devices, including sensors, actuators, therapeutic devices, energy-harvesting and storage devices, and displays. We further describe the techniques and strategies for wearable electronic systems either by hybridizing conventional off-the-shelf rigid electronic components with PCTs or by integrating multiple fibrous devices made of PCTs. Subsequently, we highlight some important wearable application scenarios in healthcare, sports and training, converging technologies, and professional specialists. At the end of the Review, we discuss the challenges and perspectives on future research directions and give overall conclusions. As the demand for more personalized and interconnected devices continues to grow, PCT-based wearables hold immense potential to redefine the landscape of wearable technology and reshape the way we live, work, and play.

Self-encapsulated ionic fibers based on stress-induced adaptive phase transition for non-contact depth-of-field camouflage sensing

Ionically conductive fibers have promising applications; however, complex processing techniques and poor stability limit their practicality. To overcome these challenges, we proposed a stress-induced adaptive phase transition strategy to conveniently fabricate self-encapsulated hydrogel-based ionically conductive fibers (se-HICFs). se-HICFs can be produced simply by directly stretching ionic hydrogels with ultra-stretchable networks (us-IHs) or by dip-drawing from molten us-IHs. During this process, stress facilitated the directional migration and evaporation of water molecules in us-IHs, causing a phase transition in the surface layer of ionic fibers to achieve self-encapsulation. The resulting sheath-core structure of se-HICFs enhanced mechanical strength and stability while endowing se-HICFs with powerful non-contact electrostatic induction capabilities. Mimicking nature, se-HICFs were woven into spider web structures and camouflaged in wild environments to achieve high spatiotemporal resolution 3D depth-of-field sensing for different moving media. This work opens up a convenient route to fabricate stable functionalized ionic fibers.

Soft Fiber/Textile Actuators: From Design Strategies to Diverse Applications

Fiber/textile-based actuators have garnered considerable attention due to their distinctive attributes, encompassing higher degrees of freedom, intriguing deformations, and enhanced adaptability to complex structures. Recent studies highlight the development of advanced fibers and textiles, expanding the application scope of fiber/textile-based actuators across diverse emerging fields. Unlike sheet-like soft actuators, fibers/textiles with intricate structures exhibit versatile movements, such as contraction, coiling, bending, and folding, achieved through adjustable strain and stroke. In this review article, we provide a timely and comprehensive overview of fiber/textile actuators, including structures, fabrication methods, actuation principles, and applications. After discussing the hierarchical structure and deformation of the fiber/textile actuator, we discuss various spinning strategies, detailing the merits and drawbacks of each. Next, we present the actuation principles of fiber/fabric actuators, along with common external stimuli. In addition, we provide a summary of the emerging applications of fiber/textile actuators. Concluding with an assessment of existing challenges and future opportunities, this review aims to provide a valuable perspective on the enticing realm of fiber/textile-based actuators.

Magnetoactive Soft Materials with Programmable Magnetic Domains for Multifunctional Actuators

Despite considerable progress having been made in the research of soft actuators, there remains a grand challenge in creating a facile manufacturing process that offers both extensive programmability and exceptional actuation capabilities. Taking inspiration from uncomplicated small organisms, this work aims to develop soft actuators that can be mobilized through straightforward design and control, similar to caterpillars or inchworms. They execute intricate actions and functions to meet survival needs in the most efficient manner possible. Here, a novel soft actuator with uniformly dispersed ferromagnetic microparticles but programmatic magnetic profile distribution is proposed by a convenient magnetization process. Benefiting from its high magnetic sensitivity and good matrix flexibility, the actuator can simultaneously achieve reversible, remote, and fast programmable shape transformation and controllable movement even in a magnetic field as low as 14 Gs. Complemented by intrinsic material properties and structural configuration, actuation employing spatial magnetization profiles can facilitate multiple modes of locomotion when subjected to magnetic fields, allowing for an efficient manipulation task of both solid and liquid media. More importantly, a finite element model is developed to assist in the design of the interaction between the alternating magnetic field and the magnetic torques. This advanced soft actuator would strongly push forward major breakthroughs in key applications such as intelligent sensors, disaster rescue, and wearable devices.

