Motorizing fibres with geometric zero-energy modes

Responsive materials(1-3) have been used to generate structures with built-in complex geometries(4-6), linear actuators(7-9) and microswimmers(10-12). These results suggest that complex, fully functional machines composed solely from shape-changing materials might be possible(13). Nonetheless, to accomplish rotary motion in these materials still relies on the classical wheel and axle motifs. Here we explore geometric zero-energy modes to elicit rotary motion in elastic materials in the absence of a rigid wheel travelling around an axle. We show that prestrained polymer fibres closed into rings exhibit selfactuation and continuous motion when placed between two heat baths due to elastic deformations that arise from rotational-symmetry breaking around the rod's axis. Our findings illustrate a simple but robust model to create active motion in mechanically prestrained objects.