Method of using surfactants as contact lens processing aids

An improved method for manufacturing hydrophilic polymeric devices wherein surfactants are added to the hydration bath to assist in the mold release and processing of hydrophilic polymeric devices, especially hydrophilic contact lenses.

FIELD OF THE INVENTION 
The present invention relates to the use of surfactants in a hydration bath 
to assist in the processing of hydrophilic polymeric devices, especially 
hydrophilic contact lenses. 
BACKGROUND 
Soft contact lenses are produced by a number of methods including static 
cast molding, spincasting, lathing and combinations of casting and 
lathing. A critical step in the production of lense using molds is 
releasing the lens from the mold without damaging the lens. One method of 
lens release is to hydrate the lens. The lens-in-mold assembly is placed 
in a hydration tank filled with water. The water is usually heated. 
Often hydration alone does not release the lenses from the molds. The 
lenses must then be gently removed from molds by hand. Such hand-assisted 
lens removal increases the liklihood of lens damage. These damaged lenses 
often fail the subsequent quality inspections due to tearing, nicks, or 
other surface defects which occur as the lenses are released from the 
molds. One surface defect which is prevalent and which is commonly 
measured is known as the POM (piece of material) defect. This defect 
occurs as a result of excess polymer debris adhering to the lens. In 
essence, the POM defect causes an irregular surface on the lens due to an 
additional piece of material adhering to the lens. 
A method of improving the release rate and decreasing the incidence of POM 
defect by facilitating the release of spincast and castmolded lenses 
which, in turn, would significantly increase the useable lens yield for 
such lenses, is not known. This invention addresses the problem of 
undesirably low cosmetic yields of contact lenses during hydration 
release. 
SUMMARY OF THE INVENTION 
The present invention relates to the discovery that the addition of 
surfactants into the aqueous hydration bath at specified concentrations, 
facilitates the release of lenses from molds. The present invention 
relates to an improved method for releasing hydrophilic polymeric devices 
from plastic molds in aqueous hydration baths the improvement of which 
comprises the addition of a surfactant to an aqueous bath in an amount 
sufficient to achieve a surfactant concentration of from 0.1 wt. % to 10 
wt. %. The present invention further reduces or eliminates the incidence 
of the POM defect on contact lenses. The resulting new aqueous hydration 
bath compositions comprising the surfactants, result in a surprisingly 
superior method of lens release. 
It was discovered that the lens yield was drastically improved when 
surfactants were incorporated in the hydration bath. This increased yield 
is due to both improved lens release and to reduction of POM defects. The 
enhanced level of lens release from the molds obviates the need to 
manually assist lens removal from the mold upon hydration. Eliminating the 
need for manual assistance eliminates lens damage which commonly occurs 
during manual lens removal. The cosmetic yield, or the number of 
acceptable lenses without nicks, tears, POM or other cosmetic defects 
therefore improved over the usual yield.

DETAILED DESCRIPTION OF THE INVENTION 
The hydration bath of the present invention comprises an aqueous solution 
of at least one surfactant. Suitable surfactants include any material 
which reduces the surface tension of water. The effective surfactants are 
those which may be incorporated into the aqueous hydration bath. Preferred 
surfactants are polymeric materials that have hydrophobic and hydrophilic 
portions. Such polymeric surfactants include polyoxyethylene lauryl 
ethers, polyoxyethylene nonylphenyl ethers, polyoxyethylene sorbitan 
monooleates, polyoxyethylene sorbitan monolaurates, polyoxyethylene 
sorbitan monopalmitates, polyoxyethylene stearyl ethers, and their 
polyoxypropylene analogs. Other surfactants found to be effective are the 
poloxamines, dioctyl sodium sulfo-succinate, and polyvinyl alcohol. 
The mold materials used for the spincasting or static cast molding may be 
one piece or more. If two part molds are used, each mold part may be made 
of a different composition with the molded polymeric device preferentially 
adhering to one of the mold parts. The mold parts may be made from any 
plastic conventionally used to mold hydrophilic polymeric devices. 
Preferred plastic materials are polyethylene, polypropylene, polystyrene, 
polyvinyl chloride, copolymers thereof, nitrile resins, and with polyvinyl 
chloride being the most preferred mold material to which the molded 
hydrophilic polymeric device adheres. 
The contemplated hydrophilic polymeric material used in the mold to make 
the hydrophilic polymeric devices are any hydrogels, silicone-containing 
hydrogels, urethane prepolymers, ethylenically terminated polysiloxane 
prepolymers and other hydrophilic polymers some of which are the subject 
of commonly assigned U.S. Pat. No. 5,034,461, issued Jul. 23, 1991. 
