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.     

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.     

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.     

Magnetic Assembly of Soft Robots with Hard Components

This paper describes the modular magnetic assembly of reconfigurable, pneumatically actuated robots composed of soft and hard components and materials. The soft components of these hybrid robots are actuators fabricated from silicone elastomers using soft lithography, and the hard components are acrylonitrile–butadiene–styrene (ABS) structures made using 3D printing. Neodymium–iron–boron (NdFeB) ring magnets are embedded in these components to make and maintain the connections between components. The reversibility of these magnetic connections allows the rapid reconfiguration of these robots using components made of different materials (soft and hard) that also have different sizes, structures, and functions; in addition, it accelerates the testing of new designs, the exploration of new capabilities, and the repair or replacement of damaged parts. This method of assembling soft actuators to build soft machines addresses some limitations associated with using soft lithography for the direct molding of complex 3D pneumatic networks. Combining the self‐aligning property of magnets with pneumatic control makes it possible for a teleoperator to modify the structures and capabilities of these robots readily in response to the requirements of different tasks.     

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.     

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.     

Fully 3D-Printed,Stretchable, and Conformable Haptic Interfaces

Integrating rich cutaneous haptic feedback enhances realism and user immersion in virtual and augmented reality settings. One major challenge is providing accurately localized cutaneous stimuli on fingertips without interfering with the user's dexterity. This sub 200 µm thick, fully printed, stretchable Hydraulically Amplified Taxels (HAXELs) enable both static indentation and vibrating haptic stimuli, localized to a 2.5 mm diameter region. The HAXELs are directly bonded to the user's skin, are soft enough to conform to any body part, and can be fabricated in dense arrays with no crosstalk. All functional materials (elastomers, stretchable conductors, and sacrificial layers) are deposited by inkjet printing, which allows rapid prototyping of multi-material, polymer-based structures. The actuators consist of oil-filled stretchable pouches, whose shape is controlled by electrostatic zipping. The 5 mm wide actuators weigh <250 mg and generate cutaneous stimuli well above reported perception thresholds, from DC to 1 kHz. They operate well even when stretched to over 50%, allowing great freedom in placement. The 2 × 2 arrays are tested on the fingers of human volunteers: the actuated quadrant is correctly identified 86% of the time. Printing soft actuators allows tailoring dense and effective cutaneous haptics to the unique shape of each user.     

Elastomeric Haptic Devices for Virtual and Augmented Reality

Since the modern concepts for virtual and augmented reality are first introduced in the 1960's, the field has strived to develop technologies for immersive user experience in a fully or partially virtual environment. Despite the great progress in visual and auditory technologies, haptics has seen much slower technological advances. The challenge is because skin has densely packed mechanoreceptors distributed over a very large area with complex topography; devising an apparatus as targeted as an audio speaker or television for the localized sensory input of an ear canal or iris is more difficult. Furthermore, the soft and sensitive nature of the skin makes it difficult to apply solid state electronic solutions that can address large areas without causing discomfort. The maturing field of soft robotics offers potential solutions toward this challenge. In this article, the definition and history of virtual (VR) and augmented reality (AR) is first reviewed. Then an overview of haptic output and input technologies is presented, opportunities for soft robotics are identified, and mechanisms of intrinsically soft actuators and sensors are introduced. Finally, soft haptic output and input devices are reviewed with categorization by device forms, and examples of soft haptic devices in VR/AR environments are presented.     

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.     

Reversible Elastomer–Fluid Transitions for Metamorphosic Robots

Endowing robots with reversible phase transition ability, especially between elastomer and fluid states, can significantly broaden their functionality and applicability. Limited attempts have been made to realize the reversible elastomer–fluid transition. Existing phase transition materials in robotics have over-hard (≈4 GPa) or over-soft (≈4 kPa) stiffness in the solid states, which should be further investigated to perform more compliant motions. To address these challenges, a reversible elastomer–fluid transition mechanism  enabled by magnetically induced hot melt materials (MIMMs) is presented. The transition principle is explained and material characterizations are conducted. MIMMs-based metamorphosic robots endow self-metamorphosing abilities, such as self-healing, spatial reshaping, self-division/assembly, and additive manufacturability. When interacting with external environments, MIMMs-based robots can perform further multifunctional abilities, such as collaborations for structure repairs, swimming by symbiosis with external objects, flowing through a narrow terrain by transiting to fluid, and working with elastomeric structures for stiffness-variable fluid soft actuators. The proposed elastomer–fluid transitions open a new path for robots to generate more flexible and metamorphosic motions, thereby addressing the cross-phase transformation challenges that soft robots face.     

