HEMA

Droxifilcon-A copolymer of HEMA and MA modified with p oly (2-vinyl-pyrrohdone) 47 Accugel Strieter Labs... 

Tetrafilcon-A terpolymer of HEMA, MMA, and 42.5 Ao s oftAquaflex American Optical Corp.Cooper Vision,... 

Vifilcon-A copolymer of HEMA and MA with PVP cross-linked with EGDM 55 Softcon American Optical Corp. 

The number of contact lens wearers has grown to an estimated 24 million in the United States and 50 million worldwide. Concurrendy, there has been a proliferation of contact lens manufacturers and products. The 1980s saw the widespread introduction of lens products made of more oxygen-permeable materials, ie, rigid gas-permeable (RGP) materials that made PMMA lenses virtually obsolete and high water content hydrogels that competed with HEMA-based lenses. 

No particular contact lens type or product is considered universally superior. In some regions of the world hard lenses dominate the market, eg, some European countries and Japan in other regions, eg. North America and Scandinavia, soft lenses dominate. Contact lens practitioners select their preferred type of lens using criteria other than just lens material properties. However, among soft lenses, HEMA-based lenses are prescribed most often, and among hard lenses, siUcone—acrylate RGP lenses are most common. 

Hydrogels, ie, gelatin and agar, have been known for a long time. In the late nineteenth century, Herschel proposed the use of jelly materials on the cornea for the correction of vision (108). In 1960, the use of synthetic hydrogels for contact lenses was proposed and several U.S. patents were obtained for the invention of cross-linked hydrophilic polymers, eg, systems based on 2-hydroxethyl methacrylate [868-77-9] (HEMA) (5) (109—112). 

In 1971, Bausch Lomb received U.S. Food and Dmg Administration approval for a soft contact lens based on the HEMA system and launched Soflens. The soft lens market has grown dramatically soft lens surpassed hard lens wear by the late 1970s, accounting for about 80% of the U.S. contact lens market in the early 1990s. 

The water content of a hydrogel depends on the hydrophilicity of the monomer, eg, cured poly(HEMA) absorbs 60% of its weight of water and thus forms a hydrogel with about 38% water content. Other hydrophilic monomers, such as A/-vinylpyrrohdinone [88-12-0] (NVP) (6), and glycerol methacrylate [100-92-5] (GM) (7), and acrylamide monomers, such as diacetone acrylamide [2873-94-9] (DAA), have also been used to form hydrogels with higher water content. 

Because of the many choices of hydrophilic monomers, cross-linkers, and hydrophobic monomers, a large number of formulations have been developed and manufactured into hydrogel lenses. The water content of these hydrogel lenses ranges from about 38%, for HEMA-based lenses, to 80%, for poly(vinyl alcohol) and partially hydrolysed acrylonitrile lenses. Table 2 gives a representative Hst of FDA approved hydrogel materials available to the consumer in the early 1990s. 

The principal problems for sdicone mbber as a viable lens material are the nonpolar nature, which gives Hpid deposits and wettabdity problems and the tendency to adhere to the cornea. Efforts to modify the sdicone lens surface for improved wettabdity have achieved limited success. These efforts include grafting hydrophilic monomers, such as HEMA, GM (150), and NVP (151—153), to the lens surface and plasma treatments of finished lenses. Efforts to improve the movement of sdicone lenses on the cornea with various lens designs have not been successfld, and the cause of lens—cornea adherence, which is not an exclusive problem of sdicone lenses, is an active area of research. 

PSS columns for aqueous separations PSS HEMA Bio and the further improved PSS SUPREMA columns are based on methacrylic ester copolymer technology and exhibit very good stability while maintaining high efficiency. 

PSS columns for medium polar or mixed solvents PSS HEMA and PSS SUPREMA Basic were designed to allow SEC separations in polar media such... 

Table 9.3 also shows that there have been some improvements in SEC sorbents for aqueous applications since the early 1990s with the availability of PSS HEMA and PSS SUPREMA columns. Table 9.4 gives an overview of available packings and their major use (this table is not complete and is intended for showing the major features and uses in SEC applications). 

Examples PSS SDV PL Gel Styragel TSK-H Shodex A,K PSS HEMA Shodex PSS HEMA Bio PSS Suprema PL Aquagel UltraHydrogel TSK-PW Shodex OH (ionic SO,H, amides) Very many ... 

Advances in aqueous column packings are reflected in this application section. PSS HEMA and HEMA Bio columns (cf. Fig. 9.21) have proven to be extremely stable columns in the middle molecular weight range. PSS HEMA columns... 

We also have to count on variations of the main monomer unit as well. For example, HEMA materials always contain traces of unesterified free carboxylic groups. These acidic groups interact with polycations. As result, retardation is observed until total adsorption of the injected sample. Consequently, polycations can be analyzed on such a material only in acidic eluents, where the dissociation of the — COOH— group is suppressed. 

DSEP direct soapless emulsion polymerization, SSEC seeded soapless emulsion copolymerization, DDC direct dispersion copolymerization, TDSC two-stage dispersion copolymerization, ATES Allyl trietoxysilane, VTES vinyl trietoxysilane, DMAEM dimethylaminoethyl-methacrylate, CMS chloromethylstyrene, GA glutaraldehyde, AAc Acrylic acid Aam Acrylamide HEMA 2-hydroxyethylmethacrylate. 

Figure 19 The scanning electron micrographs of the polystyrene seed latex and the copolymer latices carrying carboxyl, hydroxyl and amine functional groups, (a) PS/PAA, (b) PS HEMA, (c) PS/PDMAEM. The original SEM photographs were taken with 10,000 x magnification and reduced at a proper ratio to place the figure. (From Ref. 93. Reproduced with the permission of John Wiley Sons, Inc.)...

Among the pairs of monomers used in synthesizing block copolymers by this method are MMA/AN, C2F4/ MMA, and HEMA/DMA (HEMA = hydroxyethyl methacrylate and DMA = decyl methacrylate). 

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