Fig. 3. (A) Schematic design of a photoresponsive hydrogel that undergoes cyclic and reversible photoinduced transitions between a hydrogel of high stiffness and low stiffness, respectively. The stiff hydrogel is crosslinked by glucosamine–boronate ester bridges and duplex nucleic acids composed of the (2)–(4) and (3)-trans-azobenzene (5) (t-azo (5)). Photoisomerization of the t-azo (5) units to c-azo (5) (λ = 365 nm) leads to the separation of the nucleic acid duplexes, and to the formation of a low-stiffness hydrogel. The reverse photoisomerization of c-azo (5) (λ > 420 nm) restores the high-stiffness hydrogel. (B) Rheometry experiments that follow the photochemically generated hydrogels of high and low stiffness: (a) and (a′) correspond to the G′ and G′′ values of the stiff hydrogel crosslinked by the glucosamine–boronate esters and the (2)–(4)/(3)-t-azo (5) duplexes. (b) and (b′) correspond to the G′ and G′′ values of the photogenerated low-stiffness hydrogel crosslinked by the glucosamine–boronate ester only and unlocked (2)–(4) and (3)-c-azo (5). (C) Cyclic and reversible photoinduced G′/G′′ values of the stiff hydrogel crosslinked by the glucosamine–boronate ester bridges and (2)–(4) and (3)-t-azo (5) crosslinkers, and the low-stiffness hydrogel crosslinked by only the glucosamine–boronate ester crosslinking units. (D) SEM images corresponding to the photo-induced reversible transitions of the stiff hydrogel crosslinked by the glucosamine–boronate ester bridges and (2)–(4) and (3)-t-azo (5), crosslinkers, panel (I), and the low stiffness hydrogel crosslinked by the glucosamine–boronate ester only and unlocked (2)–(4) and (3)-c-azo (5), panel (II).