Contact Lens Spectrum Supplements

Special Edition 2017

Contact Lens Spectrum

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c l s p e c t r u m . c o m 29 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 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 con- tact lenses are no different. GP and soft lens evolution has concentrated on the bulk properties of the material with emphasis on oxygen permeability. Increased sili- cone 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 hydro- gel products used plasma to introduce wettability to high- ly hydrophobic surfaces, ensuring the lenses were suit- able 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 ap- plied to GP lenses for many years, as it has been shown to improve the surface behavior of the lenses, poten- tially improving initial comfort and possibly helping patients adapt to the lenses. The treatment involves plac- ing 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 leav- ing 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 dramati- cally 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 polyethyl- ene 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 sur- face to move and respond to thermodynamic conditions. When exposed to aqueous fluid, the surface presents hy- drophilic groups to interact with water molecules; how- ever, if exposed to air, the surface reorganizes and pres- ents 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 rea- son why patients stop wearing their lenses. The PEG technology maintains hydrophilic groups at the lens sur- face and the resulting fluid film creates a hydrodynamic lubrication situation during blinking. This minimizes the coefficient of friction during blinking which is be- lieved to contribute significantly to discomfort during lens wear. CONCLUSION The evolution of contact lenses has shown major steps forward, but a device truly engineered for this applica- tion 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 spe- cialty lenses still provides opportunities for the combina- tion of some novel new technologies. CLS Dr. Young has been working for more than 23 years in the field of hydrogel polymers, with a particular emphasis on ophthalmic applications. He joined Contamac as a research chemist in 2000. After 2 years, he was appointed to lead their materials development program as research and development manager. In 2009, he was appointed as technical group manager, leading research strategies as well as having responsibility for the technical aspects of the business, including quality control. In his current position, he continues to lead the research and collaborative developments of the company. G P a n d s o f t l e n s e v o l u t i o n h a s c o nc en t r a t e d o n t h e b u l k pr o p er t i e s o f t h e m a t e r i a l w i t h a n e m p h a s i s o n o x y g en p e r me a b i l i t y .

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