Biodegradable,Sustainable Hydrogel Actuators with Shape and Stiffness Morphing Capabilities via Embedded 3D Printing

Despite the impressive performance of recent marine robots, many of their components are non-biodegradable or even toxic and may negatively impact sensitive ecosystems. To overcome these limitations, biologically-sourced hydrogels are a candidate material for marine robotics. Recent advances in embedded 3D printing have expanded the design freedom of hydrogel additive manufacturing. However, 3D printing small-scale hydrogel-based actuators remains challenging. In this study, Free form reversible embedding of suspended hydrogels (FRESH) printing is applied to fabricate small-scale biologically-derived, marine-sourced hydraulic actuators by printing thin-wall structures that are water-tight and pressurizable. Calcium-alginate hydrogels are used, a sustainable biomaterial sourced from brown seaweed. This process allows actuators to have complex shapes and internal cavities that are difficult to achieve with traditional fabrication techniques. Furthermore, it demonstrates that fabricated components are biodegradable, safely edible, and digestible by marine organisms. Finally, a reversible chelation-crosslinking mechanism is implemented to dynamically modify alginate actuators' structural stiffness and morphology. This study expands the possible design space for biodegradable marine robots by improving the manufacturability of complex soft devices using biologically-sourced materials.     

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.     

Wearable Soft Technologies for Haptic Sensing and Feedback

Virtual reality (VR) and augmented reality (AR) systems have garnered recent widespread attention due to increased accessibility, functionality, and affordability. These systems sense user inputs and typically provide haptic, audio, and visual feedback to blend interactive virtual environments with the real world for an enhanced or simulated reality experience. With applications ranging from immersive entertainment, to teleoperation, to physical therapy, further development of this technology has the potential for impact across multiple disciplines. However, VR/AR devices still face critical challenges that hinder integration into everyday life and additional applications; namely, the rigid and cumbersome form factor of current technology that is incompatible with the dynamic movements and pliable limbs of the human body. Recent advancements in the field of soft materials are uniquely suited to provide solutions to this challenge. Devices fabricated from flexible and elastic bio-compatible materials have significantly greater compatibility with the human body and could lead to a more natural VR/AR experience. This review reports state-of-the-art experimental studies in soft materials for wearable sensing and haptic feedback in VR/AR applications, explores emerging soft technologies for on-body devices, and identifies current challenges and future opportunities toward seamless integration of the virtual and physical world.     

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.     

Microstructured Actuation of Liquid Crystal Polymer Networks

Smart microstructured materials enable functions such as actuation, detection, transportation, and sensing with potential applications ranging from robotics and photonics to biomedical devices. Of the many materials systems, liquid crystal polymer networks (LCN) are fascinating owing to their ability to exhibit reversible macroscopic deformation driven by a molecular order–disorder phase transition. LCN have been increasingly explored for their utility in the design and fabrication of smart actuating devices capable of complex shape changes or motions upon external stimulation of humidity, heat, light, and other stimuli, and recent studies in this field show that their actuation complexity can be enriched and actuation performance enhanced by having some sort of microstructures. Herein, the recent progress in microstructured actuation of LCN materials with substructures in scale ranging from micrometer to millimeter is reported, placing the emphasis on the main approaches to generating a microstructure in LCN, which include patterned LC director fields, patterned chain crosslinking in LCN with uniaxial orientation of mesogens, 3D/4D printing, and replica molding. The potential applications in microstructured 3D actuators and devices as well as functional LCN surfaces are also highlighted, with an outlook on important issues and future trends in smart microstructured LCN materials and actuators.     

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.     

Elastomeric Tiles for the Fabrication of Inflatable Structures

This paper describes the fabrication of 3D soft, inflatable structures from thin, 2D tiles fabricated from elastomeric polymers. The tiles are connected using soft joints that increase the surface area available for gluing them together, and mechanically reinforce the structures to withstand the tensile forces associated with pneumatic actuation. The ability of the elastomeric polymer to withstand large deformations without failure makes it possible to explore and implement new joint designs, for example “double‐taper dovetail joints,” that cannot be used with hard materials. This approach simplifies the fabrication of soft structures comprising materials with different physical properties (e.g., stiffness, electrical conductivity, optical transparency), and provides the methods required to “program” the response of these structures to mechanical (e.g., pneumatic pressurization) and other physical (e.g., electrical) stimuli. The flexibility and modularity of this approach is demonstrated in a set of soft structures that expanded or buckled into distinct, predictable shapes when inflated or deflated. These structures combine easily to form extended systems with motions dependent on the configurations of the selected components, and, when fabricated with electrically conductive tiles, electronic circuits with pneumatically active elements. This approach to the fabrication of hollow, 3D structures provides routes to new soft actuators.     

