Calcium ions combining epigallocatechin 3-gallate to enhance saliva lubrication

This in vitro study investigated the effects of the combination of calcium ions (Ca²⁺) with epigallocatechin 3-gallate (EGCG) on the adsorption and lubrication of salivary proteins as well as bacterial growth in saliva, aiming to explore a feasible approach to enhance saliva lubrication for xerostomia treatment. The adsorption and lubrication behavior of salivary proteins were examined via atomic force microscopy, quartz-crystal microbalance with dissipation, and nanoindentation–scratch techniques, and Streptococcus mutans activity in saliva was evaluated by using the flat colony counting method. The results revealed that although the adsorption and lubrication of salivary proteins improved, the addition of extra Ca²⁺ to the saliva promoted bacterial proliferation. The lubrication benefits of adding 6 mM Ca²⁺ outweigh the risks of bacterial growth, whereas the concomitant addition of EGCG in saliva at concentrations ≥ 0.046 mM inhibits bacterial proliferation. Nevertheless, the introduction of EGCG into saliva has the potential to coarsen salivary pellicles and affect oral perception, and an applicable concentration range for the combination of EGCG with 6 mM Ca²⁺ was found to be 0.046–0.08 mM. Moreover, Ca²⁺ combining EGCG contributes to an increase in the lubricity of the two-layered salivary pellicles obtained from the selective physisorption of salivary proteins through a mechanism that induces salivary protein aggregation and then increases the thickness, viscoelasticity, and energy dissipation of the outer pellicle layer. Consequently, the combination of Ca²⁺ with subperceptual EGCG has an encouraging tribological role in saliva lubrication, demonstrating great promise for xerostomia treatment.