U.S. patent number 3,700,761 [Application Number 04/880,828] was granted by the patent office on 1972-10-24 for fabrication of soft plastic contact lens blank.
This patent grant is currently assigned to Griffin Laboratories Incorporated. Invention is credited to Allan A. Isen, Kenneth F. O'Driscoll.
United States Patent |
3,700,761 |
O'Driscoll , et al. |
October 24, 1972 |
FABRICATION OF SOFT PLASTIC CONTACT LENS BLANK
Abstract
Graft or block copolymers of hydroxy alkyl methacrylate esters
and polyvinyl pyrrolidone are (1) cast in a shaping mold as a
monomer-polymer dispersion, polymerized to a solid at
40.degree.-60.degree. C. in the presence of low and medium
temperature free radical initiators, (2) the solid taken out of the
mold and heated to 90.degree.-120.degree. C., and then
post-polymerized by (3) radiation while dry and by (4) hydrogen
peroxide treatment to form hygroscopic, solid, shaped masses which
may be cut in the dry state, after step (2), into contact lenses.
The lenses may be equilibrated in the wet state by hydrating with
normal saline solution. The lenses may be maintained by treatment
with hydrogen peroxide. Steps (3) and (4) toughen the lens,
increase its elasticity and its elastic recovery and improve its
dimensional stability. From 20 to 45 percent by weight of polyvinyl
pyrrolidone imparts hygroscopic and unusual water-swelling
characteristics while retaining elastic recovery. The water-swollen
lens contains from 40-80 percent water, preferably from 50-55
percent, and in isotonic saline, the water content changes to about
52-58 percent. As a result of the polyvinyl pyrrolidone
incorporation, the lens is readily cleaned after use in the eye
with dilute hydrogen peroxide to rid it of imbibed muco-protein,
catalase and the like.
Inventors: |
O'Driscoll; Kenneth F.
(Williamsville, NY), Isen; Allan A. (Buffalo, NY) |
Assignee: |
Griffin Laboratories
Incorporated (Buffalo, NY)
|
Family
ID: |
25377194 |
Appl.
No.: |
04/880,828 |
Filed: |
November 28, 1969 |
Current U.S.
Class: |
264/1.36; 134/2;
264/2.6; 525/283; 134/42; 351/159.73; 351/159.02 |
Current CPC
Class: |
B29D
11/00 (20130101); B29D 11/00067 (20130101); C08F
271/02 (20130101); G02B 1/043 (20130101); B29D
11/00134 (20130101); B29D 11/00865 (20130101); G02B
1/043 (20130101); C08F 271/02 (20130101); G02B
1/043 (20130101); C08L 33/14 (20130101); C08L
51/003 (20130101); C08F 220/20 (20130101) |
Current International
Class: |
A61L
15/22 (20060101); A61L 15/16 (20060101); A61L
15/44 (20060101); B29D 11/00 (20060101); C08F
271/00 (20060101); C08F 271/02 (20060101); G02B
1/04 (20060101); G02C 13/00 (20060101); B29d
011/00 () |
Field of
Search: |
;264/1 ;260/885 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3462385 |
August 1969 |
Barabas et al. |
3468832 |
September 1969 |
Barabas et al. |
3417054 |
December 1968 |
Merijan et al. |
3496254 |
February 1970 |
Wichterle |
3220960 |
November 1965 |
Wichterle et al. |
|
Foreign Patent Documents
|
|
|
|
|
|
|
2,021,154 |
|
Jul 1970 |
|
FR |
|
829,565 |
|
Mar 1960 |
|
GB |
|
Primary Examiner: White; Robert F.
Assistant Examiner: Sokal; Allen M.
Claims
Having thus disclosed the invention, what is claimed is:
1. A method of shaping and cutting a contact lens comprising:
1. mixing a slurry of 20 percent up to 45 percent of a solid, high
molecular weight polyvinyl pyrrolidone of Fikentscher K value of
30-90 with 80-55 percent of monomethacrylate ester of a glycol
selected from the group consisting of ethylene glycol, propylene
glycol, diethylene glycol and dipropylene glycol, there being
present no more than about 1 percent by weight of methacrylic acid
and no more than about 0.2 percent by weight of dimethacrylate of
the aforesaid glycols and two free radical initiators, the first
effective at a temperature of 40.degree.-60.degree. C. and the
second at a temperature of 90.degree.-120.degree. C. to polymerize
said mixture, each of said two free radical initiators being
present in an amount of about 0.1 percent by weight of said
slurry;
2. casting said slurry in an open cylindrical mold and polymerizing
at a temperature of 40.degree.-60.degree. C. to form a hard,
self-supporting mass which is hygroscopic and swells
anisotropically in a water medium;
3. removing said mass and polymerizing at a temperature of
90.degree.-120.degree. C. to polymerize unreacted material from the
polymerization with the first free radical initiator and to
complete the hardening of said mass in its self-supporting
condition outside of the mold; and,
4. cutting a lens from said mass;
5. subjecting said cut mass to polymerizing radiation for at least
5 minutes up to 4 hours to toughen said cut mass in both the dry
state and when swollen with water; and
6. neutralizing and hydrating said cut mass in saline solution.
2. A method of shaping and cutting a contact lens comprising:
1. mixing a slurry of 20 percent up to 45 percent of a solid, high
molecular weight polyvinyl pyrrolidone of Fikentscher K value of
30-90 with 80-55 percent of monomethacrylate ester of a glycol
selected from the group consisting of ethylene glycol, propylene
glycol, diethylene glycol and dipropylene glycol, there being
present no more than about 1 percent by weight of methacrylic acid
and no more than about 0.2 percent by weight of dimethacrylate of
the aforesaid glycols and two free radical initiators, the first
effective at a temperature of 40.degree.-60.degree. C. and the
second at a temperature of 90.degree.-120.degree. C. to polymerize
said mixture each of said two free radical initiators being present
in an amount of about 0.1 percent by weight of said slurry;
2. casting said slurry in an open cylindrical mold and polymerizing
at a temperature of 40.degree.-60.degree. C. to form a hard,
self-supporting mass which is hygroscopic and swells
anisotropically in a water medium;
3. removing said mass and polymerizing at a temperature of
90.degree.-120.degree. C. to polymerize unreacted material from the
polymerization with the first free radical initiator and to
complete the hardening of said mass in its self-supporting
condition outside of the mold;
4. cutting a lens from said mass;
5. subjecting said cut mass to polymerizing radiation for at least
5 minutes up to 4 hours to toughen said cut mass in both the dry
state and when swollen with water;
6. neutralizing and hydrating said cut mass in saline solution;
and,
7. treating said cut mass with dilute hydrogen peroxide.
