Early contact lenses were hard lenses made of a material that had no ability to transmit oxygen or carbon dioxide, which made it unhealthy for the cornea of the wearer over the long term. It also required the wearer to use various solutions to help tears (TEERZ) adhere to the surface. Soft lenses made for daily wear are more gas-permeable, and they are also ""hydrophilic,"" meaning the lens absorbs water. However, the vision is not quite as sharp as that with a hard lens. ""Rigid Gas Permeable,"" or ""RGP"" (R-G-P) lenses were the next advancement in technology for hard lenses and were introduced to the public in 1979. They weren't as hard as the originals, so they were called ""rigid."" Also called ""semi-soft,"" they can flex more than hard lenses, and they transmit oxygen directly through the lens to the cornea. They generally offer sharp vision and can correct most vision problems. RGP lenses are made of different types of plastic and silicone or fluoride materials, depending on the manufacturer. They're more durable than soft lenses and easier to care for, but they require a longer adaptation period and consistent wear to maintain adaptation. Because they're gas-permeable, the cornea rarely becomes swollen, clear vision is maintained, and initial adaptation is easier than with hard lenses because of the oxygen transmission. These lenses are a good alternative if you can't get good visual correction with soft lenses, although some people experience drying of their eyes after wearing the lens for several hours. They can also slip off center more easily than other types, and debris can easily get under the lenses. There are also bifocal and extended-wear RGP lenses, which can be worn overnight.
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