< Part 1
This, the second part of a two-part review of silicone hydrogel
materials, discusses the differences between conventional and
silicone hydrogel lenses in terms of their surface and bulk properties.
1. Bulk Properties
|Figure 1 - click to enlarge
In silicone hydrogel materials the oxygen is primarily transmitted
through the silicone component of the lens material, resulting
in a dramatic increase in the oxygen permeability of the materials
Clinical studies have confirmed that many of the long and short-term
hypoxic problems seen with extended-wear of traditional lenses
are overcome with these novel, highly permeable materials.(1-8)
The material elasticity of currently marketed silicone hydrogels
is much less than silicone elastomer lenses, but silicone hydrogel
lenses remain “stiffer” than conventional hydrogels,
due to the incorporation of silicone. The modulus of the first two
silicone hydrogel materials is some 4-6 times greater than low rigidity
materials such as etafilcon A. The modulus of Acuvue Advance is
much closer to conventional materials, being only 1.5 times more
rigid than etafilcon.(9) According to Johnson & Johnson, this
reduced stiffness is due to the reduced amount of silicone present,
along with benefits due to the internal wetting agent HydraClear™,
which is based upon polyvinyl
pyrrolidone (PVP).(9) Figure 2 graphically indicates
the inverse relationship between water content of hydrogel materials
and oxygen permeability and material stiffness, with the materials
having the highest ratio of silicone to water being the stiffest.
|Figure 2 - click to enlarge
Increased rigidity or stiffness has some advantages, in that
the lenses handle very well. Increased rigidity might also suggest
more corneal astigmatism is masked compared to flexible hydrogels,
but that has not been our experience clinically or that of others.(10)
The mechanical properties of these lenses do pose some problems,
in that they are less able to conform easily to the shape of the
eye and fitting is critical, with loose lenses exhibiting poor
comfort.(11) Additionally, the rigidity of these materials may
be implicated in a variety of mechanical complications seen with
silicone-hydrogel lenses, including papillary conjunctivitis and
superior epithelial splits.(12-17)
Dry eye symptoms are reported by 20-50% of soft lens wearers,(18-19)
with 35% of patients permanently ceasing lens wear due to complications
associated with discomfort and dryness.(20)
The sensation of “dryness” is a complex subject and
is without question related to a variety of factors. One factor
to consider is that of lens dehydration, as the subjective symptom
of dryness appears to occur more frequently in soft lens wearers
whose lenses undergo greater dehydration during open-eye wear.(21)
Material composition influences dehydration rate and degree.(22)
In a clinical environment it has been noted that the majority
of wearers of silicone hydrogel lenses report that their lenses
feel less “dry” than their previous conventional lenses,
despite considerably longer wearing times.(23) These novel materials,
which have lower water contents than currently available materials,
may produce less subjective dryness symptoms through reduced in-eye
dehydration, enhanced wettability, reduced hydrophobic interactions
with the eye-lid, reduced deposition and/or increased oxygen performance.
Published work to-date shows that silicone-hydrogel lens materials
dehydrate at a slower rate and to a lesser extent than conventional
hydrogel materials(24,25) and may partially help to explain this
reduction in the sensation of dryness.
Historically, a huge impediment to the development of silicone
hydrogel lenses has related to the decreased wettability, increased
lipid interaction and accentuated lens binding inherent in silicon-based
materials, as previously described. In order to render the surfaces
hydrophilic, techniques incorporating plasma into the surface
processing of the lens have been developed.(26-29) The purpose
of this surface treatment is to mask the hydrophobic silicone
from the tear film, increasing the surface wettability of the
materials and reducing lipid deposition.
The surfaces of Focus Night & Day lenses are permanently
modified in a gas plasma reactive chamber to create a permanent,
ultrathin (25 nm), high refractive index, continuous hydrophilic
surface.27,30,31 PureVision lenses are surface treated
in a gas plasma reactive chamber which
|Figure 3 - click to enlarge
transforms the silicone components on the surface of the lenses
into hydrophilic silicate compounds.(26,29,32-34) Glassy, island-like,
discontinuous silicate “islands” result,(33) and the
hydrophilicity of these areas "bridges" over the underlying
hydrophobic balafilcon A material. The subtle differences in the
surfaces of these novel materials can be clearly appreciated using
very high magnification imaging techniques such as atomic force
microscopy (AFM) (Figure 3).
