Contact Lens Spectrum Supplements

Special Edition 2017

Contact Lens Spectrum

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Page 27 of 75

c l s p e c t r u m . c o m 26 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 C ontact lens materials have evolved significantly since polymers were first employed to improve vision; and certainly ocular health has benefitted from the improved prop- erties 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 compli- cated 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 high- ly 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. POLYMERIZATION 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 re- peating 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 plasticiz- ers 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 ma- terials. One approach was to incorporate silicone rub- ber because of its inherent oxygen permeability, but this proved difficult practically. The solution was to attach small sections of silicone rubber to the basic methacry- late monomer structure. The resultant monomer, com- monly 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 re- sultant contact lens materials, known as siloxy-methac- rylate 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 mono- mers 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 fluoro- silicone 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 proper- ties (Table 1). Maximizing performance in all areas si- multaneously is impossible, however, and this has led to materials being classified according to their oxygen per- meability, as this property is easiest to quantify for prac- T h e a r e a o f s p e c i a l t y l e n s e s p r o v i d e s c o n t i n u e d o pp o r t u n i t i e s f o r n o v el n e w t e c h n ol o g i e s THE EVOLUTION OF CONTACT LENS MATERIALS R I C H A R D YO U N G, C H I E F S C I E N T I FI C O FFI C E R, CO N TA M AC

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