Protein–saliva interactions govern the structure and lubrication of salivary films

HYPOTHESIS: Saliva is essential for oral lubrication and is influenced by interactions with foods, beverages and pharmaceuticals and their components. We hypothesize that increasing protein concentration increases protein adsorption and therefore reduces measured friction coefficient on the saliva coated tribopair. EXPERIMENTS: In this study, we employed our dynamic tribological protocol (DTP) to measure friction coefficient of model bovine serum albumin (BSA) solutions on ex vivo human whole saliva (HWS), saliva pellicle and proline-rich protein (PRP) components of the salivary film. This approach advances current methods that overlook saliva's complexity by testing on bare polydimethylsiloxane (PDMS) surfaces or using whole saliva. Quartz crystal microbalance with dissipation (QCM-D) monitored the mass and viscoelastic properties of these adsorbed layers. FINDINGS: We find that BSA concentrations >5.4 mg/ml correlate with decreased friction coefficients across bare PDMS and PDMS coated with the salivary pellicle and PRP layers, suggesting increased protein adsorption. This contrasts with friction measured with whole saliva, which showed no significant difference between samples from 0.6 to 10.4 mg/ml BSA concentration. QCM-D revealed substantial changes in the mass and viscoelastic properties of the adsorbed layers, highlighting a concentration-dependent interaction between BSA and salivary proteins. These interactions suggest that BSA modifies the structural properties and enhances lubrication on the saliva pellicle, impacting oral processing of foods, beverages and pharmaceuticals. These findings expand understanding of salivary lubrication mechanisms and provide an enhanced method for investigating saliva-protein interactions. This offers insight into biophysical changes at oral surfaces during food and pharmaceutical intake, informing the design of products optimized for delivery, mouthfeel, and consumer satisfaction.