Soft contact lenses and manufacture thereof

Soft contact lenses and method of making them

Contact lenses can be hard or soft in quality. Hard contact lenses are made from polymers with relatively low hydrophilicity by conventional manufacturing methods for molding them, e.g., in extruded heated molds.

When used, hard contact lenses are often uncomfortable because they are irritating to the eyes.

Soft contact lenses are made from polymers that have high hydrophilicity and are swollen with water to form a gel that contains at least 30% and often more than 70% water. Such soft, jelly-like contact lenses adapt very well to the eyes and are therefore more comfortable to wear.

U.S. Patent No. 27,401 discloses a group of hydrogels which can be formed by gelling a 20-97% aqueous liquid only with a slightly crosslinked copolymer formed by crosslinking a large amount of a hydrophilic monoester of acrylic or methacrylic acid with a small amount of a diester of these acids . Commercial contact lenses consisting of such hydrogels contain a crosslinking polymer formed from a large amount of hydroxyethyl methacrylate (HEMA) crosslinked with a small percentage of glycol methacrylate. Other known hydrogels contain acrylic-type monomers together with vinylpyrrolidone-type polymers, cf. e.g. U.S. Patents 3,639,524 and 3,621,079. U.S. Patent 4,067,839 discloses a hydrogel which is a copolymer of acrylic acid amide and acrylate or methacrylate.

Known methods for making soft contact lenses present a number of problems that make the lenses expensive and limit their use to some extent. According to a known method, the lenses are made by turning in a lathe from a cylindrical rod, which is a suitable polymer with strong hydrophilicity. Problems arise due to the relatively soft nature of the polymers and the high degree of accuracy required, as well as the small dimensions of the unexpanded lens. At a later stage of manufacture, the lenses are swollen with water, causing them to change shape and size. As a result, the optical properties of each lens must be measured after swelling.

Another known method involves casting and polymerizing a suitable lens material in an open rotating mold. In this casting technique, centrifugal and surface tension forces determine the shape of the lens, which forces act on the lens material during polymer curing. The formed lenses are subsequently swollen with water, resulting in the above-mentioned problems.

The more water the lenses contain, i.e., the higher the degree of swelling, the more comfortable the lenses are to wear. Unfortunately, the control of the final optical properties also decreases the more the lenses absorb water as they swell.

The mechanical strength of the lenses is also proportional to the polymer content, so that their mechanical strength is lower at high water contents. In addition, at high water contents, the lenses are more susceptible to microorganisms and suffer from the formation of denatured proteins and other impurities in the gel as well as other impurities in it, all of which can lead to allergic reactions or eye diseases.

The high cost of the known manufacture of soft contact lenses prevents the lenses from being systematically replaced when they become dirty. In addition, in order to achieve acceptable hygiene, careful and laborious washing of the lenses overnight and usually cooking must be performed.

A new manufacturing method has now been developed which allows easy and precise production of soft contact lenses, which are practical, have high optical accuracy and are priced to allow new lenses to be replaced at short intervals if desired. The water content of the lenses is at least 30%. At higher water contents, 70-75%, the lenses can be in place for a long time throughout the day and night, up to 14 days, while lenses with a lower water content must be removed overnight.

The present invention therefore relates to soft contact lenses and their preparation, in particular to a process for preparing a hydrophilic polymer gel for use in the manufacture of lenses, comprising preparing a substantially anhydrous polymerizable mixture of acrylic or methacrylic monomer and a preparation containing a water-substituting boronic acid ester and a polyhydroxy in a weight ratio of polyhydroxyl compound of 10:90 to 45:55, then the mixture is polymerized to form a hydrophilic polymer containing at least 30% by volume of the boronic acid ester preparation.

The invention thus makes it possible to prepare a hydrogel with improved mechanical and optical properties, which makes it suitable for use in soft contact lenses, so that it can be used in any known polymerization system only with the modification that the polymerization mixture is substantially anhydrous and contains boric acid ester and compound instead. , having three or more hydroxyl groups.

After formation of the hydrogel, the boronic acid ester is displaced by the water received by the gel up to a concentration of at least 30%, preferably at least 50% and even more preferably at least 70. The lenses used for a long time usually have a water content of 70-75

According to the present invention, for example, the acrylic or methacrylic monomer system disclosed in U.S. Patent No. 27,401, which is a combination of an acrylic or methacrylic acid monoester with a low concentration of an acrylic or methacrylic acid diester, can be used. Similarly, a polymerization system can be used in which vinyl, acrylic or methacrylic monomers are polymerized with, for example, hydroxyethyl acrylate, vinylpyrrolidone or acrylamides. By way of example, hydroxyethyl methacrylate, methyl methacrylate, hydroxypropyl acrylate, glycidyl methacrylate, diacetone acrylamide or vinyl acetate may be used in combination with acrylic acid amide, hydroxyethyl acrylate, dimethyl acrylate, acrylic acid, glyceryl methacrylate or glyceryl methacrylate.

