Silicone Hydrogel Contact Lens Materials Update - Part 2

Lyndon Jones PhD FCOptom DipCLP DipOrth FAAO (DipCL) FIACLE Lyndon Jones is currently a tenured Associate Professor at the School of Optometry and Associate Director of the Centre for Contact Lens Research at the University of Waterloo in Ontario, Canada. He graduated in Optometry from the University of Wales, UK in 1985 and gained his PhD from the Biomaterials Research Unit at Aston University, UK in 1998. He is a Fellow and Diplomate of the American Academy of Optometry, has authored over 200 papers and conference abstracts, one text-book and given over 200 invited lectures at conferences worldwide.

Brian Tighe Professor, Leader, Bio-materials Research, Aston University   Professor Tighe's research focuses on the design, synthesis and applications of biomedical polymers. Current interests include novel materials for ophthalmic applications, drug delivery systems, bioadhesive polymers, synthetic materials for articular cartilage, lung surfactant and cornea.

Part 2:

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.

Oxygen Transmissibility

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 (Figure 1). 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)

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)

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.

Surface Properties

Surface Wettability

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

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 of replacement.(48)

Increasingly, 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.

References

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    Continued < Part 1