HIGHLIGHTS FROM THE OPTOMETRIC MANAGEMENT SYMPOSIUM ON CONTEMPORARY EYE CARE

Looking at Silicone Hydrogels Across Generations

Demystify the various polymers and lens treatments that characterize silicone hydrogel lenses.

By Loretta Szczotka-Flynn, O.D., M.S., F.A.A.O.

Silicone hydrogel contact lenses are unique in many ways — even their very definition. We can't discuss silicone hydrogels as one cohesive group, as we sometimes do with low-Dk hydrogels. Nor can we consider silicone hydrogel lenses all part of a single-material family. Major differences within this class exist. In fact, in this material family, the lenses are less like sisters and more like cousins — in some cases, distant cousins.

For this reason, silicone hydrogels have evolved into first-, second- and third-generation contact lenses. Each new generation doesn't replace or necessarily build on the previous one. Instead, these three generations — all valuable options — have different polymer chemistries, treatments and material-property relationships (ie, the interaction of modulus with water content or water content with Dk values). Knowing about these different generations and their different material-property relationships will help you select the most appropriate silicone hydrogel lens for each patient.

Polymer "backbones"

Silicone hydrogels rely on silicon, rather than water, to transport oxygen through a contact lens. Polymer chemistry, which makes this possible, probably is the most important property that defines the three silicone hydrogel generations. Currently, there are two broad methods that chemists use to provide silicon for silicone hydrogel contact lenses: TRIS [(trimethylsiloxysily) propyl vinyl carbamate] and siloxy macromers.¹

The first-generation silicone hydrogels, such as balafilcon A (PureVision, Bausch & Lomb) and lotrafilcon A (Night & Day, CIBA Vision), use the TRIS molecule with or without combining it with other silicone elastomer sequences.¹ The TRIS molecule has three methacrylated trimethyl silicone groups stemming from a primary silicone group.

In a slightly modified molecule, a polar group is added without changing the trimethyl structure. Often, this is referred to as the Tanaka monomer because it was patented in 1979 by the Toyo Contact Lens Company with Kyoichi Tanaka listed as one of the inventors. Second-generation silicone hydrogels, such as galyfilcon A (Acuvue Advance, Vistakon) and senofilcon A (Acuvue Oasys, Vistakon), use the Tanaka monomer.¹ Vistakon improved the Tanaka monomer even further and added other siloxy macromers, hydrophilic monomers and polyvinyl pyrrolidone (PVP), which serves as an internal wetting agent.

The second approach uses siloxy macromer structures. These are based on a silicone rubber backbone, often interspersed with hydrophilic polyethylene glycols (PEGs) for hydrophilicity.

First-generation polymers

First-generation silicone hydrogels use some form of the TRIS molecule and may include macromers containing silicone rubber sequences. The balafilcon A material has a Dk of 99 barrers and concentrates on a TRIS-based approach to providing silicon.¹ Bausch & Lomb may have taken advantage of the TRIS molecule during plasma oxidation of the lens surface because oxidation of TRIS produces hydrophilic, glassy silicate islands.

Bausch & Lomb's PureVision lens is the only silicone hydrogel that technically falls into FDA Group III. All other silicone hydrogel lenses are in FDA Group I. I believe none of the silicone hydrogels really fit into any of the four FDA groups that were developed long ago to assess solution compatibility. Silicone hydrogels are in a class by themselves.

Lotrafilcon A concentrates on siloxy macromers in addition to a TRIS-based backbone. It has the highest Dk (140 barrers) of any silicone hydrogel material and also has the highest modulus. Lotrafilcon A took advantage of TRIS and other hydrophilic monomers to phase independently, producing a material that has two distinct phases: a water and gel phase that the manufacturer calls co-continuous phases.² This allows for high oxygen permeability, as well as high water and sodium permeability.

2nd- and 3rd-generation polymers

As mentioned, second-generation lenses are based on the Tanaka monomer, which Vistakon improved after the patent had expired. Galyfilcon A is a mixture of the Tanaka monomer, a siloxy macromer and hydrophilic monomers, such as HEMA and N,N-dimethylacrylamide (DMA).¹ The lens has a Dk of 60 barrers, the lowest of all the silicone hydrogel lenses, but it has a very high water content (47%). Vistakon uses a similar polymer, senofilcon A, for its Acuvue Oasys lens.

Comfilcon A (Biofinity, CooperVision) was the first third-generation polymer. With no TRIS-based derivatives, all of the silicon added to the material's chemistry is based on siloxy macromers¹ — a unique way to combine the lens' silicone and water properties. The patent claims that the material uses two siloxy macromers of different sizes that, when used in combination, produce very high oxygen permeability (for a given water content). The lens has a Dk of 128. Enfilcon A (Avaira, CooperVision) is another third-generation material that's naturally wettable. The enfilcon A material is 46% water. The lens has a low modulus of 0.5 MPa and a Dk of 100.

