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extended-wear (overnight) lenses--rigid or soft--increase the risk of corneal ulcers, infection-caused eruptions on the cornea that can lead to blindness. symptoms include vision changes, eye redness, eye discomfort or pain, and excessive tearing. the risk of corneal ulcers for people who keep extended-wear lenses in overnight is 10 to 15 times greater than for those who use daily-wear lenses only while awake, says the fda''s center for devices and radiological health. when the eyes are open tears carry adequate oxygen to the cornea to keep it healthy. but during sleep, the eye produces fewer tears, causing the cornea to swell. under the binding down of a rigid contact lens during sleep, the flow of tears and oxygen to the cornea is further reduced. this lack of oxygen leaves the eye vulnerable to infection. extended-wear rigid lenses also can cause unexpected, sometimes undesirable, reshaping of the cornea.

extended-wear (overnight) lenses--rigid or soft--increase the risk of corneal ulcers, infection-caused eruptions on the cornea that can lead to blindness. symptoms include vision changes, eye redness, eye discomfort or pain, and excessive tearing. the risk of corneal ulcers for people who keep extended-wear lenses in overnight is 10 to 15 times greater than for those who use daily-wear lenses only while awake, says the fda''s center for devices and radiological health. when the eyes are open tears carry adequate oxygen to the cornea to keep it healthy. but during sleep, the eye produces fewer tears, causing the cornea to swell. under the binding down of a rigid contact lens during sleep, the flow of tears and oxygen to the cornea is further reduced. this lack of oxygen leaves the eye vulnerable to infection. extended-wear rigid lenses also can cause unexpected, sometimes undesirable, reshaping of the cornea.

the water carries dissolved oxygen through these lenses to the eye surface, says an contacts optometrist and head of clinical affairs at ciba vision. "the eye needs oxygen to keep the tissue healthy." a high water content also makes soft contacts lenses more comfortable. people blink about every 5 seconds, and a wet lens surface reduces the friction between the eyelid and the plastic. for 20 years, most of the innovations in soft lenses soft boosted the hydrogel''s water content. this approach has its limits, though. too much water makes the lens fragile and difficult to handle. contact lens wearers become frustrated trying to slide a floppy lens from a fingertip to an eye. moreover, even if a lens could be made of a layer of pure water, it wouldn''t get enough oxygen to the eye. researchers decided to find new polymers that could allow greater oxygen flow.

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