Insect-scale jumping robots enabled by a dynamic buckling cascade.

Millions of years of evolution have allowed animals to develop unusual locomotion capabilities. A striking example is the legless-jumping of click beetles and trap-jaw ants, which jump more than 10 times their body length. Their delicate musculoskeletal system amplifies their muscles' power. It is challenging to engineer insect-scale jumpers that use onboard actuators for both elastic energy storage and power amplification. Typical jumpers require a combination of at least two actuator mechanisms for elastic energy storage and jump triggering, leading to complex designs having many parts. Here, we report the new concept of dynamic buckling cascading, in which a single unidirectional actuation stroke drives an elastic beam through a sequence of energy-storing buckling modes automatically followed by spontaneous impulsive snapping at a critical triggering threshold. Integrating this cascade in a robot enables jumping with unidirectional muscles and power amplification (JUMPA). These JUMPA systems use a single lightweight mechanism for energy storage and release with a mass of 1.6 g and 2 cm length and jump up to 0.9 m, 40 times their body length. They jump repeatedly by reengaging the latch and using coiled artificial muscles to restore elastic energy. The robots reach their performance limits guided by theoretical analysis of snap-through and momentum exchange during ground collision. These jumpers reach the energy densities typical of the best macroscale jumping robots, while also matching the rapid escape times of jumping insects, thus demonstrating the path toward future applications including proximity sensing, inspection, and search and rescue.

Soft Actuator with Biomass Porous Electrode: A Strategy for Lowering Voltage and Enhancing Durability.

Electroactive artificial muscles with deformability have attracted widespread interest in the field of soft robotics. However, the design of artificial muscles with low-driven voltage and operational durability remains challenging. Herein, novel biomass porous carbon (BPC) electrodes are proposed. The nanoporous BPC enables the electrode to provide exposed active surfaces for charge transfer and unimpeded channels for ion migration, thus decreasing the driving voltage, enhancing time durability, and maintaining the actuation performances simultaneously. The proposed actuator exhibits a high displacement of 13.6 mm (bending strain of 0.54%) under 0.5 V and long-term durability of 99.3% retention after 550,000 cycles (∼13 days) without breaks. Further, the actuators are integrated to perform soft touch on a smartphone and demonstrated as bioinspired robots, including a bionic butterfly and a crawling robot (moving speed = 0.08 BL s-1). This strategy provides new insight into the design and fabrication of high-performance electroactive soft actuators with great application potential.

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

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

Reversible Electrochemical Swelling of Straight Carbon Nanotube Yarns for High-Performance Linear Actuation.

Coiled artificial muscle yarns outperform their straight counterparts in contractile strokes. However, challenges persist in the fabrication complexity and the susceptibility of the coiled yarns to becoming stuck by surrounding objects during contraction and recovery. Additionally, torsional stability remains a concern. In this study, it is reported that straight carbon nanotube (CNT) yarns when driven by a low-voltage electrochemical approach, can achieve a contractile stroke that surpasses even NiTi shape memory alloy fibers. The key lies in the suitable match between a yarn consisting of randomly aligned CNTs and the reversible and substantial electrochemical swelling induced by solvated ions. Wrinkled structures are formed on the surface of the CNT yarn to adapt to the swelling process. This not only assures torsional stability but also enhances the surface area for improved electrode-electrolyte interaction during electrochemical actuation. Remarkably, the CNT artificial muscle yarn generates a contractile stroke of 8.8% and an isometric stress of 7.5 MPa under 2.5 V actuation voltages, demonstrating its potential for applications requiring low energy consumption while maintaining high operational efficiency. This study highlights the crucial impact of CNT orientation on the effectiveness of electrochemically-driven artificial muscles, signaling new possibilities in smart material and biomechanical system development.

Fiber Actuators Based on Reversible Thermal Responsive Liquid Crystal Elastomer.

