Wettability and wetting of corneal epithelium

Abstract

The wettability of cleansed corneal epithelium by aqueous solutions has been studied. The critical surface tension of corneal epithelium in situ and that of epithelial monolayers cultured from cornea has been determined with pure hydrophobic liquids. Results indicate that corneal epithelium free of adsorbed mucins exhibits a low energy surface as indicated by its critical surface tension value; 28 dyn/cm. The epithelial surface has a certain affinity toward aqueous phases, but it is insufficient for complete wetting. The possible wetting action of mucous glycoproteins has been investigated by using a commercially available glycoprotein mixture, bovine submaxillary mucin. The activity of this mucin at water-air and water-oil interfaces was measured as a function of concentration. The mucin was studied in the form of a surface film spread over water from an aqueous solution. The wettability of mucin adsorbed on various solid surfaces was also studied. Bovine submaxillary mucin lowers the surface tension considerably at both water-air and water-oil interfaces. The average area occupied by one molecule was calculated from the concentration dependence of surface tension to be between 50 and 100 square Å. The area occupied by one mucin molecule in a monolayer spread over water was measured to be about 90,000 square Å. The mucin adsorbed on low energy solids like polyethylene and corneal epithelium yields a surface with higher critical surface tension and with considerably higher affinity toward water. It is suggested that the primary role of the conjunctival glycoproteins in the eye is to transform the low-energy corneal surface into a higher energy surface via adsorption. This effect combined with increased affinity toward water and with the lowering of the surface tension of the tears is sufficient to achieve the complete wetting of the corneal surface.