A reprogrammable mechanical metamaterial with stable memory

Metamaterials are designed to realise exotic physical properties through the geometric arrangement of the underlying structural layout. Traditional mechanical metamaterials are designed to achieve functionalities such as target Poisson's ratio or shape transformation through unit cell optimisation. Once fabricated, these functionalities are programmed into the layout of the metamaterial in a way that cannot be altered. Conversely, in hard disk drives for example, each unit can be written-to or read-from on-the-fly. Here, to overcome this challenge, we demonstrate a design framework for a tileable mechanical metamaterial with stable memory at the unit cell level. Our design comprises an array of physical binary elements (m-bits), analogous to digital bits, with clearly delineated writing and reading phases. Each m-bit can be independently switched between two stable states (i.e., memory) by using magnetic actuation to move between the equilibria of a bistable shell. Under deformation, each state is associated with a significantly different mechanical response that is fully elastic. Encoding a set of binary instructions onto the tiled array yields markedly different mechanical properties. We hope that our design paradigm will instigate novel mechanical metamaterials.