Shape memory nanocomposite fibers for untethered high-energy microengines

Emerging innovations in electrically powered artificial muscle fibers

This review systematically explores the inherent structural advantages of fiber over conventional film or bulk forms for artificial muscles, emphasizing their enhanced mechanical properties and actuation, scalability, and design flexibility. Distinctive merits of electrically powered artificial muscle fiber actuation mechanisms, including electrothermal, electrochemical and dielectric actuation, are highlighted, particularly for their operational efficiency, precise control capabilities, miniaturizability and seamless integration with electronic components. A comprehensive overview of significant research driving performance enhancements in artificial muscle fibers through materials and structural innovations is provided, alongside a discussion of the diverse design methodologies that have emerged in this field. A detailed comparative assessment evaluates the performance metrics, advantages and manufacturing complexities of each actuation mechanism, underscoring their suitability for various applications. Concluding with a strategic outlook, the review identifies key challenges and proposes targeted research directions to advance and refine artificial muscle fiber technologies.

Mimosa-Inspired Body Temperature-Responsive Shape Memory Polymer Networks: High Energy Densities and Multi-Recyclability

Inspired by the Mimosa plant, this study herein develops a unique dynamic shape memory polymer (SMP) network capable of transitioning from hard to pliable with heat, featuring reversible actuation, self-healing, recyclability, and degradability. This material is adept at simulating the functionalities of artificial muscles for a variety of tasks, with a remarkable specific energy density of 1.8 J g-1-≈46 times higher than that of human skeletal muscle. As an intelligent manipulator, it demonstrates remarkable proficiency in identifying and handling items at high temperatures. Its suitable rate of shape recovery around human body temperature indicates its promising utility as an implant material for addressing acute obstructions. The dynamic covalent bonding within the network structure not only provides excellent resistance to solvents but also bestows remarkable abilities for self-healing, reprocessing, and degradation. These attributes significantly boost its practicality and environmental sustainability. Anticipated to promote advancements in the sectors of biomedical devices, soft robotics, and smart actuators, this SMP network represents a forward leap in simulating artificial muscles, marking a stride toward the future of adaptive and sustainable technology.

Self-Evolving Hierarchical Hydrogel Fibers with Circadian Rhythms and Memory Functions

The fusion of hierarchical tissues at interfaces, incorporating ultrafast selective transport channels, enables efficient matter exchange and energy transfer across multiscale structures in living organisms. However, achieving these characteristics simultaneously in an artificial multimaterial system is challenging. Here, this work presents a multimaterial hydrogel fiber with a hierarchical structure of interface fusion, which forms spontaneously through in situ hierarchy evolution induced by ionic cross-linking and molecular shear flow. Water transport occurs in the angstrom-scale confined slits created by aligned cellulose nanocrystals (CNCs) by direct Coulomb knock-on, resembling Newton's cradle motion. The fusion of interfaces enables high-efficiency water transport across multiscale layers, combined with Newton's cradle-like collective water motion, creating a highly sensitive negative feedback loop within the fiber. These fibers exhibit integrated behaviors of time-space perception, short-term memory and adaptive changes in shape. Additionally, they demonstrate rhythm characteristics, changing periodically in a 24-h day-night cycle. Composed of natural building blocks, these hierarchical hydrogel fibers exhibit a memristor effect similar to that of an elementary neuron, making them promising for applications in seamless on-skin and implantable bioelectronics.

Giant nanomechanical energy storage capacity in twisted single-walled carbon nanotube ropes

A sustainable society requires high-energy storage devices characterized by lightness, compactness, a long life and superior safety, surpassing current battery and supercapacitor technologies. Single-walled carbon nanotubes (SWCNTs), which typically exhibit great toughness, have emerged as promising candidates for innovative energy storage solutions. Here we produced SWCNT ropes wrapped in thermoplastic polyurethane elastomers, and demonstrated experimentally that a twisted rope composed of these SWCNTs possesses the remarkable ability to reversibly store nanomechanical energy. Notably, the gravimetric energy density of these twisted ropes reaches up to 2.1 MJ kg-1, exceeding the energy storage capacity of mechanical steel springs by over four orders of magnitude and surpassing advanced lithium-ion batteries by a factor of three. In contrast to chemical and electrochemical energy carriers, the nanomechanical energy stored in a twisted SWCNT rope is safe even in hostile environments. This energy does not deplete over time and is accessible at temperatures ranging from -60 to +100 °C.

Recent advances in shape memory polymeric nanocomposites for biomedical applications and beyond

Shape memory polymers (SMPs), which initiate shape transformation in response to environmental stimuli, have attracted significant attention in both academic research and technological innovation. The combination of functional nanomaterials and SMPs has led to the emergence of a variety of shape memory polymeric nanocomposites (SMPNs) with multifunctional properties. This has injected new vitality and vigor into fields such as tissue engineering, biomedicine, optical sensing, aerospace and mechanical engineering. In this review article, we present a brief introduction to the fundamentals of SMPs and SMPNs, followed by a discussion of the recent advances in their multifunctional applications in biomedical manufacturing, drug delivery devices, mechanical sensing, micro-engines, etc. The opportunities and challenges in the future development of SMPs are also discussed.

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.

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.

