Scalable 3D printing for topological mechanical metamaterials

Mechanical metamaterials are structures designed to have exotic response, for example topological soft modes at their edge. Using a combination of finite-element simulations and experimental prototyping, we show how to 3D print these structures from a single material. We begin with a 3D ball-and-spring lattice for which we compute all modes of deformation and a topological winding number. We then translate the lattice geometry into a 3D-printed structure by replacing springs by beams composed of polymerized resin, having varying cross-section, and being connected at the lattice nodes. We confirm in finite-element simulations that in the printed material, the surface is softer than the bulk. Within the linear and small non-linear regimes, the softest side is the one predicted by the ball-and-spring model. Surprisingly, we find the opposite side to be softest at large deformations. We print the structure using stereolithography (SLA) and use a universal testing machine to confirm the presence of edges that are softer than the bulk. We find the edge softness to be dependent on deformation amplitude, constituent material, and fabrication defects. Our work contextualizes the predictions of topological mechanics for real 3D materials and their potential for cushioning applications.