Contact lens materials have evolved significantly since polymers were first employed to improve vision; and certainly ocular health has benefitted from the improved properties the materials now possess. The diverse range of approaches that have been taken to develop improved soft and GP contact lenses is surprising, given the relatively simple concept of a contact lens. Our increased understanding of some of the factors involved in a successful wearing experience underscore that reality is far more complicated than the concept, and the same can be said about contact lens materials.
GP materials used for contact lenses have evolved from simple polymethyl methacrylate (PMMA) to highly involved systems containing an array of monomers that are collectively designed to bring a complex balance of often diverse properties to these materials. Soft lenses have also undergone a significant transformation, such as increased water content, inclusion of silicone monomers to improve oxygen permeability (Dk), and incorporation of functional groups that improve biocompatibility.
The materials used in the field of contact lenses are best described as polymers, and most are synthetic. A polymer results from the joining of a large number of repeating units (monomers) that contain a reactive double bond between two carbon atoms, otherwise known as a vinyl group. The resulting polymer chains each contain thousands of individual monomer units with each chain loosely entangled around one another. The properties of the resulting polymer depend on a number of factors, including the nature and structure of the component monomer repeat units, the average length of each of the polymer chains (referred to as the molecular weight), and the addition of other components, such as plasticizers and cross-linking agents, the latter of which can bind together neighboring polymer chains.
GAS PERMEABLE MATERIALS
The need to improve the oxygen permeability of PMMA led to the development of GP contact lens materials. One approach was to incorporate silicone rubber because of its inherent oxygen permeability, but this proved difficult practically. The solution was to attach small sections of silicone rubber to the basic methacrylate monomer structure. The resultant monomer, commonly referred to as TRIS, can be combined with MMA to produce hard, glassy polymers that can be machined like PMMA but are highly oxygen permeable. The resultant contact lens materials, known as siloxy-methacrylate copolymers, were successful, largely because of their increased oxygen permeability.
The greatest commercial success for GP materials has been the further addition of fluorine-containing monomers that also improve oxygen permeability. These also enhance the material’s ability to resist mucin and other deposits, and the lenses made from these materials have greater wettability than those containing TRIS alone. In the years that followed, an increasing number of fluorosilicone GP products became commercially available, and further advances have resulted in increased levels of oxygen permeability and improved surface wettability.
When formulating new materials for GP lenses, a number of competing factors must be balanced in the pursuit of a material with the ideal balance of properties (Table 1). Maximizing performance in all areas simultaneously is impossible, however, and this has led to materials being classified according to their oxygen permeability, as this property is easiest to quantify for practitioners. The characteristics of low-, mid-, and high-Dk materials follow.
|LOW Dk||MID Dk||HIGH Dk|
- Low Dk. The oxygen permeability of these materials is about 30 barrers, and they have good stability in maintaining the base curve dimensions to which they are cut. Visual acuity is usually excellent, and the lenses have good wettability and resist scratching.
- Mid Dk. The oxygen permeability of these materials is about 75 barrers, and they also have good stability in maintaining the base curve dimensions to which they are cut. Visual acuity is very good, and the lenses have reasonable wettability and scratch resistance.
- High Dk. The oxygen permeability of these materials is greater than 100 barrers, and they usually have good stability in maintaining the base curve dimensions to which they were cut. Visual acuity is good, and the lenses have adequate wettability but can be more prone to scratching.
The inclusion of silicone-containing monomers to increase permeability typically has a negative impact on surface wettability, given the hydrophobic nature of silicone. The silicone component is usually softer than other components used in GP formulations, and this may lead to increased movement in the base curve after the lens has been cut. It can result in a surface that has a lower hardness factor and is prone to surface scratches. It has also been suggested that visual acuity seems to decrease as the permeability of GP materials increases.
