Harnessing 3D printed residual stress to design heat-shrinkable metamaterials

Heat-shrinkable materials, which are typically shape-memory polymers and irreversibly shrink in response to external heating stimuli, are vital for coating and protection applications in aerospace, micro-robotics, biomedical devices, and biomimetic systems. In these applications, they are often subjected to complex environmental conditions, such as thermal and mechanical loading, which induce a certain level of deformation. It is imperative to understand the resultant deformation and internal stress state under these loading conditions. Here we report a new class of heat-shrinkable metamaterials, which utilize photochemical expansion in 3D printing to release locally residual stress with the heat-shrinkage rate of 3–8%. Planar lattice metamaterials are designed to evolve into permanent heat-shrinkage status. The fundamental mechanisms that control heat-shrinkable behavior are identified and tested in thermal experiments. The photochemical expansion strain rises with the increase of layer printing time and nozzle proceeding direction. Furthermore, analytical predictions agree well with the experiments about the unrecoverable heat-shrinkable strain and the recoverable thermal strain of the proposed unit cell. The findings presented here enable heat-shrinkable metamaterials with simple fabrication and porosity, dramatically reducing required manufacture times and materials.