Soft Actuators for Small‐Scale Robotics

This review comprises a detailed survey of ongoing methodologies for soft actuators, highlighting approaches suitable for nanometer‐ to centimeter‐scale robotic applications. Soft robots present a special design challenge in that their actuation and sensing mechanisms are often highly integrated with the robot body and overall functionality. When less than a centimeter, they belong to an even more special subcategory of robots or devices, in that they often lack on‐board power, sensing, computation, and control. Soft, active materials are particularly well suited for this task, with a wide range of stimulants and a number of impressive examples, demonstrating large deformations, high motion complexities, and varied multifunctionality. Recent research includes both the development of new materials and composites, as well as novel implementations leveraging the unique properties of soft materials.     

Slit Tubes for Semisoft Pneumatic Actuators

This article describes a new principle for designing soft or ‘semisoft’ pneumatic actuators: SLiT (for SLit‐in‐Tube) actuators. Inflating an elastomeric balloon, when enclosed by an external shell (a material with higher Young's modulus) containing slits of different directions and lengths, produces a variety of motions, including bending, twisting, contraction, and elongation. The requisite pressure for actuation depends on the length of the slits, and this dependence allows sequential actuation by controlling the applied pressure. Different actuators can also be controlled using external “sliders” that act as reprogrammable “on‐off” switches. A pneumatic arm and a walker constructed from SLiT actuators demonstrate their ease of fabrication and the range of motions they can achieve.     

Untethered Recyclable Tubular Actuators with Versatile Locomotion for Soft Continuum Robots

Stimuli‐responsive materials offer a distinguished platform to build tether‐free compact soft robots, which can combine sensing and actuation without a linked power supply. In the past, tubular soft robots have to be made by multiple components with various internal channels or complex cavities assembled together. Moreover, robust processing, complex locomotion, simple structure, and easy recyclability represent major challenges in this area. Here, it is shown that those challenges can be tackled by liquid crystalline elastomers with allyl sulfide functional groups. The light‐controlled exchange reaction between allyl sulfide groups allows flexible processing of tubular soft robots/actuators, which does not need any assisting materials. Complex locomotion demonstrated here includes reversible simultaneous bending and elongation; reversible diameter expansion; and omnidirectional bending via remote infrared light control. Different modes of actuation can be programmed into the same tube without the routine assembly of multiple tubes as used in the past. In addition, the exchange reaction also makes it possible to use the same single tube repeatedly to perform different functions by erasing and reprogramming.     

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.     

Elastic Inflatable Actuators for Soft Robotic Applications

The 20th century's robotic systems have been made from stiff materials, and much of the developments have pursued ever more accurate and dynamic robots, which thrive in industrial automation, and will probably continue to do so for decades to come. However, the 21st century's robotic legacy may very well become that of soft robots. This emerging domain is characterized by continuous soft structures that simultaneously fulfill the role of robotic link and actuator, where prime focus is on design and fabrication of robotic hardware instead of software control. These robots are anticipated to take a prominent role in delicate tasks where classic robots fail, such as in minimally invasive surgery, active prosthetics, and automation tasks involving delicate irregular objects. Central to the development of these robots is the fabrication of soft actuators. This article reviews a particularly attractive type of soft actuators that are driven by pressurized fluids. These actuators have recently gained traction on the one hand due to the technology push from better simulation tools and new manufacturing technologies, and on the other hand by a market pull from applications. This paper provides an overview of the different advanced soft actuator configurations, their design, fabrication, and applications.     

Soft Ultrathin Electronics Innervated Adaptive Fully Soft Robots

Soft robots outperform the conventional hard robots on significantly enhanced safety, adaptability, and complex motions. The development of fully soft robots, especially fully from smart soft materials to mimic soft animals, is still nascent. In addition, to date, existing soft robots cannot adapt themselves to the surrounding environment, i.e., sensing and adaptive motion or response, like animals. Here, compliant ultrathin sensing and actuating electronics innervated fully soft robots that can sense the environment and perform soft bodied crawling adaptively, mimicking an inchworm, are reported. The soft robots are constructed with actuators of open‐mesh shaped ultrathin deformable heaters, sensors of single‐crystal Si optoelectronic photodetectors, and thermally responsive artificial muscle of carbon‐black‐doped liquid‐crystal elastomer (LCE‐CB) nanocomposite. The results demonstrate that adaptive crawling locomotion can be realized through the conjugation of sensing and actuation, where the sensors sense the environment and actuators respond correspondingly to control the locomotion autonomously through regulating the deformation of LCE‐CB bimorphs and the locomotion of the robots. The strategy of innervating soft sensing and actuating electronics with artificial muscles paves the way for the development of smart autonomous soft robots.     

