Hydrophilic soft contact lenses

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. 

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. 

Chlorhexidine diglnconate is a biguanide surfactant with low toxicity, bnt it is a strong contact sensitizer. Its mncns-binding capacity hmits its nse it binds to hydrophilic (soft) contact lenses. BnUd-up of proteinaceous debris in lenses may greatly increase the binding of chlorhexidine to the lens. 

Comeal shields are medical devices that are used as a bandage for protection of the cornea and to allow healing following surgery. Initially, hydrophilic soft contact lenses used for vision correction were employed as comeal shields. Collagen was then introduced as a substitute for the plastic noneroding bandage lens. They are widely used... 

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... 

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. 

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. 

Rubin, M. and Watkins, R.,1975. Pilocarpine for the soft hydrophilic contact lens. Brit. J. Ophthalmol. 59 455. 

Hydroxyethyl methyl Methacrylate n A monomer that polymerizes to a hydrophilic polymer that is rigid when dry but when saturated with water becomes a soft, clear material (Hydron ). Applications include masonry coatings, soft contact lens, and other biomedical devices. 

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%. 

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. 

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