The surface rheology of human saliva

Saliva has a number of functions which are important in maintaining a healthy oral environment. Its composition and rheological properties are complex making the formulation of a saliva substitutes with identical properties difficult. Recently it has become apparent that saliva also forms a tough viscoelastic film at the air-saliva interface. The significance of this property is currently unknown but it may have implications for the development of better saliva substitutes and in the pathogenesis of some oral diseases. The aims of the current study were to; 1. Investigate the surface film forming properties of saliva. 2. Compare these properties with those of commercially available saliva substitutes. 3. Undertake preliminary characterisation of the composition of the surface film. Surface elasticity and surface viscosity were assessed using an Oscillating Ring Surface Shear Rheometer (ORSSR) at approximately 6Hz. Whole resting saliva and stimulated parotid saliva were collected using standard techniques from one healthy male subject. The surface rheological properties of human saliva were compared with those of commercially available saliva substitutes. Results indicated that whilst human saliva had pronounced surface film forming properties none of the commercially available saliva substitutes demonstrated any measurable surface elasticity or surface viscosity. Freezing stimulated parotid saliva at either -20°C or -70°C produced a 90% reduction in surface elasticity (from approx. 125mN/m) and viscosity (from approx. 1.75Ns/m). Dilution of saliva with normal saline produced a further reduction in rheological parameters but these were not directly related to the concentration of saliva. Isolation of the surface film of stimulated parotid saliva enabled its protein composition to be investigated with SDS page electrophoresis. Results indicated that two protein fractions were concentrated within the surface film, one with a large molecular weight (>205KDa) and one with a lower molecular weight (approx. 10-12KDa). Conclusion - Human saliva forms a strong viscoelastic film at its interface with air. The formation of the film is adversely affected by freezing and appears to involve concentration of two distinct protein fractions in the surface layer.