Shape memory mechanical metamaterials

Abstract

Shape memory materials can maintain temporary shapes without external constraints and revert to their permanent shape upon exposure to an external stimulus, such as heat, light, or moisture. This behavior, often named the shape memory effect, has led to the use of shape memory materials in a variety of applications including deployable aerospace structures, biomedical devices, flexible electronics, and untethered soft robots. Most thermally triggered reconfigurable metamaterials using shape memory polymers require a laborious process of thermomechanical programming at high temperature, above their transition value, to maintain a temporary shape. In this paper, we utilize two 3D-printable polymeric materials that do not rely upon their shape memory effect to generate robust shape memory response in a set of mechanical metamaterials. The enabling characteristic is the mismatch of the temperature-dependent moduli of the constitutive materials leveraged in rationally interconnected reconfigurable units, and their hallmark is the freedom to forego the complex programming process of typical shape memory polymers. Their shape reconfiguration and rapid recovery are solely governed by mechanical loading and temperature change, leading to sequentially programmable multistability, hyperelasticity, giant thermal deformations, and shape memory capacity. Theoretical models, numerical simulations, and thermomechanical experiments are performed to demonstrate their functionality, stability transition mechanism, and potential applications.