A new disulfide cross-linking strategy was developed to prepare hyaluronic acid (HA) hydrogel from thiolmodified HA. First, dithiobis(propanoic dihydrazide) (DTP) and dithiobis(butyric dihydrazide) (DTB) were synthesized and then coupled to HA with carbodiimide chemistry. Next, disulfide bonds of the initially formed gel were reduced using dithiothreitol (DTT) to give, after exhaustive dialysis, the corresponding thiol-modified macromolecular derivatives HA-DTPH and HA-DTBH. The degree of substitution of HA-DTPH and HA-DTBH could be controlled from 20% to 70% of available glucuronate carboxylic acid groups.
The pK a values of the HA-thiol derivatives were determined spectrophotometrically to be pK a ) 8.87 (HA-DTPH) and pK a ) 9.01 (HA-DTBH). The thiol groups could be oxidized in air to reform disulfide linkages, which resulted in HA-DTPH and HA-DTBH hydrogel films. Further oxidation of these hydrogels with dilute H 2 O 2 created additional cross-links and afforded poorly swellable films. The disulfide cross-linking was reversible, and films could be again reduced to sols with DTT. Release of blue dextran from crosslinked films was used as a model for drug release. The rapid gelation of the HA-DTPH solution under physiological conditions was also achieved, which demonstrated the capacity for in situ cell encapsulation. Thus, L-929 murine fibroblasts were encapsulated in HA-DTPH hydrogel; these cells remained viable and proliferated during 3 days of culture in vitro.