3. The method of making a contact lens comprising:
1. casting a hard concavo-planar cylinder consisting essentially of
a reaction product of 20 percent up to 45 percent of a solid, high
molecular weight polyvinyl pyrrolidone of Fikentscher K value of
30-90 with 80-55 percent of monomethacrylate ester of a glycol
selected from the group consisting of ethylene glycol, propylene
glycol, diethylene glycol and dipropylene glycol, there being
present no more than about 1 percent by weight of methacrylic acid
and no more than about 0.2 percent by weight of dimethacrylate of
the aforesaid glycols as impurities and two free radical
initiators, the first initiator polymerizing said cylinder at a
temperature of 40.degree.-60.degree. C. to form a self-supporting
mass which is hygroscopic and which will swell anisotropically when
placed in a water medium and the second initiator polymerizing said
cylinder to polymerize unreacted material from the polymerization
with the first free radical initiator, each of said two free
radical initiators being present in an amount of about 0.1 percent
by weight of said slurry at a temperature of 90.degree.-120.degree.
C. to complete the hardening of the cylinder;
2. cutting away about 90-95 percent of the material from the planar
face of the cylinder and from the circular outer periphery of the
cylinder to leave the inner concave surface of the casting as the
last surface to be cut as a lens blank having an arc of about
120.degree. and in a diameter broad enough to cover the cornea and
at least several millimeters of the limbus;
3. cutting a bevel edge at the periphery of the lens blank to form
a lens having a semi-scleral flap which is adapted to be lifted up
by tears in the scleral portion of the eye adjacent the outer edge
of the flap while simultaneously forming a circular tear vesicle in
the interior edge of the bevel which is spaced inwardly from said
flap and which provides a circular well for tears in the eye at the
limbal area, while the inner central portion of the lens is
relatively flat to contact the cornea without any tear layer
therebetween.
4. subjecting said cut lens to polymerizing radiation for at least
5 minutes up to 4 hours to toughen said cut lens in both the dry
state and when swollen with water; and
5. neutralizing and hydrating said cut lens in saline solution.
4. The method as claimed in claim 3, wherein the lens blank is
irradiated prior to cutting the bevel edge.
5. The method as claimed in claim 3, wherein the lens is irradiated
after cutting the bevel edge of the lens blank.
6. The method as claimed in claim 3, wherein the center of the lens
blank is cut and polished on both inner and outer surfaces to make
its center thinner than the edges of the corneal section to thereby
provide a minus correction.
7. The method as claimed in claim 3, wherein the lens blank is cut
to make its corneal edges thinner than its center, to thereby
provide a plus correction.
8. The method as claimed in claim 3, wherein the lens is cut and
polished to be substantially the same thickness at its center and
at its corneal edges, to thereby provide zero correction and
produce a lens useful as a corneal bandage through which eye
medicaments may be diffused.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of shaping and polymerizing, at
low (40.degree.-60.degree. C.) and medium temperatures
(90.degree.-120.degree. C.), a monomer-polymer dispersion by
casting in a mold and continuing the polymerization after removal
from the mold, the dispersion consisting preferably of 20 -45
percent of polymerized vinyl pyrrolidone and 80-55 percent of
monomethacrylate ester of a glycol selected from the group
consisting of ethylene glycol, propylene glycol, diethylene glycol
and dipropylene glycol, there being present no more than about 1
percent by weight of methacrylic acid and no more than 0.2 percent
by weight of the dimethacrylate of the aforesaid glycols. Amounts
of impurities in excess of these limits cause haze or cloudiness in
the product, undue hardness of the casting after hydration, and
lessen the amount of water which is absorbed by the hygroscopic
solid polymerized product. The monomer is essentially pure hydroxy
alkyl methacrylate ester.
An essential feature of the method of the present invention is the
stagewise post-polymerization of the bulk polymerized solid casting
after free radical initiation at low (40.degree.-60.degree. C.) and
medium (90.degree.-120.degree. C.) temperatures by means of (a)
polymerizing radiation of the dry solid. The hydrogen peroxide
treatment of the water-swollen product in an isotonic salt solution
toughens the solid post-polymerized product into completely
hydrated condition (swollen with from 45- 80 percent water which is
proportional to the polyvinyl pyrrolidone content).
2. Description of the Prior Art
Fields U.S. Pat. No. 2,136,422 shows the bulk polymerization of
ethylene glycol monomethacrylate with free radical initiator such
as benzoyl peroxide at elevated temperatures in order to obtain a
completely transparent, solid product which is cut and turned on a
lathe to make a furniture leg or the like.
Armen et al. U.S. Pat. No. 3,086,956, in Example 7, shows
polymerization of polyglycol monomethacrylate with polyvinyl
pyrrolidone and ammonium persulfate initiator at pH 5 in the
presence of water to provide a graft copolymer in the form of a
turbid aqueous solution containing 19.7 percent solids.
Ackerman et al. U.S. Pat. No. 2,923,692 shows lightly cross-linked
copolymers of esters of methacrylic acid and vinyl pyrrolidone (see
column 7, line 32). The products of Ackerman et al. contain highly
water-sensitive, cross-linked acrylic acid groups which can be
neutralized with alkali to form a smooth, non-grainy mucilage after
the product has been purified by washing, dried and then ground in
a homogenizer or colloid mill.
The bulk polymerized ester of Fields, neutralized, hydrated, washed
and ground by the method of Ackerman et al., would be expected to
give a mucilage or glue. It has been found that before grinding,
the product has limited hydration capacity (maximum of 20-30
percent) even with substantial amounts of acrylic or methacrylic
acid being present in the interpolymer or copolymer. Commercial
soft hydrophilic lenses made under the Trademark "Soflens,"
described in U.S. Pat. No. 3,408,429, are discussed more fully
below.
Robinson U.S. Pat. No. 2,941,980 shows water-soluble polymers and
copolymers of pyrrolidone with various monomers, such as acrylic
acid, vinyl acetate and the like, and these water-soluble polymers
are mixed with alkylated phenols serving as plasticizers to provide
coatings for basis of metal, paper, glass, etc., to afford
protection against water.
The accelerating effect of up to 1 percent of vinyl pyrrolidone on
the polymerization of methacrylate esters is taught by Munday et
al. U.S. Pat. No. 3,232,912, but the polymerized products which are
produced are liquids or low-melting solids, useful as detergents in
lubricating oils or as sludge dispersants in heating oils.
Copolymers of vinyl pyrrolidone and acrylic acid, as in Robinson,
or graft polymers of polyglycol methacrylate with polyvinyl
pyrrolidone, as in Armen et al., are not satisfactory as
water-swollen, tough contact lens blanks. These products form
low-strength films which, when wet with water, are easily distorted
by tensile forces and exhibit poor recovery, inadequate elongation
and inadequate toughness.
One would not expect that graft polymers in proportions taught by
Ackerman et al., Armen et al., Robinson or Munday et al. might be
useful to form tough, transparent, water-swollen contact lenses,
capable of being sterilized and cleaned with hydrogen peroxide.
