Polymers, Dk, and contact lenses: now and in the future

The polymers used in conventional contact lenses are briefly reviewed as are the factors that contribute to their gas permeability. The chemistry and gas permeability of future hydrogel contact lenses are discussed.

Interchain packed zones in polymers are less permeable than the amorphous zones. Polymers with thick bulky groups in rigid gas permeable lenses hinder interchain packing, allowing oxygen and carbon dioxide to diffuse through the polymer voids. Also, the flexibility of the siloxane groups facilitates the diffusion of gases. In hydrogel contact lenses, the polymer segments are separated by the aqueous phase. The gas diffusivity increases with hydration, but is limited by the rigid polymer network. Research on new hydrogel contact lenses aims to develop hydrogels with gas permeability enhancing polymers. In these hydrogels, the gas permeability depends not only on their hydration but also on the permeation of the gas through the polymer phase.

The new hydrogel contact lenses will contain highly permeable polysiloxane domains. A potential problem with the new hydrogels is decreased wettability due to the migration of hydrophobic moieties to the lens surface.