Programming polymorphable yet stiff truss metamaterials in response to temperature

Shape morphing in response to an external stimulus can create geometric transformations from initially flat materials. Current strategies rely on thin layers made of soft materials that are inherently floppy to provide morphability, but cannot support their own weight and must forgo structural resistance, hence limiting their use as structural materials. Here, we demonstrate thermally actuated shape morphing of a three-dimensional framework that can evolve into complex, lightweight shapes that are both stiff and morphable, hence resolving the conflict between rigidity and morphability. Our building block is a truss unit cell that leverages the mismatched thermal expansions of its rigid constituents to generate programmable spatial deformation. By encoding specific geometric and multimaterial attributes into the truss system, we realize shape morphing on target and generate reversible structural transformations with complex curvature combinations. We also explore cooling and localized heating, unveiling temperature-dependent morphological evolutions and heat-source tracking. The morphable, yet stiff, truss metamaterial here introduced has potential applications in thermally responsive smart buildings, thermal management adaptive devices, deployable shelters in extreme environments, and mirror optical systems in space.