It has long been known in the art that a contact lens must have surfaces that have a certain degree of hydrophilicity in order to be wet by tears thus providing unblurred vision.
Soft, hydrophilic contact lenses, in addition to being wettable, provide comfort to the wearer but lack the ability to correct certain visual deficiencies such as astigmatism since they tend to conform to the shape of the corneal surface.
Often hydrophilic monomers can be added to a mixture of comonomers in the formation of contact lenses so that upon polymerization optically clear contact lenses result which have a certain degree of hydrophilicity. As the hydrophilic monomer content increases where it is added directly to the lens composition, the physical characteristics of the lenses are affected by the increased hydration propensity of the polymeric composition.
In some cases, it has been known to treat a formed contact lens with a polymerizable hydrophilic monomer to form a surface coating of hydrophilic polymer grafted to an otherwise hydrophobic polymer surface. Although effective, this method of increasing the hydrophilic character of the lens surface can suffer from involved and difficult manufacturing procedures as well as lack of permanence.
Present rigid and soft contact lenses sometimes retain water on their surfaces through secondary chemical bonding and as a consequence only a very thin layer of water molecules is present between the eye and the contact lens.
Soft lenses are inherently comfortable but often times, as with hard lenses, suffer from brief surface dryness between eye blinks. State of the art technology teaches that a water soluble neutral polymer may be applied to the surfaces of a hard contact lens to provide a "cushion" layer between the lens and the eye which is equated with increased wettability as well as wearer comfort and tolerance.
Dissipation of the "cushion" layer occurs rapidly in most prior art constructions, since there is little specific interaction between the mobile polymer in this layer and the lens surface. As a result the wearer begins to feel discomfort and must rewet the lens surfaces.