Viscoelasticity of human whole saliva collected after acid and mechanical stimulation

The rheology of saliva is highly important due to its influence on oral health and physiochemical processes within the oral environment. While the rheology of human whole saliva (HWS) is considered important for its functionality, its measurement is often performed erroneously and/or limited to the viscosity at a single shear rate. To ensure accurate rheological measurements, it is necessary to test HWS immediately after expectoration and to apply a thin layer of surfactant solution around the rim of the rheometer plates so that protein adsorption is minimized at the air-liquid interface. It is shown for the first time that the viscosity and viscoelasticity of HWS depends greatly upon the method of stimulation. Mechanical action stimulates slightly shear-thinning and relatively inelastic saliva, while acidic solutions (e.g. 0.25% citric acid) stimulate secretion of saliva that is highly elastic and shear-thinning. However, both acidic solutions and mechanical action stimulate similar volumes of saliva. For acid-stimulated saliva, the ratio of the primary normal stress difference to the shear stress is of order 100 and the viscosity at high shear rates is only marginally above that of water. This extremely high stress ratio for such a low viscosity fluid indicates that saliva's elastic properties dominate its flow behavior and may assist in facilitating lubrication within the oral cavity. It is anticipated that the variation in saliva rheology arises because the individual glands secrete saliva of different rheology, with the proportion of saliva secreted from each gland depending on the method of stimulation. The steady-shear rheology and linear viscoelasticity of HWS are described reasonably well using a FENE-P constitutive model and a 3-mode Maxwell model respectively. These models indicate that there are several long relaxation modes within saliva, possibly arising from the presence of large flexible macromolecules such as mucin glycoproteins.