Recent Development of Jumping Motions Based on Soft Actuators

Drawing inspiration from the jumping motions of living creatures in nature, jumping robots have emerged as a promising research field over the past few decades due to great application potential in interstellar exploration, military reconnaissance, and life rescue missions. Early reviews mainly focused on jumping robots made of lightweight and rigid materials with mechanical components, concentrating on jumping control and stability. Herein, attention is paid to the jumping mechanisms of soft actuators assembled from various soft smarting materials and powered by different stimulus sources. The challenges and prospects of soft jumping actuators are also discussed. It is hoped that this review will contribute to the further development of soft jumping actuators and broaden their practical applications.     

Rugged Soft Robots using Tough,Stretchable, and Self-Healable Adhesive Elastomers

Soft robots are susceptible to premature failure from physical damages incurred within dynamic environments. To address this, we report an elastomer with high toughness, room temperature self-healing, and strong adhesiveness, allowing both prevention of damages and recovery for soft robotics. By functionalizing polyurethane with hierarchical hydrogen bonds from ureido-4[1H]-pyrimidinone (UPy) and carboxyl groups, high toughness (74.85 MJ m⁻³), tensile strength (9.44 MPa), and strain (2340%) can be achieved. Furthermore, solvent-assisted self-healing at room temperature enables retention of high toughness (41.74 MJ m⁻³), tensile strength (5.57 MPa), and strain (1865%) within only 12 h. The elastomer possesses a high dielectric constant (≈9) that favors its utilization as a self-healing dielectric elastomer actuator (DEA) for soft robotics. Displaying high area strains of ≈31.4% and ≈19.3% after mechanical and electrical self-healing, respectively, the best performing self-healable DEA is achieved. With abundant hydrogen bonds, high adhesive strength without additional curing or heating is also realized. Having both actuation and adhesive properties, a “stick-on” strategy for the assembly of robust soft robots is realized, allowing soft robotic components to be easily reassembled or replaced upon severe damage. This study highlights the potential of soft robots with extreme ruggedness for different operating conditions.     

A New Strategy of Discretionarily Reconfigurable Actuators Based on Self-Healing Elastomers for Diverse Soft Robots

Actuators have shown great promise in many fields including soft robotics. Since reconfiguration allows actuators to change their actuation mode, it is considered a key characteristic for new-generation adaptive actuators. However, it remains a challenge to design simple and universal methods to fabricate actuators that can be reconfigured to allow diverse actuation modes. Here, a macroscopically discretionary healing-assembly strategy to fabricate reconfigurable soft actuators based on intrinsic self-healing poly(dimethylglyoxime-urethane) (PDOU) elastomers is developed. The PDOU elastomers with different degrees of crosslinking show different responsiveness to solvents, and are seamlessly healed. Crosslinked and non-crosslinked PDOU elastomers as building units are healing-assembled into actuators/robots with diverse actuation behaviors. Notably, the assembled actuators/robots are readily reprogrammed to exhibit multiple actuation modes by simply tailoring and reassembling without any external stimuli. This work paves a new, simple, powerful, and universal method to construct sophisticated soft robots.     

Soft Actuators and Robots that Are Resistant to Mechanical Damage

This paper characterizes the ability of soft pneumatic actuators and robots to resist mechanical insults that would irreversibly damage or destroy hard robotic systems—systems fabricated in metals and structural polymers, and actuated mechanically—of comparable sizes. The pneumatic networks that actuate these soft machines are formed by bonding two layers of elastomeric or polymeric materials that have different moduli on application of strain by pneumatic inflation; this difference in strain between an extensible top layer and an inextensible, strain‐limiting, bottom layer causes the pneumatic network to expand anisotropically. While all the soft machines described here are, to some extent, more resistant to damage by compressive forces, blunt impacts, and severe bending than most corresponding hard systems, the composition of the strain‐limiting layers confers on them very different tensile and compressive strengths.     

