SILICONE HYDROGELS

The Evolution of Silicone Hydrogel Lenses

The chemistry and material characteristics of current disposable silicone hydrogels in the United States.

By Brian Chou, OD, FAAO

Dr. Chou is in group practice in San Diego at Carmel Mountain Vision Care. He is the co-developer of EyeDock.com, the online contact lens reference for eyecare professionals.

The use of silicone — polymers containing an inorganic backbone of silicon and oxygen with organic side groups — in contact lenses is nothing new. Since the late 1970s, silicone elastomer (rubber) lenses have provided exceptional oxygen permeability. In fact, levels of overnight corneal edema with silicone elastomers measured significantly less than did levels during eye closure without lenses.

Most applications of silicone elastomers have been for pediatric aphakia. Perhaps the most recognized silicone elastomer lens, Silsoft (Bausch & Lomb), entered the market in 1984 with an FDA indication as a 30-day extended wear lens for aphakia.

Unfortunately, silicone elastomer contact lenses have a checkered past. Despite high oxygen permeability, these lenses contained no water and often adhered to the ocular surface because of inadequate fluid and ion transport. Additionally, the exceedingly hydrophobic nature of silicone elastomers resulted in poor lens wetting and rapid lipid deposition, leading to limited use of these lenses.

The Rebirth of Silicone Lenses

In 1999, the market entry of PureVision (Bausch & Lomb) ushered in the silicone hydrogel era. Unlike their earlier silicone elastomer cousins, silicone hydrogels combined the benefits of highly oxygen permeable silicone monomers (such as tris[trimethylsiloxy] silylpropylmethacrylate, or TRIS) with the fluid and ion transport benefits of hydrogel monomers (such as HEMA). TRIS is the silicon-containing monomer that previously allowed material scientists to convert PMMA into GP materials. In essence, more TRIS means more silicone and greater oxygen permeability.

It was no small feat for scientists to combine monomers such as TRIS, which is hydrophobic, with HEMA, which is hydrophilic. Mixing these monomers together usually produces an opaque polymer that's unsuitable for optical correction. This is why the manufacturing process for silicone hydrogels has been likened to mixing oil with water to create an optically clear product. It was such a great challenge to develop silicone hydrogels; it took decades for chemists to arrive at materials suitable for contact lenses. And over the last nine years since the first silicone hydrogel material entered the market, the category has evolved to include several different generations of silicone hydrogel materials.

Silicone Hydrogel Diversity

At present, there are seven different disposable silicone hydrogel lens materials commercially available in the United States. In order of their initial market entry as soft spheres, they are: balafilcon A (PureVision, B&L), lotrafilcon A (Night & Day, CIBA Vision), galyfilcon A (Acuvue Advance, Vistakon), lotrafilcon B (O₂Optix, CIBA), senofilcon A (Acuvue Oasys, Vistakon), comfilcon A (Biofinity, CooperVision) and enfilcon A (Avaira, CooperVision).

Although all silicone hydrogels share the hallmark of higher oxygen permeability, there is notable diversity in each silicone hydrogel material's polymer chemistry and resulting clinical performance. This is akin to the tremendous diversity within hydrogel materials, which range from the older polymacon to newer hioxifilcon, and within GP materials, which range from older silafocon to newer tisilfocon. It's worth understanding the different chemical strategies behind the silicone hydrogel copolymers in surmounting the challenges of combining their component monomers.

First-Generation Silicone Hydrogels

Balafilcon A is a homogeneous combination of a TRIS monomer derivative that's copolymerized with the hydrophilic hydrogel monomer N-vinyl pyrrolidone (NVP). The molecular strategy with this combination is the synthesis of a TRIS-like structure with polar groups to make it hydrophilic.

The molecular strategy of lotrafilcon A involves polymerizing unmodified TRIS and macromers with silicone elastomer sequences interspersed with hydrophilic polyethylene glycols (PEGs). Lotrafilcon A has a biphasic or two-channel molecular structure that's comprised of a fluoroether macromer copolymerized with a TRIS monomer and the hydrophilic monomer DMA (N, N-dimethylacrylamide). The fluorosiloxane phase allows for high oxygen permeability while the hydrogel phase transmits ions and water, allowing adequate lens movement.

Under usual circumstances, phase separation would result in an opaque polymer. However, if the size of the phase separation is less than the wavelength of light, the resulting polymer is optically clear. Lotrafilcon A uses incredibly fine phase separation to maintain optical clarity.

Both balafilcon A and lotrafilcon A are similar in that after lens manufacture, each lens is treated to create a hydrophilic surface. With balafilcon A, plasma oxidation in a gas chamber converts the TRIS structure on the surface into hydrophilic silicate that provides water-attracting bridges over the hydrophobic balafilcon underneath. With lotrafilcon A, plasma treatment creates a permanent, ultrathin (25nm) hydrophilic layer on the lens surface, hiding the TRIS structures underneath.

