Objective: This study investigated the chemical and structural changes in the mineral phase and collagen of dentin during application of a mild universal adhesive. Particular attention was paid to the role of isopropanol and changes in water molecules.
Methods: In vitro application of the mild universal adhesive on dentin with two established etching modes (self-etch and etch-and-rinse) was studied using solid state nuclear magnetic resonance spectroscopy.
Results: It was evidenced that the etch-and-rinse mode leads to a decrease of the inorganic apatite and a reorganization of the residual mineral phase with a low amount of adhesive phosphate monoesters calcium salt formed, compared to the self-etch mode. In contrast, the adhesive interacts very similarly to the level of dentin collagen in both protocols, with a strong decrease in the amount of the free water molecules induced by the presence of isopropanol as the adhesive solvent, but without significant changes in the initial collagen structure. For both modes, the adhesive acrylates monomers remain mobile and can infiltrate the collagen.
Significance: Understanding the molecular interactions between dentin and adhesive solutions is a major challenge for designing products that lead to the formation of ideal dentin resin hybrid layer. Notably, one point considered essential is the presence of unbound water which, over time, is associated with a hydrolytic degradation of the organic matrix. Isopropanol, as an adhesive solvent, leads to a decrease in the amount of the less stable water molecules while the water molecules strongly attached to the collagen are retained, thus preserving the collagen structure.
Keywords: Adhesive; Chemical organisation; Dentin; Isopropanol; Solid state NMR; Water molecules.
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