Conductive, water-retaining and knittable hydrogel fiber from xanthan gum and aniline tetramer modified-polysaccharide for strain and pressure sensors

Herein, we explored strategies for defoaming and controllable adjustment of spinnable and mechanical properties of polyanion polysaccharide-based hydrogels to fabricate conductive, water-retaining, and knittable hydrogel fibers for next-generation flexible electronics. Xanthan gum (XG) and aniline tetramer modified-polysaccharide (TMAT38) were crosslinked with sodium trimetaphosphate (STMP) and subsequently by Fe3+/Fe2+ ions coordination to prepare conductive and spinnable hydrogels. Polypropylene glycol was introduced as chemical antifoam, and solvent displacement method was adopted to improve mechanical and water-retaining properties. The glycerol-immersed XG5-TMAT38-STMP-Fe3+/CA-PPG hydrogel exhibited conductivity of 3.55×10-3-27.30×10-3 S/cm, storage modulus at linear viscoelastic region of 573 Pa-1717 Pa and self-healing percentage of 100 %-108 %. The 2 h glycerol-immersed hydrogel fibers with good flexibility, moisture retention and freezing tolerance were ready to bend and knit into fabrics. The hydrogel fiber braid possessed better conductivity, reliability and durability than the single hydrogel fiber as strain sensors. The hydrogel fiber fabric perceived tiny vibration triggered by swallowing, speaking and writing with good sensitivity and reproducibility. Furthermore, a three-component model was developed to evaluate response sensitivity and recoverability of the hydrogel fiber fabric-based pressure sensors, which facilitated understanding transient response of polymer-based hydrogel strain and pressure sensors.

Continuous Spinning of High-Tough Hydrogel Fibers for Flexible Electronics by Using Regional Heterogeneous Polymerization

Hydrogel fibers have attracted substantial interest for application in flexible electronics due to their ionic conductivity, high specific surface area, and ease of constructing multidimensional structures. However, universal continuous spinning methods for hydrogel fibers are yet lacking. Based on the hydrophobic mold induced regional heterogeneous polymerization, a universal self-lubricating spinning (SLS) strategy for the continuous fabrication of hydrogel fibers from monomers is developed. The universality of the SLS strategy is demonstrated by the successful spinning of 10 vinyl monomer-based hydrogel fibers. Benefiting from the universality of the SLS strategy, the SLS strategy can be combined with pre-gel design and post-treatment toughening to prepare highly entangled polyacrylamide (PAM) and ionic crosslinked poly(acrylamide-co-acrylic acid)/Fe3+ (W-PAMAA/Fe3+ ) hydrogel fibers, respectively. In particular, the W-PAMAA/Fe3+ hydrogel fiber exhibited excellent mechanical properties (tensile stress > 4 MPa, tensile strain > 400%) even after 120 days of swelling in the pH of 3-9. Furthermore, owing to the excellent multi-faceted performance and one-dimensionality of W-PAMAA/Fe3+ hydrogel fibers, flexible sensors with different dimensions and functions can be constructed bottom-up, including the one-dimensional (1D) strain sensor, two-dimensional (2D) direction sensor, three-dimensional (3D) pressure sensor, and underwater communication sensor to present the great potential of hydrogel fibers in flexible electronics.

Nanoarchitectonics composite hydrogels with high toughness, mechanical strength, and self-healing capability for electrical actuators with programmable shape memory properties

Hydrogel materials show promise in various fields, including flexible electronic devices, biological tissue engineering and wound dressing. Nevertheless, the inadequate mechanical properties, recovery performance, and self-healing speed still constrain the development of intelligent hydrogel materials. To tackle these challenges, we designed a composite hydrogel with high mechanical strength, rapid self-recovery and efficient self-healing ability based on multiple synergistic effects. With the synergistic effect of hydrogen bonds, metal coordination bonds and electrostatic interaction, the synthesized hydrogel could reach a maximum tensile strength of 6.2 MPa and a toughness of 50 MJ m-3. The interaction between the weak polyelectrolyte polyethyleneimine and polyacrylic acid aided in improving the elasticity of the hydrogel, thereby endowing it with prompt self-recovery attributes. The multiple reversible effects also endowed the hydrogel with excellent self-healing ability, and the fractured hydrogel could achieve 95% self-healing within 4 h at room temperature. By the addition of glycerol, the hydrogel could also cope with a variety of extreme environments in terms of moisture retention (12 h, maintaining 80% of its water content) and freeze protection (-36.8 °C) properties. In addition, the composite hydrogels applied in the field of shape memory possessed programmable and reversible shape transformation properties. The polymer chains were entangled at high temperatures to achieve shape fixation, and shape memory was eliminated at low temperatures, which allowed the hydrogels to be reprogrammed and achieve multiple shape transitions. In addition, we also assemble composite hydrogels as actuators and robotic arms for intelligent applications.

