Composition of Contact Lenses

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.

However, to avoid infringement of existing patents, many comonomers, as, for example, methyl methacrylic acid or a blend of comonomers, are used. Comonomers produce changes in the water content or ionic nature of lenses that is significantly different from HEMA lenses. For example, addition of acrylic acid in HEMA increases the water content and ionic nature of lenses. Some lenses are made from n-vinylpyrrolidone and have high water contents. Such lenses have pore sizes that are much larger than low water content lenses. Cross-linkers, such as ethyleneglycol dimethacrylate, and initiators as, for example, benzyl peroxide, in appropriate amounts are added for polymerization and to achieve desirable physical and chemical properties. Recently, contact lenses made from HEMA and silicone were made available. These lenses combine the properties of hydrogel and gas-permeable polymers (374). Table 5 gives a list of monomers, comonomers, and cross-linkers along with their effects on polymer properties. In 1985, the FDA published a classification for soft hydrophilic lenses based on their water content and ionic nature. Groupings for soft lenses and their generic names are listed in Table 6. Adequate levels of oxygen are necessary to maintain normal corneal metabolism (375). Lenses that are poorly designed and worn overnight deprive the cornea of oxygen, causing edema (376). Contact lenses made from PMMA materials are virtually impermeable to gases (377). The PMMA lenses are also inflexible, causing discomfort in a large percentage of individuals while the lens is worn. During the 1980s, lenses were made from either cellulose acetate butyrate (CAB) or silicone elastomer. Although comfortable and flexible, such lenses accumulated lipids, were nonwettable, and adhered to the cornea. Several reports detailing difficulty in removing CAB and silicone lenses appeared in the published literature (378). Lenses made from fluorocarbons and in various combinations of fluorocarbon, silicone, methyl methacrylate, and acrylic acid are currently available. Desired properties of these lenses include flexibility, wettability, and gas transmissibility. Grouping for RGP lenses was published by the FDA in 1989. The generic names and oxygen permeabilities of RGP lens materials are provided in Table 7 (379). The development of silicone hydrogels has initiated discussions among regulatory agencies worldwide to add additional groups. Recently, the International Standards Organization (ISO) has recognized that silicone hydrogels do not effectively "fit" within the existing FDA lens grouping and that a further "group V" category should be added for this class of high oxygen permeability hydrogel materials.

Table 5 Commonly Used Monomers, Comonomers, and Cross-Linkers in Contact Lens Polymers

Name Abbreviation Lens properties

Acrylic acid AA Flexibility

Hydrophilicity pH sensitivity: acidic

Reactivity: ionically interacts with positively charged tear components Wettability

Butyl methacrylate BMA Softness

Flexibility

Hydrophobicity: attracts lipids

Wettability

Gas transmissibility

Cellulose acetate butyrate CAB Clarity

Wettability

Gas transmissibility

Physical stability

Table 5 Commonly Used Monomers, Comonomers, and Cross-Linkers in Contact Lens Polymers (Continued)

Name

Abbreviation

Lens properties

Dimethyl siloxane

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