Effect of adsorption time on the lubricating properties of the salivary pellicle on human tooth enamel

Lubrication is the primary function of saliva in the mouth, which is closely associated with the adsorbed salivary proteins. Previous studies have suggested that the amount of adsorbed salivary proteins on the surfaces of human teeth depends on adsorption time. The objective of this study is to investigate the effect of adsorption time on the lubricating performance of human whole saliva. In this paper, nano-scratch tests were performed on the salivary pellicle formed on human tooth enamel after different adsorption times. Adsorption was conducted in vitro by immersing the enamel specimens in human saliva under different time intervals. The results showed that at the early stage of adsorption, a thin and dense salivary pellicle was formed on the enamel surface. As the adsorption time increased, some scattered protuberances appeared on the surface of the initial layer. With the adsorption time further increasing, more and more protuberances appeared. Finally the initial layer was completely covered by the protuberances. The amount of proteins in the salivary pellicle increased with the adsorption time, but no significant variation occurred to the kinds of the adsorbed proteins. Friction-wear tests by a nano-scratch tester indicated that both the friction coefficient and wear loss of the enamel surface significantly decreased after the surface was covered with salivary pellicle. For the saliva-adsorbed enamel surface, the friction coefficient increased with the adsorption time, but no obvious changes happened to either the wear morphology or the wear loss. In summary, the salivary pellicle changed with the adsorption time, and the initial salivary pellicle exhibited an excellent lubricating effect on human tooth enamel. The results would be helpful to extend the understanding of the lubricating mechanism of human whole saliva and then to develop new artificial saliva.