Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating
Reexamination Certificate
1997-09-23
2001-01-30
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
C134S031000, C264S002600, C264S085000, C264S344000, C528S480000, C528S483000
Reexamination Certificate
active
06180031
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to improved methods of manufacturing or processing contact lens materials employing a supercritical fluid.
Polymerized contact lens materials must not only have sufficient optical clarity, but also must be suitable for contact with the eye for extended periods. The contact lenses made from such materials must be sufficiently hydrophilic at the lens surface to properly “wet”. Wetting is the characteristic understood to relate to the contact lens' ability to be lubricated by the eye's natural tears so that the lens may move freely over the eye during its use. This freedom of movement over the eye keeps the lens from adhering to the eye and allows a continuous stream of tears to wash under and over the lens, resulting in maximum comfort.
The ability of a lens to properly wet and be “comfortable” in the eye is difficult to predict. Much work in the field has been directed to achieving and maintaining a hydrophilic environment on the contact lens surface. Incorporation of a variety of hydrophilic monomers into the monomer mix, as well as post-treatments such as plasma treatments have been attempted, with some success, to maintain hydrophilicity at the lens surface. However, attempts to improve and maintain adequate hydrophilicity must not compromise the other important lens characteristics, such as optical clarity. Conversely, attempts to improve or retain a high degree of optical clarity, oxygen permeability, tear strength, etc., while maintaining the proper modulus, must not adversely affect the lens' wetting capabilities.
A method of lens treatment which would not adversely affect the lens' wetting or other physical characteristics, but which could improve the lens' optical clarity would be desirable. In addition, a lens treatment which could improve the hydrophilicity of a lens, thereby eliminating or at least reducing the need for additives or post-treatment would be advantageous.
Because of certain processing limitations, most soft hydrogel contact lens production results in a molded lens which must be handled, inspected and shipped in a wet state. This processing limitation adds time and cost to the overall lens production. However, a cost-effective lens treatment which could produce a dry lens could simplify lens production and reduce the cost of lenses.
It will also be appreciated that large-scale manufacturing processes require disposal of relatively high volumes of the extraction solutions used to remove impurities and residual materials (sometimes referred to as residuals or extractables) from the lenses. In addition, various materials used as solvents provide potentially hazardous conditions to working environments due to material toxicity to humans or flammability, for example. An extraction technique using a non-toxic, preferentially recyclable extraction solvent would be desirable.
SUMMARY OF THE INVENTION
The present invention provides methods of treating contact lenses and contact lens materials. More specifically, in one aspect of the invention, a method of treating a contact lens material is disclosed comprising treating the contact lens material with a supercritical fluid.
In another aspect of the invention a method of producing an optically clear contact lens is disclosed comprising treating the contact lens with a supercritical fluid.
In a still further aspect of the invention a method of dry releasing a contact lens from a mold in which the lens was molded is disclosed, comprising treating the contact lens and mold with a supercritical fluid.
In yet another aspect of the present invention, a method of extracting incompletely polymerized materials from a contact lens is disclosed comprising treating the contact lens with a supercritical fluid.
DETAILED DESCRIPTION OF THE INVENTION
A gas is considered to be in a supercritical state when it is subjected to such a combination of pressure and temperature that its density approaches that of a liquid (where a liquid and gas state coexist). When a gas is subjected to such conditions, it is called a supercritical fluid (SCF). It has now been discovered that exposing a contact lens to supercritical carbon dioxide (CO
2
) produces a contact lens with enhanced optical clarity. It was also discovered that supercritical fluids could extract the unreacted or incompletely polymerized material in the polymeric structure of various contact lens materials.
Carbon dioxide is a preferred candidate for supercriticality since its critical temperature is 31° C. A gas' critical temperature is the temperature at which a change in phases takes place causing an appreciable change in the gas' physical properties. While all of the experimentation discussed below was conducted with CO
2
it is understood that many other gases under supercritical conditions may be used to treat contact lens polymeric materials, including supercritical nitrous oxide (N
2
O), ethane and propane, or a combination thereof.
As is known in the field of supercritical fluids, the solubility of a particular solid in a SCF depends upon the density and polarity of the SCF. Therefore, when a supercritical fluid is to be used to extract a particular component from a material, the specific solubility of the component must be experimentally determined.
For the extraction of incompletely polymerized residual material from contact lens materials, it has now been determined that the CO
2
should be pressurized to from about 1000 psi to about 4000 psi, and is preferably from about 2000 psi to about 4000 psi when the temperature is kept within the range of from about 40° C. to about 90° C., and preferably from about 50° C. to about 80° C.
Generally, in the manufacture of contact lenses, some of the monomer mix is not fully polymerized. The incompletely polymerized material from the polymerization process may affect optical clarity or may be harmful to the eye. Residual material may include solvents or unreacted monomers from the monomeric mixture, or oligomers present as by-products from the polymerization process.
The residual material may be hydrophilic or hydrophobic. Conventional methods to extract such residual materials from the polymerized contact lens material include extraction with water (for extraction of hydrophilic residual material) or an alcohol solution (for extraction of hydrophobic residual material). However, some of the alcohol extraction solution remains in the polymeric network of the polymerized contact lens material, and must also be extracted from the lens material before the lens may be worn safely and comfortably on the eye. This requires an additional extraction of the alcohol from the lens, generally using heated water for up to 4 hours or more. Frequently, however, the conventional methods may not adequately remove the residual material.
Contact lens materials are formed from the polymerization product of a mixture of monomers or prepolymers. (For purposes of convenience, the term “monomer” as used hereafter shall include prepolymers.) The monomeric mixture may also include materials other than monomers that aid in the polymerization process, such as a solvent or a diluent. Contact lens materials include materials for “hard” and “soft” lenses. The hard lens classification typically includes lenses such as rigid gas permeable (RGP) contact lenses, which are generally formed of crosslinked silicone acrylate or fluorosilicone acrylate copolymers. Soft lenses include “soft” hydrogel contact lenses. Hydrogels are hydrophilic polymers that absorb water to an equilibrium value and are insoluble in water due to the presence of a crosslinked three-dimensional network. Hydrogels are generally formed of a copolymer of at least one hydrophilic monomer and a crosslinking monomer. The hydrophilicity is due to the presence of hydrophilic groups, such as alcohols, carboxylic acids, amides and sulfonic acids. The swollen equilibrated state results from a balance between the osmotic driving forces that cause the water to enter the hydrophilic polymer and the forces exerted by th
Bawa Rajan S.
Hahn Dennis
Tasber Frank
Bausch & Lomb Incorporated
Thomas John E.
Vargot Mathieu D.
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