Contact lens and method of molding

A soft contact lens made of a hydrated polymer with an overall water content in the range of 40% to 90% by weight has a water content at the front surface of the lens which is at least 3 percentage points higher than the water content at its rear cornea-contacting surface, as measured by use of an Abbe refractometer. The lens is made from a monomer capable of polymerizing to form a clear transparent partially swollen hydrogel polymer and a diluent forming from 15 to 50% by weight of the moulding mixture, which is moulded between a convex and a concave mould member of different materials chosen so that the lens has the different water contents at its surfaces, as mentioned above. After curing, the mould members are separated with the partially swollen lens adhering to the convex mould member and the whole immersed in an aqueous medium, which may be a weak alkali solution, so as to cause the lens to separate from the convex mould member, to substitute water for any replaceable non-aqueous diluent in the lens, and to cause the lens to swell to substantially its fully hydrated state. The swollen lens is then transferred into saline solution which replaces the water and is packaged and sealed in saline solution in a container. The lens may be sterilized by treatment in an autoclave before and/or after packaging and sealing.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to soft contact lenses made of a hydrated 
polymer or hydrogel and methods of forming such lenses. 
2. Description of the Prior Art 
Soft contact lenses must have a high level of oxygen transmissibility so as 
to avoid any adverse effect on the eyes of the wearer through insufficient 
oxygen being supplied to the cornea. Present lenses rely on using 
materials which are permeable to gases such as oxygen and carbon dioxide, 
since the cornea is avascular and acquires oxygen directly from the 
atmosphere in order to respire. Soft or hydrogel lenses are formed from 
polymers which when hydrated swell to a soft jelly like consistency and 
can be worn comfortably on the eye. It is known that the oxygen 
permeability of a hydrated contact lens is related to its equilibrium 
water content. A high water content is therefore desirable as the 
"dissolved" oxygen permeability of hydrogels increases almost 
exponentially with increasing water content up to a limiting value equal 
to the oxygen permeability of water. Much work has been devoted to 
developing polymeric materials which will form hydrogels with a high water 
content yet be sufficiently strong to withstand the physical stresses 
which will inevitably occur as the lens is handled by the wearer in 
placing or removing the lens from the eye. The transfer of oxygen and 
other gaseous species through known lenses is related to the fall in 
oxygen concentration from the lens surface in contact with the atmosphere 
to the concentration at the surface in contact with the eyes. 
As stated above, the dissolved oxygen permeability of a hydrogel material 
is related to the water content of the lens. In turn the oxygen flux (F) 
through the lens into the corneal epithelium required for any contact lens 
thickness (L) is related to the difference (.DELTA.p) of oxygen tension 
across the lens and to the dissolved oxygen permeability (Pd) of the 
material forming the lens. F may be expressed as: 
##EQU1## 
The transfer of oxygen to the corneal epithelium is a complex physiological 
process and while calculations can be made of oxygen flux demand, these 
can be based on an over-simplification of the real in vivo situation which 
occurs when the lens is in place on the eye. Measurements are difficult as 
the oxygen requirements of one person can differ from those of another. in 
fact there are groups of people who find existing soft contact lenses 
impossible to wear because there is insufficient transfer of oxygen. Many 
papers discuss the requirements for "dissolved oxygen permeability" and in 
one such paper Ng and Tighe (British Polymer journal December 1976 page 
118 to 123), apart from discussing the design requirements of hydrogel 
materials to be used for contact lenses, point out a number of factors 
which can complicate a theoretical approach to the problem of oxygen 
transfer namely: "(a) the equilibrium water content of a hydrogel contact 
lens, such as Hydron or Bionite, has been found to be lower in the eye 
than in the saline soaking solution, which is in turn lower than in 
distilled water. (b) The evaporation of water from the anterior surface of 
the contact lens during wear might result in the back flow of water from 
the layer of the tear fluid between the contact lens and the cornea, and 
hence a reduction in the oxygen tension at the epithelial surface. (c) 
Because of the presence of solutes in the tear fluid, the solubility of 
oxygen in the tear fluid may be lower than that in distilled water. (d) 
Because of the presence of solutes in the tear fluid, again, the structure 
of water in the hydrogel contact lens worn in the eye may not be the same 
as in distilled water and so may affect the oxygen permeability. (e) The 
oxygen consumption rate of the human cornea varies from one person to 
another and is not constant for a given individual. (f) Under closed-eye 
conditions, the eye-ball movement may contribute to tear fluid 
replenishment behind the lens. 
A calculation of minimum oxygen flux for particular requirements can be 
made, and general guidance given that such a flux is likely to be attained 
or exceeded at the temperature of the eye with lens materials having 
particular levels of oxygen permeability. The paper quoted suggests a 
water content of 65% at 34.degree. C. "provides a reasonable basis for 
meeting problems of manufacture and visual stability with the oxygen 
consumption requirements of the cornea over successive day and night 
cycles." 
One of the complications referred to is the fact that at certain levels of 
loss of water from the front surface of the lens there can be a back flow 
of water from the cornea contacting surface of the lens thereby causing a 
reduction of oxygen tension at the epithelial surface. This is a 
recognised problem which has been mentioned by several authors as having a 
limiting effect on the spread of the use of soft hydrogel contact lenses. 
All soft contact lenses available at present have virtually the same water 
content at both major surfaces of the lens. Once placed in the eye, there 
is a drop in water content but this occurs in a uniform manner so that the 
content at both surfaces remains the same. However, if due to climatic 
conditions or any other reason there is a loss of water from the front 
surface of the lens, this may be sufficient to reduce or reverse the 
transfer of oxygen to the cornea. 
Oxygen permeability (DK) is an intrinsic property of the materials used for 
hydrogel lenses, and the limiting factor to date in increasing the 
permeability of the hydrogel has been the fact that the maximum level will 
be that of the DK of pure water. At present if one has two lenses, namely 
a thick lens and a thin lens made of the same hydrogel material, the thin 
lens will have a higher oxygen transmissibility (DK/.sub.L), i.e. it will 
allow greater amounts of oxygen to pass through, but there are limits to 
how thin one can make a high water content lens and still have a 
sufficiently robust lens to withstand the stresses of even one insertion 
in the eye. 
SUMMARY OF THE INVENTION 
The present invention aims to provide lenses which will provide a 
particularly high rate of transfer of oxygen through the lens and will 
therefore be capable of being kept in the eye for extended periods. 
According to the invention, there is provided a soft contact lens made of a 
hydrated polymer, having a front surface and a rear cornea-contacting 
surface and an overall water content in the range of 40% to 90% by weight, 
wherein the hydrated polymer has a water content at the front surface of 
the lens which is at least 3 percentage points higher than the water 
content at its rear cornea-contacting surface, as measured by use of an 
Abbe refractometer. Preferably the water content at the front surface is 
from 4 to 25 percentage points higher than the water content at the rear 
cornea-contacting surface. Because the water content at the rear surface 
is lower than that at the front surface, it is believed that there will be 
a diffusive flow from front surface to back surface of the lens when in 
use, thus actively promoting the transfer of tear fluid containing 
dissolved oxygen directly through the lens to the cornea. Thus there is 
not sole reliance on the passive transfer of oxygen based on the oxygen 
concentration or tension at the epithelium being lower than at the front 
surface of the lens. This transfer of tear fluid will increase the oxygen 
flux value over that of a conventional lens of the same overall water 
content but with the same or substantially the same water content at both 
surfaces. 
