Release of Interface Confined Water Significantly Improves Dentin Bonding

Water residue and replacement difficulty cause insufficient adhesive infiltration in demineralized dentin matrix (DDM), which produces a defective hybrid layer and thus a bonding durability problem, severely plaguing adhesive dentistry for decades. In this study, we propose that the unique properties of a highly hydrated interface of the porous DDM can give rise to 1 new type of interface, confined liquid water, which accounts for most of the residue water and may be the main cause of insufficient infiltration. To prove our hypothesis, 3 metal ions with increasing binding affinity and complex stability (Na+, Ca2+, and Cu2+) were introduced respectively to coordinate negatively charged groups such as -PO43-, -COO- abundant in the DDM interface. Strong chelation of Ca2+ and Cu2+ rapidly released the confined water, significantly improving penetration of hydrophobic adhesive monomers, while Na+ had little effect. A significant decrease of defects in the hybrid layer and a much decreased modulus gap between the hybrid layer and the adhesive layer greatly optimized the microstructure and micromechanical properties of the tooth-resin bonding interface, thus improving the effectiveness and durability of dentin bonding substantially. This study paves the way for a solution to the core scientific issue of contemporary adhesive dentistry: water residue and replacement in dentin bonding, both theoretically and practically.