It is now preferred that the acrylic monomer be hydroxyethyl methacrylate (HEMA) and even more preferably a combination of a larger amount of HEMA with a smaller amount of another monomer, most preferably hydroxyethyl acrylate (HEA).

A small amount of a crosslinking agent having a functionality of 2 or more can be added to the monomer or monomer mixture. An example of such a substance is ethylene glycol dimethacrylate.

By adding a crosslinking agent, the dimensional stability of the lens is improved, but at the same time, the breaking strength is reduced. It has been found that polymerization in the presence of the boronic acid esters used according to the present invention usually results in better dimensional stability and that the amount of crosslinking agent can be kept very small. However, some commercially available monomers can achieve the desired properties without the addition of a crosslinking agent.

The polymerization is carried out mainly in the absence of water. Namely, it has been found that the presence of water usually leads to incomplete polymerization and very low strength. Clear lenses with better properties have been obtained by carrying out the polymerization in the presence of water-soluble substances, e.g., ethylene glycol, diethylene glycol or glycerol, which do not react during the process and can be replaced by water. However, there are drawbacks in the method because the desired low use of crosslinker has no direct relationship between the amount of water-soluble material used and the amount of water in the hydrogel after washing (substitution reaction). Also, there is no sufficient compatibility of the polymer with the high water content, resulting in cloudy or opaque lenses. This is a serious drawback when it is desired to manufacture a lens with a high final water content. Or it has been found possible to reduce turbidity by using a mixture of glycerol and a solvent, e.g. 2-ethoxyethanol, but in this case the lenses become mechanically inferior because they become weak and brittle. In addition, it is common to remove 2-ethoxyethanol from the lens.

According to the present invention, both turbidity and poor mechanical strength are avoided by using glycerol in the form of a reaction product with boric acid as a solvent, diluent or filler during the polymerization. The method also allows for the adjustment of the water content of the finished lens and the manufacture of lenses having the same shape and size as the cavity of the mold in which they are made.

The reaction product or ester is prepared in a conventional manner, e.g., by heating in vacuo boric acid and a polyhydroxyl compound having three or more hydroxyl groups, e.g., glycerol, trimethylolpropane, glucose or the like, or a diethylene or a mixture of such polyhydroxyl compounds with two hydroxyls. with butadiene and removing the water formed in the reaction. Boric acid can also be reacted with a mixture of two or more polyhydroxyl compounds, glycerol and sorbitol, or mixtures containing more than two polyhydroxyl compounds can be used. The weight ratio of boric acid to hydroxyl compound or compounds may be about 10:90 to 50:50.

Most preferably, the weight ratio of boric acid to total hydroxyl compounds is about 20:80 to 45:55 and the weight ratio of boric acid to polyhydroxyl compound is about 15:85 to 40:60.

As a simple example, a suitable ester can be prepared by heating 160 parts by weight of glycerol and 40 parts by weight of boric acid in vacuo to 80 ° C and distilling off 33 parts by weight of water.

The boronic acid ester has a relatively high viscosity relative to water, at least 500 and preferably about 2000 centipoise. The viscosity of boric acid is an important factor in the process because it apparently controls the strength of the finished polymer gel and also determines the time at which the lens to be made in the mold is separated from the monomer feed. A highly useful boric acid ester having a viscosity of about 20,000 centipoise can be prepared from boric acid, glycerol and 1,2-propanediol and having a viscosity of about 30,000 boric acid, sorbitol and 1,2-propanediol. The choice of boric acid ester to be used depends on the acrylic or vinyl monomer or a combination of two or more copolymerizable monomers. Preferably, the monomers should remain compatible with the ester throughout the polymerization, which can be readily determined by simple experiments.

As a general rule, the boronic acid ester will be about 30-90% by volume, preferably 45-80% by volume, based on the total volume of the monomer and boric acid ester.

Without being bound by this theory, it is possible that the effect of a boronic acid ester depends on its high viscosity and solvent properties. Thus, it is conceivable that the ester produces a gel effect at a very early stage of the reaction, thereby preventing the movement of the reactive polymer chains and thus the polymerization can terminate by mutual reaction, so that only monomers can move in the reaction mixture and form long chains at high rate and only sporadic. Water and other hitherto known diluents act rather than solvents, so that the reactive polymer chains can move freely and thus, by mutual reaction, most often terminate their polymerization and give a polymer with a lower molecular weight. On the other hand, it is also conceivable that a boric acid ester can cause a stereo irregularity in the hydrogel. Despite the reaction mechanism, it is highly advantageous that the boronic acid ester, despite its high viscosity, can be easily washed off the lenses and replaced with water. In water, the ester is broken down into its constituents, e.g., boric acid and glycerol, which have low molecular weight and are readily soluble, so they are easily removed from the lenses.