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.

Recent Advances in Sources of Bio-Inspiration and Materials for Robotics and Actuators

Bionic robotics and actuators have made dramatic advancements in structural design, material preparation, and application owing to the richness of nature and innovative material design. Appropriate and ingenious sources of bio-inspiration can stimulate a large number of different bionic systems. After millennia of survival and evolutionary exploration, the mere existence of life confirms that nature is constantly moving in an evolutionary direction of optimization and improvement. To this end, bio-inspired robots and actuators can be constructed for the completion of a variety of artificial design instructions and requirements. In this article, the advances in bio-inspired materials for robotics and actuators with the sources of bio-inspiration are reviewed. The specific sources of inspiration in bionic systems and corresponding bio-inspired applications are summarized first. Then the basic functions of materials in bio-inspired robots and actuators is discussed. Moreover, a principle of matching biomaterials is creatively suggested. Furthermore, the implementation of biological information extraction is discussed, and the preparation methods of bionic materials are reclassified. Finally, the challenges and potential opportunities involved in finding sources of bio-inspiration and materials for robotics and actuators in the future is discussed.

Hydrogel fibers for wearable sensors and soft actuators

Owing to superior softness, wetness, responsiveness, and biocompatibility, bulk hydrogels are being intensively investigated for versatile functions in devices and machines including sensors, actuators, optics, and coatings. The one-dimensional (1D) hydrogel fibers possess the metrics from both the hydrogel materials and structural topology, endowing them with extraordinary mechanical, sensing, breathable and weavable properties. As no comprehensive review has been reported for this nascent field, this article aims to provide an overview of hydrogel fibers for soft electronics and actuators. We first introduce the basic properties and measurement methods of hydrogel fibers, including mechanical, electrical, adhesive, and biocompatible properties. Then, typical manufacturing methods for 1D hydrogel fibers and fibrous films are discussed. Next, the recent progress of wearable sensors (e.g., strain, temperature, pH, and humidity) and actuators made from hydrogel fibers is discussed. We conclude with future perspectives on next-generation hydrogel fibers and the remaining challenges. The development of hydrogel fibers will not only provide an unparalleled one-dimensional characteristic, but also translate fundamental understanding of hydrogels into new application boundaries.

Ambient-conditions spinning of functional soft fibers via engineering molecular chain networks and phase separation

Producing functional soft fibers via existing spinning methods is environmentally and economically costly due to the complexity of spinning equipment, involvement of copious solvents, intensive consumption of energy, and multi-step pre-/post-spinning treatments. We report a nonsolvent vapor-induced phase separation spinning approach under ambient conditions, which resembles the native spider silk fibrillation. It is enabled by the optimal rheological properties of dopes via engineering silver-coordinated molecular chain interactions and autonomous phase transition due to the nonsolvent vapor-induced phase separation effect. Fiber fibrillation under ambient conditions using a polyacrylonitrile-silver ion dope is demonstrated, along with detailed elucidations on tuning dope spinnability through rheological analysis. The obtained fibers are mechanically soft, stretchable, and electrically conductive, benefiting from elastic molecular chain networks via silver-based coordination complexes and in-situ reduced silver nanoparticles. Particularly, these fibers can be configured as wearable electronics for self-sensing and self-powering applications. Our ambient-conditions spinning approach provides a platform to create functional soft fibers with unified mechanical and electrical properties at a two-to-three order of magnitude less energy cost under ambient conditions.

