Oxygen permeable contact lens and method of manufacture therefor

Abstract: An oxygen permeable bifocal contact lens which has a distance vision portion and a near vision portion is made from materials having differing refractive indexes. The near vision portion has a higher refractive index than the distance portion with a Dk of at least 10. The distance portion forms the major portion of the lens and is formed as a crosslinked copolymer with a refractive index of not more than 1.49 from monomeric materials which impart oxygen permeability to the finished lens, and the near vision portion similarly is formed as a crosslinked copolymer from a monomer mixture which includes one or more monomers at a concentration and with a refractive index such that the near vision portion has a refractive index of at least 1.54. The monomer of the near vision portion includes at least one vinyl monomer having a substituted aromatic ring structure to make the monomer bulky and ensure there is only a limited shrinkage on polymerization of such monomers and mixtures containing them.

Description: The present invention relates to oxygen permeable contact lenses and a method of manufacture therefor, and in particular oxygen permeable bifocal contact lenses. The manufacture of bifocal contact lenses using a single polymer or copolymer material for the lens means that the near vision portion of the lens must be thick relative to the distance vision portion. In turn, this results in discomfort, due to the increased pressure on the eye and the lack of oxygen permeability, to the wearer. It is appreciated in the industry that the use of two differing materials for the near and distance vision portions of the lens enables the weight of the lens to be reduced and thus increasing wearer comfort. Previously, the use of polymethyl methacrylate (refractive index 1.49) for the distance portion and polystyrene (refractive index 1.59) for the near portion has been proposed. However such a combination, while lighter, still has a low oxygen permeability and can still result in the wearer suffering discomfort and edema. In fact, the less than desirable oxygen permeability has resulted in very limited acceptance of such lenses in the marketplace. A suitable contact lens must not only have an acceptable level of oxygen permeability but, in addition, good visual acuity at both near and far distances, adequate surface wettability, good dimensional stability, resistance to surface deposits, and a durability which is good enough to withstand handling in use. There is a need for an effective, and suitable, oxygen permeable bifocal contact lens. A bifocal contact lens which takes the form of a lens made of a bulk material of one refractive index for observing distant images and a segment of a material of a higher refractive index for observing near images is known as a two-piece lens. Various ways have been suggested for making such two-piece lenses and are disclosed in the following US patent specifications: 3,560,598 3,726,587 3,597,055 4,302,081 3,684,357 4,921,205 3,984,506 The use of a polymerized block of a monomeric material in which a recess has been filled with a high refractive index monomer and then polymerised to form a lens blank with a high index portion for near vision and a lower refractive index portion for distance vision has been described in USP 3,560,598. There is no suggestion in this disclosure to use materials which are oxygen permeable. USP 4,614,624 discloses a one-piece bifocal lens and suggests the use of silicone acrylates, but does not suggest how these materials might be used in forming a two-piece lens. USP 3,984,506 and 4,302,081 describe the manufacture of what are described as fused bifocal contact lenses. The method of making these lenses disclosed in the specification involves the use of a solid preform made of a high refractive index material against which is cast a low index material to form the bifocal blank. There is no suggestion that this process can be used to form an oxygen permeable bifocal contact lens blank, and it appears that the choice of the method used has been dictated by the need to avoid dimensional stability problems that arose with the type of high index materials used by the patentee. It has now been found that a two-piece oxygen permeable bifocal contact lens can be formed by first forming a low index crosslinked oxygen permeable copolymer preform with a recess therein, into which there is cast a high index material. This process differs from the prior art in that it has been found that warping and general distortion of the lens can be avoided, and a good bond formed between the two materials through careful selection of the materials. Suitable high refractive index materials are those which include a substituted aromatic ring group in their structure; these groups confer bulkiness on the monomer. Copolymers incorporating these materials tend to eliminate the dimensional stability problems referred to in the prior art discussion. The present invention differs from those disclosed in the prior art in that the use of the copolymer systems of the present invention results not only in the formation of a highly stable bond while maintaining the required optical properties at the interface between the differing materials used, but provides adequate oxygen permeability for user comfort, and that the copolymer system used for one or both of the distance and near vision portions is an interpenetrating polymer network. A comprehensive review of interpenetrating polymer networks and their applications is provided in "Interpenetrating polymer networks and related materials." L. H. Sperling, Plenum Press, New York and London, 1981. The use of an interpenetrating network mixture in at least one of the materials to be bonded together means that a material is formed which undergoes little dimensional change on polymerization, and which helps to prevent weakening and rupture of any bond formed as the polymerization proceeds. In European Specification 345,994 the formation of interpenetrating networks by the addition of one monomer mixture to another using viscosity as a means of determining the degree of polymerization of the one mixture which is to be added to the other unpolymerized mixture is disclosed by the present applicants. It has been found that the extent of polymerization, as indicated by the relative viscosity of the mixture, is also important in determining not only the optimum point at which to mix two mixtures to form an interpenetrating network, but also when forming one interpenetrating network at an interface with a fully polymerized material. Therefore the viscosity of the partially polymerized mixture is not only important as an indicator of when a satisfactory interpenetrating network can be formed, but in determining when a satisfactory bond will occur at the interface between materials. USP 4365074 discloses an oxygen permeable bifocal contact lens having a distance vision portion of a