SILICONE HYDROGEL MATERIALS
Silicone- and fluorine-containing lenses provide an alternative transport mechanism for oxygen as opposed to the aqueous phase in conventional soft materials, and this was originally exploited in GP materials with TRIS. The principle of combining silicone-containing monomers to increase oxygen permeability with hydrophilic monomers to increase comfort on the eye is simple, but combining these two components has proved extremely challenging. The simplest approach is to try to combine hydrophilic HEMA with TRIS, the molecule widely used in the production of GP materials. Unfortunately, these two compounds are immiscible, and the hydrated polymer is often opaque, indicating that phase separation has taken place in this system. Various approaches have been used to solve this fundamental problem, resulting in the successful evolution of these materials.
One approach has been the modification of the TRIS molecule to make it more hydrophilic. Many patients have described the insertion of hydrophilic groups in the linking region of this molecule between the methacrylate polymerizable group and the branched silicone section analogous to silicone rubber. Another approach was the use of macromers (preassembled, large monomers that contain specific functional units), which often cannot be combined in the form of simple monomers. In this way, the elements of silicone rubber and fluorine can be combined with hydrophilic polyethylene glycol units. These macromers have also been developed over many years, illustrating that despite another relatively simple concept, its execution proved difficult.
More recent materials use a combination of these approaches. They contain a modified TRIS derivative, and they also make use of macromer molecules. In addition, many of these products contain hydrophilic polymers that act as wetting agents such as polyvinylpyrrolidone (PVP). This component is a linear homopolymer composed entirely of vinylpyrrolidone, which is added to the monomer formulation prior to the lens molding process. The monomer mixture is then polymerized around the linear polymer to form what is generally termed an interpenetrating network. The result is that the hydrophilic properties of the PVP can exist in combination with the permeability properties of the material so that a high degree of surface wettability exists. This approach enables the production of lenses without the added cost of a post-fabrication surface treatment process.
THE MOLDING PROCESS
The traditional lathe-cutting techniques used for PMMA and subsequently GP materials can be used to produce soft lenses. However, a key revolutionary aspect to these materials is that they are also suitable for molding, which offers considerable efficiency advantages with minimal waste of raw materials. The hydration of the lens and the subsequent ability of a soft lens to conform to the surface of the cornea mean that soft lens fitting is typically simpler than GP lens fitting. A relatively small number of base curves can be used to fit most eyes, and this limits the number of individual lens sizes or SKUs that are required for a product, making molding a viable manufacturing option.
The initial costs of establishing a molded lens production facility are significant, but because lenses are produced on a large scale, this form of manufacture gave rise to the evolution of disposable products, and many molded products are now available with daily disposable options. The convenience of a disposable lens comes at the cost of compromised prescribing of lenses that may not exactly meet a patient’s requirements, leading to reduced visual performance. Only by providing a true specialty lens in any base curve and optical correction does the custom fitting of a lens become possible.
The process to mold a contact lens has evolved over the years. For example, many lenses are no longer formed as the dry polymer but often are formed containing a substantial amount of a nonreactive solvent. Originally, this approach sought to improve the consistency of soft lenses by forming them in a fully expanded state, whereupon the nonreactive solvent was replaced by water during the lens hydration process. Recently, the solvents have assumed another more important role in the manufacture of silicone hydrogel lenses. They are now used as a compatibility bridge between the hydrophilic monomers and the hydrophobic silicone monomers to prevent them from phase-separating during polymerization, which can lead to non-homogeneous lenses that can appear cloudy or even opaque. These solvents can also help remove the nonreacted monomers from the lens, as silicone-containing monomers are often not readily soluble in water. Solvents are required to clean the finished lenses so that they can be worn safely without species migrating from the lens to the eye during wear.
The success of molded silicone hydrogel contact lenses has resulted in a demand for these types of materials for the specialty lens sector. However, developing a silicone hydrogel material for the specialty lens industry is not a straightforward proposition. Various obstacles must be overcome before a material produced in button form can be processed into a contact lens.
The lathing of silicone hydrogel materials presents a major challenge, as inclusion of what is essentially a silicone elastomer component results in these materials being relatively soft. Raw materials for silicone hydrogel lenses are relatively expensive, which is not ideal because producing a lens through lathing techniques usually leads to wasted material. Including compatibility-enhancing solvents or internal wetting agents is also not possible, because they render the polymerized material relatively soft, given the plasticizing nature of these compounds.
