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A contact lens, or simply contact, is a thin lens placed directly on the surface of the eye. Contact lenses are considered medical devices and can be worn to correct vision, or for cosmetic or therapeutic reasons. In 2004, it was estimated that 125 million people (2%) use contact lenses worldwide, including 28 to 38 million in the United States.〔Barr, J. ("2004 Annual Report" ). ''Contact Lens Spectrum''. January, 2005.〕 In 2010, worldwide contact lens market was estimated at $6.1 billion, while the U.S. soft lens market is estimated at $2.1 billion.〔Nichols, Jason J., et al ("ANNUAL REPORT: Contact Lenses 2010" ). January 2011.〕 Multiple scientists have estimated that the global market will reach $11.7 billion by 2015.〔 , the average age of contact lens wearers globally was 31 years old and two thirds of wearers were female.〔Morgan, Philip B., et al. ("International Contact Lens Prescribing in 2010" ). ''Contact Lens Spectrum''. October 2011.〕 People choose to wear contact lenses for many reasons.〔Agarwal, R. K. (1969), Contact Lens Notes, Some factors concerning patients' motivation, The Optician, January 10, pages 32-33 (published in London, England).〕 Aesthetics and cosmetics are often motivating factors for people who would like to avoid wearing glasses or would like to change the appearance of their eyes. Other people wear contacts for functional or optical reasons. When compared with spectacles, contact lenses typically provide better peripheral vision, and do not collect moisture such as rain, snow, condensation, or sweat. This makes them ideal for sports and other outdoor activities. Contact lens wearers can also wear sunglasses, goggles, or other eyewear of their choice without having to fit them with prescription lenses or worry about compatibility with glasses. Additionally, there are conditions such as keratoconus and aniseikonia that are typically corrected better by contacts than by glasses. ==History== Leonardo da Vinci is frequently credited with introducing the idea of contact lenses in his 1508 ''Codex of the eye, Manual D'',〔 where he described a method of directly altering corneal power by either submerging the head in a bowl of water, or wearing a water-filled glass hemisphere over the eye. Neither idea was practically implementable in Da Vinci's time.〔 He did not suggest his idea be used for correcting vision, being more interested in learning about the mechanisms of accommodation of the eye.〔Heitz, RF and Enoch, J. M. (1987) "Leonardo da Vinci: An assessment on his discourses on image formation in the eye." ''Advances in Diagnostic Visual Optics'' 19—26, Springer-Verlag.〕 René Descartes proposed another idea in 1636, in which a glass tube filled with liquid is placed in direct contact with the cornea. The protruding end was to be composed of clear glass, shaped to correct vision; however, the idea was impracticable, since it would make blinking impossible. In 1801, Thomas Young made a basic pair of contact lenses on the model of Descartes. He used wax to affix water-filled lenses to his eyes. This neutralized his own refractive power. He then corrected for it with another pair of lenses. However, like Leonardo's, Young's device was not intended to correct refraction errors. Sir John Herschel, in a footnote of the 1845 edition of the ''Encyclopedia Metropolitana'', posed two ideas for the visual correction: the first "a spherical capsule of glass filled with animal jelly", and "a mould of the cornea" that could be impressed on "some sort of transparent medium".〔("The History of Contact Lenses." ) eyeTopics.com. Accessed October 18, 2006.〕 Though Herschel reportedly never tested these ideas, they were both later advanced by several independent inventors such as Hungarian Dallos with Istvan Komàromy (1929), who perfected a method of making molds from living eyes. This enabled the manufacture of lenses that, for the first time, conformed to the actual shape of the eye. It was not until 1887 that a German glassblower, F.E. Muller, produced the first eye covering to be seen through and tolerated.〔(【引用サイトリンク】title=Contact Lenses - A Consumer Guide from AllAboutVision.com )〕 In 1888, the German ophthalmologist Adolf Gaston Eugen Fick constructed and fitted the first successful contact lens.