Gas permeablecontact lenses (GP Lenses), also known as RGPs, rigid gas permeable, or sometimes oxygen permeable lenses. Today these are the most advanced hard contact lenses. But even though we call them hard contact lenses these should never be confused with the hard contact lenses from years ago.

The old type of  hard contacts were made of PMMA. Before 1970, when soft contact lenses were introduced, almost all contact lenses were made from PMMA.

One issue with PMMA lenses is that they are difficult for the wearer to get used to and uncomfortable to wear. Also, PMMA does not allow oxygen to pass through it, and healthy eyes need plenty of oxygen.