Characterization of the chemical degradation of hyaluronic acid during chemical gelation in the presence of different cross-linker agents.

Dynamic light scattering (DLS) and rheological experiments have been performed on semidilute aqueous hyaluronic acid (HA) solutions during the chemical cross-linking process with a water-soluble carbodiimide (WSC) to produce a hydrogel. The formation and destruction of the gel are characterized. The results suggest that the gel is cross-linked via ester linkages and at later stage in the process, the omnipresent hydrolysis of interpolymer ester linkages and glycosidic bonds prevails, leading to disruption of the gel. The process of forming and breaking the gel is affected by the cross-linker concentration and pH. The cross-linking of HA with WSC in the presence of L-lysine methyl ester produced a gel with a longer time of gelation and the degradation of the gel was prolonged because of the more stable amide bond formation as the cross-link. By using the Ugi multicomponent condensation reaction, interpolymer cross-linking occurs via the formation of amide linkages and a stable gel evolves, which is only slightly degraded over an extended time window. DLS measurements on HA solutions with WSC show the emergence of a long-time power-law tail in the correlation functions at conditions both before and beyond the viscosity maximum. At a late stage in the gel-breakage regime, the power-law profile of the decay disappears and the long-time tail of the correlation function can be portrayed by a stretched exponential. The findings indicate that the power-law feature is associated with the confinement of chain dynamics and anomalous diffusion in the system. At later times, the connectivity is lost due to fragmentation of the network, and the long-time stretched exponential decay in the correlation function reflects the relaxation of clusters of various sizes.