Improving Ocular Health and Comfort With Silicone Hydrogel Contact Lenses
Experts discuss the attributes of various lens materials and the tools used to measure lens wettability in the laboratory and in practice.
Three groups of silicone hydrogel lens materials are distinguished by different chemistries and surfaces. Here are insights from an expert.
By Loretta Szczotka-Flynn, OD, MS, FAAO
Silicone hydrogels are very different from other classes of contact lenses. And within the silicone hydrogel class itself, major differences exist as well. The silicone hydrogel materials currently on the market are so distinct, in fact, that they have been placed in three different groups of families to distinguish between them.
Similarities exist between materials developed from the same manufacturer, but this does not apply to all brands. Low-Dk hydrogels, for example, are considered more of a single-material family. They behave more homogenously in that increased water content results in improved oxygen permeability. Alternatively, the silicone hydrogel materials that have been commercially released thus far contain a variety of polymer chemistries, surface treatments and material properties that result in less predictable relationships.
In this article, I will discuss the different material chemistries and properties of silicone hydrogel lenses. Because of the differences within the silicone hydrogel class, you cannot predict properties, such as modulus and oxygen permeability from water content. In fact, one new material (comfilcon A) stands out from the others in breaking the Dk paradigm.
Combining hydrophobic silicone-based monomers with hydrophilic monomers, such as HEMA, has been challenging due to phase separation and impaired optical clarity. However, various methods have been developed to solve this incompatibility. The first method entails modifying the TRIS (trimethylsilyl) monomer, a long-used component of contact lens chemistry, routinely used to convert PMMA into gas permeable rigid materials. Soft contact lens companies modify TRIS by inserting polar groups into the molecule to aid its miscibility with hydrophilic components.1 The second approach involves using macromer technology and components from the silicone rubber backbone.1 Higher oxygen permeabilities are achieved with siloxy macromers other than the TRIS approach, although both methods can be used in a given lens.
■ First generation. The first two commercially released lens materials include lotrafilcon A (Night & Day) and balafilcon A (PureVision). These lenses take advantage of the phase separation inherent in silicone hydrogel monomer compatibility. In other words, the hydrophilic and silicone phases are separate and distinct within the material.
Lotrafilcon A, which is in FDA Group I, is a mixture of fluoroether macromer1,2 and TRIS that creates a low-water and nonionic material with a Dk of 140. Balafilcon A is the only silicone hydrogel lens in FDA Group III. This lens material primarily uses technology based on modified TRIS structures.1,2 It is a low-water, ionic lens and has a Dk of 99.
■ Second generation. The second phase of commercially released silicone hydrogel technology includes different materials using a combination of silicone-based and hydrophilic monomers and macromer technology.3 These materials — galyfilcon A (Acuvue Advance) and senofilcon A (Acuvue Oasys) — incorporate various monomers and macromers together with an internal wetting agent (polyvinyl pyrrolidone [PVP]).3 Galyfilcon A has a Dk of 60, whereas senofilcon A has a Dk of 103.
■ Latest generation: The newest material, comfilcon A (Biofinity), has no TRIS-based or PVP-based chemistry. Macromers are the only source of silicone.3 The lens has a Dk of 128, which is perhaps higher than some of its counterparts because of the non-TRIS based and non-PVP approach.
As you can see, silicone hydrogel lens materials can vary significantly from one another, and their distinct physical properties can affect performance. In fact, some materials need certain surface treatments to improve their wettability, while others do not.
Surface treatments for silicone hydrogel lenses vary in composition and effect. The first two materials released (lotrafilcon A and balafilcon A) as well as lotrafilcon B (O2Optix), require surface treatments to hide the silicone from the surface of the lens to keep it wettable.4 The treatment process must occur after the lens is manufactured, making it an expensive process.
