The design of polymers for contact lens applications

The use of thermoplastics, elastomers and hydrogel polymers in contact lens applications is discussed in terms of the balance of properties required for the lens to function in a satisfactory manner. Although there are several thermoplastics that would appear to offer advantages over poly(methyl methacrylate), the most widely used polymer, the inherent disadvantages are in each case too great. The clarity, toughness, physiological inactivity and ease of fabrication are shared by many polymers, but its critical surface tension governing as it does the maintenance of a coherent tear film on the lens surface is unique amongst materials that are otherwise acceptable. The development of hydrogel polymers is discussed in terms of the potential use of this group of polymers in lenses designed for continuous as distinct from daily wear. The specific requirements of such a lens are considered in the light of the properties possessed by the cornea. Apart from the optical properties of thin (∼0.2 mm) sections of material, the most important parameters are oxygen permeability (since the cornea is avascular a relatively high permeability is required), mechanical properties (the material must be soft enough to avoid discomfort but rigid enough to resist deformation during eyelid movement) and surface hydrophilicity (most conveniently described in terms of the critical surface tension phenomena).