Active reconfiguration of multistable metamaterials for linear locomotion

Buckling-driven shape-shifting is increasingly being used in metamaterials to achieve mechanical programmability and novel functionality. Here, we show that the post-buckling response and the ensuing behavior, of lattice metamaterials can be programed through so-called passive and active modal nudging. Numerical continuation is first used to explore exhaustively the bifurcation manifold of a compressed, elastomeric lattice metamaterial. We then tailor the natural postcritical behavior of the metamaterial by judiciously altering the baseline geometry using modal information extracted from the bifurcation landscape (passive modal nudging). Experimental tests verify the effectiveness of the approach. Subsequently, shape change is induced by actively nudging the metamaterial between two stable postcritical states using an embedded actuator, i.e., by changing the local topology of the metamaterial rather than by controlling a global field. Fundamentally, we use passive modal nudging to bias the metamaterial towards one of three possible postcritical states under applied compression and then employ active nudging to switch between different postcritical states. Based on this paradigm, we manufacture and test a crawling soft robot and demonstrate that locomotion through local actuation to switch between two postcritical states is significantly more energy efficient than the alternative strategy of switching between a precritical and a postcritical state through global actuation. Overall, this paper demonstrates the benefits and promise of programming the behavior of soft metamaterials by exploiting principles of bifurcation theory and using tailored imperfections and embedded actuators.