Contact lenses, hydrogel polymers

Water content indirectly affects other lens characteristics. Water evaporation from the lens can result in a dry eye sensation and subsequent desiccative erosion of the cornea. Clinical studies have shown the incidence of corneal erosion as a result of lens desiccation to be a material-dependent and water-content-dependent phenomenon (25,26). The nature of water and sodium ions in hydrogels has been studied primarily by nmr and thermal techniques (27,28). An empirical relationship between water mobility in contact lens polymers and desiccative staining has been proposed (29). 

While the silicone polymer phase provides a high enough oxygen permeability, it also dominates the surface characteristics, i.e., the surface of silicone hydrogel is too hydrophobic to be used as a contact lens. The silicone phase also readily absorbs... 

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. 

In addition, many of the known hydrophilic monomers have been used to prepare synthetic hydrogel polymers and copolymers of a wide range of compositions, mostly for contact lens uses. In such materials, water insol-... 

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. 

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. 

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... 

So-called soft contact lenses are actually made from a polymer hydrogel with a composition of up to about 50% water (the older hard lenses were a solid piece of plastic). If you are a contact lens wearer, you will know what happens when you leave a soft lens to dry out it shrinks and becomes hard like plastic. Dried up lenses will regain their former shape if soaked in water as the polymer network swells and refills with water. Some of the most current contact lens technologies, such as overnight or continuous wear lenses, are based on silicone hydrogels (polymers with a silicon backbone). 

Knowledge of the swelling characteristics of a polymer is of utmost importance in biomedical and pharmaceutical applications since the equilibrium degree of swelling influences the solute diffusion coefficient through these hydrogels [1], the surface properties and surface mobility [1,2,8], the optical properties, especially in relation to contact lens applications, and the mechanical properties. 

Keywords Biocompatibility, contact lens, silicone hydrogel, hydrophilic polymers... 

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. 

Use of preserved tear substitutes with contact lenses is a concern because the preservatives may bind to the lens polymer, prolonging ocular retention and exposure, which may result in toxic or hypersensitivity reactions. BAK is more readily absorbed than are thimerosal and chlorhexidine in most hydrogel 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. 

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