Contact lenses can also be used medically for the treatment of certain diseases. In such cases they are called therapeutic or bandage lenses . Contact lenses range from hard to soft. Hard lenses contain mainly poly(methy1 methacrylate) (PMMA) and are impermeable to oxygen. Hard and semirigid lenses permeable to oxygen are made from copolymers of siloxanes and methacrylates. Flexible, oxygen permeable lenses are made of silicones. Soft contact lenses are prepared from polymers that absorb large quantities of water to become hydrogels. The aqueous phase of the hydrogel is oxygen permeable. Hard and soft hydrophobic lenses require a relatively thick tear film between their posterior surface and the cornea of the eye.