Bioinspired Stimuli-Responsive Materials for Soft Actuators

Biological species can walk, swim, fly, jump, and climb with fast response speeds and motion complexity. These remarkable functions are accomplished by means of soft actuation organisms, which are commonly composed of muscle tissue systems. To achieve the creation of their biomimetic artificial counterparts, various biomimetic stimuli-responsive materials have been synthesized and developed in recent decades. They can respond to various external stimuli in the form of structural or morphological transformations by actively or passively converting input energy into mechanical energy. They are the core element of soft actuators for typical smart devices like soft robots, artificial muscles, intelligent sensors and nanogenerators. Significant progress has been made in the development of bioinspired stimuli-responsive materials. However, these materials have not been comprehensively summarized with specific actuation mechanisms in the literature. In this review, we will discuss recent advances in biomimetic stimuli-responsive materials that are instrumental for soft actuators. Firstly, different stimuli-responsive principles for soft actuators are discussed, including fluidic, electrical, thermal, magnetic, light, and chemical stimuli. We further summarize the state-of-the-art stimuli-responsive materials for soft actuators and explore the advantages and disadvantages of using electroactive polymers, magnetic soft composites, photo-thermal responsive polymers, shape memory alloys and other responsive soft materials. Finally, we provide a critical outlook on the field of stimuli-responsive soft actuators and emphasize the challenges in the process of their implementation to various industries.

Chain-Level Analysis of Reinforced Polyethylene through Stretch-Induced Crystallization

Herein, we propose a large-scale simulation approach to perform the stretch-induced crystallization of entangled polyethylene (PE) melts. Sufficiently long (1000 ns) united-atom molecular dynamics (UAMD) simulations for 16000 chains of 1000 consecutive CH2 united-atom particles under periodic boundary conditions were performed to achieve the crystallinity observed in experiments. Before the isothermal crystallization process, we applied uniaxial stretching as pre-elongation to the embedded strain memory on the entangled PE melts. We confirmed significant differences in the morphologies of crystal domains and scattering patterns for pre-elongation ratios of 400% and 800%. The obtained scattering patterns were consistent with the experimental results. Uniaxial stretching MD simulations revealed that the elastic modulus at 800% pre-elongation was stronger than that at 400% pre-elongation. From this observation, we can derive the structure-property relationship, wherein the magnitude of the pre-elongation governs the crystal domain structures and mechanical properties.

Air-Working Electrochromic Artificial Muscles

Artificial muscles are indispensable components for next-generation robotics to mimic the sophisticated movements of living systems and provide higher output energies when compared with real muscles. However, artificial muscles actuated by electrochemical ion injection have problems with single actuation properties and difficulties in stable operation in air. Here, air-working electrochromic artificial muscles (EAMs) with both color-changing and actuation functions are reported, which are constructed based on vanadium pentoxide nanowires and carbon tube yarn. Each EAM can generate a contractile stroke of ≈12% during stable operation in the air with multiple color changes (yellow-green-gray) under ±4 V actuation voltages. The reflectance contrast is as high as 51%, demonstrating the excellent versatility of the EAMs. In addition, a torroidal EAM arrangement with fast response and high resilience is constructed. The EAM's contractile stroke can be displayed through visual color changes, which provides new ideas for future artificial muscle applications in soft robots and artificial limbs.

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.

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

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

Toward a Unified Naming Scheme for Thermo-Active Soft Actuators: A Review of Materials, Working Principles, and Applications

Soft robotics is a rapidly growing field that spans the fields of chemistry, materials science, and engineering. Due to the diverse background of the field, there have been contrasting naming schemes such as "intelligent," "smart," and "adaptive" materials, which add vagueness to the broad innovation among literature. Therefore, a clear, functional, and descriptive naming scheme is proposed in which a previously vague name-Soft Material for Soft Actuators-can remain clear and concise-Phase-Change Elastomers for Artificial Muscles. By synthesizing the working principle, material, and application into a naming scheme, the searchability of soft robotics can be enhanced and applied to other fields. The field of thermo-active soft actuators spans multiple domains and requires added clarity. Thermo-active actuators have potential for a variety of applications spanning virtual reality haptics to assistive devices. This review offers a comprehensive guide to selecting the type of thermo-active actuator when one has an application in mind. In addition, it discusses future directions and improvements that are necessary for implementation.

Shape memory polymer with programmable recovery onset

Stimulus-responsive shape-shifting polymers1-3 have shown unique promise in emerging applications, including soft robotics4-7, medical devices8, aerospace structures9 and flexible electronics10. Their externally triggered shape-shifting behaviour offers on-demand controllability essential for many device applications. Ironically, accessing external triggers (for example, heating or light) under realistic scenarios has become the greatest bottleneck in demanding applications such as implantable medical devices8. Certain shape-shifting polymers rely on naturally present stimuli (for example, human body temperature for implantable devices)8 as triggers. Although they forgo the need for external stimulation, the ability to control recovery onset is also lost. Naturally triggered, yet actively controllable, shape-shifting behaviour is highly desirable but these two attributes are conflicting. Here we achieved this goal with a four-dimensional printable shape memory hydrogel that operates via phase separation, with its shape-shifting kinetics dominated by internal mass diffusion rather than by heat transport used for common shape memory polymers8-11. This hydrogel can undergo shape transformation at natural ambient temperature, critically with a recovery onset delay. This delay is programmable by altering the degree of phase separation during device programming, which offers a unique mechanism for shape-shifting control. Our naturally triggered shape memory polymer with a tunable recovery onset markedly lowers the barrier for device implementation.