crosslinked copolymer with a refractive index of 1.48 or 1.475 which is formed from monomeric materials which include silicone acrylates capable of imparting oxygen permeability to the finished lens, and a near vision portion of a crosslinked copolymer with a refractive index of 1.540 including at least one vinyl monomer having substituted aromatic ring structure (benzylmethacrylate), said near vision portion being totally surrounded by low refractive index material. There remains a definite need for an oxygen permeable bifocal lens which will provide suitable visual acuity at near and far distances as well as being safe, comfortable, durable, wettable, and resistant to protein deposits and that can also be readily fabricated in an economical manner. The present invention is based on the realisation that a combination of high and low refractive index copolymer compositions, which are formulated to have good oxygen permeability, can be bonded together in a cast molding process to form a preform (button) from which high quality bifocal contact lenses can be readily manufactured. Accordingly, the present invention is concerned with providing an oxygen permeable bifocal contact lens materials composed of selected high and low refractive index components of such a nature that they bond tightly at the interface therebetween in such a manner as to provide a smooth transition between the high and low refractive index segments. The present invention also aims to provide oxygen permeable bifocal contact lens materials having near and far vision segments of such a nature that bifocal lenses cut from such materials have, in addition to excellent visual qualities over a range of distances without aberration, an adequate oxygen permeability to ensure comfort and safety in the eye. By adequate oxygen permeability, we mean a permeability which when measured as Dk is at least 10 x 10⁻¹¹. According to the present invention, there is provided an oxygen permeable bifocal contact lens, lens blank or lens preform, having a distance vision portion with one refractive index and a near vision portion with another higher refractive index and a Dk of at least 10 x 10⁻¹¹ (cm²/sec)(10² x mm Hg) at 35° to 37°C, the distance portion forming the major portion of the lens and being formed as a crosslinked copolymer with a refractive index of not more than 1.49 from monomeric materials which includes one or more silicon acrylate monomers capable of imparting oxygen permeability to the finished lens, and the near vision portion similarly being formed as a crosslinked copolymer from a monomer mixture which includes one or more monomers at a concentration and with a refractive index such that the near vision portion has a refractive index of at least 1.540 including at least one vinyl monomer having substituted aromatic ring structure which makes such monomers bulky and ensures there is only minimal shrinkage on polymerization of such monomers and mixtures containing them, the distance vision portion and/or the near vision portion being formed as an interpenetrating network. Preferably, both the near vision and the distance vision portion are formed as an interpenetrating network copolymer. The near vision portion, however, while it is preferably formed from an interpenetrating network can be a simple crosslinked copolymer, but in such a case the distance vision portion is preferably formed as an interpenetrating network. The interpenetrating networks are formed by polymerizing monomer mixtures which are formed by mixing two monomer mixtures, one of which is partially polymerized to a selected viscosity relative to the other monomer mixture which is in an unpolymerized state. In forming the distance vision portion of oxygen permeable bifocal contact lens according to the invention, an interpenetrating network can be chosen with a refractive index of not more than 1.49 to form the distance portion and formed into a lens with a material with a refractive index of at least 1.540 forming the near portion. The interpenetrating network with a refractive index of not more than 1.49 is formed from two monomer mixtures A and B. The interpenetrating network composition is selected so that Mixture A contains 10 to 90% of total monomer formula weight. (By total monomer formula weight, we mean the total weight of monomers, including cross linking agents, but not initiator making up the total mixture of A and B.) B provides the balance of the mixture of A and B. It is preferred to use a mixture of A and B, where A contains 50 to 90% of the total monomer formula weight, and B contains 10 to 50% of the total monomer formula weight. An interpenetrating network polymer with a refractive index of at least 1.540 can be formed from two monomer mixtures C and D, and the resultant composition is selected so that mixture C contains 10 to 90% of the total monomer formula weight and the balance of the total is mixture D. The present invention is particularly directed to an oxygen permeable bifocal contact lens with a Dk of at least 10 x 10⁻¹¹ in which the higher index segment is surrounded by an oxygen permeable low index material by forming the lens in a series of steps comprising first forming a preform from low index material in which there is provided a recess, filling the recess with a partially polymerised high index material formed from a monomer mixture which includes at least one vinyl monomer with a refractive index of at least 1.540 and having a substituted aromatic ring structure completing the polymerisation of the high index material, and thereafter covering both the low index material and the high index material with further low index material in a partially polymerised form, and finally completing polymerisation of the low index material to form a lens blank which is then machined into a bifocal contact lens. The present invention also includes an oxygen permeable bifocal contact lens with a Dk of at least 10 x 10⁻¹¹ in which a base curve is cut in the polymerised high index material before the step of covering the high index material with low index material is carried out. The present invention also includes a method of producing an oxygen permeable bifocal contact lens having a near vision portion and a distance vision portion, in which the near vision portion is formed in the body of the lens by polymerizing a partially polymerized mixture of interpenetrating network forming comonomers in contact with a surface shaped to leave a recess in the material both after polymerisation and after removal from contact with the shaped surface, the recess being of the same shape as the desired shape of the near vision portion of the finished lens and a partially polymerized mixture of interpenetrating network comonomers including at least one vinyl monomer with refractive index of at least 1.54 and a substituted aromatic ring structure is placed in the recess and polymerized. If desired in accordance with the present invention the near vision portion is covered with