Contact lenses hydrophilic

Water Content and Refractive Index. The water content of a hydrophilic contact lens is a determinant of other properties. The relationship of water content and Dk is discussed above. Water content in lenses is inversely related to refractive index (23), a key property for vision correction. A lens material with a higher refractive index refracts light to a greater degree, allowing more vision correction with a thinner material. The water content of a lens is generally determined gravimetricaHy or inferred from the relationship to refractive index, measured with a refractometer (24). 

Hydrogels used for contact lens appHcations can be classified based on the chemical stmcture of hydrophilic monomers and on the water content of the hydrogel lens. These classifications are not unique because the design of hydrogels is based on the performance desired in most cases, more than one component is used for the same or complementary purpose. In fact, some hydrophilic monomers are used in both high water and low water lenses. 

Most contact lenses are worn for optical reasons as an alternative to spectacles. Contact lenses are of two types, namely hard lenses, which are hydrophobic, and soft lenses, which may be either hydrophilic or hydrophobic. The surfaces of lenses mnst be wetted before nse, and wetting solntions (section 4.5.1) are nsed for this pnrpose. Hard and, more especially, soft lenses become heavily contantinated with protein material dnring nse and must therefore be cleaned (section 4.5.2) before disinfection (section 4.5.3). Contact lenses are potential sonrees of eye infection and conseqnentiy nticroorganisms should be removed before the lens is again inserted into the eye. Lenses mnst also be clean and easily wettable by the lacrimal secretions. Contact-lens solntions are thns sterile solutions of the varions types described below. Apart fiom... 

Food and Drug Administration, Draft Testing Guidelines for Class III Soft (Hydrophilic) Contact Lens Solutions, U.S. Food and Drug Administration, Silver Springs, MD, 1985. 

Observations Soft contact lens materials represent a balance between hydrophilic... 

With the development of neutral hydrophilic methacrylates in Prague, originally for contact lens applications in the early 60 s113>, considerable interest was generated in the application of these materials in the cardiovasular environment. The qualitative argument was that such soft, water-rich surfaces must be relatively non-traumatic to proteins and cells. The development of neutral hydrophilic polysaccharide-based particles for protein chromatography in the late 60 s provided evidence that such surfaces do indeed show minimal binding of proteins. 

Presoaked lenses are considered a more efficient and reliable delivery system. However, the soaking of lenses in ophthalmic formulations to incorporate the drag into the lens may cause toxicity to corneal epithelium because preservatives, such as benzalkonium chloride, have a great affinity for the hydrophilic contact lens material and are concentrated in the contact lens. Contact lens for sensitive wearers may also cause foreign-body sensation, blurring and decreased oxygen tension on the corneal surface resulting from occlusion by contact lens. 

Employees should be cautioned about wearing contact lens in certain, if not all, pesticide environments. Dusty or chemical environments may represent particular hazards to contact lens wearers. Soft contact lenses can be especially problematic. Their design makes them hydrophilic (absorb fluids) , as a result they will also absorb chemical mists. If an employee is working in these conditions, it is best that he or she not wear contact lenses. 

The hydrophilic-lipophilic balance (HLB) and molecular dimensions of a preservative should also be considered in the design of contact lens products. For example, chlorobutanol (a non-ionic preservative used... 

To be suitable for long-term wear, a contact lens material has to satisfy several criteria. It must be hydrophilic enough to maintain a stable, continuous tear film on its surface, resist fouling by tear components, not irritate the eye, and be comfortable to wear. It should also be realized that the metabolism of the cornea is highly dependent on dissolution and adsorption of atmospheric oxygen and therefore any lens material must have sufficient oxygen permeability to maintain this, else corneal anoxia will set in. 

A soft contact lens comprises a water-swollen gel of a polymer prepared by polymerising one or more hydrophilic monomers such as 2-hydroxyethyl methacrylate, one or more crosslinking monomers, a monomer that... 

Recent experiments indicate that polymers that contain a balance of hydrophobic (nonpolar) and hydrophilic (polar) chemical groups show minimal protein adsorption and cell adhesion (6). With the intent of rationally designing a contact lens material that would minimize protein adsorption, the adsorption of lysozyme, albumin, and immunoglobulin G (IgG) to a series of hydrophobic and hydrophilic polymers and copolymers was measured. The polymers ranged from 100% poly(methyl methacrylate) (PMMA) to 100% poly(2-hydroxyethyl methacrylate) (PHEMA). Adsorption varied significantly for each protein, as did the elutability of the proteins from the surfaces. 

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. 

Iliis paper descnbes a novel contact lens materim based on a methacrylate end capped poly (trifluoropropylmethylsiloxane)(TET), octafluoropentyl-methacrylate (OFPMA) and a wettmg agent 2-vinyl-4,4-dimethyl-2-oxazolin-5-one (VDMO). The trifluoropropylmethyl siloxane, fluorinated methacrylate methacrylate and VDMO in the correct concentration provide a lipid resistant, oxygen permeable, hydrophilic contact lens material. 

A colored printing ink for contact lenses has been detailed (91,92). A method for making a hydrophilic contact lens is comprised of the steps of providing a polymeric contact lens and coating a portion of a surface of the lens with a coat. This coat contains a colorant, and a crosslmked binder polymer. The binder polymer entraps the colorant and adheres to the lens. A binder polymer for a colored base ink is summarized in Table 7.12. 

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