Artificial Muscle Applications Research Articles

Design and Scalable Fast Fabrication of Biaxial Fabric Pouch Motors for Soft Robotic Artificial Muscle Applications

Design and Scalable Fast Fabrication of Biaxial Fabric Pouch Motors for Soft Robotic Artificial Muscle Applications

Satisfactory Tensile Strength and Strain of Recyclable Polyurethane with a Trimaleimide Structure for Thermal Self-Healing and Anticorrosive Coatings.

Bismaleimide (BMI) is often used as the cross-linking reagent in Diels-Alder (D-A)-type intrinsic self-healing materials (DISMs) to promote the connectivity of damaged surfaces based on reversible D-A bond formation on the molecular scale. Until now, although DISMs have exhibited great potential in the applications of various sensors, electronic skin, and artificial muscles, it is still difficult to prepare DISMs with satisfactory self-healing abilities and high tensile strengths and strains at the same time, thus largely limiting their applications in self-healing anticorrosive coatings. Herein, symmetrical trimaleimide (TMI) was successfully synthesized, and trimaleimide-structured D-A self-healing polyurethane (TMI-DA-PU) was prepared via the reversible D-A reaction (cycloaddition of furan and maleimide). As a DISM, TMI-DA-PU exhibits apparently higher self-healing efficiency (98.7%), tensile strength (25.4 MPa), and strain (1378%) compared to bismaleimide-structured D-A self-healing polyurethane (BMI-DA-PU) (self-healing efficiency, 90.2%; tensile strength, 19.3 MPa; strain, 1174%). In addition, TMI-DA-PU shows a high recycling efficiency (>95%) after 4 cycles of recycling. A series of characterizations indicate that TMI provides more monoene rings as the self-healing sites, forms denser cross-linked structures compared to BMI, and is, thus, more appropriate to be used for DISM applications. Moreover, the barrier abilities of coatings can be semi-quantitatively expressed by the impedance value at 0.01 Hz (|Z|0.01Hz). The |Z|0.01 Hz value of the TMI-DA-PU coating is 3.93 × 109 Ω cm2 on day 0, which is significantly higher than that of the BMI-DA-PU coating (6.76 × 108 Ω cm2 on day 0), indicating that the denser rigid cross-linked structure of TMI results in the small porosity in the TMI-DA-PU coating, thus effectively improving the anticorrosion performance. The construction of DISMs with the structure of TMI demonstrates immense potential in self-healing anticorrosive coatings.

Electromechanical strain response of phosphorene nanotubes

Nanomaterials that undergo structural or other property changes upon application of external stimuli are called stimuli responsive materials and are particularly suited for drug delivery, biosensing or artificial muscle applications. Two-dimensional (2D) black phosphorus is an ideal material for such applications due to its remarkable electromechanical response. Given that one-dimensional (1D) black phosphorus nanotubes (PNTs) are calculated to be energetically stable, it is possible that they can undergo similar electromechanical responses to their 2D counterparts, allowing their potential application as nanochannel devices for drug delivery. Using first-principles density functional theory, we investigated the electromechanical response of different-sized PNTs upon charge injection. Upon hole injection, the diameter of the PNTs expands up to a maximum of 30.2% for a (0,15) PNT that is 0.24 nm in diameter. The PNTs become highly p-doped as the valence band maximum crosses the Fermi level and undergoes switching from a direct to indirect band gap. The mechanism behind the electromechanical response was determined through analysis of the structural deformations, charge density distribution and Bader partial charges. It was shown that injection of charge alters the Young’s Modulus of the PNTs, as hole injection weakens the structural integrity of the nanotube, allowing a greater electromechanical response, with PNT-15 showing the largest decrease in the Young’s Modulus of 15.34%. These findings show that 1D PNTs are promising materials for the development of nanoelectromechanical actuators which could be used for drug delivery, energy harvesting or similar applications.