Soft actuators inspired by the movement of organisms have attracted extensive attention in the fields of soft robotics, electronic skin, artificial intelligence, and healthcare due to their excellent adaptability and operational safety. Liquid crystal elastomer fiber actuators (LCEFAs) are considered as one of the most promising soft actuators since they can provide reversible linear motion and are easily integrated or woven into complex structures to perform pre-programmed movements such as stretching, rotating, bending, and expanding. The research on LCEFAs mainly focuses on controllable preparation, structural design, and functional applications. This review, for the first time, provides a comprehensive and systematic review of recent advances in this important field by focusing on reversible thermal response LCEFAs. First, the thermal driving mechanism, and direct and indirect heating strategies of LCEFAs are systematically summarized and analyzed. Then, the fabrication methods and functional applications of LCEFAs are summarized and discussed. Finally, the challenges and technical difficulties that may hinder the performance improvement and large-scale production of LCEFAs are proposed, and the development opportunities of LCEFAs are prospected.

Bio-Inspired Soft-Rigid Hybrid Smart Artificial Muscle Based on Liquid Crystal Elastomer and Helical Metal Wire.

Artificial muscles are of significant value in robotic applications. Rigid artificial muscles possess a strong load-bearing capacity, while their deformation is small; soft artificial muscles can be shifted to a large degree; however, their load-bearing capacity is weak. Furthermore, artificial muscles are generally controlled in an open loop due to a lack of deformation-related feedback. Human arms include muscles, bones, and nerves, which ingeniously coordinate the actuation, load-bearing, and sensory systems. Inspired by this, a soft-rigid hybrid smart artificial muscle (SRH-SAM) based on liquid crystal elastomer (LCE) and helical metal wire is proposed. The thermotropic responsiveness of the LCE is adopted for large reversible deformation, and the helical metal wire is used to fulfill high bearing capacity and electric heating function requirements. During actuation, the helical metal wire's resistance changes with the LCE's electrothermal deformation, thereby achieving deformation-sensing characteristics. Based on the proposed SRH-SAM, a reconfigurable blazed grating plane and the effective switch between attachment and detachment in bionic dry adhesion are accomplished. The SRH-SAM opens a new avenue for designing smart artificial muscles and can promote the development of artificial muscle-based devices.

High Performance and Multifunction Moisture-Driven Yin-Yang-Interface Actuators Derived from Polyacrylamide Hydrogel.

High actuation performance of a moisture actuator highly depends on the presence of a large property difference between the two layers, which may cause interfacial delamination. Improving interfacial adhesion strength while increasing the difference between the layers is a challenge. In this study, a moisture-driven tri-layer actuator with a Yin-Yang-interface (YYI) design is investigated in which a moisture-responsive polyacrylamide (PAM) hydrogel layer (Yang) is combined with a moisture-inert polyethylene terephthalate (PET) layer (Yin) using an interfacial poly(2-ethylhexyl acrylate) (PEA) adhesion layer. Fast and large reversible bending, oscillation, and programmable morphing motions in response to moisture are realized. The response time, bending curvature, and response speed normalized by thickness are among the best compared with those of previously reported moisture-driven actuators. The excellent actuation performance of the actuator has potential multifunctional applications in moisture-controlled switches, mechanical grippers, and crawling and jumping motions. The Yin-Yang-interface design proposed in this work provides a new design strategy for high-performance intelligent materials and devices.

The Role of Crosslinker Molecular Structure on Mechanical and Light-Actuation Properties in Liquid Crystalline Networks.

Phase behavior modulation of liquid crystalline molecules can be addressed by structural modification at molecular level. Starting from a rigid rod-like core reduction of the symmetry or increase of the steric hindrance by different substituents generally reduces the clearing temperature. Similar approaches can be explored to modulate the properties of liquid crystalline networks (LCNs)-shape-changing materials employed as actuators in many fields. Depending on the application, the polymer properties have to be adjusted in terms of force developed under stimuli, kinetics of actuation, elasticity, and resistance to specific loads. In this work, the crosslinker modification at molecular level is explored towards the optimization of LCN properties as light-responsive artificial muscles. The synthesis and characterization of photopolymerizable crosslinkers, bearing different lateral groups on the aromatic core is reported. Such molecules are able to strongly modulate the material mechanical properties, such as kinetics and maximum tension under light actuation, opening up to interesting materials for biomedical applications.

Muscular Damping Distribution Strategy for Bio-Inspired, Soft Motion Control at Variable Precision.

