Lubrication and load-bearing properties of human salivary pellicles adsorbed ex vivo on molecularly smooth substrata

Abstract

In a series of Surface Force Balance experiments, material from human whole saliva was adsorbed to molecularly smooth mica substrata (to form an ‘adsorbed salivary film’). Measurements were taken of normal (load bearing, F n) and shear (frictional, F s*) forces between two interacting surfaces. One investigation involved a salivary film formed by overnight adsorption from undiluted, centrifuged saliva, with the adsorbed film rinsed with pure water before measurement. Measurements were taken under pure water and 70 mM NaNO3. In a second investigation, a film was formed from and measured under a solution of 7% filtered saliva in 10 mM NaNO3. F n results for both systems showed purely repulsive layers, with an uncompressed thickness of 35–70 nm for the diluted saliva investigation and, prior to the application of shear, 11 nm for the rinsed system. F s* was essentially proportional to F n for all systems and independent of shear speed (in the range 100–2000 nm s−1), with coefficients of friction μ ∼ 0.24 and μ ∼ 0.46 for the unrinsed and rinsed systems, respectively. All properties of the rinsed system remained similar when the pure water measurement environment was changed to 70 mM NaNO3. For all systems studied, shear gave rise to an approximately threefold increase in the range of normal forces, attributed to the ploughing up of adsorbed material during shear to form debris that stood proud of the adsorbed layer. The results provide a microscopic demonstration of the wear process for a salivary film under shear and may be of particular interest for understanding the implications for in vivo oral lubrication under conditions such as rinsing of the mouth cavity. The work is interpreted in light of earlier studies that showed a structural collapse and increase in friction for an adsorbed salivary film in an environment of low ionic strength.