Soft contact lenses monomers

The Effect of Monomer Purity on Protein Adsorption onto Poly-(HEMA). The importance of relatively minor contamination of the monomers used in formulating hydrogels to be used in biomedical applications has not been recognized widely as yet, although Bruck has referred to this problem in connection with the soft contact lens (26). Protein adsorption studies performed with hydrogels made with monomers of typical commercial quality illustrate this potential problem. 

A soft contact lens comprises a water-swollen gel of a polymer prepared by polymerising one or more hydrophilic monomers such as 2-hydroxyethyl methacrylate, one or more crosslinking monomers, a monomer that... 

YNTHETIC HYDROGEL POLYMERS, first introduced in the early 1960s, made a major impact, initially in the soft contact lens field, and more recently in other biomedical or specialty applications. The first synthetic poly(2-hy-droxyethyl methacrylate) [poly(HEMA)] hydrogel developed by Wichterle (i) remains very important, as do its copolymers with monomers such as N-vinyl-2-pyrrolidinone, acrylic and methacrylic acids, glycerol methacrylate, various acrylamides, and alkoxyalkyl methacrylates. 

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. 

Table 2 Representative monomers and polymers used as soft contact lens materials.

The successful development of eye contact lenses led in turn to a demand for soft contact lenses. Such a demand was eventually met by the preparation of copolymers using a combination of an acrylic ester monomer such as methyl methacrylate, a cross-linkable monomer such as a dimethacrylate, and a monomer whose homopolymer is soluble or highly swollen in water such as N-vinyl pyrrolidone. Such copolymers swell in water (hence the term hydrophilic), the degree of swelling being controlled by the specific type and amount of the monomers used. In use the lens is swollen to equilibrium in water, a typical soft lens having a water content of about 75%. 

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. 

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). 

Soft contact lenses (SCLs) are one of the major products constructed from poly(HEMA) hydrogels. Since their introduction, a variety of such poly-(HEMA)-based hydrogels have been developed to improve lens properties. Both excellent protein adsorption resistance and wettability are required for SCLs, making MFC a good candidate as a suitable monomer. Biocompatible Co. has produced MFC polymer-based SCLs that contain 20% MFC, 80% HEMA, and a small amount of crosslinker (Froclear , omafilcon A), with such lenses now commercially available from CooperVision Co. The Froclear is the only contact lens for reduction of dry-eye syndrome that has been approved by the FDA in the USA. 

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. 

Contact lenses are broadly classified as PMMA, RGP, and soft hydrogel (HEMA) lenses. Dyes may be added during polymerization or after fabrication to improve lens handling or to change the color of the lens wearer s eyes. Lenses made from numerous polymers are available today (374). In soft hydrogel lenses, HEMA is a commonly used monomer. 

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