Preparation and properties of azobenzene liquid crystal networks with stable and reversible deformation, self-healing characteristics

Photo-responsive liquid crystal networks (LCNs) can respond to optical stimuli and show significant macroscopic reversible deformation, which is promising for applications in flexible actuators, artificial muscles, and other fields. To date, most of the photo-responsive LCNs have been constructed using azobenzene as photo-responsive group. However, how to prepare azobenzene-LCNs (azo-LCNs) with stable deformation remains a challenge. In this article, we provide a new strategy for preparing azo-LCNs with stable and reversible deformation. We copolymerize azobenzene monomer M6AZ1, commercially butyl acrylate monomer BA, and dynamic covalent cross-linker monomer M6HABI to obtain azo-LCNs. When irradiated with 365 nm light, the uniaxially oriented LCNs fibers undergo bending towards light attributed to the photo-isomerization of azobenzene, and the bending angle increases with azobenzene component content in LCNs. With the highly reversible process of dynamic C–N bonds dissociation-reconjugation, the rearrangement and regeneration of covalent crosslinked network can be realized. Hence, the newly formed cross-linked network locks the deformation of azo-LCNs, and this deformation can be maintained for more than 90 days. When heated, the deformation can return to the original state. In addition, the azo-LCNs also exhibit self-healing and reprocessing properties.

Dielectric Elastomer Actuators with Enhanced Durability by Introducing a Reservoir Layer.

A Dielectric Elastomer Actuator (DEA) consists of electrodes with a dielectric layer between them. By controlling the design of the electrodes, voltage, and frequency, the operating range and speed of the DEA can be adjusted. These DEAs find applications in biomimetic robots, artificial muscles, and similar fields. When voltage is applied to the DEA, the dielectric layer undergoes compression and expansion due to electrostatic forces, which can lead to electrical breakdown. This phenomenon is closely related to the performance and lifespan of the DEA. To enhance stability and improve dielectric properties, a DEA Reservoir layer is introduced. Here, stability refers to the ability of the DEA to perform its functions even as the applied voltage increases. The Reservoir layer delays electrical breakdown and enhances stability due to its enhanced thickness. The proposed DEA in this paper is composed of a Reservoir layer and electrode layer. The Reservoir layer is placed between the electrode layers and is independently configured, not subjected to applied voltage like the electrode layers. The performance of the DEA was evaluated by varying the number of polymer layers in the Reservoir and electrode designs. Introducing the Reservoir layer improved the dielectric properties of the DEA and delayed electrical breakdown. Increasing the dielectric constant through the DEA Reservoir can enhance output characteristics in response to electrical signals. This approach can be utilized in various applications in wearable devices, artificial muscles, and other fields.

Programming Motion into Materials Using Electricity-Driven Liquid Crystal Elastomer Actuators.

As thermally driven smart materials capable of large reversible deformations, liquid crystal elastomers (LCEs) have great potential for applications in bionic soft robots, artificial muscles, controllable actuators, and flexible sensors due to their ability to program controllable motion into materials. In this article, we introduce conductive LCE actuators using a liquid metal electrothermal layer and a polyethylene terephthalate substrate. Our LCE actuators can be stimulated at low currents from 2 to 4 A and produce a maximum work density of 9.4 . We illustrate the potential applications of this system by designing a palm-activated artificial muscle gripper, which can be used to grasp soft objects ranging from 5 to 55 mm in size, and even ring-shaped workpieces with precise external or internal support. Furthermore, inspired by the movement of fruit fly larvae, we designed a new soft robot capable of bioinspired crawling and turning by inducing anisotropic friction with an asymmetric design. Finally, we illustrate advanced motional control by designing an autonomously rotating wheel based on the asymmetric contraction of its spokes. To assist in the production of autonomously moving robots, we provide a thorough characterization of its motion dynamics.

Effective On-Line Performance Modulation and Efficient Continuous Preparation of Ultra-Long Twisted and Coiled Polymer Artificial Muscles for Engineering Applications.