the material used to form the distance vision portion of the lens by polymerizing a further quantity of that material in contact with the already formed near and distance vision portions. Also, a base curve can be cut in the near vision portion before it is covered. As will appear from the detailed description below, one method of making a bifocal contact lens according to the invention includes a method in which a quantity of material used to form the distance vision portion is placed in an open topped mold and a recess formed by providing the mold with a cover whose surface contacting the material is the recess forming surface. Using an open topped mold means that, if desired, a lens blank can be formed within the open topped mold and the mold used as a carrier during machining the lens blank to a final lens form. The use of an open topped mold and a cover to form a lens casting cavity means that one or both optical surfaces can be formed during the lens casting operation making it possible to produce both finished and semi-finished lenses by simply shaping one or both major casting surfaces to the shape necessary to produce the desired optical surface. Further, the present invention includes a method for producing an oxygen permeable bifocal contact lens with a Dk of at least 10 x 10⁻¹¹ by embedding a high refractive index polymer segment within a lens blank which has a substantially lower refractive index than that of the internal segment, the lens blank having been prepared by casting a partially polymerized mixture of low refractive index interpenetrating network forming comonomers including a silicone acrylate monomer against a mold with a protrusion so as to form a recess in the blank and polymerizing to a solid state, the recess being then filled with a relatively high refractive index monomer mixture, which includes at least one vinyl monomer with a refractive index of at least 1.540 and a substituted aromatic ring structure, the polymerization of which is then completed in the recess, a sufficient quantity of partially polymerized low refractive index monomer mixture being next added to completely cover the high refractive index segment, and the resulting composition is cured to form the final bifocal contact lens blank, which is in turn machined to form a bifocal contact lens. If desired before covering the high refractive index segment with low index material, the base curve of the add segment may be lathe cut to a desired configuration. The present invention is of particular value in producing a gas permeable bifocal contact lens with a Dk of at least 10, formed from at least two materials joined by a stable optically clear bond. Dk is defined as (Y)10⁻¹¹ (cm²/sec)(mlO₂ X mm Hg) at 35°-37°. It is preferred to reduce undesirable shrinkage during polymerisation by employing for both near and distance vision portions of the lens separate comonomer mixtures capable of forming copolymers which are interpenetrating networks. They are designated as stated above Mixture A and Mixture B for the distance vision portion and Mixture C and Mixture D for the near vision portion. In the first step of the preparation of the distance vision copolymer, Mixture A is partially polymerized to a predetermined relative viscosity and then added to Mixture B and the resulting composition is then copolymerized. In a similar manner in the preparation of the distance segment, Mixture C is partially polymerized to the desired relative viscosity and then added to Mixture D. The polymerisation of the resulting composition is then effected. The relative viscosity referred to in the present instance is defined as the viscosity of the partially polymerized comonomer mixture divided by the viscosity of the corresponding unpolymerized comonomer mixture at a given temperature. The viscosity is measured during the polymerization, and techniques are readily available for doing this. The reaction mixture may be polymerised under a blanket of nitrogen so that when it is exposed to obtain a sample, polymerization ceases or is substantially slowed down due to the inhibiting effect of oxygen in the air. A sample is removed and the viscosity measured to determine the relative viscosity. In addition to reducing undesirable shrinkage of the comonomer mixtures during polymerization as indicated above, the formation of an interpenetrating polymer network system makes it possible to control the penetratability of the high refractive index comonomer composition into the low refractive index solid portion during final polymerisation of the high index portion. Further, the partial polymerisation of the comonomers during the first step of the polymerisation of the comonomers for the high refractive index segment reduces the time required for polymerization of the mixture of C and D in contact with the low index solid portion. This further reduces the possibility of warpage or separation of the two components and determines the extent of penetration into the solid portion, though this may also be related to the nature of the polymerized state of the solid portion which may contain minor amounts of extractable unpolymerized material. In another form of our invention, the Mixture C is not partially polymerized and then added to Mixture D. Instead, all of the components which make-up the combination of C and D are simply mixed and cast into the recess without any deliberate partial polymerisation. Monomers useful in the preparation of the low refractive index copolymers A/B used for the distance vision portion of the invention include hydrophobic low molecular weight acrylates of the general formula: where R is H or a monovalent organic radical having no more than seven carbon atoms and R' is a monovalent organic radical having fewer than 20 carbon atoms. Such acrylates include, methyl methacrylate, isobornyl acrylate, methyl alpha carboxymethyl acrylate, and cyclohexyl acrylate. Use of such acrylates in the comonomer mixture results in improved surface hardness and increased scratch resistance. In general, up to 30%, preferred up to 25% of the total monomer formula weight of such acrylates are present in the comonomer mixture and from 3 to 8% is a preferred range since this amount results in improved physical properties of the copolymer without any great reduction in the Dk of the copolymer. It is also desirable to include in the low refractive index comonomer composition at least one hydrophylic monomer such as N-vinylpyrrolidone, 2-hydroxyethyl methacrylate or methacrylic acid in a concentration of 1 to 15%, and preferably from 3 to 12% of the total comonomer mixture. The use of acryloxyalkylpolysiloxane monomers in the low refractive index comonomer mixture is necessary to provide a copolymer with an oxygen permeability Dk of at least 10 x 10⁻¹¹, the silicone acrylates suitable for use in this invention have the general formula: where R = H or monovalent organic radical having from one to seven carbon atoms; X is a monovalent organic radical having