How surface electronegativity and calcium release of enamel mediate the adsorption and lubrication of salivary proteins: The role of interfacial water

This study investigated the effect of surface electronegativity and calcium release of human enamel on the adsorption and lubrication of salivary proteins from the perspective of interfacial water using three model substrates, calcium-release electronegative hydroxyapatite (which represented enamel), calcium-free electronegative silica, and calcium-free electropositive zirconia. The interfacial water layer was probed using ATR-IR, and the adsorption and lubrication of salivary proteins were examined using AFM, QCM-D and nano-indentation/scratch techniques. Strong affinity of electropositive substrates to water contributed to a thick interfacial water layer, which served as a physical barrier to weaken electrostatic attraction to salivary proteins. Thus, the proteins randomly adsorbed forming a pellicle without multi-layered structure and good lubricity. The interfacial water layer on electronegative substrates tends to be thin. Driven by a strong electrostatic interaction, the salivary proteins adsorbed through self-assembly to form a pellicle with a two-layered structure. While the hydrated calcium ions caused by substrate calcium release thickened the interfacial water layer, but they served as a bridge to connect proteins. Consequently, a two-layered pellicle, both stiff and viscoelastic, formed to provide excellent lubricating action. In summary, surface electronegativity and calcium release of enamel benefit the adsorption and lubrication of salivary proteins through regulating interfacial water.