SUMMARY OF THE INVENTION
In general, the method converts a free radial initiated solid
polymer containing polyvinyl pyrrolidone and polyhydroxy alkyl
methacrylate to a highly permeable, soft, hydrated, shaped mass
having improved toughness, elasticity and recovery by treating the
dry solid mass with radiation to aid densification and thereafter
hydrating the mass in saline solution and treating with hydrogen
peroxide to cause further toughening by chemical interaction
between the polyvinyl pyrrolidone and polymerized methacrylate.
In a preferred form, a tough, soft, hydrated, fluid-permeable
contact lens cut from a hard blank is prepared by casting a
composition consisting essentially of 20-45 percent of solid, high
molecular weight polyvinyl pyrrolidone in a network of 80-55
percent of hydroxy ethylmethacrylate, hydroxy propylmethacrylate,
or diethylene glycol monomethacrylate which may contain, as
impurities, less than 1 percent of methacrylic acid, preferably not
more than 0.2 percent, and up to 0.2 percent of ethylene glycol
dimethacrylate. The polymerization of the bulk matrix and preformed
polymer is carried out in stages, first in a casting mold and then
outside of the mold, as follows:
1. In the open cylindrical casting mold with a low-temperature
peroxide, such as acetyl peroxide, di-secondary butyl peroxy
dicarbonate, cyclohexanone peroxide, etc., at 40.degree.-60.degree.
C. for a period of 4-24 hours;
2. Out of the mold with a medium-temperature, free radical
initiator, such as benzoyl peroxide, diethyl peroxide,
asoisobutyronitrile, orthotolyl peroxide, etc., at a temperature of
90.degree.-120.degree. C. for a period of one-half to 2 hours;
3. Out of the mold as a shaped, hard, polymerized mass with actinic
or high energy radiation, such as ultraviolet radiation, gamma
radiation, etc., after the lens has been cut to size; and,
4. Finally with hydrogen peroxide in hydrated condition and in the
presence of salt which produces the osmotic equivalent of normal
saline solution, whereby the cut, water-swollen lens, containing
from 40- 80 percent water, preferably 50-60 percent water, is
toughened in the wet condition.
Step (3) increases the toughness of the lens in hydrated condition,
as measured by a bubble bursting test which blows compressed air
against the lens to break or burst it and step (4) further
increases the toughness of the water-swollen lens and aids in
cleaning the lens of debris which accumulates thereon from the
eye.
It was completely unexpected to find that a low methacrylic acid
medium will provide a graft of hydroxy ethylmethacrylate which is
formed with polyvinyl pyrrolidone and thereby provide castings from
these materials in bulk which are tough, dimensionally stable and
uniformly reproducible in hydrated, swollen form to contain from
above 40 percent and preferably 50-60 percent water in which the
permeability of the hydrated product has been increased by
post-polymerization treatment, first with radiation when dry and
then with hydrogen peroxide in isotonic salt solution.
The function of polyvinyl pyrrolidone in responding to hydrogen
peroxide treatment which, in the preferred embodiment of the
invention, has a Fikentscher K value of from 30 to 90, appears to
be a critical aspect of the new and unexpected properties of
toughness, elastic recovery and elasticity developed by the graft
copolymer with hydroxy alkyl acrylate.
Polyvinyl pyrrolidone is comparable to gelatin and albumen in
respect to its high affinity for water and its low toxicity and
general biochemical inertness. The polyvinyl pyrrolidone which is
preferred for the present contact lens manufacture has a
Fikentscher K value of 33, corresponding to a molecular weight
between about 25,000 and 50,000, the number average molecular
weight by osmosis being about 37,000 which is about half of that of
bovine serum albumen. The carbonamide groups present in gelation
are responsible for thread-like structures in the hydration of
gelatin emulsions which have been detected under the
ultramicroscope in grainless photographic emulsions containing from
5-10 percent of gelatin in solutions adjusted to the isoelectric
point. Surprisingly, electron microscope photographs of the
hydrogen peroxide-treated lens structures of the present invention
show no threads. Polyvinyl pyrrolidone imparts hygroscopic
characteristics to the product is distributed in the matrix and
takes part in chemical interaction with hydrogen peroxide. There
appears to be increased permeability and diffusibility of solutes
in water through the polymer membrane. This polymer membrane, used
as a contact lens, can be cleaned with hydrogen peroxide to remove
catalase deposited from tears and organic debris which tend to
accumulate in the Wichterle hydrophilic contact lenses.
The hydrogen peroxide has the effect of clearing the lens of any
catalase or of any other mucoprotein of the eye. At the same time,
it increases the strength at a slow rate without affecting the
fluid permeability which is so important to the performance. This
is over and above the initial toughening of the product which is
achieved by the first wash in hydrogen peroxide. The use of the
peroxide, therefore, becomes a maintenance procedure which not only
sterilizes the lenses but maintains clarity, transparency and fluid
permeability.
Accordingly, this treatment permits freedom from eye irritation and
prevents the development of edema under the lens when the lens is
worn continuously for 24 hours and longer.
The hydrated hydrogen peroxide-treated product of the present
invention appears to possess significantly different properties
from gelatin in its resistance to change by acids, alkalis and
relatively high temperatures in the wet condition. Gelatin, being
amphoteric, reacts with acid and alkali and reversibly dissolves on
heating unless it is denatured and flocculated by overheating when
wet. In contrast, the present product withstands boiling water for
periods up to 72 hours without alteration of its desirable
permeable characteristics. Although the chemical mechanism of
alteration of carbonamide linkages by radiation and hydrogen
peroxide polymerization is not fully understood, it is clear that a
critical and significant enhancement of physical properties has
been achieved and this could not be obtained by any other
method.
BRIEF DESCRIPTION OF THE DRAWING
In the fabrication of a soft, water-swollen, plastic contact lens
from a hard, dry blank by the preferred method of the invention, a
simple casting apparatus is used for shaping the blank and for
polymerizing the blank which is illustrated in the attached
drawing, in which:
FIG. 1 is a flow diagram showing mixing of ingredients, stirring,
degassing, pouring into the mold and placing the filled mold into
the oven for the first and second stage curings at low and medium
temperatures, respectively, to produce a hard, transparent, shaped
mass which is cut and polished;
FIG. 2 is a diagrammatic view showing the placement of male and
female mold parts to shape the hard, transparent solid which is
subsequently cut;
FIG. 3 is a flow diagram showing the manufacturing steps taken in a
particularly preferred embodiment whereby radiation treatment,
water-swelling in alkaline medium, osmotic swelling and hydrogen
peroxide hardening are carried out to improve the physical
properties and water permeability and to diminish osmotic swelling
of the cut and polished lens;
FIGS. 4a, 4b, 4c and 4d show stages in the cutting of the shaped
mass ejected from the mold in FIG. 2 in achieving the finished
lens; and,
FIG. 5 is a sectional view which shows the relation of the mold
parts and the hard, transparent, shaped mass after one-stage curing
and before ejection from the mold.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Example 1
To 120 parts of distilled hydroxyethylmethacrylate containing up to
0.2 percent of ethylene glycol dimethacrylate and less than 1
percent of methacrylic acid as impurities were added 40 parts of
powdered polyvinyl pyrrolidone (Plasdone C Grade, supplied by GAF
Corporation) having a Fikentscher K value of 33, number molecular
weight of 37,000, molecular weight range of 25,000-50,000 with the
upper 15 percent of K value distribution being 39 weight of the
polymer and the lower 25 percent of K value distribution being 18.5
percent. This polymer, hereinafter termed PVP, is hygroscopic and
had a moisture content of about 3 percent, but picked up 1 or 2
percent additional moisture from the atmosphere in the plant.