from one to seven carbon atoms; Y is a monovalent organic radical having one to seven carbon atoms or Z where where R1, R2 and R3 are the same or different monovalent organic radicals having from one to seven carbon atoms and R4 is OH or a monovalent organic radical having from one to seven carbon atoms; where k = 0-1; m = 1-3; n = 1-5; and p = 1-3. Examples of representative suitable acryloxyalkylpolysiloxanes include: 3-methacryloxypropyl-tris-(trimethylsiloxy)silane; bis-(trimethylsiloxy)-3-methacryloxypropylsilanol, 3-acryloxypropoxy-tris-(trimethylsiloxy)silane and 3-acryloxypropyl-1,1,1-trimethyl-3,3,5,5-tetrakis (trimethylsiloxy)trisiloxane. In general from 10 to 70%, preferred 10-65 % of the acryloxyalkylpolysiloxanes are used in the low refractive index comonomer mixture when forming an oxygen permeable material. Best overall properties are obtained when the acryloxyalkyl polysiloxanes make up 35 to 55% (w/w) of the total of the comonomer mixture and where k = 0. Addition of fluoro substituted acrylic monomers to the low refractive index comonomer polymerisation mixture increases the resistance of the final bifocal contact lenses to protein and other surface deposits and contributes to lowering the refractive index of the resulting copolymer. Such fluoro containing acrylic monomers have the general formula: where R = H or a monovalent organic radical having up to eight carbon atoms; A = H or E; and E is a fluorinated monovalent organic group having up to 20 carbon atoms. Suitable specific fluoroacrylates include: 2,2,2-trifluorethyl acrylate; monafluorocyclohexyl methacrylate; 2-(N-ethylperfluorooctanesulfamido)ethyl acrylate; alpha-fluoroacrylonitrile; and 1,1-dihydroperfluorononyl methacrylate. When present, up to about 35% (w/w) of the fluoroacrylate of the total comonomer concentration is used in the low refractive index polymerization mixture and a concentration of about 2 to 25% is preferred. Examples of suitable formulations of both Mixture A and Mixture B may be found in European Specification 345,994, as well as in the examples appended hereto. The comonomer mixture used in the preparation of the high refractive index portion C/D contains at least one polymerizable vinyl monomer having a substituted aromatic ring structure so as to form a copolymer with a refractive index of at least 1.540. Suitable monomers include: carbazoles such as, N-vinylcarbazole, N-ethyl-3-vinylanisole, 3-(N-carbozoyl)propyl acrylate, 2-(N-carbozyl)-1-methylethyl acrylate, and 6-(N-carbazoyl) hexyl methacrylate; 2-(N-phenothiazinyl) ethyl acrylate; styrene derivatives such as, vinyl anisole; and p-t-butylstyrene; vinyl naphthalene; and aromatic acrylates such as, phenyl and benzyl methacrylates. The use of carbazoles having a polymerisable vinyl or acrylate group is preferred. The comonomer mixture for the preparation of the high refractive index copolymer contains other monomers in addition to monomers conferring high refractive index on the copolymer of the type listed above. It is desirable, for example, to include a monomer such as, N-vinylpyrrolidone or methacrylic acid as a wetting agent and monomers such as, methyl methacrylate which can contribute strength and durability to the final copolymer, and crosslinking agents such as, ethylene dimethacrylate or divinylbenzene, which provide enhanced dimensional stability. As noted above, the partially polymerised comonomer mixture used to prepare the high index copolymer segment is preferably prepared in two distinct steps: Mixture C is partially polymerised before Mixture D is added to it. The total monomer content of mixtures C and D is made-up of: a) from 30 to 90% of at least one polymerisable vinyl monomer having a refractive index of at least 1.540 and having a substituted aromatic ring structure. b) from 1 to 60% of an acrylate having the formula: where R = H or a monovalent organic radical having not more than seven carbon atoms and R' = a monovalent organic radical having up to 20 carbon atoms. c) from 0.1 to 15% of a polymerizable polyvinyl crosslinking agent. d) from 0 to 2% of a polymerizable vinyl silane coupling agent. e) from 0.1 to 3.0% of a vinyl polymerisation initiator. Mixture C as indicated above contains about 10 to 90% (w/w) of the total comonomers used in the preparation of the high refractive index portion. Mixture D contains the balance of the required comonomers. Mixture C preferably contains a major proportion of the comonomers employed, and it is preferred to form a mixture in which 50 to about 90% of the total comonomers employed are in Mixture C. The use of crosslinking agents in both the low and high refractive index comonomer polymerisation mixture is necessary so as to prepare copolymers having satisfactory dimensional stability and a suitable hardness, qualities which promote good visual actuity and improved resistance to scratching on the lens surface. The total amount of crosslinking agent used in the practice of this invention will normally vary from about 0.05% up to about 35% (w/w) and preferably from about 0.10% to about 25% (w/w) of the total comonomers used in the preparation of the bifocal contact lens blank. The precise amount will depend upon the specific crosslinking agents used and the properties desired in the bifocal contact lens. In the preparation of partially polymerized Mixture A of the comonomer mixture, it is preferable to use diacryl crosslinking agents in which the terminal vinyl groups are separated by at least ten atoms. Suitable crosslinking agents of this type included tetraethylene dimethacrylate and 1,3-bis(methacryloxypropyl-1,1,3,3-tetrakis-(trimethylsiloxy)disiloxane. Use of such crosslinking agents permits greater latitude in processing conditions. It is preferable not to use shorter chain crosslinking agents such as, ethylene dimethacrylate, in Mixture A and if they are used, they should only form a very small fraction of the comonomer mixture. In comonomer Mixture B, shorter chain crosslinking agents having nine or fewer atoms between the vinyl groups are effective in concentrations from 0.05% to 10% and preferably from 0.1% to 8% of the total comonomers used in the preparation of the low index portion. Suitable short chain crosslinking agents include 1,2 ethylene dimethacrylate; 1,4-butylene dimethacrylate and diethylene dimethacrylate. Both short and long chain crosslinking agents can be used in comonomer Mixture B. Mixture A always forms the partially polymerized component in the Mixture A/B and where short chain crosslinking agents are used always contain less of such crosslinking agents. The use of a crosslinking agent in Mixture C is optional, because Mixture D will always contain crosslinking agent. It is necessary to avoid gelling of C, but crosslinking agents having at least 10 atoms between the vinyl groups are suitable for use in Mixture C if desired up to a concentration of 20% of the total comonomers used in Mixture C. Shorter chain crosslinking agents such as, ethylene dimethacrylate