Lens treatments

Some silicone hydrogel lenses are designed with the use of surface treatments to modify the surface and shield the silicone. After all, silicone and water don't mix, making it difficult for the polymer chemists to produce an optically clear product. Surface treatments were required on the first silicone hydrogels to hide the silicone from the tear layer and to make the lenses wettable. Lenses manufactured more recently have employed other types of treatments, such as internal wetting agents and chemistry changes, to promote clarity and better wettability.

Figure 1. As water content increases, modulus decreases. This relationship is the same for all silicone hydrogel lenses. The more water a lens contains, the softer it is.

Figure 2. Oxygen permeability of silicone hydrogel materials increases linearly as the modulus increases. The highest Dk lens is also the stiffest although Biofinity lenses break this rule.

• First generation. Balafilcon A, lotrafilcon A and lotrafilcon B (O2 Optix, CIBA Vision) lenses have a treated surface to buffer the silicone from the ocular surface and the tear layer. These polymers go through a plasma treatment process, which adds to the expense of the contact lens because it must be performed after the lens is manufactured.

Lotrafilcon A and lotrafilcon B lenses undergo a plasma surface treatment in which a permanent coating is embedded on the surface of the plastic. The result is a chemically uniform, dense, highly refractive lens coating.³

Balafilcon A undergoes a different process called plasma oxidation, which involves oxidizing the TRIS structures. This creates hydrophilic, glassy silicate islands on the lens with hydrophobic areas in between. The hydrophilicity of the islands overcomes the hydrophobicity of the channels, so the lens remains wettable.³

• Second generation. Galyfilcon A and senofilcon A lenses aren't surface treated. They use PVP as an internal wetting agent. PVP is a long chain, high molecular weight molecule that sequesters the silicone within the core of the lens. In other words, it provides a hydrophilic layer on the surface of the material, enabling the lens surface to remain hydrophilic and wettable without an added coating.³

• Third generation. Comfilcon A and enfilcon A don't use surface treatments or internal wetting agents. Rather than shield the silicone from the surface or sequester it within the lens, these third-generation silicone hydrogel materials begin with a completely different backbone that's naturally wettable. The non-TRIS chemistry allows for enhanced compatibility between silicone moieties and hydrophilic domains.¹ In other words, lenses made of this polymer are inherently wettable.

Material-property relationships

Now that we know the differences in polymers and lens treatments, the question is how do the relationships between modulus, water content and Dk differ across silicone hydrogel materials? The answers contain some of the defining characteristics of the lens generations.

• Modulus vs. water content. Some relationships are predictable. If we look at modulus vs. water content, we find that as the water content increases, modulus decreases. More water means the lens is softer. This relationship holds true for all silicone hydrogel lenses on the market (Figure 1).

• Dk and modulus. Oxygen permeability of the silicone hydrogel materials increases linearly as the modulus — and lens stiffness — increases. For example, the highest-Dk silicone hydrogel lens, lotrafilcon A (Night & Day), is also the stiffest. But there are two silicone hydrogel lens materials that break this rule: comfilcon A (Biofinity) and enfilcon A (Avaira). Both lenses have a relatively high Dk value but a low modulus. Comfilcon A has the second highest Dk value, yet it has a low modulus of 0.75 MPa, so you don't have to sacrifice one for the other (Figure 2). Before the introduction of these newer silicone hydrogels, we had to choose between lens softness and oxygen permeability, but the comfilcon A and enfilcon A materials provide both.

Keep in mind that modulus isn't always our enemy. Sometimes practitioners, or patients, prefer a stiffer lens. I have patients that tell me they want a good old-fashioned lens that sits up nicely on their finger. Sometimes practitioners choose stiffer lenses to mask a little bit of corneal irregularity. However, modulus can cause some problems. Focal contact lens-induced papillary conjunctivitis, superior epithelial arcuate lesions or sudden epithelial defects are more frequent with higher modulus lenses — in particular the higher modulus silicone hydrogel materials.^1,2

• Dk and water content. With traditional hydrogels, water transports oxygen, and Dk increases with increasing water content. But there are limits to how much oxygen water can transport. For example, the Dk of a lens that's 100% water is still limited to approximately 90 or 100 barrers.¹

Silicone hydrogels practically reverse this relationship. Increased water content results in decreased Dk because silicone is predominantly responsible for transporting oxygen. The Night & Day lens has the highest Dk and the lowest water content, and the Acuvue Advance lens has the lowest Dk and the highest water content. However, Biofinity is an exception to the rule. Compared to other silicone hydrogels, Biofinity has the highest water content and a much higher water content for its Dk than expected, which contributes to a lower modulus.

Differences in generations to come

It's clear that first-, second- and third-generation silicone hydrogel lenses aren't mutually exclusive. But each generation has been developed with different polymers and lens treatments. As we greet the fourth generation, we're certain to see more advances in lens materials and lens capabilities. Until then, a thorough understanding of silicone hydrogels will continue to help meet patients' needs and solve their vision problems.

Loretta Szczotka-Flynn, O.D., M.S., F.A.A.O., is associate professor at Case Western Reserve University, Department of Ophthalmology in Cleveland, and director of the contact lens service at the University Hospitals Case Medical Center in Cleveland.