A 40 part portion of the liquid methacrylate, hereinafter termed
HEMA, was mixed with catalyst, e.g. 0.2 grams of benzoyl peroxide
in powdered form and 0.2 grams of di-secondary butyl peroxy
dicarbonate, available under the trade name of Lupersol 225 from
Lucidol Chemical Corporation, Buffalo, New York.
The catalyst-liquid mixture, 40 parts, was added to the PVP-HEMA
mixture, 120 parts, and was mixed carefully to provide 160 parts of
monomer with 40 parts of polymer dispersion or slurry. These
proportions cut down the shrinkage as compared with liquid HEMA
alone. The amount of secondary butyl peroxy dicarbonate catalyst
which is particularly effective at a temperature of
40.degree.-60.degree. C. and of benzoyl peroxide catalyst which is
particularly effective at a temperature of 90.degree.-120.degree.
C. is 0.2 part of each catalyst per 200 parts of liquid PVP mixture
containing 160 parts of liquid monomer and 40 parts of PVP, e.g. a
proportion of about 0.1 percent for each of these catalysts. PVP
was present in an amount of 20 percent by weight of the dispersion
The dispersion was de-aerated to permit air bubbles to escape, and
the mold was filled in the manner shown diagrammatically in FIG. 1.
The tray of molds was then placed in an air circulating oven for 20
hours at a temperature of 40.degree. C. At the end of this time,
the molds were removed from the oven and taken apart by using a
small arbor press against the flat end of the Teflon core at the
bottom of the mold unit. This forced the cast blank out of the
other end of the sleeve. The cast blanks were placed on aluminum
sheets and returned to the oven where they were post-cured at a
temperature of 110.degree. C. for 11/2 hours. When the trays were
removed from the oven, the finished cast plastic blanks were a
polymer consisting of PVP to which poly-HEMA had been grafted.
EXAMPLE 2
The casting procedure set out in Example 1 was carried out, but
instead of using 20 percent by weight of PVP, 30 percent by weight
was used. The lens made by the technique of Example 1 contained
about 55 percent of water as measured in isotonic saline and
resulted in a lens that came up to the exacting standards set for
Example 1.
EXAMPLE 3
The process of Example 1 was repeated, except that diethylene
glycol monomethacrylate was used with 25 percent by weight of PVP.
This lens, too, met the standards set in Example 1.
EXAMPLE 4
The process of Example 1 was repeated except that instead of 20
percent of PVP, 35 percent by weight was used. The resulting lens
came up to the standards set for Example 1.
EXAMPLE 5
A mixture of 80 parts of hydroxy propyl methacrylate and 80 parts
of HEMA was used for the monomer phase with 20 percent by weight of
PVP and the process of Example 1 was repeated. The resulting lens
came up to the high standards set for Example 1.
The water content of the lens made and tested in Example 1 was
about 51 percent in water and about 49.5 percent in 0.9 percent
saline solution. In contrast, the water content of poly-HEMA from
which PVP has been excluded is about 38 percent and 36 percent. The
water content of the remaining examples was substantially the same
as that in Example 1. Generally, the main differences imparted by
substituting propylene glycol monomethacrylate or diethylene glycol
monomethacrylate is to lower the refractive index and to make the
polymerized solid slightly more flexible.
If, in the foregoing examples, PVP is used in an amount less than
20 percent, the water uptake value of about 50-60 percent is not
achieved in the hydrated water-swollen polymerized mass and the
desired toughening and increase in strength are not achieved by
subsequent hydrogen peroxide treatment.
If the low temperature initiator is omitted and polymerization is
carried out at 90.degree.-120.degree. C. for one-half to 2 hours,
the solid product is not uniform in physical properties nor does it
provide the toughening and improved strength necessary to come up
to the desired standard. Without both low and medium temperature
initiators, the improvement in permeability over the commercial
"Soflens" made under the Wichterle patents is not achieved, nor is
consistent reproducibility possible.
Accordingly, the critical two-stage initiation at temperatures of
40.degree.-60.degree. C. in the mold to form the solid rod and at
90.degree.-120.degree. C. out of the mold, in a tray in an oven, to
harden the self-sustaining rod provides a rod of stock material of
Shore A Hardness value between 70 and 90 which can be cut and
polished into lenses by the familiar technique used with hard
acrylic material. Even without further treatment, such lenses can
be hydrated and water-swollen to surpass the performance of the
presently commercially available hydrophilic lenses.
If more than 45 percent of PVP is used in the polymerized mass, the
mass after hydration becomes excessively soft. Even after
post-polymerization, the product cannot be toughened to match the
high strength and elasticity values of the preferred examples
above. Only in the range of 20-45 percent PVP can the present
polymerized composition, free from cross-linker, match the strength
properties of the commercial "Soflens." The "Soflens" material does
not possess the same high permeability rate of diffusion which in
the present examples is from 10-15 times that of the "Soflens"
material.
MANUFACTURING THE LENS FROM THE BLANK
The lenses are made by cutting and polishing the shaped masses in
the hard state, as shown in the flow diagram of FIG. 1 and in the
views of FIGS. 2, 4a-4d, and 5, so that they appear to be exactly
like a hard acrylic contact lens.
The molding ingredients, comprising the mixture of hydroxy alkyl
methacrylate, PVP, low and medium temperature initiators, cure in
the first stage in the molds and in the second stage in the trays
to produce hard, transparent, shaped, plano-concave cylinders 20,
as illustrated in FIG. 2. The concave surface is formed by male
mold member 10 fitting into female mold member 11. The interior
surface of the female member 11 is coated with Teflon, as is the
outer surface of the male member 10. After curing, the Shore A
Hardness value of the shaped mass 20 is between about 80 and 90.
Subsequent post-polymerization treatment increases the Shore A
Hardness value by from 3 to 10 points. At higher PVP
concentrations, lower initial Shore A Hardness values are obtained.
Subsequent treatment, e.g., post-polymerization, by radiation
causes a greater increase in Shore A Hardness at these higher PVP
concentrations and this demonstrates that radiation is especially
effective in post-polymerization of the PVP moiety in the product.
Radiation also tends to cause slight embrittlement so that if is
easier to cut the entire mass 20 before radiation treatment.
Because of the PVP content in the lens and the two stages of
curing, there is no trace of unreacted material in the cast blank
20 and cut lenses which are formed by the steps shown in FIGS.