Fibrous wearable and implantable bioelectronics

Fibrous wearable and implantable devices have emerged as a promising technology, offering a range of new solutions for minimally invasive monitoring of human health. Compared to traditional biomedical devices, fibers offer a possibility for a modular design compatible with large-scale manufacturing and a plethora of advantages including mechanical compliance, breathability, and biocompatibility. The new generation of fibrous biomedical devices can revolutionize easy-to-use and accessible health monitoring systems by serving as building blocks for most common wearables such as fabrics and clothes. Despite significant progress in the fabrication, materials, and application of fibrous biomedical devices, there is still a notable absence of a comprehensive and systematic review on the subject. This review paper provides an overview of recent advancements in the development of fibrous wearable and implantable electronics. We categorized these advancements into three main areas: manufacturing processes, platforms, and applications, outlining their respective merits and limitations. The paper concludes by discussing the outlook and challenges that lie ahead for fiber bioelectronics, providing a holistic view of its current stage of development.

Plug & play origami modules with all-purpose deformation modes

Three basic deformation modes of an object (bending, twisting, and contraction/extension) along with their various combinations and delicate controls lead to diverse locomotion. As a result, seeking mechanisms to achieve simple to complex deformation modes in a controllable manner is a focal point in related engineering fields. Here, a pneumatic-driven, origami-based deformation unit that offers all-purpose deformation modes, namely, three decoupled basic motion types and four combinations of these three basic types, with seven distinct motion modes in total through one origami module, was created and precisely controlled through various pressurization schemes. These all-purpose origami-based modules can be readily assembled as needed, even during operation, which enables plug-and-play characteristics. These origami modules with all-purpose deformation modes offer unprecedented opportunities for soft robots in performing complex tasks, which were successfully demonstrated in this work.

Multi-Stimuli Dually-Responsive Intelligent Woven Structures with Local Programmability for Biomimetic Applications

This work focuses on multi-stimuli-responsive materials with distinctive abilities, that is, color-changing and shape-memory. Using metallic composite yarns and polymeric/thermochromic microcapsule composite fibers, processed via a melt-spinning technique, an electrothermally multi-responsive fabric is woven. The resulting smart-fabric transfers from a predefined structure to an original shape while changing color upon heating or applying an electric field, making it appealing for advanced applications. The shape-memory and color-changing features of the fabric can be controlled by rationally controlling the micro-scale design of the individual fibers in the structure. Thus, the fibers' microstructural features are optimized to achieve excellent color-changing behavior along with shape fixity and recovery ratios of 99.95% and 79.2%, respectively. More importantly, the fabric's dual-response by electric field can be achieved by a low voltage of 5 V, which is smaller than the previously reported values. Above and beyond, the fabric is able to be meticulously activated by selectively applying a controlled voltage to any part of the fabric. The precise local responsiveness can be bestowed upon the fabric by readily controlling its macro-scale design. A biomimetic dragonfly with the shape-memory and color-changing dual-response ability is successfully fabricated, broadening the design and fabrication horizon of groundbreaking smart materials with multiple functions.

Tethering of twisted-fiber artificial muscles

Twisted-fiber artificial muscles, a new type of soft actuator, exhibit significant potential for use in applications related to lightweight smart devices and soft robotics. Fiber twisting generates internal torque and a spiral architecture, exhibiting rotation, contraction, or elongation as a result of fiber volume change. Untethering a twisted fiber often results in fiber untwisting and loss of stored torque energy. Preserving the torque in twisted fibers during actuation is necessary to realize a reversible and stable artificial muscle performance; this is a key issue that has not yet been systematically discussed and reviewed. This review summarizes the mechanisms for preserving the torque within twisted fibers and the potential applications of such systems. The potential challenges and future directions of research related to twisted-fiber artificial muscles are also discussed.

Polymer Complex Fiber: Property, Functionality, and Applications

Polymer complex fibers (PCFs) are a novel kind of fiber material processed from polymer complexes that are assembled through noncovalent interactions. These can realize the synergy of functional components and miscibility on the molecular level. The dynamic character of noncovalent interactions endows PCFs with remarkable properties, such as reversibility, stimuli responsiveness, self-healing, and recyclability, enabling them to be applied in multidisciplinary fields. The objective of this article is to provide a review of recent progress in the field of PCFs. The classification based on chain interactions will be first introduced followed by highlights of the fabrication technologies and properties of PCFs. The effects of composition and preparation method on fiber properties are also discussed, with some emphasis on utilizing these for rational design. Finally, we carefully summarize recent advanced applications of PCFs in the fields of energy storage and sensors, water treatment, biomedical materials, artificial actuators, and biomimetic platforms. This review is expected to deepen the comprehension of PCF materials and open new avenues for developing PCFs with tailor-made properties for advanced application.

Shape-memory effect in twisted ferroic nanocomposites

The shape recovery ability of shape-memory alloys vanishes below a critical size (~50 nm), which prevents their practical applications at the nanoscale. In contrast, ferroic materials, even when scaled down to dimensions of a few nanometers, exhibit actuation strain through domain switching, though the generated strain is modest (~1%). Here, we develop freestanding twisted architectures of nanoscale ferroic oxides showing shape-memory effect with a giant recoverable strain (>8%). The twisted geometrical design amplifies the strain generated during ferroelectric domain switching, which cannot be achieved in bulk ceramics or substrate-bonded thin films. The twisted ferroic nanocomposites allow us to overcome the size limitations in traditional shape-memory alloys and open new avenues in engineering large-stroke shape-memory materials for small-scale actuating devices such as nanorobots and artificial muscle fibrils.