Overall water content has generally been measured by a method involving 
dehydration of the lens, using the equation: 
##EQU2## 
Such a method is time consuming and whatever system is used to dry the 
lens one can never be sure that all water has been driven off, and only 
solid material remains. More recently, a method has been described of 
measuring the water content of a hydrogel lens by means of an Abbe 
Refractometer. 
The refractometer method is based on the use of an instrument which 
measures the refractive index of the surface of the hydrogel. For an 
article of uniform water content, this can be considered to measure the 
overall water content of the article, but in fact the measurement is made 
only at the surface. The conversion of the measured refractive index value 
into a water content is based on the accepted hypothesis that the 
refractive index of the hydrated material is a non-linear function of 
water content and can be calculated using the refractive indices and 
densities of the dry polymer and the solution in which the lens is held in 
a hydrated state. The technique is described in detail in a paper by Dr. 
Wilhelm Teurle entitled "Refractive Index Calculation of Hydrogel Lenses", 
International Contact Lens Clinic Vol 11 Number 10 pages 625-628 (1984). 
For any particular polymer it is possible to construct a graph from which 
the approximate water content may be read off once a refractive index 
reading is given. Certain available refractometers also have a scale 
giving a solids content reading which can be used in the comparison of one 
lens with another. Thus it is possible to determine for any lens surface 
simply and rapidly the water content of the surface of the lens presented 
to the instrument for measurement. The instrument may even be calibrated 
for the particular polymeric composition used to form the hydrogel but 
when both surfaces are being measured to determine whether a difference in 
water content exists, the absolute value for each surface is unimportant. 
Thus one can take a lens purchased in the open market, or a lens removed 
from a subject's eye, and rapidly determine whether a water content 
difference exists. 
Lenses in accordance with the invention have been found to be capable of 
being kept in the eye for extended periods without any adverse effect on 
the cornea epithelium structure. 
We prefer to form the lens of the invention by a casting process comprising 
the steps of (a) introducing into a lens casting mould a mixture 
comprising at least one monomer capable of polymerising to form a clear, 
transparent, partially swollen hydrogel polymer and a diluent forming from 
15 to 50% by weight of the mixture, and causing the monomer or monomers to 
polymerise to form said hydrogel polymer, the mould cavity being formed 
between a convex member and a concave member, said mould members having 
opposed mould surfaces made of different materials chosen so that the lens 
formed in the mould from the hydrogel polymer has a water content at its 
convex front surface which is at least 3 percentage points higher than the 
water content at its concave rear surface, as measured by use of an Abbe 
refractometer, (b) separating the mould members with the partially swollen 
lens adhering to the convex mould member, (c) immersing the partially 
swollen lens, while still adhering to the convex mould member, in an 
aqueous medium so as to cause the lens to separate from the convex mould 
member, to substitute water for any replaceable non-aqueous diluent in the 
lens and to cause the lens to swell to substantially its fully hydrated 
state, (d) transferring the swollen lens into saline solution to replace 
substantially all the water by saline solution, and (e) packaging and 
sealing the lens in saline solution in a container. 
Casting the lens in this way, we have found it possible to achieve a 
satisfactory water content difference between the two lens surfaces. The 
process preferably also comprises the step of sterilising the lens by 
treatment in an autoclave before and/or after packaging and sealing. In 
some cases, it has been found advantageous to immerse the lens in a weak 
alkali solution so as to replace hydrogen on carboxyl groups within the 
polymer matrix by the alkali cation. The immersion in weak alkali solution 
may be effected by employing the weak alkali solution as the aqueous 
medium in which the partially swollen lens is immersed while still 
adhering to the convex mould member. These steps have been found to tend 
to increase the water content difference between the surfaces. 
It would appear probable that polymerisation and/or cross-linking at the 
rear surface takes place to a greater degree than at the front surface, 
thus ensuring that the hydrogel formed has a lower capacity for water at 
the back surface. 
The diluent has been found essential to ensure that the lens is directly 
formed in a partially swollen or partially hydrated state, and not as a 
xerogel. Any known diluent for hydrogels can be used. These materials are 
also described as extenders or space creaters, and include water, 
glycerol, ethylene glycol, diethylene glycol and the borate ester of 
glycerol. We prefer to use a mixture of glycerol and water as the diluent. 
The diluent content of the partially swollen lens when formed is 
preferably at least 35% and in the range 15 to 50%, but water alone must 
not exceed 40% or adverse effects occur. 
Casting is preferably carried out in a rigid cavity formed between a convex 
mould and a concave mould, there being some means to allow flow of 
additional polymerisable mixture into the mould cavity as polymerisation 
occurs. 
The use of two mould members of different materials is essential for 
achieving a water content difference between the front and back surfaces 
of the lens. 
As the measurement of the final lens to determine whether there is a 
satisfactory water content difference from back to front surfaces is so 
simple, the selection of mould materials for pairs of materials from which 
to form lenses is merely a matter of time and effort. As a general 
guidance in selecting materials for screening, there are certain general 
characteristics which will contribute to the achievement of a difference 
in water content. This guidance is in no way intended to be limiting. The 
convex mould surface should generally be made of a material having a 
higher surface energy than the material of the opposed concave mould 
surface. The material likely to be found satisfactory for the concave 
mould surface will have a hydrophobic character when in contact with the 
casting mix, and should desirably have an open surface morphology. On the 
other hand, the convex mould surfaces used to cast the rear cornea 
contacting surface are likely to be chosen from materials having a 
hydrophilic character and a fine surface finish such as glass. 
Materials may be simply screened for their suitability as mould surfaces by 
casting monomer mix between two flat plates of the materials in question 
with a ring gasket separating them and containing the monomer mix in 
place. It is clearly impossible to investigate all the possible 
permutations and combinations of materials and surface treatments but the 
simple screening method using two flat plates reduces such an 
investigation to the level of a routine overnight experiment. 
As the difference in hydrophilicity and hydrophobicity between two 
materials is a matter of degree, two polymeric materials, e.g. nylon and 
polypropylene, can be sufficiently different in character to obtain a 
water content difference between the lens surfaces when used in casting. 
When the materials are the same or are not sufficiently different in their 
ability to influence the polymerisation, no real difference in water 
content is found between one surface and the other of the lens. We prefer 
to mould the front surface of the lens by contact with polypropylene and 
the rear cornea contacting surface by contact with glass. 
