Soft contact lenses hydrogels

Do not use hydrogel soft contact lenses during ophthalmic diclofenac therapy... 

Because fluorescein sodium can penetrate into many hydrogel contact lenses, the lenses become discolored, which raises bacterial growth issues and renders the lenses cosmetically objectionable. In addition, the boimd-ary between lens and tears becomes obscured, which precludes the use of fluorescein in soft contact lens fitting. Fluorexon, a molecule similar in fluorescent characteristics to that of fluorescein, is less readily absorbed by the soft lens material, which renders it useful in fitting and evaluating soft and hybrid design lenses. 

All ages and both genders may be affected. Although study results vary widely, 3% to 15% of rigid lenses wearers and 5% to 10% of soft contact lens wearers are reported to develop GPC. Eighty-five percent of GPC occnrs in hydrogel lens wearers.The incidence of GPC is lower in those wearing disposable versus conventional contact lenses and is lower with more frequent versus less frequent lens replacement. 

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. 

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. 

A hydrophilic polymer (especially the aoss-linked form) may transition from hard and rigid to soft and elastic when immersed in aqueous media. A good example of this is cross-linked poly(2-hydroxyethyl methacrylate) (pHEMA), the original soft contact lens polymer. When dehydrated, pHEMA is a hard, hrittle polymer. When hydrated, it is a soft elastomer. The hydrated (swollen) form of cross-linked pHEMA contains about 40% by weight of water. Polymers that swell to an equiUhrium level in aqueous solutions are referred to as hydrogels. 

Hyon S-H, Cha W-I, Ikada Y, Kita M, Ogura Y, Honda Y (1994) Poly(vinyl alcohol) hydrogels as soft contact lens material. J Biomater Sci Polym Ed 5 397... 

Many materials are employed in the production of soft contact lenses. The material with the widest application in the industry is HEM A [13]. HEM A became a popular soft contact lens because of its clarity, comfort, good vision, and excellent wetting characteristics. HEMA is a hydrogel, and hydrogels are cross-linked polymer networks that are swollen in water. Hydrogels are typically classified by whether they are neutral or ionic. Anyone who has had a piece of dirt in the eye understands the need for good wettability when it comes to a contact lens. The wettability of a soft contact lens in the ocular environment is a function of the surface chemistry while it is hydrated, which determines the biocompatibility of the lens [6]. A lens surface that does not wet will cau.se discomfort and potentially increased deposits. These deposits can affect vision and the physical properties of the lens [3,4]. The major drawback of HEMA is the permeability of the material to oxygen [12]. Extended wear can result in increased vascularization of the cornea... 

Salamone, J. C. Fluorine Containing Soft Contact Lens Hydrogels. PCX Int. AppL, WO 9211300, A1 19920709, 1992. 

As one can see from Eq. 6, the permeability of a soft contact lens is directly proportional to its water content (Figure 5.53). Whereas the moisture content of early PHEMA lenses was only ca. 35%, more recent silicone-based hydrogels contain up to 70% water. Though hydrophilic lenses are able to absorb water and permit extended wear without comeal irritation, an increase in water content also corresponds to lower mechanical strength that may result in tearing/scratching. 

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. 

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. 

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