Contact lens Swell

During the application of the various guidelines for ophthalmic, contact lens, and intraocular products, ocular examination and biomicro scopic examination of rabbit eyes are completed with objective reproducible grading for conjunctival congestion, conjunctival swelling, conjunctival discharge, aqueous (humor) flare, iris involvement, severity and area of corneal opacity or cloudiness, pannus, and intensity of fluorescein staining [39,103]. Other available methods... 

Polyesters synthesized from terephthaloyl chloride and isohexides were prepared, and characterized by differential scanning calorimetry, n.m.r. spectroscopy, and viscosity measurement.25,128 Another polyester, prepared by copolymerization of (2-hydroxyethyl)methacrylate with isoman-nide dimethacrylate (172), forms a hydrogel when allowed to swell in isotonic sodium chloride solution. This gel has useful properties for manufacturing contact-lens material.265... 

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. 

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. 

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%. 

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