Friction coefficient and wettability of oral mucosal tissue: Changes induced by a salivary layer

The friction coefficient, the wettability and the surface free energy of lingual mucosa while deprived of its natural removable salivary layer were evaluated ex vivo and the influence of a salivary coating on these characteristics assessed. Piglet tongue was used as support for the experiments due to its known similarities with human tongue and the availability of a large number of specimens. Whole stimulated saliva (fresh human saliva) was collected and used as such. The friction and wettability properties of the oral mucosal tissue were tested before and after organic salivary layer formation using devices developed to characterise human skin properties. Tongue surface wettability was determined by advancing contact angle measurements. Distilled water, diiodomethane and ethylene glycol were used as probe liquids. Under experimental conditions, the tongue surface has a hydrophobic tendency and is weakly polar. When coated with a salivary layer, the surface of the tongue becomes significantly more hydrophilic. Salivary coatings increase the total surface free energy of the dorsal surface of the tongue and at the same time impart a more basic character. Moreover, the presence of salivary layers reduces the dynamic coefficient of friction by a factor of 1.6, underlying the lubricant function of saliva. In conclusion, friction and wetting properties of tongue surface provide valuable insight into how the tongue interacts with other surfaces as a function of the coating.