Any monomer or monomer mixture capable of forming a clear transparent 
hydrogel after polymerisation and hydration can be used. We prefer to use 
as the main component, the commercially available major constituent of 
most soft lenses, namely hydroxy ethyl methacrylate (HEMA). This material, 
when cross-linked, forms a hydrogel. Other monomeric materials which can 
be added to the monomer mix include: 
N-vinyl pyrrolidone 
Methacrylic acid and its esters 
Monomers, particularly useful in the practice of this invention for 
admixture with HEMA, include hydrophobic acrylic esters, such as lower 
alkyl acrylic esters, the alkyl moiety containing 1-5 carbon atoms, such 
as methyl acrylate or methacrylate, ethyl acrylate or methacrylate, 
n-propyl acrylate or methacrylate, isopropyl acrylate or methacrylate, 
isobutyl acrylate or methacrylate, n-butyl acrylate or methacrylate or 
various mixtures of these monomers. For increased dimensional stability 
the above monomers or monomer mixtures are further admixed with a minor 
proportion of di- or polyfunctional polymerisable species to cause 
cross-linking of the polymeric matrix as polymerisation proceeds. Examples 
of such di- or polyfunctional species include: 
divinylbenzene, ethylene glycol diacrylate or di-methacrylate, propylene 
glycol diacrylate or di-methacrylate, trimethylol propane trimethacrylate 
Other monomeric materials suitable for producing lenses by processes 
according to this invention are hydrophilic monomer mixtures forming 
three-dimensional cross-linked networks such as those disclosed in U.S. 
Pat. No. 3,822,089. Illustrative hydrophilic monomers include 
water-soluble monoesters of an acrylic acid or methacrylic acid with an 
alcohol having an esterifiable hydroxyl group and at least one additional 
hydroxyl group such as the mono- and polyalkylene glycol monoesters of 
methacrylic acid and acrylic acid, e.g. ethylene glycol monomethacrylate, 
ethylene glycol monoacrylate, diethylene glycol monomethacrylate, 
diethylene glycol monoacrylate, propylene glycol monomethacrylate and 
dipropylene glycol monoacrylate; the N-alkyl and N,N-dialkyl substituted 
acrylamides and methacrylamides such as N-methylmethacrylamide, 
N,N-dimethyl-methacrylamide, and the like; N-vinylpyrrolidone-; the alkyl 
substituted N-vinyl pyrro1idones, e.g. methyl-substituted N-vinyl 
pyrrolidone, glycidyl methacrylate glycidyl acrylate; and others known to 
the art. 
It is essential, when polymerising such hydrophilic monomers or mixtures 
thereof, that a three-dimensional cross-linked network be formed since the 
polymerised materials absorb water and become soft and flexible and would 
lack shape retention if not cross-linked. For this purpose it is necessary 
to employ small amounts of cross-linking monomers such as those 
illustrated above. 
We believe that the use of an organic initiator such as di-isopropyl 
peroxydicarbonate is advantageous. This material is preferably used as the 
mixture of materials produced by the reaction of sec-butyl chloroformate, 
isopropyl chloroformate, and hydrogen peroxide in the presence of alkali. 
The other components formed are isopropyl sec-butyl peroxydicarbonate and 
di-sec-butyl peroxydicarbonate. There may be a differential distribution 
of catalyst due to either the catalyst having an affinity for the 
hydrophilic mould surface without being deactivated by that surface, 
and/or the catalyst being deactivated in contact with the hydrophobic 
mould surface, the end result being a greater degree of polymerisation of 
the rear surface than the front surface of the lens. 
The curing conditions, the method of removal of the lens from the moulds, 
and the nature of the monomer mix used to form the polymer all influence 
the ability of the lens to hold water, and hence the measured water 
content. It is believed however that they have no major influence on 
whether a difference in water content can be measured between the front 
surface and the rear cornea contacting surface of the lens. The results of 
our comparative experiments support the view that it is the nature of the 
casting surfaces which determine whether a difference will exist, and the 
other variables operate no differently in their influence on water content 
than they would in a casting system where both mould surfaces are made of 
the same materials. Curing in an air oven, curing in a water bath and 
curing using U.V. light only produce a water content difference when the 
mould surfaces used to form the surfaces of the lenses are different in 
character. One problem in attempting to specify more closely how to select 
pairs of casting surfaces beyond the simple screening test is that it is 
sometimes difficult to determine the exact nature of the surface of a 
polymeric material. With a particular sample of polyethylene used in 
conjunction with glass, no water content difference was obtained, although 
the only variable from previous experiments was the substitution of 
polyethylene for polypropylene. The fact that polyethylene behaves 
differently from polypropylene could be related to a basic difference in 
the structure of both polymers, e.g. polyethylene has no pendant groups. 
However differences in manufacturing techniques could have a major 
influence on the surface character of a polymeric material. It is 
therefore impossible to determine easily what is the factor that 
influences the polymer mix and results in the polymerisation and hence the 
water content at one surface of the lens being different from the other. 
It is apparent from the work carried out that while the process is 
complex, its effect can be measured simply and rapidly by means of the 
Abbe Refractometer. There is therefore no need to provide the man 
practiced in the art with a theoretical basis for predicting whether a 
particular pair of mould surfaces will provide a satisfactory water 
content difference with a particular monomer mix, under particular curing 
conditions, as this can be determined by one casting. We do believe, 
however, that by ensuring that the mould materials chosen have a major 
difference in surface energy, it will be found that as long as the surface 
energy of the surface/surfaces has not been altered by any treatment 
either during or after manufacture of the shaped mould, a difference in 
water content from one surface to the other of the cast lens will be 
found. 
The casting of lenses is preferably carried out in such a manner as to 
obtain lenses at a cost such that they can be disposed of by the wearer at 
regular intervals. Such a manner must produce lenses in a precisely 
reproduceable way so that one lens can replace another without the need 
for a fitting whenever a new lens, or pair of lenses is adopted. We have 
found that lenses of the kind described above can be produced by the 
casting processes described so that not only are the desired properties to 
achieve satisfactory oxygen transfer obtained but the lens are also 
produced in a sufficiently precise manner as to be readily replaceable and 
at a cost which enables them to be discarded after a limited period of 
use. The lenses are designed to be handleable for perhaps four or five 
insertions but to have a relatively limited life so avoid use beyond a 
desirable change over period. The ability of the lens to transfer oxygen 
at a high rate will further reduce the risk of damage to the cornea should 
a wearer continue to wear a lens beyond the normally recommended period of 
wear. 
The polymerisation of the moulding mixture may be caused by subjecting it 
to a standard curing procedure e.g. by heating the moulding mixture in the 
mould at a temperature rising from substantially 65.degree. C. to 
substantially 100.degree. C. over a period of substantially 11/4 hours, 
followed by a secondary cure at substantially 120.degree. C. for 
substantially 1/2 hour. 
We have found by finishing the polymerisation with a secondary cure at 
substantially 120.degree. C., the lens is caused to adhere to the convex 
mould member when the mould members separate. We prefer to remove the lens 
from the convex mould member by immersion in de-ionised water or water of 
substantially equivalent purity so as to cause the lens to separate from 
the mould member, substitute water for any replaceable non-aqueous 
diluent, and cause the lens to swell to substantially its fully hydrated 
state. In those cases where there are carboxyl groups within the polymer 
matrix, this treatment may be followed by immersing the lens in weak 
alkali so as to replace hydrogen on all or substantially all of the 
carboxyl groups within the polymer matrix by the alkali cation present. It 
is also possible to combine the two steps and use an aqueous solution of a 
weak alkali to separate the lens, hydrate the lens, and replace hydrogen 
on all or substantially all of the carboxyl groups with the alkali cation 
all in one step. 
