Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-11-12
2004-07-13
Moore, Margaret G. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C528S032000, C351S16000R, C623S001210, C623S001210
Reexamination Certificate
active
06762264
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to silicone hydrogel compositions useful as biomedical devices, such as contact lenses and intraocular lenses.
Polymeric silicone materials have been used in a variety of biomedical applications, including, for example, in contact lenses and intraocular lenses. Such materials can generally be subdivided into hydrogels and non-hydrogels. Silicone hydrogels constitute crosslinked polymeric systems that can absorb and retain water in an equilibrium state and generally have a water content greater than about 5 weight percent and more commonly between about 10 to about 80 weight percent. Such materials are usually prepared by polymerizing a mixture containing at least one silicone-containing monomer and at least one hydrophilic monomer. Either the silicone-containing monomer or the hydrophilic monomer may function as a crosslinking agent (a crosslinker being defined as a monomer having multiple polymerizable functionalities) or a separate crosslinker may be employed.
Silicone hydrogels combine the beneficial properties of hydrogels with those of silicone-containing polymers (Kunzler and McGee, “Contact Lens Materials”,
Chemistry & Industry,
pp. 651-655, 21 August 1995). Silicone hydrogels have been used to produce a contact lens that combines the high oxygen permeability of polydimethylsiloxane (PDMS) materials with the comfort, wetting and deposit resistance of conventional non-ionic hydrogels.
Monomers that have been found to be particularly useful for preparing silicone-containing contact lenses are described in U.S. Pat. Nos. 4,136,250; 4,153,641; 4,189,546; 4,208,506; 4,217,038; 4,277,595; 4,327,203; 4,355,147; 4,740,533; 4,780,515; 5,034,461; 5,070,215; 5,310,779; 5,346,976; 5,374,662; 5,358,995; 5,387,632; 5,420,324; and 5,496,871.
U.S. Pat. No. 4,153,641 (Deichert et al) discloses contact lenses made from poly(organosiloxane) monomers which are &agr;, &ohgr; terminally bonded through a divalent hydrocarbon group to a polymerized activated unsaturated group. Various hydrophobic silicone-containing prepolymers such as 1,3-bis(methacryloxyalkyl)-polysiloxanes were copolymerized with known hydrophilic monomers such as 2-hydroxyethyl methacrylate (HEMA). These materials were used to produce lenses which had a low water content and a high modulus (greater than 300 g/mm
2
).
U.S. Pat. No. 5,358,995 (Lai et al) describes a silicone hydrogel which is comprised of an acrylic ester-capped polysiloxane prepolymer, polymerized with a bulky polysiloxanyalkyl (meth)acrylate monomer, and at least one hydrophilic monomer. The acrylic ester-capped polysiloxane prepolymer, commonly known as M
2
D
x
consists of two acrylic ester end groups and “x” number of repeating dimethylsiloxane units. The preferred bulky polysiloxanyakyl (meth)acrylate monomers are TRIS-type (methacryloxypropyl tris(trimethylsiloxy)silane) with the hydrophilic monomers being either acrylic- or vinyl-containing. While the properties of these lenses are acceptable, the modulus of these lenses can be high, which may result in damage to the epithelial layer and poor comfort.
Designing silicone based hydrogels utilizing M
2
D
x
as the base prepolymer has mainly involved copolymerizing the prepolymer with hydrophilic monomers, such as dimethylacrylamide and N-vinylpyrrolidone. Silicone is hydrophobic and has poor compatibility with hydrophilic monomers, especially when the M
2
D
x
prepolymer is of high molecular weight. Poor compatibility results in phase separated, opaque materials. This can be particularly problematic when preparing hydrogels to be used as optically clear contact lenses.
Reducing the molecular weight of the M
2
D
x
prepolymer can improve the incompatibility. Unfortunately, low molecular weight M
2
D
x
prepolymers typically result in hydrogels of high modulus. This is a direct result of the higher crosslink density of these low molecular weight M
2
D
x
based hydrogels.
