No abstract

No 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.

The ‘dissolved’ oxygen permeabilities (Pd) of a range of hydrogels have been studied at 25 and 34°C (the temperature of the eye). At both temperatures the equilibrium water content (W) was found to be the major controlling factor in determining oxygen permeability and log (Pd) was found to be linearly related to W. The values of Pd at 34°C were found to be approximately twice those at 25°C throughout the range of water contents studied. Available information on corneal oxygen consumption rates is used as a basis for the prediction of oxygen permeability/thickness requirements in continuous wear lenses. These data lead in turn to predicted minimum equilibrium water contents for hydrogels in this, type of application.

Some of the problems and advantages in the use of non‐hydrogel polymers in contact lenses are discussed together with studies on a series of such polymers which have potential advantages over the established material, poly(methyl methacrylate), in that they are both more flexible and more oxygen‐permeable. Of the polymers examined which are all too hydrophobic for direct use, poly(4‐methylpent‐l‐ene) proved to be the most readily modified in such a way that its surface became sufficiently wettable to sustain a coherent tear film without reducing its optical qualities to an unacceptable level. The ‘dissolved’ and ‘gaseous’ oxygen permeability coefficients of this polymer were studied as a function of film thickness, surface hydrophilicity and temperature. A pronounced boundary layer effect was observed in ‘dissolved’ oxygen permeability studies, although this decreased as the surface was treated to make it more wettable (as indicated by the equilibrium advancing water contact angle). The ‘gaseous’ permeability coefficients of oxygen were found to be some 4‐6 times greater than those for nitrogen. A discontinuity corresponding to the glass transition temperature was observed at 28°C with both permeants and apparent activation energies for permeation were determined both above and below this temperature.