Bio-inspired and compliant control approaches have been studied by roboticists for decades to achieve more natural robot motion. Independent of this, medical and biological researchers have discovered a wide variety of muscular properties and higher-level motion characteristics. Although both disciplines strive to better understand natural motion and muscle coordination, they have yet to meet. This work introduces a novel robotic control strategy that bridges the gap between these distinct areas. By applying biological characteristics to electrical series elastic actuators, we developed a simple yet efficient distributed damping control strategy. The presented control covers the entire robotic drive train, from abstract whole-body commands to the applied current. The functionality of this control is biologically motivated, theoretically discussed, and finally evaluated through experiments on the bipedal robot Carl. Together, these results demonstrate that the proposed strategy fulfills all requirements that are necessary to continue developing more complex robotic tasks based on this novel muscular control philosophy.

Simultaneous Sensing and Actuating Capabilities of a Triple-Layer Biomimetic Muscle for Soft Robotics.

This work presents the fabrication and characterization of a triple-layered biomimetic muscle constituted by polypyrrole (PPy)-dodecylbenzenesulfonate (DBS)/adhesive tape/PPy-DBS demonstrating simultaneous sensing and actuation capabilities. The muscle was controlled by a neurobiologically inspired cortical neural network sending agonist and antagonist signals to the conducting polymeric layers. Experiments consisted of controlled voluntary movements of the free end of the muscle at angles of ±20°, ±30°, and ±40° while monitoring the muscle's potential response. Results show the muscle's potential varies linearly with applied current amplitude during actuation, enabling current sensing. A linear dependence between muscle potential and temperature enabled temperature sensing. Electrolyte concentration changes also induced exponential variations in the muscle's potential, allowing for concentration sensing. Additionally, the influence of the electric current density on the angular velocity, the electric charge density, and the desired angle was studied. Overall, the conducting polymer-based soft biomimetic muscle replicates properties of natural muscles, permitting simultaneous motion control, current, temperature, and concentration sensing. The integrated neural control system exhibits key features of biological motion regulation. This muscle actuator with its integrated sensing and control represents an advance for soft robotics, prosthetics, and biomedical devices requiring biomimetic multifunctionality.

Fast Light-Driven Motion of Polydopamine Nanomembranes.

The actuation of micro- and nanostructures controlled by external stimuli remains one of the exciting challenges in nanotechnology due to the wealth of fundamental questions and potential applications in energy harvesting, robotics, sensing, biomedicine, and tunable metamaterials. Photoactuation utilizes the conversion of light into motion through reversible chemical and physical processes and enables remote and spatiotemporal control of the actuation. Here, we report a fast light-to-motion conversion in few-nanometer thick bare polydopamine (PDA) membranes stimulated by visible light. Light-induced heating of PDA leads to desorption of water molecules and contraction of membranes in less than 140 µs. Switching off the light leads to a spontaneous expansion in less than 20 ms due to heat dissipation and water adsorption. Our findings demonstrate that pristine PDA membranes are multiresponsive materials that can be harnessed as robust building blocks for soft, micro-, and nanoscale actuators stimulated by light, temperature, and moisture level.

One-Step Manufacturing of Supramolecular Liquid-Crystal Elastomers by Stress-Induced Alignment and Hydrogen Bond Exchange.

Liquid-crystal elastomers (LCEs) capable of performing large and reversible deformation in response to an external stimulus are an important class of soft actuators. However, their manufacturing process typically involves a multistep approach that requires harsh conditions. For the very first time, LCEs with customized geometries that can be manufactured by a rapid one-step approach at room temperature are developed. The LCEs are hydrogen bond (H-bond) crosslinked main chain polymers comprising flexible short side chains. Applying a stretching/shear force to the LCE can simultaneously induce mesogen alignment and H-bond exchange, allowing for the formation of well-aligned LCE networks stabilized by H-bonds. Based on this working principle, soft actuators in fibers and 2D/3D objects can be manufactured by mechanical stretching or melt extrusion within a short time (e.g. <1 min). These actuators can perform reversible macroscopic motions with large, controlled deformations up to 38 %. The dynamic nature of H-bonds also provides the actuators with reprocessability and reprogrammability. Thus, this work opens the way for the one-step and custom manufacturing of soft actuators.

Realizing the potential of dielectric elastomer artificial muscles.