Artificial muscle is a kind of thread-like actuator that can produce contractile strain, generate force, and output mechanical work under external stimulations to imitate the functions and achieve the performances of biological muscles. It can be used to actuate various bionic soft robots and has broad application prospects. The electrically controlled twisted and coiled polymer (TCP) artificial muscles, with the advantages of high power density, large stroke and low driving voltage, while also being electrolyte free, are the most practical. However, the relationship between the muscle performances and its preparation parameters is not very clear yet, and the complete procedure of designing and preparing TCP muscles according to actual needs has not been established. Besides, current preparation approaches are very time-consuming and cannot make ultra-long TCP muscles. These problems greatly limit wide applications of TCP artificial muscles. In this study, we studied and built the relationship between the actuating performances of TCP muscles and their preparation parameters, so that suitable TCP muscles can be easily designed and prepared according to actual requirements. Moreover, an efficient preparation method integrating one-step annealing technique has been developed to realize on-line performance modulation and continuous fabrication of ultra-long TCP muscles. By graphically assembling long muscles on heat-resist films, we designed and produced a series of fancy soft robots (butterfly, flower, starfish), which can perform various bionic movements and complete specific tasks. This work has achieved efficient on-demand preparation and large-scale assembly of ultra-long TCP muscles, laying solid foundations for their engineering applications in soft robot field.

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 ±4V 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.

Cover Picture

Spider silk is made of natural protein polymers and is known for its tradeoff between mutually exclusive strength and toughness. To simulate the spinning process of spiders in nature, the strong fibers are prepared by gene protein biotechnology and chemical synthesis. With their excellent properties, artificial spider silks can realize potential applications in biomedicine, smart wearables, artificial muscles and sensors, and aerospace. More details are discussed in the article by Liu et al. on page 3401—3418.image

Liquid crystal elastomer film actuators with anti-strain robustness

Thermotropic monodomain liquid crystal elastomers (mLCEs) exhibit reversible contractile actuation properties along the collimated direction of mesogens when stimulated by external temperature, similar to those of biological skeletal muscle, and have great potential applications in flexible actuators and artificial muscles. However, the existence of flexible chain segments causes mLCEs to elongate under a small load, making calibration under different loads necessary when they are used as an actuator. In this study, a physical crosslinking point between flexible segments is provided through the introduction of designed long-chain molecules (LHS) containing amide bonds on the main chain of LCEs. The 5% LHS-mLCEs with enhanced anti-strain robustness achieved essentially constant original length over a range of loads (0–0.25 MPa) and maintained a reversible contraction ratio of 39.4%, comparable to biological skeletal muscle. Furthermore, a nonreconfigurable topological structure of a metal electrothermal layer with lossless compression properties was designed and prepared on the surface of mLCEs. The film actuators exhibit higher frequency oscillatory actuation (0.1 Hz 9.2%; 0.2 Hz 6.3%; 0.5 Hz 2%) relative to previously reported actuators, benefitting from the optimized heat dissipation structure of LCEs/Ag/air. This study provides valuable references and ideas for applying thermotropic mLCEs in practice.

Research Progress on Spider‐Inspired Tough Fibers†

Comprehensive SummarySpider silk has attracted increasing attention due to its fascinating combination of ultra‐high tenacity high strength, and excellent elasticity. Based on the fundamental biological studies on spider silk, significant research efforts have been devoted to biotechnology and chemical synthesis to mimic or even exceed the properties of natural spider silk fibers. Moreover, the natural spider silk fiber has been simulated with the burgeoning development of numerous spinning technologies, including wet spinning, dry spinning, electrostatic spinning, and microfluidic spinning, which continuously help to optimize the properties of synthetic spider silk. The unique characteristics of natural spider silk include high refraction transmission, heat resistance, antimicrobial properties, biocompatibility, and super shrinking. Biconical recreation of spider silk with special features and extraordinary capabilities demonstrates potential applications in biomedicine, smart wearables, artificial muscles and sensors, aerospace and other domains.

Tough and Photo-Plastic Liquid Crystal Elastomer with a 2-Fold Dynamic Linker for Artificial Muscles.