Second-Generation Silicone Hydrogels

Galyfilcon A and senofilcon A appear to use a more hydrophilic derivative of the TRIS monomer. This monomer was originally patented in 1979 by the Toyo Contact Lens Company with Kyoichi Tanaka and four others listed as inventors. Following the expiration of the original patent, Vistakon moved forward with synthesizing the Tanaka monomer through an improved process, combining it with a siloxy macromer and hydrophilic monomers such as HEMA and DMA.

Both galyfilcon and senofilcon are differentiated from the first-generation silicone hydrogels because no surface treatment is applied. Instead, both of these lenses incorporate a long-chain, high-molecular-weight polyvinyl pyrrolidone (PVP), which serves as an internal wetting agent for galyfilcon A (Hydraclear) and senofilcon A (Hydraclear Plus). PVP acts as a hydrophilic humectant, meaning that it attracts and retains moisture with the intent of keeping the lenses hydrated throughout the wearing day.

Finally, galyfilcon A (0.4 MPa) and senofilcon A (0.73 MPa) both have a significantly lower modulus than do balafilcon A (1.1 MPa) and lotrafilcon A (1.4 MPa). The relatively high modulus of first-generation silicone hydrogels has been associated with mechanical issues such as superior epithelial arcuate lesions, mucin balls, contact lens papillary conjunctivitis, conjunctival epithelial flaps and reports of initial lens awareness.

In light of these modulus issues, CIBA introduced lotrafilcon B (1.2 MPa) with a modulus lower than that of lotrafilcon A (1.4 MPa), although the two are chemically similar. Additionally, lotrafilcon B has a higher water content and lower Dk value compared to lotrafilcon A.

Published and anecdotal reports of the clinical performance of second-generation silicone hydrogels is impressive. A recent study (Young et al, 2007) of 496 hydrogel soft lens wearers refit into the aforementioned second-generation silicone hydrogel lenses for daily wear found that comfort improved significantly with the second-generation silicone hydrogels in various challenging environments.

Third-Generation Silicone Hydrogels

Comfilcon A (Biofinity, CooperVision) and enfilcon A (Avaira, CooperVision) are the latest silicone hydrogel entries. Both use a unique long-chain siloxane macromer combined with other components to result in a lens that features high oxygen permeability and a relatively low modulus. These materials are inherently wettable — no internal wetting agent or surface treatment is required.

Comfilcon A and enfilcon A have a fundamentally different chemistry that has allowed these third-generation silicone hydrogels to break the relationship between oxygen permeability and water content that other silicone hydrogel materials follow. Because oxygen is transmitted most efficiently through the silicone component of silicone hydrogels, increasing water composition generally decreases the oxygen permeability of the material. That is why for most silicone hydrogels, higher water content is associated with lower Dk. For example, lotrafilcon A has a water content of 24 percent with a Dk of 140 units. In comparison, lotrafilcon B has a higher water content of 33 percent and a Dk of 110 units. In contrast, comfilcon A has a water content of 48 percent and a Dk of 128 units, while enfilcon A has a water content of 46 percent and a Dk of 100 units. For silicone hydrogel materials, these are unexpectedly high Dk values for their corresponding water contents.

In turn, the relatively high water content affords comfilcon A a modulus of 0.75 MPa and enfilcon A a modulus of 0.5 MPa, both of which are less than the modulus of lotrafilcon A and B and balafilcon A, but greater than that of galyfilcon A.

What's Next?

Over the years, several trends have emerged with silicone hydrogels. One is the shift in chemistry — from surface treatments to internal wetting agents and now, to an inherently wettable silicone hydrogel material. Furthermore, our industry no longer holds a mentality of pursuing high Dk at all costs. Rather, practitioners and manufacturers recognize that material characteristics such as wettability, modulus, water content and deposit resistance together exert a significant influence on clinical performance.

Our industry has also undergone other notable silicone hydrogel trends:

• A shift from prescribing silicone hydrogels for extended wear toward daily wear. Strong evidence exists that high oxygen transmissibility hasn't significantly decreased the rate of microbial keratitis in extended wear. Daily removal of contact lenses remains the safest wearing modality.
• Increasing parameter availability, including toric (Acuvue Advance for Astigmatism and Acuvue Oasys for Astigmatism, Vistakon; PureVision Toric, B&L; and Air Optix for Astigmatism, CIBA) and multifocal (PureVision Multi-Focal, B&L) options as well as limited custom parameters (O₂Optix Custom, CIBA). We can expect the future to bring daily disposable, multifocal toric and cosmetically tinted silicone hydrogel lens options.
• Some multipurpose solutions have incompatibilities with specific silicone hydrogel materials. Be sure to prescribe specific care systems and to devote attention to new contact lens solutions.

Prescribe the Best Lens for Each Patient

As we shift our emphasis away from selling contact lenses and toward instilling value in rendering contact lens services, it's more important than ever for us to understand the intricacies of each silicone hydrogel material. Doing so allows us to appropriately prescribe the best contact lenses of any type for each patient. In turn, patients will recognize our expertise as a contact lens professional rather than a simple dispenser of contact lenses. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #151.