M.Sc Thesis

M.Sc StudentLangzam-Sinai Ronit
SubjectNeutral and Ionizable Polymeric Hydrogels in Biomedical
DepartmentDepartment of Biomedical Engineering
Supervisors PROFESSOR EMERITUS Yoram Lanir
MS. Sarit Sivan


            The main goals of this research were to further understand the relationship among the mode of preparation of hydrogels, their composition and mechanical properties and to find a biomaterial the mechanical properties of which resembles those of the human cartilage.

            As part of this study, hydrogels were prepared using acrylamide as monomer and N,N`-methylenebis (acrylamide) or diethyleneglycol divinyl ether as cross-linker. These materials differed from one another in their polymer content (up to 30 %), cross-link density (up to 35 mol % ) and cross-linking agent.  Such a combination of high concentrations of polymer and cross-linker seem not to have been reported before. In order to achieve a high cross-link density, either addition of an organic solvent to the polymerization mixture or heating this mixture were performed.  A few of the gels prepared carry ionizable sulfonate groups.

            All hydrogels prepared were studied for their compression and swelling characteristics. We found that the compression moduli and swelling ratios increase with increasing polymer concentration. However, their dependence on the cross-link density is complex. Thus, upon increasing the cross-link density, swelling ratios decrease monotonically, while compression moduli increase, decrease and increase again. This behavior might stem from structural non-homogeneity of the tri-dimensional polymeric network, an outcome of the specific copolymerization kinetics of the participating monomers.  The modulus was found to be lower when polymerization was performed in the presence of an organic solvent, but unaffected by heating . The swelling characteristics of ionizable hydrogels are highly dependent on the pH of the equilibrating medium.

            The data obtained were analyzed using the statistical theory of Flory.  The thus calculated cross-link density is lower than expected from the concentrations of the reacting components.  Probably, not all the vinylic groups of the cross-linking molecules participate in formation of the polymeric network.

            Among the materials produced, five exhibit compression moduli in the range of 1-2 MPa,  similar to that of human cartilage.  Two of these are electro-neutral, contain 20% polymer and have experimental cross-link densities of 30 and 35 mol%, respectively.  The other three carry charged sulfonate groups, also contain 20% polymer, yet their experimental cross-link densities are only 5, 7 and 10 mol%, respectively.  Hence, to some extent, the presence of ionized groups leads to mechanical characteristics similar to the ones promoted by more extensive cross-linking.  Possibly, this is due to the electrostatic repulsion prevailing between the negatively charged sulfonate moieties.