Strong Glycosaminoglycan Hydrogels for Tissue Engineering Applications

Hydrogels are hydrophilic materials that typically contain over 95% water and are widely used in tissue engineering and drug delivery applications. However, due to high water content, hydrogels are weak and brittle. Therefore, methods to improve their mechanical performance (especially moduli and fracture strain) are highly sought. For tissue engineering scaffolding purposes, glycosaminoglycans (GAGs), such as hyaluronic acid (HA) and chondroitin sulfate (CS) have been of great interest for hydrogels since they are biocompatible and abundant in the mammalian extracellular matrix. However, like many other hydrogels, GAG based hydrogels typically do not possess the mechanical properties that are needed for a mechanically robust tissue engineering scaffold. To address this problem, in this study functionalized (methacrylated) HA and CS were photo-copolymerized with acrylate oligomers of ethylene glycol. The hypothesis was that acrylate oligomers of ethylene glycol could improve the crosslink density of the copolymer gels by improving the reaction yield through bridging the macromere functional groups with the crosslinker and grow the kinetic chains, which otherwise due to steric limitations would not be possible. 

Methacrylated hyaluronic acid (MHA) was co-polymerized with acrylate ethylene glycol (EG) oligomers with lengths varying from (EG)₂DA to (EG)₁₃DA . It was found that as the lengths of the EG oligomers increased, the shear modulus, Young’s modulus, and crosslink density of the co-polymerized gels improved. When the crosslinker spacer length increased from (EG)₂DA to (EG)₁₃DA, the shear modulus increased from 191 kPa to 268 kPa while the Young’s modulus increased from 658 kPa to 949 kPa. The crosslink density was increased from 0.824 mol/cm³ to 0.962 mol/cm³ for (EG)₂DA and (EG)₁₃DA respectively.

As buffer solutions are used in tissue engineering practices, the salt effect on swelling, moduli, and the crosslink density of these gels were studied. For this purpose, methacrylated chondroitin sulfate (MCS) was equilibrated in salt solutions of different ionic strengths (0.01M to 1.4M sodium chloride solutions). For a MCS 13 wt% gel, it was found that as the salt concentration increased, the swelling degree decreased from 42 to 8. Additionally, for the same range of salt concentration the shear modulus decreased from 47 kPa to 9 kPa and the Young’s modulus decreased from 141 kPa to 27 kPa. With increasing salt concentration, the crosslink density decreased from 0.256 mol/cm³ to 0.0277 mol/cm³.