Fig. 5: The combinatorial effects of distribution of residual stresses and bi-material on the out-of-plane deformation of 4D printed disks.

The distribution of residual stresses across the thickness determines the bending orientation of the PLA disks. We introduced higher residual stress at the bottom layer of the PLA-disk (i.e., the surface which is in contact with the printing build-plate) by using a faster printing strategy for the initial layers. Such manipulation of the imperfection resulted in an inverse bending of the PLA disks (a–left, position 2) with an opposite localized curvature as compared to the case where the higher gradient was located at the top layer of the PLA disks (a–left, position 1). By precisely positioning the imperfections in the PLA disks and integrating the concept of inverse bending with that of out-of-plane deformation, one can achieve more complex shape transformations (b–left) (see Supplementary Movie 2 for the dynamic visualization of the shape-shifting process). Even more complex shapes can be achieved by including the soft polymeric phase in the design of the 4D printed disks with alternating the position of the residual stress (b–right). The distribution of the soft phase around the periphery of the PLA disks can also contribute to its overall shape-shifting behavior (c).