Any ophthalmic lens must meet a variety of criteria in order to be acceptable for wear. Foremost for a contact lens, any material placed over the cornea of the eye must in some way provide for the passage of oxygen to the eye as well as remove waste products away from the eye. With hydrated soft contact lenses this is accomplished by having a material that, inherent with its high water content (sometimes over 50%), passes oxygen to the eye via the water contained in the lens.
Hydrated soft contact lenses, however, can act as a wick, drawing water way from the tear fluid in the eye and hastening its evaporation. This results in the "dry eye" effect, wherein an excess of moisture is drawn away from the eye by the hydrophilic lens.
In contrast, hard contact lenses do not exhibit this wicking effect because water does not absorb and pass through the lens, but rather is underneath the lens. A hard lens, however, can have a deleterious effect on the eye because of its non-pliable nature and the movement of the lens over the cornea whenever the wearer blinks can cause mechanical agitation.
Other desirable and undesirable characteristics are divided between hard and hydrated soft contact lenses.
For example, hard contact lenses do not absorb proteins and lipids to the extent that a high water content hydrogel does. The semi-rigid and hard lenses do adsorb some surface proteins and lipids, but these low water content materials absorb no proteins or lipids into the bulk material. Proteins and lipids are taken into the material of the soft lenses along with the tear fluid where they may be deposited. In general, this necessitates cleaning of the hydrated lens to remove protein and lipid deposits. Furthermore, hard contact lenses typically exhibit a higher strength and higher refractive index because they contain more plastic and less water allowing them to be made thinner.
Soft hydrated contact lenses have enjoyed wide acceptance because of the high degree of comfort and extended period of wear that such lenses exhibit. Most soft hydrophilic contact lens polymers produced over the last decade have strived to increase the water content of the material because of the water's contribution to wearer comfort and the passage of oxygen and carbon dioxide through the lens. This increase in water content, however, leads to the aforementioned problem with wicking of moisture away from the eye and also reduces the refractive index of the lens (i.e., the ability of the lens to bend light), and decreases the stiffness of the lens resulting in poorer handling properties. This in turn requires the lens to be thicker in order to meet the refractive index requirements necessary for the optical correction needed by the wearer.
If a lens material is either not permeable enough to oxygen and carbon dioxide, or does not provide the "tear pumping" action required to move the tear layer between the cornea and the lens to transport oxygen and carbon dioxide, negative physiological responses occur, which include: acidosis, decreased metabolic rates, thinning of the cornea, microcysts, and stromal edema.
Other physiological problems can occur even with high permeable lenses from effects such as protein deposits, lens ageing, occlusions, mechanical abrasion and bacteria contamination such as acute inflammation, acute red eye, and 3 and 9 o'clock staining of the central cornea.
The importance of water content for oxygen permeability in a hydrogel contact lens is shown in FIG. 1. Permeability of a gas through a material is expressed as a quantitative value given by Dk, which is equal to the diffusion constant, D, times the solubility, k. At 35.degree. C., Dk for a typical hydrogel lens is quantitatively expressed as (2.0.times.10.sup.-11)e.sup.(0.0442("%H.sbsp.2.sup.O")) (cm.times.mm/s)(ml O.sub.2 /ml.times.mm Hg).
Despite the increased water content of hydrogel contact lenses, current hydrogel lenses may not supply the cornea with enough oxygen, and corneal edema, during wear, may not be as low as desired.
It is believed that extended wear contact lenses would at a minimum need to have a Dk/L (where L being the thickness of the lens) between 75.times.10.sup.-9 and 90.times.10.sup.-9 (cm.times.ml O.sub.2)/(s.times.ml.times.mm Hg) to reduce corneal edema to an acceptable level.
Current high water contact lenses, for example, those that are approximately 70% water or higher, need to be made at approximately 140 to 250 microns thickness to achieve the necessary optical and physical properties. With this water content and at this thickness, it is seen in FIG. 2 that the Dk/L is about 55.times.10.sup.-9. Even with a hydrogel material having a water content of 80% and with a Dk equal to 53, a lens would have to be produced at approximately 70 microns in order for Dk/L to be 75.times.10.sup.-9.
As stated above, however, increasing the water content tends to lower the refractive index of the contact lens material and therefore requires an increase in lens thickness. Even if this were not the case, however, thinner contact lenses have lower strength, less desirable handling properties and, at high water content, tend to dehydrate to such an extent that corneal staining may occur.
Examples of the current practice in the art of producing polymers for contact lenses is shown in European Patent Application Nos. 0 330 614 and 0 330 615. These publications describe contact lens polymers containing polyoxyalkylene and having the usual desirable properties of a soft contact lens, but both are described as containing, in the hydrated state, between 10% and 90% water, preferably between 35% and 55%, water by weight.
European Patent Application No. 0 263 061 also describes a contact lens material consisting of a polyoxyalkylene backbone unit which absorbs less than 10% water by weight. This polyoxyalkylene backbone forms a polymer which requires the addition of carbonyl containing monomers to induce surface wettability, but which also lowers oxygen permeability. EP-A Nos. 330614, 330615 and 330618 use polyether and carbamate linkages to produce contact lens polymers of both low and high water content but also use small molecular weight monomers to increase the water content of the base polymer. Each of the aforementioned references however fail to teach the use of more biocompatible materials such as sugars which contain carbon atoms bonded to two oxygen atoms (geminal) as part of their structures. The materials of the references also require large amounts of hydrophilic modifiers to induce wettability and silicon materials require surface treatment of some type.
U.S. Pat. No. 3,225,012 discloses a polymer that is prepared by polymerizing 1,2: 5,6-di-O-isopropylidene-3-O-methacryloyl-D-glucose and then removing the isopropylidene groups from the glucose by acid hydrolysis. U.S. Pat. No. 3,356,652 describes a polymer that is derived from 2-(D-glucose)oxyethyl methacrylate. Both U.S. Pat. Nos. 3,225,012 and 3,356,652 use the glucose component of the polymer as a terminated pendant group off of a repeating carbon backbone, and not as the primary repeating group from which the polymer chain is formed.
U.S. Pat. No. 5,196,458 provides a lower water content lens material having high O.sub.2 permeability, a lower water content and reduced polymer matrix size. The lens provided in this reference is prepared by polymerizing and crosslinking a prepolymer which contains a cyclic polyol such as an alkoxylated glucose or sucrose with polyalkylether segments.
Polysiloxane compounds (co)-polymerized with various monomers such as acrylic esters and alkyl acrylic esters have also been employed in the prior art to provide contact lenses with higher oxygen permeabilities. Such lenses containing polysiloxane compounds are disclosed, for example, in U.S. Pat. Nos. 3,808,178; 4,153,641; 4,740,533; and 5,070,169.
Despite the current state in the art there is still a continued need to provide soft contact lenses which are comfortable to wear, strong, have low water content, and extremely high oxygen permeability, but do not wick water away from the eye nor allow protein or other tear components to penetrate and deposit on the cornea of the eye.
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