4a-4d have water-swelling characteristics far beyond the commercial
hydrophilic lenses available in the prior art. The commercial
"Soflens," from the Bausch & Lomb Company, has a water uptake
of 38 percent in comparison with a 50-55 percent water uptake for
the cut lens of FIG. 4 herein, prior to radiation and hydrogen
peroxide treatment. This combination of high hardness in the dry
state and high water uptake in the wet state of the present lens
blank material permits an entirely different and simpler
manufacturing process to be carried out than with the Wichterle
lens which must be cut when mounted on a support, as described in
U.S. Pat. No. 3,361,858. The present material contains no
cross-linker as is required in the lens composition of this patent
and it is surprising that the substantially pure poly-HEMA matrix
of the present invention, having a Shore A Hardness value close to
90, can be easily cut in the dry state to very small tolerances of
about one one-hundredth mm. and thereafter can be hydrated to
imbibe at least 50 percent more water than the prior art lens. This
cutting in the dry state permits thinner edge sections to be
produced and permits uniformity in lens manufacture which cannot be
achieved in the prior art manufacturing methods.
In the hard state, the index of refraction of the cast blank 20 is
approximately 1.49. In the hydrated state, the index of refraction
of the lens is approximately 1.39 to 1.40. The lenses become
larger, thicker and flatter when they are changed to the soft state
by hydration. In the cutting and polishing processing of the
lenses, and additional reversal in shape occurs and they become
somewhat steeper and slightly smaller in diameter. All of these
changes are taken into account to produce the desired
specifications of curvature and dimension in the hydrated
state.
The lens blank 20 is a small cylinder with a concave curve at one
end which must be optically finished and a small amount of stock is
removed from this surface, e.g., a minimum amount of about 0.2 mm.
in thickness and a maximum of 0.5 mm., the removal being
symmetrical and from the entire surface.
The following are the steps in manufacturing after the low
temperature and medium temperature cures of the blank 20. After the
low temperature cure, the blank 20 is removed in the manner shown
in FIG. 5 wherein ram 13 pushes against the planar surface of the
blank STEP eject the blank from the female mold member 10. The
blank is removed from the same end into which the dispersion or
slurry was poured and this removal is completely different from
that which is carried out in U.S. Pat. No. 3,361,858, wherein the
lens is molded to size on a mount and is removed from the mount by
immersing in water to swell it and allow it to be peeled from the
mount. After this, the blank of the present invention is cured in
the medium temperature stage on trays and the steps below are
followed:
STEP 8c 1 - CUTTING
The cutting steps are shown diagrammatically in FIGS. 4a-4d. The
dotted lines in FIG. 4a show removal of peripheral or radial
portions of the blank in order to cut the 180.degree. arc down to
about 120.degree., as shown in FIG. 4a and in FIG. 4b. The planar
surface is then cut along the dotted line shown at the bottom of
FIG. 4b in order to faciliate mounting the blank on a lathe, and
this blank is shown in FIG. 4c. Where the chuck of the lathe can
handle the form shown in FIG. 4b, the planar cut shown in FIG. 4c
is not required. The concave cut is made along the dotted line
shown in FIG. 4c and cut lens is shown in FIG. 4d.
In the lens mad in accordance with the present invention, the flat
surface on the back of the blank is faced off in the manner shown
in FIGS. 4b and 4c and the center of the flat surface is tapped to
make a pivot depression on a small jeweler's lathe. The diameter of
the blank was reduced in a Levin lathe to a size 0.1 mm. larger
than the desired finished lens size (see FIG. 4a). The radius cut
on the concave surface matched the surface.
STEP 2 - POLISHING OF INNER OR BACK Surface SURFACE
The base curve blank was then mounted on an optical polishing
machine and it was polished against a brass lap coated with
adhesive tape whose curve matched the base curve of the blank. In
the polishing, the lens blank rocked back and forth on the
polishing lap, and spun at the same time. Two polishing cycles were
used, The first employed Snow Floss Compound, made by Johns
Manville, mixed with odorless kerosene to the consistency of a
thick paste. This rough polish procedure was used for 3 minutes.
The finish polish was done with U.S. Patent Grade of Zinc Oxide,
made by Merck Pharmaceuticals, mixed with vaseline to form a paste.
This cycle also lasted for 3 minutes. When the curve was finished,
it was measured with a radiuscope for radius and quality to have a
radius of curvature within plus or minus 0.4 mm. of the original
lathe radius cut.
STEP 3 - RADIUS CUTTING THE OUTER OR FRONT SURFACE
The finished base curve was now mounted onto a brass or plastic
chuck preparatory to radius cutting and polishing the front
surface. This chuck has a finished, polished, convex surface which
matches the base curve of the blank. The rear of the chuck fits
into a collet of a small, high precision jeweler's lathe made by
Levin & Sons. The chuck was heated slightly, sufficiently to
melt some mounting wax on the surface of the chuck. The finished
base curve was pressed firmly onto this surface and was allowed to
cool. The chuck was then mounted in a Levin lathe and its front
surface was radius cut in two stages. In the first stage, rough
cutting removed all of the stock. In the second stage, finish
cutting produced a highly smooth radius cut and an exact center
thickness. The thickness was measured by a regular plunger
thickness gauge through the small diameter hole in the center of
the chuck, which was also used for evaluating the optics as the
lens was being finished. The front surface was then polished on an
optical polishing machine. The chuck mounted on a vertical spindle
and rotated as a polishing lap coated with adhesive tape rocked
back and forth over the surface, spinning at the same time. The
polish used was Snow Floss Compound mixed with odorless kerosene to
the consistency of this paste. It required 3 minutes to complete
the surface, and the optical quality was judged by removing the
chuck from the spindle and viewing the optics in a lensometer
through the hole in the chuck.
STEP 4 - CUTTING THE LENTICULAR GROOVE ON MINUS LENSES
On minus lenses, it is necessary to thin the edge by adding a minus
lenticular cut to the peripheral region of the front of the lens.
This is done with a single edge razor blade and polished with
molefoam and the zinc oxide polishing solution. The width of this
front lenticular region should not be greater than one-half of the
diameter of the lens.
STEP 5 - ADDING INSIDE BEVEL AND INSPECTION
The lens was then removed from the chuck by heating the bottom of
the chuck until the wax softened and the lens could be slid off the
chuck easily, and was then cleaned in a test tube with Xylene which
was put into an ultrasonic cleaner.
The finishing of the lens included the addition of a small flat
bevel to the inside aspect of the edge approximately 0.3 mm. wide.
This was put on by grinding against an emery sphere or a diamond
impregnated lap, and polishing against a matching felt lap.
Following this, the edge was rounded and polished against a
polyurethane sponge saturated with the polish mixture or zinc oxide
and odorless kerosene. The lens was then recleaned in Xylene in the
ultrasonic cleaner. It was measured and inspected for base curve
radius, optical value and quality, thickness and surface
scratches.
