A Hierarchical Programmable Mechanical Metamaterial Unit Cell Showing Metastable Shape Memory

Hierarchically structuring materials open the door to a wide range of unexpected and uniquely designed properties. This work presents a novel mechanical metamaterial unit cell with strain-dependent solid-solid phase changes resultant from hierarchically structured mechanisms built into an auxetic unit cell, and further presents a realization of this kind. The interaction of auxetic structure and mechanism allows stable or metastable elastic energy states to be reached as a result of mechanical deformation. The result is a principally elastic analog to a shape memory material with a functional dependency on its negative Poisson's ratio. Prototypes are additively manufactured using direct laser writing, and are subsequently subjected to uniaxial compression with a customized micromechanical test set up. Experimental results depict reversible states initially triggered by deformation; the unit cell is a building block for a programmable material with a nonlinear if horizontal ellipsis then horizontal ellipsis relationship. Implementing interior mechanisms as a hierarchical level unlocks new directions for mechanical metamaterials research, and the authors see potential impacts or applications in multi-scale modeling, medicine, micro-actuation and -gripping, programmable matter/materials.