Title: Ultra-light, Ultra-stiff Mechanical Metamaterials

Abstract: The mechanical properties of ordinary materials degrade substantially with reduced density, due to the bending of their structural elements under applied load. We report a class of micro-architected materials that maintain a nearly constant stiffness per unit mass density, even at ultra-low density. This performance derives from a network of nearly isotropic microscale unit cells with high structural connectivity and nanoscale features, whose structural members are designed to carry loads in tension or compression. Production of these microlattices, with constituent materials ranging from polymers to metals and ceramics, is made possible by using projection microstereolithography, an additive micromanufacturing technique, combined with nanoscale coating and post-processing. We found that these materials exhibit ultra-stiff properties across more than three orders of magnitude in density, regardless of the constituent material. One Sentence Summary: We report a class of micro-architected materials that change their stiffness linearly with reduced density. Main Text: Nature has found a way to achieve mechanically efficient materials by evolvin