Molecular Engineering of Injectable, Fast Self-Repairing Hydrogels with Tunable Gelation Time: Characterization by Diffusing Wave Spectroscopy

Injectable and self-repairing hydrogels with tunable gelation time have excellent application prospects in drug release and tissue engineering. In this study, we synthesized the above hydrogels through acylhydrazone bonds between the polyacrylate containing the aldehyde group and azobenzene group (P(Azo-co-EGMA)) and the P(N-isopropylacrylamide-co-acrylhydrazide) containing the hydrazide group (P(NIPAM-co-AH)). The gelation time (tsol–gel) could be controlled from a few seconds (<5 s) to several hours (20 h) by changing the total polymer concentration or the molecular weight of the polymers. After a period of hydrogel formation observed by the vial inversion method, the hydrogel reached a stable state, and the stable hydrogel state time (tstable) recorded by diffusing wave spectroscopy (DWS) was much longer than tsol–gel. The elasticity index (EI = 217391) of stabilized hydrogel Gel 7 formed by polymers with higher molecular weight was about five times that of hydrogel Gel 4 formed by polymers with lower molecular weight, and the elastic modulus (G′ = 1766 Pa) of Gel 7 at the same frequency was about four times that of Gel 4. Besides, the gelation behavior could also be adjusted by adding β-cyclodextrins (β-CDs) and UV irradiation. The hydrogels displayed fast self-healing ability without any external stimuli and achieved pH-responsive sol–gel transition. Moreover, the hydrogels exhibited injectable ability and biocompatibility, which made the hydrogels more promising.