Rotational multimaterial printing of filaments with subvoxel control

Helical structures are ubiquitous in nature and impart unique mechanical properties and multifunctionality¹. So far, synthetic architectures that mimic these natural systems have been fabricated by winding, twisting and braiding of individual filaments^1-7, microfluidics^8,9, self-shaping^1,10-13 and printing methods^14-17. However, those fabrication methods are unable to simultaneously create and pattern multimaterial, helically architected filaments with subvoxel control in arbitrary two-dimensional (2D) and three-dimensional (3D) motifs from a broad range of materials. Towards this goal, both multimaterial^18-23 and rotational²⁴ 3D printing of architected filaments have recently been reported; however, the integration of these two capabilities has yet to be realized. Here we report a rotational multimaterial 3D printing (RM-3DP) platform that enables subvoxel control over the local orientation of azimuthally heterogeneous architected filaments. By continuously rotating a multimaterial nozzle with a controlled ratio of angular-to-translational velocity, we have created helical filaments with programmable helix angle, layer thickness and interfacial area between several materials within a given cylindrical voxel. Using this integrated method, we have fabricated functional artificial muscles composed of helical dielectric elastomer actuators with high fidelity and individually addressable conductive helical channels embedded within a dielectric elastomer matrix. We have also fabricated hierarchical lattices comprising architected helical struts containing stiff springs within a compliant matrix. Our additive-manufacturing platform opens new avenues to generating multifunctional architected matter in bioinspired motifs.

Adaptive IR- and Water-Gating Textiles Based on Shape Memory Fibers

Thermoregulation is an essential function of the human body for adapting to the surrounding temperature. Stimuli-responsive smart textiles can provide effective protection of the human skin temperature from a continuously changing environment. Herein, we develop a smart textile based on shape memory polymer (SMP) fibers for adaptive regulation of IR and water transmission on human skin. An SMP textile is fabricated with hierarchical micro/nanoporous structures to enhance thermal insulation performance, and silver nanowires are coated on one side to provide asymmetric IR reflectivity and hydrophilicity. The porous SMP textile shows great tunability of thermal insulation and asymmetric wettability by deformation and recovery of the shape and structure in response to stimuli. The degree of thermal insulation is controlled by 65.7% of the original value, and the surface temperature of the SMP textile on a hot plate is successfully controlled in the IR images due to adaptive IR reflectivity. Additionally, the directional transportation of water droplets can be switched on/off according to the shape of the SMP textiles, which can be employed for sweat removal from the human skin. This IR- and water-gating smart textile can provide a feasible strategy for protecting the human skin from external environmental changes.

Smart-Temporary-Film-Based Local-Delivery System with Controllable Drug-Release Behavior

The development of a simple local drug-delivery system that exhibits the advantages of macro- and microscale carriers with controllable drug-release behavior is still highly desired. Herein, in this work, a smart temporary film was prepared from doxorubicin (DOX)-loaded shape-memory microgels via a simple hot-compression programming method. The temporary film showed a very smooth surface and easy handing, as well as macroscopy mechanical properties, which could disintegrate into the microgels with heating at 45 °C. In this case, the temporary film showed a controllable DOX release behavior when compared with the microgels, which could release the DOX on demand. Consequently, the temporary film exhibited weaker cytotoxicity to normal cells and a much longer antitumor capability, as well as a higher drug-utilization efficiency when compared with microgels. Therefore, the smart temporary film has high potential as a candidate for use as a local drug-delivery system.

Human-muscle-inspired single fibre actuator with reversible percolation

Artificial muscles are indispensable components for next-generation robotics capable of mimicking sophisticated movements of living systems. However, an optimal combination of actuation parameters, including strain, stress, energy density and high mechanical strength, is required for their practical applications. Here we report mammalian-skeletal-muscle-inspired single fibres and bundles with large and strong contractive actuation. The use of exfoliated graphene fillers within a uniaxial liquid crystalline matrix enables photothermal actuation with large work capacity and rapid response. Moreover, the reversible percolation of graphene fillers induced by the thermodynamic conformational transition of mesoscale structures can be in situ monitored by electrical switching. Such a dynamic percolation behaviour effectively strengthens the mechanical properties of the actuator fibres, particularly in the contracted actuation state, enabling mammalian-muscle-like reliable reversible actuation. Taking advantage of a mechanically compliant fibre structure, smart actuators are readily integrated into strong bundles as well as high-power soft robotics with light-driven remote control.

Stepwise Artificial Yarn Muscles with Energy-Free Catch States Driven by Aluminum-Ion Insertion

Present artificial muscles have been suffering from poor actuation step precision and the need of energy input to maintain actuated states due to weak interactions between guest and host materials or the unstable structural changes. Herein, these challenges are addressed by deploying a mechanism of reversible faradaic insertion and extraction reactions between tetrachloroaluminate ions and collapsed carbon nanotubes. This mechanism allows tetrachloroaluminate ions as a strong but dynamic "locker" to achieve an energy-free high-tension catch state and programmable stepwise actuation in the yarn muscle. When powered off, the muscle nearly 100% maintained any achieved contractile strokes even under loads up to 96,000 times the muscle weight. The actuation mechanism allowed the programmable control of stroke steps down to 1% during reversible actuation. The isometric stress generated by the yarn muscle (14.6 MPa in maximum, 40 times that of skeletal muscles) was also energy freely lockable and step controllable with high precision. Importantly, when fully charged, the muscle stored energy with a high capacity of 102 mAh g^-1, allowing the muscle as a battery to power secondary muscles or other devices.

