Article Date: 9/1/2011

The Many Faces of Silicone Hydrogel Contact Lenses
SILICONE HYDROGEL CONTACT LENSES

The Many Faces of Silicone Hydrogel Contact Lenses

More than a decade after their introduction, researchers are still working to enhance the lens-wearing experience.

By Loretta Szczotka-Flynn, OD, PhD

Silicone hydrogel (SiHy) contact lenses made of highly oxygen permeable polymers were approved and introduced into the United States meant to provide a rebirth for the extended wear modality. They provided more oxygen transmissibility in a soft lens than the contact lens market had ever experienced. Manufacturers hoped to decrease the swelling response, improve the physiologic response to lens wear, and even decrease the incidence of microbial keratitis during extended wear. Silicone hydrogel lenses were successfully shown to decrease the swelling response1 and improve the corneal physiology as measured by limbal hyperemia,2 limbal neovascularization,3 epithelial microcysts,4 tear lactate dehydrogenase (LDH)—an enzyme used as a marker to signify cell stress,5 endothelial cell morphology,6 and Pseudomonas aeruginosa binding to exfoliated corneal surface cells.7-12 However, we now know that that incidence of microbial keratitis remains unchanged from that experienced with traditional hydrogels.13

The positive attributes of these lenses made some of us wonder why they were released and heavily marketed mainly for extended wear over a decade ago. In fact, practitioners embraced the breakthrough polymers for daily wear, making them the most prescribed class of lenses today.14 The two “first generation” lenses made of balafilcon A (PureVision, Bausch + Lomb) and lotrafilcon A (Night & Day, CibaVision) became rapidly prescribed for daily wear and are still widely prescribed today. Subsequently, other manufacturers followed with new polymers of their own. We now have 10 distinct polymers available in the United States (Table 1), including one manufactured as a daily disposable lens and two that can be lathe cut and customized.

Relationships Between Material Properties

The newer SiHy polymers vary from the traditional hydrogel materials such that they could be classified into a different group. By now, most of us are familiar with the unpredictable relationships between water content, oxygen permeability and modulus in silicone hydrogel materials. Unlike low oxygen permeable (Dk) hydrogels—which behave more homogeneously, (for example, increasing water content linearly increases oxygen permeability)–silicone hydrogel materials have variations between the material property relationships that make certain lenses stand out from the linear predicted relationships.

Dk vs. water content: With traditional hydrogel lenses, Dk increases as water content increases because the water transports oxygen. The opposite trend occurs with silicone hydrogel lenses: Oxygen permeability decreases as water content increases. In other words, adding silicone components into hydrogel materials enhances oxygen transmission without increasing water content because oxygen is more soluble in silicone than it is in water.15. Within first generation lenses, the lotrafilcon A material provides the highest Dk (140 X10−11 (cm2/sec)[mlO2/mlXmmHg]), yet has the lowest water content (24%). However, the newest lathe cut material, efrofilcon A, deviates greatly from the linear regression line with unusually high water content (74%) for its Dk value (60 X10−11 (cm2/sec)[mlO2/mlXmmHg]) (Figure 1).

Figure 1. Dk vs. Water Content.

Dk vs. modulus: In Figure 2, one can appreciate that increased Dk is associated with increased modulus in silicone hydrogel lenses. Lotrafilcon A with the highest Dk also has the greatest modulus (1.4 MPa). The next generation of lenses provided significantly softer materials such galyfilcon A (modulus 0.4 MPa) which subsequently had the lowest Dk available at that time (60 X10−11 (cm2/sec)[mlO2/mlXmmHg]). However, not all lenses follow this pattern. For example a third generation product made of a siloxy macromer backbone (comfilcon A) has an unusually high Dk (128 X10−11 (cm2/sec)[mlO2/mlXmmHg]) for its given modulus (0.75 MPa).

Figure 2. Dk vs. Modulus.

Modulus vs. water content: This is the most predictable relationship in silicone hydrogel lens dynamics. As water content increases, the modulus decreases with most points hovering closely to the predicted regression line (Figure 3). This means that lenses with higher water content generally are softer.

Figure 3. Modulus vs. Water Content.

It is important to note these properties, then choose materials and lens designs selectively for your patients. For example, the highest Dk material may be required for patients with high prescriptions or a history of hypoxic complications. A higher modulus lens may be desirable for handling or masking mild corneal irregularity. Alternatively, a low modulus lens may be desired to eliminate mechanically driven adverse responses such as superior epithelial arcuate lesions and focal papillary conjunctivitis when Dk values are secondary needs.

Polymer Structures

Any hydrogel can be thought as having a “washing line” polymer structure as described by Tighe.15 One can think of hydrogels as having a long backbone from which different chemical groups, or monomers, are suspended. For example, HEMA, methacrylic acid, and N-vinyl pyrrolidone (NVP) are monomers used to attract and bind water in conventional hydrogels.15 In silicone hydrogel lenses, (hydrophobic) silicones are attached to the “washing line” to increase oxygen permeability.

