From the standpoint of comfort, durability and optical performance, an ideal contact lens should be soft, very wettable yet low in bulk water content, clear and have an oxygen permeability greater than 50 [barrers]. In present day soft contact lenses comfort and wettability are achieved by using hydrogels with 30-80% water content, but at the expense of limited oxygen-permeability, physical strength and formation of protein-deposits. High oxygen permeability is achieved in contact lenses made entirely from silicone rubber, as described for instance in U.S. Pat. No. 3,996,187, but at the expense of wettability. In addition, silicone rubber, because of its great inherent hydrophobicity and chain mobility, rapidly and tenaciously interacts with lipids in the tear liquid, resulting in quick dewetting and potential adhesion to the cornea.
It would be highly desirable to produce a contact lens composed entirely of silicone rubber since it offers the highest oxygen permeability of all presently available materials. The enhancement of eye comfort and long wear capability would represent a significant improvement in contact lens development. Silicone rubber lenses, although possessing high oxygen permeability, suffer from poor comfort characteristics because they are not soft enough to conform to the cornea in the same way a typical hydrogel lens does. On the other hand, silicone-free polyurethane rubbers are known for their excellent bio-compatibility, as examplified by the use of BIOMER--a linear polyurethane rubber--to coat artificial hearts to reduce blood clotting.
Silicone rubbers are made by crosslinking liquid polysiloxanes. These rubbers are thermoset polymers which can be made to varying degrees of hardness and elasticity by controlling the degree of crosslinking and the molecular weight of the silicone fluid. Silicone rubber is usually made by vulcanizing polydimethylsiloxane fluids with organic peroxides at elevated temperatures. Another approach to crosslinking employs hydrosilation in which poly(vinylalkyl siloxanes) are cured with poly(hydridoalkylsiloxanes) in the presence of transition metal catalysts. Silicone rubber has also been formed by chemically reacting, .alpha.,.omega.-difunctional poly(diorganosiloxanes) with polyfunctional silanes and siloxanes. Typically the crosslinking reaction is a condensation which forms a siloxane bond and a volatile by product. Common examples of this type of cure reaction are silanol-alkoxylsilane (French Pat. 1,042,019), silanol-acetoxysilane (German Appl. 1,121,803), silanol-silane (Brit. Pat. 804,199), and silanol-silanol (via the corresponding acetone oxime) (Belg. Pat. 614,394). Suitable catalysts for these reactions are amines and carboxylic acid salts of Sn, Pb, Zn, Sb, Fe, Cd, Ba, Ca and Mn.
Organosiloxane isocyanates have been prepared (U.S. Pat. No. 3,179,622) which vulcanize when exposed to moisture. In these cases the isocyanate group is joined to the siloxane through an alkyl group, rendering it unhydrolyzable. Besides moisture cured polyurethanes, silicone rubber, which are 2-component polyurethanes are conveniently prepared by reaction of di-, tri- or tetra hydroxyalkyl-substituted polysiloxanes with di-isocyanates, using the well known techniques of conventional polyurethane synthesis. Most commonly these poly-hydroxyalkyl-polysiloxanes are used as additives to conventional polyurethane formulations, for instance to regulate pore size in polyurethane foams, to impart lubricity or gloss to polyurethane coatings or achieve water repellency.
The silicone rubbers, which are described for use as soft, oxygen permeable contact lenses, are obtained by hydrosilation reaction between poly-(vinyl-alkyl siloxanes) and poly-(hydridoalkyl siloxanes), as described in U.S. Pat. No. 3,996,187. Silicone rubber like materials for use as soft contact lenses which are free-radical polymerized copolymers of methacrylate-functionalized polydimethylsiloxanes have also been described in U.S. Pat. No. 4,605,712.
It has now been found that polysiloxane-polyurethane rubbers also make excellent soft contact lens material, combining clarity, high elongation and softness with exceptional oxygen-permeability.
Such contact lenses, which are entirely based on poly(dialkylsiloxane) di- or tri-alkanols and diisocyanates have not been described before.
All silicone containing contact lens materials, be they conventional silicone rubbers or the polysiloxane-polyurethanes of this invention are extremely hydrophobic and attract lipid deposits when worn in the eye. In order to overcome the hydrophobic nature of silicone rubber, silicone rubber lenses, such as SILSOFT (DOW-CORNING) have to be surface treated to render them wettable. Numerous such surface treatments have been described:
Relevant United States patents in this area include: U.S. Pat. No. 3,925,178 describes surface treatment by water-vapor subjected to an electrical discharge. U.S. Pat. No. 4,099,859 describes grafting a hydrophilic polymer onto a silicone-rubber contact lens by coating the lens with a hydrophilic monomer followed by exposure to UV. U.S. Pat. No. 4,229,273 describes grafting of a acrylic polymer onto a silicone rubber lens pre-irradiated in the presence of oxygen. U.S. Pat. No. 4,168,112 describes the formation of a poly-electrolyte complex on the surface of a contact lens in order to render it more hydrophilic and comfortable to wear. U.S. Pat. No. 4,217,038 describes the formation of a glass-coating on a silicone contact lens to improve wettability. U.S. Pat. No. 4,409,258 describes a hydrophilizing treatment of contact lenses by bombardment with nitrogen and oxygen ions. U.S. Pat. No. 4,388,164 describes coating the surface of a silicone rubber with a thin metal film by vacuum decomposition in the stretched state. U.S. Pat. No. 4,332,922 describes hydrophilization of silicone contact lenses by a gas-discharge. U.S. Pat. No. 4,143,949 describes putting a hydrophilic coating onto a hydrophobic contact lens by radiation induced polymerization. U.S. Pat. Nos. 4,311,573 and 4,589,964 describe hydrophilization of a hydrophobic polymer by ozone treatment, followed by a grafting of vinyl monomers through decomposition of the formed peroxy groups.
Although these methods allow fabrications of wettable silicone rubber lenses, most of them have major drawbacks; for instance polymerizations can occur not only on the surface, but also in the liquid polymerization medium. Also, because the graft-polymerization depends on active centers, which decompose at different times, inhomogeneous surfaces result because of preferential absorption of monomer into already modified surface regions. In cases where exact depth-control and uniformity of the grafted region is important, such as for example in the surface modification of contact lenses, such uncontrollable grafting reactions are not acceptable. On the other hand, if to reduce inhomogeneities grafting is carried out for a short time only, the grafted surface regions are too thin and in many applications the desired effect soon wears off. Exact control over reaction conditions is therefore very important.
It has now also been discovered, that polysiloxane-polyurethane rubbers are especially well suited to make soft contact lenses not only with excellent oxygen permeability, but excellent wettability and hydrogel-like softness as well, when they are prepared in contact lens molds which have previously been coated with a reactive hydrophilic polymer, which is transfer-grafted during cure.
It has been further discovered, that polysiloxane-polyurethane rubbers can be made in form of an interpenetrating polymer network (IPN) with a free-radical polymerized vinyl polymer; these IPN's are often clear and besides being highly oxygen permeable, allow the physical properties of the polysiloxane-polyurethane rubber to be varied over a wide range; they include water swellable compositions and compositions bearing polar groups which are otherwise difficult to incorporate into a polyurethane.