and divinyl benzene or longer chain crosslinking agents or both types up to a concentration of 20% are used in Mixture D to provide the desired dimensional stability of the resulting copolymer. In an embodiment of the invention where both or either of the high and low refractive index copolymers are prepared as interpenetrating networks, it is essential that the partial copolymerization of Mixture A and of Mixture C be closely monitored so that the relative viscosity at which A and C are mixed, respectively, with B and D is controlled between selected limits. While the optimum relative viscosity will vary with the particular comonomers used in Mixture A and in Mixture C and is easily determinable by experiment, it is normally kept between 1.10 and 25 and preferably between 1.50 and 12. The relative viscosity is defined as the viscosity of the partially polymerized Mixture A or Mixture C divided by the viscosity of the initial Mixture A or Mixture C, respectively, just before it is purged with nitrogen. If the relative viscosity of Mixture A or Mixture C falls below 1.10, undesirable shrinkage will occur during the polymerization process. On the other hand, if the relative viscosity of Mixture A or Mixture C greatly exceeds 25, the partially polymerised Mixture A, or Mixture C will be difficult to dispense properly and a dimensionally unstable bifocal contact lens blank may result. This particularly applies to Mixture C. The measurement of relative viscosity of A and C enables the degree of partial polymerization to be achieved to be controlled in a consistent manner, so that there is reproducibility from one batch of material to another and the formation of the interpenetrating network takes place starting substantially from the same degree of polymerization of A and C, respectively. Care must also be taken that the addition of the other mixture, B to A, and D to C does not result in a material with a viscosity which interferes with the ability to form a satisfactory interpenetrating network or interfacial bond. If by the addition of B and D, respectively, the relative viscosity is increased to above 25, the mixture will become more difficult to cast against the solid surface and the interfacial bond may be unsatisfactory while below 1.10, there may be no formation of an interpenetrating network. Free radical polymerization initiators of the type commonly used in the polymerization of ethylenically unsaturated monomers are suitable for use in the present invention. They include such representative polymerization initiators as 2,2'-azobis(methyl-butyronitrile)(Vazo 67); 2,2'-azobis-(isobutyronitrile(AIBN); benzoyl peroxide and tertiary butyl peroxypivalate. Initiators, sold under the trade name "Vazo," are preferred since they give uniformly good results and do not produce oxygenated residues. From 0.1% to 3% and preferably from 0.2 to 1.5% (w/w) of initiator is used. This invention makes possible the preparation of superior, high quality novel oxygen permeable bifocal contact lens by a new, economical cast molding process. The two-stage polymerization processes for the preparation of the near and for the far distance viewing segments permits the polymerization process to be controlled so as to yield copolymers which readily join together at the interface to form a strong bond which results in excellent optical properties. As the monomers used in forming the mixtures can differ to a large extent from one another in refractive index, this factor must be taken account of in selecting the proportions used in a formulation e.g., if the proportion of high index material say with a refractive index of 1.540 or higher is being kept low, then e.g., a methacrylate monomer can be chosen to compensate for this e.g., one could use benzyl methacrylate with an index of 1.514 as opposed to methyl methacrylate with an index of 1.414. Description of the Drawings Figure 1 is a front view of a bifocal contact lens preform or button made in accordance with the invention in which the position of the segment is shown by a dotted line. Figures 2-7 are sectional views of the different molding stages in a manufacturing process to manufacture the embodiment of the invention shown in Figure 1. One method by which a bifocal contact lens according to the present invention can be produced is that described in USP 3,560,598. The method consists of casting a lens forming material against a curved optical surface in a mold cavity where the optical surface is provided with a protrusion in the shape of the high index segment desired in the finished lens or preform. This forms a depression in the cast lens or preform which is subsequently filled with high index lens forming material. The process involves removing the lens or preform from the mold before filling the depression as it is formed on the surface which is cast against the optical surface on the mold. In using this method, it is essential to use the monomer mixtures disclosed in this specification in order to obtain a user satisfying oxygen permeable two-piece bifocal contact lens. A preferred method, according to the present invention, is one in which a recess in the lens or preform is formed by providing a protrusion on a cover for an open top mold. The open mold is filled with lens forming material which will form the distance portion of the lens or preform, the cover is put on and the protrusion forms a recess in the lens material. The assembly can then be held together by a spring and heated in an oven in a nitrogen atmosphere for the time necessary to polymerize the lens material. The recess then formed is filled with the material used to form the near portion of the lens. Figure 2-7 illustrate this method of forming a bifocal contact lens preform or button according to the present invention. A mold cavity 4 in an open top mold 1 is partially filled with a material which will form a low index portion 5 of a bifocal contact lens. A cover 2 with a protruding portion 3 is now placed on the open top mold and held in place with a spring clip. The mold assembly thus formed is heated in a nitrogen atmosphere to polymerize the material which forms the low index portion 5 of the lens. After polymerization, the assembly is opened and the cover removed. The depression 7 formed in the low index material is now filled with material 9 of a higher refractive index than that used to form the low index portion 5 of the lens. A cover 6 is placed on the open top mold and held in place with a spring clip. This assembly is then heated in a nitrogen atmosphere to polymerize the high index material. Once polymerization is complete the button or preform may be removed from the mold assembly and machined into the desired configuration for a finished lens. It can also be machined in the mold. In our preferred process, a further step is undertaken so as to encapsulate a high index portion of the lens preform in the low index material. Once the cover 6 is removed, low index lens forming material is placed