Soft robotics represents a new set of technologies aimed at operating in natural environments and near the human body. To interact with their environment, soft robots require artificial muscles to actuate movement. These artificial muscles need to be as strong, fast, and robust as their natural counterparts. Dielectric elastomer actuators (DEAs) are promising soft transducers, but typically exhibit low output forces and low energy densities when used without rigid supports. Here, we report a soft composite DEA made of strain-stiffening elastomers and carbon nanotube electrodes, which demonstrates a peak energy density of 19.8 J/kg. The result is close to the upper limit for natural muscle (0.4-40 J/kg), making these DEAs the highest-performance electrically driven soft artificial muscles demonstrated to date. To obtain high forces and displacements, we used low-density, ultrathin carbon nanotube electrodes which can sustain applied electric fields upward of 100 V/µm without suffering from dielectric breakdown. Potential applications include prosthetics, surgical robots, and wearable devices, as well as soft robots capable of locomotion and manipulation in natural or human-centric environments.

Strong and Robust Electrochemical Artificial Muscles by Ionic-Liquid-in-Nanofiber-Sheathed Carbon Nanotube Yarns.

To address the lack of a suitable electrolyte that supports the stable operation of the electrochemical yarn muscles in air, an ionic-liquid-in-nanofibers sheathed carbon nanotube (CNT) yarn muscle is prepared. The nanofibers serve as a separator to avoid the short-circuiting of the yarns and a reservoir for ionic liquid. The ionic-liquid-in-nanofiber-sheathed yarn muscles are strong, providing an isometric stress of 10.8 MPa (about 31 times the skeletal muscles). The yarn muscles are highly robust, which can reversibly contract stably at such conditions as being knotted, wide-range humidity (30 to 90 RH%) and temperature (25 to 70 °C), and long-term cycling and storage in air. By utilizing the accumulated isometric stress, the yarn muscles achieve a high contraction rate of 36.3% s-1 . The yarn muscles are tightly bundled to lift heavy weights and grasp objects. These unique features can make the strong and robust yarn muscles as a desirable actuation component for robotic devices.

Ultrafast, High-Contractile Electrothermal-Driven Liquid Crystal Elastomer Fibers towards Artificial Muscles.

Liquid crystal elastomer (LCE) fibers are capable of large and reversible deformations, making them an ideal artificial muscle. However, limited to stimulating source and structural design, current LCE fibers have not yet achieved both large contraction ratio and fast contraction rate to perform the intense motion. In this work, electrothermal-responsive liquid metal (LM) containing LCE (LM-LCE) fibers is reported. By introducing flexible liquid metal, LM-LCE fibers retain deformability with a large contraction ratio similar to that of pure LCE fibers and are endowed with electrical responsiveness. Applying precisely controlled electrical stimulation, the contraction ratio and rate of LM-LCE fibers can be programmed by adjusting voltage value and pulse time. Under electrical stimulation at 1.25 V cm-1 , 0.1 s, LM-LCE fibers can produce over 40% contraction ratio at an ultrafast contraction rate of up to 280% s-1 . Furthermore, LM-LCE fibers mimic human triceps muscle and can conduct precise ball shooting. LM-LCE fibers with excellent contraction ratio and rate extend their functionality as artificial muscles to perform intense movements and are expected to enrich the challenging applications of soft robots.

Matching Magnetic Heating and Thermal Actuation for Sequential Coupling in Hybrid Composites by Design.

Sequentially coupling two material functions requires matching the output from the first with the input of the second function. Here, magnetic heating controls thermal actuation of a hybrid composite in a challenging system environment causing an elevated level of heat loss. The concept is a hierarchical design consisting of an inner actuator of nanocomposite material, which can be remotely heated by exposure to an alternating magnetic field (AMF) and outer layers of a porous composite system with a closed pore morphology. These porous layers act as heat insulators and as barriers to the surrounding water. By exposure to the AMF, a local bulk temperature of 71 °C enables the magnetic actuation of the device, while the temperature of the surrounding water is kept below 50 °C. Interestingly, the heat loss during magnetic heating leads to an increase of the water phase (small volume) temperature. The temperature increase is able to sequentially trigger an adjacent thermal actuator attached to the actuator composite. In this way it could be demonstrated how the AMF is able to initiate two kinds of independent actuations, which might be interesting for robotics operating in aqueous environments.

