Development of techniques to compare mechanical properties of reversible hydrogels with spherical, square columnar and ocular lens geometry

We have developed a new experimental technique for determining the elastic modulus of the ocular lens by using hydrogel phantoms. We successfully prepared disulfide-reversible polyacrylamide hydrogels in the forms of a square column, a sphere, and a lens. The M_n (number average molecular weight) of the reduced copolymer ranged from 696,800 to 870,900 Da. The physical and swelling properties of the gels were independent of shape. The M_c (molecular weight between cross-links) ranged from 1776 to 1887 compared with the theoretical M_c of 1638 Da. The gels exhibited non-linear rubber elasticity, but at low strains the elastic moduli (E) were 4680±150, 5010±280, and 4870±220 Pa for the square column, sphere, and lens, respectively. The shear modulus (G) was 1531±70 Pa with an E/G ratio of approximately 3:1, indicating an incompressible gel at low strains. At high strains (∼15%) the Mooney-Rivlin plot was linear and the magnitude of 2C₁ was 1515 Pa, which was comparable to the shear modulus of the gels. Finally, the Tatara mechanical model for large deformation of rubber spheres was successfully applied to extract the elastic modulus of the lens. The modulus of the lens obtained with this technique was consistent with the moduli for the square column and sphere. The new technique will be used to determine the mechanical properties of the ocular lens.