STEP 6 - IRRADIATION TREATMENT AS SHOWN IN FIG. 3
Irradiation is preferably carried out under an ultraviolet lamp
which provides a high energy source in the spectral range of 2,000
to 4,000 Angstrom units, for at least one-half hour, preferably 2-4
hours. The bursting strength is increased from 7 p.s.i. gauge to
10.5-11.0 p.s.i. gauge, an increase of at least about 50 percent of
the original value and the hydrated lens loses practically none of
its elasticity and rapid recovery. In contrast, the commercial
"Soflens," the lens of the prior art, does not improve under
irradiation in its bursting strength. Therefore, it is clear that
the PVP component in the original composition, as well as the
unique two-step polymerizing procedure, e.g., the low temperature
initiation and the medium temperature initiation referred to in
FIG. 1, coact in a new way with irradiation treatment in the solid
polymerized state to produce this new and unexpected result.
Utraviolet sources, such as the mercury vapor tube, a Xenon lamp,
or a carbon arc tube, may be used.
Other irradiation sources which may be used are a cobalt 60 source
which emits gamma radiation, spent reactor elements from a uranium
pile which also emit gamma radiation, or high energy ionizing
radiation from a commercially available source, e.g., Radiation
Dynamics, Long Island, New York X-Rays may be used at exposure
dosages of 10.sup.7 roentgens for a period of 15 minutes to 1 hour.
Gamma radiation dosage for post-polymerization treatment is
preferably from about 5 to about 95 megarads for 5 minutes to 1
hour. All treatments by irradiation are carried out at room
temperature.
In this irradiation treatment, as shown in FIG. 3, the hard
finished lens was placed under a pure ultraviolet light for a
period of 31/2 hours. The light source was 6 inches from the lens.
The unit was covered to prevent light loss and the polymerization
of poly-HEMA and PVP was completed in 2 hours. The light source was
a 250 watt Spectraline utraviolet lamp.
STEP 7 - NEUTRALIZATION, HYDRATION AND HYDROGEN PEROXIDE
TREATMENT
This step accomplished the neutralization and hydration of the
lenses. Lenses were placed in a 0.8 percent saline bath mixed with
sufficient bicarbonate of soda to produce a pH of 8. They remained
in this bath for 2-20 hours. Each lens was held by a small
"flo-thru" basket made of polypropylene. The lenses were then
placed in a bath of normal saline at approximately 200.degree. F.
for 1 hour. The bath was then changed to fresh normal saline for a
period of 3 hours and then changed again for an additional 4 hours.
This boiling was done in a pressure cooker to which a condensing
column had been added to prevent evaporation and to increase the
concentration of the saline.
The lenses were then placed, for 4 hours, in a bath of 10 volume
reagent grade hydrogen peroxide (3% H.sub.2 o.sub.2) to which had
been added sufficient pure sodium chloride to produce the
equivalent of a normal saline solution. This caused the lenses to
shrink in size and become hypertonic. Following this, they were
boiled in the pressure cooker in distilled water for 2 hours and
again in normal saline for 2 hours. See FIG. 3.
BURSTING STRENGTH OF THE LENS
The post-polymerizaiton steps (6) and (7), irradiation and hydrogen
peroxide, illustrated in FIG. 3 of the drawing, contribute to
significant strengthening and toughening of the water-swollen lens
and provide thereby advantages not available in any commercial lens
of the soft hydrophilic type.
The hard cut lens resulting from the manufacturing operations shown
in FIGS. 1 and 4a-4d can be tested for bursting strength by binding
the edges about the opening of one-fourth inches pipe and measuring
the air pressure required to burst the lens which has been boiled
in water of 4 hours to hydrate it. This test carried out with the
cut lens of FIGS. 1 and 4a-4d of the invention showed a bursting
strength of 7 p.s.i. gauge pressure. This cut lens, based on
poly-HEMA matrix containing 20 percent PVP, matched the bursting
strength (7 p.s.i. gauge) of the centrifugally cast "Soflens"
containing dimethacrylate cross-linker, thereby showing unexpected
improvement in the absence of cross-linker and at almost 50 percent
higher water content (52 percent uptake in the present lens against
37 percent in the "Soflens" ).
After irradiation and hydrogen peroxide treatment by the process of
the invention, the bursting strength by the above mentioned test is
increased from 10.5-11 p.s.i. up to 16-17 p.s.i., an increase of at
least 250 percent based upon the original bursting strength and an
increase of about 100 percent f the original value as compared with
the 50 percent increase achieved by irradiation.
This enhancement of strength by hydrogen peroxide is uniquely based
upon the PVP content since the commercial "Soflens" does not show
such enhancement in bursting strength. Uniquely, both the
irradiation and the hydrogen peroxide treatments are essential if
the maximum bursting strength is to be achieved and if the other
lens properties, e.g., controlled elongation in lateral and
vertical dimensions in water-swollen state, are to be
maintained.
These other lens properties, in Water-swollen state, will be more
clearly visualized in comparison with the shape and dimensions of
the hard lens shown in FIG. 4d. Upon immersing the lens in water at
pH 8 after radiation (see Block 3, FIG. 3), the radius of curvature
of the lens expands 26 percent, the core diameter expands 35
percent, and the thickness expands 23 percent. These values of
anisotropic expansion do not change if the lens is immersed in
saline (Block 4, FIG. 3). The water uptake is between 48 and 55
percent.
The hydrogen peroxide treatment accomplishes the most surprising
improvements in the physical properties of the hydrated,
water-swollen lens which facilitate maintenance and cleaning of the
lens by the patient. By this treatment, there is accomplished, as
mentioned above, an increase in bursting strength after irradiation
of 10.5-11 p.s.i. up to 16-17 p.s.i., gauge pressure. If alkaline
bicarbonate solution (1 percent) at pH 8 is repeatedly applied to
the lens, a slight softening occurs and the bursting strength falls
to about 12.5-13.5 p.s.i. This effect is reversed by immersion in 3
percent hydrogen peroxide to regain the 16-17 p.s.i. value.
Repeated treatment with hydrogen peroxide interspersed with
softening treatment by alkaline bicarbonate solution increases the
bursting strength still further up to values of 19-20 p.s.i.
FLUID PERMEABILITY CHARACTERISTICS
The fluid permeability of the lenses of Example 1 was studied and
compared with the Bausch & Lomb "Solfens," made by the process
of Wichterle U.S. Pat. No. 3,408,429.
Individual vials of sterile fluorescein solution were made up
various disodium fluorescein concentrations in 0.1M phosphate
buffered saline at a pH of 7.4.
An objective slit lamp fluorophotometer measured fluorescein
concentration in the lenses, in the interior segment of the eye,
and in the solution. The fluorophotometer consists of a light
sensing device built into the eye piece of the lamp and measures
the fluorescein concentration in an area 0.08 mm. across. The
instrument is accurate to within plus or minus 2 percent. The
unknown is compared with a fresh, stable, standard fluorescein
solution.