As indicated above, the materials to be used as the mould members may be 
selected for screening on the basis of differences in surface energy. A 
method of calculating the surface energy of a solid is described in 
POLYMER SURFACES by B. W. CHERRY, CAMBRIDGE UNIVERSITY PRESS 1981, chapter 
one pages 13 to 17. D. A. KAEBLE, PHYSICAL CHEMISTRY OF ADHESION, WILEY 
1971, chapter five, pages 149 to 170 contains an extensive discussion of 
the surface energy of polar and non-polar solids. 
Materials which have a high surface energy and can be used for the convex 
mould part include machineable ceramics, silica or quartz glass, 
boro-silicate glass, soda lime glass, and electroformed nickel. Low 
surface energy materials include polymeric materials which can be easily 
formed to the desired shape by injection moulding, such as 
polytetrafluoroethylene and polypropylene. 
The casting cavity is preferably formed by providing a rigid step which 
separates one mould member from the other and thus prevents the casting 
surfaces directly contacting one another, while forming a rigid casting 
cavity. The shape of the step is used to determine the profile of the edge 
of the finished lens. The mould members are designed so that the part of 
the convex member extending from the casting surface is partly surrounded 
by the monomeric mixture when the mould cavity has been filled so that, on 
shrinkage within the cavity, mixture is available to flow into the cavity 
and ensure that the cast, partially swollen, lens reproduces the shape of 
the mould cavity. The rigid step is preferably provided on the concave 
mould casting surface. 
The casting to a partially swollen state using a mixture which contains 
particular quantities of diluent assists, in conjunction with other 
factors, the relatively easy release of the partially swollen lens from 
the mould surface to which it adheres. The presence of diluent provides 
effective control over adhesion to mould surfaces when associated with an 
appropriate choice of mould materials. The mould cavity is kept rigid and 
shrinkage of the moulding mixture during polymerisation is accommodated by 
the monomer mixture flowing from a reservoir into the cavity by capillary 
action at the junction of the two mould parts. This ensures a good control 
over centre thickness of the finished lens and avoids problems with edge 
quality. We have found that the use of a casting mixture with an effective 
diluent content combined with particular choices of mould material gives 
us optimum yields and reproducibility. 
The amount of added diluent is limited to that which can be added without 
the mix becoming cloudy due to the diluent beginning to separate in the 
form of an emulsion rather than simply acting to dilute the mixture. We 
have found that if the quantity of diluent falls below 15%, it is 
impossible to remove the lens after polymerisation from the mould surface. 
If water is used on its own, removal from the mould surface takes longer 
than when water is used in admixture with other diluents. In addition, the 
use of water alone in quantities of more than 40% results in a hazy 
material, while casting problems are experienced when the total quantity 
of diluent exceeds 55%. It is also found that when using glycerol on its 
own, separation from the mould surface during polymerisation can occur to 
a limited extent, thereby reducing yields. We have found that mixtures of 
water and glycerol in which glycerol is in excess result in the ability to 
remove the lens from the mould surface to which it is adhered in a 
satisfactory time and with no reduction in yields due to separation. A 
preferred ratio is 2 parts glycerol to 1 part water. 
The presence of both methacrylic acid and glycerol in the polymerised lens 
after the initial cure will, it is believed, provide the reactants 
necessary for an esterification reaction to take place when the 
temperature is raised further. This will occur if the polymerised lens is 
heated to 120.degree. C. for times of the order of 15 to 30 minutes during 
the secondary cure. Such an esterification could explain the reduction in 
adherence of the lens to the mould surfaces and the ease of achieving 
separation. We therefore prefer to have methacrylic acid and glycerol 
present in the monomer mix and to ensure that some free methacrylic acid 
is present when the secondary cure is commenced. 
As a polymerisation initiator we use an organic polymerisation initiator in 
preference to an inorganic initiator. 
It is possible that there may be a difference in the distribution of 
monomer species either related to the reactivity of the monomers or to the 
influence of the mould surfaces. In the case of a mix containing as a 
major components hydroxy ethyl methacrylate (HEMA), N-vinyl pyrrolidone 
and methacrylic acid, Fourier Transformation Infra Red Analysis of the 
concave and convex lens surfaces shows one surface to be HEMA rich 
polymers while the other is richer in methacrylic acid and N-vinyl 
pyrrolidone derived polymers. Hence the refractive index difference may 
reflect a difference in polymer structure as well as a water content 
difference. 
The rigidity or form stability of a soft contact lens can be related to the 
flexibility of the lens. Flexibility is measured in terms of the lens 
displacement l, produced by a saline column of height h. Flexibility can 
be used to measure the handleability of a lens and to check on its form 
stability in relation to changes in polymer composition or process of 
formation. Flexibility is of course also related to the centre thickness 
of the lens. We have found that as higher water contents are approached at 
particular ranges of centre thickness, the displacement l increases 
drastically, going from a lens of 75% final average water content to 85% 
final average water content. It is therefore better to decrease the centre 
thickness of a 73-75% water content lens than to increase the water 
content to 80% or over to obtain the same or higher oxygen permeability. 
The effects of various changes in the conditions under which polymerisation 
is carried out such as curing cycle, curing initiator concentration, and 
nature of diluent have shown that for the same thickness and average water 
content, changes in these parameters once they have been selected to 
achieve a particular water content have in general little effect on 
overall mechanical strength. 
Examples of monomer mixtures which have been found satisfactory are given 
in the examples below. The use of these mixtures under varying curing 
conditions is also illustrated. 
As indicated above we prefer to achieve the condition where the polymerised 
lens adheres preferentially to the convex mould member, and is removed 
from the convex mould member by immersing the convex mould without cooling 
in de-ionised water at 60.degree. C. for ten minutes. We have found that 
where carboxyl groups are present a subsequent treatment which consists of 
immersing the lens in a 2% solution of sodium bicarbonate means that in 
about 30 minutes any mobile hydrogen is replaced in carboxyl groups by 
sodium and the lens has also reached a fully swollen state. The lens in 
the course of such treatment is hydrated to an overall water content of at 
least 60% in order to achieve a consistently satisfactory oxygen 
permeability. We prefer to hydrate to an overall water content in the 
range 60 to 75%. The water content is governed by the nature of the 
monomer mix and the curing conditions chosen and is now an area where 
there is sufficient guidance in the literature for the man practiced in 
the art. The lens floats free from the mould during the deionised water 
treatment and can then be placed in saline solution after the alkaline 
treatment (if this is carried out) to exchange water within the lens for 
saline solution. The lens can also be removed from the mould surface by 
omitting the treatment with de-ionised water, and utilising the treatment 
with mild alkali to cause the lens to separate from the mould surface and 
swell to its fully swollen state. In addition to a 2% solution of sodium 
bicarbonate other equivalent mild alkali treatments can be used e.g. a 
weak ammonia solution. 
The lens is then packaged in a sealed container in saline solution and 
supplied to the wearer in that form so that it is untouched until it is to 
be inserted by the wearer in the eye for the first time. This can only be 
done because of the high degree of reproducibility obtained from one lens 
to the next by using the processes of the present invention. Up to eight 
sets of lenses may be supplied after the first fitting with change over 
dates marked on the packages. 
