Minimal-actuation feather star-inspired soft swimmers for multimodal 3D maneuverability

Complex, three-dimensional (3D) motions typically require actuator arrays and complex control architectures. Here, we present a feather star-inspired soft robotic swimmer that uses only two pneumatic inputs to produce three distinct and switchable swimming modes: jellyfish-like pulsation, fishlike propulsion, and rotor-like reorientation. The robot owes this ability to mechanical intelligence: It leverages a monostable instability in its flexible arms to convert two control actuation inputs into 3D swimming modes, including ascension and descension, forward and backward swimming, hovering, and rotation. The robot achieves a maximum swimming speed of 1.64 body lengths per second, minimum cost of transport of 17.6, and peak rotation speed of 90° per second. Particle image velocimetry analyses and computational fluid dynamics simulations reveal distinct vortex structures governing thrust generation and/or rotational torque in each swimming mode. The robot's minimal input yet multimodal output demonstrates how mechanical intelligence can enable adaptive and multifunctional, yet simple and energy-efficient, robotic and biological swimming mechanisms.