Fabricating lightweight and ultrastrong mechanical metamaterials

In the search for materials that are both light and strong, classic material design—such as optimizing the chemistry and/or microstructure of bulk materials—has been systematically exploited over centuries, leaving limited room for further improvements.1 Although major advancements have been made with respect to mechanical strength and density, light materials generally remain weak and heavy materials strong; hence, the two properties have historically been considered to be connected. However, in recent years, the field of so-called ‘metamaterials’ (materials engineered to possess properties not usually found in nature) has made considerable advances in the development of materials that are both light and strong. Metamaterials usually consist of assemblies of multiple repeating elements, and their special properties are primarily determined by their topology rather than their composition. Initially, these materials were designed to display unique optical, electromagnetic, or acoustic characteristics. Recently, mechanical metamaterials have also emerged, with principally opposing mechanical properties, such as both high stiffness and high damping (mechanical energy dissipation) capability2 or a negative Poisson’s ratio (i.e., a material that expands laterally when stretched).3 In addition, a class of lightweight mechanical metamaterials has been developed, inspired by natural hierarchical cellular materials and triggered by the recent evolution of high-resolution 3D printing technologies that enable the miniaturization of lattice structures. The properties of these lightweight metamaterials depend on the microscopic length scales of their patterns as well as their topologies.5–9 Because of their specifically designed architectures, these lattice materials reach remarkable strengths at low densities that might never be achieved using classic material Figure 1. Scanning electron microscopy images of a glassy carbon nanolattice. (a,b) A polymer microlattice fabricated by 3D printing. (c,d) Vacuum pyrolysis transforms the polymer to glassy carbon and isotropically shrinks the lattice by 80%, producing a nanolattice. Lattice distortion during pyrolysis is eliminated by including pedestals and coiled spring supports, distancing the lattice from the substrate. Scale bars: (a,c) 5 m, (b,d) 1 m. Reproduced with permission.4