We have found the design of the profile of the peripheral edge the lens is 
of particular importance in relation to the following factors: 
(a) lens casting performance 
(b) edge strength or toughness 
(c) wearer comfort 
Lens casting performance is measured by the yield of saleable lenses from 
the casting operation. This yield can be reduced by separation of the lens 
from the mould surface during curing. Such separation results in surface 
defects, and produces an unacceptable lens. Separation usually arises from 
a lack of adhesion at the part of the mould where the edge of the lens is 
being formed and one way of reducing losses due to separation is by 
choosing a particular shape for the edge forming part of the cavity in 
which the lens is cast. 
As regards the overall shape of the peripheral edge, we have found that a 
blunt edge is to be preferred to a sharp knife edge. 
Such a blunt edge shape of lenses is determined by the configuration at the 
interface between the convex and concave mould members. When casting in a 
rigid cavity, the cavity is bounded by the casting surfaces of the convex 
member and the concave member. The step which prevents the convex member 
contacting the concave member other than at the edge of the step in 
particular defines the peripheral edge profile of the lens. One design 
which has proved satisfactory in respect of all three factors is described 
later by reference the accompanying drawings. The step is so small that 
its dimensions are stable and any shrinkage after mould manufacture is 
minimal and has no measureable affect on the performance characteristics 
of the lens. We have found that the use of a rigid mould assembly in which 
there is a reservoir for the casting mix means that no edge defects are 
formed which cause irritation of the wearer's eye, nor affect the optical 
quality of the lens. 
The design of the lenses of this invention is not restricted to any 
particular set of parameters. Both anterior and posterior surfaces of the 
lenses may consist of entirely spherical curves or aspherical curves or 
combinations of both. For example, the central portion of the lens may 
consist of spherical curves on both the anterior and posterior surface and 
the periphery of the anterior surface may consist of a steeper or flatter 
spherical curve, and the periphery of the posterior surface may be 
aspheric to achieve improved fitting characteristics. In addition, one or 
both of the surfaces may be toric in the central or optical zone; however, 
the peripheral portion should preferably be symmetrical with respect to 
the central axis of the lens to achieve proper positioning of the lens on 
the eye. Multi-focal lenses may also be made. 
The design of the lens must be reproduced in the mould, taking into account 
the change in dimensions produced on hydrating the finished lens. The 
concave mould member may be formed by reproducing the design configuration 
on a convex metal mould, and then injection moulding the moulds from 
thermo-plastic material. The convex mould member may be formed directly by 
grinding and polishing a blank to the desired configuration. 
One form of the lens of the present invention and of a casting process for 
forming that lens will be described in more detail with reference to the 
accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The mould assembly shown in FIG. 1 in which the lenses are cast rests on a 
cylindrical base 3, which is integrally connected by an annulus 4 to the 
concave mould member 2, the casting surface 6 of which is provided with a 
rigid step or rim 7. The convex mould member 1 when in the operative 
moulding position rests against the rim 7, and is shaped relative to the 
concave member 2 to provide between them a reservoir 8 adjacent to the 
rim. 
The convex mould member 1 is shaped at the end remote from the casting 
surface to fit within the concave mould member 2 so as to substantially 
seal the reservoir 8, and reduce any evaporation of materials from the 
reservoir. 
The positions of the critical dimensions of a concave mould member surface 
6 are identified in FIG. 2. The surface is designed to mould a lens of the 
kind having different central and peripheral curves. 
The positions of the identified dimensions on the mould surface in FIG. 2 
are as follows: 
16. Diameter inside step 7. 
12. Radius of peripheral area. 
13. Radius of central area. 
14. Diameter of central area. 
15. Point of change of curve from peripheral area radius to central area 
radius. 
17. Centre line of mould. 
The preferred form of step design is shown in FIG. 3, a major identifiable 
feature being a step chamfer 9 at the step edge. The height 10 of the step 
edge may vary between typically 0.05 to 0.07 mm, and the length 11 of the 
step can be between 0.02 and 0.04 mm. Angle A can lie between 30.degree. 
and 35.degree., while Angle B can fall in the range 15.degree. to 
42.degree.. We have found it satisfactory to use a value of 0.06 mm for 
10, and 0.04 mm for 11. 
The process of the present invention is carried out by placing a plurality 
of the polypropylene concave mould members 2 on a flat surface. The 
concave mould members 2 are then partially filled with a moulding mixture 
containing as the major component preferably once distilled special grade 
hydroxy-ethyl methacrylate monomer (containing about 1% ethylene glycol 
dimethacrylate), a diluent which forms from 15% to 50% by weight of the 
mixture, a polymerisation initiator, preferably an organic initiator such 
as iso-propyl percarbonate, and a cross-linking agent selected from those 
mentioned above. Other materials, such as methacrylic acid and N-vinyl 
pyrrolidone, are preferably present to vary the nature of the final form 
of the hydrogel produced (their choice and the proportion of each that can 
be used is discussed in the more detailed examples below). Sufficient 
mixture is added so that when the glass convex moulds 1 are placed in the 
concave moulds 2 as shown in FIG. 1, the reservoirs 8 above the inside 
step 7 are substantially filled with mixture. The filled moulds are then 
placed in a circulating oven and cured for 1-4 hours as the temperature is 
raised to 99.9.degree. C., followed by secondary curing for 15-30 minutes 
at 120.degree. C. The mould parts can then be separated with the partially 
swollen lens adhered to the convex mould. Polymerisation of the moulding 
mixtures can take place at temperatures over the range 20.degree. C. to 
120.degree. C. for varying times, depending on the rate at which 
polymerisation is desired. 
After the mould members are separated, the convex mould members with the 
cast lenses adhering to them are placed in de-ionised water at 60.degree. 
C. until the lenses separate from the mould members. This takes about 10 
minutes. 
The lenses are then immersed in a weak alkaline solution to replace 
hydrogen by the alkali metal cation in carboxyl groups in the polymer 
matrix, and then in physiological saline solution. During this stage, the 
water content of the lenses is increased from of the order of 40% to a 
value in the range 60% to 75% which is related to the nature of the 
polymeriseable mixture and the curing conditions. The lenses have a water 
content difference between front and back surfaces of at least 3 
percentage points due to the use of glass and polypropy1ene for the convex 
and concave mould members, respectively. The lenses are then packed singly 
with saline solution in containers of the kind shown in FIG. 5, sealed, 
and the container and contents sterilised in an autoclave. Alternatively, 
the lenses may be autoclaved before packaging. Apart from providing 
sterilisation, autoclaving appears to tend to increase the water content 
difference between the lens surfaces. The containers after cooling are 
labelled with the lens characteristics. The lenses are then ready, after 
transmission to the wearer, to be transferred direct by the wearer from 
the container to the eye. The lenses are supplied as pairs to fit the left 
and right eye, and the packages can be marked with a tactile symbol to 
indicate which lens is for which eye. 
Each polymerised lens is clear as glass of good optical quality, fully 
transparent, soft, and resilient. It has an edge which blends smoothly 
with the cornea surface when the lens is positioned on the eye. The lens 
does not irritate the eyelid of of the wearer, nor the cornea and moves 
freely on the eye. 