Miniature Soft Electromagnetic Actuators for Robotic Applications

Electromagnetic actuators (EMAs) serve the majority of motion control needs in fields ranging from industrial robotics to automotive systems and biomedical devices, due to their unmatched combination of speed, precision, force, and scalability. This paper describes the design and fabrication of miniature soft EMAs that operate based on the Lorentz force principle. The actuators are fabricated from silicone polymer, liquid metal (LM) alloy (eutectic gallium indium, EGaIn), and magnetic (NdFeB) powder. They are small, intrinsically deformable, and can be fabricated using simple techniques. The central elements of the actuators are fine, 3D helical coil conductors, which are used as electromagnetic inductors. The coils are formed from stretchable filaments that are filled with a LM alloy. To achieve high power densities, the filaments themselves may be fabricated from colloids of EGaIn microdroplets in a silicone polymer matrix, allowing them to dissipate heat and accommodate high currents, and thus high forces. Millimeter‐scale cylindrical actuators are demonstrated for linear high frequency motion and articulated devices for bending motion. These actuators are applied in a vibrotactile feedback display and in a miniature soft robotic gripper.     

Highly Conductive MXene/PEDOT:PSS-Integrated Poly(N-Isopropylacrylamide) Hydrogels for Bioinspired Somatosensory Soft Actuators

Sophisticated sensing and actuation capabilities of many living organisms in nature have inspired scientists to develop biomimetic somatosensory soft robots. Herein, the design and fabrication of homogeneous and highly conductive hydrogels for bioinspired somatosensory soft actuators are reported. The conductive hydrogels are synthesized by in situ copolymerization of conductive surface-functionalized MXene/Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) ink with thermoresponsive poly(N-isopropylacrylamide) hydrogels. The resulting hydrogels are found to exhibit high conductivity (11.76 S m⁻¹), strain sensitivity (GF of 9.93), broad working strain range (≈560% strain), and high stability after over 300 loading–unloading cycles at 100% strain. Importantly, shape-programmable somatosensory hydrogel actuators with rapid response, light-driven remote control, and self-sensing capability are developed by chemically integrating the conductive hydrogels with a structurally colored polymer. As the proof-of-concept illustration, structurally colored hydrogel actuators are applied for devising light-driven programmable shape-morphing of an artificial octopus, an artificial fish, and a soft gripper that can simultaneously monitor their own motions via real-time resistance variation. This work is expected to offer new insights into the design of advanced somatosensory materials with self-sensing and actuation capabilities, and pave an avenue for the development of soft-matter-based self-regulatory intelligence via built-in feedback control that is of paramount significance for intelligent soft robotics and automated machines.     

High‐Strain Peano‐HASEL Actuators

Soft robots are intrinsically safe for use near humans and adaptable when operated in unstructured environments, thereby offering capabilities beyond traditional robots based on rigid components. Soft actuators are key components of soft robots; recently developed hydraulically amplified self‐healing electrostatic (HASEL) actuators provide a versatile framework to create high‐speed actuators with excellent all‐around performance. Peano‐HASEL actuators linearly contract upon application of voltage, closely mimicking the behavior of muscle. Peano‐HASEL actuators, however, produce a maximum strain of ≈15%, while skeletal muscles achieve ≈20% on average. Here, a new type of HASEL is introduced, termed high‐strain Peano‐HASEL (HS‐Peano‐HASEL) actuator, that achieves linear contraction up to ≈24%. A wide range of performance metrics are investigated, and the maximum strain of multiunit HS‐Peano‐HASEL actuators is optimized by varying materials and geometry. Furthermore, an artificial circular muscle (ACM) based on the HS‐Peano‐HASEL acts as a tubular pump, resembling the primordial heart of an ascidian. Additionally, a strain‐amplifying pulley system is introduced to increase the maximum strain of an HS‐Peano‐HASEL to 42%. The muscle‐like maximum actuation strain and excellent demonstrated all‐around performance of HS‐Peano‐HASEL actuators make them promising candidates for use in artificial organs, life‐like robotic faces, and a variety of other robotic systems.     

Elastomeric Origami: Programmable Paper‐Elastomer Composites as Pneumatic Actuators

The development of soft pneumatic actuators based on composites consisting of elastomers with embedded sheet or fiber structures (e.g., paper or fabric) that are flexible but not extensible is described. On pneumatic inflation, these actuators move anisotropically, based on the motions accessible by their composite structures. They are inexpensive, simple to fabricate, light in weight, and easy to actuate. This class of structure is versatile: the same principles of design lead to actuators that respond to pressurization with a wide range of motions (bending, extension, contraction, twisting, and others). Paper, when used to introduce anisotropy into elastomers, can be readily folded into 3D structures following the principles of origami; these folded structures increase the stiffness and anisotropy of the elastomeric actuators, while being light in weight. These soft actuators can manipulate objects with moderate performance; for example, they can lift loads up to 120 times their weight. They can also be combined with other components, for example, electrical components, to increase their functionality.     

