Properties of Poly(ethylene glycol) Hydrogels Cross-Linked via Strain-Promoted Alkyne–Azide Cycloaddition (SPAAC)

A series of poly(ethylene glycol) (PEG) hydrogels was synthesized using strain-promoted alkyne-azide cycloaddition (SPAAC) between PEG chains terminated with either aza-dibenzocyclooctynes or azide functionalities. The gelation process was found to occur rapidly upon mixing the two components in aqueous solution without the need for external stimuli or catalysts, making the system a candidate for use as an injectable hydrogel. The mechanical and rheological properties of these hydrogels were found to be tunable by varying the polymer molecular weight and the number of cross-linking groups per chain. The gelation times of these hydrogels ranged from 10 to 60 s at room temperature. The mass-based swelling ratios varied from 45 to 76 at maximum swelling (relative to the dry state), while the weight percent of polymer in these hydrogels ranged from 1.31 to 2.05%, demonstrating the variations in amount of polymer required to maintain the structural integrity of the gel. Each hydrogel degraded at a different rate in PBS at pH = 7.4, with degradation times ranging from 1 to 35 days. By changing the composition of the two starting components, it was found that the Young's modulus of each hydrogel could be varied from 1 to 18 kPa. Hydrogel incubation with bovine serum albumin showed minimal protein adsorption. Finally, a cell cytotoxicity study of the precursor polymers with 3T3 fibroblasts demonstrated that the azide- and strained alkyne-functionalized PEGs are noncytotoxic.