Swelling pressure of hydrogels that degrade through different mechanisms

This study compares the behavior of dextran-based hydrogels that degrade through different mechanisms. The major aim is to investigate how the degradation mechanism influences their swelling pressure. The release of degradation products, the mechanical and swelling properties, and swelling pressure of the degrading gels are measured. Two types of dextran-based hydrogels are investigated: dextran methacrylate (dex-Ma) hydrogels with entrapped dextranase which degrade by hydrolysis of the polymer backbone and dextran hydroxyethyl methacrylate (dex-HEMA) hydrogels which degrade at their cross-links. The release of degradation products, but especially the swelling pressure profile, seems to be strongly dependent on the mechanism underlying the degradation of the gels. In the case where the dextran gels are degraded at their backbone the swelling pressure increases rather continuously; in the case where they are degraded at the cross-links it increases more discontinuously as a sudden increase occurs when the gels are (nearly) completely degraded. This study reveals that the increase in swelling pressure in degrading dex-MA/dextranse gels is (nearly) completely attributed to an increase in osmotic pressure. However, in degrading dex-HEMA gels the increasing swelling pressure seems to be mostly attributed to the decrease in elastic pressure (i.e., elasticity) of the gels. Indeed, during a substantial period the osmotic pressure of degrading dex-HEMA gels does not change. At complete degradation the maximum swelling pressure is obtained and equals the osmotic pressure of the solution of degradation products. A much higher maximal swelling pressure is obtained when the gels are degraded at their backbone than when degraded at their cross-links.