Lightweight Self‐Forming Super‐Elastic Mechanical Metamaterials with Adaptive Stiffness

Scarcity of stiff, yet compliant, materials is a major obstacle toward biological‐like mechanical systems that perform precise manipulations while being resilient under excessive load. A macroscopic cellular structure comprising two pre‐stressed elastic “phases” is introduced, which displays a load‐sensitive stiffness that drops by 30 times upon a “pseudoductile transformation” and accommodates a fully recoverable compression of over 60%. This provides an exceptional 20 times more deformability beyond the linear‐elastic regime, doubling the capability of previously reported super‐elastic materials. In virtue of the pre‐stressing process based on thermal‐shrinkage, it simultaneously enables a heat‐activated self‐formation that transforms a flat laminate into the metamaterial with 50 times volumetric growth. The metamaterial is thereby inherently lightweight with a bulk density in the order of 0.01 g cm−3, which is one order of magnitude lower than existing super‐elastic materials. Besides the highly programmable geometrical and mechanical characteristics, this paper is the first to present a method that generates single‐crystal or poly‐crystal‐like 3D lattices with anisotropic or isotropic super‐elasticity. This pre‐stress‐induced adaptive stiffness with high deformability could be a step toward in situ deployed ultra‐lightweight mechanical systems with a diverse range of applications that benefit from being stiff and compliant. A type of self‐stressed cellular structure achieves both high stiffness and high deformability through stiffness‐adaptation upon external load. Transformed from a stack of membranes under heat, it is inherently lightweight and realizes superelastic 3D crystal‐like lattices. It allows soft machines at a wide range of scales to carry load in a stiff manner while being resilient under excessive load.