Untethered soft robots that swim by snapping

Mobile robots usually need a tether or onboard power and control. We built small soft swimmers with none of these, each printed in one piece and following a trajectory fixed at design time. A shape-memory polymer muscle slowly loads a bistable element whose snap-through releases stored energy as a fast paddle stroke, so a slow thermal stimulus yields rapid propulsion, and an asymmetric energy landscape rectifies the stroke into net motion. Tuning each actuator's threshold fires strokes in sequence and fin geometry sets heading, so the whole path is encoded in the morphology; a demonstrator swims out, releases its cargo, and returns with no feedback. The structure is the controller. The work appeared in PNAS.