Lotrafilcon A and B lenses have a plasma surface treatment, which is a chemically uniform, dense, high-refractive index coating. Balafilcon A lenses receive their surface treatment through a plasma oxidation process. Oxidation of the TRIS molecules causes glassy silicate islands to form over the surface of the material.3
The next phase of materials, which includes galyfilcon A and senofilcon A, saw the first release of inherently wettable lenses. These lenses use an internal wetting agent, which sequesters the silicone within the center of the lens and provides a hydrophilic layer on the outside surfaces of the lens. The internal wetting agent is PVP (Hydraclear), a long-chain, high molecular weight, flexible, humectant molecule.
The newest lens, made of comfilcon A, is wettable in its own right and does not require a surface treatment. In terms of the lens chemistry, this non-TRIS structure with siloxy macromers and other hydrophilic monomers permits enhanced compatibility between the silicone moieties and the hydrophilic domains.
Figure 1:As Dk increases, modulus usually increases. However, the newest silicone hydrogel technology does not necessarily follow this mold. Comfilcon A has the second highest Dk of all silicone hydrogel lenses, yet a relatively low modulus.
Just as silicone hydrogels differ from one another when it comes to polymer chemistry and surface treatments, the relationships between material properties also differ. These variations in material chemistry relate directly to differences in physical properties. In addition, certain relationships between physical properties, such as modulus, water content and oxygen permeability, can be predicted in the low-Dk material family. But this is not strictly followed in the silicone hydrogel class of lenses.
■ Modulus vs. water content: Across all silicone hydrogel lenses, as water content increases, the modulus decreases. Therefore, as in low-Dk silicone hydrogels, the relationship between water content and modulus is fairly linear as traditional theory dictates.
■ Dk vs. modulus: Generally, increased Dk is linked to increased modulus in silicone hydrogel lenses. For example, the highest Dk material (lotrafilcon A) is the stiffest, and the lowest-Dk material (galyfilcon A) is the softest. However, one lens material stands outside this continuum: comfilcon A. This lens is unusually soft for its given Dk value (Figure 1).
■ Dk vs. water content: With traditional low-Dk hydrogel lenses, Dk increases as water content increases because the water transports oxygen. The opposite relationship occurs with silicone hydrogel lenses: Dk typically decreases as water content increases. Silicone transports the oxygen, and lenses with higher water content traditionally have less silicone.
Comfilcon A, once again, breaks the trend with an unusually high water content for its Dk/t value (Figure 2). This lens has an entirely different chemistry.
Figure 2: DK typically decreases as water content increases with silicone hydrogels. However, senofilcon A has a higher water content than expected for its oxygen transmissibility. Comfilcon A also has an unusually high water content for its Dk/t value.
A NEW PARADIGM
As you can see, we have vastly different contact lenses in the silicone hydrogel family. And although the lenses are related, they are not alike. In fact, they are more like distant cousins. The earlier released products have TRIS-based chemistry, are surface-treated or contain PVP. The first nonsurface treated lenses were released in 2004. And in 2007, we see an inherently wettable lens that does not follow the traditional water, Dk and modulus relationship.
Understanding the unique characteristics of these silicone hydrogel materials and how they behave on the eye will enable you to prescribe the best lens for your patients' individual needs. CLS
|Dr. Szczotka-Flynn is associate professor in the department of ophthalmology at Case Western Reserve University and director of the Contact Lens Service at the University Hospitals Case Medical Center in Cleveland.|
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2. Suwala M, Glasier M, Subbaraman L, Jones L. Quantity and conformation of lysozyme deposited on conventional and silicone hydrogel contact lens materials using an in vitro model. Eye Contact Lens. 2007;33:138-143.
3. Brian Tighe. Trends and developments in silicone hydrogel materials. Editorial. September 2006. http://www.siliconehydrogels.com. Last accessed 5/18/07.
4. Jones L, Dumbleton K. Contact lens fitting today. Silicone hydrogels Part 1: Technological developments. Optometry Today. 2005;23-29.