Contact lenses polymeric materials

Silicone—Fluorosilicone Lenses. Sdicone mbber has long been considered a unique contact lens material (55), and the development of sdicone mbber lenses has been reviewed in earHer editions of the Eniyclopedia. The oxygen permeabdity of sdicone mbber, >300 barrers, is virtually unsurpassed by any other polymeric material considered for contact lens appHcations. 

Surface modification of a contact lens can be grouped into physical and chemical types of treatment. Physical treatments include plasma treatments with water vapor (siUcone lens) and oxygen (176) and plasma polymerization for which the material surface is exposed to the plasma in the presence of a reactive monomer (177). Surfaces are also altered with exposure to uv radiation (178) or bombardment with oxides of nitrogen (179). Ion implantation (qv) of RGP plastics (180) can greatiy increase the surface hardness and hence the scratch resistance without seriously affecting the transmission of light. 

The number of options for contact lens wearers has been transformed by innovative advances in the chemistry of the polymeric materials used to create these vision products. 

The first soft contact lenses were also constructed with a polymeric material containing a single monomeric unit. The added pliability of the soft lens was derived from the more hydrophilic nature of the monomer, enhancing the ability of the polymer to absorb water and provide greater comfort to the lens wearer. This monomer is a derivative of MMA known as hydroxyethyl methacrylate (HEMA). A number of hydrophilic monomers are used in soft lenses today these materials are referred to as hydrogels because of their ability to absorb significant amounts of water yet remain insoluble. 

Contact lenses are made of polymeric materials designed and fabricated to correct vision. Because these lenses are removed from the eye after a prescribed wear time, lens care products are required to clean, disinfect and rinse them prior to reinsertion to avoid ocular infections and other complications. Lens care products are also required to enhance the comfort of lens wear. 

The design of new polymeric materials for contact lens application requires an extensive knowledge of polymer chemistry, polymer properties and the physiology of the eye (1-4). The properties that must be optimized in designing a new contact lens material are optical transparency, chemical and thermjd stability, wettability to tears, mechanical properties, dimensional stability, biol( cal compatibility and oxygen permeability. 

The use of a polymerizable surface-active material during the fabrication of the contact lens was evaluated to render the contact lens surface more wettable and resistant to deposit formation. This material preferentially migrated to the surfaces presented to a monomer mix during lens formation. During the polymerization process(es) to form the lens, the surface-active material was covalently bound to the gel matrix at the surfaces. In this manner, the surface-active material should displace the silicones from the surface and be permanently affixed to the lens. 

Hydrophilic contact lens materials have been prepared by graft polymerizing 2-hydroxyethyl methacrylate onto poly(N-vinylpyrrolidone) using tert hnty perbenzoate as an initiator [42]. A more complex cross-linked contact lens material has been prepared by polymerizing a composition of PVP, N-vinylpyrrolidone, 2-hydroxyethyl methacrylate, and ethylene dimethacrylate with benzoyl peroxide [43]. 

Hydroxyethyl methyl Methacrylate n A monomer that polymerizes to a hydrophilic polymer that is rigid when dry but when saturated with water becomes a soft, clear material (Hydron ). Applications include masonry coatings, soft contact lens, and other biomedical devices. 

There hcis been extensive resecuch on polymeric materials for use cis contact lenses in the pjist two decades. The ocular environment places high demjmds on the biomaterial, both bulk md smface properties, when used as a contact lens. Proper hydration, sufficient oxygen permeability to... 

Cast molding is an increasingly used manufacturing process for both rigid gas-permeable and hydrogel contact lenses. In this process, two molds, made from a variety of plastics, are used. A female mold forms the lens front surface (convex) and a male mold forms the lens back surface (concave). The plastic molds are made from metal tools or dies that are usually stainless steel, precision lathed, and polished to the specified lens design. A variety of mold materials are used. The polymerized, hardened lens is released from the mated molds and is processed in much the same way as the spin-cast lenses described above. 

Contact lenses are optical devices that are either fabricated from preformed polymers or polymerized during lens manufacture. The main purpose of contact lenses is to correct defective vision, and they are generally referred as corrective lenses. However, due to the recent advances in material manufacturing, reduction in fabrication cost, and then-availability in many colors, they are extensively used to change the appearance of the eye. For this application, they are called cosmetic lenses. Contact lenses used medically for the treatment of certain comeal diseases are called bandage lenses. 

Next-generation soft contact lenses, dental polymers, surface coatings, and similar materials are produced from compounds of varying structure and reactive functionality. For example, currently in development are new soft lenses that will be manufactured from monomers synthesized with dimethylsil-oxane backbones. The dimethylsiloxane backbone is terminated with a methacryloxy functionality that supplies the site for polymerization. The siloxane provides lens softness. Occasionally the functionality is formed on both ends of the monomer, resulting in undesired properties. The compound BisGMA is a monomer that is polymerized to form hard dental structures. In the monomer synthesis process impurities are coproduced that interfere with the polymerization. Finally, diacetone acrylamide used in a copolymerization process is another specialty monomer that is occasionally contaminated with difficult-to-remove impurities. These three monomers are quite reactive at modest temperature and cannot be purified by distillation. The three examples that are presented here derive from as yet unpublished research (Krukonis, 1982c). 

Polymeric soft contact lenses came into existence in the 1950s (12]. Otto Wich-terle discovered the hydrogel poly(hydroxyethyl methacrylate) (HEMA), a transparent, soft, hydrophilic material that could be used to prepare contact lenses, Wichterle utilized a free radical polymerization of the HEMA monomer (including cross-linker, solvent, initiator, and stabilizer) with either thermal or ultraviolet initiation of the reaction. Initially, the len.ses were produced via spin casting, which involved the use of a concave mold that is spun at a particular rate. The rate of the mold spin determines the resultant lens power (13). After production of the lens in the mold, the lens would be hydrated from the mold in a warm water solution. Once hydrated, the lens would float free from the mold. Each lens is inspected for rips, tears, and clarity. Finally, the lens is packaged, sterilized, and boxed for shipping. The surface quality of the mold determines the surface chemistry and morphology on the anterior surface of the lens produced. 

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