Large Shape Transforming 4D Auxetic Structures

Three-dimensional (3D) printing of active materials is a rapidly growing research area over the last few years. Numerous works have shown potential to revolutionize the field of four-dimensional (4D) printing and active self-deploying structures. Conventional manufacturing technologies restrict the geometric complexity of active structures. 3D printing allows the fabrication of complex active structures with no assembly required. In this study, we propose active 3D printed auxetic meta-materials that are capable of achieving area changes up to 200%. With these meta-materials, we design geometrically complex active structures that can be programmed into versatile shapes and recover their original shape given an external stimulus. We simulate the proposed meta-materials based on thermoviscoelastic material properties obtained by experimental characterization. A reduced beam model is constructed to predict forces and deformations of complex active structures. Excellent correlation is found between finite element simulation and experimental data from a 3-point bending test. Rectilinear tiling of the proposed meta-materials achieves the desired shape transformation. To demonstrate versatile programming, a selected meta-material is tiled into a complex contour, programmed into an arbitrary shape, and recovers as predicted. Simulation results verify this behavior. Such programmability in conjunction with 3D printing may be further exploited for applications such as biomedical devices, civil structures, and aerospace.