Surface epithelium, dehydration

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. 

Figure 9.32 is a diagrammatic representation of those parts of the eye involved in dmg absorption. The cornea and the conjuctiva are covered with a thin fluid film, the tear film, which protects the cornea from dehydration and infection. Cleansed comeal epithelium is hydrophobic, physiological saline forming a contact angle of about 50° with it, and it has, in this clean condition, a critical surface tension of 28 mN m b The aqueous phase of tear fluid is spread by blinking. 

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