Anti-wear performance of human enamel in the mouth is closely related to the lubrication of salivary pellicle. It is well known that the inorganic hydroxyapatite (HA) of the enamel plays an important role in the adsorption and pellicle-forming of salivary proteins on the enamel, but the role of enamel matrix proteins remains unclear. In this study, the adsorption and lubrication behavior of salivary proteins on original, heated, and deproteinated enamel surfaces was comparatively investigated using an atomic force microscopy and nano-indentation/scratch techniques. Compared with that on the original enamel surface, the adsorption and lubrication behavior of salivary proteins remains almost unchanged on the heated enamel surface (where the enamel matrix proteins are denatured but the size of HA crystalline nanoparticles keeps constant) but exhibits an obvious compromise on the deproteinated enamel surface (where the enamel matrix proteins are removed and agglomeration of HA crystallites occurs). The HA agglomeration weakens the electrostatic interaction of enamel surfaces with salivary proteins to cause a distinct negative influence on the adsorption and pellicle-forming of salivary proteins. Further, the negative effect is confirmed with a quartz crystal microbalance with dissipation. In summary, by regulating enamel nanostructure for appropriate electrostatic interactions between salivary proteins and enamel surfaces, the enamel matrix proteins play an essential role in the adsorption and pellicle-forming of salivary proteins on human enamel, and then contribute to saliva lubrication, which provides the enamel with an anti-wear mechanism. The findings will promote and assist the design of enamel-inspired anti-wear materials.

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.

Over the past two decades, superhydrophobic surfaces that are easily created have aroused considerable attention for their superior performances in various applications at room temperature. Nowadays, there is a growing demand in special fields for the development of surfaces that can resist wetting by high-temperature molten droplets (>1200 ℃) using facile design and fabrication strategies. Herein, bioinspired directional structures (BDSs) were prepared on Y₂O₃-stabilized ZrO₂ (YSZ) surfaces using femtosecond laser ablation. Benefiting from the anisotropic energy barriers, the BDSs featured with no additional modifiers showed a remarkable increase from 9.2° to 60° in the contact angle of CaO–MgO–Al₂O₃–SiO₂ (CMAS) melt and a 70.1% reduction in the spreading area of CMAS at 1250 ℃, compared with polished super-CMAS-melt-philic YSZ surfaces. Moreover, the BDSs demonstrated exceptional wetting inhibition even at 1400 ℃, with an increase from 3.3° to 31.3° in contact angle and a 67.9% decrease in spreading area. This work provides valuable insight and a facile preparation strategy for effectively inhibiting the wetting of molten droplets on super-melt-philic surfaces at extremely high temperatures.

In this in vitro study, the restoration of acid-eroded enamel surface morphology and anti-wear properties under two conditions, mono-remineralization (treated with remineralization alone) and impact-remineralization (treated with cyclic impact followed by remineralization), are characterized to determine the effect of occlusal loading on enamel remineralization. Compared with the mono-remineralized surface, the impact-remineralized surface demonstrates better anti-wear performance, as manifested by a higher hardness and elastic modulus, as well as a lower friction coefficient and wear volume. Loading on the eroded enamel surface induces the fragmentation of hydroxyapatite nanoparticles, which aids crystal deposition and fusion during subsequent remineralization. In summary, owing to the enamel microstructure, occlusal loading can promote the restoration of enamel anti-wear properties by enhancing remineralization. Remineralization enhancement through occlusal-loading-induced nanoparticle fragmentation plays a significant role in preventing human teeth from excessive wear.

This study investigated the influence of two polyphenols on the structure and lubrication of the salivary pellicle, aiming to extend the understanding of astringency mechanisms. The salivary pellicle was prepared by the adsorption of human whole saliva on the enamel substrate. Low-astringency catechin and high-astringency tannic acid were used as astringents. The changes induced by the two polyphenols in the structure and lubrication of the salivary pellicle were examined using quartz crystal microbalance with dissipation (QCM-D) and nano- indentation/scratch technique. The salivary pellicle suffers from changes in structure and physical properties owing to protein dehydration and protein-polyphenol complexation when encountering polyphenolic molecules, causing increases in the roughness and contact angle but a decrease in the load-bearing capacity. Therefore, the lubrication performance of the salivary pellicle is impaired, leading to an increase and fluctuation of the friction coefficient. The intensity of astringency has a strong positive correlation with the water contact angle, surface roughness, and friction coefficient of the salivary pellicle. In summary, astringency is a tactile perception driven by the roughness and wettability of the salivary pellicle rather than oral lubrication, and increased intraoral friction is an inevitable consequence of astringency. The findings of this study will help promote and assist the objective evaluation of astringency.

In this study, the protective effects of two food hydrocolloids, Xanthan gum and Arabic gum, on dental erosion are investigated from the perspective of the nanomechanical properties and microtribological behavior of acid-eroded enamel. Enamel specimens prepared from extracted human teeth were immersed in citric acid solution (CAS), CAS with 0.03% w/v Xanthan gum and CAS with 0.03% w/v Arabic gum, respectively, for 10 min to obtain three groups of eroded specimens. The nanomechanical properties and microtribological behavior of enamel were examined using nano-indentation/scratch techniques. The results show that compared with Arabic gum, Xanthan gum inhibits enamel surface demineralization and acid permeation more effectively because of a more uniform and denser adsorption on the surface of the enamel. The impairment of the nanomechanical and microtribological properties of the enamel surface by acid erosion is mitigated more significantly by adding trace amounts of Xanthan gum than Arabic gum. In summary, adding trace food hydrocolloids reduces enamel surface demineralization and inhibits acid permeation to mitigate the influence of erosion on the mechanical and tribological properties of enamel. The adsorption state of food hydrocolloids is the determining factor in the permeability of acid agents into the enamel and plays a significant role in preventing dental erosion.