Scalable functionalized liquid crystal elastomer fiber soft actuators with multi-stimulus responses and photoelectric conversion

Liquid crystal elastomer (LCE) fibers exhibit large deformation and reversibility, making them an ideal candidate for soft actuators. It is still challenging to develop a scalable strategy and endow fiber actuators with photoelectric functions to achieve tailorable photo-electro-thermal responsiveness and rapid large actuation deformation. Herein, we fabricated a multiresponsive actuator that consists of LCE long fibers obtained by continuous dry spinning and further coated it with polydopamine (PDA)-modified MXene ink. The designed PDA@MXene-integrated LCE fiber is used for shape-deformable and multi-trigger actuators that can be photo- and electro-thermally actuated. The proposed LCE fiber actuator combines an excellent photothermal and long-term electrically conductive PDA@MXene and a shape-morphing LCE fiber, enabling their robust mechanical flexibility, multiple fast responses (∼0.4 s), and stable and large actuation deformation (∼60%). As a proof-of-concept, we present near-infrared light-driven artificial muscle that can lift 1000 times the weight and an intelligent circuit switch with stable controllability and fast responsiveness (∼0.1 s). Importantly, an adaptive smart window system that integrates light-driven energy harvesting/conversion functions is ingeniously constructed by the integration of a propellable curtain woven by the designed fiber and solar cells. This work can provide insights into the development of advanced intelligent materials toward soft robotics, sustainable energy savings and beyond.

Environmentally adaptive polysaccharide-based hydrogels and their applications in extreme conditions: A review

Polysaccharide hydrogels are one of the most promising hydrogel materials due to their inherent characteristics, including biocompatibility, biodegradability, renewability, and easy modification, and their structure and functional designs have been widely researched to adapt to different application scenarios as well as to broaden their application fields. As typical wet-soft materials, the high water content and water-absorbing ability of polysaccharide-based hydrogels (PHs) are conducive to their wide biomedical applications, such as wound healing, tissue repair, and drug delivery. In addition, along with technological progress, PHs have shown potential application prospects in some high-tech fields, including human-computer interaction, intelligent driving, smart dressing, flexible sensors, etc. However, in practical applications, due to the poor ability of PHs to resist freezing below zero, dehydration at high temperature, and acid-base/swelling-induced deformation in a solution environment, they are prone to lose their wet-soft peculiarities, including structural integrity, injectability, flexibility, transparency, conductivity and other inherent characteristics, which greatly limit their high-tech applications. Hence, reducing their freezing point, enhancing their high-temperature dehydration resistance, and improving their extreme solution tolerance are powerful approaches to endow PHs with multienvironmental adaptability, broadening their application areas. This report systematically reviews the study advances of environmentally adaptive polysaccharide-based hydrogels (EAPHs), comprising anti-icing hydrogels, high temperature/dehydration resistant hydrogels, and acid/base/swelling deformation resistant hydrogels in recent years. First, the construction methods of EAPHs are presented, and the mechanisms and properties of freeze-resistant, high temperature/dehydration-resistant, and acid/base/swelling deformation-resistant adaptations are simply demonstrated. Meanwhile, the features of different strategies to prepare EAPHs as well as the strategies of simultaneously attaining multienvironmental adaptability are reviewed. Then, the applications of extreme EAPHs are summarized, and some meaningful works are well introduced. Finally, the issues and future outlooks of PH environment adaptation research are elucidated.

Chinese Tofu-Inspired Biomimetic Conductive and Transparent Fibers for Biomedical Applications

Conductive fibers are vital for next-generation wearable and implantable electronics. However, the mismatch of mechanical, electrical, and biological properties between existing conductive fibers and human tissues significantly retards their further development. Here, the concept of neuro-like fibers to meet these aforementioned requirements is proposed. A new wet spinning process is established to continuously produce pure gelatin hydrogel fibers. The key is the controllable and rapid gelation of spinning solutions based on the salting-out effect, which is inspired by the Chinese food tofu. The resultant fibers exhibit neuro-like features of soft-while-strong mechanical properties, high ionic conductivity, and superior biological properties including biodegradability, biocompatibility, and edibility, which are crucial for implanted applications but seldom reported. Furthermore, all-weather suitable neuro-like fibers with excellent anti-freezing and water retention properties are developed by introducing glycerol for wearable applications. The optical fiber, transient electronics, and electronic data glove made of neuro-like fibers profoundly demonstrate their potential in biomedical applications.