Laser-Induced Graphene for Electrothermally Controlled,Mechanically Guided, 3D Assembly and Human–Soft Actuators Interaction

Mechanically guided, 3D assembly has attracted broad interests, owing to its compatibility with planar fabrication techniques and applicability to a diversity of geometries and length scales. Its further development requires the capability of on-demand reversible shape reconfigurations, desirable for many emerging applications (e.g., responsive metamaterials, soft robotics). Here, the design, fabrication, and modeling of soft electrothermal actuators based on laser-induced graphene (LIG) are reported and their applications in mechanically guided 3D assembly and human–soft actuators interaction are explored. Over 20 complex 3D architectures are fabricated, including reconfigurable structures that can reshape among three distinct geometries. Also, the structures capable of maintaining 3D shapes at room temperature without the need for any actuation are realized by fabricating LIG actuators at an elevated temperature. Finite element analysis can quantitatively capture key aspects that govern electrothermally controlled shape transformations, thereby providing a reliable tool for rapid design optimization. Furthermore, their applications are explored in human–soft actuators interaction, including elastic metamaterials with human gesture-controlled bandgap behaviors and soft robotic fingers which can measure electrocardiogram from humans in an on-demand fashion. Other demonstrations include artificial muscles, which can lift masses that are about 110 times of their weights and biomimetic frog tongues which can prey insects.     

Artificial Muscles: Mechanisms,Applications, and Challenges

The area of artificial muscle is a highly interdisciplinary field of research that has evolved rapidly in the last 30 years. Recent advances in nanomaterial fabrication and characterization, specifically carbon nanotubes and nanowires, have had major contributions in the development of artificial muscles. However, what can artificial muscles really do for humans? This question is considered here by first examining nature's solutions to this design problem and then discussing the structure, actuation mechanism, applications, and limitations of recently developed artificial muscles, including highly oriented semicrystalline polymer fibers; nanocomposite actuators; twisted nanofiber yarns; thermally activated shape‐memory alloys; ionic‐polymer/metal composites; dielectric‐elastomer actuators; conducting polymers; stimuli‐responsive gels; piezoelectric, electrostrictive, magnetostrictive, and photostrictive actuators; photoexcited actuators; electrostatic actuators; and pneumatic actuators.     

Kirigami-Based Light-Induced Shape-Morphing and Locomotion

The development of stimuli-responsive soft actuators, a task largely undertaken by material scientists, has become a major driving force in pushing the frontiers of microrobotics. Devices made of soft active materials are oftentimes small in size, remotely and wirelessly powered/controlled, and capable of adapting themselves to unexpected hurdles. However, nowadays most soft microscale robots are rather simple in terms of design and architecture, and it remains a challenge to create complex 3D soft robots with stimuli-responsive properties. Here, it is suggested that kirigami-based techniques can be useful for fabricating complex 3D robotic structures that can be activated with light. External stress fields introduce out-of-plane deformation of kirigami film actuators made of liquid crystal networks. Such 2D-to-3D structural transformations can give rise to mechanical actuation upon light illumination, thus allowing the realization of kirigami-based light-fuelled robotics. A kirigami rolling robot is demonstrated, where a light beam controls the multigait motion and steers the moving direction in 2D. The device is able to navigate along different routes and moves up a ramp with a slope of 6°. The results demonstrate a facile technique to realize complex and flexible 3D structures with light-activated robotic functions.     

