Oligomodal metamaterials with multifunctional mechanics

Abstract

Significance Mechanical metamaterials are man-made materials with extraordinary properties that come from their geometrical structure rather than their chemical composition. For instance, they can be engineered to be extremely light and stiff; to shrink sideways when compressed, instead of expanding; or to exhibit programmable shape changes. Such properties often rely on zero-energy modes. In this work, we created a class of mechanical metamaterials with zero-energy modes that can exhibit multiple properties at the same time within a single structure. In particular, we created a metamaterial that can either shrink or expand on the side when compressed, depending on how fast it is compressed. These metamaterials could lead to novel adaptable devices for, for example, robotics and energy absorption applications. Mechanical metamaterials are artificial composites that exhibit a wide range of advanced functionalities such as negative Poisson’s ratio, shape shifting, topological protection, multistability, extreme strength-to-density ratio, and enhanced energy dissipation. In particular, flexible metamaterials often harness zero-energy deformation modes. To date, such flexible metamaterials have a single property, for example, a single shape change, or are pluripotent, that is, they can have many different responses, but typically require complex actuation protocols. Here, we introduce a class of oligomodal metamaterials that encode a few distinct properties that can be selectively controlled under uniaxial compression. To demonstrate this concept, we introduce a combinatorial design space containing various families of metamaterials. These families include monomodal (i.e., with a single zero-energy deformation mode); oligomodal (i.e., with a constant number of zero-energy deformation modes); and plurimodal (i.e., with many zero-energy deformation modes), whose number increases with system size. We then confirm the multifunctional nature of oligomodal metamaterials using both boundary textures and viscoelasticity. In particular, we realize a metamaterial that has a negative (positive) Poisson’s ratio for low (high) compression rate over a finite range of strains. The ability of our oligomodal metamaterials to host multiple mechanical responses within a single structure paves the way toward multifunctional materials and devices.