GB1601087A - Soft contact lens - Google Patents

Description

(54) SOFT CONTACT LENS (71) 1, WILLIAM MICHAEL FOLEY, Jr., a citizen of the United States of America, of 2551 Sleepy Hollow Road, Glendale, State of California 91206, United States of America, do hereby declare the invention for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to soft contact lenses and to hydrogels comprised of loosely cross-linked co-polymers for producing such lenses.

Of the 90100 million people in the United States wearing eye glasses, about 10 million have, to date, tried contact lenses typically a "hard" lens of the type developed many years ago. However, such "hard" lenses are not suitable for correcting all eye difficulties and, moreover, can lead to corneal edema, discomfort and eye irritation for many wearers.

Because of this, "soft" contact lenses have recently become popular being first licensed by the F.D.A. for commercial use a- few years ago. Soft lenses are an improvement in many respects, although they have some limitations as currently produced; and some lens fitters claim that only one-half to three-fourths of the candidates for contact lenses can successfully use "soft" lenses.

A good, soft lens should be made of a water-absorbing polymer that is rendered soft and pliable when water-saturated so that when placed upon one's cornea, it will mold itself to the corneal shape and remain there, and thus not likely be dislodged and lost or broken. Of course, corneal shapes can vary widely, as can their diameters: typically from about 12 to 15 mm. A soft lens is conventionally made slightly larger than the cornea whereas hard lenses are made somewhat smaller than the cornea.

Soft lenses should, ideally, be custom-fitted to the shape and size of a wearer's eyes; however, as yet, this is done rather imperfectly. In practice, some soft lens wearers experience excellent visual acuity, whereas for others adequate visual acuity cannot be achieved. Unlike "hard" lenses, a soft lens will customarily minimize any transitory irritations. such as "corneal edema", staining and abrasions caused by extended lens wear. Because they cover the cornea, all contact lenses should help protect it from foreign matter like dirt particles; however this is accomplished better with soft lenses than with hard lenses. Also, while hard lenses often admit foreign matter at their corneal-interface, this is relatively rare with a soft lens, since it fits and conforms more closely.

One problem with today's soft lens material, is that they are not strong enough nor sufficiently tear-resistant, and can too easily be torn, broken or split during handling, such as during fitting or when removed for cleaning and disinfecting (e.g., during nightly sterilization). Such breakage is common and is a common cause of dissatisfaction with soft lenses.

The present invention is directed towards correcting this problem and providing a soft lens material which is stronger and longer-lasting, being more tolerant to such handling without tearing or breaking-while yet maintaining acceptable hydration and good lens properties.

Today's soft lenses are, for the most part, incapable of being adequately cleaned. For instance, their surfaces will pick up proteinaceous material and other undesired deposits with cannot be completely, or adequately, removed; either by immersion disinfecting or by steam sterilization. Moreover, repeated steam sterilization rapidly degrades current lens materials, since they are not sufficiently "heat-stable". The present invention is directed towards producing a material having the improved strength mentioned, as well as being more resistant to surface deposits and, also, being more susceptible to thorough cleaning and being quite stable at the temperature of boiling water. In addition, soft lenses produced with the material of this invention surprisingly exhibit these properties without any significant degradation of other lens properties, such as hydration and optical quality.

According to this invention, it has been discovered that replacement of a few percent of the "HEMA" constituent in a soft lens formulation with MMA can have surprising, advantageous results, such as: Curing typical problems resulting from HEMA contamination, with methacrylic acid (i.e., irregular hydration alleviating the tendency of a HEMA lens to collect irreversible proteinaceous surface contaminants, greatly improving heat-stability (e.g., resistance to breakdown when boiled for sterilization); affording substantial grains in lens rigidity (e.g., sufficient to enable a soft lens to include astigmatic correction); while providing adequate strength (e.g., tear-resistance "handle-ability" and flexibility).