Remotely Light‐Powered Soft Fluidic Actuators Based on Plasmonic‐Driven Phase Transitions in Elastic Constraint

Materials capable of actuation through remote stimuli are crucial for untethering soft robotic systems from hardware for powering and control. Fluidic actuation is one of the most applied and versatile actuation strategies in soft robotics. Here, the first macroscale soft fluidic actuator is derived that operates remotely powered and controlled by light through a plasmonically induced phase transition in an elastomeric constraint. A multiphase assembly of a liquid layer of concentrated gold nanoparticles in a silicone or styrene–ethylene–butylene–styrene elastic pocket forms the actuator. Upon laser excitation, the nanoparticles convert light of specific wavelength into heat and initiate a liquid‐to‐gas phase transition. The related pressure increase inflates the elastomers in response to laser wavelength, intensity, direction, and on–off pulses. During laser‐off periods, heating halts and condensation of the gas phase renders the actuation reversible. The versatile multiphase materials actuate—like soft “steam engines”—a variety of soft robotic structures (soft valve, pnue‐net structure, crawling robot, pump) and are capable of operating in different environments (air, water, biological tissue) in a single configuration. Tailored toward the near‐infrared window of biological tissue, the structures actuate also through animal tissue for potential medical soft robotic applications.     

25th Anniversary Article: A Soft Future: From Robots and Sensor Skin to Energy Harvesters

Scientists are exploring elastic and soft forms of robots, electronic skin and energy harvesters, dreaming to mimic nature and to enable novel applications in wide fields, from consumer and mobile appliances to biomedical systems, sports and healthcare. All conceivable classes of materials with a wide range of mechanical, physical and chemical properties are employed, from liquids and gels to organic and inorganic solids. Functionalities never seen before are achieved. In this review we discuss soft robots which allow actuation with several degrees of freedom. We show that different actuation mechanisms lead to similar actuators, capable of complex and smooth movements in 3d space. We introduce latest research examples in sensor skin development and discuss ultraflexible electronic circuits, light emitting diodes and solar cells as examples. Additional functionalities of sensor skin, such as visual sensors inspired by animal eyes, camouflage, self‐cleaning and healing and on‐skin energy storage and generation are briefly reviewed. Finally, we discuss a paradigm change in energy harvesting, away from hard energy generators to soft ones based on dielectric elastomers. Such systems are shown to work with high energy of conversion, making them potentially interesting for harvesting mechanical energy from human gait, winds and ocean waves.     

Actively Perceiving and Responsive Soft Robots Enabled by Self‐Powered,Highly Extensible,and Highly Sensitive Triboelectric Proximity‐ and Pressure‐Sensing Skins

Robots that can move, feel, and respond like organisms will bring revolutionary impact to today's technologies. Soft robots with organism‐like adaptive bodies have shown great potential in vast robot–human and robot–environment applications. Developing skin‐like sensory devices allows them to naturally sense and interact with environment. Also, it would be better if the capabilities to feel can be active, like real skin. However, challenges in the complicated structures, incompatible moduli, poor stretchability and sensitivity, large driving voltage, and power dissipation hinder applicability of conventional technologies. Here, various actively perceivable and responsive soft robots are enabled by self‐powered active triboelectric robotic skins (tribo‐skins) that simultaneously possess excellent stretchability and excellent sensitivity in the low‐pressure regime. The tribo‐skins can actively sense proximity, contact, and pressure to external stimuli via self‐generating electricity. The driving energy comes from a natural triboelectrification effect involving the cooperation of contact electrification and electrostatic induction. The perfect integration of the tribo‐skins and soft actuators enables soft robots to perform various actively sensing and interactive tasks including actively perceiving their muscle motions, working states, textile's dampness, and even subtle human physiological signals. Moreover, the self‐generating signals can drive optoelectronic devices for visual communication and be processed for diverse sophisticated uses.     