Liquid crystal elastomers (LCEs) have been optimized by combining cross-linkers and dynamic bonds to achieve a reversible actuation behavior comparable to living skeletal muscles. In this study, one unique type of segment with 2-fold dynamic properties was introduced into LCEs, which offered not only dynamic diselenide covalent bonds for thermo-/photoplasticity but also H-bond arrays for dynamic cross-linking and mechanical robustness. Besides self-healing, self-welding, and recyclability, the LCEs were reprogrammable with elevated temperature or intensive visible light irradiation. The resultant LCEs gave an actuation blocking stress of 1.96 MPa and an elastic modulus of 14.4 MPa at 80 °C. The actuation work capacity reached 135.2 kJ m-3. When incorporating the Joule electrode or photothermal materials, the LCEs could be programmed as the electricity-driven and photothermal artificial muscles and thereby promised the application both as a biomimetic artificial hand and as an energy collector from sunlight. Thus, the 2-fold dynamic LCEs offered the pathway of enabling the reversible actuation behavior comparable to living skeletal muscles and promising applications in sustainable actuators, artificial muscles, and soft robots.

Finite element modeling of programmable liquid crystal elastomer plates and its application in text display

AbstractLiquid crystal elastomers (LCEs) have wide and potential applications in artificial muscles, soft robots, biomedical devices, and so on. On the application of LCE plate in text display, different from others' work where specially shaped ultraviolet (UV) lighting is used, in this paper, the programmable deformation of LCE plates with arbitrary liquid crystal orientation subjected to uniform light illumination is investigated. A photomechanical coupling finite element method (FEM) is developed, in which the impact of the shear strains induced by light is taken into account. Inspired by the FEM, the LCE plate is divided into finite small regions. By altering the liquid crystal orientation of each region in the LCE plate, the ability to generate 24 letters and 10 numbers through light‐induced deformations is demonstrated. Matrixes of the liquid crystal orientation corresponding to each letter and number are provided. This work indicates that it is possible for blind people to read text information by touching the deformed LCE plate.

Photocontrollable Elongation Actuation of Liquid Crystal Elastomer Films with Well-Defined Crease Structures.

Although liquid crystal elastomers (LCEs) have demonstrated various applications in artificial muscles and soft robotics, their inherent flexibility and orientation-dependent forces limit their functions. For instance, LCEs can sustain a high actuation force when they contract but cannot elongate to drive loads with large displacements. In this study, it is demonstrated that photocontrollable elongation actuation with a large strain can be achieved in polydomain LCEs by programming the crease structures in a well-defined order to couple the actuation forces. Efficient photoactuation without overheating-induced damage to the materials is favored, based on the well-designed photosensitive molecular switch crosslinker via the synergy of photochemical and photothermal effects. The LCE actuator can jack up heavy loads, elongate freely, and contract back to manipulate distant objects. Theoretical analysis based on a finite element simulation of the deformation energy during the actuation process reveals a trade-off between the abilities of jacking-up and withstanding load. More importantly, this study simplifies the design of a single material with functions inherent only in other soft robotic devices based on the assembly of multiplemodules, thus providing a design strategy for surpassing instinctive properties of conventional soft materials to expand the functions of soft robotics.

Actuation performances and microscopic mechanism of a stimuli-responsive artificial muscle based on the chelation of sodium alginate with CaI2·4H2O