Various polymeric materials have been suggested for producing contact lenses, including soft lenses (as examples, see U.S. Patent No, 3,503,942 to Seiderntan : and U.S. Patent Nos. 3,220,960, 3,260,945, 2,976,576; 3,496,254; 3,669,089; and 3,499,862 all to Wichterle et al).

Now, the present invention involves polymeric hydrogel. It is known in the art to adapt gel material for contact lenses e.g., preparing the gel from HEMA. HEMA formulations which do not include any methyl methacrylate polymer, suffer in terms of mechanical fragility (being relatively breakable), and in thermal stability; for instance, becoming discolored and subject to tearing after repeated boiling as opposed to formulations according to the present invention.

It is also known that formulations including HEMA may be combined with a cross-linker to produce contact lenses; yet such lenses are all too fragile, tearing readily; also they don't clean up thorougly enough as compared with lenses according to the present invention-these comprising HEMA co-polymerized with MMA and cross-linked with "TGD" (triethylene glycol dimethacrylate and being far stronger, cleaning up readily, with less adhesion of foreign deposits, and moreover, capable of being repeatedly sterilized in boiling water with degradation.

This is new in the art.

For example, by comparison with a current, well-known commercial soft lens, using HEMA without the mentioned MMA additive, lenses according to the invention can, advantageously, be made: more rigid and incorporate features for correction of astigmatism; less hydrated; relatively insensitive to methacrylic acid contamination (in polymer formulation); to be readily cleaned and polished; much more resistant to undesired deposits, like the mucoprotein film that can commonly impair lens quality; steam-sterilizable, having sufficient thermal stability to tolerate prolonged boiling without degradation.

Also, by comparison with a second extant soft lens formulation (including HEMA and polyvinyl pyrolidone, with no added cross-linker) this invention provides much greater lens strength and heat-stability (lenses from this second formulation split readily and disintegrate on prolonged boiling-making it necessary to sterilize them with a chemical, like H2O2).

It is also known in the art, to copolymerize HEMA with a hydrophilic monomer (as opposed to an MMA co-polymer); however, the product of such a copolymerization is not particularly useful for soft lenses. By contrast, formulations according to this invention rather surprisingly copolymerize MMA, a nonhydrophilic monomer, with HEMA (or with HPMA) to produce a superior soft lens material-one having improved lens properties, satisfactory hydration, and remarkable strength-all via the addition ofjust a few percent MMA (0.5 to 7 O by weight).

Thus, it is an object of this invention to meet some or all of the above difficulties and to provide some or all of the above features in an improved hydrophilic, polymeric hydrogel producing a soft lens material that is hard enough to cut and polish, that is tough enough to withstand extensive handling without breakage or tearing and that nonetheless maintains good hydration and optical properties.

The invention, as specified below, will be seen to produce such an improved hydrogel by copolymerizing HEMA (or HPMA) with a few percent MMA; preferably, together with about one percent of a dimethacrylate-type cross-linker.

According to the present invention, there is provided a soft contact lens comprises a copolymer of from 92 to 98.5 weight percent of HEMA or HPMA (2 hydroxyethylmethacrylate or 2 - hydroxypropylmethacrylate) and from 0.5 to 7 weight percent of MMA (methylmethacrylate).

In a preferred form of the invention, the lens comprises a copolymer of from 96 to 98.5 weight percent of 2 - hydroxyethylmethacrylate and 0.5 to 3 weight percent of methylmethacrylate.

Preferably also, the copolymer contains up to 1 weight percent of triethylene glycol dimethacrylate (TGD) as a cross linking agent.

As will be seen below, it is preferred to use 96-98.5 wt.% HEMA, with 3-0.5 wt.% MMA plus about one percent, or slightly less, of a compatible cross-linker such as "TGD".

In certain cases where a slight lens discoloration may be tolerated, and increased hydration is desired, up to 1 percent methacrylate acid (M-Acid) may also be added according to the invention; this being done preferably in conjunction with increased concentration of MMA to maintain strength. As will be seen, preparation of a lens according to this invention is relatively simple involving initiation of the polymerization (e.g., chemically) together with a heat-cure at an elevated temperature, preferably in lens blanks, followed by the usual lens fabrication steps.