The optimal concentrations of the surfactants used depend upon the 
temperature of the hydration bath and the amount of time that the 
lens-in-mold piece is allowed to be exposed to the hydration bath. Use of 
various surfactants at elevated temperatures, generally those above 98 
degrees C. have experimentally been observed to cloud the hydration bath 
which makes retrieval of the released lenses from the hydration bath more 
complicated. 
The contemplated concentration of surfactants in the aqueous hydration 
baths ranges from about 0.1 wt % to about 10 wt %, with the preferred 
concentration ranging from about 0.25 wt % to about 10.0 wt %, and with a 
0.5 wt % concentration being most preferred when the bath temperature is 
40 degrees C. The contemplated hydration bath temperatures range from 
about 30 degrees C. to about 100 degrees C., with the preferred 
temperature ranging from about 35 degrees C. to about 80 degrees C., and 
with 40 degrees C. being the most preferred operating temperature. 
The surfactant used in the present invention may be selected from the group 
of polyoxyethylene lauryl ethers, polyoxyethylene nonylphenyl ethers, 
polyoxyethylene sorbitan monooleates, polyoxyethylene sorbitan 
monolaurates, polyoxyethylene sorbitan palmitates, polyoxyethylene stearyl 
ethers, and their polyoxypropylene analogs, dioctyl sodium sulfosuccinate 
and polyvinyl alcohol, with polyoxyethylene sorbitan monooleate being most 
preferred. 
It is contemplated that the mold releasing hydration bath of the present 
invention will assist in the release of cast hydrophilic polymers from 
molds made from polyethylene, polypropylene and polyvinyl chloride, 
copolymers thereof, nitrile resins such as Barex (BP Chemicals) which are 
described in U.S. Pat. No. 3,426,102. Other resins as are known in the 
field may also be used as the mold material, such as the nitriles and 
nitrile resins some of which are the subject of co-pending and commonly 
assigned U.S. Ser. No. 618,448 filed Nov. 27, 1990. The hydrophilic 
polymeric device being released from the mold may be any device which can 
be static cast molded or spin cast molded, but is preferably a contact 
lens. 
While this invention has been described with particular reference to 
contact lenses, the method of this invention is useful in forming a whole 
variety of shaped articles used in biomedical applications. Such articles 
include dialyzer diaphragms, artificial kidneys and other implants (see 
U.S. Pat. Nos. 2,976,576 and 3,220,960), therapeutic bandages (U.S. Pat. 
No. 3,428,043), surgical devices such as heart valves, vessel substitutes, 
interuterine devices, membranes and other films, catheters, mouth guards, 
and denture liners (U.S. Pat. Nos. 3,520,949 and 3,618,231), and 
ophthalmic prostheses (U.S. Pat. No. 3,679,504). Particularly preferred 
shaped articles for the purposes of this invention are contact lenses and 
preforms wherein one surface of the lens is formed in the molding process. 
The following examples serve only to further illustrate aspects of the 
present invention and should not be construed as limiting the invention. 
EXAMPLES 
Surfactant concentration, bath temperature and hydration time were the 
factors studied. Number of lenses displaying POM when viewed through an 
optical comparator at 10x magnification, and total cosmetic yield were the 
measured responses. Cosmetic yield represents those lenses observed to be 
free from nicks, tears and POM at said 10x magnification. All listed 
Examples used cast-molded combinations. The lens material used was 
uniformly a HEMA (hydroxyethylmethacrylate) based contact lens material. 
The molds were made from polyvinyl chloride. 
EXAMPLE 1 
Pure Aqueous Control Bath--No Surfactant 
Twenty lens-in-mold assemblies were added to a 7 liter control hydration 
bath at 80 degrees C. with no surfactant present. The lenses were removed 
from the bath and inspected for POM defect. The results showed that 33% of 
the lenses had POM defect. Total cosmetic yields were not measured for 
this example. 
EXAMPLE 2 
Polyoxyethylene sorbitan 20 monooleate--10 wt% 
Sixty lens-in-mold assemblies were added to a 7 liter hydration bath 
containing a 10 wt % concentration of poloxyethylene sorbitan 20 
monooleate (Tween 80) (Ruger Chemical Co.) at 80 degrees C. An equal 
number of lens-in-mold assemblies were added to a completely aqueous 
control hydration bath under similar temperatures for comparison. The 
results showed that 7 % of the lenses in the bath with surfactant 
displayed the POM defect, while 52 % of the lenses in the pure aqueous 
hydration control bath had the POM defect. Total cosmetic yields were not 
measured for this example. 
EXAMPLE 3 
Poloxyethylene sorbitan 20 monooleate 
Increasing concentrations of polyoxyethylene sorbitan 20 monooleate were 
added to a 7 liter aqueous hydration bath. Twenty lens-in-mold assemblies 
were added to the hydration bath with a 1 wt % concentration of 
polyoxyethylene sorbitan 20 monooleate. The bath was maintained at 40 
degrees C. for 6 minutes. The cosmetic yield of the lenses was 60%, or 
greater than the cosmetic yield of the lenses released upon hydration 
without surfactant. No POM defect was observed. The further results of the 
9 experimental runs are shown in Tables 1 and 2. 
