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Contact Lens Materials Update 2008

A look at the characteristics of and technology behind today's soft and gas permeable contact lens materials.

BY GREGORY J. NIXON, OD, FAAO

In today's eyecare arena, contact lens practitioners have the luxury of choosing from a multitude of contact lens designs and materials to meet the needs of almost any contact lens patient. We can attribute our vast array of choices to renewed and growing interest in contact lens research and development. Almost all major contact lens manufacturers employ or consult with polymer chemists and bench scientists to produce new and innovative materials to advance contact lens performance.

Contemporary contact lens materials have become highly sophisticated chemical compounds with complex polymer combinations to add unique characteristics to lens stability and functioning. However complex the structure, the goals of any lens material are the same: to produce a stable, comfortable lens with crisp optics that reduces or eliminates any physiological compromise to ocular health. Researchers continue to explore the key chemical components of contact lens materials to provide optimal design, oxygen permeability, wettability, surface integrity and deposit resistance.

This article will discuss lens material technology and characteristics for both soft and GP lens materials.

Soft Contact Lens Materials

Basics By definition, a soft or hydrophilic lens absorbs and contains water within the matrix of the core polymer chains, creating what we commonly refer to as a hydrogel material. A main water-absorbing chemical monomer of many soft lenses is hydroxyethyl-methacrylate (HEMA).

Having water within the matrix of a lens results in many advantages and disadvantages. First, water embedded within the polymer chains of the material provides the "soft" flexible nature of a lens that translates to greater immediate patient comfort compared to GP lenses. Water also provides a mechanism for oxygen delivery to the cornea due to its ability to dissolve and transmit oxygen.

On the other hand, water can also diminish lens durability, which often requires the addition of ethylene glycol or other crosslinking agents to stiffen the lens and provide structural stability. Additionally, water can serve as a breeding ground for microbial growth and therefore requires stringent disinfection products and protocols as well as care system and lens replacement compliance to avoid and limit lens contamination and ocular infection.

The balance of these advantages and disadvantages has resulted in manufacturers experimenting with the degree of water content to optimize these various factors of lens performance. In fact, the Food and Drug Administration defines its classification of soft lenses according to the material properties of water content and electrostatic charge (Table 1).

Oxygen Permeability The oxygen permeability of a contact lens is characterized by its Dk value. Dk is defined as the diffusion coefficient (D) multiplied by the solubility constant (k). A lens Dk value represents the chemical properties of the lens material itself. A more clinically applicable measure is the oxygen transmissibility, or Dk/t, which represents the combined properties of the lens material and lens design. That is, oxygen transmissibility values account for the thickness of the lens through which oxygen must travel to reach the corneal surface. By default, the thickness value (t) is the center thickness of the lens. Because the center thickness of a lens can vary widely for a given lens power, most Dk/t values are referenced for a –3.00D lens.

As with lens power, there is a large discrepancy in lens thickness profile from the center to the edge of a given lens. For example, a minus powered lens with a thicker edge will certainly have less peripheral oxygen transmissibility than that lens' given Dk/t value as measured from the thinner lens center. Therefore, investigations into more advanced methods of determining the best indicator of the oxygen performance of a finished lens design are ongoing. As this review focuses mainly on the material properties of lenses, I will use the published Dk values of the products discussed throughout the remainder of the article.

Traditional hydrogel lenses derive all of their oxygen permeability from their oxygen solubility, which is solely determined by their water content. Therefore, one of the perceived benefits of increasing water content is to maximize oxygen permeability. However, as Snyder (2004) points out, "because the highest practical water content for any material is about 80 percent, maximum permeability is approximately 40 Dk." It is this permeability limitation of traditional hydrogels that sparked the research on advancing chemical combinations that resulted in the development of silicone hydrogel materials.

This advancement of material chemistry resulted in a tremendous increase in oxygen permeability because silicone hydrogels have both oxygen solubility from the hydrogel component of the lens and oxygen diffusion from the silicone component of the lens. Dk values of current spherical silicone hydrogels range from 60 to 140 and, unlike traditional hydrogels, are not solely influenced by water content (Figure 1). To the contrary, most silicone hydrogels increase in their oxygen permeability as water content decreases secondary to a subsequent increase in silicone content. As mentioned above, it's important to note that while many listings refer only to lens Dk, it is the Dk/t of a lens that is the relevant clinical factor because this measurement takes into account the thickness through which the oxygen must travel.

Figure 1. Silicone hydrogel lenses are different from hydrogel lenses in that there is not always a direct link between water content and Dk.

