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.

Macroscale robotic systems have demonstrated great capabilities of high speed, precise, and agile functions. However, the ability of soft robots to perform complex tasks, especially in centimeter and millimeter scale, remains limited due to the unavailability of fast, energy-efficient soft actuators that can programmably change shape. Here, we combine desirable characteristics from two distinct active materials: fast and efficient actuation from dielectric elastomers and facile shape programmability from liquid crystal elastomers into a single shape changing electrical actuator. Uniaxially aligned monoliths achieve strain rates over 120%/s with energy conversion efficiency of 20% while moving loads over 700 times the actuator weight. The combined actuator technology offers unprecedented opportunities towards miniaturization with pre-1 arXiv:1904.09606v1 [cond-mat.soft] 21 Apr 2019 cision, efficiency, and more degrees of freedom for applications in soft robotics and beyond.Force generation, efficiency, strength-to-weight ratio, work capacity, and shape programmability will be key for the next generation of soft robots to perform complex functions. Despite significant advances in robots, including gymnastic feats (1), the underlying rigid actuation mechanisms, use of electric motors or hydraulic and pneumatic actuators, remain relatively unchanged and potentially hinder their miniaturization and, more importantly, their use in human collaborative environments (2). Efficient and programmable soft actuators, like an artificial muscle, would significantly increase the capabilities and potential applications of soft robotic systems in aerospace, industrial, or medical technologies (3-5). Among many soft actuation mechanisms that have been explored, dielectric elastomer (DE) actuators appear promising and even outperform skeletal muscle in some aspects (6-9). Separately, liquid crystal elastomers (LCEs) have demonstrated reversible large mechanical deformation by thermal and optical actuation. Recent advances in photoalignment and top-down microfabrication techniques have enabled pre-programming of LC alignment in microdomains for complex shape morphing (10-12). However, both actuator types have their drawbacks: DE films need to be prestretched, making it difficult to program local actuation behaviors microscopically (13). Meanwhile, directly converting electrical energy to mechanical work utilizing LCEs has, until now, remained limited due to the small strain generated (14-19). Typically, DE actuators function by electrostatic attraction between two compliant electrodes coated on opposing sides of an isotropic DE to form a variable resistor-capacitor (20).High voltage applied to the compliant electrodes induces an electrostatic pressure called Maxwell stress. The electrical actuation mechanism can result in much higher operating efficiency (ratio of mechanical work to input electrical energy) and faster actuation speed than those of LCEs (8,9). Besides functioning as soft linear actuators, DE actuators could be applied...

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Most soft robotic systems are currently dependent on bulky compressors or pumps. A soft actuation method is presented combining hyperelastic membranes and dielectric elastomer actuators to switch between stable deformations of sealed chambers. This method is capable of large repeatable deformations, and has a number of stable states proportional to the number of actuatable membranes in the chamber.

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