---

Gas permeable lens

Both hard and soft contact lenses are available. Hard lenses are rigid or semi-rigid and are commonly made from methyl methacrylate polymers which are optically excellent and easily machined and molded. Soft lenses are made from hydrophilic polymeric compositions, for example a hydroxyalkylmethacrylate polymer. In recent years, corneal contact lenses having high oxygen permeability have become well known. Among the most important properties and requirements of contact lenses are substantial permeability to oxygen (commonly referred to as DK), and a highly wettable and deposition-resistant surface. High oxygen permeability has been sought to provide a contact lens which could be worn by a wearer for an extended period of time, desirably weeks or even months. High oxygen permeability also has advantages even where extended wear is not necessary or desirable; these include greater comfort and good overall permeability even in relatively thick lenses. Extended wear contact lenses such as hydrophilic (soft) and rigid gas permeable (RGP) contact lenses have been a major development in vision care. Conventional hydrophilic soft lenses are primarily hydrogels derived from a variety of hydrophilic monomers or polymers, which have either been cross-linked or insolubilized in water by some mechanism, such as by the introduction of crystallinity or by varying hydrophobic/hydrophilic properties. Allowing sufficient amounts of oxygen to pass through the lenses to the cornea of the wearer helps eliminate several problems that afflict soft lens and ordinary hard lens wearers, such as Corneal edema (swelling of the cornea), etc. High oxygen permeability is needed because the pupil of the eye has no blood circulation and extracts the oxygen needed by its cells directly from the atmosphere. Wettability is essential for wearer comfort. Resistance to protein and mucus deposits from fluids that bathe the outer surface of the eye is essential since without this property the deposits will irritate the cornea and in some severe cases the deposits will scratch the cornea surface. Also as a result of the deposits, both visual acuity and the oxygen permeability of the lens can suffer. In addition, some other applications and benefits of gas permeable contact lenses include: correction of special vision problems, post-surgical treatment, superior visual acuity and health benefits afforded by the lenses. Gas permeable contact lenses replaced hard lenses because they allow gases such as oxygen to pass through the plastic.

---