Lightweight 3D Hierarchical Metamaterial Microlattices.

Hierarchical auxetic metamaterials are a class of materials which are characterized by a multi-tiered architecture and have the capability of exhibiting enhanced mechanical properties in comparison to their single-geometry counterparts. In this work, three distinct new classes of hierarchical auxetic metamaterials designed by incorporating cubic crystal lattice geometries, namely, Body-Centred Cubic (BCC), Face-Centred Cubic (FCC) and Tetrahedral Cubic (TC) are proposed into 3D rotating cube structures. Through the introduction of hierarchy, these relatively dense mechanical metamaterials are rendered lightweight, through a volume fraction reduction of over 90% in the majority of cases from their full-block (FB) counterparts, while retaining their original auxetic capabilities. These systems are also demonstrated to possess the ability to exhibit a wide range of stiffnesses and Poisson's ratios, including giant negative values, as well as superior stiffness/density ratios making them ideal for implementation in lightweight applications. Furthermore, a two-photon lithography 3D-printer is used to fabricate these new lattice structures at the microscale and test them in-situ. The results obtained provide clear and comprehensive evidence of the improvement imparted through the introduction of hierarchy and the advantages of using this method to design lightweight 3D rotating unit auxetic structures.