Realizing three-dimensional chiral metamaterials with tailored macroscopic behaviour
Modern manufacturing methods like additive manufacturing make it possible to precisely control the shape of three-dimensional (3D) microstructures. With the help of these tools, materials with surprising and unprecedented mechanical properties can be rationally designed and realized. Our contribution focuses on the geometrical feature chirality. The lack of mirror symmetry in a periodic 3D microstructure leads to a behavior strictly forbidden by the laws of classical “Cauchy” continuum mechanics – it can couple pure expansions or compressions with rotational deformation modes.
Starting with a very simple chiral unit cell, we present numerical studies of the strong size dependence of the chiral response, which causes the twist-to-strain coupling to vanish if a sample’s dimensions become much larger than the characteristic length scale of the metamaterial. By combining chiral and achiral structural elements, we develop a design principle that allows tailoring the magnitude of the chiral response itself as well as the size effects over orders of magnitude . Furthermore, we show that the decay of the twist can be avoided by building non-periodic annular structures. Building on these studies, we add achiral coupling elements to a previously studied blueprint  of a chiral metamaterial and optimize the design for fabrication via multifocus direct laser writing . The chiral response of the built structures can be tailored to specific requirements. By directly comparing numerical simulations with mechanical in-situ tests, we show that the resulting metamaterial exhibits a large twist-to-strain coupling up to twists of several degrees in samples comprising more than one hundred thousand unit cells.
 P. Ziemke et al., Tailoring the characteristic length scale of 3D chiral mechanical metamaterials, Extreme Mech. Lett., (2019)
 T. Frenzel et al., Three-dimensional mechanical metamaterials with a twist, Science, (2017)
 V. Hahn et al., Rapid Assembly of Small Materials Building Blocks (Voxels) into Large Functional 3D Metamaterials, Advanced Functional Materials, (2020)