It will be seen that the invention may be very convenientlv used to control important lens properties. For instance, to increase toughness one simply increases the concentration of MMA; or to maximize percent hydration, the concentration of HEMA is maximized (with up to about 40% hydration being achievable, with about 0.5% MMA and 0.50.9% cross-linker).

Workers in the art will be surprised at the improvement in toughness and tearresistance, and in the improvedceanability of.lenses according to this invention, with less adsorption of foreign matter, and less cloudiness and yellowing-all as a result of merely adding a very few percent MMA copolymers. It is also surprising that this MMA additive yields such significant improvements, yet without any significant sacrifice in lens strength, optical clarity or percent hydration (maintaining the minimum 3032% hydration thought necessary).

EXAMPLES The following examples further illustrate the invention. The examples are summarized in Table I below indicating the approximate concentration of ingredients as related to lens properties in terms of percent hydration (water content), strength, etc.

TABLE I (Add MMA; diff. conc.)

Weight of Components (%) Testing Results Example HEMA MMA M-Acid TGD Hyd Elast. Str. Hard Cut Pol Col Opt No. (%) (X) (D) 1 99.0 0 0 1.0 38 5.2 Str. 86 OK Good OK Good 2 98.5 0.5 0 1.0 37 5.7 Str. 87 OK Good OK Good 3 97.5 1.5 0 1.0 37 5.0 V.Str. 86 OK Good OK Good 4* 96 3.0 0 1.0 36 5.7 V.Str. 87 OK Good OK Good 5 94 5.0 0 1.0 34 5.4 V.Str. 86 OK Good OK Good 6 92 7.0 0 1.0 34 6.2 V.Str. 87 OK Good OK Good ="desirable" * "Standard" Abbreviations: HEMA -2-hydroxyethylmethacrylate or 2-hydroxypropyimethacrylate instead of HEMA.

MMA -Methylmethacrylate M-ACID -Methacrylic Acid (ord. 0.1-0.3% present in HEMA) TGD -Triethylene glycol dimethacrylate HYD -Hydration; prefer minimum of 3W38 wt.%; 3031% rather rigid for eye comfort, less than 30% not generally acceptable.

ELAST -Elasticity; as lens is stretched (e.g., 5x) to "rupture point"; is measure of flexibility STR strength: Subjective evaluation of rigidity and elasticity relative to HYD% (e.g., a 30% HYD lens with 3x stretch; and a 50% HYD lens with 6x stretch can both be rated "Strong") HARD -Hardness (ASTM: Durometer Reading; D-scale) CUT -Can lens be satisfactorily cut? POL -Can lens be satisfactorily polished? COL -Clarity and color (e.g. versus translucent and yellowed): visual examination.

OPT -Lens optical quality (e.g., power and transmittance tested on Lensometer").

V. Str. -Very Strong S. Rig -Slightly Rigid Example W"Standard" Lens Co-polymer To produce tougher, more readily cleaned, more temperature stable, soft-lens polymeric hydrogels, a novel composition is taught, comprising in wt.% approximately: HEMA, 96; MMA, 3; TGD, 1.0 as summarized in Table I above, and explained as follows: TABLE A (Polymer Preparation) A typical formulation for this polymerization is as follows: 2-Hydroxyethyl Methacrylate (HEMA) 480 gm. 95.90% Methyl Methacrylate (MMA) 15 gm. 3.0 % Triethylene Glycol Dimethacrylate 5 gm. 1.0 % 2,5-Dimethylhexane-2,5-Diper-, 2 Ethyl Hexoate (free rad. initiator) I gm. 0.2 % The HEMA and MMA will be understood as conventionally distilled and purified to remove inhibitor and like contaminants. All components are to be mixed together to be homogeneously blended.

