Fig. 3. Degradation of multimodal degradable hydrogels.

Degradation of hydrogels in response to different stimuli was studied by monitoring the storage modulus as a function of time. (A) Hydrogels exhibited a rapid decrease in the modulus in response to externally applied, low doses of light (10 mW/cm² at 365 nm, 30 sec pulse, closed symbols), and (B) complete degradation is observed after ~ 4.5 minutes of continuous exposure. The negative control hydrogels, which lack a photolabile group, show no significant change in modulus over time in response to the same light exposure (open symbols). The hydrogels also were responsive to low intensity visible light (10 mW/cm² at 400 to 500 nm, continuous irradiation, inset). (C) In a thiol-rich reducing microenvironment like that observed in tumors (ca. 10 mM GSH), a decrease in the modulus of the multimodal degradable hydrogel (closed symbols) was observed, indicating responsive degradation due to reversible click and thiol exchange reactions of aryl thiol based thioether succinimide linkages with GSH under reducing conditions. Complete degradation is observed in approximately 2 days. (D) In an aqueous microenvironment (phosphate buffer), the multimodal degradable hydrogel exhibited slower degradation due to hydrolysis of ester linkages with complete degradation in approximately 4 days (closed symbols). The negative control (no degradable groups), after initial swelling, was relatively stable during the experimental time frame (open symbols). The data shown illustrate the mean (n ≥ 3) with error bars showing the standard error.