Fast‐Response,Stiffness‐Tunable Soft Actuator by Hybrid Multimaterial 3D Printing

Soft robots have the appealing advantages of being highly flexible and adaptive to complex environments. However, the low‐stiffness nature of the constituent materials makes soft robotic systems incompetent in tasks requiring relatively high load capacity. Despite recent attempts to develop stiffness‐tunable soft actuators by employing variable stiffness materials and structures, the reported stiffness‐tunable actuators generally suffer from limitations including slow responses, small deformations, and difficulties in fabrication with microfeatures. This work presents a paradigm to design and manufacture fast‐response, stiffness‐tunable (FRST) soft actuators via hybrid multimaterial 3D printing. The integration of a shape memory polymer layer into the fully printed actuator body enhances its stiffness by up to 120 times without sacrificing flexibility and adaptivity. The printed Joule‐heating circuit and fluidic cooling microchannel enable fast heating and cooling rates and allow the FRST actuator to complete a softening–stiffening cycle within 32 s. Numerical simulations are used to optimize the load capacity and thermal rates. The high load capacity and shape adaptivity of the FRST actuator are finally demonstrated by a robotic gripper with three FRST actuators that can grasp and lift objects with arbitrary shapes and various weights spanning from less than 10 g to up to 1.5 kg.     

Multiresponsive MXene Actuators with Asymmetric Quantum-Confined Superfluidic Structures

MXene, which is known for its high electrical/thermal conductivity, surface hydrophilicity, excellent mechanical flexibility, and chemical stability, is a versatile and smart material for soft actuators. However, most MXene actuators are fabricated by combining MXene with other inert materials to form a bilayer or multilayer structure. Considering the strain mismatch at multimaterial interfaces under frequent deformation, MXene-based actuators are generally associated with poor stability, which limits their practical applications. Herein, inspired by the natural quantum-confined superfluidic (QSF) effect, a multiresponsive MXene actuator that can be driven by moisture, light, and electricity by engineering an asymmetric QSF structure on both sides of the MXene film is reported. The actuation mechanism of the MXene film can be attributed to nonuniform water adsorption, transport, and desorption within the asymmetric QSF channels under moisture, photothermal, and electrothermal stimuli. Interestingly, MXene actuators can be flexibly formed into various shapes under moisture-assisted mechanical compression, which not only enhances their multiresponsive actuation, but also permits a more complex deformation. As proof-of-concept demonstrations, various intriguing applications including a dual-role robot, a smart shielding curtain, and a dragonfly robot, are fabricated, revealing the potential of MXene actuators for soft robotics.     

Light‐Driven Liquid Crystalline Networks and Soft Actuators with Degree‐of‐Freedom‐Controlled Molecular Motors

Design and fabrication of photomechanical soft actuators has attracted intense scientific interest because of their potential in the manufacture of untethered intelligent soft robots and advanced functional devices. Trifunctional and monofunctional polymerizable molecular motors are judiciously designed and synthesized. Novel light‐driven liquid crystalline networks (LCN) are prepared by crosslinking overcrowded‐alkene‐based molecular motors with different degrees of freedom into the anisotropic LCN. The photoisomerization and thermal helix inversion of light‐driven molecular motors are reversible when only the upper part of the molecular motor is linked to the network, endowing the LCN film with remarkable photoactive performance. However, photochemical geometric change of the light‐driven molecular motor does not work after crosslinking both the upper and lower part of the motor by polymer chains. Interestingly, it is found that the fastened motor can transfer the light energy into localized heat instead of performing photoisomerization. The light‐driven molecular‐motor‐based LCN soft actuators are demonstrated to function as a grasping hand, where the continuous motions of grasping, moving, lifting, and releasing an object are successfully achieved. This work may provide inspiration to the preparation of next‐generation photoactive advanced functional materials toward their wide applications in the areas of photonics, optoelectronics, soft robotics, and beyond.