An epigenetic bi-stability of structural metamaterials undergoing dual matching shape-memory effects

With the development of structural metamaterials and 3D printing technology, the polymorphic shape-memory metamaterials have attracted extensive attention. This study aims to design a structural 3D printed shape-memory metamaterial, of which the epigenetic bi-stability of shape-fixity and recovery behaviors have been achieved by means of the dual matching nominal moduli and geometrical size optimization. Epigenetic bi-stability and dual matching refer to the combination of bi-materials in a specific structure, in response to external stimuli to show adjustable bi-stability. Elastic thermoplastic polyurethane (TPU) and viscoelastic polylactic acid (PLA) both are thermally responsive shape-memory polymers (SMPs) and have been employed to fabricate the structural metamaterial, of which the nominal modulus is tailorable owing to the dual matching shape-memory effects (SMEs) of two SMP components. Furthermore, the effects of structural parameters, i.e., width of framework and width of internal support, on the nominal modulus have been investigated for the structural metamaterials, of which the shape-fixity and recovery ratios have been characterized using finite element method (FEM) analyses and experimental measurements. Finally, a constitutive relationship among structural parameters, dual matching SME and nominal modulus has been identified to explore the working principle of epigenetic bi-stability in shape-memory metamaterials. This study provides a design strategy for a shape-memory metamaterial with a post-switchable bi-stability, through dual matching SME and geometrical size optimization.