The trend of using silicone hydrogel lenses for daily wear is accompanied by a reduction in the Dk required in the final lens product. With this reduced requirement, a lathe-cut material becomes more practical. This resulted in the launch of efrofilcon A, the only latheable silicone hydrogel approved by the U.S. Food and Drug Administration. This material is intended for the production of specialty lenses for daily wear. Efrofilcon A has a relatively high water content with an oxygen permeability of 60 barrers, which is significantly higher than observed in a traditional hydrogel with a similar water content and is comparable to some molded products. As the material is lathe-cut, the center thickness depends on the lens power and the design used by the manufacturer. The high water content also contributes to the low modulus of this material. In fact, it is the lowest modulus of all silicone hydrogels currently available.
SURFACE TECHNOLOGY APPROACHES
For many medical devices, the property requirements for the bulk of the device can differ significantly from the properties required at the surface of the device, and contact lenses are no different. GP and soft lens evolution has concentrated on the bulk properties of the material with emphasis on oxygen permeability. Increased silicone content for this bulk property will inevitably cause the surface to be more hydrophobic, and this will reduce the ability of water to spread on the surface of the lens, potentially affecting comfort. This creates a compromise between bulk and surface considerations; however, a process to modify or change the surface of the material can provide a means to break this relationship.
The first two commercially successful silicone hydrogel products used plasma to introduce wettability to highly hydrophobic surfaces, ensuring the lenses were suitable to be worn on the eye. One product approached the problem by converting the organic silicone groups on the surface to an inorganic, glassy form of silicon, creating islands of silicate, which possesses good wettability. The other product used plasma energy to react the surface of the material with a volatile monomer that was introduced into the plasma discharge, and this created a thin layer of a wettable material on the surface of the lens.
In the specialty lens production arena, GP materials have received most of the attention for potential surface modifications. A simple plasma treatment has been applied to GP lenses for many years, as it has been shown to improve the surface behavior of the lenses, potentially improving initial comfort and possibly helping patients adapt to the lenses. The treatment involves placing the lenses in a plasma chamber at the conclusion of the standard production process and exposing them to the plasma for a few minutes before wet-shipping them to practitioners. The process is achieved using an oxygen plasma; however, the effect is relatively short-lived, because rubbing the surface of the lens or leaving the lens in air results in the surface reverting to its original character.
Recently, a new technology was introduced in the specialty lens sector. This technology aims to dramatically improve comfort by applying a coating on both GP and soft lens materials. The basic concept is to generate a thin polymer layer around the entire surface of the lens, a process that could be considered encapsulation. This polymer layer incorporates elements of polyethylene glycol (PEG) chemistry and is about 40 nanometers thick. The PEG groups are highly hydrophilic, which results in the surface polymer having a water content of about 90%; however, the polymer is constructed as a cross-linked network, which means hydrophilic groups are always presented at the surface, even if the material is exposed to air. This enables the material to maintain a structured aqueous surface layer that resists dehydration far better than a conventional contact lens material.
In conventional soft lens materials, rotation of the bonds between carbon atoms enables groups at the surface to move and respond to thermodynamic conditions. When exposed to aqueous fluid, the surface presents hydrophilic groups to interact with water molecules; however, if exposed to air, the surface reorganizes and presents hydrophobic groups at the surface. These changes can result in the material developing a dry surface quite rapidly between blinks, and such a surface is then prone to lipid deposits, which can impact many aspects of soft lens wear including comfort, which is still the major reason why patients stop wearing their lenses. The PEG technology maintains hydrophilic groups at the lens surface and the resulting fluid film creates a hydrodynamic lubrication situation during blinking. This minimizes the coefficient of friction during blinking which is believed to contribute significantly to discomfort during lens wear.
The evolution of contact lenses has shown major steps forward, but a device truly engineered for this application is still to be developed. In addition, the disposable nature of lenses can limit the amount of sophistication that can be engineered into each lens. The field of specialty lenses still provides opportunities for the combination of some novel new technologies. CLS