4D printing of transformable re-entrant auxetic metastructures using shape memory polymers for wearable protective devices

4D printing on shape memory polymers (SMPs) enables transformable structures capable of programmable and reversible shape morphing, offering significant potential for adaptive wearable protection. In this study, transformable re-entrant auxetic metastructures based on SMP were designed and fabricated via 4D printing to simultaneously achieve high stretchability and reliable shape recoverability. Finite element analyses incorporating a thermo-viscoelastic constitutive model were conducted to accurately capture temperature-dependent deformation during thermal programming. Both numerical and experimental results demonstrate that the re-entrant auxetic structure exhibits superior stretchability and recovery performance, owing to its bending-dominated deformation mechanism, which enables large ligament rotation while mitigating stress concentration. Compared with conventional porous structures, the re-entrant design achieved substantially larger recoverable deformation and improved structural integrity during repeated transformations. A transformable wrist splint was developed as a representative application, which can be programmed from a compact 2D configuration (50 × 60 mm) into a personalized 3D shape and subsequently recovered within 10 s at 60°C, maintaining a recovery ratio exceeding 97% over repeated cycles. These results demonstrate that combining SMPs with 4D-printed re-entrant auxetic metastructures provides an effective strategy for developing lightweight, portable, and reusable wearable protective devices with enhanced transformability and mechanical robustness.