TABLE 1 
______________________________________ 
Exp. # 
(Min.) Conc.(%) Temp.(C.) Time 
______________________________________ 
1 1 40 6 
2 1 60 10 
3 1 80 14 
4 5 40 10 
5 5 60 14 
6 5 80 6 
7 10 40 14 
8 10 60 6 
9 10 80 10 
______________________________________ 
TABLE 2 
______________________________________ 
Number of 
Lenses with 
Cosm. 
Exp. # POM Yield(%) 
______________________________________ 
1 0 60 
2 0 55 
3 0 70 
4 0 60 
5 0 45 
6 0 75 
7 1 55 
8 0 55 
9 0 55 
______________________________________ 
EXAMPLE 4 
Polyvinyl alcohol 
Increasing concentrations of polyvinyl alcohol were added to a 7 liter 
hydration bath. Twenty lens-in-mold assemblies were added to a hydration 
bath maintained at 40 degrees C. for 6 minutes. The released lenses 
displayed a 95% acceptable POM rate, meaning that only 1 lens out of 
twenty had apiece of material deposition. The cosmetic yield of the lenses 
was 50%, or greater than the ordinary cosmetic yield of released cast 
lenses hydrated without surfactant. The further results of the 9 
experimental runs are shown in Tables 3 and 4. 
TABLE 3 
______________________________________ 
Exp. # 
(Min.) Conc.(%) Temp.(C.) Time 
______________________________________ 
1 1 40 6 
2 1 60 10 
3 1 80 14 
4 5 40 10 
5 5 60 14 
6 5 80 6 
7 10 40 14 
8 10 60 6 
9 10 80 10 
______________________________________ 
TABLE 4 
______________________________________ 
Number of 
Lenses with 
Cosm. 
Exp. # POM Yield(%) 
______________________________________ 
1 1 50 
2 1 70 
3 0 55 
4 0 80 
5 3 50 
6 0 65 
7 0 80 
8 0 55 
9 1 80 
______________________________________ 
EXAMPLE 5 
Polyvinyl alcohol 
Eighty-six lens-in-mold assemblies were hydrated in a 0.1 wt % polyvinyl 
alcohol (PVA) in buffered saline solution at 80 degrees C. An equal number 
of lens-in-mold assemblies were hydrated under the same conditions in a 
pure aqueous control hydration bath for comparison. The results showed 
that 62% of the lenses in the hydration bath with surfactant had the POM 
defect. Thirty-four percent (34%) of the lenses hydrated in the control 
bath had the POM defect. The low concentration of PVA coupled with a 
saline solution did not produce satisfactory results. 
EXAMPLE 6 
Poloxyethylene sorbitan 20 monoleate 
The results of experiments 4 and 5 led to the conclusion that a smaller 
surfactant concentration should be evaluated. Once again, increasing 
concentrations of polyoxyethylene sorbitan 20 monooleate were added to a 7 
liter hydration bath. Thirty lens-in-mold assemblies were added to the 
hydration bath with a 0.5% concentration of polyoxyethylene sorbitan 20 
monooleate. The bath was maintained at 40 degrees C. for 10 minutes. The 
released lenses displayed a 90% acceptable POM rate, meaning that no 
significant piece of material deposition had occurred. The cosmetic yield 
of the lenses was 66.7%, or greater than the ordinary cosmetic yield of 
hydrated cast lenses released without surfactant. The first three runs of 
this experiment were conducted without any surfactant present. The further 
results of the 9 experimental runs are shown in Tables 5 and 6. 
TABLE 5 
______________________________________ 
Exp. # 
(Min.) Conc.(%) Temp.(C.) Time 
______________________________________ 
1 0 40 6 
2 0 60 10 
3 0 80 14 
4 0.5 40 10 
5 0.5 60 14 
6 0.5 80 6 
7 1.0 40 14 
8 1.0 60 6 
9 1.0 80 10 
______________________________________ 
TABLE 6 
______________________________________ 
Number of 
Lenses with 
Cosm. 
Exp. # POM Yield(%) 
______________________________________ 
1 0 76.7 
2 10 53.3 
3 18 26.7 
4 3 66.7 
5 8 70 
6 2 63.3 
7 8 63.3 
8 3 66.7 
9 1 83.3 
______________________________________ 
Many other modifications and variations of the present invention are 
possible and will be readily apparent to the skilled practitioner in the 
field in light of the teachings herein. It is therefore understood that, 
within the scope of the claims, the present invention can be practiced 
other than as herein specifically prescribed.