In designing a low modulus silicone hydrogel based on low molecular weight M
2
D
x
prepolymers, one approach can be to use high concentrations of hydrophilic monomers. The lower modulus for these materials is a result of the higher water content and lower cross-link density. The major drawback of this approach is that the higher water content materials possess lower levels of oxygen permeability, due to the lower concentration of silicone in these materials. The low levels of oxygen permeability are not suitable for continuous wear contact lens application.
Another approach in the development of low modulus silicone hydrogels based on low molecular weight M
2
D
x
prepolymers is through the incorporation of the monomer methacryloxypropyl tris(trimethylsiloxy)silane (“TRIS”). Higher concentrations of TRIS results in hydrogels of lower modulus, but lenses made with high TRIS levels overall tend not to perform well in clinical studies.
The development of low modulus hydrogels based on low molecular weight M
2
D
x
prepolymers may be accomplished through the addition of silicone macromonomers, such as those taught by Y. Kawakami in
Polymer Journal,
v. 14, p. 913, 1982. High levels of silicone macromonomer may reduce the modulus by lowering the cross-link density of the resultant hydrogel without a significant reduction in oxygen permeability. The major disadvantage of this route is that the methacrylate based silicone macromonomers are very difficult to synthesize. The synthesis of siloxane macromonomers requires several steps.
SUMMARY OF THE INVENTION
There remains a need for a contact lens material having the high oxygen permeablity of a polysiloxane-containing prepolymer, yet have a modulus low enough to be used as a contact lens. The approach taken in this invention alters the silicone-containing monomer to affect the polymer properties. By lowering the methacrylate functionality of M
2
D
x
the cross-linking density is reduced. This can be done by removing a percentage of the methacrylate groups on the end of the prepolymer.
These improved polymer silicone hydrogel compositions are formed from the polymerization product of a monomer mixture comprising a silicone prepolymer having the general formula:
wherein;
A is an activated unsaturated radical;
A′ is either an activated unsaturated radical or an alkyl group;
R
1
-R
10
are independently an alkyl, fluoroalkyl, alcohol, ether, or fluoroether group having 1-10 carbons, or an aromatic group having 6-18 carbons;
m, n, and p are independently 0 to 200, m+n+p being from about 15 to 200;
a is 1 to 10; and
b is 0 to 10,
wherein the silicone prepolymer is prepared by the reaction of dimethacrylate disiloxane (M
2
) and cyclic siloxane (D) in the presence of an catalyst, the improvement comprising adding at least one disiloxane (T
2
) having the formula:
wherein
R
11
-R
16
are independently an alkyl group having 1-5 carbons, to the reaction mixture used to form the silicone prepolymer.
In particular, this invention is directed to preparing a M
2
D
x
based prepolymer that is endcapped with trimethylsilyl (TMS) as shown in formula II:
wherein m+n+p is 15 to 200.
Note that prepolymers of formula II are a species of formula I wherein b is zero and R
9
,R
10
and A′ are methyl groups.
Applicants have found that the above preparation of making the prepolymer is especially effective in improving the flexibility of polymer silicone materials and hence lowering the modulus of silicone hydrogel copolymers, in contrast to previous siloxane compounds which were methacrylate endcapped and not endcapped with trimethyl silyl. The synthesis of the M
2
D
x
, TMS-endcapped prepolymer is easy, requiring fewer steps and components than previous methods.
The hydrogel material is especially useful in biomedical devices such as soft contact lenses, intraocular lenses, heart valves and other prostheses.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
This invention describes a novel approach to the design of low modulus silicone hydrogels based on M
2
D
x
prepolymers. The M
2
D
x
prepolymers of this inve
Künzler Jay F.
Martin Arthur W.
Bausch & Lomb Incorporated
Moore Margaret G.
Smith Glenn D.
Thomas John E.
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