Contact lenses bulk properties

Although surface treatments, both physical and chemical, have demonstrated the abiUty to alter specific properties of contact lens surfaces, most treatments fail as a result of alteration of bulk lens properties, instabifity of surface treatment, or poor ocular compatibifity. Research is expected to continue in the characterization and modification of contact lens surfaces. 

The three key features of LCVD coating ideally suited for biomaterial surface, and the important balance between the bulk properties and the surface properties, could be best illustrated by examples of nanofilms of methane plasma polymer on a contact lens made of polydimethylsiloxane elastomer. Hence, some details of processing factors and their influence on the overall properties of the product are described in the following sections. 

The hydrophobicity of the surface prevents the wetting by tear and tends to expose dry surface of a contact lens. Therefore, rapid dehydration of the corneal tissues could occur, which could cause the damage of corneal epithelium. However, this explanation seems to be oversimplified in light of the adsorption of protein, which makes a hydrophobic surface wettable by tear fluid, as described in Chapter 26. Moreover, the highly hydrophobic surface characteristic of silicone rubber tends to encourage the deposition of protein and mucus of the tear on the surface of the lens. Lipids and lipid-soluble materials follow the same track and eventually penetrate into the bulk phase of the contact lens. Because of these undesirable factors, the use of silicone contact lenses of various chemical compositions and with surface treatments has not been successful but rather disastrous because of the interfacial characteristics of silicone contact lens on the cornea, which cannot be oflfset by these efforts. It indicates that more profound surface modification to cope with the problems rather than mere surface treatment is needed in capitalizing on the advantageous bulk properties of silicone polymers. 

There hcis been extensive resecuch on polymeric materials for use cis contact lenses in the pjist two decades. The ocular environment places high demjmds on the biomaterial, both bulk md smface properties, when used as a contact lens. Proper hydration, sufficient oxygen permeability to... 

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