Polymers in contact lens applications. V. Design and calibration of a technique for “dissolved” oxygen permeability measurements

Abstract

The importance of ‘in vitro’ measurements of ‘dissolved’ oxygen permeabilities of polymers for use in contact with the cornea (as in contact lenses) is discussed and a description given of the design and calibration of a technique suitable for the measurement of oxygen permeabilities in both the ‘dissolved’ (i.e. the transport of oxygen across a membrane separating two aqueous phases) and ‘gaseous’ (in which the membrane separates two gas phases) states. By the use of a thin film of hydrophobic but highly oxygen permeable polymer even very fragile high water content hydrogels can be supported and studied. Sources of error are discussed together with the conflicting literature values for the oxygen permeability of poly(2‐hydroxyethyl methacrylate) hydrogels. On the basis of the present work a value of 75 × 10−10 cm3 (STP) mm cm−2 s−1 cm−1 Hg is suggested for the ‘dissolved’ oxygen permeability of this polymer at 25 °C.

A Review of the Theoretical Concepts, Measurement Systems and Application of Contact Lens Oxygen Permeability

“…In this system, two chambers are separated by the sample to be measured Morrison and Edelhauser, 1972;Ng and Tighe, 1976;Yang et al, 1981;Yang and Peppas, 1983). Either air or aqueous media may be used in these chambers.…”

“…Initially both chambers are free of oxygen, and when oxygen is introduced into the feed-side chamber, the rate of change of oxygen concentration in the downstream chamber can be used to estimate the permeability of the lens. Ng and Tighe (1976) have given the following equation for the determination of oxygen permeability when a polarographic sensor is used to monitor oxygen pressure:…”

“…(6) Incomplete mixing in the downstream chamber can result in the formation of stagnant layers of liquid (or air) at the surfaces of the sample being measured (Ng and Tighe, 1976) these are known as boundary layers. This introduces a potential error in the determination of oxygen permeability, which may be overcome by ensuring complete mixing of the media within this chamber or allowing for the resistances of the boundary layers by calculation.…”

“…According to the data from these authors, the error introduced by failure to account for the edge effect in general will be less than 10% for commonly used contact lens thicknesses, although it will depend on both the electrode size and the lens thickness. Ng and Tighe (1976) used the dual chamber system to take measurements of the oxygen transmissibility of various hydrogel contact lenses. They reported certain inconsistencies in their measurements, which were probably due to some of the above mentioned error sources.…”

“…where r l and r2 are the boundary layer resistances at the surfaces in phase I and phase 11, respectively. There are several instances in the literature where the description of permeability as a phenomenological coefficient has lead to the belief that the transport properties of the membrane itself will vary depending on the media at each surface of the lens during measurement (Ng and Tighe, 1976;Yang et al, 1981;Yang and Peppas, 1983;Hitchman et al, 1984;Hamano et al, 1985). For example, the distinction between "gaseous oxygen permeability'' and "dissolved oxygen permeability" is often made to distinguish between the permeability measured when the membrane is bounded at both surfaces by gases and liquids, respectively.…”