Spatio-Selective Reconfiguration of Mechanical Metamaterials Through the Use of Dynamic Covalent Chemistries

Mechanical metamaterials achieve unprecedented mechanical properties through their periodically interconnected unit cell structure. However, their geometrical design and resulting mechanical properties are typically fixed during fabrication. Despite efforts to implement covalent adaptable networks (CANs) into metamaterials for permanent shape reconfigurability, emphasis is given to global rather than local shape reconfiguration. Furthermore, the change of effective material properties like Poisson's ratio remains to be explored. In this work, a non-isocyanate polyurethane elastomeric CAN, which can be thermally reconfigured, is introduced into a metamaterial architecture. Structural reconfiguration allows for the local and global reprogramming of the Poisson's ratio with change of unit cell angle from 60° to 90° for the auxetic and 120° to 90° for the honeycomb metamaterial. The respective Poisson's ratio changes from -1.4 up to -0.4 for the auxetic and from +0.7 to +0.2 for the honeycomb metamaterial. Carbon nanotubes are deposited on the metamaterials to enable global and spatial electrothermal heating for on-demand reshaping with a heterogeneous Poisson's ratio ranging from -2 to ≈0 for a single auxetic or +0.6 to ≈0 for a single honeycomb metamaterial. Finite element simulations reveal how permanent geometrical reconfiguration results from locally and globally relaxed heated patterns.