RESULTS OF IN VITRO STUDIES - FULLY HYDRATED
1. uptake
Fully hydrated lenses were placed in solutions of various
fluorescein concentrations, were rinsed briefly after the test time
in saline and were then mounted on the end of a glass test tube for
measurement of the fluorescein concentration in the lens.
The lenses themselves absorb less than 3 percent of the emitted
light and do not interfere with the test by reason of light
absorption. The volume of the soaking solution is large in
comparison to the lens volume.
Lenses fluoresced uniformly under the slit lamp after 90 seconds of
soaking on a fluorescein (5 .times. 10.sup.-.sup.3 mg./ml.)
solution. Three distinct zones were observed in the "Soflens"
lenses soaked for 30 minutes due to the fluorescein slowly
diffusing to the interior of the lenses. The lenses of the present
invention take up fluorescein quite rapidly; uptake is complete in
about 2 hours. The Bausch & Lomb lenses ("Soflens" ) take up
fluorescein slowly and continue to do so for over 24 hours,
reaching a final concentration approximately 2.3 times that in the
lenses of the present invention.
2. Elution Studies
After a 24 hour presoaking of the lenses in 5 .times.
10.sup.-.sup.3 mg./ml. fluorescein solution, they were placed in 4
cc. of buffered saline and the time rate of change of fluorescein
concentration in the lenses and the eluting solutions were
measured. At the end of 1 hour, the lenses of the present invention
had released 70 percent of the fluorescein into solution, while the
"Soflens" lenses released 25 percent. Only after 8 hours did the
"Soflens" lenses release 90 percent of the bound fluorescein.
After elution from each type of lens, it was determined that the
"Soflens" lenses took up twice as much fluorescein as did the
lenses of the present invention, on a weight basis.
The total uptake or fluorescein was linearly related to the
concentration of the soaking solution over a 4,000-fold
concentration range between 5 .times. 10.sup.-.sup.3 and 20 mg./ml.
The present lenses were air dried, place in fluorescein solution
and the uptake by the dried lenses was substantially identical to
the fully hydrated lenses.
RESULTS OF THE VIVO STUDIES
A young female who had worn both conventional and hydrophilic
lenses without difficulty was studied. On the first day, she wore a
lens of the present invention in one eye and no lens in the other.
A single drop of sterile 2 percent fluorescein was instilled in
each eye at 0, 2, 4, 6 and 11 hours. The corneal and anterior
chamber concentrations were measured at 2, 4, 6 and 24 hours, each
time removing the lens 10 minutes prior to measurement. ONe week
later, the subject wore the Bausch & Lomb "Soflens" in one eye
and a methyl methacrylate conventional hard lens in the other.
Drops were instilled at 0, 2 and 4 hours and measurements were made
at 0, 2, 4 and 6 hours, at which time the Bausch & Lomb lens
was removed, The corneal and anterior chamber concentrations of
fluorescein are shown in Tables 1 and 2 below. The corneal and
anterior chamber concentrations were higher with the lens of the
present invention than with the other lenses. There was essentially
no difference between using no lens, standard methacrylate lens, or
the Bausch & Lomb lens. At the end of 6 hours, the corneal and
anterior chamber concentrations of fluorescein in the eye with the
lens of the present invention were six to eight times that attained
with any other mode of treatment. Furthermore, the lens of the
present invention was able to maintain the fluorescein
concentration in the ocular tissues for 24 hours despite the known
rapid exit of fluorescein from the eye. It should be noted that the
lens had not been presoaked in fluorescein prior to insertion.
In other studies, the present lenses were presoaked in solutions of
0.1 percent and 0.01 percent fluorescein and inserted into the
right eyes of three rabbits. At the same time, a drop of the 0.01
percent solution was put into each of the left eyes. 90 minutes
later, the lenses were removed, the eyes were irrigated with saline
and the corneal and anterior chamber concentrations were
determined. The lenses were the reinserted and the rabbits received
one drop of the 0.01 percent solution in the left eye every 30
minutes for 2 additional hours. Saline solution was instilled into
the right eye. The corneal and aqueous humor concentrations of
fluorescein at 1 1/2 and 3 1/2 hours are shown in Tables 3 and 4
below. The ocular concentrations attained with the presoaked lenses
were four times higher than those attained with frequent drops.
Increasing the concentration of the soaking solution ten-fold
resulted in an 800 percent increase in the ocular concentrations.
It took much less fluorescein to get the same ocular concentration
if the fluorescein was permeated into a lens that if it was
instilled topically.
TABLE 1
Corneal Concentration of Fluorescein With Different Types of
Contact Lenses in Place
(All Values Are .times. 10.sup.-.sup.5 mg./ml.)
Time Present Bausch and Methyl No (hours) Lens Lomb Lens
Methacrylate Lens
__________________________________________________________________________
2 14 6 4.5 9 4 81 17 14 13 6 150 25 13 24 24 220 -- - 11
__________________________________________________________________________
TABLE 2
Anterior Chamber Concentration of Fluorescein With Various Types of
Contact Lenses in Place
(All Values are .times. 10.sup.-.sup.6 mg./ml.)
Time Present Bausch and Methyl No (hours) Lens Lomb Lens
Methacrylate Lens
__________________________________________________________________________
2 17 6 4 13 4 115 19 19 26 6 235 27 21 28 24 184 -- 14
__________________________________________________________________________
TABLE 3
Corneal Fluorescein Concentration In Rabbits With Presoaked Lenses
Of Present Invention
(All VAlues are .times. 10.sup.-.sup.5 mg./ml.
(Number of Animals in Parenthesis)
Lens Presoaked Lens Presoaked Time in 0.1% in 0.01% Topical 0.01%
(hours) Fluorescein Fluorescein Fluorescein
__________________________________________________________________________
1.5 -- 107 (3) 9 (6) 3.5 616 (3) 70 (3) 19 (6)
__________________________________________________________________________
--
TABLE 4
Fluorescein Concentration In Aqueous Humor Of Rabbits With
Presoaked Lenses of the Present Invention
(All Values are .times. 10.sup.-.sup.6 mg./ml.)
(Number of Animals in Parenthesis)
Lens Presoaked Lens Presoaked Time in 0.1% in 0.01% Topical 0.01%
(hours) Fluorescein Fluorescein Fluorescein
__________________________________________________________________________
1.5 -- 42 (3) 12 (6) 3.5 490 (3) 59 (3) 16 (6)
__________________________________________________________________________
OXYGEN PERMEABILITY STUDIES
The lens of the present invention appears to permit higher
transmission of oxygen than the commercially available hydrophilic
lens and, as a consequence, is of value in permitting oxygen access
across the lens to the cornea.
