Fig. 3: The effects of distribution of residual stresses on the out-of-plane deformation of 4D printed disks.

The PLA disks were divided into two regions of which the inner part was printed at the minimum printing speed (i.e., 20 mm s⁻¹) while the outer was printed with the maximum speed (i.e., 80 mm s⁻¹). Four different ratios were used (i.e., \({R}_{{in}}/R\) = 60%, 70%, 80%, and 90%). Such a segmentation of the disks resulted in a discontinuous out-of-plane deformation of the specimens (a–left) and the development of a positive Gaussian curvature (i.e., \(K \, > \, 0\)) and a negative mean curvature (i.e., \(H \, < \, 0\)) (a–right). We then switched the printing speed of the inner (80 mm s⁻¹) and outer (20 mm s⁻¹) parts of the disk. This change resulted in a completely different shape transformations (b–left) and local curvatures (b–right). The first deposited layers (F.D.L) are illustrated in (a) and (b).