Optometry in Practice Vol 4 (2003) 101 - 115
Hydrogel Lenses – Materials and Manufacture:
A Review
C Maldonado-Codina
MSc PhD MCOptom FAAO
& N Efron PhD DSc MCOptom FAAOD
Department of Optometry & Neuroscience, UMIST, Manchester, UK
Accepted for publication 17 June 2003
Introduction
Hydrogel (or ‘soft’) lenses have had a huge impact on the
contact lens market since they were first introduced in the
late 1960s. In the UK, a recent survey has indicated that
they currently make up 92% of all new contact lens fittings
(Morgan & Efron 2002). This trend is also echoed around
the world (Morgan et al. 2002). These lenses are termed
‘soft’ since they are made from water-swollen, cross-linked,
hydrophilic polymers.
Polymers are solid materials made from high-molecularweight chains, and consist of repeating units known as
monomers. Materials made from one monomer are termed
homopolymers and materials made from more than one
type of monomer are termed copolymers. Their long chain
length, the kinds of atoms they are made up of (eg carbon,
oxygen, hydrogen, nitrogen), their geometric arrangement
and the stability of the bonds created all combine to give
polymers unique and distinctive properties which in turn
dictate the material’s particular use and function.
In the dehydrated state most hydrogels are hard and
brittle. However, because the polymer network in a
hydrogel contains hydrophilic (water-loving) groups, it
swells in water, which causes it to become soft and to take
on elastic properties (ie the water acts as a plasticiser).
The polymers are cross-linked in order to give them
increased physical stability. Cross-links can be thought of
as a chemical bridge that links one chain to another. In
addition to cross-links, chains are also connected to each
other by chain entanglements and loops which occur as a
result of the macromolecular order of these amorphous
polymers. The polymer chains intermingle randomly,
rather like individual strands of wool being rolled up
together to form a ball.
A hydrogel polymer must possess certain physical
properties if it is going to be suitable as a contact lens
material. These include:
•being optically transparent
•having a refractive index similar to that of the cornea,
ie approximately 1.37
•being sufficiently oxygen-permeable
•having sufficient hydraulic permeability
•having sufficient dimensional stability
•having adequate mechanical properties
•being biocompatible in the ocular environment
Conventional Hydrogel Materials
Contemporary hydrogel materials can be conveniently
divided into two groups: firstly, conventional hydrogel
materials (now often referred to as low-Dk materials: note
that for the purposes of this review, the term ‘conventional’
should not be thought of as meaning non-disposable) and
secondly, silicone hydrogels (high-Dk materials).
Hydrogel lenses were born thanks to the extraordinary
pioneering efforts of Professor Otto Wichterle and
Dr Drahoslav Lim of the Institute of Macromolecular
Chemistry of the Czechoslovak Academy of Sciences
in Prague in the mid-1950s. The story of how such a
significant breakthrough in the history of contact lens
development came about has been well documented.
Essentally, Wichterle and Lim were working on the
synthesis of a new material that they hoped could be used
for implantation into the human body (Wichterle & Lim
1960). That material was poly(hydroxyethyl methacrylate)
or PHEMA. They soon realised that the material had
potential applications in the manufacture of contact lenses
but were prevented from researching such a project by the
directors of the Institute, who perceived this work as being
a petty distraction from fundamental studies in chemistry.
Wichterle was forced to carry out his experiments at home.
Despite such difficult circumstances, he was triumphant,
producing the first spun-cast lens (made from his son’s toy
construction set) in 1961 (Wichterle & Lim 1961). The
enormity of his breakthrough for the contact lens industry
cannot be understated.
PHEMA (Figure 1) is made by polymerising 2-hydroxyethyl
methacrylate monomer with a cross-linker such as
ethylene glycol dimethacrylate (EGDMA) (Figure 1).
Most of the hydrophilic behaviour of PHEMA is due to the
hydroxyl group (OH). At this location hydrogen bonding
Address for correspondence: C Maldonado-Codina, Department
of Optometry & Neuroscience, UMIST, PO Box 88, Manchester
M60 1QD, UK.
© 2003 The College of Optometrists
101
C Maldonado-Codina & N Efron
with water molecules occurs, causing them to be drawn
into the polymer matrix. The result is that contact
lenses made from PHEMA contain approximately 38–40%
water in the fully hydrated state. This property is what is
commonly termed the equilibrium water content (EWC)
of a hydrogel and is defined as:
EWC =
weight of water in polymer
total weight of hydrated polymer
x 100
The EWC of a hydrogel may vary with temperature, pH
and osmolality but in PHEMA hydrogels, this variation is
minimal.
PHEMA lenses were first distributed in western Europe in
1962 but sales were disappointing. In 1965 the National
Patent Development Corporation (NPDC) bought the
licence for the American rights to the technology from
the Czechs. This was subsequently sold on to Bausch &
Lomb (B&L), who at that time manufactured ophthalmic
equipment and spectacle lenses. B&L dramatically refined
Wichterle’s spin-casting process and finally obtained
approval from the Food and Drug Administration (FDA)
for their PHEMA lenses in 1971. This time, the lenses
became very popular very quickly – both practitioners
and patients enjoyed the benefits of increased comfort,
reduced adaptation time and easier fitting procedures.
With time, more and more companies developed their own
PHEMA lenses and soon enough it became obvious that
these lenses too would cause ocular complications. Most
of the problems stemmed from the fact that the lenses
caused hypoxia but other problems relating to solution
toxicity and lens spoliation were also common.
Figure 1. Conventional hydrogel monomers.
