(236d) Enhancing Crosslinking Efficiency and Mechanical Performance of Glycosaminoglycan Hydrogels
Chondroitin sulfate (CS) and hyaluronic acid (HA) are the main glycosaminoglycan (GAG) components of mammalian extracellular matrix (ECM) and are found in all connective tissues. Thus hydrogels based on these glycosaminoglycans have been widely used as tissue engineering scaffolds. However, for load bearing applications GAG based hydrogels do not have adequate mechanical properties as they are polyelectrolytes and are typically lightly crosslinked, which leads to a high water content and a brittle nature. Thus efficient crosslinking methods are required to improve the mechanical performance of such gels. Several crosslinking strategies have been employed to improve the mechanical properties of GAG structures. The primary technique used in current study is to functionalize the GAGs with methacrylate groups and crosslink the macromolecules with low levels of oligo (ethylene glycol) (meth)acrylate monomers to improve the crosslinking efficiency.
Compressive elastic and shear moduli of the CS and HA gels were significantly increased by copolymerization of oligo (ethylene glycol) acrylate as crosslinkers. In contrast, analogous methacrylate crosslinkers inhibited crosslinking. The difference is correlated with differences in reactivity ratios of acrylates and methacrylates. It was further demonstrated that the increases in moduli was due to participation of the acrylate groups of the OEGDAs in the kinetic chains of MHA/MCS methacrylate groups, as monoacrylate PEGs increased the moduli in the same way as the diacrylates. The increase in moduli was determined to be a function of the number of EG repeats in the OEGDA and the degree of substitution of functional group on the GAG chain. For example, the shear modulus of a 13 wt% methacrylated chondroitin sulfate (MCS) gel was increased from 2kPa to 45kPa by 10 % increase in the substitution degree. Using poly (ethylene glycol) diacrylate 700 Da as the crosslinker, the shear modulus of crosslinked MCS was 4 fold higher than the parent MCS gel. After crosslinking with tetra (ethylene glycol) diacrylate, elastic and shear modulus of methacrylated hyaluronic acid (MHA) 13 wt% gels increased from 586 kPa and 170 kPa to 777kPa and 212 kPa respectively. While the moduli could be increased by reaction with OEGDAs, the fracture strains were largely unaffected by the crosslinking. This was attributed to the extended chain conformation of the GAGs in water, as the fracture strains increased as ionic strength was increased. GAG crosslinking by photopolymerization will also be contrasted with crosslinking by other methods which do not involve growth of kinetic chains in free radical polymerization, but rather link the GAG chains directly with difunctional reagents like vinyl sulfone-modified PEGs.