With the popularization of energy conservation and environmental protection, a stimuli-responsive artificial muscle (SRAM) prepared by green process of the chelation of sodium alginate (SA) with calcium iodide tetrahydrate (CaI2·4H2O), provides new ideas and prospects for the development and application of smart artificial muscles. In this paper, actuation performances including force density (Fρ , mN/g), working life (τ, s), rise time (τ r, s) and response speed (V F, mN/g·s) of the SRAM with different concentrations of CaI2·4H2O was researched through the test platform of electrically bending force. Furthermore, the microscopic mechanism on mechanical and electrochemical characteristics of the SRAM was analyzed and verified comprehensively by microstructure, energy dispersive spectroscopy, ion channel, infrared spectroscopy and diffraction of x-rays. The experimental results were demonstrated that when the concentration of CaI2·4H2O was within the range of 1.5 g l−1–2.0 g l−1, the SRAM achieved optimal modification, where at 1.5 g l−1 CaI2·4H2O, its specific capacitance and τ were both the maximum values of 93.7 mF g−1 and 1080 s, but internal resistance was the minimum of 3.09 Ω; at 2.0 g l−1 CaI2·4H2O, the Fρ, V F, elastic modulus, yield strength and ion channel of the SRAM reached the largest values of 22.807 mN g−1, 0.1046 mN g−1·s, 20 MPa, 0.18091 MPa and 60.2%, respectively, but τ r obtained the lest of 98 s. Because after being chelated by CaI2·4H2O, the α-L-guluronic blocks in SA molecular chains coagulated with Ca2+ ions, making the synergy between molecular chains of the SRAM stronger, thus forming a three-dimensional ‘egg box’ network structure of polymeric hydrogel.

Fuel-Driven Redox Reactions in Electrolyte-Free Polymer Actuators for Soft Robotics.

Polymers that undergo shape changes in response to external stimuli can serve as actuators and offer significant potential in a variety of technologies, including biomimetic artificial muscles and soft robotics. Current polymer artificial muscles possess major challenges for various applications as they often require extreme and non-practical actuation conditions. Thus, exploring actuators with new or underutilized stimuli may broaden the application of polymer-based artificial muscles. Here, we introduce an all-solid fuel-powered actuator that contracts and expands when exposed to H2 and O2 via redox reactions. This actuator demonstrates a fully reversible actuation magnitude of up to 3.8% and achieves a work capacity of 120 J/kg. Unlike traditional chemical actuators, our actuator eliminates the need for electrolytes, electrodes, and the application of external voltage. Moreover, it offers athermal actuation by avoiding the drawbacks of thermal actuators. Remarkably, the actuator maintains its actuated position under load when not stimulated, without consuming energy (i.e., catch state). These fuel-powered fiber actuators were embedded in a soft humanoid hand to demonstrate finger-bending motions. In terms of two main actuation metrics, stress-free contraction strain and blocking stress, the presented artificial muscle outperforms reported polymer redox actuators. The fuel-powered actuator developed in this work creates new avenues for the application of redox polymers in soft robotics and artificial muscles.

Structure-induced Intelligence of Liquid Crystal Elastomers.

Liquid crystal elastomers (LCEs) are active soft matter-based materials with strong stimulus responsiveness and reversible, large-shape morphing capabilities. LCEs have demonstrated broad and growing applications in soft robotics, wearable devices, artificial muscles, and optical machines. The actuation intelligence and advanced functionality of LCEs depend on the smartness and properties of structures. In this review, we discuss recent advances in structure-induced intelligence of LCEs, specifically the integration of structural properties with the alignment and processing of LCEs. The structural design principles for three categories consisting of common structures (film, fiber, and tubule), smart structures (origami, kirigami, mechanical metamaterial, topology, and topography), and complex structures (monolithic and integrated) are presented. Various alignment controls of LCEs, including mechanical, surface, field-assisted, and shear alignment, are capable of inducing structural properties. The coupling and collaboration mechanisms of the LCE structures and the generated functions are discussed. The review concludes with perspectives on current challenges and emerging opportunities.

