Contact Lens Spectrum Supplements

Special Edition 2017

Contact Lens Spectrum

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Page 29 of 75

c l s p e c t r u m . c o m 28 C O N T A C T L E N S S P E C T R U M S P E C I A L E D I T I O N 2 0 1 7 LENS MA TERIALS linear polymer to form what is generally termed an interpenetrating network. The result is that the hydro- philic 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 advan- tages with minimal waste of raw materials. The hydra- tion 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 produc- tion facility are significant, but because lenses are pro- duced 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 re- duced visual performance. Only by providing a true spe- cialty 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 sub- stantial 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 as- sumed another more important role in the manufacture of silicone hydrogel lenses. They are now used as a com- patibility 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-con- taining 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. SPECIALTY LENSES The success of molded silicone hydrogel contact lenses has resulted in a demand for these types of mate- rials 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 require- ment, a lathe-cut material becomes more practical. This resulted in the launch of efrofilcon A, the only lathe- able 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 prod- ucts. 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 low- est modulus of all silicone hydrogels currently available. T h e c o n v en i enc e o f a d i s p o sa b l e l en s c o me s a t t h e c o s t o f c o m p r o m i s e d p r escr i b i n g o f l e n s es t h a t m a y n o t e x a c tl y m ee t a p a t i en t ' s r e q u i r emen t s , l ea d i n g t o r ed u c ed v i s u a l p er f o r m a n c e .

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