-11
Dk (barrer) =
Oxygen permeability is essentially governed by EWC in
conventional hydrogels. This occurs since oxygen is able to
pass through the water rather than through the material
itself. Oxygen permeability is described as the Dk, where
D is the diffusivity of the material and k is the solubility of
the material. The relationship between EWC and oxygen
permeability has been found to be (Morgan & Efron 1998):
Dk=1.67e 0.0397EWC
where e is the natural logarithm. Oxygen transmissibility
is a property of a lens and is defined as the oxygen
permeability of the material divided by the lens thickness
(t). The units of Dk are known as Fatt units (after
Professor Irving Fatt) or Barrer:
Dk
t
10 (cm2 x mlO2)
sec x ml x mmHg
-9
Dk (barrer/cm) =
10 (cm x mlO2)
sec x ml x mmHg
Contact lens manufacturers, therefore, had two possible
avenues to follow to increase the oxygen transmissibility
of lenses: develop hyperthin lenses or develop materials
with higher water contents. Producing lenses which were
thinner was a relatively straightforward matter for lens
designers and several such lenses were launched, eg. the
Hydrocurve thin lens (Soft Lenses, Inc.) in 1977 and
subsequently the O3 Series (B&L). These lenses were in the
region of 0.035–0.06mm thick, which was less than half the
thickness of the original B&L PHEMA lenses.
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Optometry in Practice
Developing materials with higher EWC led to HEMA
copolymers. One of the first successful copolymerisations
was with N-vinyl pyrrolidone (NVP) (Figure 1). The amide
(N-C=O) moiety is very polar and two molecules of water
can become hydrogen-bonded to it. Hydrogels with EWC of
up to about 90% have been produced.
NVP-based copolymers lose the slippery feel of PHEMA and
consequently can feel quite rubbery. These copolymers
also tend to have relatively high evaporation rates of water,
which may be seen as a problem for lens stability and
comfort. This occurs because the amide group does not
bind water as strongly as a hydroxyl group. In addition,
these polymers are also temperature-sensitive, ie their
parameters tend to change with increasing or decreasing
temperature. This is important when fitting a lens as its
parameters may change on-eye. NVP-based lenses have
also been associated with increased corneal staining
and decreased comfort when used in conjunction with
solutions containing higher levels of polyhexanide (Jones
et al. 1997). This does not mean that polyhexanide-based
solutions cannot be used with NVP-containing lenses,
rather the interaction should be borne in mind and if
any significant corneal staining or discomfort symptoms
arise, these can usually be treated simply by changing the
solution to one containing a lower level of polyhexanide or
one free from polyhexanide.
At about the same time as the patents for NVP-based
polymers were being filed, development was continuing
in a different direction. A useful combination of methyl
methacrylate (MMA) and VP was discovered (Tighe 1989).
MMA is familiar to most contact lens practitioners as the
material hard lenses are made from (PMMA; Figure 1). When
these two monomers were copolymerised into an MMA/VP
copolymer, a completely new material was obtained with
very different characteristics to the HEMA/VP copolymer.
Depending on their composition, contact lenses made from
MMA/VP copolymers can contain 60–85% water.
Another hydrophilic monomer that has been very
successfully used in contact lens hydrogels is methacrylic
acid (MAA) (Figure 1). When added to a soft lens polymer
formulation, it results in a soft lens with ionised groups
(negatively charged) within the polymer matrix, allowing
the lens to absorb more water. The higher the amount of
MAA, the higher the EWC of the resulting polymer or lens.
Amounts of MAA in the region of 1.5–2.5% will increase
the water content of a HEMA material into the mid
water content range of 50–60%, thereby allowing oxygen
permeability to increase significantly.
Once HEMA/MAA lenses have been manufactured they need
to be ionised (ie the hydrogen atom in the carboxyl group
is removed). The conversion of the carboxyl group (CO2H)
to the more hydrophilic ionised form (the carboxylate
anion, CO2-) produces an increase in water content. This
is commonly achieved by washing the lenses in sodium
bicarbonate solution.
During this bicarbonate wash, the carboxylic acid groups
are neutralised, as illustrated in the following equation:
R-CO2H + NaHCO3
R-CO2Na + H2O + CO2
Unlike the unreacted methacrylic acid moieties, the
sodium methacrylate groups are ionised, with the negative
methacrylate ions being counterbalanced by the positive
charge in the sodium ions. However, this negative charge
causes the carboxylate ions to repel each other – often
referred to as ‘expanding the network’. This has the effect
of allowing the network to take in more water. If the
polymer was not ionised or if this step were omitted, then
the material would have an unexpanded network whose
water content was similar to that of PHEMA.
Unfortunately, using MAA to increase the water content of
a polymer also has its disadvantages. These include:
•a lens which is extremely sensitive to changes in
tonicity (McCarey & Wilson 1982, McKenney 1990).
The Na+ ions present in saline solution have the effect
of ‘shielding’ the carboxylate anions. In hypotonic
solutions (eg pure water), since these shielding ions are
present to a far lesser degree, more chain repulsion will
occur, which increases the swelling of the network and
consequently the EWC of the material. In hypertonic
solutions, the reverse situation occurs and the material
network shrinks, causing its EWC to decrease.
•a pH-sensitive lens (McCarey & Wilson 1982, McKenney
1990). If the pH of the solution in which the lens is
immersed is decreased (ie the hydrogen ion
concentration is increased), the carboxylate anions are
more shielded and the network becomes less expanded.
This will cause a decrease in the lens EWC.
•a very significant level of protein build-up both on the
lens surface and within the lens matrix (Maissa et al.
1998, Sack et al. 1987).
•dimensional instability when the lens is heat-disinfected
(McCarey & Wilson 1982).
Glyceryl methacrylate (GMA) (Figure 1) is more
hydrophilic than HEMA due to the fact that the monomer
contains two hydroxyl groups. This monomer has been
used in contact lens materials in two main ways. The
first method has used GMA in combination with MMA to
produce materials which have water contents in the range
of 30–42%. These materials are thought to be stiffer and
stronger than PHEMA hydrogels. An example of such a
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material is crofilcon A, which is used in the CSI™ lens
(CIBA Vision).