Regardless, the improved oxygen characteristics of the silicone hydrogels results in significantly more oxygen to the ocular surface compared to any other soft lenses. At their introduction, many silicone hydrogel Dk values exceeded the long-held minimum Dk value of 87 for limited overnight corneal swelling as determined through the work of Holden and Mertz (1984). Therefore, their entrance into the market provoked a resurgence of interest in overnight wear. While many patients have enjoyed the use of these new materials in a flexible wear, extended wear or 30-day continuous wear modality, studies have shown no decrease in risk for developing microbial keratitis with these new materials in overnight wear.

Modulus The unique feature of contact lens modulus or material stiffness reemerged with the introduction of silicone hydrogels. While lenses with the highest silicone content typically enjoyed the highest oxygen permeability, those lenses also had the highest modulus subsequent to the inherent stiffness of the silicone molecule (Figure 2). Initially, the highest silicone content lenses also had the lowest water content, which also contributed to a higher modulus. The increase in lens modulus has effects on initial lens comfort and persistent lens awareness from both lens stiffness and increased lens movement. The stiffness of the silicone hydrogels also brought about a return of GPC-like irritation from stiff lens interaction with the superior palpebral conjunctiva.

Figure 2. Modulus versus Dk for various silicone hydrogel materials.

The negative impact of high-modulus silicone hydrogels on some patients provoked two companies to develop additional products with lower silicone content. CIBA Vision introduced O₂Optix and Air Optix Aqua (lotrafilcon B: Dk = 110, 33 percent water) as a lower-modulus silicone hydrogel alterative to the company's Night and Day lens (lotrafilcon A: Dk = 140, 24 percent water) by increasing the water content.

Avaira (enfilcon A: Dk = 100, 46 percent water), the lower-modulus silicone hydrogel offering from CooperVision, actually has a lower water content than CooperVision's Biofinity lens (comfilcon A: Dk = 128, 48 percent water). CooperVision explains that the lower modulus results from using longer siloxane polymer chains that require a lower overall silicone content.

Lens Wettability Incorporating silicone molecules into contact lens materials creates challenges to overall contact lens performance. Silicone molecules are hydrophobic in nature, which requires silicone hydrogel lenses to have some degree of chemical alteration to allow for adequate compatibility with the corneal epithelium on the posterior lens surface and with the tear film and palpebral conjunctiva on the anterior lens surface. Failure to create a smooth, water-loving surface can result in lens dehydration, poor wettability, increased lens friction and lens deposition. These factors can result in clinical manifestations of lens-induced dry eye, corneal staining, lid wiper epitheliopathy, GPC, increased lens awareness and overall reduced contact lens tolerance.

In general, wettability of a contact lens relates to its ability to attract and interact with the precorneal tear film. The goal is for the lens to attract the tear film so that it spreads evenly, consistently and completely across the lens surface. Clinical researchers estimate a surface's wettability by testing its interaction with water. Water has an inherently high surface tension as a result of its strong cohesive forces, or its molecular attraction to itself. The greater the adhesion forces, or attraction, a given material has for water, the better that material will counter the surface tension effects of the water and "bind" the water molecules to coat the lens surface. One measure of wettability is the contact angle formed when a drop of water is placed onto the lens surface. A highly wettable lens will attract the water, spreading it across the lens surface and creating a low contact angle (the ideal angle is 0 degrees); a poorly wetting surface (with low adhesive forces for water) will "repel" the water and allow the droplet to rest on the lens surface, creating a large contact angle (such as ≥90 degrees).

Based on these wettability characteristics, the challenge for contact lens manufacturers is to turn the hydrophobic silicone surface into one that will attract and spread the precorneal tear film. The first silicone hydrogels on the market, Bausch & Lomb's PureVision (balafilcon A: Dk = 101, 36 percent water) and CIBA's Night & Day lens (lotrafilcon A: Dk = 140, 24 percent water) addressed this issue of lens wettability by utilizing lens surface treatments. CIBA's method involves a proprietary surface plasma treatment that binds to the lens surface. This results in a newly formed hydrophilic, yet electrically neutral surface that falls under the FDA group 1 material classification. Bausch & Lomb's patented Performa surface process uses additive agents that become an inherent part of the lens matrix, so it is not a surface coating subject to removal through lens wear or rubbing. The surface remains electrically charged and is classified as an FDA group 3 material.

