Polymeric Materials for Contact Lenses

A contact lens is an optical device that is placed over the cornea of the eye in such a manner that the lens remains on the eye's surface throughout blinking. The main purpose of wearing a contact lens is to correct vision deficiencies; in this application they are called cosmetic lenses. Contact lenses can also be used medically for the treatment of certain diseases. In such cases they are called therapeutic or bandage lenses [l]. Contact lenses range from hard to soft. Hard lenses contain mainly poly(methy1 methacrylate) (PMMA) and are impermeable to oxygen. Hard and semirigid lenses permeable to oxygen are made from copolymers of siloxanes and methacrylates. Flexible, oxygen permeable lenses are made of silicones. Soft contact lenses are prepared from polymers that absorb large quantities of water to become hydrogels. The aqueous phase of the hydrogel is oxygen permeable. Hard and soft hydrophobic lenses require a relatively thick tear film between their posterior surface and the cornea of the eye. Soft hydrogel lenses adhere closely to the cornea with a tear film of only capillary thickness between the lens and the corneal surface. With any kind of contact lens, the cornea's surface must be wet and oxygenated at all times to remain transparent and healthy. The cornea is an avascular tissue with an active aerobic metabolism. Oxygenation of the corneal surface is normally accomplished through the tear film that supplies the oxygen to the epithelium of the cornea. When the eyelids are closed, oxygen is supplied to the cornea surface by blood capillaries of the palpebral conjunctiva (the posterior part of the eyelids). When the cornea surface is deprived of oxygen, epithelium glycogen decreases and lactic acid production increases as a result of anaerobic glycolysis [2]. The cornea then swells and its surface becomes hazy. Thus, with any type of contact lens, disruption of the Oz supply to the cornea surface must be minimized, either by oxygen-rich tear exchange under the lens, by oxygen permeation through the lens, or by both.