Oxygen Permeability of a New Type of High Dk Soft Contact Lens Material

Abstract

Lotrafilcon A is a biphasic block-copolymer, comprising a highly permeable siloxane-based polymeric phase, coupled with a water phase (hydrogel phase). The high oxygen permeability of this material, and the fact that it is a hydrogel, places it outside the applicability of both the polarographic ISO standard and coulometric ISO draft standards for contact lens Dk determination. The oxygen permeability (Dk) and transmissibility (Dk/t) of lotrafilcon A lenses were determined by an adaptation of the standard coulometric method. Lenses with a thickness (t) range from 30 μm to over 300 μm were measured in a liquid-to-gas and a gas-to-gas configuration in an effort to combine features of the ISO (draft) standards to yield a valid measurement of the intrinsic material's oxygen transmission characteristics. The following results, for lotrafilcon A, are the mean values and SE for the oxygen permeability coefficient (Dk) determined at 34°C: 140 ± 2 barrer with water overlay (liquid-to-gas) at 2100 rpm stirring speed, 150 ± 5 barrer with water overlay (liquid-to-gas) at “infinite” stirring speed, and 170 ± 2 barrer without water overlay (gas-to-gas), where barrer=10-11 (mlO2 · cm)/(sec · cm2 · mm Hg). Clinical lenses produced from this material are expected to be in the 60 to 90 μm thickness range, with an average center thickness of 80 μm. Given that the Dkmaterial is greater than or equal to 140 barrers, for this material, a parallel sided lens of 80 μm central thickness would exhibit a lens transmissibility (Dk/t) of at least 170 x 10-9 mlO2/(sec · cm2 · mm Hg) at 34°C. This transmissibility is well in excess of the 87 x 10-9 mlO2/(sec · cm2 · mm Hg) value postulated to be sufficient to prevent overnight lens-induced corneal swelling and places it in the hyper-permeable material category.