The caveats of combining hydrophobic silicone based monomers with hydrophilic monomers such as HEMA revolve around phase separation and optical clarity. Two general methods have been used in many of the silicone hydrogel materials to date, either separately or in combination, to achieve compatibility between the hydrophobic and hydrophilic components. The first is to modify a “TRIS” (trimethylsilyl) monomer, a long-used component of contact lens material chemistry that was routinely used to convert PMMA into gas permeable rigid materials. In fact, in 1979, the “Tanaka patent” described a specific modification of TRIS by inserting polar groups into the molecule to aid its miscibility with hydrophilic components.16 The second approach uses macromer technology in which large monomers of structural units taken from the silicone rubber backbone are often interspersed with poly-ethylene glycol (PEG) sequences to provide hydrophilicity.16 In direct comparisons, higher oxygen permeability is achieved using siloxy macromers compared to the “TRIS” approach, but both methods may be used in a given lens.

Thus far, one can see that increasing silicone content brings with it an increased modulus and most importantly increases in Dk. Although the benefit of increasing silicone is to increase oxygen permeability, higher silicone content brings with it challenges of decreased wettability, increased lipid interaction, and fears of decreased water transport and lens binding. Lens binding has been addressed in at least one patent describing hydraulic and ionic permeability of biphasic materials.15 As the water content decreases in silicone hydrogel lenses, ionic permeability becomes critical for lens movement. Hydraulic flow through the lens is capable of maintaining the hydrodynamic boundary layer between the lens and the eye to avoid hydrophobic binding.15 Manufacturers have done an admirable job achieving this as lens binding among all silicone hydrogel materials has been rarely encountered.

Figure 4. A moist environment attracts hydrophilic groups.

Figure 5. A dry environment stabilizes hydrophobic groups, causing them to predominate on the lens surface. FIGURES ORIGINALLY APPEARED IN “CONTACT LENS WETTING: WHY IT MATTERS, WHAT WORKS, WHAT DOESN'T WORK,” PUBLISHED BY ETHIS COMMUNICATIONS.

Alternatively, decreased wettability and increased lipid deposits have been addressed through surface treatments and internal wetting agents. To understand these issues on a molecular level, I like to recall the “washing line” analogy. Most of us think of contact lens plastics as static, that is, not in any state of motion. However, the molecules at the lens surface are constantly changing.17 The hydrophilic and hydrophobic moieties within the polymer can rotate about their single bonds until a stable state is found. (Figure 4) Since likes attract, the hydrophobic groups (demonstrated in yellow) will tend to cluster where they encounter each other, air, or otherwise dry surroundings (Figure 5).

Lens manufacturers have addressed such problems with different methodologies. In the first-generation lenses, surface treatments are used to “hide” the silicone from the surface of the lens to keep it wettable. For example, lotrafilcon A lenses have a plasma surface treatment, which is a chemically uniform, dense, highly refractive coating. Balafilcon A lenses receive their surface treatment through a plasma oxidation process. Oxidation of the TRIS molecules in balafilcon A causes glassy silicate islands to form over the surface of the material.16 The next generation of materials—galyfilcon A and senofilcon A—are not surface-coated. They use an internal wetting agent, which sequesters the silicone within the center of the lens and keeps the outside surfaces of the lens very hydrophilic. Later generations of SiHy materials, for example comfilcon A, do not require surface treatments or internal wetting agents. Such lenses typically are composed of siloxy macromers interspersed with other monomers to maintain hydrophilicity.

Solution manufacturers are keenly aware of these hydrophobic phenomena and are constantly finding ways to reverse the hydrophobic tendencies of silicone hydrogel lenses. For example, they can incorporate surface active agents that have both hydrophobic and hydrophilic domains. The hydrophobic regions are capable of attracting to the hydrophobic surfaces on the lens, while the hydrophilic domains effectively hold moisture to the lens surface.

In summary, the majority of all lenses—including silicone hydrogel lenses—are prescribed for daily wear. The “rebirth of extended wear” secondary to the launch of silicone hydrogel lenses never came into reality. In fact, in 2010, soft lens extended wear accounted for only 7% of all fits internationally.14 So although silicone hydrogel lenses have improved corneal physiology1-5 and comfort as some authors have claimed,18 they have not overcome the traditional problems of infection and inflammation associated with extended wear.13,19 Microbial contamination of the silicone hydrogel lens surfaces continues to plague the contact lens wearer resulting in various adverse events.20 Solution manufacturers have provided us with care products that are biocompatible with these intriguing polymers. Contact lens care solutions have to maintain microbial efficacy, while providing enhanced wettability on these silicone hydrogel products. I look forward to another decade of progress on these materials, their surface coatings, antimicrobial characteristics of lenses and solutions, and methods to enhance the wetting of these inherently hydrophobic surfaces. CLS

