Comparative Study on Adhesive Performance of Functional Monomers

TLDR

Mild self‑etch adhesives partially demineralize dentin, leaving hydroxyapatite adjacent to collagen in a sub‑micron hybrid layer. The study tests whether residual hydroxyapatite can chemically interact with functional monomers to enhance adhesive performance beyond micromechanical hybridization. The authors chemically characterized interactions of three functional monomers with synthetic hydroxyapatite using XPS and AAS, and examined their dentin interactions ultra‑morphologically by TEM. 10‑MDP forms a stable, low‑dissolution bond with hydroxyapatite, whereas 4‑MET shows much lower bonding and phenyl‑P lacks hydrolytic stability, indicating that specific functional monomers provide additional chemical bonding that enhances adhesive potential.

Abstract

Mild self-etch adhesives demineralize dentin only partially, leaving hydroxyapatite around collagen within a submicron hybrid layer. We hypothesized that this residual hydroxyapatite may serve as a receptor for chemical interaction with the functional monomer and, subsequently, contribute to adhesive performance in addition to micro-mechanical hybridization. We therefore chemically characterized the adhesive interaction of 3 functional monomers with synthetic hydroxyapatite, using x-ray photoelectron spectroscopy and atomic absorption spectrophotometry. We further characterized their interaction with dentin ultra-morphologically, using transmission electron microscopy. The monomer 10-methacryloxydecyl dihydrogen phosphate (10-MDP) readily adhered to hydroxyapatite. This bond appeared very stable, as confirmed by the low dissolution rate of its calcium salt in water. The bonding potential of 4-methacryloxyethyl trimellitic acid (4-MET) was substantially lower. The monomer 2-methacryloxyethyl phenyl hydrogen phosphate (phenyl-P) and its bond to hydroxyapatite did not appear to be hydrolytically stable. Besides self-etching dentin, specific functional monomers have additional chemical bonding efficacy that is expected to contribute to their adhesive potential to tooth tissue.