The second method has been to use GMA in combination
with HEMA to produce a high-water non-ionic contact lens
material (up to approximately 70% has been possible).
These contact lenses claim to show a low rate of dehydration
and a rapid rate of rehydration, i.e. they have good water
balance ratios (Benz & Ors 2000). In addition, the
materials are thought to be relatively deposit-resistant and
seem to be relatively insensitive to pH changes in the range
of pH 6–10.
In the early 1970s John de Carle proposed that if the EWC
of hydrogel lenses could be sufficiently increased, then
these lenses could be worn successfully on an overnight or
continuous basis. He developed the first extended-wear lens
to be distributed in the UK (1975) known as the Permalens
(de Carle 1975). The lens material had an EWC of 71% and
was made from a HEMA/VP/MAA copolymer. In 1981 the lens
was given FDA approval for extended wear of up to 30 days
along with another lens, the Hydrocurve II (Wesley Jessen).
Slowly, other lenses were given approval for extended wear
during the 1980s but along with the increase in demand for
these lenses, so too came an increase in complications. In
1989 studies were published which showed that the risk of
microbial keratitis was 5–15 times greater for extended wear
than daily wear (Poggio et al. 1989, Schein et al. 1989). As a
result, the FDA recommended that extended wear be limited
to six consecutive nights and with that, the enthusiasm for
extended wear died down – that is, until the emergence of
silicone hydrogel lenses in the late 1990s.
Silicone Hydrogels
Holden & Mertz (1984) defined the critical oxygen levels
in order to avoid corneal oedema for daily and extended
wear. They concluded that 24.1Barrer/cm was the oxygen
transmissibility required for daily wear and 87Barrer/cm
was that required for overnight wear. These values have
been re-evaluated by Harvitt & Bonanno (1999), who found
that the minimum oxygen transmissibility required to
avoid anoxia throughout the entire cornea was 35Barrer/
cm for the open eye and 125Barrer/cm for the closed eye.
Figure 2 shows the relationship between the EWC and
the Dk of conventional hydrogels. From the graph, it is
obvious that there is an upper limit to how much oxygen
permeability can be attained simply by increasing the
EWC of conventional hydrogel materials. A hydrogel
with a theoretical EWC of 90% and a central thickness
of 0.1mm would have an oxygen transmissibility in the
region of 60Barrer/cm, which still falls far short of that
required to avoid additional overnight corneal oedema.
Such a lens would need to be in the region of 0.06mm
Figure 2. Relationship between Dk and equilibrium water content for conventional and
silicone hydrogels (redrawn from Tighe 2000).
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Optometry in Practice
Figure 3. Silicone-based materials.
thick, which is unrealistic from both a manufacturing
and a clinical point of view (Holden et al. 1986).
If reducing the thickness of lenses made from conventional
hydrogels was not an option for achieving success in
extended wear, then polymer scientists had to come
up with an altogether new kind of material. Silicone
rubber (polydimethyl siloxane: PDMS; Figure 3) has been
used with limited success as a contact lens material
for some time in the form of silicone elastomer lenses
(Dow Corning Corporation 1967). These lenses can be
thought of as having material properties in between
rigid and soft materials and have not become popular
mainly due to the intractable problem of lens tightening
and poor surface wettability. PDMS has an oxygen
permeability in the region of 600Barrer but is unwettable
by tears and needs to be surface-treated. Silicone,
however, has been very successfully incorporated into
rigid lens materials. In 1974, Norman Gaylord at Polycon
Laboratories successfully developed the first siloxanebased rigid lens material – merging the properties of
MMA with the increased oxygen performance of silicone
rubber (Gaylord 1974). In particular, a material that
he successfully copolymerised with MMA was a silicone
acrylate, commonly referred to as trimethylsiloxy silane
(TRIS: Figure 3). His work still forms the basis of most
existing rigid materials used in the production of contact
lenses today.
The patent literature shows that combining silicone with
hydrogel monomers has been a goal for polymer scientists
for some time. In theory the principle was simple; however,
the difficulty lay in actually achieving the combination in
the laboratory. Many years of hard work eventually led to
the launch of two silicone hydrogels for continuous wear –
the PureVision™ lens (B&L) in 1999 (UK) and the Focus®
Night & Day™ lens (CIBA Vision: 1999 UK).
The PureVision™ lens (balafilcon A) has an EWC of
36% and an oxygen transmissibility of 110Barrer/cm
(at –3.00D). The material is based on a carbamatesubstituted TRIS-based material known as TPVC (Figure
3). The TPVC is then copolymerised with NVP (Figure
1) to form the balafilcon material. The Focus® Night
& Day™ lens (lotrafilcon A) has an EWC of 24% and an
oxygen transmissibility of 175Barrer/cm (at –3.00D)
and is described as ‘biphasic’. The formulation of the
lotrafilcon material takes a rather different approach to
that of the balafilcon material. Tighe describes the lens
as being a fluoroether macromer copolymerised with
TRIS and dimethyl acrylamide (DMA) in the presence of
a diluent (Tighe 2000). Its biphasic structure means that
oxygen and water permeability channels are not reliant on
each other. The silicone-containing phase allows passage
of oxygen whilst the water phase primarily allows the lens
to move.
Without further treatment, both of these lenses would
be unsuitable for wear due to the fact that the resultant
material surfaces are not wettable by the tears. To this
end, both lenses are surface-treated using gas plasma
techniques. High-energy gases or gas mixtures (the
plasma) are used to modify the lens surface properties
without changing the bulk properties. The result
for the balafilcon lens is that surface wettability is
gained via plasma oxidation which produces glassy
silicate islands on the lens surface (Tighe 2000). The
lotrafilcon lens is coated with a dense 25nm-thick
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C Maldonado-Codina & N Efron
coating. Both resultant surfaces have low molecular
mobility which minimises the migration of hydrophobic
silicone groups to the surface.
