Inspired by the snap-through action of a steel hairclip, we propose a design method for in-plane prestressed mechanisms that exhibit biomimetic morphing and high locomotion performance. Compliant bistable flapping mechanisms are fabricated using this method and are mounted on our untethered soft robotic fish. Using this mechanism, we achieve life-like undulation with a Strouhal number (1) of St = 0.28 and a velocity of 2.03 body lengths per second (43.6 cm/s), a three-fold improvement over past compliant fish robots. A tethered pneumatic version indicates that this mechanism is compatible with soft actuators. We study the mechanism both computationally and experimentally and suggest that elastic instability may offer a path to overcome the speed challenge of soft and compliant robots.

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