Biohybrid toggle switches: leveraging bistability in muscle-actuated machines

Tissue-engineered skeletal muscle offers robotics unique advantages in energy density, adaptability, and self-repair, but current biohybrid machines couple muscle directly to artificial skeletons, limiting net force output and requiring continuous stimulation maintain mechanism positions. Here, we introduce a biohybrid bistable switching mechanism that converts antagonistic muscle contractions to snap-through actuation of an elastomeric four-bar flexure, maintaining stable positions without energy consumption. We decouple passive muscle pre-tensioning from flexure operation with thin-film elastomer resistance bands, enabling fine tuning of spring stiffness and force to match muscle mechanical output envelopes. Leveraging mesoscale additive manufacturing, spin coating, and replica molding techniques, components are integrated into a millimeter-scale linear stage to axially align muscles with artificial components. Our prototype achieves reversible toggling between bistable states with a 130 ms switching time, establishing skeletal muscle actuators as viable electromechanical switches, and broadening biohybrid robotics towards novel applications such as grippers, valves, and mechanical computers.