Plasmonic‐Assisted Graphene Oxide Artificial Muscles

Muscles and joints make highly coordinated motion, which can be partly mimicked to drive robots or facilitate activities. However, most cases primarily employ actuators enabling simple deformations. Therefore, a mature artificial motor system requires many actuators assembled with jointed structures to accomplish complex motions, posing limitations and challenges to the fabrication, integration, and applicability of the system. Here, a holistic artificial muscle with integrated light‐addressable nodes, using one‐step laser printing from a bilayer structure of poly(methyl methacrylate) and graphene oxide compounded with gold nanorods (AuNRs), is reported. Utilizing the synergistic effect of the AuNRs with high plasmonic property and wavelength‐selectivity as well as graphene with good flexibility and thermal conductivity, the artificial muscle can implement full‐function motility without further integration, which is reconfigurable through wavelength‐sensitive light activation. A biomimetic robot and artificial hand are demonstrated, showcasing functionalized control, which is desirable for various applications, from soft robotics to human assists.     

Soft Pneumatic Actuators with Controllable Stiffness by Bio-Inspired Lattice Chambers and Fused Deposition Modeling 3D Printing

This article shows how changing 3D printing parameters and using bio-inspired lattice chambers can engineer soft pneumatic actuators (SPAs) with different behaviors in terms of controlling tip deflection and tip force using the same input air pressure. Fused deposition modeling (FDM) is employed to 3D print soft pneumatic actuators using varioShore thermoplastic polyurethane (TPU) materials with a foaming agent. The effects of material flow and nozzle temperature parameters on the material properties and stiffness are investigated. Auxetic, columns, face-centered cubic, honeycomb, isotruss, oct vertex centroid, and square honeycomb lattices are designed to study actuators’ behaviors under the same input pressure. Finite-element simulations based on the nonlinear hyper-elastic constitutive model are carried out to precisely predict the behavior, deformation, and tip force of the actuators. A closed-loop pneumatic system and sensors are developed to control the actuators. Results show that lattice designs can control the bending angle and generated force of actuators. Also, the lattices increase the ultimate strength by controlling the contact area inside the chambers. They demonstrate variable stiffness behaviors and deflections under the same pressure between 100 and 500 kPa. The proposed actuators could be instrumental in designing wearable hand rehabilitative devices that assist customized finger and wrist flexion-extension.     

Rubber-Like Soft Lattice Structure for Anti-Ballooning in Fluidic Elastic Soft Robots

Additive manufacturing of lattice structures provides materials with enhanced strength, stiffness, and lightweight properties. While most research focuses on stiff, low-stretch materials like metals and acrylonitrile butadiene styrene, herein, the tensile behavior of soft, elastomeric lattice structures is explored. Soft-material 3D-printing advancements have enabled increased usage of directly printed soft robots. Traditional fluidic elastic actuators, however, face limitations due to the ballooning effect of soft polymers, causing potential explosions or leakages. To mitigate this, the study proposes using a soft lattice structure to reinforce soft inflatable robots, thereby reducing the ballooning effect and increasing design freedom. Herein, soft lattices are fabricated using Agilus30 in a Stratasys J735 printer and their behaviors under compression and stretching are compared. It is indicated in the results that lattice reinforcement maintains the soft robot's shape under higher pressure and allows tunability of stiffness with variable internal pressure. The implementation of this method in non-convex soft robots successfully demonstrates its anti-ballooning effect.     

Magnetically and Electrically Responsive Soft Actuator Derived from Ferromagnetic Bimetallic Organic Framework

The advancement in smart devices and soft robotics necessitates the use of multiresponsive soft actuators with high actuation stroke and stable reversibility for their use in real-world applications. Here, this work reports a magnetically and electrically dual responsive soft actuator based on neodymium and iron bimetallic organic frameworks (NdFeMOFs@700). The ferromagnetic NdFeMOFs@700 exhibits a porous carbon structure with excellent magnetization saturation (166.96 emu g⁻¹) which allows its application to a dual functional material in both magnetoactive and electro-ionic actuations. The electro-ionic soft actuator, which is fabricated using NdFeMOFs@700 and PEDOT-PSS, demonstrates 4.5 times higher ionic charge storage capacity (68.21 mF cm⁻²) and has excellent cycle stability compared with the PEDOT-PSS based actuator. Under a low sinusoidal input voltage of 1 V, the dual-responsive actuator displays bending displacement of 15.46 mm and also generates deflection of 10 mm at 50 mT. Present results show that the ferromagnetic bimetallic organic frameworks can open a new way to make dual responsive soft actuators due to the hierarchically porous structures with its high redox activity, superior magnetic properties, and larger electrochemical capacitance. With the NdFeMOFs@700 based soft actuators, walking movement of a starfish robot is demonstrated by applying both the magnetic and electric fields.     

