Tensile properties of soft contact lens materials

The strength of contact lens materials is an important consideration with respect to resistance to damage during lens handling and long term durability, and may govern some aspects of in-eye lens performance. The tensile properties of hydrogel contact lenses manufactured from eight different materials were examined in a series of clinical and laboratory experiments using the Instron 1122 Universal Testing Instrument. Lenses from the following eight materials (and nominal water contents) were used: HEMA/VP 40%, HEMA/VP 55%, HEMA/VP 70%, VP/MMA 55%, VP/MMA 70%, HEMA 40%, HEMA/MAA 55% and HEMA/MAA 70% (HEMA: 2-hydroxy-ethyl methacrylate, VP: vinyl pyrrolidone, MMA: methyl methacrylate, MAA: methacrylic acid). Tensile strength, elongation-at-break and Young's modulus were measured. A technique was devised that enables three parallel-sided specimens of identical width to be cut from a single contact lens with good accuracy. It was found that materials made from HEMA/MAA--although having a very low tensile strength and elongation-at-break--exhibit only a moderate Young's modulus. Materials made from HEMA/VP exhibit high-to-moderate tensile strength, high elongation-at-break and moderate-to-low Young's modulus. Materials made from VP/MMA exhibit high tensile strength and high-to-moderate elongation-at-break, but the Young's modulus is high for the 55% water content and low for the 70% water content materials. The HEMA 40% material exhibits a moderate tensile strength, a low elongation-at-break and a high Young's modulus. This experiment highlights the necessity of developing an accepted standard test methodology for contact lens material stiffness evaluation, in order to derive useful comparative information. Six subjects were fitted with the same lenses for one day. In vitro measurements of total diameter and back optic zone radius were taken at 35 degrees C before lens fitting and after 6h of lens wear. Lens water content, the relative change in lens total diameter (%deltaTD) and relative change in lens back optic zone radius (%deltaBOZR) were calculated and correlated with the tensile properties of the eight hydrogel lens materials. It is concluded that hydrogel materials with high stiffness and strength display less tendency to change their geometric parameters and materials with a high water content do not necessarily have the weakest mechanical properties.