SILICONE HYDROGELS
Silicone Hydrogel Material and Surface Properties
Lens surface and movement, wettability, oxygen transmission and mechanical properties set these lenses apart.
By Desmond Fonn, Dip Optom, MOptom, Kathryn Dumbleton, MCOptom, Lyndon Jones, PhD, FCOptom, Renée du Toit, Dip Optom, MPhil, Deborah Sweeney, B Optom, PhD

The major challenges to successful extended wear (EW) of soft contact lenses has been to increase oxygen transmissibility (Dk/t) and surface biocompatibility as well as the elimination of adverse responses, especially microbial keratitis. Attempts to increase Dk/t of hydrogel lenses have been through increased water content or decreased thickness, but unfortunately even the most oxygen transmissible soft lenses have fallen far short of the Holden/Mertz criteria of 87x10^-9 (cm x mlO₂)/(s x ml x mmHg) established in 1984, as the minimum oxygen transmissibility required to prevent closed-eye lens-induced corneal edema. More recently a level of 125x10^-9 has been reported by Harvitt and Bonanno as a requirement to prevent stromal hypoxia.

The introduction of silicone elastomer in the 1980s was promising for EW because the material had an oxygen permeability of approximately 300x10^-9, which was well above the limits for eliminating hypoxia. However, the flexible large diameter lens designs were never successful as EW devices for the correction of refractive error (other than pediatric aphakia). This was mainly due to lens adhesion, discomfort and hydrophobicity of the surface because of the material's physical properties. Silicone has been successfully incorporated in rigid gas permeable (RGP) materials, and while there are a number of commercially available lenses that will satisfy the Holden/Mertz oxygen transmission criteria, these lenses have not provided the answer for the majority of contact lens wearers again primarily due to levels of discomfort.

Of course, elimination of hypoxia is not the only criterion for successful contact lens wear. Wettability, comfort, material durability and stability, as well as optical quality, are also essential for a viable contact lens. It has taken about 20 years since the original patent by Tanaka for the first commercial silicone hydrogel lens to emerge. Not only has it been a challenge to combine the bulk properties of the material, but surface modifications were required to transform the concept into a commercial reality.

Figure 1: Example of a wet and undeposited lens, typifiying an optimal surface.

Materials

Monomers commonly used in hydrogel contact lens materials include N-vinyl pyrrolidone (NVP), methacrylic acid (MA) and poly-2-hydroxyethyl me-thacrylate (polyHEMA). These components allow the lens materials to absorb and bind water. In silicone hydrogel materials, silicone rubber is combined with conventional hydrogel monomers. The silicone component provides high oxygen permeability, while the hydrogel component facilitates fluid permeation that enables the lens to move on the eye. Unfortunately, this "mixture" proved to be difficult, which is why commercial development of these materials has taken time. The process of combining these monomers has been likened to combining oil with water.

Contact lens materials must permit the transmission of both oxygen and ions. One approach involves the incorporation two "phases" into the materials. Phase separation occurs when the interconnections between the chemically similar molecules in the material are stronger than the adhesive connections between them and the different molecules. This approach to material development was historically avoided because it usually resulted in an opaque material. However, techniques to limit the phase separation to shorter than the wavelength of light made optically clear contact lens materials possible.

CIBA Vision's Focus Night & Day material, lotrafilcon A, employs such a biphasic or two- channel molecular structure. The fluorosiloxane phase facilitates the storage and transmission of oxygen, and the hydrogel phase transmits water and oxygen, facilitating lens movement. The two phases work concurrently to allow the co-continuous transmission of oxygen and ions. Lotrafilcon A is comprised of a fluoroether macromer co-polymerized with the monomer trimethyl-siloxy silane (TRIS - used in the preparation of RGP materials) and the solvent N,N-dimethyl acrylamide (DMA) in the presence of a diluent. The resultant silicone hydrogel material has a water content of 24 percent and an oxygen permeability (Dk) of 140 barrers.

Bausch & Lomb's PureVision material, balafilcon A, is a homogeneous combination of the silicone containing monomer poly-methylsiloxane (a vinyl carbamate derivative of TRIS) co-polymerized with the hydrophilic hydrogel monomer N-vinyl pyrrolidone (NVP). This silicone hydrogel material has a water content of 36 percent and a Dk of 110 barrers.

	Figure 2: Haze and globule-type deposits as an example of poor wettability.

Lens Surface

Decreased wettability, increased lipid interaction and accentuated lens binding inherent in silicone based materials also impeded their development. A technology referred to as "gas plasma surfacing" was developed in order to render the surfaces hydrophilic.

Focus Night & Day lenses are manufactured using a standard molding process. The surfaces are permanently modified in a gas plasma reactive chamber to create a permanent, ultrathin (25nm) continuous hydrophilic surface.

Cast-molded PureVision lenses are surface treated in a gas plasma reactive chamber which transforms the silicone components on the surface of the lenses into hydrophilic silicate compounds. Glassy silicate "islands" result, and the hydrophilicity of these areas "bridges" over the underlying hydrophobic balafilcon material.

Both surface treatments are an integral part of the lens and are not surface coatings that can be "stripped" away from the base material. The flow of oxygen and fluid through the lens is not impeded by the surface modifications.