The lens can be inserted by holding it on an applicator and placing it on 
the cornea of the open eye. The lens clings firmly to the eye surface, and 
centres itself on the cornea because of the almost perfect fit of the 
corresponding surfaces of lens and eye. A small difference between the 
curvature of a cornea and of a lens of the invention, as in cases of 
corneal astigmatism, is automatically compensated for by elastic 
deformation of the lens rim. 
Properly fitted contact lenses of the invention may be worn for long 
continuous periods without discomfort, and need only be removed after 3 to 
8 weeks of wear for disposal and replacement by a new lens from an 
unopened package. 
FIG. 5 shows a lens package containing two sets of lenses, each of the 
pairs (18, 18.sup.1, 19, 19.sup.1) of containers containing a single lens, 
and in each pair, a lens for the left eye, and a lens for the right eye. 
Each pair is accompanied by a disposal applicator (20, 20.sup.1) and space 
is provided for inserting the date of changeover, at 21 and 21.sup.1, 
along with a panel 22 for the patient's name. The individual lens 
containers can either be marked with a symbol indicating whether the lens 
is for the left or right eye, or the package may be marked to the side of 
each container. 
The accompanying Graph (FIG. 6) compares the flexibility of various 
commercially available high water content soft contact lenses with a lens 
made in accordance with the present invention. 
The displacement 1 in mm is plotted against the height h of saline solution 
column in mm producing that displacement. The curves plotted on the graph 
have been numbered and relate to the following lenses: 
______________________________________ 
Overall 
Water Content 
Centre Thickness 
% (mm) 
______________________________________ 
(1) Hydron 04 38 0.036 
(2) X-Ten 38 0.092 
(3) Lens according to 
73 0.104 
invention 
(4) Permalens 71 0.236 
(5) Danalens 68 0.124 
______________________________________ 
It will be seen that the lens according to the invention has a flexibility 
comparable with that of most of the other soft lenses with which it has 
been compared. 
Experiments comparing lenses commercially available with a lens according 
to the invention have shown clearly the basic dependence of tear strength 
on water content and thickness. Tear strength is determined by clamping 
the lens so that it acts as a seal to a column of saline solution. The 
pressure under the lens is increased by increasing the height of the 
column of saline solution, and the lens is pre-notched with a needle. The 
tear strength is recorded as the height of the saline column when the 
notch starts to propagate. Table I below compares a lens of the present 
invention which was made as described in Example 8 (below) with 6 
commercial lenses of varying thickness and water content. 
TABLE I 
______________________________________ 
Overall Lens 
Water Thick- Burst Tear 
Lens Type Content ness Strength 
Strength 
By Origin % mm (kpa) (mm Saline) 
______________________________________ 
"Permalens" 
71% 0.23 20 660 
"X-Ten" 70% 0.15 27 702 
"Igel" 64% 0.108 60 750 
"Hydrocurve II" 
55% 0.118 26 600 
"Hydron 04" 
38% 0.050 90 600 
"Danalens" 68% 0.144 10 230 
Lens of 75% 0.110 15 320 
Example 8 
(below) 
______________________________________ 
The above table shows that the lens of the invention is not as robust as 
all except one of the lenses with which it has been compared. This is 
desirable in order to ensure that a lens has life which results in 
replacement being needed within a predetermined time. All of the above 
lenses with an overall water content of above 64% will have what is 
considered to be a satisfactory oxygen transmissibility, as will the very 
thin "Hydron 04" lens. However the lens which is an example of the present 
invention will provide a particularly high rate of transfer of oxygen and 
consequently be capable of being kept in the eye for extended periods. 
The following examples illustrate but do not limit the invention. 
EXAMPLE 1 
Mould members 1 and 2 were manufactured to produce a lens having a power of 
-1.00 Dioptre and diameter of 13.8 mm. The convex mould members 1 were 
made by polishing and grinding glass. The concave mould members 2 were 
made by inJection moulding from polypropylene using a metal die having a 
convex shape, an outside diameter of 10.327 mm, a central radius of 
curvature of 6.986 mm and a peripheral radius of curvature of 6.924 mm, 
and optic zone diameter of 8.312 mm. The dimensions of a finished hydrated 
lens cast from mould members made in this way when swollen to a water 
content of 73% are given in Table II below, see lens No. 1. 
The dimensions of two lenses of different powers and diameters made by the 
process of the present invention are also shown in Table II. The 
dimensions quoted are identified in FIG. 4 (all lengths being in mm). 
TABLE II 
______________________________________ 
Lens Number 1 2 3 
______________________________________ 
Lens Power Dioptres 
-1.00 -3.00 -6.00 
Inside Diameter AB 
13.8 14.5 13.8 
Centre Curve Diameter OP 
11.0 11.0 8.13 
Periphery Radius C 
8.95 9.01 9.01 
Centre Radius D 9.04 9.51 10.33 
Optic Zone Curvature 
8.8 8.8 8.8 
(inside Radius) E 
Centre Thickness GF 
0.12 0.078 0.07 
Periphery Thickness HI 
0.147 0.21 0.213 
Edge Dimension 
JK 0.07 0.07 0.07 
LM 0.04 0.04 0.04 
Angle B 33.degree. 30.degree. 
33.degree. 
______________________________________ 
The moulds were manufactured to obtain lenses of the above dimensions, 
taking into account the changes of dimensions on the further hydration of 
the lenses after casting to reach a water content of 73%, and any changes 
in the dimensions of the concave mould parts during manufacture. 
Examples 2 to 12 below illustrate various changes that can be made in raw 
materials used, the proportions of such materials, the conditions chosen 
for polymerisation, and the overall and surface water contents of the 
lenses. It should be noted that overall water content was measured by 
dehydrating the lens and measuring the loss in weight, which can give a 
low figure due to the difficulty of driving off the last few percent of 
water. The surface water contents were measured using an Abbe 
Refractometer. 
EXAMPLE 2 
A lens was formed from a monomer portion as listed under A below, and with 
diluent portion as listed under B. The proportions used for A:B were 85:15 
parts by weight. 
______________________________________ 
A. Hydroxy ethyl methacrylate (HEMA) 
90% 
N--vinyl pyrrolidone (NVP) 
4.5% 
Methacrylic acid (MAA) 3.6% 
TMT (Trimethylol Propane Trimethacrylate) 
1.7% 
Isopropyl percarbonate (SIP) 
0.2% 
B. Diluent = water alone. 
______________________________________ 
The monomer mixture was degassed and filled into a series of moulds, made 
in the same manner as in Example 1, before curing and cured by raising the 
moulds and their contents from 75.degree. C. to 99.9.degree. C. in 1 hour, 
followed by a secondary curing at 120.degree. C. for 15 minutes. The 
convex mould carrying the lens was immersed in a 2% solution of sodium 
bicarbonate for 30 minutes. On removal the lens was found to have an 
overall water content of 60% and a tear strength of 400 mm saline at a 
thickness of 0.1 mm. At the lens surfaces, the water contents as measured 
by an Abbe Refractometer were: Front 76%, Rear 61%. 
EXAMPLE 3 
This was carried out in the same manner as Example 2, except that the 
diluent portion B used was a mixture of water and glycerol in 1:1 
proportions, and the ratio of A:B was 85:15. The lens produced had a tear 
strength of 400 mm saline at a thickness of 0.1 mm and an overall water 
content of 61%. Front surface water content was 79% and rear surface water 
content 62%. 
