Synthesis and in vitro degradation of new polyvalent hydrazide cross-linked hydrogels of hyaluronic acid

New polyvalent hydrazide cross-linkers were synthesized, characterized, and used to prepare hydrazide cross-linked hydrogels derived from hyaluronic acid (HA). First, the chemical synthesis and characterization of the di-, tri-, tetra-, penta-, and hexahydrazides are presented. Second, KA concentration, buffer type and concentration, and ratio of HA to carbodiimide to cross-linker were varied to obtain HA-hydrogels with different chemical and physical properties. Third, two new assays are described to monitor the stability of HA-hydrogels toward hyaluronidase (HAse) and other media. These assays were used to evaluate the stability of cross-linked HA-hydrogels to HAse solutions and different pH values. Hydrophobic cross-linkers gave the most stable gels, and the susceptibility of the gels to HAse was independent of cross-linker concentration. HAse does not significantly penetrate the HA-hydrogels and acts primarily at the gel-solution interface. The HA-hydrogels are stable in acid environments and dissolve gradually above pH 7.0.