Existing soft actuators have persistent challenges that restrain the potential of soft robotics, highlighting a need for soft transducers that are powerful, high-speed, efficient, and robust. We describe a class of soft actuators, termed hydraulically amplified self-healing electrostatic (HASEL) actuators, which harness a mechanism that couples electrostatic and hydraulic forces to achieve a variety of actuation modes. We introduce prototypical designs of HASEL actuators and demonstrate their robust, muscle-like performance as well as their ability to repeatedly self-heal after dielectric breakdown-all using widely available materials and common fabrication techniques. A soft gripper handling delicate objects and a self-sensing artificial muscle powering a robotic arm illustrate the wide potential of HASEL actuators for next-generation soft robotic devices.

Soft robotic systems are well suited to unstructured, dynamic tasks and environments, owing to their ability to adapt and conform without damaging themselves or their surroundings. These abilities are crucial in areas such as humanrobot interaction. Soft robotic systems are currently limited by the soft actuators that power them. To date, most soft actuators are based on pneumatics or shape-memory alloys, which have issues with efficiency, response speed, and portability. Dielectric elastomer actuators (DEAs) are controlled and powered electrically and excel with musclelike actuation, but they typically require a rigid frame and prestretch to perform effectively. In addition, DEAs require complex stacks or structures to achieve linear contraction modes. We present a class of soft electrohydraulic transducers, termed Peano-HASEL (hydraulically amplified self-healing electrostatic) actuators, that combine the strengths of fluidic actuators and electrostatic actuators, while addressing many of their issues. These actuators use both electrostatic and hydraulic principles to linearly contract on application of voltage in a muscle-like fashion, without rigid frames, prestretch, or stacked configurations. We fabricated these actuators using a facile heat-sealing method with inexpensive commercially available materials. These prototypical devices demonstrated controllable linear contraction up to 10%, a strain rate of 900% per second, actuation at 50 hertz, and the ability to lift more than 200 times their weight. In addition, these actuators featured characteristics such as high optical transparency and the ability to self-sense their deformation state. Hence, this class of actuators demonstrates promise for applications such as active prostheses, medical and industrial automation, and autonomous robotic devices.

For soft robots to have ubiquitous adoption in practical applications they require soft actuators that provide well‐rounded actuation performance that parallels natural muscle while being inexpensive and easily fabricated. This manuscript introduces a toolkit to rapidly prototype, manufacture, test, and power various designs of hydraulically amplified self‐healing electrostatic (HASEL) actuators with muscle‐like performance that achieve all three basic modes of actuation (expansion, contraction, and rotation). This toolkit utilizes easy‐to‐implement methods, inexpensive fabrication tools, commodity materials, and off‐the‐shelf high‐voltage electronics thereby enabling a wide audience to explore HASEL technology. Remarkably, the actuators created from this easy‐to‐implement toolkit achieve linear strains exceeding 100%, a specific power greater than 150 W kg −1 , and ≈20% strain at frequencies above 100 Hz. This combination of large strain, extreme speed, and high specific power yields soft actuators that jump without power‐amplifying mechanisms. Additionally, an efficient fabrication technique is introduced for modular designs of HASEL actuators, which is used to develop soft robotic devices driven by portable electronics. Inspired by the versatility of elephant trunks, the above capabilities are combined to create an untethered continuum robot for grasping and manipulating delicate objects, highlighting the wide potential of the introduced methods for soft robots with increasing sophistication.

Figure 1. Selected sources of inspiration for hydraulically amplified self-healing electrostatic (HASEL) actuators. The timeline shows key sources of inspirations for the invention of the HASEL technology. Photo of the hummingbird and elephant by Günter Oesterling and reproduced with permission. Photo of the octopus by Jeahn Laffitte on Unsplash and reproduced with permission. "Soft, electrostatic actuator studied by Röntgen" image:

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.

The impressive locomotion and manipulation capabilities of spiders have led to a host of bioinspired robotic designs aiming to reproduce their functionalities; however, current actuation mechanisms are deficient in either speed, force output, displacement, or efficiency. Here-using inspiration from the hydraulic mechanism used in spider legs-soft-actuated joints are developed that use electrostatic forces to locally pressurize a hydraulic fluid, and cause flexion of a segmented structure. The result is a lightweight, low-profile articulating mechanism capable of fast operation, high forces, and large displacement; these devices are termed spider-inspired electrohydraulic soft-actuated (SES) joints. SES joints with rotation angles up to 70°, blocked torques up to 70 mN m, and specific torques up to 21 N m kg −1 are demonstrated. SES joints demonstrate high speed operation, with measured roll-off frequencies up to 24 Hz and specific power as high as 230 W kg −1 -similar to human muscle. The versatility of these devices is illustrated by combining SES joints to create a bidirectional joint, an artificial limb with independently addressable joints, and a compliant gripper. The lightweight, low-profile design, and high performance of these devices, makes them well-suited toward the development of articulating robotic systems that can rapidly maneuver.

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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.