Calcium ions combining epigallocatechin 3-gallate to enhance saliva lubrication

This <em>in</em> <em>vitro</em> study investigated the effect of combination of calcium ions (Ca²⁺) with epigallocatechin 3-gallate (EGCG) on the adsorption and lubrication of salivary proteins as well as the 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 using atomic force microscope, quartz-crystal microbalance with dissipation and nano-indentation/scratch techniques, and the Streptococcus mutans activity in saliva was evaluated by using flat colony counting method. Results showed that although improving the adsorption and lubrication of salivary proteins, extra addition of Ca²⁺ in saliva causes flagrant bacterial proliferation. The lubrication benefits of adding 6 mM Ca²⁺ outweigh the risks of bacterial growth, while a concomitant addition of EGCG in saliva at concentrations of ≥ 0.046 mM inhibits the resulting bacterial proliferation. Nevertheless, introduction of EGCG into saliva has the potential to coarsen salivary pellicles and affect oral perception, and an applicable concentration range for the EGCG in combination with 6 mM Ca²⁺ was found to be 0.046 mM to 0.08 mM. Moreover, Ca²⁺ combining EGCG contributes to an increase in the lubricity of the two-layered salivary pellicles obtained from 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 sub-perceptual EGCG has an encouraging tribological role in saliva lubrication, demonstrating great promise for xerostomia treatment.