EXAMPLE 4 
This was carried out in the same manner as Example 3, except that the ratio 
of A:B was 63:37. The lens produced had a tear strength of 380 mm saline 
at a thickness of 0.1 mm and a water content of 68%. Front surface water 
content was 79% and rear 68%. 
EXAMPEL 5 
This was carried out in the same manner as Example 2, except the A portion 
was: 
______________________________________ 
HEMA 85% 
NVP 6.8% 
MAA 5.9% 
TMT 1.7% 
Allyl Methacrylate 
0.4% 
SIP 0.2% 
______________________________________ 
Methylmethacrylate (MMA): 4% by weight of A portion. 
The B portion was water. 
The ratio of the portions used was: 
(A+MMA) B=63:37 
The finished lens had an overall water content of 73%, and a tear strength 
of 320 mm saline at 0.1 mm thickness. Front surface water content was 84% 
and rear 74%. 
EXAMPLE 6 
This example only differed from Example 5 in that (1) the B diluent portion 
was a mixture of water and glycerol in the proportion 1:2, (2) the ratio 
of (A+MMA):B was 63:37 and (3) the method of releasing the lens from the 
mould. The finished lens had an overall water content of 69% and a tear 
strength of 320 mm saline at 0.1 mm. Front surface water content was 85% 
and rear 70%. While the tear strength of the lens was of the same order as 
the lens of Example 9, the lens was removed more rapidly from the mould by 
immersion in de-ionised water for 10 minutes at 60.degree. C. The lens was 
treated with 2% sodium bicarbonate solution after removal from the mould 
surface. 
EXAMPLE 7 
This Example was carried out in the same manner as Example 6, except that 
the B portion was water and glycerol in the ration 1:4 by weight and the 
ratio of (A+MMA):B was 45:55. The finished lens had a tear strength of 280 
mm saline at a thickness of 0.1 mm and an overall water content of 74%. 
Front surface water content was 90% and rear 75%. 
EXAMPLE 8 
This Example was carried out to demonstrate the use of ethylene glycol as 
the component B in various proportions as the only variant in the 
procedure of Example 8. The results obtained at various proportions of (A 
+MMA):B are given in the Table below. 
______________________________________ 
Tear Strength 
Overall Surface 
(mm saline) Water Water Content 
(A + MMA):B 
0.1 mm thickness 
Content Front Rear 
______________________________________ 
(a) 75:25 300 68% 91 67 
(b) 63:37 280 75% 88 76 
(c) 40:60 140 77% 87 76 
______________________________________ 
This Example shows the importance of avoiding excessive amounts of diluent 
as the water content rises drastically giving a poor tear strength as the 
proportion of diluent is increased. 
EXAMPLE 9 
Example 6 was repeated with a modified curing cycle, the mould and its 
contents were heated from 40.degree. C. to 99.9.degree. C. over four 
hours, and then given a secondary cure at 120.degree. C. for 15 minutes. 
No significant difference was detectable in the measured properties of the 
finished lens of the Example from those measured for the lens of Example 
6. 
EXAMPLE 10 
This was carried out in the same manner as Example 2, except that the lens 
was removed by the water treatment and treated with alkali in the manner 
of Example 6, and the A portion was as follows: 
______________________________________ 
HEMA 84.5% 
NVP 6.8% 
MAA 6.8% 
TMT 1.7% 
SIP 0.2% 
______________________________________ 
The finished lens had a water content of 61% and tear strength of 300 mm 
saline at a thickness of 0.1 mm. Front surface water content was 76% and 
rear 60%. 
EXAMPLE 11 
This Example only differs from Example 10 in that the borate ester of 
glycol was used instead of water as a diluent. No measureable difference 
could be detected in the finished lens. 
EXAMPLE 12 
This was carried out in the same manner as Example 10 with different 
composition for A. 
The A portion was: 
______________________________________ 
HEMA 84.7% 
NVP 6.8% 
MAA 5.9% 
Polyethylene glycol (400) dimethacrylate 
1.9% 
Allyl methacrylate 0.4% 
SIP 0.3% 
______________________________________ 
The finished lens had a tear strength of 300 mm saline, an overall water 
content of 77% and a thickness of 0.1 mm. Front surface water content was 
96% and rear 68%. 
The above examples and other experimental work have demonstrated that the 
mechanical strength of the lens shows relatively little change for changes 
in concentration of SIP from 0.1 to 0.4%, curing times as varied in the 
Examples above, and change in the nature of the diluent. We have found 
from our experimental work that it is preferable to choose the conditions 
and proportions of raw materials which result in the lens having an 
overall water content of at least 60% and not more than 75%. At 0.1 mm 
thickness, we need to achieve a tear strength of at least 280 mm saline 
and preferably the tear strength should not fall below 320 mm saline at 
0.1 mm thickness. A combination, e.g. of a water content of 77% and a tear 
strength of 140 mm of saline as obtained in Example 8(c) is unacceptable 
as a saleable lens due to its low fragility. 
Oxygen transmissibility is related to both lens thickness and water 
content. Lenses made by the process of the present invention with a centre 
thickness of 0.086 to 0.142 mm will have an oxygen transmissibility higher 
than previous soft contact lenses and as long as the water content falls 
in the preferred range 60 to 75% no problems are likely to be experienced 
with oxygen permeability. 
Examples 1 to 12 demonstrated the use of varying casting mixes, and curing 
conditions while always using a glass convex mould and a polypropylene 
concave mould. 
The following examples demonstrate the effects of changes in curing 
temperature, mould material, and hydration method on the surface water 
content of the cured polymer. As in examples 1 to 12, the casting mix was 
made up from component A and component B. Component A was in all cases 
made up as follows: 
______________________________________ 
HEMA 81.56% 
MAA 5.61% 
NVP 6.52% 
Trimethylol Propane 
1.63% 
Methacrylate 
Allyl Methacrylate 
0.42% 
SIP 0.33% 
Methyl Methacrylate 
3.83% 
______________________________________ 
In all except Example 13, the proportion of components A:B was 60.08:39.92. 
In the case of Example 13, the proportion of A:B was 63:37. 
Component B was a mixture of glycerol and water in the proportions 1 part 
water to 2 parts glycerol. 
The curing conditions used when referred to as standard were 1.25 hours in 
a heated circulating air oven as the temperature is raised from 65.degree. 
C. to 99.9.degree. C., followed by 30 minutes at 119.degree. C. The glass 
moulds with lenses adhering were placed in deionised water at 60.degree. 
C. for up to ten minutes to allow the lenses to float free. The standard 
hydration conditions were immersion in a 2% solution of sodium bicarbonate 
at 68.degree. C. for 30 minutes. 
EXAMPLE 13 
5 lenses cast using convex glass and concave poly-propylene moulds and the 
standard curing and hydration conditions in February 1985 and then stored 
in saline were removed from the saline solution in December 1985 and their 
surface water content measured using an Abbe Refractometer. The average 
values obtained from measurement of the five lenses was: 
______________________________________ 
nd % H2O 
______________________________________ 
Front lens surface (cast in contact with 
1.365 81.3 
polypropylene) 
Rear lens surface (cast in contact with 
1.3835 70.2 
glass) 
______________________________________ 
EXAMPLE 14 
8 lenses were cast, cured and hydrated in January 1986 in the same manner 
as the lenses of example 13, except the quantity of diluent was 40%. 