Another important difference between these materials
and conventional hydrogels is the fact that they have
significantly greater elastic moduli, ie they are stiffer. Such
mechanical characteristics mean that the lenses are easy
to handle but have also been implicated in the aetiology
of a number of clinical complications observed with these
lenses.
The development of these materials has highlighted a
new issue – the importance of the so-called hydraulic
permeability or water transport of a contact lens material.
Essentially, a minimum level of hydraulic (as well as
ionic) permeability is necessary to maintain adequate lens
movement. This is important in allowing the postlens tear
film to reform between blinks, thus reducing the likelihood
of these quite elastic lenses from binding to the cornea.
Both of the new silicone hydrogel lenses achieve this
minimum hydraulic permeability, albeit in quite different
ways.
Polymerisation
Initiation
The monomer mixture must contain an initiator. This is a
chemical whose role is to start off the chemical process.
These substances readily fragment into free radicals when
activated by heat or some other form of radiation, eg
ultraviolet (UV) light. This is schematically represented
by the following equation, where I represents the initiator
molecule and I represents a free radical.
∆
I-I The active radicals formed are then able to combine
with the monomer (M), resulting in a free radical of the
monomer (this is why the polymerisation of hydrogels is
sometimes referred to as free radical polymerisation):
I• + M
Chain polymerisation
Chain polymers are formed by the reaction of the
monomeric units with each other, without the
elimination of small byproduct molecules. The monomer
concentration, therefore, decreases steadily with time,
resulting in a reaction mixture that contains monomer,
high-molar-mass polymer and a low concentration of
growing chains.
The monomers used in chain polymerisation are
unsaturated and are sometimes referred to as vinyl
monomers. Essentially this means that the monomer
has one or more carbon-to-carbon double bonds. Chain
polymerisation is characterised by three distinct stages:
initiation, propagation and termination.
IM•
Propagation
The monomer radical, which is a transient compound, is
now able to combine with (ie add to) another monomer
unit, resulting in another new compound:
IM• + M
Polymerisation describes the chemical reaction which
monomers undergo in order to form long-chain
polymers. Broadly speaking, polymerisaton reactions can
be classified into two types: step (condensation) and
chain (addition) processes. Condensation polymers are
produced by the reaction of monomeric units with each
other, with the elimination of a small molecule such as
water. Hydrogels are not generally formed through this
method of polymerisation but through addition (chain)
polymerisation.
2I
IMM•
By the continuation of this process, the polymer chain is
propagated. The resultant chain may consist of thousands
of monomer units:
IMn• + M
IM•
(n+1)
Termination
Polymerisation does not usually continue until all of the
monomer has been used up because the free radicals involved
are so reactive that they inevitably find a variety of ways of
losing their reactivity. Polymerisation can be terminated
in two main ways. The first method is recombination.
This occurs when two growing molecules containing free
radicals meet, share their unpaired electrons and so form a
stable covalent bond, thereby extinguishing their reactivity
(shown schematically below). The growing polymer chain is
represented by the symbol P.
IP• + IP•IP – PI
The second method of termination is known as
disproportionation. This occurs when two radicals interact
via hydrogen abstraction, leading to the formation of two
reaction products, one of which is saturated and one of
which is unsaturated:
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Optometry in Practice
PCH2CH2• + PCH2CH2•
PCH2CH3 + PCH=CH2
As initiation, propagation and termination steps are taking
place during chain polymerisation, other reactions can
also occur because of the reactivity of the free radicals.
These processes include chain transfer and free radical
combination with retarders or inhibitors.
Polymerisation considerations in different methods
of manufacture
Lenses made by different methods of manufacture will
undergo very different polymerisation conditions which
may have an effect on the resultant material. The way in
which a material is processed is likely to affect almost
every aspect of a lens, from its clinical performance to its
physical and chemical characteristics.
Lathe-cutting
Lathed lenses are formed from solid buttons of dehydrated
material. The buttons are bulk-polymerised over relatively
long periods, usually in water tanks. Thermal initiators are
used which have low activation energies, therefore allowing
water baths or ovens to be set to lower temperatures. This
type of polymerisation is likely to lead to longer chains
(higher molecular weights) and, therefore, more chain
entanglements.
During polymerisation, the walls of the polymerisation
vessel may hinder the supply of fresh monomer. This may
cause the molecular weight at the surfaces of the button
to be decreased relative to the bulk. In addition, some
oxygen degradation may occur at the surfaces which
will further add to this surface degradation. However, a
button will have a relatively high volume to surface ratio
which means that the surfaces can be discarded during
the lathing process and the lens can be formed from the
centre of a button, thus obviating the effects of surface
degradation.
Spin-casting
Many manufacturers incorporate a diluent into the
monomer mixture during the spin-casting process. This will
affect what are known as the gel point and the vitrification
point of the polymer. Essentially, this promotes more
complete polymerisation since the monomers have better
access to the growing polymer chains. The diluent is also
used as an aid to demoulding and can also be used to
change the swell factor of the material.
Spin-casting usually takes place in anaerobic conditions (ie
the spinning machinery is nitrogen-purged). This has the
effect of reducing the surface degradation effects, which
would otherwise take place in the presence of oxygen. Since
spun-cast lenses have a low volume to surface area ratio,
this is an important consideration, ie. surface degradation
effects cannot be removed, as occurs in the lathing process.
Compared to button manufacture in the lathing process,
spin-casting is very quick – usually taking less than an hour
to polymerise the material.
Cast-moulding
In the cast-moulding process a small amount of monomer
is placed between two casts to form the lens directly.