Morphology modulation of artificial muscles by thermodynamic-twist coupling

Human muscles can grow and change their length with body development; therefore, artificial muscles that modulate their morphology according to changing needs are needed. In this paper, we report a strategy to transform an artificial muscle into a new muscle with a different morphology by thermodynamic-twist coupling, and illustrate its structural evolution during actuation. The muscle length can be continuously modulated over a large temperature range, and actuation occurs by continuously changing the temperature. This strategy is applicable to different actuation modes, including tensile elongation, tensile contraction and torsional rotation. This is realized by twist insertion into a fibre to produce torsional stress. Fibre annealing causes partial thermodynamic relaxation of the spiral molecular chains, which serves as internal tethering and inhibits fibre twist release, thus producing a self-supporting artificial muscle that actuates under heating. At a sufficiently high temperature, further relaxation of the spiral molecular chains occurs, resulting in a new muscle with a different length. A structural study provides an understanding of the thermodynamic-twist coupling. This work provides a new design strategy for intelligent materials.

A Thermo-Mechanically Robust Compliant Electrode Based on Surface Modification of Twisted and Coiled Nylon-6 Fiber for Artificial Muscle with Highly Durable Contractile Stroke

In this paper, we propose a novel and facile methodology to chemically construct a thin and highly compliant metallic electrode onto a twisted and coiled nylon-6 fiber (TCN) with a three-dimensional structure via surface modification of the TCN eliciting gold-sulfur (Au-S) interaction for enabling durable electro-thermally-induced actuation performance of a TCN actuator (TCNA). The surface of the TCN exposed to UV/Ozone plasma was modified to (3-mercaptopropyl)trimethoxysilane (MPTMS) molecules with thiol groups through a hydrolysis-condensation reaction. Thanks to the surface modification inducing strong interaction between gold and sulfur as a formation of covalent bonds, the Au electrode on the MPTMS-TCN exhibited excellent mechanical robustness against adhesion test, simultaneously could allow overall surface of the TCN to be evenly heated without any significant physical damages during repetitive electro-thermal heating tests. Unlike the TCNAs with physically coated metallic electrode, the TCNA with the Au electrode established on the MPTMS-TCN could produce a large and repeatable contractile strain over 12% as lifting a load of 100 g even during 2000 cyclic actuations. Demonstration of the durable electrode for the TCNA can lead to technical advances in artificial muscles for human-assistive devices as well as soft robots those requires long-term stability in operation.

Nanostructured block copolymer muscles

High-performance actuating materials are necessary for advances in robotics, prosthetics and smart clothing. Here we report a class of fibre actuators that combine solution-phase block copolymer self-assembly and strain-programmed crystallization. The actuators consist of highly aligned nanoscale structures with alternating crystalline and amorphous domains, resembling the ordered and striated pattern of mammalian skeletal muscle. The reported nanostructured block copolymer muscles excel in several aspects compared with current actuators, including efficiency (75.5%), actuation strain (80%) and mechanical properties (for example, strain-at-break of up to 900% and toughness of up to 121.2 MJ m^-3). The fibres exhibit on/off rotary actuation with a peak rotational speed of 450 r.p.m. Furthermore, the reported fibres demonstrate multi-trigger actuation (heat and hydration), offering switchable mechanical properties and various operating modes. The versatility and recyclability of the polymer fibres, combined with the facile fabrication method, opens new avenues for creating multifunctional and recyclable actuators using block copolymers.

Bistable and Multistable Actuators for Soft Robots: Structures, Materials, and Functionalities

Snap-through bistability is often observed in nature (e.g., fast snapping to closure of Venus flytrap) and the life (e.g., bottle caps and hair clippers). Recently, harnessing bistability and multistability in different structures and soft materials has attracted growing interest for high-performance soft actuators and soft robots. They have demonstrated broad and unique applications in high-speed locomotion on land and under water, adaptive sensing and fast grasping, shape reconfiguration, electronics-free controls with a single input, and logic computation. Here, an overview of integrating bistable and multistable structures with soft actuating materials for diverse soft actuators and soft/flexible robots is given. The mechanics-guided structural design principles for five categories of basic bistable elements from 1D to 3D (i.e., constrained beams, curved plates, dome shells, compliant mechanisms of linkages with flexible hinges and deformable origami, and balloon structures) are first presented, alongside brief discussions of typical soft actuating materials (i.e., fluidic elastomers and stimuli-responsive materials such as electro-, photo-, thermo-, magnetic-, and hydro-responsive polymers). Following that, integrating these soft materials with each category of bistable elements for soft bistable and multistable actuators and their diverse robotic applications are discussed. To conclude, perspectives on the challenges and opportunities in this emerging field are considered.

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.