〔(【引用サイトリンク】title=Adolf Eugen Fick (1852-1937) )〕 While working in Zürich, he described fabricating afocal scleral contact shells, which rested on the less sensitive rim of tissue around the cornea, and experimentally fitting them: initially on rabbits, then on himself, and lastly on a small group of volunteers. These lenses were made from heavy blown glass and were 18–21 mm in diameter. Fick filled the empty space between cornea/callosity and glass with a dextrose solution. He published his work, "Contactbrille", in the journal ''Archiv für Augenheilkunde'' in March 1888. Fick's lens was large, unwieldy, and could only be worn for a couple of hours at a time. August Müller in Kiel, Germany, corrected his own severe myopia with a more convenient glass-blown scleral contact lens of his own manufacture in 1888. Also in 1887, Louis J. Girard invented a similar scleral form of contact lens. Glass-blown scleral lenses remained the only form of contact lens until the 1930s when polymethyl methacrylate (PMMA or Perspex/Plexiglas) was developed, allowing plastic scleral lenses to be manufactured for the first time. In 1936, optometrist William Feinbloom introduced plastic lenses, making them lighter and more convenient.〔Robert B. Mandell. ''Contact Lens Practice'', 4th Edition. Charles C. Thomas, Springfield, IL, 1988.〕 These lenses were a combination of glass and plastic. In 1940, German optometrist Heinrich Wöhlk produced plastic lenses, based on experiments performed during the 1930s. In 1949, the first "corneal" lenses were developed.〔U.S. Patent No. (2,510,438 ), filed February 28, 1948.〕〔"The Corneal Lens", ''The Optician'', September 2, 1949, pp. 141–144.〕〔"Corneal Contact Lenses", ''The Optician'', September 9, 1949, p. 185.〕〔"New Contact Lens Fits Pupil Only", ''The New York Times'', February 11, 1952, p. 27.〕 These were much smaller than the original scleral lenses, as they sat only on the cornea rather than across all of the visible ocular surface, and could be worn up to sixteen hours per day. PMMA corneal lenses became the first contact lenses to have mass appeal through the 1960s, as lens designs became more sophisticated with improving manufacturing (lathe) technology. Early corneal lenses in the 1950s and 1960s were relatively expensive and fragile, resulting in the development of a market for contact lens insurance. Replacement Lens Insurance, Inc. (now known as RLI Corp.) phased out its original flagship product in 1994 after contacts became more affordable and easier to replace. One important disadvantage of PMMA lenses is that no oxygen is transmitted through the lens to the conjunctiva and cornea, which can cause a number of adverse clinical effects. By the end of the 1970s, and through the 1980s and 1990s, a range of oxygen-permeable but rigid materials were developed to overcome this problem. Chemist Norman Gaylord played a prominent role in the development of these newer, permeable contact lenses. Collectively, these polymers are referred to as "rigid gas permeable" or "RGP" materials or lenses. Although all the above lens types – sclerals, PMMA lenses and RGPs – could be correctly referred to as being "hard" or "rigid", the term hard is now used to refer to the original PMMA lenses, which are still occasionally fitted and worn, whereas rigid is a generic term that can be used for all these lens types: hard lenses (PMMA lenses) are a sub-set of rigid lenses. Occasionally, the term "gas permeable" is used to describe RGP lenses, but this is potentially misleading, as soft lenses are also gas permeable in that they allow oxygen to move through the lens to the ocular surface. The principal breakthrough in soft lenses was made by the Czech chemists Otto Wichterle and Drahoslav Lím who published their work "Hydrophilic gels for biological use" in the journal ''Nature'' in 1959. In 1965 National Patent Development Corporation (NPDC) bought the American rights to produce the lenses and then sublicenced the rights to Bausch & Lomb which started to manufacture them in the USA.〔(【引用サイトリンク】title=CONTACT LENS HISTORY - Otto Wichterle )〕 The work of the Czech scientists led to the launch of the first soft (hydrogel) lenses in some countries in the 1960s and the first approval of the Soflens material by the United States Food and Drug Administration (FDA) in 1971. These lenses were soon prescribed more often than rigid lenses, mainly due to the immediate comfort of soft lenses; by comparison, rigid lenses require a period of adaptation before full comfort is achieved. The polymers from which soft lenses are manufactured improved over the next 25 years, primarily in terms of increasing the oxygen permeability by varying the ingredients. In 1972, British optometrist Rishi Agarwal was the first to suggest disposable soft contact lenses. In 1998, an important development was the launch of the first silicone hydrogels onto the market 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 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.〔(【引用サイトリンク】title=Looking at Silicone Hydrogels Across Generations )〕 Comfilcon A (Biofinity, CooperVision) was the first third-generation polymer. The patent claims that the material uses two siloxy macromers of different 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. The enfilcon A material is 46% water.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Contact lens」の詳細全文を読む スポンサード リンク
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