Conjugated Polymer Actuators and Devices: Progress and Opportunities

Conjugated polymers (CPs), as exemplified by polypyrrole, are intrinsically conducting polymers with potential for development as soft actuators or “artificial muscles” for numerous applications. Significant progress has been made in the understanding of these materials and the actuation mechanisms, aided by the development of physical and electrochemical models. Current research is focused on developing applications utilizing the advantages that CP actuators have (e.g., low driving potential and easy to miniaturize) over other actuating materials and on developing ways of overcoming their inherent limitations. CP actuators are available as films, filaments/yarns, and textiles, operating in liquids as well as in air, ready for use by engineers. Here, the milestones made in understanding these unique materials and their development as actuators are highlighted. The primary focus is on the recent progress, developments, applications, and future opportunities for improvement and exploitation of these materials, which possess a wealth of multifunctional properties.     

Soft Somatosensitive Actuators via Embedded 3D Printing

Humans possess manual dexterity, motor skills, and other physical abilities that rely on feedback provided by the somatosensory system. Herein, a method is reported for creating soft somatosensitive actuators (SSAs) via embedded 3D printing, which are innervated with multiple conductive features that simultaneously enable haptic, proprioceptive, and thermoceptive sensing. This novel manufacturing approach enables the seamless integration of multiple ionically conductive and fluidic features within elastomeric matrices to produce SSAs with the desired bioinspired sensing and actuation capabilities. Each printed sensor is composed of an ionically conductive gel that exhibits both long‐term stability and hysteresis‐free performance. As an exemplar, multiple SSAs are combined into a soft robotic gripper that provides proprioceptive and haptic feedback via embedded curvature, inflation, and contact sensors, including deep and fine touch contact sensors. The multimaterial manufacturing platform enables complex sensing motifs to be easily integrated into soft actuating systems, which is a necessary step toward closed‐loop feedback control of soft robots, machines, and haptic devices.     

Photodeformable Azobenzene‐Containing Liquid Crystal Polymers and Soft Actuators

Photodeformable liquid crystal polymers (LCPs) that adapt their shapes in response to light have aroused a dramatic growth of interest in the past decades, since light as a stimulus enables the remote control and diverse deformations of materials. This review focuses on the growing research on photodeformable LCPs, including their basic actuation mechanisms, the various deformation modes, the newly designed molecular structures, and the improvement of processing techniques. Special attention is devoted to the novel molecular structures of LCPs, which allow for easy processing and alignment. The soft actuators with various deformation modes such as bending, twisting, and rolling in response to light are also covered with the emphasis on their photo‐induced bionic functions. Potential applications in energy harvesting, self‐cleaning surfaces, sensors, and photo‐controlled microfluidics are further illustrated. The existing challenges and future directions are discussed at the end of this review.     

Living Materials Herald a New Era in Soft Robotics

Living beings have an unsurpassed range of ways to manipulate objects and interact with them. They can make autonomous decisions and can heal themselves. So far, a conventional robot cannot mimic this complexity even remotely. Classical robots are often used to help with lifting and gripping and thus to alleviate the effects of menial tasks. Sensors can render robots responsive, and artificial intelligence aims at enabling autonomous responses. Inanimate soft robots are a step in this direction, but it will only be in combination with living systems that full complexity will be achievable. The field of biohybrid soft robotics provides entirely new concepts to address current challenges, for example the ability to self‐heal, enable a soft touch, or to show situational versatility. Therefore, “living materials” are at the heart of this review. Similarly to biological taxonomy, there is a recent effort for taxonomy of biohybrid soft robotics. Here, an expansion is proposed to take into account not only function and origin of biohybrid soft robotic components, but also the materials. This materials taxonomy key demonstrates visually that materials science will drive the development of the field of soft biohybrid robotics.