The polymerisation process often involves placing the
lenses in an oven, during which time they are subjected
to a very fast temperature rise and curing process. Rapid
polymerisation times are likely to produce shorter chains,
more chain ends and less efficient cross-links. If the
procedure is carried out in the presence of oxygen (eg in
a tunnel oven), oxygen can diffuse through the casts and
degrade the surface of the lenses. Cast-moulded lenses have
a low volume to surface area ratio, and thus, the oxygen
degradation is likely to have a major effect on the polymer
network. Unlike lathed lenses and like spun-cast lenses,
this degradation cannot be removed. Polymerisation of
the lenses in anaerobic conditions is likely to reduce these
surface effects. Alternatively, polymerisation during castmoulding can take place in anaerobic conditions and be
initiated by UV light.
History of Soft Lens Manufacturing
Lathe-cutting
Lathing was used in the manufacture of corneal PMMA
and rigid lenses before it was used for manufacturing
soft hydrogel lenses. The variety of lens designs that can
be made by this method is vast. Soft lenses have been
lathed since the early 1970s and this process is still used,
particularly in the production of custom lenses. Since
the 1970s lathes have become more sophisticated and
automated but the general principles remain similar to
those used for the first soft lenses in the 1970s.
Spin-casting
As described earlier, Wichterle patented this completely
new method of soft lens manufacture in 1961. B&L then
refined Wichterle’s crude spin-casting process and finally
obtained FDA approval for their PHEMA lenses in 1971,
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which they called SofLens. Lenses manufactured by spincasting today use a process similar to the one developed
by B&L.
Cast-moulding
During the 1980s manufacturers and researchers put a lot
of effort into solving the hypoxia-related problems seen
with conventional hydrogel materials that were available.
However, not all contact lens-related complications were
due to hypoxia. Hydrogel materials were particularly prone
to spoliation, which was implicated in the aetiology of
hypersensitivity reactions (Allansmith et al. 1977, Ballow et
al. 1989) as well as decreased vision and comfort (Nason et
al. 1994, Solomon et al. 1996). It was a natural progression,
then, to consider the possibility of replacing lenses more
frequently. B&L commenced the first formal replacement
programme when they launched their FreshLens scheme
in 1985. This allowed extended-wear patients to replace
their O3 or O4 series lenses every 3 months and was later
expanded to include daily-wear lenses. Other frequent
replacement programmes soon followed.
The clinical need for more frequent replacement of lenses
led the way forward for investment into technology that
would allow the mass manufacture of lenses at a costeffective price whilst maintaining a high level of quality.
This has inevitably led to the demise of many small
manufacturing companies which have been unable to
develop low-cost manufacturing technologies and have,
therefore, been unable to move into the disposable lens
market.
Before its application to the production of contact lenses,
cast-moulding had been used in many industries to
manufacture plastic goods cost-effectively. During the
process, liquid monomer is polymerised between two casts
to form a contact lens. The lens is then further processed
before packaging.
American Hydron was the first contact lens company to
receive FDA approval for cast-moulding contact lenses
in 1980. The company, which was owned by the NPDC,
claimed that the new process increased its production
capacity 10-fold without requiring additional labour (Mertz
1997). The company was sold to Allergan, Inc. in 1985.
Johnson & Johnson entered the contact lens market in
1981 when the company bought a small contact lens
manufacturer named Frontier Contact Lenses based in
Jacksonville, Florida, USA. The contact lens division,
renamed Vistakon, then purchased the rights to a Danish
manufacturing technology known as stabilised soft
moulding (SSM).
SSM can essentially be thought of as a cast-moulding
process, but it uses techniques which set it apart from
conventional cast-moulding technology. The principal
difference is that the lenses are not allowed to dry after
polymerisation. This is achieved through the addition
of a diluent into the monomer mix which is then simply
replaced by water. The Acuvue™ lens was the first lens
made using this new technique and was launched in 1987.
After acquiring the cast-moulding technology in place
at American Optical in 1985, CIBA Vision launched its
first disposable lens, NewVues™, in 1989 and the Focus®
disposable lens 2 years later. Meanwhile, B&L were
developing their new cast-moulding process, Performa,
which they used to produce SofLens 66™ (Medalist 66™),
which was launched in 1995.
Following on from the success of the Acuvue™ lens,
Vistakon decided to make the move into the daily
disposable market. The SSM technology was improved and
became a fully automated process known as Maximise,
where each ‘line’ now became a stand-alone production
module. The 1-Day Acuvue™ lens was launched in the UK
in 1995. However, this was not the first daily disposable
lens to reach the UK market. The Premier™ lens, made by
the Scottish company, Award plc, was launched in 1994.
During the manufacturing process, the male part of the
mould became part of the blister packaging. The lenses
were made from a HEMA/VP copolymer with an EWC of
73%. The company was bought in 1996 by B&L which
currently manufactures an altered version of the lens
under the trade name SofLens™ One Day. Ron Hamilton,
the former managing director of Award plc, has formed a
new company, Provis Ltd, which currently manufactures
the daysoft™ uv daily disposable lens which was launched
in the UK at the beginning of 2001. The lens has an EWC
of 72% and is a HEMA/VP copolymer.
CIBA Vision launched their daily disposable lens, Focus®
Dailies™ in the UK in 1997 using a variant of the castmoulding process known as Lightstream™ Technology. The
technique employs reusable glass moulds, polymerisaton
in the fully hydrated state and a masking technique (to
UV light) to form the final lens. The company claims to
produce lenses of superior edge quality when compared
with the mechanical separation of moulds used in other
manufacturing processes. The lens is made from polyvinyl
alcohol (PVA).
The most recent daily disposable lens to be launched on
to the UK contact lens market is the Biomedics™ One
108
Optometry in Practice
Day (Ocular Sciences Inc.) which was launched in 2002.
