As materials for hard contact lenses polymethylmethacrylate has hitherto been used widely for their excellency in optical properties, physical strength and mechanical processability. However, polymethylmethacrylate has very low oxygen permeability and therefore oxygen supply to ectocornea through these contact lenses can hardly be expected. Accordingly, in case of using these contact lenses for long hours or sleeping with these contact lenses put on, cornea comes to be short of oxygen causing at times congestion, edema and other cornea disorders. Being an avascular tissue, cornea receives supply of the oxygen necessary for metabolism from the oxygen dissolving in lacrima covering the front surface of cornea. Since hard contact lenses generally have a size covering about half the area of cornea, oxygen supply to cornea is made by (a) lacrima exchange at the backside of lens by the pumping action of lens and (b) the lacrima at the cornea part not covered by lens. As a matter of course, it is desirable for the metabolism of cornea that lens materials themselves have high oxygen permeability and, through hard contact lenses made of these materials, oxygen by supplied to cornea.
In recent years, with a view to replace conventional hard contact lenses mainly composed of polymethylmethacrylates and to provide lenses capable of supplying oxygen to cornea through lenses by using lens materials of high oxygen permeability, there have been disclosed (a) a silicone methacrylate type hard contact lens, namely, a hard contact lens whose oxygen permeability has been enhanced by introducing a siloxane bond to the ester portion of a methacrylic acid ester (Japanese Patent Publication No. 33502/1977), (b) an oxygen-permeable hard contact lens mainly composed of a cellulose such as cellulose acetate butylate (CAB) and (c) an oxygen-permeable hard contact lens using a fluorine-containing methacrylate (Japanese Laid-Open Patent Application No. 51705/1982). In general, oxygen-permeable hard contact lenses mainly composed of silicone methacrylates such as disclosed in Japanese Patent Publication No. 33502/1977, have oxygen permeability higher by several tens to several hundreds times compared with conventional hard contact lenses mainly composed of polymethylmethacrylate, but are usually inferior to them in hardness and hydrophilicity and further tend to pick up stains of lipids and the like. Generally, oxygen-permeable hard contact lenses mainly composed of silicone methacrylates are copolymers between a silicone methacrylate and a methyl methacrylate and, when the proportion of the silicone methacrylate becomes higher, these contact lenses will have improved oxygen permeability but reduced hardness, higher possibility of scratch formation during handling and worsened polishability. Further, their hydrophilicity becomes worse and therefore their hydrophobicity is increased, and consequently their wettability by water is deteriorated resulting in higher, tendency of adsorbing up lipids such as lecithin and the like, and resultantly oxygen-permeable hard contact lenses containing higher contents of silicone methacrylates are liable to adsorb stains more easily. Since oxygen-permeable hard contact lenses mainly composed of silicone methacrylates have poor wettability by water which is characteristic of silicone, among these lenses there are those whose surface have been treated so as to have higher hydrophilicity. However, this treated surface layer having higher hydrophilicity is as very thin as 1/1000 the lens thickness and accordingly the hydrophilicity is gradually lost during use of lens necessitating, in cases, surface retreatment for higher hydrophilicity.
The present inventors previously found out in Japanese Laid-Open Patent Application No. 51705/1982 that an oxygen-permeable hard contact lens can be obtained from a copolymer comprising a fluorine-containing monomer. However, further improvements in oxygen permeability, surface hardness and hydrophilicity have been desired for this contact lens.
A hard contact lens made of cellulose acetate butylate is somewhat superior in oxygen permeability and hydrophilicity compared with conventional hard contact lenses made of polymethylmethacrylate, but has small hardness and therefore is damaged more easily and consequently lens parameters are liable to change during use of the lens.