the next 25 years, primarily in terms of increasing oxygen permeability, by varying the ingredients. In 1972, British optometrist Rishi Agarwal was the first to suggest disposable soft contact lenses. In 1998, the first silicone hydrogel contact lenses were released by Ciba Vision in Mexico. These new materials encapsulated the benefits of silicone which has extremely high oxygen permeability—with the comfort and clinical performance of the conventional hydrogels that had been used for the previous 30 years. These contact lenses were initially advocated primarily for extended (overnight) wear, although more recently, daily (no overnight) wear silicone hydrogels have been launched. In a slightly modified molecule, a polar group is added without changing the structure of the silicone hydrogel. This is referred to as the Tanaka monomer because it was invented and patented by Kyoichi Tanaka of Menicon Co. of Japan in 1979. Second-generation silicone hydrogels, such as galyfilcon A (Acuvue Advance, Vistakon) and senofilcon A (Acuvue Oasys, Vistakon), use the Tanaka monomer. Vistakon improved the Tanaka monomer even further and added other molecules, which serve as an internal wetting agent. Comfilcon A (Biofinity, CooperVision) was the first third-generation polymer. Its patent claims that the material uses two siloxy macromers of diverse sizes that, when used in combination, produce very high oxygen permeability (for a given water content). Enfilcon A (Avaira, CooperVision) is another third-generation material that is naturally wettable; its water content is 46%. Types Contact lenses are classified in diverse ways: by their primary function, material, wear schedule (how long a lens can be worn), and replacement schedule (how long before a lens needs to be discarded). Functions Correction of refractive error Corrective contact lenses are designed to improve vision, most commonly by correcting refractive error. This is done by directly focusing light so it enters the eye with the proper power for clear vision. A spherical contact lens bends light evenly in every direction (horizontally, vertically, etc.). They are typically used to correct myopia and hypermetropia. There are two ways that contact lenses can correct astigmatism. One way is with toric soft lenses that work essentially the same way as eyeglasses with cylindrical correction; a toric lens has a different focusing power horizontally than vertically, and as a result can correct for astigmatism. Another way is by using a rigid gas permeable lens; since most astigmatism is caused by the shape of the cornea, rigid lenses can improve