Achieving the Optimum

The introduction by Contamac of the Optimum range of RGP materials came about as the result of a lengthy and extensive research and development programme. This article outlines the various factors involved in such a programme. During this process the team drew upon the experience gained through previous work to develop the Hybrid FS (fluid surface technology) materials (see optician, January 9, 2004). The Hybrid FS project had confirmed the need in the RGP lens sector for new materials, combining the traditional wearer benefits of RGP stability and good optical performance with high wettability. Wettability is also important, not only because a highly wettable lens surface is more comfortable (as good surface wettability of a lens promotes tear film maintenance), but because wettable surfaces have better deposit resistance. Oxygen permeability is also vital for successful material performance. This is provided by the incorporation of molecules containing silicone and fluorine within the lens material. Oxygen permeability is represented as Dk, where D is the diffusion co-efficient of a gas, such as oxygen, through a polymer, and k is the solubility of the gas in the polymer. For practical purposes, permeability of a contact lens is represented in terms of transmissibility Dk/L. Here the term L is introduced, such that the transmissibility at a specific thickness can be expressed. This is important, as a highly permeable material may not allow good transmission of oxygen if it is very thick. All contact lens materials necessarily represent a balance between different, to some extent opposing, desirable properties. In the case of RGP lenses, these are wettability, oxygen permeability and stability. Other important properties are the refractive index of the material (for its optical properties) and its specific gravity (weight per cubic gram of lens material is significant in terms of comfort, lens movement and overall performance). At the outset of the Optimum project, past experience and industry feedback indicated that there was increasing demand in the market for a range of new materials of varying permeability, to allow for different lens prescribing, fitting and wearing requirements. It was hoped that such a range would represent a significant contribution to the growth of the RGP lens sector throughout the world.