The average values for the 14 lenses were as follows: 
______________________________________ 
nd % H2O 
______________________________________ 
Front Lens surface 1.363 82.5 
Rear Lens surface 1.3818 72.6 
______________________________________ 
EXAMPLE 15 
In order to evaluate the use of flat plates as a means of identifying 
satisfactory mould surfaces, a series of commparative tests were carried 
out. These involved casting the monomer mix between: 
(a) 2 glass plates 
(b) 2 polypropylene plates 
(c) 2 glass plates--cured with U.V. light for 1 hour with an irradiance of 
765 W/M.sup.2. 
(d) glass plate and polypropylene plate 
(e) glass plate and polypropylene plate cured for 24 hours in water at 
50.degree. C. 
(f) glass plate and polypropylene plate cured with U.V. light for 1 hour 
with an irradiance of 765 W/M.sup.2. 
(a), (b) and (d) were cured in the standard manner. 
The monomer was placed in a space between the two plates defined by a 
plastic gasket, and the sheets clamped together by spring clips. 
The results of surface measurement of the cured material after hydration in 
the monomer described above were as follows: 
______________________________________ 
nd % H2O 
______________________________________ 
(a) 2 glass plates 1.3895 66.8 
1.390 66.6 
(b) 2 Polypropylene plates 
1.376 74.8 
1.376 74.8 
(c) 2 Glass plates (U.V. Cure) 
1.383 70.5 
1.383 70.5 
(d) Glass plate 1.3865 68.6 
Polypropylene plate 1.376 74.8 
(e) Glass plate 1.3775 74 
Polypropylene plate 1.3445 79 
Water cure at 50.degree. C. for 24 hours 
(material undercured when 
measurements taken) 
(f) Glass plate (U.V. Cure) 
1.38 72.4 
Polypropylene plate 1.356 87.2 
______________________________________ 
The first three tests show how with the same surfaces under the same 
conditions, approximately the same water content is achieved. The last 
three simply demonstrate that even under different curing conditions, the 
values obtained will be different but that different mould surfaces will 
still produce a differing solids/water content. The examples also 
demonstrate the value of the use of flat casting surfaces as a means of 
comparing one casting surface with another. 
EXAMPLE 16 
A series of experiments were carried out to examine the effect of 
alternative methods of treating a glass/polypropylene mould assembly after 
curing in a situation where the monomer mix was cured in the standard 
manner but the secondary cure step of 30 minutes at 120.degree. C. was 
replaced by an alternative procedure and hydration methods were varied. In 
all of the following examples when the mould was opened the lens was found 
to adhere to the polypropylene mould. The conditions used and the results 
obtained were as follows: 
______________________________________ 
nd % H2O 
______________________________________ 
(a) Assembly soaked in water 
Front 1.359 
85.4 
overnight at 65.degree. C., opened 
Rear 1.376 
74.8 
and hydrated in standard manner 
(b) As (a) but hydrated in 
Front 1.385 
69.4 
saline after opening 
Rear 1.405 
60.5 
(c) Assembly soaked in water 
Front 1.355 
87.8 
at room temperature over 
Rear 1.379 
73.0 
night, opened and hydrated 
in standard manner 
(d) As (c) but hydrated in 
Front 1.376 
74.8 
saline after opening 
Rear 1.402 
60.6 
______________________________________ 
The above examples demonstrate that both the method of removing the lens 
from the mould, and the method of hydration can cause a difference in 
overall average water content, but that there seems to be little influence 
on the differential in water content when one compares (a) to (b) and (c) 
to (d). This demonstrates the major importance of the casting surfaces in 
determining the difference in water content between the two cast surfaces. 
The removal of the lens from the concave polypropylene mould was more 
liable to result in damage to the lens and we therefore prefer to operate 
when using a glass convex mould and a polypropylene concave mould with a 
secondary cure at about 120.degree. C. so as to cause the assembly to 
separate with the lens adhering to the glass convex mould. 
EXAMPLE 17 
In order to determine whether there was any possibility of a temperature 
effect during curing influencing the water content of the surfaces, two 
castings were carried out where the polymer surfaces temperatures were 
measured during curing. In both cases glass and polypropylene plates were 
used as the casting surfaces, with in one case the cure being in a water 
bath at 40.degree. C. for 36 hours, and the other in air at 40.degree. C. 
for 36 hours. The cast plates were hydrated in the standard manner and the 
results of measurements with the Abbe refractometer were as follows: 
______________________________________ 
nd % H2O 
______________________________________ 
Water cure Polypropylene 1.352 89.6 
Glass 1.3775 74.0 
Air cure Polypropylene 1.345 79.0 
Glass 1.379 73.0 
______________________________________ 
No significant difference in polymer surface temperature was detected 
during either curing cycle. 
EXAMPLE 18 
The following examples illustrate the use of different plates under 
standard conditions: 
______________________________________ 
nd % H2O 
______________________________________ 
(a) Nylon 1.371 78.9 
Polypropylene 1.362 83.5 
(b) Glass 1.379 73.0 
Polyethylene (additives unknown) 
1.377 74.2 
(c) Glass 1.3791 73 
Polytetrafluorethylene 
1.367 80.4 
______________________________________ 
The surface cast against the polytetrafluorethylene plate was difficult to 
measure and the value given is approximate. 
______________________________________ 
(d) GLASS A 1.3783 73.5 
GLASS B 
GLASS A was twice the thickness of 
GLASS B 
______________________________________ 
It can be seen from the above that is was possible to obtain A substantial 
difference in water content by casting between nylon and polypropylene, 
and between glass and PTFE, but not between glass and the particular 
sample of polyethylene. It is important therefore when considering the use 
of alternative mould surfaces to evaluate the material by casting a test 
piece in the manner described above and comparing performance with e.g. a 
test piece cast between glass and polypropylene. 
As the result of the measurements made on the sample produced by casting 
between plates of glass and polyethylene in Example 18(b) was to give a 
difference in refractive index value which could have been due to the 
experimental error in making one or both of the measurements, three 
concave moulds were injection moulded from high density polyethylene 
(Hoechst GA 7260H) and used with glass convex moulds to cast lenses in 
otherwise the same manner as the lenses of Example 14. The results 
obtained were as follows: 
______________________________________ 
Water 
nd Content 
______________________________________ 
Mould pair (a) 
Polyethylene 
1.374 76 
Glass 1.376 75.8 
Mould pair (b) 
Polyethylene 
1.371 77.8 
Glass 1.374 76.0 
Mould pair (c) 
Polyethylene 
1.3700 78.5 
Glass 1.3765 75.5 
______________________________________ 
Comparison with Example 14 shows that the glass/polypropylene mould pairs 
produce a major difference in refractive index compared to the difference 
produced even in the case of Mould pair (c). Polyethylene of the grade 
used is clearly not equivalent in performance to polypropylene, and while 
it is possible some forms of polyethylene may produce a useable lens, its 
use is clearly not to be preferred over polypropylene.