The lens material is a HEMA/MA copolymer with an EWC
of 52%. The lens is dry-moulded in a fully automated
production facility.
Methods of Soft Contact Lens Manufacture
Lathe-cutting
Lathing can be used to make soft or rigid lenses from most
types of contact lens material.
Material preparation
The first step in the lathing process is the preparation of
the material that will eventually be lathed into the final
contact lens. Monomers are usually bulk-polymerised into
individual button-shaped moulds or cast into rods from
which buttons can then be sliced. Polymerisation is usually
a slow process and buttons or rods are typically left in a
water bath for several hours at a specified temperature
which will vary with the material being cured.
Once cured, buttons are usually post-cured or annealed.
This involves heating the material to above its glass
transition temperature and then cooling it back down
to near room temperature. The purpose of this step is
to soften the material which in turn has the effect of
relieving stress in the buttons. If this step was not carried
out the swelling behaviour of the buttons would be very
unpredictable. Annealing also prevents other effects that
are sometimes seen in finished lenses, such as fluting of
lens edges or rolling up of lenses like a cigarette when
hydrated.
When a soft contact lens is lathed, it is done so in the dry
state. This means that a smaller, steeper lens of greater
power is made so that when it is hydrated, it swells to the
required dimensions and powers.
Back surface cutting of a lens blank
The next step in the process is the cutting of the lens
back surface from the buttons (or lens blanks), usually
on a computer numerically controlled (CNC) lathe. The
lathes can be programmed to cut innumerable design and
parameter variations.
The back surface is cut using diamond cutting tools. The
first cut is a rough cut, performed by a roughing tool
which removes most of the excess button material and
then a finishing tool performs the final cut which will
usually cut secondary curves and edge bevels. The whole
process takes only a few minutes. Following on from this,
the back surface is polished on a polishing machine. The
composition of the polishing compound will vary between
manufacturers, but it is typically made up of a lanolin base
with abrasive diamond dust. The semifinished lenses are
dipped into a solvent to remove the polish.
Front surface cutting of a lens
The next step in the process is blocking. The semifinished
lens blank is attached by the newly formed back surface
to a front surface chuck by means of hot, melted wax.
When the wax cools and solidifies, a strong bond is formed
between the chuck and the lens blank. The lenses are
loaded back on to the lathe for front surface cutting
(Figure 4). A diamond tool will generally cut about 7000
lenses before it needs to be replaced.The front lens surface
is polished and then deblocked by immersing the arbor
with the attached contact lens into a deblocking solvent.
All of the procedures described so far place enormous
demands on the material. The mechanical properties of
the material must be such that it is able to cope with highspeed cutting and polishing. The process (in particular
the generation of heat) places a lower limit on the glass
transition temperature of the polymers used. For example,
if the glass transition temperature of a material were very
low, the buttons would be soft at room temperature and
consequently very difficult to lathe. It is equally important
that the material is not too brittle – this would lead to
cracking and breaking of the lenses during the process.
Wet processing of the lenses
Figure 4. Lathing of the front lens surface.
The dry lenses undergo hydration and wet processing.
Hydration steps will vary according to which material
the lens has been made from. Most non-ionic lenses
will be washed in deionised water (perhaps containing a
surfactant) and then saline. These steps are often carried
out in ultrasonic baths to speed up the hydration process.
109
C Maldonado-Codina & N Efron
If the lens material contains MAA, the lenses will be
washed in tanks containing sodium bicarbonate in order
to ionise the lenses.
Lens packaging and sterilisation
Lenses are either packaged in individual glass vials or now,
more commonly, in blisters. Blister packing tends to be
used for all disposable products whereas glass vials are
mainly used for non-disposable products.
The label on a contact lens needs to display the following
information:
•Name and address of manufacturer
•Name of contact lens
•Material that the lens is made from
•Lot number
•EWC of the lens in physiological saline
•Expiry date
•Back optic zone radius (BOZR)
•Overall size
•Back vertex power (BVP)
•CE markings
processing of the lenses. Manufacturers take steps, in
varying degrees, to minimise swelling of the dry material,
such as environmental humidity control and use of finger
cots and masks by employees to minimise the effects of
moisture from the skin and breath. Uncontrolled swelling
of the dry material will have a significant effect on the
reproducibility of lathed lenses and consequently on the
cost of manufacturing.
Although the development of automated lathes has
reduced the time taken to cut the lenses and has improved
the reproducibility of the final product, other aspects of
the process such as polishing and blocking continue to
be sources of error. A totally automated lathing process
for hydrogel lenses has not yet been developed. Therefore,
lathing remains impractical for the mass production of
high-quality inexpensive lenses required for disposability.
The technique, however, is essential for the manufacture
of low-volume, high-prescription custom lenses.
Contact lenses are classed as medical devices and as
such are regulated under the Medical Device Directory
93/42/EEC (MDD). The MDD is one of three directives for
medical devices and was enacted to provide a harmonised
regulatory environment for all medical devices sold within
Europe. All products falling within the scope of the
Directive must meet certain safety and administrative
requirements and are CE-marked to indicate that they
comply.
Insert and cast
manufacture
Dosing cast with
monomer
Sterilisation procedures are just one area where
manufacturers must meet certain standards for CE
marking. This is usually achieved using an autoclave which
produces steam under pressure.
Spinning
UV cure
In the USA, contact lenses are regulated by the FDA which
sets out its own standards.
and
Possible manufacturing errors
Despite its many advantages, the lathing process has some
disadvantages. It is labour-intensive and, therefore, both
expensive and susceptible to significant human error. Also,
the lathing process is slow. Since hydrogel lenses have to
be lathed in the dry state, the processing steps required to
hydrate the lenses finally can also be a source of potential
manufacturing error. Environmental humidity will always
be hydrating (swelling) any hydrophilic material when it
is in a state of hydration less than its EWC. This creates a
‘moving target’ for the manufacturer during the dry state
Lens hydration
and packaging
Figure 5. The spin-casting process
(courtesy of CooperVision).