Current State of Robotics in Hand Rehabilitation after Stroke: A Systematic Review

Among the methods of hand function rehabilitation after stroke, robot-assisted rehabilitation is widely used, and the use of hand rehabilitation robots can provide functional training of the hand or assist the paralyzed hand with activities of daily living. However, patients with hand disorders consistently report that the needs of some users are not being met. The purpose of this review is to understand the reasons why these user needs are not being adequately addressed, to explore research on hand rehabilitation robots, to review their current state of research in recent years, and to summarize future trends in the hope that it will be useful to researchers in this research area. This review summarizes the techniques in this paper in a systematic way. We first provide a comprehensive review of research institutions, commercial products, and literature. Thus, the state of the art and deficiencies of functional hand rehabilitation robots are sought and guide the development of subsequent hand rehabilitation robots. This review focuses specifically on the actuation and control of hand functional rehabilitation robots, as user needs are primarily focused on actuation and control strategies. We also review hand detection technologies and compare them with patient needs. The results show that the trends in recent years are more inclined to pursue new lightweight materials to improve hand adaptability, investigating intelligent control methods for human-robot interaction in hand functional rehabilitation robots to improve control robustness and accuracy, and VR virtual task positioning to improve the effectiveness of active rehabilitation training.

Microfluidic manipulation by spiral hollow-fibre actuators

A microfluidic manipulation system that can sense a liquid and control its flow is highly desirable. However, conventional sensors and motors have difficulty fitting the limited space in microfluidic devices; moreover, fast sensing and actuation are required because of the fast liquid flow in the hollow fibre. In this study, fast torsional and tensile actuators were developed using hollow fibres employing spiral nonlinear stress, which can sense the fluid temperature and sort the fluid into the desired vessels. The fluid-driven actuation exhibited a highly increased response speed (27 times as fast as that of air-driven actuation) and increased power density (90 times that of an air-driven solid fibre actuator). A 0.5 K fluid temperature fluctuation produced a 20° rotation of the hollow fibre. These high performances originated from increments in both heat transfer and the average bias angle, which was understood through theoretical analysis. This work provides a new design strategy for intelligent microfluidics and inspiration for soft robots and smart devices for biological, optical, or magnetic applications.

Miniature coiled artificial muscle for wireless soft medical devices

Wireless small-scale soft-bodied devices are capable of precise operation inside confined internal spaces, enabling various minimally invasive medical applications. However, such potential is constrained by the small output force and low work capacity of the current miniature soft actuators. To address this challenge, we report a small-scale soft actuator that harnesses the synergetic interactions between the coiled artificial muscle and radio frequency-magnetic heating. This wirelessly controlled actuator exhibits a large output force (~3.1 N) and high work capacity (3.5 J/g). Combining this actuator with different mechanical designs, its tensile and torsional behaviors can be engineered into different functional devices, such as a suture device, a pair of scissors, a driller, and a clamper. In addition, by assuming a spatially varying magnetization profile, a multilinked coiled muscle can have both magnetic field-induced bending and high contractile force. Such an approach could be used in various future untethered miniature medical devices.

Liquid Crystal Elastomer Twist Fibers toward Rotating Microengines

Untethered twist fibers do not require end-anchoring structures to hold their twist orientation and offer simple designs and convenient operation. The reversible responsiveness of these fibers allows them to generate torque and rotational deformation continuously upon the application of external stimuli. The fibers therefore have potential in rotating microengines. In practical applications, high torque and rotational deformation are desirable to meet work capacity requirements. However, the simultaneous endowment of reversible responsiveness and high rotational performance to untethered twist fibers remains a challenge. In this study, a liquid crystal elastomer twist fiber (LCETF) is designed and developed with a fixed twisting alignment of mesogens to provide untethered and reversible responsiveness. Outstanding rotational performance can be achieved when the mesogenic orientation is disrupted through heat triggering. Owing to the significant intrinsic contractile ratio of the LCE material, the rotational deformation of the LCETF can reach 243.6° mm^-1 . More importantly, the specific torque can reach 10.1 N m kg^-1 , which exceeds previously reported values. In addition, the LCETF can be exploited in a rotating microengine to convert heat into electricity with an induction voltage as high as 9.4 V. This work broadens the applications of LCEs for energy harvesters, micromachines, and soft robots.

Soft actuators for real-world applications

Inspired by physically adaptive, agile, reconfigurable and multifunctional soft-bodied animals and human muscles, soft actuators have been developed for a variety of applications, including soft grippers, artificial muscles, wearables, haptic devices and medical devices. However, the complex performance of biological systems cannot yet be fully replicated in synthetic designs. In this Review, we discuss new materials and structural designs for the engineering of soft actuators with physical intelligence and advanced properties, such as adaptability, multimodal locomotion, self-healing and multi-responsiveness. We examine how performance can be improved and multifunctionality implemented by using programmable soft materials, and highlight important real-world applications of soft actuators. Finally, we discuss the challenges and opportunities for next-generation soft actuators, including physical intelligence, adaptability, manufacturing scalability and reproducibility, extended lifetime and end-of-life strategies.

Programmable Knot Microfibers from Piezoelectric Microfluidics

Microfibers have demonstrated significant application values in a large number of areas. Current efforts focus on developing new technologies to prepare microfibers with controllable morphological and structural features to enhance their functions. Here, a piezoelectric microfluidic platform is presented for consecutive spinning of functional microfibers with programmable spindle-knots. In this platform, a jet of a pregel-solution flowing in the channel can be subjected to a programmable piezoelectric signal and vibrates synchronously. Following a rapid polymerization of the wavy jet, microfibers with corresponding morphologies can be generated, including uniform, gradient, and symmetrical knots. Such a unique knot structure contributes to a water-collection mechanism. Thus, it has been observed that microfibers with programmed knots enable even more flexible droplet handling and active water transport. In addition, by constructing higher-order knot fiber networks, practical applications including spray reaction, lab-on-a-chip vapor detection, etc., can also be demonstrated. it is believed that this platform opens a new avenue for fiber spinning, and the programmable microfibers would be highly applicable in chemical, biomedical, and environmental areas.