The polymerization can be catalyzed in some hitherto known manner. Although conventional, thermally accelerated polymerization can be used, it has been found that this process tends to lead to internal stresses in the lenses and reduce their homogeneity. Therefore, in the systems used according to the invention, it is preferred to accelerate the polymerization by means of ultraviolet light. Also, if desired or necessary, a catalyst operating in conjunction with ultraviolet light can be added to the polymerization system. This photocatalyst can be any known catalyst that is sufficiently soluble in the monomer mixture to achieve substantially complete polymerization. Typical examples are commercially available UV catalysts under the trade names Darocur 1173 and 1176. When the catalyst particles become an integral part of the polymer, the catalyst can be selected to provide the desired light filtration effect.

Although soft contact lenses can be made in any of the previously mentioned and known ways, it is preferred to perform immediate polymerization with the finished lenses in a mold cavity of substantially the same size and shape. This mold cavity will thus correspond to the shape of the water-expanded final product and has a volume of 60-125%, more suitably 95^-105 % and more preferably about 100% of the volume of the final product.

It is well known that a negative volume change occurs during the polymerization of the monomer systems in question. To prevent cavitation and easily open the mold without adversely affecting the soft lens, the second mold part must be thin and elastic, which can lead to complications when shaping the lenses. It has now been found that shrinkage can be mainly compensated for by providing a small amount of monomer in the mold cavity in contact with the polymerization system. This amount of monomer acts as a reservoir from which monomer can be absorbed into the mold cavity as a result of the vacuum caused by shrinkage.

The edge of the contact lens must be thin and yet smooth without any irregularities that could irritate the eyelid or conjunctiva. It is practically impossible to make a suitable edge by machining. However, it has been found that if the mold has a suitable shape, the slight vacuum created by shrinkage can be used as a spacer to cut the edge at a later stage of curing and to guide the monomer into the mold to compensate for shrinkage.

Figure 1 schematically shows a cross-section of an embodiment of a mold used in the manufacture of contact lenses.

The casting mold 1 has an upper part or a lid 2 which is relatively heavy and a lower part 3 of more complex shape. The surfaces outside the casting cavity 4 formed in this lower part 3 have a shape such that there is contact between the mold parts 2 and J along the annular edge 5 around the casting cavity 4 otherwise, the distance between the opposite surfaces of the parts 2 and J increases uniformly in all radial directions away from the center of the mold. More specifically, the lower part 3 has a concave surface part 6 with a relatively sharp edge part 7 around the concave surface 6. The cover 2 has a convex surface part 8 and when the cover 2 is placed on the lower part 3, the concave surface 6 and the convex surface 8 together define a contact lens-shaped cavity 4 when the edge part 7 comes into contact with the convex surface 8 around it. The edge portions 9 and 10 circumferentially surround the convex and concave surface of the mold at a mutual distance which increases away from the edge portion 7. The cavity defined by the edge portions 9 and 10 is used as a container for the monomer during the polymerization. As the polymerization progresses, the shrinkage will create a small vacuum, allowing a sufficient amount of monomer to be absorbed into the mold cavity 4 to compensate for the shrinkage, after which the lid 2 again closes tightly over the mold cavity 4. At a later stage in the polymerization, the viscosity of the polymer solution is increased and the monomer flow decreases, resulting in an increase in vacuum, causing the relatively sharp edge 7 to strike the lid 2 irregularities, followed by cross-linking of the polymer terminating the reaction.

The lid 2 and the lower part 3 can be made of any suitable mouldable material. However, since it is preferred that the curing of the polymer takes place under ultraviolet light, at least the second mold part, preferably the lid 2, must be made of a material which is transmitted by the ultraviolet light. Today, polypentene is considered the best, but other substances, such as polystyrene, can also be used.

After casting the contact lens material, it is treated with water or water with small amounts of additives, e.g., salt or antibacterial agents, for a time sufficient to replace the entire amount of boronic acid ester with water.

It has further been found that the described method for manufacturing the soft contact lenses of the invention provides such precise control of the dimensions of the lens that ultra-thin soft contact lenses can be manufactured. For example, lenses with a negative diopter and a thickness in the central optical region of less than 0.1 mm, typically 0.07 mm, and often even less than 0.04 mm, can be repeatedly fabricated.

The small thickness of the lenses makes them very comfortable to use and also provides improved oxygen permeation. A living eye requires a certain amount of oxygen to come in, otherwise conjunctivitis and corneal edema can occur. The amount of oxygen required varies from person to person, and a few groups of users can use the hitherto known contact lenses only for a short time in a single pass. The transfer of oxygen through the contact glass is to some extent inversely proportional to the thickness. Therefore, the ultra-thin contact lenses according to the invention allow new groups of persons to wear contact lenses for an even longer period of time at a time.

Oxygen transfer has proven to be so high in practice that comfortable contact lenses with sufficient oxygen permeability can be produced with as low a water content as e.g. 30%. Such contact lenses can be made, for example, from hydroxymethyl acrylate accompanied by a low polar monomer, e.g., vinyl acetate, methyl methacrylate or acrylonitrile.

The following examples further illustrate the subject matter of the invention. It should be noted that, unless otherwise indicated, percentages throughout the specification and claims are by weight. In the manufacture of the contact lenses according to the examples, the mold described above is used.