OXYGEN PERMEABILITY OF A NEW-TYPE OF HIGH DK SOFT CONTACT-LENS MATERIAL

Lotrafilcon A is a biphasic biock-copolymer, comprising a highly perme able siloxane-based polymeric phase, coupled with a water phase (hydro gel phase). The high oxygen permeability of this material, and the fac t that it is a hydrogel, places it outside the applicability of both t he polarographic ISO standard and coulometric ISO draft standards for contact lens Dk determination. The oxygen permeability (Dk) and transm issibility (Dk/t) of lotrafilcon A lenses were determined by an adapta tion of the standard coulometric method. Lenses with a thickness (t) r ange from 30 mu m to over 300 Crm were measured in a liquid-to-gas and a gas-to-gas configuration in an effort to combine features of the IS O (draft) standards to yield a valid measurement of the intrinsic mate rial'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 degrees C: 140 +/- 2 barrer with wa ter overlay (liquid-to-gas) at 2100 rpm stirring speed, 150 +/- 5 barr er with water overlay (liquid-to-gas) at ''infinite'' stirring speed, and 170 +/- 2 barrer without water overlay (gas-to-gas), where barrer = 10(-11) (mlO(2) . cm)/(sec . cm(2) . mm Hg). Clinical lenses produce d from this material are expected to be in the 60 to 90 mu m thickness range, with an average center thickness of 80 mu m. Given that the Dk (material) is greater than or equal to 140 barrers, for this material, a parallel sided lens of 80 mu m central thickness would exhibit a le ns transmissibility (Dk/t) of at least 170 x 10(-9) mlO(2)/(sec . cm(2 ) mm Hg) at 34 degrees C. This transmissibility is well in excess of t he 87 x 10(-9) mlO(2)/(sec . cm(2) mm Hg) value postulated to be suffi cient to prevent overnight lens-induced corneal swelling and places it in the hyper-permeable material category.