4D printing of shape memory alloy metamaterials: Mechanisms, structures, and applications

Additive manufacturing (AM), also referred to as 3D printing, is an emerging fabrication technology that enables the construction of complex multiscale architectures with high material utilization through a layer-by-layer deposition strategy. Based on 3D printing, 4D printing introduces a stimulus-responsive functionality in the time dimension, thereby achieving the integrated design of material, structure, and intelligent functional response. With reversible deformation, reusable functionality, and a high degree of topological freedom, 4D printing exhibits promising advantages for lightweight and adaptive applications, and has consequently attracted extensive research interest. Following a technology-material-structure-application framework, the paper first reviews additive manufacturing methods for shape memory alloys (SMAs), together with their transformation behavior and principal alloy systems. The concept of metamaterials is then introduced, highlighting representative lattice structures and design strategies. Subsequently, emerging applications of SMA metamaterials are discussed. Finally, current challenges in 4D-printed SMA metamaterials are outlined, and perspectives on future development are proposed, intended to provide a reference for ongoing and forthcoming research. • A comprehensive review is presented on 4D printing of shape memory alloy metamaterials. • The mechanisms of SME and SE, along with representative SMA systems, are introduced. • Different types of metamaterial structures are discussed. • Emerging applications in medicine and engineering are highlighted, and the outlook is proposed.