Photothermal Bimorph Actuators with In‐Built Cooler for Light Mills,Frequency Switches,and Soft Robots

Photothermal bimorph actuators are widely used for smart devices, which are generally operated in a room temperature environment, therefore a low temperature difference for actuation without deteriorating the performance is preferred. The strategy for the actuator is assembling a broadband‐light absorption layer for volume expansion and an additional water evaporation layer for cooling and volume shrinkage on a passive layer. The response time and temperature‐change‐normalized bending speed under NIR, white, and blue light illumination are at the same level of high performance, fast photothermal actuators based on polymer or polymer composites. The classical beam theory and finite element simulations are also conducted to understand the actuation mechanism of the actuator. A new type of light mill is designed based on a wing‐flapping mechanism and a light‐modulated frequency switch. A fast‐walking robot (with a speed of 26 mm s^?1) and a fast‐and‐strong mechanical gripper with a large weight‐lifting ratio (≈2142), respectively, are also demonstrated.     

A Versatile Ionomer-Based Soft Actuator with Multi-Stimulus Responses,Self-Sustainable Locomotion,and Photoelectric Conversion

The prospects of endowing artificial robotics or devices with increasingly complex and emergent life-like behaviors have attracted growing interest in the soft functional materials that mimic the versatile motions of living creatures in the iridescent nature. However, despite the flourishing achievements so far, soft actuators capable of sensitive multi-stimulus responses and self-sustainable movements, have been extensively pursued to reduce control complexity yet remains a challenging target. Here, through material-structural synergistic design incorporating stress-mismatching structure, high pseudo-negative coefficient of thermal expansion of perfluoro-sulfonic acid ionomer, comprehensive converting properties of carbon nanotube, and anisotropic large thermal expansion of PE polymer, an ionomer-based bilayer actuator is proposed, presenting high-performance actuation of various forms and nice stability, responsive to light (including sunlight without focusing, LED light), low voltage, mild heating, and humidity/solvent change. With a built-in structural feedback loop, the actuation performances are further explored to realize intelligent systems, including: 1) self-sustainable locomotion under sunlight irradiation with adjustable photophobic and phototropic direction as well as adaption to different topographies and loading conditions, 2) self-sustainable oscillation and solar-electric generating, and 3) bionic floristic reaction according to environmental change. These diversified actuation modes allow promising following-up designs for bio-hybrid soft robotics fueled by and harmonized with natural environments.     

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.     

Bioinspired Electrically Activated Soft Bistable Actuators

Movement and morphing in biological systems provide insights into the materials and mechanisms that may enable the development of advanced engineering structures. The nastic motion of plants in response to environmental stimuli, e.g., the rapid closure of the Venus flytrap's leaves, utilizes snap‐through instabilities originating from anisotropic deformation of plant tissues. In contrast, ballistic tongue projection of chameleon is attributed to direct mechanical energy transformation by stretching elastic tissues in advance of rapid projection to achieve higher speed and power output. Here, a bioinspired trilayered bistable all‐polymer laminate containing dielectric elastomers (DEs) is reported, which double as both structural and active materials. It is demonstrated that the prestress and laminating strategy induces tunable bistability, while the electromechanical response of the DE film enables reversible shape transition and morphing. Electrical actuation of bistable structures obviates the need for continuous application of electric field to sustain their transformed state. The experimental results are qualitatively consistent with our theoretical analyses of prestrain‐dependent shape and bistability.     

Bioinspired and Hierarchically Textile-Structured Soft Actuators for Healthcare Wearables

Soft pneumatic actuators possess the increasing potential for various healthcare applications, such as smart wearable devices, safe human-robot interaction, and flexible manipulators. However, it is difficult to translate the existing technologies to commercial applications due to their inefficient volumetric power, sophisticated control with high operation pressure, slow production, and high cost. To overcome these issues, herein, a caterpillar-inspired actuator using hierarchical textile architectures based on simple fabrication and low-cost strategy is designed. Unlike the existing textile-based pneumatic actuators, the designed actuators are constructed by combining boucle fancy yarns with a novel trilayer-knit architecture. The as-prepared actuators concurrently possess fast response (1100° s⁻¹), large bending actuation strain (1080° m⁻¹), high-power density (272 W m⁻³), mechanical robustness, easy-programmable motions, and human-tactile comfort, which outperforms currently reported textile-based pneumatic actuators. Furthermore, due to the geometrical transition of the engineered hierarchical structure, the developed actuators exhibit superior dual-stiffness effect with stress evolution, providing a facile approach to addressing the conflict of flexibility and force output in soft fluidic actuators. This concept as a paradigm provides new insights to develop soft actuators with outstanding design flexibility, adaptability, and multifunctionality using engineered textile-structure, which has great potential for real-world applications in medical rehabilitation, physiotherapy, and soft robotics.     

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.     

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.     

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.     

