Rheological Evaluation of Artificial Saliva Formulations

Artificial saliva formulations are commonly used in medicine, food, and pharmaceutical research to emulate the biochemical, tribological, and rheological properties of human saliva. Like other viscoelastic fluids, saliva is known to possess properties such as stringiness or spinnbarkeit, governed by extensional flow parameters which cannot be extracted using conventional torsional rheometry. Even though extensional flows involving saliva are commonly encountered in situations such as swallowing, licking, drooling, and sneezing, rheological evaluations of saliva substitutes in the current literature are usually based on shear viscosity alone. In this contribution, we provide a comprehensive examination of the shear and extensional rheology of twelve commercially available artificial saliva formulations and dry mouth treatments using rate-dependent torsional rheometry and dripping-onto-substrate (DoS) protocols, and evaluate their properties based on pioneering studies of saliva's viscoelasticity. Despite the majority of these formulations being marketed as having enhanced rheology, only a select few displayed measurable viscoelasticity and strain-hardening, and did so at viscosities significantly higher than that of saliva.