Hydrogel-Based Flexible Electronics

Flexible electronics is an emerging field of research involving multiple disciplines, which include but not limited to physics, chemistry, materials science, electronic engineering, and biology. However, the broad applications of flexible electronics are still restricted due to several limitations, including high Young's modulus, poor biocompatibility, and poor responsiveness. Innovative materials aiming for overcoming these drawbacks and boost its practical application is highly desirable. Hydrogel is a class of 3D crosslinked hydrated polymer networks, and its exceptional material properties render it as a promising candidate for the next generation of flexible electronics. Here, the latest methods of synthesizing advanced functional hydrogels and the state-of-art applications of hydrogel-based flexible electronics in various fields are reviewed. More importantly, the correlation between properties of the hydrogel and device performance is discussed here, to have better understanding of the development of flexible electronics by using environmentally responsive hydrogels. Last, perspectives on the current challenges and future directions in the development of hydrogel-based multifunctional flexible electronics are provided.

Aqueous spinning of robust, self-healable, and crack-resistant hydrogel microfibers enabled by hydrogen bond nanoconfinement

Robust damage-tolerant hydrogel fibers with high strength, crack resistance, and self-healing properties are indispensable for their long-term uses in soft machines and robots as load-bearing and actuating elements. However, current hydrogel fibers with inherent homogeneous structure are generally vulnerable to defects and cracks and thus local mechanical failure readily occurs across fiber normal. Here, inspired by spider spinning, we introduce a facile, energy-efficient aqueous pultrusion spinning process to continuously produce stiff yet extensible hydrogel microfibers at ambient conditions. The resulting microfibers are not only crack-insensitive but also rapidly heal the cracks in 30 s by moisture, owing to their structural nanoconfinement with hydrogen bond clusters embedded in an ionically complexed hygroscopic matrix. Moreover, the nanoconfined structure is highly energy-dissipating, moisture-sensitive but stable in water, leading to excellent damping and supercontraction properties. This work creates opportunities for the sustainable spinning of robust hydrogel-based fibrous materials towards diverse intelligent applications.

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.

Stretchable Semi-Interpenetrating Carboxymethyl Guar Gum-Based Composite Hydrogel for Moisture-Proof Wearable Strain Sensor

Wearable strain sensors of conductive hydrogels have very broad application prospects in electronic skins and human-machine interfaces. However, conductive hydrogels suffer from unstable signal transmission due to environmental humidity and inherent shortcomings of their materials. Herein, we introduce a novel moisture-proof conductive hydrogel with high toughness (2.89 MJ m-3), mechanical strength (1.00 MPa), and high moisture-proof sensing performance by using dopamine-functionalized gold nanoparticles as conductive fillers into carboxymethyl guar gum and acrylamide. Moreover, the hydrogel can realize real-time monitoring of major and subtle human movements with good sensitivity and repeatability. In addition, the hydrogel-assembled strain sensor exhibits stable sensing signals after being left for 1 h, and the relative resistance change rate under different strains (25-300%) shows no obvious noise signal up to 99% relative humidity. Notably, the wearable strain sensing is suitable for wearable sensor devices with high relative humidity.

Stretchable One-Dimensional Conductors for Wearable Applications

Continuous, one-dimensional (1D) stretchable conductors have attracted significant attention for the development of wearables and soft-matter electronics. Through the use of advanced spinning, printing, and textile technologies, 1D stretchable conductors in the forms of fibers, wires, and yarns can be designed and engineered to meet the demanding requirements for different wearable applications. Several crucial parameters, such as microarchitecture, conductivity, stretchability, and scalability, play essential roles in designing and developing wearable devices and intelligent textiles. Methodologies and fabrication processes have successfully realized 1D conductors that are highly conductive, strong, lightweight, stretchable, and conformable and can be readily integrated with common fabrics and soft matter. This review summarizes the latest advances in continuous, 1D stretchable conductors and emphasizes recent developments in materials, methodologies, fabrication processes, and strategies geared toward applications in electrical interconnects, mechanical sensors, actuators, and heaters. This review classifies 1D conductors into three categories on the basis of their electrical responses: (1) rigid 1D conductors, (2) piezoresistive 1D conductors, and (3) resistance-stable 1D conductors. This review also evaluates the present challenges in these areas and presents perspectives for improving the performance of stretchable 1D conductors for wearable textile and flexible electronic applications.

