Millimeter-Scale Soft Electrohydraulic Actuators for Locomotion in Aquatic Environments

Undulating fin and tail propulsion—used by marine animals across many size scales—represents an efficient locomotion method, both on and under water. It is silent and enables a variety of maneuvers. Robotic swimmers can achieve such life-like locomotion using soft actuators but typically measure several tens of centimeters, preventing applications that benefit from smaller scales. Here we present the design, material approaches, and fabrication processes of soft mm-scale electrohydraulic actuators. Our actuators use zipping of metalized polymers films to displace a dielectric fluid, which generates bending of fins, creating a periodic undulation. We developed a fabrication strategy based on multilayer lamination to enable actuators which operate reliably when submerged in water at voltages from 500 V to 2 kV. The actuators are fast, capable of operation up to 20 Hz, and provide suitable amplitudes for swimming, with bending angles up to 15 degrees. With these actuators, we demonstrate flatworm-inspired robots, only a few tens of millimeters long, that locomote both on or under water.