in the open topped mold and a cover 8 placed on the mold and held in place by a spring clip. The assembly thus formed is then heated in a nitrogen atmosphere to polymerize the material added to the mold. Once polymerisation is complete, the preform or button can be removed from the mold and machined to any desired configuration as a finished bifocal contact lens or left in the mold and machined in the mold. The mold and cover surfaces, which contact the lens forming materials, may, if desired, be formed with a radius surface corresponding to the desired optical surface of the finished lens so that a fully finished or semi-finished lens may be cast in situ depending on whether all or only one of the lens material contacting surfaces are shaped so as to cast a finished lens surface. The open topped mold may be made of a material which is more easily machined than the copolymers forming the lens preform or button such as a polyester. Preferred materials for forming the open topped mold are acrylate blends such as, acrylate styrene acrylonitrile ("ASA") and polycarbonate ("PC"); ASA and poly(methylmethacrylate) ("PMMA"). It is preferred to make the mold from one of the acrylic blends disclosed above and the button need not be removed from the mold before machining. The mold acts as a carrier for the button or preform and supports the button during machining, so that it can be made thinner reducing the amount of diamond wear experienced in machining the actual button. The acrylic material causes substantially less wear. The mold covers may be made of a polyolefin such as, low or high density polypropylene. The lens materials used are those described above so that unlike the prior art two-piece bifocal contact lenses formed with a high index segment, the lenses of the present invention are oxygen permeable with a Dk of at least 10 x 10⁻¹¹. The following examples are presented to facilitate an understanding of the present invention and not to limit its scope. Example 1 Mixture A containing 10 parts of methyl methacrylate; 20 parts of 2,2,2-trifluoroethyl methacrylate; 37 parts of 3-methacryoxpropyl-tris-(trimethylsiloxy)silane; 5.6 parts of bis(trimethylsiloxy)-3-methacryloxypropylsilanol; 4.4 parts of 1.3-bis (methacryloxypropyl)-1,1,3,3 tetrakis (trimethylsiloxy)silane; 8 parts of methacrylic acid and 1 part of 2,2'-azobis-(isobutyronitrile) was gradually warmed to a temperature of 40°C and then purged with a stream of dry nitrogen. The viscosity was measured at regular intervals relative to the viscosity of the initial Mixture A before purging with nitrogen. The polymerization was permitted to proceed until the relative viscosity reached 3.30. At that time inhibited Mixture B which contained 3 parts of methyl methacrylate, 4 parts of 2,2,2,-trifluoroethyl methacrylate, 2.4 parts of 3-methacryloxypropyl-tris-(trimethylsiloxy) silane; 0.3 parts of 3-methacryloxypropyl-bis-(trimethylsiloxy) silanol; 5 parts of ethylene dimethacrylate and 0.3 part of 1;3bis-(methacryloxypropyl)-1,1,3,3-tetrakis-(trimethylsiloxy)silane, was added to the partially polymerized Mixture A in the presence of air to stop the polymerization process. The resulting partially polymerized combined mixture of A and B was cooled to room temperature and dispensed into an open top mold made of acrylic blend. The casting was then covered with a propylene mold cover that has a crescent-shape protrusion and a predetermined radius surface, and the two mold parts held together by a spring clip. The assembly was heated at 40 to 45°C for 10 hours in a nitrogen atmosphere in an oven. The temperature was then raised to 60°C for one hour. The assembly was removed from the oven and cooled to room temperature. The mold cover was then removed leaving a low refractive index copolymer contact lens button with a recess into which a higher index material can be cast. Mixture C containing 30 parts of 3-(N-carbazoyl)-propyl acrylate; 50 parts of 4-vinylanisole; 4 parts of methyl methacrylate; 4 parts of N-vinylpyrrolidone, and 0.1 part of benzoyl peroxide was gradually raised to a temperature of 60°C and then purged with nitrogen. The polymerization was continued at 60°C until a relative viscosity of 4.1 was reached in about 2.5 hours. At this juncture, Mixture D, which contained 3 parts of 4-vinylanisol; 2 parts of methyl methacrylate; 5 parts of ethylene glycol dimethacrylate; 0.5 part of 2,2'-azobis(isobutyronitrile); 1 part of vinyltrimethoxysilane and 1 part of a 2% solution 2,2'(2,5-thiophenediyl)bis 5-tertiary-butyl benzoxazole (an optical brightener), was added to Mixture C. The combined Mixture C and Mixture D was cooled to room temperature and kept at 5°C until used for cast molding of the near vision segment. A portion of the partially polymerized Mixture C and Mixture D, prepared as described above, was cast into the recess in the button prepared from Mixtures A and B which is still in the open top mold. The filled mold was covered with a polypropylene mold cover. The assembly, held together by a spring clip, was heated in an oven at 40°C for 10 hours in a nitrogen atmosphere. The temperature was then raised to a temperature of 60°C over a period of 90 minutes and kept at that temperature for one hour. The assembly was removed from the oven and the mold cover was removed and discarded. An additional casting was then made by adding more of the partially polymerized combined Mixture A and Mixture B to the open top mold to encapsulate the near segment. The filled mold was covered with a mold cover and the assembly held together by a spring clip was placed in an oven and heated at 41°C for 8 hours in a nitrogen atmosphere and then heated at 69°C for 5.5 hours and finally at 83°C for 5 hours. The assembly was removed from the oven and allowed to cool to 35°C over a period of 2.5 hours. After removal of the mold cover the filled mold was cut to desired parameters for a finished lens. The copolymer of the near vision segment has a refractive index lens of 1.600, and the distance vision portion a refractive index of 1.475. The finished lens had a Dk of 30 x 10⁻¹¹. Example 2 Mixture A containing 22 parts of methyl methacrylate; 12 parts of 2,2,2-trifluoroethyl methacrylate; 35 parts of 3-methacryoxpropyl-tris-(trimethylsiloxy)silane; 4 parts of bis-(trimethylsiloxy)-3-methacryloxypropylsilanol; 4 parts of 1,3-bis(methacryloxypropyl)1,1,3,3 tetrakis (trimethylsiloxy)silane; 9 parts of methacrylic acid and 1 part of 2,2'-azobis(isobutyronitrile) was gradually warmed to a temperature of 40°C and then purged with a stream of dry nitrogen. The viscosity was measured at regular intervals relative to the viscosity of the initial Mixture A before purging with nitrogen. The polymerization was permitted to proceed until the relative viscosity reached 1.94. At that time, inhibited Mixture B, which contained 3 parts of methyl methacrylate; 2 parts of 2,2,2-trifluoroethyl methacrylate; 2 parts of 3-methacryloxypropyl-tris-(trimethylsiloxy)silane; 1 part of 