Liquid Crystal Soft Actuators and Robots toward Mixed Reality

Liquid crystals (LCs) are soft but smart materials that can adjust its chemical or physical properties in response to various external stimuli. Using these materials to construct soft actuators and robots, referred as LC actuators and robots, is expected to replace current machinery part, obtaining lighter and smaller equipment with adjustable and complex functions. Especially, combining these LC actuator and robots with existing virtual reality and augmented reality technologies will produce a new world of mixed reality (MR) with the visual, auditory, and somatosensory interaction. In this review, the recent work on responsive LC actuators and robots is introduced, emphasizing on their potentials in haptic use. By discussing their programmable control via suitable stimuli, the LC actuators and robots are summarized for mechanical outputs, environmental mimic, and fine-tuning of surface texture and roughness. It is anticipated that the continuous development on LC actuators and robots will accelerate the MR technology toward practical application.     

Zero-Power Shape Retention in Soft Pneumatic Actuators with Extensional and Bending Multistability

Power-free shape retention enables soft pneumatic robots to reduce energy cost and avoid unexpected collapse due to burst or puncture. Existing strategies for pneumatic actuation cannot attain motion locking for trajectories combining extension and bending, one of the most common modes of operation. Here, a design paradigm is introduced for soft pneumatic actuators to enable zero-power locking for shape retention in both extension and bending. The underpinning mechanism is the integration of a pneumatic transmitter and a multistable guider, which are programmed to interact for balanced load transfer, flexural and extension steering, and progressive snapping leading to state locking. Through theory, simulations, and experiments on proof-of-concept actuators, the existence of four distinct regimes of deformation is unveiled, where the constituents first interact during inflation to attain locking in extension and bending, and then cooperate under vacuum to enable fully reversible functionality. Finally, the design paradigm is demonstrated to realize a soft robotic arm capable to lock at desired curvature states at zero-power, and a gripper that safely operates with puncture resistance to grasp and hold objects of various shapes and consistency. The study promises further development for zero-power soft robots endowed with multiple deformation modes, sequential deployment, and tunable multistability.     

Biomimetic Color Changing Anisotropic Soft Actuators with Integrated Metal Nanowire Percolation Network Transparent Heaters for Soft Robotics

To add more functionalities and overcome the limitation in conventional soft robots, highly anisotropic soft actuators with color shifting function during actuation is demonstrated for the first time. The electrothermally operating soft actuators with installed transparent metal nanowire percolation network heater allow easy programming of their actuation direction and instantaneous visualization of temperature changes through color change. Due to the unique direction dependent coefficient of thermal expansion mismatch, the suggested actuator demonstrates a highly anisotropic and reversible behavior with very large bending curvature (2.5 cm^?1) at considerably low temperature (≈40 °C) compared to the previously reported electrothermal soft actuators. The mild operating heat condition required for the maximum curvature enables the superior long‐term stability during more than 10 000 operating cycles. Also, the optical transparency of the polymer bilayer and metal nanowire percolation network heater allow the incorporation of the thermochromic pigments to fabricate color‐shifting actuators. As a proof‐of‐concept, various color‐shifting biomimetic soft robots such as color‐shifting blooming flower, fluttering butterfly, and color‐shifting twining tendril are demonstrated. The developed color‐shifting anisotropic soft actuator is expected to open new application fields and functionalities overcoming the limitation of current soft robots.     

Rapid 3D Printing of Electrohydraulic (HASEL) Tentacle Actuators

A comprehensive material system is introduced for the additive manufacturing of electrohydraulic (HASEL) tentacle actuators. This material system consists of a photo‐curable, elastomeric silicone‐urethane with relatively strong dielectric properties (ε_r ≈ 8.8 at 1 kHz) in combination with ionically‐conductive hydrogel and silver paint electrodes that displace a vegetable‐based liquid dielectric under the application of an electric field. The electronic properties of the silicone material as well as the mechanical properties of the constitutive silicone and hydrogel materials are investigated. The hydraulic pressure exerted on the dielectric working fluid in these capacitive actuators is measured in order to characterize their quasi‐static behavior. Various design features enabled by 3D printing influence this behavior—decreasing the voltage at which actuation begins or increasing the force density in the system. Using a capacitance change of >35% across the actuators while powered, a demonstration of self‐sensing inherent to HASELs is shown. Antagonistic pairs of the 3D printed actuators are shown to exert a blocked force of over 400 mN. An electrohydraulic tentacle actuator is then fabricated to demonstrate the use of this material and actuation system in a synthetic hydrostat. This tentacle actuator is shown to achieve motion in a multi‐dimensional space.     