After the formulation components are weighed and checked, they are combined in a flask with the initiator and stirred mechanically.

The resin formulation is cured by placing in high density polyethylene (or "Teflon"-Registered Trade Mark) molds, 1/2 inch in diameter and 3/16 inch in depth, which are placed side-by-side on aluminum trays. The resin is transferred to the molds by means of a syringe or a polyethylene squeeze bottle.

Curing is then performed in an oven equipped with a blower and automatic temperature controls. Prior to placing the trays of resin molds in the oven, it is preheated to 95 C (#3 ) in air.

Cure is typically effected in about one hour at the mentioned 95 C (-2 hours for the other formulations). Overheating should be avoided since MMA will start boiling off at about 100 C; too low a curing temperature inhibits a complete cure.

Results (See also Table 1): A number of very strong, tough, lens blanks are produced, these being very suitable for making soft lenses as follows: Each blank is annealed to optimize hardness and relieve stress (e.g., 3 hours at about 85 C). Then it is trimmed to size, top and sides. The "back curve" is then cut and is polished leaving a peripheral flange, preferably. Next, the "anterior curve" is cut and polished. The lens is edged to complete the finished dehydrated lens.

The lens may then be hydrated to saturation (here typically 35-37 wt.",,) and is autoclaved while in saline solution to sterilize, killing micro-biological contaminants.

The toughness and tear-resistance of the lenses will be seen as surprisingly high, allowing them to be handled more readily than conventional soft lenses, yet their hydration is kept adequate. Clean-up is unexpectedly effective, too-and the lens surface is more susceptible to polishing and removal of deposits. The lens material may be repeatedly disinfected by boiling without discoloration or other degradation.

Within limits, replacing some HEMA with a few percent MMA significantly- and surprisingly-enhances "soft lens properties".

For instance, in general, as MMA is increased, hydration goes down and strength and elasticity increase. Ex. 1-2 indicate that (here) less than about 0.5-1 wit.% MMA is disfavored, the lens being inferior in strength (e.g., like conventional HEMA-only lenses).

More surprisingly, it appears that a concentration of only a few percent MMA is all that is needed to enhance lens properties. The preferred MMA concentration (only a few %) is relatively critical in range and affords surprising improvements (especially in lens resilience-yet maintaining good hydration plus satisfactory rigidity) and surprising "balance", i.e., the comfort (hydration) and resilience desired in a conventional soft lens together with the strength, toughness, corneaconformability and "lens suitability" of a better lens.

Even more: since many lots of HEMA are found to contain a minor (but varying) concentration (e.g., 0 to about 0.1-0.3 wt.%) of methacrylic acid, which can cause associated undesired variance in lens hydration and strength, so adding MMA can, fortuitously, counteract this loss in strength when added in approximately the same concentration range ("only a few percent") as prescribed above (see below for explanation).

Thus, according to this embodiment, an improved "soft" contact lens is manufactured from a novel hydrophilic co-polymer of HEMA, plus a few percent MMA, plus conventional additives. This co-polymer (a poly-2-hydroxyethyl methacrylate with methyl methacrylate) is adapted to be hydrated about 3040% when fully equilibrated in normal (isotonic) silane. The novel co-polymer will be understood as a cross-linked co-polymer of 2-HEMA and MMA with the copolymer chains coupled by TGD cross-links at a density of about one cross-link for approximately every 225 monomer units. The good hydration is believed largely due to the HEMA constituent (-OH and -CO groups) and the improved mechanical strength due (at least in great measure) to the MMA constituent. Still the interaction between these constituents and their remarkable ability to, together, render the resultant enhanced, yet balanced lens properties is not, as yet, fully understood.

EXAMPLE 7-HPMA replacing HEMA Example I is repeated with hydroxy-propyl methacrylate (HPMA) being substituted for HEMA at the same concentration. The results in terms of hydration and strength are essentially the same.