3-methacryloxypropyl-bis-(trimethylsiloxy) silanol; 5 parts of ethylene dimethacrylate and 1 part of 1,3 bis(methacryloxypropyl)-1,1,3,3-tetrakis-(trimethylsiloxy)silane, was added to the partially polymerized Mixture A in the presence of air to stop the polymerization process. The resulting partially polymerized combined mixture of A and B was cooled to room temperature and dispensed into an open topped mold made of an acrylic blend. The casting was then covered with a polypropylene mold cover that has a crescent-shape protrusion and a predetermined radius surface, and the two mold parts held together by spring clip. The assembly was heated at 40 to 45°C for 10 hours in a nitrogen atmosphere in an oven. The temperature was then raised to 60°C for one hour. The assembly was removed from the oven and cooled to room temperature. The mold cover was then removed to leave a low refractive index copolymer contact lens button with a recess to receive high index material. The button had a refractive index of 1.4750 and a Dk of 35, and was left in the open topped mold. A mixture with a refractive index of 1.5490 that contained 30 parts of 3-(N-carbazoyl)-propyl acrylate; 53 parts of 4-vinylanisole; 6 parts of methyl methacrylate; 4 parts of N-vinylpyrrolidone; 5 parts of ethylene glycol dimethacrylate; 1 part of vinyl trimethylsilane; 1 part 2,2'(2,5-thiopene diyl) bis(5-tertiary-butyl benzoxazole) solution; 0.1 part benzoyl peroxide and 0.5 parts 2,2', azo bis(isobutyronitrile) was cast into the recess. The open topped mold was covered with a radius polypropylene mold cover. The assembly, held together by a spring clip, was heated in an oven at 40°C for 10 hours in a nitrogen atmosphere. The temperature was then raised to a temperature of 60°C over a period of 90 minutes and kept at that temperature for one hour. The assembly was removed from the oven and the cover was removed and discarded. An additional casting was then made by adding more partially polymerized combined Mixture A and Mixture B to the open top mold in order to encapsulate the near segment. The filled mold was covered with a polypropylene cover and the assembly held together by a spring clip was placed in an oven and heated at 41°C for 8 hours in a nitrogen atmosphere and then heated at 69°C for 5.5 hours and finally at 83°C for 5 hours. The assembly was removed from the oven and allowed to cool to 35°C over a period of 2.5 hours. After removal of the cover, the resulting bifocal lens blank was cut to desired parameters without removal from the mold. The copolymer of the near vision segment had a refractive index of 1.600 and the resulting bifocal contact lens had a Dk of 10 x 10¹¹. Examples 3-7 Examples 3-7 were carried out in the same manner as Example 1 using the formulations of A/B and C/D set out in Table I. Example 8 A distance vision gas permeable monomer mixture with a refractive index of 1.4175 that contained 27 parts of methyl methacrylate; 12 parts of 2,2,2-trifluoroethyl methacrylate; 37 parts of 3-methacryloxypropyl-tris (trimethsiloxy) silane; 5 parts of bis(trimethyl siloxy)-3-methacryloxypropylsilanol; 5 parts of 1,3-bis (methacryloxypropyl)-1,1,3,3,tetrakis (trimethylsiloxy) silane; 8 parts of methacrylic acid; 6 parts of ethylene dimethacrylate; and 0.7 parts of 2,2'-azobis (isobutyronitrile) was cast into polymerization tubes and cured at 40°C for 30 hours and post-cured at 70°C for 20 hours. The copolymer rods thus formed were removed from the tubes and cut into optical blanks. A near vision segment cavity was machined in one of the blanks and a near vision monomer mixture with a refractive index of 1.5490 that contained 30 parts of 3-(N-carbazoyl)-propyl acrylate; 53 parts of 4-vinylanisole; 6 parts of methyl methacrylate; 4 parts of N-vinylpyrrolidone; 5 parts of ethylene dimethacrylate; 1 part of vinyl trimethoxysilane; 1 part 2,2;(2,5-thiopene diyl) bis(5-tertiary-butyl bensoxazole) solution; 0.1 part benzoyl peroxide and 0.5 parts 2,2',azo bis(isobutyronitrile) was cast into the cavity recess. The blank and filled recess were covered with a radius polypropylene mold cover. The assembly, held together by a spring clip, was placed in an oven and heated at 41°C for 8 hours in a nitrogen atmosphere and then heated at 69°C for 5.5 hours and finally at 83°C for 5 hours. The assembly was removed from the oven and allowed to cool to 35°C over a period of 2.5 hours. After removal of the cover, the resulting bifocal lens blank was cut to desired parameters. The bifocal lens had a Dk of 12 x 10⁻¹¹.

Claims: An oxygen permeable bifocal contact lens, lens blank or lens preform, having a distance vision portion with one refractive index and a near vision portion with another higher refractive index and a Dk of at least 10 x 10⁻¹¹ (cm²/sec)(10² x mm Hg) at 35° to 37°C, the distance portion forming the major portion of the lens and being formed as a crosslinked copolymer with a refractive index of not more than 1.49 from monomeric materials which includes one or more silicon acrylate monomers capable of imparting oxygen permeability to the finished lens, and the near vision portion similarly being formed as a crosslinked copolymer from a monomer mixture which includes one or more monomers at a concentration and with a refractive index such that the near vision portion has a refractive index of at least 1.540 including at least one vinyl monomer having substituted aromatic ring structure which makes such monomers bulky and ensures there is only minimal shrinkage on polymerization of such monomers and mixtures containing them, the distance vision portion and/or the near vision portion being formed as an interpenetrating network. An oxygen permeable bifocal contact lens material as claimed in claim 1, in which the interpenetrating network is formed by polymerizing a mixture of two monomer mixtures, one of which is partially polymerized to a relative viscosity of from 1.1 to 25 before being mixed with the other. An oxygen permeable bifocal contact lens material as claimed in claim 2, in which the distance vision portion is formed as an interpenetrating network from two monomer Mixtures A and B where monomer Mixture A is said partially polymerized mixture and the composition is selected so that Mixture A contains 10% to 90% (w/w) of the total monomer formula weight, the balance being Mixture B. An oxygen permeable bifocal contact lens material as claimed in Claim 3, where Mixture A contains 50% to 95% of the total monomer formula weight, and the balance is Mixture B. An oxygen permeable bifocal contact lens material as claimed in claims 2, in which the near vision portion is formed as an interpenetrating network with a refractive index of at least 1.540 from two monomer Mixtures C and D where monomer Mixture C is said partially polymerized mixture and the resulting composition is selected so that Mixture C contains 10% to 90% of the total monomer formula weight, and the balance is Mixture D. An oxygen permeable bifocal contact lens material as claimed in claim 