Electrical stimulation of untethered muscle ring actuator. Muscle rings produce significant contractile force even when untethered to a mechanical skeleton, as demonstrated here in response to electrical stimulation (2 Hz) on day 12 after seeding. These muscle rings were maintained in the injection mold throughout the growth and differentiation processes. In this movie, some net locomotion of the ring is observed, but directionality is not controlled. Coupling the modular ring to a forward-engineered 3D printed skeleton would enable controllable directional locomotion.