Nanoscale Kevlar Liquid Crystal Aerogel Fibers

Aerogel fibers, the simultaneous embodiment of aerogel porous network and fiber slender geometry, have shown critical advantages over natural and synthetic fibers in thermal insulation. However, how to control the building block orientation degree of the resulting aerogel fibers during the dynamic sol-gel transition process to expand their functions for emerging applications is a great challenge. Herein, nanoscale Kevlar liquid crystal (NKLC) aerogel fibers with different building block orientation degrees have been fabricated from Kevlar nanofibers via liquid crystal spinning, dynamic sol-gel transition, freeze-drying, and cold plasma hydrophobilization in sequence. The resulting NKLC aerogel fibers demonstrate extremely high mechanical strength (41.0 MPa), excellent thermal insulation (0.037 W·m^-1·K^-1), and self-cleaning performance (with a water contact angle of 154°). The superhydrophobic NKLC aerogel fibers can cyclically transform between aerogel and gel states, while gel fibers involving different building block orientation degrees display distinguishable brightness under polarized light. Based on these performances, digital textiles woven or embroidered with high- and low-orientated NKLC aerogel fibers enable up to 6.0 Gb information encryption in one square meter and on-demand decryption. Therefore, it can be envisioned that the tuning of the building blocks' orientation degree will be an appropriate strategy to endow performance to the liquid crystal aerogel fibers for potential applications beyond thermal insulation.

Responsive Hydrogels Based on Triggered Click Reactions for Liver Cancer

Globally, liver cancer, which is one of the major cancers worldwide, has attracted the growing attention of technological researchers for its high mortality and limited treatment options. Hydrogels are soft 3D network materials containing a large number of hydrophilic monomers. By adding moieties such as nitrobenzyl groups to the network structure of a cross-linked nanocomposite hydrogel, the click reaction improves drug-release efficiency in vivo, which improves the survival rate and prolongs the survival time of liver cancer patients. The application of a nanocomposite hydrogel drug delivery system can not only enrich the drug concentration at the tumor site for a long time but also effectively prevents the distant metastasis of residual tumor cells. At present, a large number of researches have been working toward the construction of responsive nanocomposite hydrogel drug delivery systems, but there are few comprehensive articles to systematically summarize these discoveries. Here, this systematic review summarizes the synthesis methods and related applications of nanocomposite responsive hydrogels with actions to external or internal physiological stimuli. With different physical or chemical stimuli, the structural unit rearrangement and the controlled release of drugs can be used for responsive drug delivery in different states.

Bio-inspired hydrogels with fibrous structure: A review on design and biomedical applications

Numerous tissues in the human body have fibrous structures, including the extracellular matrix, muscles, and heart, which perform critical biological functions and have exceptional mechanical strength. Due to their high-water content, softness, biocompatibility and elastic nature, hydrogels resemble biological tissues. Traditional hydrogels, on the other hand, have weak mechanical properties and lack tissue-like fibrous structures, limiting their potential applications. Thus, bio-inspired hydrogels with fibrous architectures have piqued the curiosity of biomedical researchers. Here, we review fabrication strategies for fibrous hydrogels and their recent progress in the biomedical fields of wound dressings, drug delivery, tissue engineering scaffolds and bioadhesives. Challenges and future perspectives are also discussed.

Skeletal Muscle Fibers Inspired Polymeric Actuator by Assembly of Triblock Polymers

Inspired by the striated structure of skeletal muscle fibers, a polymeric actuator by assembling two symmetric triblock copolymers, namely, polystyrene-b-poly(acrylic acid)-b-polystyrene (SAS) and polystyrene-b-poly(ethylene oxide)-b-polystyrene (SES) is developed. Owing to the microphase separation of the triblock copolymers and hydrogen-bonding complexation of their middle segments, the SAS/SES assembly forms a lamellar structure with alternating vitrified S and hydrogen-bonded A/E association layers. The SAS/SES strip can be actuated and operate in response to environmental pH. The contraction ratio and working density of the SAS/SES actuator are approximately 50% and 90 kJ m-3 , respectively; these values are higher than those of skeletal muscle fibers. In addition, the SAS/SES actuator shows a "catch-state", that is, it can maintain force without energy consumption, which is a feature of mollusc muscle but not skeletal muscle. This study provides a biomimetic approach for the development of artificial polymeric actuators with outstanding performance.

