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In page Contact lens:

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Glass lenses were never comfortable enough to gain widespread popularity. The first lenses to do so were those made from polymethyl methacrylate (PMMA or Perspex/Plexiglas), now commonly referred to as "hard" lenses. Their main disadvantage is they do not allow oxygen to pass through to the cornea, which can cause a number of adverse, and often serious, clinical events. Starting in the late 1970s, improved rigid materials which were oxygen-permeable were developed. Contact lenses made from these materials are called rigid gas permeable lenses (RGPs).[citation needed]