Polymer interdigitated pillar electrostatic (PIPE) actuators

This work reports a three-dimensional polymer interdigitated pillar electrostatic actuator that can produce force densities 5-10× higher than those of biological muscles. The theory of operation, scaling, and stability is investigated using analytical and FEM models. The actuator consists of two high-density arrays of interdigitated pillars that work against a restoring force generated by an integrated flexure spring. The actuator architecture enables linear actuation with higher displacements and pull-in free actuation to prevent the in-use stiction associated with other electrostatic actuators. The pillars and springs are 3D printed together in the same structure. The pillars are coated with a gold-palladium alloy layer to form conductive electrodes. The space between the pillars is filled with liquid dielectrics for higher breakdown voltages and larger electrostatic forces due to the increase in the dielectric constant. We demonstrated a prototype actuator that produced a maximum work density of 54.6 µJ/cc and an electrical-to-mechanical energy coupling factor of 32% when actuated at 4000 V. The device was operated for more than 100,000 cycles with no degradation in displacements. The flexible polymer body was robust, allowing the actuator to operate even after high mechanical force impact, which was demonstrated by operation after drop tests. As it is scaled further, the reported actuator will enable soft and flexible muscle-like actuators that can be stacked in series and parallel to scale the resulting forces. This work paves the way for high-energy density actuators for microrobotic applications.

A comparative review of artificial muscles for microsystem applications

Artificial muscles are capable of generating actuation in microsystems with outstanding compliance. Recent years have witnessed a growing academic interest in artificial muscles and their application in many areas, such as soft robotics and biomedical devices. This paper aims to provide a comparative review of recent advances in artificial muscle based on various operating mechanisms. The advantages and limitations of each operating mechanism are analyzed and compared. According to the unique application requirements and electrical and mechanical properties of the muscle types, we suggest suitable artificial muscle mechanisms for specific microsystem applications. Finally, we discuss potential strategies for energy delivery, conversion, and storage to promote the energy autonomy of microrobotic systems at a system level.

Switching microobjects from low to high aspect ratios using a shape-memory effect

Spherical particles from shape-memory polymers (SMP) can be stretched to ellipsoids with high aspect ratio (AR) and temporarily stabilized. They can switch back to low AR upon thermal stimulation. Here, the creation of an alternative shape-switching capability of particles from low to high AR is introduced, where a SMP matrix from polyvinyl alcohol (PVA) is used to create crosslinked high AR particles and to program the embedded micrometer-sized particles from a second SMP (oligo(ε-caprolactone) micronetworks, MN) with a low switching temperature T_sw. This programming proceeds through shape-recovery of the PVA matrix, from which the MN are harvested by PVA matrix dissolution. The use of a dissolvable SMP matrix may be a general strategy to efficiently create systems with complex moving capabilities.

Textiles in soft robots: Current progress and future trends

Soft robotics have substantial benefits of safety, adaptability, and cost efficiency compared to conventional rigid robotics. Textiles have applications in soft robotics either as an auxiliary material to reinforce the conventional soft material or as an active soft material. Textiles of various types and configurations have been fabricated into key components of soft robotics in adaptable formats. Despite significant advancements, the efficiency and characteristics of textile actuators in practical applications remain unsatisfactory. To address these issues, novel structural and material designs as well as new textile technologies have been introduced. Herein, we aim at giving an insight into the current state of the art in textile technology for soft robotic manufacturing. We firstly discuss the fundamental actuation mechanisms for soft robotics. We then provide a critical review on the recently developed functional textiles as reinforcements, sensors, and actuators in soft robotics. Finally, the future trends and current strategies that can be employed in textile-based actuator manufacturing process have been explored to address the critical challenges in soft robotics.

Twisting and Braiding Fluid-Driven Soft Artificial Muscle Fibers for Robotic Applications

Research on soft artificial muscles (SAMs) is rapidly growing, both in developing new actuation ideas and improving existing structures with multifunctionality. The human body has more than 600 muscles that drive organs and joints to achieve desired functions. Inspired by the human muscles, this article presents a new type of SAM fiber formed from twisting and braiding soft hydraulic filament artificial muscles with high aspect ratio, high strain, and high energy efficiency. We systematically investigated the relationship between input pressure and output elongation as well as contraction force of the new muscles using different configurations in terms of an array of single and multiple muscles arranged in nontwisting (or straight), twisting, and braiding variants. Experimental results revealed that the twisting and braiding configurations greatly enhanced the muscle elongation and generated force compared with their nontwisting/braiding counterparts. To demonstrate the new muscles' usability, we implemented several muscle variants to bidirectionally manipulate 3D-printed human fingers and elbow, mimicking the human upper limb with a full range of motion. We also created a bioinspired growing soft tubular muscle that could simultaneously exert longitudinal and radial expansion upon pressurization, similar to that of auxetic metamaterial structures. The new growing soft tubular muscles were experimentally validated and the results showed that they could be potentially implemented in several emerging applications, including smart compression garments, stent-like supporting devices, and tubular grippers for medical use.

