Figure 5: Asymmetric patterning of mechanically heterogeneous structures results in stiffness mismatch-induced hydrogel folding/unfolding.

(a) Various 3D folding patterns of Ca-alginate PAAm hydrogels via localized Fe³⁺ ion solution exposure to encode anisotropy into a hydrogel. Adapted from Ref. 82 with permission from the Royal Society of Chemistry. (b) Thermal actuation of a circular photo-patterned PNIPAM hydrogel ring. Between heating to 50° C and cooling to 22° C, the hydrogel de-swells, adopting a nearly planar shape, and swells into triple coiled state, respectively. Adapted from Ref. 81 with permission from Elsevier (c) Shape deformation of poly(NIPAM) and the poly(HEMA) induced with temperature changes. The hydrogel swollen state and adopts a folded 3D box conformation at room temperature and unfolds into the planar conformation upon heating to 60° C. Adapted from Ref. 77 with permission from of John Wiley & Sons.