3D Knitting for Pneumatic Soft Robotics

Soft robots adapt passively to complex environments due to their inherent compliance, allowing them to interact safely with fragile or irregular objects and traverse uneven terrain. The vast tunability and ubiquity of textiles has enabled new soft robotic capabilities, especially in the field of wearable robots, but existing textile processing techniques (e.g., cut-and-sew, thermal bonding) are limited in terms of rapid, additive, accessible, and waste-free manufacturing. While 3D knitting has the potential to address these limitations, an incomplete understanding of the impact of structure and material on knit-scale mechanical properties and macro-scale device performance has precluded the widespread adoption of knitted robots. In this work, the roles of knit structure and yarn material properties on textile mechanics spanning three regimes–unfolding, geometric rearrangement, and yarn stretching–are elucidated and shown to be tailorable across unique knit architectures and yarn materials. Based on this understanding, 3D knit soft actuators for extension, contraction, and bending are constructed. Combining these actuation primitives enables the monolithic fabrication of entire soft grippers and robots in a single-step additive manufacturing procedure suitable for a variety of applications. This approach represents a first step in seamlessly “printing” conformal, low-cost, customizable textile-based soft robots on-demand.     

Robotic Locomotion and Piezo1 Activity Controlled with Novel Liquid Marble-based Soft Actuators

A novel soft actuator is designed, fabricated, and optimized for applied use in soft robotics and biomedical applications. The soft actuator is powered by the expansion and contraction of a graphene-containing and encased liquid marble using the photothermal effect. Unfortunately, conventional liquid marbles are found to be too fragile and prone to cracking and failure for such applications. After experimentation, it is possible to remedy this problem by synthesizing liquid marbles encased with polymeric shells–polymerized in situ–for added mechanical strength and robustness. These marbles are shown to have intrinsic photothermal activity. They are then situated in bimorph-type soft actuators where one side of the actuator has a dramatically different Young's modulus than the other, leading to directional actuation which is successfully demonstrated in multistep walking soft robots. The soft actuators are shown to successfully activate the mechanosensitive Piezo protein in a transfected human cell line with high effectiveness and no toxicity. Overall, the liquid marble-powered soft actuators described here represent a new soft actuation methodology and a novel tool for mechanobiological studies, such as stem cell fate and organoid differentiation.     

Textile Technology for Soft Robotic and Autonomous Garments

Textiles have emerged as a promising class of materials for developing wearable robots that move and feel like everyday clothing. Textiles represent a favorable material platform for wearable robots due to their flexibility, low weight, breathability, and soft hand-feel. Textiles also offer a unique level of programmability because of their inherent hierarchical nature, enabling researchers to modify and tune properties at several interdependent material scales. With these advantages and capabilities in mind, roboticists have begun to use textiles, not simply as substrates, but as functional components that program actuation and sensing. In parallel, materials scientists are developing new materials that respond to thermal, electrical, and hygroscopic stimuli by leveraging textile structures for function. Although textiles are one of humankind's oldest technologies, materials scientists and roboticists are just beginning to tap into their potential. This review provides a textile-centric survey of the current state of the art in wearable robotic garments and highlights metrics that will guide materials development. Recent advances in textile materials for robotic components (i.e., as sensors, actuators, and integration components) are described with a focus on how these materials and technologies set the stage for wearable robots programmed at the material level.     

Directed Motility of Hygroresponsive Biomimetic Actuators

The capability of cellulose microfibrils to elicit directionality by anisotropically restricting the deformation of amorphous biogenic matrices is central to the motility of many plants as motoric and shape‐restoring elements. Herein, an approach is described to control directionality of artificial composite actuators that mimic the hygroinduced motion of composite plant tissues such as the opening of seed pods, winding of plant tendrils, and burial of seed awns. The actuators are designed as bilayer structures where single or double networks of buried parallel glass fibers reinforce the composite. By anisotropically restricting the expansion along certain directions they also effectively direct the mechanical reconfiguration, thereby determining the mechanical effect. A mathematical model is developed to quantify the kinematic response of fiber‐reinforced actuators. Within a broader context, the results of this study provide means for control over mechanical deformation of artificial dynamic elements that mimic the oriented fibrous architectures in biogenic motoric elements.