A new torsion tester based on an electronic autocollimator for characterizing the torsional behaviors of microfibers

Torsional properties of single fibers have considerable effects on the performance of fabrics. A new torsion tester with high resolution is developed based on the torsion-balance method and the autocollimation principle for measuring the torsional properties of single fibers. The tester can measure the torque accurately using the torsion wire of known torsional rigidity. The angular displacement of the torsion wire is measured by a non-contact electronic autocollimator. The calibration results show that the maximum torque capability is around 6 × 10^-6 N m with a theoretical resolution of about 8 × 10^-11 N m. The instrument's validity is verified by comparing the measurement result with that obtained by a torsion pendulum. Torsion tests on four industrially microfibers, including glass fiber, polyimide fiber, carbon fiber, and gold wire, are performed with the tester, which gives shear moduli 71.0 ± 0.4, 5.6 ± 0.1, 22.6 ± 3.1, and 34.3 ± 0.5 GPa, respectively. Young's moduli of these fibers are also provided. It is demonstrated that these fibers have a high anisotropy in mechanical properties.

One-Step, Continuous Three-Dimensional Printing of Multi-Stimuli-Responsive Bilayer Microactuators via a Double-Barreled Theta Pipette

Although there has been extensive development and exploration of small-scale robots, the technological challenges associated with their complicated and high-cost fabrication processes remain unresolved. Here, we report a one-step, bi-material, high-resolution three-dimensional (3D) printing method for the fabrication of multi-stimuli-responsive microactuators. This method exploits a two-phase femtoliter ink meniscus formed on a double-barreled theta micropipette to continuously print a freestanding bilayer microstructure, which undergoes an asymmetric volume change upon the adsorption or desorption of water. We show that the 3D-printed bilayer microstructures exhibit reversible, reproducible actuation in ambient humidity or under illumination with infrared light. Our 3D printing approach can assemble bilayer segments for programming microscale actuation, as demonstrated by proof-of-concept experiments. We expect that this method will serve as the basis for flexible, programmable, one-step routes for the assembly of small-scale intelligent actuators.

Recent progress of biomimetic motions-from microscopic micro/nanomotors to macroscopic actuators and soft robotics

Motion is a basic behavioral attribute of organisms, and it is a behavioral response of organisms to the external environment and internal state changes. Materials with switchable mechanical properties are widespread in living organisms and play crucial roles in the motion of organisms. Therefore, significant efforts have been made toward mimicking such architectures and motion behaviors by making full use of the properties of stimulus-responsive materials to design smart materials/machines with specific functions. In recent years, the biomimetic motions based on micro/nanomotors, actuators and soft robots constructed from smart response materials have been developed gradually. However, a comprehensive discussion on various categories of biomimetic motions in this field is still missing. This review aims to provide such a panoramic overview. From nano-to macroscales, we summarize various biomimetic motions based on micro/nanomotors, actuators and soft robotics. For each biomimetic motion, we discuss the driving modes and the key functions. The challenges and opportunities of biomimetic motions are also discussed. With rapidly increasing innovation, advanced, intelligent and multifunctional biomimetic motions based on micro/nanomotors, actuators and soft robotics will certainly bring profound impacts and changes for human life in the near future.

Magnetic Dynamic Polymers for Modular Assembling and Reconfigurable Morphing Architectures

Shape-morphing magnetic soft materials, composed of magnetic particles in a soft polymer matrix, can transform shape reversibly, remotely, and rapidly, finding diverse applications in actuators, soft robotics, and biomedical devices. To achieve on-demand and sophisticated shape morphing, the manufacture of structures with complex geometry and magnetization distribution is highly desired. Here, a magnetic dynamic polymer (MDP) composite composed of hard-magnetic microparticles in a dynamic polymer network with thermally responsive reversible linkages, which permits functionalities including targeted welding for magnetic-assisted assembly, magnetization reprogramming, and permanent structural reconfiguration, is reported. These functions not only provide highly desirable structural and material programmability and reprogrammability but also enable the manufacturing of functional soft architected materials such as 3D kirigami with complex magnetization distribution. The welding of magnetic-assisted modular assembly can be further combined with magnetization reprogramming and permanent reshaping capabilities for programmable and reconfigurable architectures and morphing structures. The reported MDP are anticipated to provide a new paradigm for the design and manufacture of future multifunctional assemblies and reconfigurable morphing architectures and devices.

Tuning the reversibility of hair artificial muscles by disulfide cross-linking for sensors, switches, and soft robotics

Tensile and torsional artificial muscles from biocompatible and biodegradable materials are highly desired for soft robotics, sensors, and controllers in bio-related applications. Twisted fibers can be used to prepare tensile and torsional artificial muscles, while torsional tethering is always required to avoid release of the inserted twist, which adds complexity to the device design. Moreover, the tuning of the reversibility of twisted fiber artificial muscles has not been realized. Here disulfide cross-linking was used to prepare novel tether-free hygroresponsive tensile and torsional fiber artificial muscles in twisted hair fibers. Increasing the cross-linking level converted the fiber artificial muscle from irreversible to reversible actuation. Different types of actuations including rotation, contraction, and elongation were realized for the twisted, the homochirally coiled, and the heterochirally coiled hair fibers, respectively. A reversible torsional fiber artificial muscle showed 122.4° mm^-1 rotation, homochiral and heterochiral fiber artificial muscles showed 94% contraction and 3000% elongation, respectively, and a maximum work capacity and energy density of 6.35 J kg^-1 and 69.8 kJ m^-3, respectively, were realized, on exposure to water fog. This work provides a new strategy for preserving the inserted twist in bio-fiber artificial muscles and for tuning of muscle reversibility, which show application perspectives in biocompatible smart materials, sensors, and robotics.