3 or 4, in which the total monomer content of Mixtures A and B is selected from: a) 0 to 30% of an alkyl acrylate having the formula: where R = H or a monovalent organic radical having not more than 7 carbon atoms and R' = a monovalent organic radical having up to 20 carbon atoms. b) 0 to 35% of a fluoroacrylate monomer having the formula: where R = H or a monovalent organic radical having not more than 7 carbon atoms; A = H or E and E is a fluorinated monovalent radical having no more than 20 carbon atoms. c) 10 to 70% of a silicone acrylate of the general formula: where R = H or monovalent organic radical having from 1 to 7 carbon atoms or Z; X is a monovalent organic radical having from 1 to 7 carbon atoms; Y is a monovalent organic radical having from 1 to 7 carbon atoms or Z; and where R1, R2 and R3 are the same or different monovalent organic radicals having from 1 to 7 carbon atoms and R4 is OH or a monovalent organic radical having from 1 to 7 carbon atoms; and k = 0-1; m = 1-3; n = 1-5; and p = 1-3. d) from 0.1 to 20% of at least one crosslinking agent. e) from 1 to 10% of a polymerizable vinyl hydrophilic wetting agent. An oxygen permeable bifocal contact lens material as claimed in claim 5, in which the total monomer content of Mixtures C and D is selected from: a) from 30 to 90% of at least one polymerizable vinyl monomer having a refractive index of at least 1.540 and having a substituted aromatic ring structure. b) from 1 to 60% of an acrylate having the formula: where R = H or a monovalent organic radical having not more than 7 carbon atoms and R' is a monovalent organic radical having up to 20 carbon atoms. c) from 0.1 to 15% of a polymerizable polyvinyl crosslinking agent. d) from 0 to 2% of a polymerizable vinyl silane coupling agent. e) from 0.1 to 3.0% of a vinyl polymerization initiator. An oxygen permeable bifocal contact lens material as claimed in claim 7, in which the polymerizable vinyl monomer having a refractive index of at least 1.540 comprises one or more carbazoles selected from N-vinyl carbazole and 3-(N-carbazoyl) propyl acrylate. An oxygen permeable bifocal contact lens material as claimed in claim 7, in which at least one polymerizable vinyl monomer having a refractive index of at least 1.540 is selected from 2-(N-phenothiazinyl) ethyl acrylate, p-t-butylstyrene, vinyl naphthalene, and phenyl and benzyl methacrylates. An oxygen permeable bifocal contact lens material as claimed in any one of claims 1 to 9, in which the distance vision portion of the lens surrounds the near vision portion. A method of producing an oxygen permeable bifocal contact lens having a near vision portion and a distance vision portion as claimed in any one of claims 1 to 10 in which the near vision portion is formed in the body of the lens by polymerizing a partially polymerized mixture of the interpenetrating network forming comonomers in contact with a surface shaped to leave a recess in the material after both polymerization and removal from contact with the shaped surface, the recess being of the same shape as the desired shape of the near vision portion of the finished lens, and a partially polymerized mixture of interpenetrating network comonomers, including at least one vinyl monomer with refractive index of at least 1.540 and a substituted aromatic ring structure is placed in the recess and polymerized. A method as claimed in claim 11 in which the near vision portion is covered with the material used to form the distance vision portion of the lens by polymerizing a further quantity of said distance vision material in contact with the already formed near and distance vision portions. A method as claimed in claim 12, in which a base curve is cut in the near vision portion before it is covered. A method as claimed in claim 11, 12 or 13, in which a quantity of the material used to form the distance vision portion is placed in an open topped mold and a recess formed by providing the mold with a cover whose surface contacting the material is the recess forming surface. A method as claimed in claim 14, in which a lens blank is formed within the open topped mold and the mold is used as a carrier during machining the lens blank to a final lens form. A process for preparing an oxygen permeable bifocal contact lens with a Dk of at least 10 x 10⁻¹¹ (cm²/sec)(10² x mm Hg) at 35° to 37°C in which the higher index segment is surrounded by an oxygen permeable low index material by forming the lens in a series of steps comprising first forming a preform from a crosslinked copolymer material having a refractive index of not more than 1.49 in which there is provided a recess wherein said low refractive index material is formed from monomeric materials which includes one or more silicon acrylate monomers capable of imparting oxygen permeability to the low index material, filling the recess with a partially polymerized high index material formed from a monomer mixture which includes at least one vinyl monomer with a refractive index of at least 1.540 and having a substituted aromatic ring structure, completing the polymerization of the high index material to provide for a polymer composition having a refractive index of at least 1.540, thereafter covering both the low index material and the high index material with further low index material in a partially polymerized form, and finally completing polymerization of the low index material to form a lens blank which is then machined into a bifocal contact lens. A process of producing an oxygen permeable bifocal contact lens with a Dk of t least 10 x 10⁻¹¹ (cm²/sec)(10² x mm Hg) at 35° to 37°C by embedding a high refractive index polymer segment, having a refractive index of at least 1.540, within a lens blank having a refractive index of not more than 1.49, the lens blank having been prepared by casting a partially polymerized mixture of low refractive index interpenetrating network forming comonomers including a silicon acrylate monomer against a mold with a protrusion so as to form a recess in the blank and polymerizing to a solid state, the recess being then filled with a high refractive index monomer mixture, which includes at least one vinyl monomer with a refractive index of at least 1.540 and a substituted aromatic ring structure, the polymerization of which is then completed in the recess, a sufficient quantity of partially polymerized low refractive index monomer mixture being next added to completely cover the high refractive index segment, and the resulting composition is cured to form the final bifocal contact lens blank, which is in turn machined to form a bifocal contact lens.

Inventors: BURKE WILLIAM E, PETTIGREW LISA, RATKOWSKI DONALD A, WEINSCHENK JOSEPH I III, BURKE, WILLIAM E., PETTIGREW, LISA, RATKOWSKI, DONALD A., WEINSCHENK, JOSEPH I.III
Applicants: PILKINGTON BARNES HIND INC, PILKINGTON BARNES HIND, INC.
Application year: 1991
Main classification: B29D11
IPC codes: C08F226, G02B1, B29D11, B29C39, C08F26, C08F230, G02C7, C08F30, C08F220, C08F20