Multicomponent chiral hydrogel fibers with block configurations based on the chiral liquid crystals of cellulose nanocrystals and M13 bacteriophages

Integrating the advantages unique to CNCs and the M13 virus into blockwise chiral hydrogel fibers, which have block dependent chiral fingerprints, birefringence, (de)swelling behaviors, mechanical strength and stretchability.

Electrospun liquid crystal elastomer microfiber actuator

Fibers capable of generating axial contraction are commonly seen in nature and engineering applications. Despite the broad applications of fiber actuators, it is still very challenging to fabricate fiber actuators with combined large actuation strain, fast response speed, and high power density. Here, we report the fabrication of a liquid crystal elastomer (LCE) microfiber actuators using a facile electrospinning technique. Owing to the extremely small size of the LCE microfibers, they can generate large actuation strain (~60 percent) with a fast response speed (<0.2 second) and a high power density (400 watts per kilogram), resulting from the nematic-isotropic phase transition of liquid crystal mesogens. Moreover, no performance degradation is detected in the LCE microfibers after 10⁶ cycles of loading and unloading with the maximum strain of 20 percent at high temperature (90 degree Celsius). The small diameter of the LCE microfiber also results in a self-oscillatory behavior in a steady temperature field. In addition, with a polydopamine coating layer, the actuation of the electrospun LCE microfiber can be precisely and remotely controlled by a near-infrared laser through photothermal effect. Using the electrospun LCE microfiber actuator, we have successfully constructed a microtweezer, a microrobot, and a light-powered microfluidic pump.

Smart Hydrogel Bilayers Prepared by Irradiation

Environment-responsive hydrogel actuators have attracted tremendous attention due to their intriguing properties. Gamma radiation has been considered as a green cross-linking process for hydrogel synthesis, as toxic cross-linking agents and initiators were not required. In this work, chitosan/agar/P(N-isopropyl acrylamide-co-acrylamide) (CS/agar/P(NIPAM-co-AM)) and CS/agar/Montmorillonite (MMT)/PNIPAM temperature-sensitive hydrogel bilayers were synthesized via gamma radiation at room temperature. The mechanical properties and temperature sensitivity of hydrogels under different agar content and irradiation doses were explored. The enhancement of the mechanical properties of the composite hydrogel can be attributed to the presence of agar and MMT. Due to the different temperature sensitivities provided by the two layers of hydrogel, they can move autonomously and act as a flexible gripper as the temperature changes. Thanks to the antibacterial properties of the hydrogel, their storage time and service life may be improved. The as prepared hydrogel bilayers have potential applications in control devices, soft robots, artificial muscles and other fields.

Novel Graphene Oxide Nanohybrid Doped Methacrylic Acid Hydrogels for Enhanced Swelling Capability and Cationic Adsorbability

Novel versatile hydrogels were designed and composited based on covalent bond and noncovalent bond self-assembly of poly(methacrylic acid) (PMAA) networks and nanohybrids doped with graphene oxide (GO). The structures and properties of the neat PMAA and the prepared PMAA/GO hydrogels were characterized and analyzed in detail, using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, swelling and cationic absorption, etc. The swelling results showed that the water penetration follows the non-Fick transport mechanism based on swelling kinetics and diffusion theory. The swelling capacity of PMAA and composited PMAA/GO hydrogels toward pH, Na+, Ga2+, and Fe3+ was investigated; the swelling ratio was tunable between 4.44 and 36.44. Taking methylene blue as an example, the adsorption capacity of PMAA/GO hydrogels was studied. Nanohybrid doped GO not only self-associated with PMAA via noncovalent bonding interactions and had a tunable swelling ratio, but also interacted with water molecules via electrostatic repulsion, offering a pH response of both the network and dye absorption. Increases in pH caused a rise in equilibrium swelling ratios and reduced the cumulative cationic dye removal.