Clinical Implications of Surface Modification

Clinical observations of the tear film's affinity for the lens surface are usually made with a biomicroscope. This subjective assessment is based on a combination of factors that include the transparency and stability of the tear film, the speed of tear film break-up on the anterior lens surface and the absence of debris or deposits, primarily lipid, that can be loosely adherent. Figure 1 is an example of a silicone hydrogel that shows a typically desirable optically transparent and "wet" surface. Most studies have reported that silicone hydrogel lenses have wettability equal to hydrogel lenses but, like all soft contact lenses, they will occasionally appear to lose their "wet" appearance. Figure 2 is an example of a silicone hydrogel lens that shows a pattern of haze and globule-type appearance on its surface that is associated with lipid deposition. This lens will have a reduced tear film break-up time and generally poor wettability. Reduced wettability and the resultant deposits are more likely to occur toward the end of the one-month wearing cycle, the recommended replacement period for current generation silicone hydrogels.

Oxygen Transmissibility and Corneal Health

Traditional contact lenses have relied on water to carry the oxygen through the lens. This has been a limiting factor, since 100 percent water has a Dk of about only 80 barrers. As a result, conventional hydrogel lens materials do not deliver sufficient oxygen during extended wear, and a number of clinical signs of chronic hypoxia may occur. It is the close relationship between water content and oxygen permeability that has impeded hydrogel lens material development for extended wear for more than 20 years.

In silicone hydrogel materials, the oxygen is transmitted through the silicone component of the lens material, resulting in a dramatic increase in the oxygen permeability. Pure silicone rubber has a Dk in excess of 300 barrers, and this provides silicone hydrogel materials with Dk/t values of 110 to 175x10^-9, which is six times more transmissible than conventional hydrogel contact lenses. Figure 3 demonstrates the Dk of these materials compared with conventional hydrogel materials, in which the Dk is directly related to the water content of the lens material.

Corneal swelling is highly dependent on the contact lens oxygen transmissibility. Studies conducted by Fonn et al at the Centre for Contact Lens Research, University of Waterloo, have found overnight edema levels with silicone hydrogel contact lenses to be similar to the levels with no lens wear (2.7 ± 1.9 percent) and to be significantly lower than those measured with Vistakon Acuvue contact lenses (8.7 ± 2.8 percent). Figure 4 shows central corneal swelling after overnight wear and the relative recovery period after lens removal.

In a similar study, Comstock et al showed that the overnight central corneal swelling induced by PureVision on adapted contact lens wearers was 2.8 ± 2.0 percent, compared to 8.7 ± 2.7 percent with a 70 percent water content lens (Dk/t = 22).

Ionic and Hydraulic Permeability and Lens Movement

The transport of fluid and ions through contact lenses is necessary for on-eye lens movement and comfort. In homogenous silicone hydrogel contact lens materials such as balafilcon A, as the oxygen permeability increases, the hydraulic permeability decreases due to the decreased water content. This is because fluids and ions are transported through the hydrogel component of the contact lens material. A minimum sodium ion and hydraulic permeability of 0.2 x 10^-6 cm₂sec^-1 was claimed to be necessary for contact lens movement, which is similar to the value for polyHEMA (0.18 x 10^-6 cm₂sec^-1). A balance therefore has to be reached between maximizing oxygen transmission while still allowing sufficient hydraulic flow to prevent hydrophobic binding of the contact lens to the cornea.

In biphasic co-continuous silicone hydrogel materials such as lotrafilcon A, the oxygen and fluid permeability are "uncoupled," allowing a greater level of hydraulic and ionic permeability than would be available through a polyHEMA with an equivalent water content. As a result, contact lenses made from this material with a water content of 24 percent display adequate lens movement while still benefiting from the additional oxygen permeability. Although PureVision has a water content of 36 percent, it does not appear to have different movement characteristics to Focus Night & Day. Both silicone hydrogel lenses appear to move more than conventional hydrogel contact lenses.

Mechanical Properties

Pure silicone materials are extremely elastic and tend to adhere to the cornea with a suction effect. The materials of silicone hydrogel contact lenses are fortunately much less elastic and similar to HEMA. Thus adhesion is not an issue with these lenses enabling unrestricted movement and tear flow beneath the lens. However, following overnight wear, it is very likely that lenses will adhere for a few minutes after eye opening (as with other soft and rigid lenses). This is because of reduction in tear production overnight and the absence of a post-lens tear film.

Silicone hydrogels contact lenses are much stiffer than their conventional hydrogel contact lens counterparts. It is this property that gives these lenses their excellent handling characteristics. The modulus of silicone hydrogel materials is 110 to 120 g/mm (1.1-1.2 MPa) which is more than twice that of polyHEMA. As a consequence, the stiffer material does not drape over the cornea as easily.

	Figure 5: Lens fluting (edge lift) of a silicone hydrogel lens

When silicone hydrogel contact lenses are too flat, the lens can exhibit edge lift or slight fluting and cause discomfort (Figure 5). Poor fit has been cited as a cause of post-dispensing discontinuations with Focus Night & Day contact lenses. The additional steeper base curve that has recently been introduced by CIBA Vision appears to have overcome the flat fitting and associated discomfort. In a recent clinical trial at the CCLR, University of Waterloo, 98 percent of subjects could be satisfactorily fitted with either an 8.4mm or 8.6mm base curve.

Summary

Table 1 summarizes the differences between the two silicone hydrogel contact lens materials and compares them with the Acuvue lens material.

Visit www.siliconehydrogels.org for more information.

Desmond Fonn, Kathryn Dumbleton and Lyndon Jones are at the Centre for Contact Lens Research, School of Optometry, at the University of Waterloo in Waterloo, Ontario, Canada.

Renée du Toit and Deborah Sweeney are at the Cornea and Contact Lens Research Unit at the University of New South Wales in Sydney, Australia.