Soft robots achieve muscle-like performance, self-healing through electrohydraulic coupling
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Soft robots achieve muscle-like performance, self-healing through electrohydraulic coupling
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obots made from soft materials have potential for use in applications that require high adaptability and conformability, or close interactions with humans. To achieve this, soft robotic actuators need to replicate the diverse capabilities and performance of natural muscles, which include high speed, power, strength, efficiency, and the ability to self-heal. Existing soft robotic systems—based on fluidic, thermal, or dielectric elastomer actuators—might exhibit outstanding performance according to certain metrics, but have critical limitations in others. To address this issue, the research team of Christoph Keplinger, a professor at the University of Colorado Boulder, has introduced a versatile actuation technology that combines high strains (>100%), high strength (0.3 MPa), high speed (>50 Hz), long lifetime (>1 million cycles), selfhealing, and self-sensing capabilities. These artificial muscles, called hydraulically amplified self-healing electrostatic (HASEL) actuators, are promising especially for advancements in soft robotics. HASEL actuators consist of a stretchable or flexible shell filled with a liquid
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dielectric, a vegetable oil. Electrodes, which can be made from conductive hydrogels, cover parts of the shell. When a voltage is applied to the electrodes, electrostatic forces displace the liquid deforming the soft shell. This coupling between electrostatic and hydraulic forces allows for a variety of actuation modes. As a first design, donut-type HASEL actuators have a disc-shaped shell with electrodes located at the center (see Figure a). When a voltage is applied, the liquid is pushed to the outer region of the shell, causing a local increase in thickness. This transition from disc to donut occurs rapidly as the result of a “pull-in” instability triggered by an imbalance between the electrostatic and mechanical restoring forces. A 5-cm round disc-shaped HASEL actuator is already able to lift 150 g repeatedly over a million cycles. A planar-type HASEL actuator is another design explored by the research team, where the electrodes cover the entire surface of the shell. When voltage is applied, the actuator reduces in thickness and expands in width. Scaling up these devices, heavy objects such as a 4-kg jug of water can be lifted (see Figure b). In addition to the mechanical performance, what makes the technology unique is the capacity of the actuator for
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Hydraulically amplified self-healing electrostatic (HASEL) actuation principle (a) used to deliver high forces (b). Credit: Science.
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self-healing and self-sensing. Under high electric fields, dielectric breakdown can occur. Instead of creating a permanent conducting path between the electrodes, the liquid dielectric simply reflows and returns to an insulating state. This way, the liquid dielectric can self-heal from 50 cycles of dielectric breakdown with “the 50th breakdown event occurring at a higher voltage than the 1st breakdown event,” observes Er
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