A New Adaptive Synergetic Control Design for Single Link Robot Arm Actuated by Pneumatic Muscles.

This paper suggests a new control design based on the concept of Synergetic Control theory for controlling a one-link robot arm actuated by Pneumatic artificial muscles (PAMs) in opposing bicep/tricep positions. The synergetic control design is first established based on known system parameters. However, in real PAM-actuated systems, the uncertainties are inherited features in their parameters and hence an adaptive synergetic control algorithm is proposed and synthesized for a PAM-actuated robot arm subjected to perturbation in its parameters. The adaptive synergetic laws are developed to estimate the uncertainties and to guarantee the asymptotic stability of the adaptive synergetic controlled PAM-actuated system. The work has also presented an improvement in the performance of proposed synergetic controllers (classical and adaptive) by applying a modern optimization technique based on Particle Swarm Optimization (PSO) to tune their design parameters towards optimal dynamic performance. The effectiveness of the proposed classical and adaptive synergetic controllers has been verified via computer simulation and it has been shown that the adaptive controller could cope with uncertainties and keep the controlled system stable. The proposed optimal Adaptive Synergetic Controller (ASC) has been validated with a previous adaptive controller with the same robot structure and actuation, and it has been shown that the optimal ASC outperforms its opponent in terms of tracking speed and error.

New twist on artificial muscles.

Lightweight artificial muscle fibers that can match the large tensile stroke of natural muscles have been elusive. In particular, low stroke, limited cycle life, and inefficient energy conversion have combined with high cost and hysteretic performance to restrict practical use. In recent years, a new class of artificial muscles, based on highly twisted fibers, has emerged that can deliver more than 2,000 J/kg of specific work during muscle contraction, compared with just 40 J/kg for natural muscle. Thermally actuated muscles made from ordinary polymer fibers can deliver long-life, hysteresis-free tensile strokes of more than 30% and torsional actuation capable of spinning a paddle at speeds of more than 100,000 rpm. In this perspective, we explore the mechanisms and potential applications of present twisted fiber muscles and the future opportunities and challenges for developing twisted muscles having improved cycle rates, efficiencies, and functionality. We also demonstrate artificial muscle sewing threads and textiles and coiled structures that exhibit nearly unlimited actuation strokes. In addition to robotics and prosthetics, future applications include smart textiles that change breathability in response to temperature and moisture and window shutters that automatically open and close to conserve energy.

Shape Memory Alloy (SMA)-Based Microscale Actuators with 60% Deformation Rate and 1.6 kHz Actuation Speed.

Shape memory alloys (SMAs) are widely utilized as an actuation source in microscale devices, since they have a simple actuation mechanism and high-power density. However, they have limitations in terms of strain range and actuation speed. High-speed microscale SMA actuators are developed having diamond-shaped frame structures with a diameter of 25 µm. These structures allow for a large elongation range compared with bulk SMA materials, with the aid of spring-like behavior under tensile deformation. These actuators are validated in terms of their applicability as an artificial muscle in microscale by investigating their behavior under mechanical deformation and changes in thermal conditions. The shape memory effect is triggered by delivering thermal energy with a laser. The fast heating and cooling phenomenon caused by the scale effect allows high-speed actuation up to 1600 Hz. It is expected that the proposed actuators will contribute to the development of soft robots and biomedical devices.

Fabrication of Light-Triggered Soft Artificial Muscles via a Mixed-Matrix Membrane Strategy.

Artificial muscles triggered by light are of great importance, especially for the development of non-contact and remotely controlled materials. Common materials for synthesis of photoinduced artificial muscles typically rely on polymer-based photomechanical materials. Herein, we are able to prepare artificial muscles using a mixed-matrix membrane strategy to incorporate photomechanical molecular crystals with connective polymers (e.g. PVDF). The formed hybrid materials inherit not only the advantages of the photomechanical crystals, including faster light response, higher Young's modulus and ordered structure, but also the elastomer properties from polymers. This new type of artificial muscles demonstrates various muscle movements, including lifting objects, grasping objects, crawling and swimming, triggered by light irradiation. These results open a new direction to prepare light-driven artificial muscles based on molecular crystals.