Both surface treatments become an integral part of the lens and
are not surface coatings that can be easily “stripped”
away from the base material during daily handling and cleaning.
The Acuvue Advance material is the first non surface-treated
silicone hydrogel to become a commercial reality. Acuvue Advance
uses an internal wetting agent (Hydraclear™) based upon
PVP, which is designed to provide a hydrophilic layer at the surface
of the material that “shields” the silicone at the
material interface, thereby reducing the degree of hydrophobicity
typically seen at the surface of siloxane-hydrogels.(35-38) Details
on the Advance material are scanty thus far, but some information
can be gleaned from patents issued or pending. The silicone content
is derived from a tailor-made silicone macromer, used in conjunction
with TRIS monomer, copolymerised with hydrogel-forming monomers
such as N,N-dimethyl acrylamide and HEMA, in the presence of around
5% of PVP and an organic diluent together with a small amount
of cross-linking agent. Extraction of the diluent and hydration
leads to a silicone hydrogel which is sufficiently wettable to
avoid the need for subsequent surface treatment.
Analysis of the surfaces of both PureVision and Focus Night
& Day has shown that these surface treatments have only been
partially effective at masking the silicone, with the lenses having
significantly more silicon exposed at the surface than conventional
lenses(39) and a more hydrophobic surface.(40,41) No details yet
exist on the amount of silicon exposed on the Advance surface.
Lipid and Protein Deposition
What of the future?
It is important that silicone hydrogel materials do not deposit
to the degree that silicone elastomer lenses did as these lenses
are primarily intended for overnight use for up to 30 days and
such deposition would require frequent lens removal for cleaning.
To date, the degree of in-eye biocompatibility achieved with silicone-hydrogel
materials has received relatively minimal attention, with the
published results indicating that the deposition of protein on
these materials is less than that seen with conventional materials,(42-47)
but that lipid deposition can be a problem for certain patients,(44)
particularly if they are refitted from an ionic material such
as etafilcon that deposits very little lipid. If subjects are
seen to be depositing their lenses with lipid then moving to non-NVP-containing
materials (such as Proclear or Acuvue) will reduce lipid deposition.
Further options include adding surfactant cleaners containing
alcohol (such as Miraflow) or moving to more frequent periods
contact lens companies are looking at developing novel silicone-based
hydrogels and the foreseeable future for this group of lens materials
looks promising, with several other new hydrogel materials already
registered with the USAN Council. Whilst the exact details of
such materials (eg acquafilcon, lenefilcon and senofilcon) are
unknown, it is likely that the next 10 years will be dominated
by the release of silicone-based hydrogels from all manufacturers.
These materials will likely have stiffness levels closer to conventional
hydrogels and have better surface treatments that truly make the
surfaces hydrophilic. Ideally, such materials would support a
tear film for longer than the typical 7-8 seconds seen with currently
available materials and consist of polymers that would resist
contamination with pathogenic organisms.(49) Such materials would
result in increased comfort and reduced inflammatory complications
compared with currently available materials and would have a significant
impact on growing the contact lens market.
- Papas E, Vajdic C, et al.: High oxygen-transmissibility
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- Dumbleton KA, Chalmers RL, et al.: Changes in myopic
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- Dumbleton KA, Chalmers RL, et al.: Vascular response
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- Dumbleton K: Adverse events with silicone hydrogel continuous
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- du Toit R, Simpson TL, et al.: Recovery from hyperemia
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- Keay L, Sweeney DF, et al.: Microcyst response to
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- Fonn D, du Toit R, et al.: Sympathetic swelling
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- Covey M, Sweeney DF, et al.: Hypoxic effects on
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- Dumbleton KA, Chalmers RL, et al.: Effect of lens
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< Part 1