February 2008 - Polarographic Cell
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Believe it or not, the vital component to make this laboratory testing device work properly was a dampened cigarette paper. How would we ever manage today? The equipment pictured is the original polarographic cell manufactured by Global Vision (UK) Ltd and used by Irving Fatt and Judith Morris, FCOptom at the Contact Lens and Prosthetics Department at the Moorfields Eye Hospital during the academic year September 1976-June 1977. Taking advantage of the fact that gas will dissolve in a polymer, their task was to make a scientific comparison of oxygen permeabilities (DK values) for various branded contact lenses commercially available in the UK at that time. The introduction of the lens material CAB (Cellulose acetate butyrate) in the 1970s meant that rigid gas permeable (RGP) contact lenses became a viable alternative to the more commonly dispensed lenses. Some contact lens practitioners spoke of this as the dawn of the 'Oxygen Era'. There were many positive outcomes, including a great reduction in corneal trauma for contact lens wearers.
An American chemist resident in London, Irving Fatt (1920-1996), is remembered for his ground-breaking studies into oxygen tension and permeability with Jennifer Chaston, FCOptom at St Thomas' Hospital and Judith Morris, later a President of the College of Optometrists, at Moorfields. Fatt and St. Helen had already, in 1971, pioneered the polarographic method for use with gel lenses. Now he and Morris were to apply a similar analysis to a wide variety of lenses. The brands they tested included the Hydron lens by Smith & Nephew, the Igel 78 by Optiflex, the Permalens by Global Vision, the Plano T by Bausch & Lomb, the Scanlens, the Soflens and the Weicon 38 by Titmus-Eurocon.
The laws of diffusion of gases across a fluid membrane were derived by the German scientist Adolf Fick (1829-1901) in 1855. He is not to be confused with Adolf Eugen Fick the contact lens pioneer, though they were related. Applying diffusion theory Fatt and Morris measured the transmissibility of lenses of the same material but different thicknesses to plot a true graph of permeability that took account of thickness dependence. The transmissibility measurements were taken at room temperature of 20-25oc and again at 35-37oc. (which gave less precise results). The permeability was then calculated using electronic micrometer readings of the lens thickness. The lens sample was placed horizontally on the electrode above a thin film of water (the diffusion resistance of this layer had to be taken into account when measuring ultra thin hydrogel lenses) and held in place by a PMMA cylinder. This is where the wet cigarette paper came in, to provide the thin film. There was some doubt as to whether the cylinder pressed down effectively over the entire lens. In some later experiments at the University of California, Berkeley, highly permeable Styrofoam was used between the cylinder and the sample material. In fact the transmissibilty of the lens on the eye might be affected by the tear layer...the thinner the lens the more this was so.
There is now an alternative method to measure permeability, the cuolometric method, and it says something for the speed of developments in the field that DK values are already referred to as 'traditional' units. For the historical record, D = how fast molecules will dissolve as they move through a material and k = the number of molecules dissolved in the material. The unit of transmissibility, the Dk/t, includes a measure of the thickness of the material at the average lens centre. Account needs to be taken of the nature of the lens surface, the edge and the boundary. The end result of research of this kind? ....the optimum lens made from a material of a particular water ratio.
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