Some of the later silicone hydrogel entries into the market incorporate wetting agent compounds directly into the lens matrix from the beginning of the polymer process before the lenses are molded. Vistakon's Acuvue Advance (galyfilcon A: Dk = 60, 47 percent water) and Acuvue Oasys (senofilcon A: Dk = 103, 38 percent water) utilize Hydraclear, a wetting agent with polyvinyl-pyrrolidone (PVP). Alternatively, CooperVision's Biofinity and Avaira incorporate the company's Aquaform technology, a hydrogen bonding process, as part of its comfilcon A and enfilcon A materials that result in a silicone hydrogel polymer that is hydrophilic without the need for any surface treatments or wetting agents.

Regardless of the method (Table 2), by addressing the poor wettability of silicone, the new silicone hydrogel materials have proven to stabilize the silicone molecules, unlike the silicone elastomer lenses of the 1980s and 1990s. The net effect is that patients not only benefit from improved oxygen performance, but most patients tend to have better comfort and less contact lens-induced dry eye with silicone hydrogels. The availability of these materials in increasing parameters including multifocal and toric lens designs (Table 3) allows more individuals to benefit from these high performance lenses.

The issue of lens wettability is one of the myriad of factors related to contact lens-induced dry eye. Because lens-induced dryness or discomfort remains a leading cause of contact lens intolerance and cessation of lens wear, manufacturers are making efforts to improve many attributes of contact lens solutions and materials. In addition to the work being done on silicone hydrogels, manufacturers are employing some novel innovations in traditional hydrogel materials as well.

High-water-content materials have a strong association with dry eye complaints, so it's not surprising that many unique lens alterations are made for group 2 and group 4 materials. For example, CooperVision adds phosphorylcholine into the lens polymer for its Proclear lens and other lenses made of omafilcon A (62 percent water). Phosphorylcholine is a structural component of the phospholipid bilayer within human cell membranes. It's thought that adding an agent with inherent similarities of corneal and palpebral epithelial tissue would result in enhanced lens biocompatibility with the ocular surface. As a result, the Proclear lens has received an FDA indication for providing improved comfort to individuals who experience symptoms related to dryness during lens wear. Recently, the company used phosphorylcholine in a 60-percent-water version of omafilcon A and introduced this as a daily disposable lens (Proclear 1 Day).

Manufacturers of other high-water daily disposable lenses are making efforts to reduce or eliminate dry eye-induced side effects as well. CIBA's Focus Dailies with AquaRelease (nelfilcon A, 69 percent water) utilizes polyvinyl alcohol (PVA) as an internal wetting agent that is released during the natural blink cycle to help maintain surface lens wettability. CIBA's newest offering in this category, Dailies AquaComfort Plus (nelfilcon A, 69 percent water) adds hydroxypropyl methylcellulose (HPMC) as a lubricant and polyethylene glycol (PEG) as an additional wetting agent to the PVA to further enhance lens performance throughout the day.

Vistakon's 1-Day Acuvue Moist (etafilcon A, 58 percent water) uses the company's patented Lacreon technology, which embeds a hydrophilic polymer into the lens matrix to maintain lens moisture and decrease surface friction.

The issue of surface friction of a contact lens is complex. Friction occurs with opposing forces of adjacent surfaces. Certainly, the wettability of the lens surface plays a role. Less friction is created when a lubricant exists between two surfaces. Therefore, having an intact tear film spread evenly over a contact lens can reduce the friction between the lens and the adjacent palpebral conjunctiva during the blink. Discounting the presence of a lubricant, every surface has its own inherent degree of friction as indicated by a coefficient of friction. An example of a solid surface with one of the lowest coefficients of friction is Teflon. The fact that Teflon is not very wettable proves that there are additional factors to friction than just maintaining a smooth wettable surface. The Teflon surface has its electrostatic charge arranged to reduce attraction between it and adjacent surfaces. We don't have to look too far for an eye-related example of this same principle. The epithelial surface of the cornea with its relatively uneven, bumpy glycocalyx has a similar arrangement of its ionic components to produce little friction with the blink.

Gas Permeable Lens Materials

Almost all of today's GP lenses contain a methylmethacrylate (MMA) backbone that constitutes the material stiffness and strength. Unlike soft lenses, MMA polymers are hydrophobic and do not absorb water into the matrix of the material. Thus, the resultant firm, stiff materials of the first polymethyl-methacrylate (PMMA) lenses were deemed "hard" lenses. PMMA lenses allowed no gas permeability through the material itself, but their smaller diameters and unique fitting characteristics did allow for some oxygen distribution to the cornea through the tear-pump mechanism.

Like soft lenses, today's GP lenses incorporate the use of silicone into the MMA polymer to increase oxygen permeability. The advent of silicone acrylate (SA) lenses represented the first true GP lenses that allowed oxygen to reach the corneal surface through direct penetration of the lens material.

