Bistable Design in Topological Mechanical Metamaterials

Quantum topological states of matter have been extensively used for studying novel mechanical metamaterials with topologically protected properties. Mechanical metamaterials exhibit exotic properties governed by structures rather than constituents. Maxwell lattices represent a class of topological mechanical metamaterials (TMMs) that exhibit floppy modes localized on the soft edge when they are topologically polarized. Achieving topological transformation in these materials can enable on-and-off switching of hard and soft edge states, providing new paths of exploring programmable mechanical response and wave propagation. However, it is desirable yet extremely challenging to control the topological polarization transition of a Maxwell lattice. Here a Maxwell lattice with bistable units is designed to implement synchronized transitions. These bistable units in the lattice exhibit two stationary configurations, at the topologically polarized and non-polarized phases, respectively. Remarkably, the bistable units allow the lattice to be conveniently and swiftly switched between the polarized and non-polarized phases. And we demonstrate dramatically different stiffnesses at the opposite surfaces in the topologically polarized phase both theoretically and experimentally. This new design will help provide novel TMMs with potential applications on stiffness tuning, impact mitigation, mechanological and neuromorphic computation, and flexible robotics.