110
Optometry in Practice
Most errors that occur during lathing will be picked up
during the various checking steps (quality control and
quality assurance) that are carried out throughout the
process. Manufacturing errors that may occur include:
•Inclusions (eg rust)
•Debris (eg fibres)
•Edge defects
•Watermarks, bubbles or holes
•Fractures
•Lathe rings
•Distortion
•Discoloration
•Creases
•Arbor marks, scratches or surface marks
Spin-casting
The procedure involves spinning a cast, at a computercontrolled speed, into which the mixture of monomers
is injected. The centripetal force causes the monomer to
climb the walls of the cast to form into the required shape
whilst polymerisation takes place (Figure 5).
a certain centre thickness and BVP. The BVP is essentially
determined by the spin speed and the dose mainly
determines the centre thickness of the final lens. The
inner surface of the cast provides the front surface of the
contact lens. The back surface of the lens is aspheric and
is determined by the factors listed above. The power of the
lens which best approximates a sphere is usually close to
a –3.00D lens. It is therefore an ideal process for making
minus-powered lenses. Positive-powered lenses can be
made, although casts with more complicated designs need
to be produced. Alternatively some manufacturers spin
their lenses and then lathe either the front or the back
surface to achieve the desired profile and power required.
Some manufacturers will further lathe and polish the
edges of their lenses after they have been spun-cast.
The lenses are demoulded – either manually or as part
of an automated production line and wet-processed in a
similar way to lathed lenses.
Insert manufacture
The spin-casting process begins with the production of
high-quality inserts or master tools. The inserts are used as
the mould from which all the casts are made. The quality
of the inserts has to be extremely good since they will
ultimately determine the front surface and edges of each
lens produced.
Female
insert
Male insert
Cast manufacture
Male mould
Female
mould
Casts can be made from various materials, eg
poly(propylene) or poly(vinyl chloride), but the plastic
usually has a surface energy similar to the material that is
being spun. This will facilitate wetting of the casts by the
monomer mix. It is sometimes necessary to surface-treat
the resulting casts to ensure this, but such a process will
increase production costs.
Heat cure
Dry lens
released
from mould
Casts are usually made in a multi-impression bolster
set into an injection-moulder which makes hundreds of
finished casts every hour.
The spinning process
The final shape and power of the resulting lens are due
to the combination of gravity, centrifugal force, surface
tension, the amount of liquid monomer in the cast and the
rate of spin (spin speed). By knowing the radius of the cast
produced, the speed and the dose can be assigned to give
Figure 6. The cast-moulding process
(courtesy of CooperVision).
111
C Maldonado-Codina & N Efron
Cast-moulding
Conclusion
Since the unit production cost is potentially low, this
technique and variants of it are now the most important
form of high-volume soft lens production (Figure 6).
Hydrogel materials and the complex technology behind
making them have come a long way since the pioneering
efforts of Professor Otto Wichterle in the late 1950s.
Insert manufacture
Thick PHEMA lenses which were replaced every year or two
are now a thing of the past. Hypoxia-related problems with
conventional hydrogels have been solved by reducing the
thickness of the lenses and/or utilising more hydrophilic
monomers to produce the highly successful lenses that
are on the market today. Problems related to spoliation
have been solved by frequent replacement, which has
spurred a huge investment by manufacturers to invest in
new high-volume, automated, cost-effective cast-moulding
technologies. However, these materials in general still
fall short of the oxygen requirements needed for safe
continuous wear. Enter silicone. The combination of this
highly oxygen-permeable material with hydrogel monomers
has enabled the industry to promote continuous wear for
the second time – so far, very successfully.
Tooling is again the key to successful contact lens
fabrication using this manufacturing method. For castmoulding, both a male and a female insert need to be
made as well as the auxiliary insert housings. The female
cast (formed by the female insert) creates the front
surface of the final contact lens and the male mould
(formed by the male insert) creates the back surface of
the final lens. Relatively few male inserts are made since a
particular lens will usually only be available with a limited
number of BOZR options. However, hundreds of female
inserts are made. It is by changing the female insert that
different powers of lenses will ultimately be obtained
because this changes their front surface radii and overall
thickness profile.
Cast manufacture
Typically, hundreds of casts are made every hour. The
material that the casts are made from is integral to
the manufacturing process. In the past, manufacturers
have had problems with the dimensional stability of
casts. This manifests itself in a low yield of lenses at the
correct specification for a particular batch. It is vital,
therefore, that the chemical structure of the polymer
used is carefully selected. The Ciba Vision Lightstream™
production method uses glass moulds.
Acknowledgements
The authors wish to thank Dr Trevor Glasbey, formerly of
Biocompatibles Hydron, and Dr Paul Riggs at CooperVision
for their contributions to this article.
The design of the casts is critical to the final form of
the lens. The cast design will determine not only the
surface curvatures, optical quality and the diameter of
the lens, but also the edge design of the final lens. The
reproducibility of a particular lens, particularly the edge
quality, is directly related to the cast design. The patent
literature is full of patents which describe different
designs for cast manufacture, all using slightly different
techniques for forming the edges of a lens (Larson 1993,
Sealy 1989, Seden & Hamilton 1993).
Possible manufacturing errors
These are much the same as for the lathe-cut lenses. Efron
& Veys (1992) demonstrated that defects in disposable
lenses could cause increased corneal and conjunctival
staining when worn. It is believed that advances in castmoulding manufacturing technology over the past decade,
fuelled by the attention drawn to the above issues, has led
to a substantial overall improvement in lens quality.