While silicone increases oxygen permeability, it also increases hydrophobicity, which reduces lens surface wettability and increases attraction to deposits. SA lenses are still commonly used today, but many contemporary materials also incorporate fluorine molecules into the lens matrix to further enhance gas permeability while also providing improved wettability, surface integrity and deposit resistance. The low coefficient of friction and low surface tension of fluorine helps prevent deposits from sticking. By virtue of their maximal Dk values, fluorosilicone acrylate (FSA) lenses represent the majority of GP materials with indications for extended wear (Table 4).

PMMA adds stability, strength and optical clarity to GP materials, and cross-linkers bind the polymer chains and add to material stability. To increase material wettability, methacrylic acid (MA), n-vinyl pyrollidone (NVP), polyvinyl alcohol (PVA) or HEMA may be incorporated into the material. A balance of the amount of these wetting agents must be achieved; too little does not produce sufficient wetting, while too much produces lens softness.

Various in vitro tests have been used to measure GP lens wetting angle, or a material's surface affinity for water. These include the sessile drop, the captive bubble and the Wilhelmy Plate. While these water wettability tests provide some indication of whether tears will form a stable and coherent layer on the surfaces of GP lenses when on the eye, they do not accurately predict in vivo wetting.

In the United States, after receiving FDA Pre-Market Approval (PMA), most of the companies that are PMA holders choose to produce their materials in semi-finished buttons that they supply to independent laboratories for custom manufacturing and distribution. These laboratories usually provide their own brand name to the finished lenses. However, it is important for practitioners to know the characteristics of each material, such as Dk and available tints. Whereas tints of soft lenses are on the surface, tints of GP lenses are throughout the material.

It's accepted that the rigid nature of GP lenses can adversely influence initial lens awareness and increase the length of time for patient adaptation. But it's also the rigid nature of the materials that give these lenses their greatest advantage: optical quality. The material stiffness of GPs has the ability to create a stable optical surface that can provide clear acuity to patients who have irregular corneal surfaces from keratoconus, corneal dystrophies and degenerations, or any condition resulting in an irregular corneal surface.

The material stiffness of GPs also allows these lenses to be manufactured in advanced custom designs. With the precision capabilities of computer-guided lathes, GP lenses can be sculpted into complex designs such as back torics, bitorics, simultaneous multifocals, translating multifocals, reverse geometry lenses, orthokeratology lenses and many others. Therefore, GP lenses are often the best and sometimes the only opportunity to maximally correct some patients' vision.

One of the most exciting advances in material science was the development of hybrid GP lenses with a hydrogel skirt. Hybrid lenses maximize all of the optical benefits of a GP lens while also providing the improved initial comfort and patient adaptation of a soft lens. Newer, more oxygen-permeable GP components of hybrid lens materials along with better overall stability and durability through improved manufacturing have allowed hybrids to make a resurgence in the contact lens marketplace.

The FDA has approved daily wear of SynergEyes lenses for myopia and hyperopia (SynergEyes A), presbyopia (SynergEyes Multifocal), keratoconus (SynergEyes KC) and post-trauma/surgery (SynergEyes PS). The GP central portion is made of paflufocon D, which has a water content of less than 1 percent and a Dk of 100; the soft skirt is made of poly-HEMA (liberfilcon) with a water content of 32 percent and a Dk of 8.

GP lenses must overcome many of the same challenges as soft lenses, namely they must maintain a smooth, wettable surface throughout the day to retain clarity and comfort while minimizing physiological compromise to the ocular surface. Like soft lenses, many GP lenses employ plasma treatments to improve surface properties. GP plasma treatments are delivered by exposing a newly manufactured lens to gases in a reaction chamber. The process removes manufacturing residue and smoothes the lens surface, which can improve initial wettability and translate into better initial comfort and patient adaptation. There is some question about how the long the wettability benefits of GP plasma treatments last and their role in helping patients who have chronic dry eye. However, in my experience plasma treatments appear to benefit the longterm cleanliness of the lens surface, providing effective management against lens deposition.

Take Advantage of Your Options

Admittedly, some chemical properties of the newly developed lens materials may seem complex and confusing. But it's the continual advancements from contact lens researchers and manufacturers that will provide us with high performance products to offer patients reliable, healthy, comfortable vision correction with contact lenses. It's important for all practitioners to have a basic understanding of contact lens materials so we can pass along their benefits to patients. CLS

Dr. Nixon is an associate professor of clinical optometry and the extern coordinator at The Ohio State University College of Optometry. He is also in a group private practice in Westerville, Ohio.

For references, please visit www.clspectrum.com/references.asp and click on document #156.

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