Degradation behavior of covalently cross-linked poly(aldehyde guluronate) hydrogels

There is a significant need in the biomedical field for hydrogels with controllable mechanical and degradative properties. We now report the degradation behavior of novel hydrogels formed by the cross-linking of poly(aldehyde guluronate) (PAG). PAG was prepared from alginate by acid hydrolysis and oxidation and was covalently cross-linked with adipic acid dihydrazide (AAD) to form hydrogels. These hydrogels were degradable in aqueous media due to the hydrolysis of hydrazone bonds formed between the aldehyde of PAG and the hydrazide of AAD. We confirmed the degradation behavior of FAG hydrogels cross-linked with different concentrations of AAD by monitoring the changes of dry weight, modulus, and degree of swelling. The degradation behavior generally depends on the cross-linking density as calculated from the Flory-Rehner equation. However, hydrogels with many dangling single-end molecules showed a retarded degradation behavior irrespective of their low initial modulus and low degree of cross-linking density. We hypothesize this is due to re-cross-linking of dangling single-end molecules during degradation. The mechanical properties and degradation time of hydrogels typically increase or decrease together as the degree of cross-linking is varied. However, the findings of the current study suggest that the mechanical properties and degradation time can be decoupled by utilizing partially bound cross-linking molecules that are capable of reversibly cross-linking the polymer to form the hydrogel. This approach to control the properties of hydrogels may find wide utility in the biomedical field and other applications of these materials.