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 NaNO 3 . In a second investigation, a film was formed from and measured under a solution of 7% filtered saliva in 10 mM NaNO 3 . 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 71 ), with coefficients of friction m * 0.24 and m * 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 NaNO 3 . 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.

The Pellicle and Erosion

“…Bar represents 200 nm. force spectroscopy and friction force spectroscopy data from in vitro investigations [9][10][11][12]. Also, complementary surface characterization methods (surface plasmon resonance and quartz crystal microbalance with dissipation monitoring) for the analysis of the in vitro adsorbed salivary films clearly support this two-layer model [13].…”

Influence of substratum hydrophobicity on salivary pellicles: organization or composition?

“…For a wide variety of substrata of different hydrophobicities, there is growing evidence (Cárdenas, Arnebrant et al 2007;Macakova et al 2010;Sotres et al 2011;Harvey et al 2012) that, at least in vitro, pellicles consist of an inner thin dense layer (formed mainly of proteins of relatively low molecular weight) and an outer thick diffuse layer (mainly composed of mucins). The presence of mucins in the outer layer is further supported by studies showing that pellicles and mucin films exhibit similar long-range steric normal forces (Nylander et al 1997;Dedinaite et al 2005;Pettersson & Dedinaite 2008) and friction coefficients (Hahn Berg et al 2003;Pettersson & Dedinaite 2008).…”

Salivary Pellicles

“…This in turn supports the loss of cohesion as the mechanism underlying the increase in wear at low ionic strengths. Harvey et al [58] also observed that, after performing shear measurements on a spot of a film, the range of normal forces increased. This is indicative of ploughing-up, i.e.…”

“…A different mechanism would be that adhesion between adsorbed components on opposing surfaces may arise due to conformational changes at low ionic strengths. However, this adhesion mechanism could be discarded from the work of Harvey et al [58] where the lubrication and load-bearing properties of salivary films were studied by means of surface force balance. In this work they showed a net repulsive interaction between salivary films and, in agreement with the work of Nylander et al [36], a lack of adhesion.…”