A formula that relates weight percent hydration (%H2O) and linear swelling (%LE) on wet basis for any hydrogel which swells isotropically is presented. This formula is valid for all hydrations, but requires a knowledge of the dry and hydrate densities of the hydrogel and involves the solution of a cubic root equation. A much simplified formula in the form of a first-order equation, %LE = −0.9 + 0.5χ(%H2O), was also obtained. This formula can be applied as a first approximation to any hydrogel of isotropic swelling up to about 50% hydration. In addition, similar order equations were obtained for several hydrogel contact lens materials. Elongation, a ratio of one linear hydrate dimension to the same dimension in the dry state, is related to %LE by a simple formula. These formulas can be used to calculate changes in the diameter, radius of curvature, and thickness of hydrogel contact lenses as a function of their water content.
© Williams & Wilkins 1975. All Rights Reserved.