Reversible shape change structures by grayscale pattern 4D printing

Structures and devices with reversible shape change (RSC) are highly desirable in many applications such as mechanical actuators, soft robotics, and artificial muscles. In this paper, we propose to use 3D grayscale printing method to create reversible self-folding structures. The grayscale pattern was used to control the light intensity distribution of a UV projector in a digital light processing 3D printer such that the same photo irradiation time leads to different curing degrees and thus different crosslinking densities at different locations in the polymer during 3D printing. After leaching the uncured oligomers inside the loosely crosslinked network, bending deformation could be induced due to the volume shrinkage. The bending deformation was reversed if the bent structure absorbed acetone and swelled. Using this method, we designed and created RSC structures such as reversible pattern transformation and self-expanding/shrinking structures, auxetic metamaterial, structures mimicking the blossom of a flower. The grayscale 4D printing method provides us a simple and efficient way to create active structures and has great potential in the application of smart structures, composite materials, soft robotics and endovascular stent.