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lenses are classified into three basic types according to their chemical composition and physical properties. these are hard, soft and rigid gas permeable. soft lenses are further classified into daily wear, extended wear, and disposable lenses.hard lenses. hard lenses are hydrophobic and rigid. chemically, these lenses are polymerized products of esters of acrylic acid or methacrylic acid. the most common plastic employed in hard lenses is polymethyl-methacrylate (pmma), a rigid plastic resin with the advantages of being transparent, physiologically safe, stable and having a surface wettability about the same as the cornea. it is essentially contacts impermeable to moisture and oxygen, though online typically absorbing contacts only about 0.5 percent water.

lenses are classified into three basic types according to their chemical composition and physical properties. these are hard, soft and rigid gas permeable. soft lenses are further classified into daily wear, extended wear, and disposable lenses.hard lenses. hard lenses are hydrophobic and rigid. chemically, these lenses are polymerized products of esters of acrylic acid or methacrylic acid. the most common plastic employed in hard lenses is polymethyl-methacrylate (pmma), a rigid plastic resin with the advantages of being transparent, physiologically safe, stable and having a surface wettability about the same as the cornea. it is essentially contacts impermeable to moisture and oxygen, though online typically absorbing contacts only about 0.5 percent water.

the water carries dissolved oxygen through these lenses to the eye surface, says an optometrist and head of clinical affairs at ciba vision. "the eye needs oxygen to keep the online tissue healthy." a high water content also makes soft 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 boosted the hydrogel''s water content. this approach has its limits, though. too much water makes the lens fragile and difficult contacts to handle. contact lens wearers become frustrated trying to slide a floppy lens from a fingertip online 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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