Selective Stiffening in Soft Actuators by Triggered Phase Transition of Hydrogel-Filled Elastomers

Nature has inspired a new generation of robots that not only imitate the behavior of natural systems but also share their adaptability to the environment and level of compliance due to the materials used to manufacture them, which are typically made of soft matter. In order to be adaptable and compliant, these robots need to be able to locally change the mechanical properties of their soft material-based bodies according to external feedback. In this work, a soft actuator that embodies a highly controllable thermo-responsive hydrogel and changes its stiffness on direct stimulation is proposed. At a critical temperature, this stimulation triggers the reversible transition of the hydrogel, which locally stiffens the elastomeric containment at the targeted location. By dividing the actuator into multiple sections, it is possible to control its macroscopic behavior as a function of the stiffened sections. These properties are evaluated by arranging three actuators into a gripper configuration used to grasp objects. The results clearly show that the approach can be used to develop soft actuators that can modify their mechanical properties on-demand in order to conform to objects or to exert the required force.     

Adaptive Magnetoactive Soft Composites for Modular and Reconfigurable Actuators

Magnetoactive soft materials, typically composed of magnetic particles dispersed in a soft polymer matrix, are finding many applications in soft robotics due to their reversible and remote shape transformations under magnetic fields. To achieve complex shape transformations, anisotropic, and heterogeneous magnetization profiles must be programmed in the material. However, once programmed and assembled, magnetic soft actuators cannot be easily reconfigured, repurposed, or repaired, which limits their application, their durability, and versatility in their design. Here, magnetoactive soft composites are developed from squid-derived biopolymers and NdFeB microparticles with tunable ferromagnetic and thermomechanical properties. By leveraging reversible crosslinking nanostructures in the biopolymer matrix, a healing-assisted assembly process is developed that allows for on-demand reconfiguration and magnetic reprogramming of magnetoactive composites. This concept in multi-material modular actuators is demonstrated with programmable deformation modes, self-healing properties to recover their function after mechanical damage, and shape-memory behavior to lock in their preferred configuration and un-actuated catch states. These dynamic magnetic soft composites can enable the modular design and assembly of new types of magnetic actuators, not only eliminating device vulnerabilities through healing and repair but also by providing adaptive mechanisms to reconfigure their function on demand.     

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.     

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.     

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.     

Pneumatic Networks for Soft Robotics that Actuate Rapidly

Soft robots actuated by inflation of a pneumatic network (a “pneu‐net”) of small channels in elastomeric materials are appealing for producing sophisticated motions with simple controls. Although current designs of pneu‐nets achieve motion with large amplitudes, they do so relatively slowly (over seconds). This paper describes a new design for pneu‐nets that reduces the amount of gas needed for inflation of the pneu‐net, and thus increases its speed of actuation. A simple actuator can bend from a linear to a quasi‐circular shape in 50 ms when pressurized at ΔP = 345 kPa. At high rates of pressurization, the path along which the actuator bends depends on this rate. When inflated fully, the chambers of this new design experience only one‐tenth the change in volume of that required for the previous design. This small change in volume requires comparably low levels of strain in the material at maximum amplitudes of actuation, and commensurately low rates of fatigue and failure. This actuator can operate over a million cycles without significant degradation of performance. This design for soft robotic actuators combines high rates of actuation with high reliability of the actuator, and opens new areas of application for them.     

Biotissue-Inspired Anisotropic Carbon Fiber Composite Hydrogels for Logic Gates,Integrated Soft Actuators,and Sensors with Ultra-High Sensitivity

Natural biotissues like muscles, ligaments, and nerves have highly aligned structures, which play critical roles in directional signal transport, sensing, and actuation. Inspired by anisotropic biotissues, composite hydrogels with outstanding mechanical properties and conductivity are developed by compositing thermo-responsive poly (N-isopropylacrylamide) (PNIPAM) hydrogels with highly aligned carbon fibers (CFs). The anisotropic hydrogels show superior tensile strength (3.0 ± 0.3), modulus (74 ± 7.0 MPa), excellent electrical conductivity (≈670 S m⁻¹), and ultra-high sensitivity (gauge factor up to 647) along CFs, with an anisotropic ratio (AR) up to 740 over those in perpendicular direction. The extremely high AR in conductivity (more than 400) produces high-level output in parallel direction and low-level output in perpendicular direction with a direct current (DC) power supply, which is used to fabricate AND and OR gates. Moreover, the composite hydrogels are converted into thermo-responsive actuators with CFs twisted before compositing with PNIPAM/clay network. The pre-twisted CF helices impart internal stress that drives reversible actuation of hydrogel helices upon thermo-stimulating. The actuation is self-sensed due to the extremely high sensitivity of the composite hydrogels. Such biomimetic anisotropic self-sensing hydrogel actuators resemble natural biotissues with both actuation and sensing capabilities, and have promise applications for artificial robotics.