References

1. Steffen RB, Schnider CM. The impact of silicone hydrogel materials on overnight corneal swelling. Eye Contact Lens. 2007;33(3):115-120.
2. Papas EB, Vajdic CM, Austen R, Holden BA. High-oxygen-transmissibility soft contact lenses do not induce limbal hyperaemia. Curr Eye Res. 1997;16(9):942-948.
3. Dumbleton K, Richter D, Simpson, Fonn D. A Comparison of the vascular response to extended wear of conventional lower dk and experimental high dk hydrogel contact lenses. Optom Vis Sci. 1998;75(12s):170.
4. Keay L, Sweeney DF, Jalbert I, Skotnitsky C, Holden BA. Microcyst response to high Dk/T silicone hydrogel contact lenses. Optom Vis Sci. 2000;77(11):582-585.
5. Ladage PM, Yamamoto K, Ren DH, Li L, Jester JV, Petroll WM, Cavanagh HD. Effects of rigid and soft contact lens daily wear on corneal epithelium, tear lactate dehydrogenase, and bacterial binding to exfoliated epithelial cells. Ophthalmology. 2001;108(7):1279-1288.
6. Doughty MJ, Aakre BM, Ystenaes AE, Svarverud E. Short-term adaptation of the human corneal endothelium to continuous wear of silicone hydrogel (lotrafilcon a) contact lenses after daily hydrogel lens wear. Optom Vis Sci. 2005;82(6):473-480.
7. Latkovic S, Nilsson SE. The effect of high and low dk/l soft contact lenses on the glycocalyx layer of the corneal epithelium and on the membrane associated receptors for lectins. CLAO J. 1997;23(3):185-191.
8. Ren DH, Petroll WM, Jester JV, Ho-Fan J, Cavanagh HD. The relationship between contact lens oxygen permeability and binding of Pseudomonas aeruginosa to human corneal epithelial cells after overnight and extended wear. CLAO J. 1999;25(2):80-100.
9. Ren DH, Petroll WM, Jester JV, Ho-Fan J, Cavanagh HD. Short-term hypoxia downregulates epithelial cell desquamation in vivo, but does not increase Pseudomonas aeruginosa adherence to exfoliated human corneal epithelial cells. CLAO J. 1999;25(2):73-79.
10. Ren DH, Yamamoto K, Ladage PM, Molai M, Li L, Petroll WM, Jester JV, Cavanagh HD. Adaptive effects of 30-night wear of hyper-O(2) transmissible contact lenses on bacterial binding and corneal epithelium: a 1-year clinical trial. Ophthalmology. 2002;109(1):27-39; discussion 39-40.
11. Imayasu M, Petroll WM, Jester JV, Patel SK, Ohashi J, Cavanagh HD. The relation between contact lens oxygen transmissibility and binding of Pseudomonas aeruginosa to the cornea after overnight wear. Ophthalmology. 1994;101(2):371-388.
12. Ren H, Petroll WM, Jester JV, Cavanagh HD, Mathers WD, Bonanno JA, Kennedy RH. Adherence of Pseudomonas aeruginosa to shed rabbit corneal epithelial cells after overnight wear of contact lenses. CLAO J. 1997;23(1):63-68.
13. Stapleton F, Keay L, Edwards K, Naduvilath T, Dart JK, Brian G, Holden BA. The incidence of contact lens-related microbial keratitis in Australia. Ophthalmology. 2008;115(10):1655-1662.
14. Morgan PB. Woods C, Tranoudis I, et al. International contact lens prescribing in 2010. Contact Lens Spectrum. January 2011.
15. Tighe B. Silicone hydrogels-what are they and how should they be used in everyday practice? Optician. 1999;218(5726):31-32.
16. Tighe B. Trends and developments in silicone hydrogel materials. September 2006. siliconehydrogels.org/editorials/sep_06.asp
17. Tonge S, Jones L, Goodall S, Tighe B. The ex vivo wettability of soft contact lenses. Curr Eye Res. 2001;23(1):51-59.
18. Dillehay SM. Does the level of available oxygen impact comfort in contact lens wear? A review of the literature. Eye Contact Lens. 2007;33(3):148-155.
19. Szczotka-Flynn L, Diaz-Insua M. Risk of corneal inflammatory events with silicone hydrogel and low dk hydrogel extended contact lens wear: a meta-analysis. Optom Vis Sci. 2007;84(4):247-256.
20. Szczotka-Flynn L, Lass JH, Sethi A, Debanne S, Benetz BA, Albright M, Gillespie B, Kuo J, Jacobs MR, Rimm A. Risk factors for corneal infiltrative events during continuous wear of silicone hydrogel contact lenses. Invest Ophthalmol Vis Sci. 2010;51(11):5421-5430.

Dr. Szczotka-Flynn is a professor at the Case Western Reserve University Department of Ophthalmology & Visual Sciences and is director of the Contact Lens Service at University Hospitals Case Medical Center. She has received research funding from Alcon, Ciba Vision, CooperVision and Vistakon.


Contact Lens Spectrum, Issue: September 2011