112
Optometry in Practice
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Sealy MJ (1989) Contact lens mould. UK patent 2 187 999
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contact lenses. CLAO J 22(4), 250–7
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Multiple Choice Questions
This paper is reference C4687b (College). Three College credits are available. Please use the inserted answer sheet.
Copies can be obtained from Optometry in Practice Administration, PO Box 6, Skelmersdale, Lancashire WN8 9FW.
There is only one correct answer for each question.
1. Which one of the following statements is NOT true?
(a) All hydrogel materials used in the production of
conventional soft lenses are copolymers.
(b) All hydrogel materials are polymers.
(c) All hydrogel materials used in the production of
conventional soft lenses are plasticised by water.
(d) All hydrogel materials used in the production of
conventional soft lenses have elastic properties.
(e) All hydrogel materials used in the production of
conventional soft lenses are amorphous polymers.
2. A cross-linker commonly used in the production of
conventional hydrogels is:
(a) poly(hydroxy ethyl methacrylate)
(b)glycerol
(c) ethylene glycol dimethacrylate
(d) benzoin methyl ether
(e) azo-bis-isobutyronitrile
3. Which one of the following with regards to oxygen
permeability is true?
(a) Oxygen permeability is positively correlated to EWC
in all hydrogels.
(b) Oxygen is only able to pass through water rather
than the material itself in conventional hydrogels.
(c) In silicone hydrogels, oxygen permeability increases
with increasing water content.
(d) In silicone hydrogels, oxygen transmissibility is not
affected by lens thickness.
(e) None of the above.
4. Which one of the following monomers will NOT
increase the EWC of a HEMA-based conventional
hydrogel?
(a) acrylic acid
(b) methacrylic acid
(c) methyl methacrylate
(d) glycerol methacrylate
(e) N-vinyl pyrrolidone
5. Lenses incorporating NVP:
(a) tend to lose water from their surfaces
(b) are relatively insensitive to temperature changes
(c) attract water into the polymer matrix due to the
hydroxyl moiety
(d) have been associated with increased corneal staining
when used with all multipurpose solutions
(e) cannot be used in conjunction with solutions
containing polyhexanide
6. Which one of the following statements is NOT true
regarding lenses containing methacrylic acid?
(a) The lenses are negatively charged.
(b) Lenses containing approximately 2% MAA together
with PHEMA will have an EWC of approximately
55%.
(c) Increasing pH will cause the EWC to increase.
(d) Increasing pH will cause the EWC to decrease.
(e) The lenses will attract lysozyme into the polymer network.
7. Which of the following is true regarding glycerol
methacrylate?
(a) Lenses containing HEMA/GMA are always more
oxygen-permeable than PHEMA lenses.
(b) GMA monomer contains two hydroxyl groups per
repeating unit.
(c) GMA has been successfully combined with MMA to
produce conventional lenses which are currently on
the market today.
(d) GMA has been successfully combined with PHEMA
to produce conventional lenses which are currently
on the market today.
(e) All of the above.
8. Conventional hydrogels have been unsuccessful as
continuous wear lenses in the past mainly because:
(a) The lenses deposited too much protein.
(b) The lenses were too uncomfortable.
(c) The lenses bound to the eye after sleeping.
(d) The lenses were not permeable enough to oxygen to
avoid corneal oedema beyond that normally seen
overnight.
(e) The lenses were too thick.
9. The minimum oxygen transmissibility to avoid
anoxia throughout the entire cornea is currently
thought to be:
(a) 24.1Barrer/cm for daily wear
(b) 87Barrer/cm for overnight wear
(c) between 120 and 140Barrer/cm for overnight wear
(d) between 40 and 50Barrer/cm for daily wear
(e) none of the above
114
Optometry in Practice
10. Hydraulic permeability in silicone hydrogels is
essential for which one of the following reasons?
(a) lens movement
(b) lens surface wettability
(c) oxygen permeability
(d) oxygen transmissibility
(e) lens spoliation
(d) are used to produce silicate islands on the surface of
lotrafilcon lenses
(e) are used to achieve the property of surface
wettability for silicone hydrogel lenses
15. Which of the following statements is correct?
(a) The purpose of annealing soft lens polymer buttons
is to harden the material prior to lathing.
(b) It is a requirement of CE-marking that
manufacturers meet specified standards for their lens
sterilisation procedures.
(c) Manual demoulding of spin cast lenses has
contributed to reducing the costs of disposable
lenses.
(d) Cast material and design are only of minor
consideration in determining the final form of a
lens.
(e) The BVP of spin cast lenses is determined by the
shape of the inner surface of the cast.
11. Most conventional disposable soft lenses are
manufactured using which polymerisation method?
(a) bulk polymerisation
(b) solution polymerisation
(c) addition polymerisation
(d) free radical polymerisation
(e) step polymerisation
12. Which one of the following statements is true for
lenses which are lathed?
(a) The polymerisation process is very fast.
(b) Buttons have a low surface to volume ratio.
(c) Lathes are cheaper than spun-casting or cast
moulding equipment and are, therefore, the most
cost-effective way of manufacturing all lens types.
(d) Lathing is now generally only used for low-volume,
high-prescription custom lenses.
(e) Lathed lenses are better quality than spun-cast or
cast-moulded lenses.
13. A diluent is often used during the spin-casting
process in order to:
(a) aid the demoulding process
(b) alter the swell factor of the resulting hydrogel
(c) prolong or prevent the gel point during
polymerisation
(d) effect more complete polymerisation
(e) all of the above
14. Which of the following statements is true? Gas
plasma techniques:
(a) are used to improve the gas permeability of silicone
hydrogels
(b) temporarily alter the biphasic structure of the bulk
of a silicone hydrogel lens
(c) are responsible for making silicone hydrogel lenses
‘stiffer’ than conventional hydrogels
115
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