Polyurethane Shape Memory Polymer/pH-Responsive Hydrogel Hybrid for Bi-Function Synergistic Actuations

Stimuli-responsive actuating hydrogels response to the external stimulus with complex deformation behaviors based on the programmable anisotropic structure design are one of the most important smart soft materials, which have great potential applications in artificial muscles, smart values, and mini-robots. However, the anisotropic structure of one actuating hydrogel can only be programmed one time, which can only provide single actuating performance, and subsequently, has severely limited their further applications. Herein, we have explored a novel SMP/hydrogel hybrid actuator through combining polyurethane shape memory polymer (PU SMP) layer and pH-responsive polyacrylic-acid (PAA) hydrogel layer by a napkin with UV-adhesive. Owing to both the super-hydrophilicity and super-lipophilicity of the cellulose-fiber based napkin, the SMP and the hydrogel can be bonded firmly by the UV-adhesive in the napkin. More importantly, this bilayer hybrid 2D sheet can be programmed by designing a different temporary shape in heat water which can be fixed easily in cool water to achieve various fixed shapes. This hybrid with a fixed temporary shape can achieve complex actuating performance based on the bi-functional synergy of temperature-triggered SMP and pH-responsive hydrogel. The relatively high modulus PU SMP achieved high to 87.19% and 88.92% shape-fixing ratio, respectively, correspond to bending and folding shapes. The hybrid actuator can actuate with the 25.71 °/min actuating speed. Most importantly, one SMP/hydrogel bi-layer hybrid sheet was repeatedly programmed at least nine times in our research to fix various temporary 1D, 2D and 3D shapes, including bending, folding and spiraling shapes. As a result, only one SMP/hydrogel hybrid can provide various complex stimuli-responsive actuations, including the reversable bending-straightening, spiraling-unspiraling. A few of the intelligent devices have been designed to simulate the movement of the natural organisms, such as bio-mimetic "paw", "pangolin" and "octopus". This work has developed a new SMP/hydrogel hybrid with excellent multi-repeatable (≥9 times) programmability for high-level complex actuations, including the 1D to 2D bending and the 2D to 3D spiraling actuations, which also provides a new strategy to design other new soft intelligent materials and systems.

AgNWs/Ti3C2Tx MXene-based multi-responsive actuators for programmable smart devices

Thermal bimorph actuators have important applications in artificial muscles, switches, and microsensors for robotics and biomimetic devices. To broaden the adaptability of environments, it is essential to fabricate thermal actuators with multiple stimulus responses, including electricity, near-infrared (NIR) light, and ultraviolet (UV) light, with fast and large deformations. Herein, a thermal actuator with programmable stimuli response was designed by combining silver nanowires (AgNWs) with Ti3C2Tx MXene. Due to the synergistic effect of AgNWs with high plasmonic properties and electrical conductivity as well as Ti3C2Tx MXene with good flexibility and photothermal properties, the obtained actuator can roll up for 2200° at a low driving voltage (2 V) and shows a fast response speed of 0.4 s for a bending angle of 360° under NIR light. More remarkably, the obtained thermal actuator can curl 360° in 1.2s under UV light which has never been reported before. We believe that such thermal actuators are attractive for a wide range of innovative technologies involving soft-bodied robots, intelligent grippers, bionic worms, and information cryptography.

Ultrarobust subzero healable materials enabled by polyphenol nano-assemblies

Bio-inspired self-healing materials hold great promise for applications in wearable electronics, artificial muscles and soft robots, etc. However, self-healing at subzero temperatures remains a great challenge because the reconstruction of interactions will experience resistance of the frozen segments. Here, we present an ultrarobust subzero healable glassy polymer by incorporating polyphenol nano-assemblies with a large number of end groups into polymerizable deep eutectic solvent elastomers. The combination of multiple dynamic bonds and rapid secondary relaxations with low activation energy barrier provides a promising method to overcome the limited self-healing ability of glassy polymers, which can rarely be achieved by conventional dynamic cross-linking. The resulted material exhibits remarkably improved adhesion force at low temperature (promotes 30 times), excellent mechanical properties (30.6 MPa) and desired subzero healing efficiencies (85.7% at −20 °C). We further demonstrated that the material also possesses reliable cryogenic strain-sensing and functional-healing ability. This work provides a viable approach to fabricate ultrarobust subzero healable glassy polymers that are applicable for winter sports wearable devices, subzero temperature-suitable robots and artificial muscles.