Mechanically Programmable Composite Metamaterials with Switchable Positive/Negative Poisson's Ratio

Design of mechanical metamaterials with a negative Poisson's ratio (NPR), known as auxetics, is of great importance in the development of flexible electronics and tissue engineering. However, the mechanical performance of conventional auxetic metamaterials is largely hindered by the high porosity and lack of tunability, which restricts their practical engineering applications. Here, this study presents a programmable approach for designing integrated two-phase auxetic composite metamaterials with pore-free structure, robust interfaces, and customized mechanical properties by substituting the vacancy of the auxetic lattices (frame phase) with hydrogel-based soft fillers (matrix phase). Manipulated by two-phase stiffness contrast, integrated composite metamaterials exhibit a functional transition between auxetic and non-auxetic behaviors. Rationally designing Young's modulus ratios in spatial domains endows the programming of composite metamaterials with the desired spatial heterogeneity. Furthermore, the incorporation of temperature-responsive hydrogels to auxetic frames also enbles the development of dynamic programmable composites with the thermo-driven adaptability. As a practical application demonstration, designed programmable composites are used to dynamically tune the phononic bandgaps. The design strategy of integrated two-phase composite metamaterials paves the way for the development of advanced auxetic engineering materials with programmable mechanical properties, providing opportunity for conformal electronic patches, bio-scaffolds, and smart functionality applications.