Self-oscillating poroelastic instabilities

Responsive materials changing shape in response to physical stimuli such as light, heat or chemical reactions are ubiquitous in nature and have recently sparked a myriad of applications, e.g. in smart coatings and soft robotics. So far, stimuli-responsive materials are not “active”: they cannot exhibit a perpetual motion unless the environmental conditions change. Here we show experimentally, numerically and theoretically how a simple system - a confined beam at the boundary of a solvent interface - can harness a combination of poroelastic swelling and elastic instabilities to generate perpetual oscillations. These oscillations are realised by rational design and optimisation of the time-dependent hysteretic loop coupling mechanical deformation to solvent transport within the structure. Our work provides general principles for active systems that operate autonomously, harnessing energy from their environment and as such offers new vistas for active metamaterials and soft robots.