Three-Dimensionally Printed Mechanical Metamaterials with Thermally Tunable Auxetic Behavior

The extraordinary properties of mechanical metamaterials are created from the geometrical design of their internal structures. As a result, the nature of classical mechanical metamaterials usually would not change and one geometrical design only corresponds to one set of mechanical behaviors. This restricts the application of metamaterials where an adaptive behavior is desirable. In this work, we demonstrate the dynamic and on-demand regulation of mechanical metamaterials through a combination of structural design and multimaterial 3D printing. We design a soft periodic lattice metamaterial containing two distinct deformation modes. One deformation mode is controlled by the zig-zagged topological defects, and the other one is controlled by the thermal activation of the responsive materials imbedded in the lattice. By regulating the deformation mode with ambient temperature, the effective Poisson ratio of the lattice can be intentionally switched between negative values and positive values. The mechanism of thermally tunable deformation is investigated both experimentally and theoretically, resulting in a predictive design space of the metamaterial. We believe our design principle opens up a new route for the exploration of metamaterials exhibiting extraordinary and programmable mechanical behaviors.