Methacrylic Acid Additive (M-Acid) Table 11 summarizes the results of adding different amounts of M-Acid to a soft lens HEMA/MMA formulation. As a control, the preferred formulation of Ex.

4 is again shown here. The abbreviations and other symbols here will be understood as the same for the Examples of Table I, as are the preparation methods.

TABLE II MMA plus M-Acid

Weight of Components (%) Testing Results Example HEMA MMA M-Acid TGD Hyd Elast. Str. Hard. Cut Pol Col Opt.

No. (%) (X) (Rel) (D) 4* 96.0 3.0 0 1.0 36 5.7 Str. 87 OK Good OK Good 8 95.7 3.0 0.3 1.0 39 5.4 Str. 87 OK Good OK Good *"Standard" Example 8 is representative of a common concentration of M-Acid found, as an impurity, in commercial HEMA shipments. They also indicate that at least a bit of MMA (3%) present can counteract the loss in lens strength induced by the M Acid (more elasticity, less tear-prone), even when the M-Acid concentration varies (as it often does, from zero to about .3%).

Also, Example 8 indicates what a striking effect a relatively tiny concentration of M-Acid has on hydration-e.g., compare Example 4 versus Example 8.

EXAMPLE 8-M-Acid Added to Example 4 Formulation As seen in Table II, adding a mere 0.3% M-Acid to the formulation of Example 4 produced a gain in hydration, with attendant loss of strength. The enormous increase in percent hydration (up to about 60% is possible) will be looked-upon as quite surprising and unexpected by those skilled in the art, especially in view of the tiny amount of M-Acid causing it. The loss in strength need not be viewed as problematical, since an increase in MMA concentration can be undertaken to compensate for this, to a degree.

However, it should be noted that a lens produced by addition of M-Acid are apt to suffer some loss in optical clarity and become yellowed in use. Thus, the concentration of M-Acid added should be minimized where this is important. The mechanical properties of the resultant lens so produced are quite remarkable, being extremely flexible (elastomeric) and much tougher than conventional "high hydration" lenses (e.g., compare to those having a hydration of about 50"', or more).

As workers in the art will recognize, a stronger "M-Acid" lens may be produced by replacing more of the conventional HEMA with MMA (e.g., about up to about 15% MMA here, though hydration will suffer somewhat, of course; e.g., down to the order of 3040%). Conversely, where a smaller enhancement of hydration is acceptable, considerably less M-Acid may be used (e.g., up to a few tenths of 1% here). Again, to a surprising degree, a very tiny concentration of M Acid will produce enormous amplificiation in hydration properties.

Claims (5)

1. A prescribed range (about 0.5 to 1% TGD) of cross-linker appears needed in such "HEMA type" lens formulations (for minimum strength); yet when used in conjunction with a "few percent " MMA, the needed portion of cross-linker is minimized, its effect on hydration, elasticity and rigidity is moderated (less sensitivity) and-quite fortuitously (for Examples like the above at least) roughly the same concentration of MMA is apt for all purposes. This coincidence is striking indeed! WHAT I CLAIM IS: 1. A soft contact lens comprising a copolymer of from 92 to 98.5 weight percent of 2-hydroxyethyl methacrylate or 2-hydroxypropylmethacrylate and from 0.5 to 7 weight percent of methyl methacrylate.
2. 2. A contact lens as claimed in Claim I comprising a copolymer of from 96 to 98.5 weight percent of 2 - hydroxyethylmethacrylate and 0.5 to 3 weight percent of methylmethacrylate.
3. 3. A contact lens as claimed in Claim I or 2 in which the copolymer also contains up to 1 weight percent of triethylene glycol dimethacrylate as a cross- linking agent.
4. 4. A contact lens as claimed in any one of Claims 1 to 3 in which the copolymer also contains up to 1 weight percent of methacrylic acid.
5. 5. A contact lens as claimed in Claim I and substantially as hereinbefore described with reference to any one of Examples 2 to 6 and 8.