The lenses of the present invention, made by the two-stage
initiation process shown in FIG. 1 without the further steps of
irradiation and hydrogen peroxide treatment, exhibit permeability
and diffusion characteristics comparable to those pointed out in
the studies above; and these lenses, cut from the polymerized rod,
as shown in FIGS. 4a-4d, respond to hydrogen peroxide toughening
and cleaning, althouth to a degree substantially less than the
lenses which are made by the preferred method of the invention as
shown in FIG. 3.
The diffusibility of solutes through the lenses made by the methods
of FIGS. 1 and 3 is from about 6 to about 20 times as great as the
diffusibility of the commercially available "Soflens," this
diffusibility being expressed as the rate of elution of a dilute
tracer material through the lens. A comparative diffusion value is
demonstrated where a dye is seen to completely diffuse in a few
hours through the lens of the present invention, while such
diffusion through the presently commercially available lenses takes
24 hours or longer.
The significance of such diffusion is demonstrated when the novel
circular lens of our copending application, mentioned hereinabove,
is placed in contact with the cornea, over the super-sensitive
limbal area, with its thin flap or edge extending a few millimeters
beyond the limbus, the limbal area of the lens defining a circular
tear vesicle which is cut out from the material of the lens
adjacent the flap. This novel lens vesicle provides a clear
solution of liquid tears adjacent the cornea and osmotic pressure
is created in a direction from the less dense tear liquid to the
more dense liquid in the cornea to aid in bathing the eye. The
semi-scleral flap is held to the scleral portion of the eye by
capillary attraction. Tears can enter under the flap to replenish
the vesicle well which is immediately adjacent the inner edge of
the lens. Hypertonic eyedrops instilled into the eye stimulate the
washing and cleaning function of the tears, and any medication in
these eyedrops diffuses rapidly, in mere minutes, through the
permeable structure of the lens.
The hydrogen peroxide treatment appears not only to toughen the
lens and raise the bursting strength values, as mentioned above,
from 16 p.s.i. gauge to 19-20 p.s.i. gauge, but it also opens and
cleans the pores or microvoids in the lens material through which
diffusion takes place. Therefore, hydrogen peroxide at 3 percent
dilution constitutes a maintenance fluid which is used in
conjunction with 1 percent sodium bicarbonate solution, the latter
relaxing the pores and softening the lens to aid in cleaning and
reducing the bursting strength by 3 or 4 p.s.i. gauge units and the
former reversing this decrease to bring the lens back to its
maximum toughness after cleaning. Surprisingly, aging of the lens
through normal use, e.g., wearing and cleaning, has been found to
slightly increase the bursting strength value of the lens by 2 or 3
p.s.i. gauge. Whether the lens is a newly manufactured lens having
a bursting strength of 16 p.s.i. gauge or an aged lens with
bursting strength of 20 p.s.i. gauge, no difference is found in the
corrective function of the lens or in its comfort.
The lenses of the present invention resist the substantial
dimensional changes which ordinarily occur when different osmotic
salt concentrations are applied. The anisotropic swelling and
shrinking characteristics at 50-55 percent water uptake appear to
provide a unique environment for resisting osmotic dimensional
change which would cause the lens to shift its position or to flex
in response to normal movement of the eyelid or illumination by
strong light, irritation and the like.
If the lens is too stiff and insufficiently hydrated, which is the
case with the hydrophilic, highly cross-linked presently available
commercial lens, the inside curve of the lens must be steeper than
the curve of the cornea with a space under the lens in the central
region in order to shape the lens to the cornea. If is this space
which flexes with each blink. The edge of the hydrophilic lens
grabs the cornea at the sensitive limbal area. The inadequate water
uptake causes irritation and discomfort. Only by increasing the
lens diameter and by providing a very thin, flexible edge can the
presently available commercial lens to improved, but the liquid
permeability is still insufficient and the optical tolerance can
never be as good as in the present lens because the present lens is
cut in the hard state with a greater degree of accuracy than can be
obtained with the prior art lens which is cut in the soft state.
Suprisingly, there is no dimensional change in the present lenses
with small changes in hydration, while significant dimensional
changes occur with the prior art lenses. Consequently, the fit is
flatter and better with the present lens; the optical correction is
more accurate; substantially no shifting occurs; and flexing by
blinking is completely avoided. In the present lens, the
elimination of flexing, the smoother optical finish, the flatter
corneal curve, and the limbal tear vesicle make it possible to
correct astigmatism in a manner far better than with any lens now
available.
The chemical composition of the present hygroscopic lens, e.g., the
critical PVP and HEMA content, combined with the critical method of
two-step polymerization to permit precision grinding and
uniformity, and the post-polymerization with radiation and hydrogen
peroxide, provide precision fitting, maximum strength, elasticity
and elastic recovery properties, all of which are essential to eye
comfort when the lens is worn for long periods of time.
Instead of the polyvinyl pyrrolidone homopolymer, various lower
alkyl derivatives thereof may be employed. Such derivatives
include:
3-methyl-N-vinyl-2-pyrrolidone
4-methyl-N-vinyl-2-pyrrolidone
3,3-dimethyl-N-vinyl-2-pyrrolidone
4-ethyl-N-vinyl-2-pyrrolidone
5-methyl-N-vinyl-2-pyrrolidone
5-ethyl-N-vinyl-2-pyrrolidone and the like. These examples support
any PVP having lower alkyl in the 3, 4 or 5 positions.
The composition also has utility as a liquid, hygroscopic coating
material which strongly adheres to glass, plastic or metal when
cured, after application, in two stages under the temperature
condition shown in FIG. 1. The liquid slurry can be applied in a
thickness of 1-10 mils onto a glass tumbler to provide a
"frost-free" drinking glass for cold drinks, the glass being free
from outside condensation. The coating may be applied to an
automobile windshield to make it fog-free on the inside. The
coating may be applied to the polycarbonate lenses used in ski
goggles or to contact lenses made of CR-39 (polycarbonate)
plastic.
The membrane composition may be cast as a desalination membrane and
used to remove salt by reverse osmosis.
The membrane may be used as a germicide-carrying bandage for
internal and external wounds. In the treatment of the eye, these
drugs include pilocarpine, belladonna alkaloids, dibenzyline,
hydergine, methacholine, carbachol, bethanechol, a sulfonamide and
similar medicaments.
The precision fitting advantage of the lens of the present
invention carries over to prevent liquid build-up behind the
bandage when a medicament-carrying bandage is formed for the
eyeball which extends over the scleral area under the lid. Edema
which is encountered with hard, impermeable acrylic bandages is
eliminated. The medicines are not concentrated in the present
hydrated plastic membrane to cause osmotic swelling thereof, but
are readily diffused to bathe the affected eye portion with the
proper concentrations for therapeutic effectiveness. There is no
dimensional change of the bandage when hypertonic concentrations of
salts and medicines are applied to the eye, and this aids in the
healing process.
The liquid slurry composition may be used to cast or coat an
artificial eye, limb or appendage without shrinkage and to precise
dimensions. In all of these uses, the coating, membrane, casting,
etc. can be cleaned with hydrogen peroxide at an appropriate
time.
* * * * *