Redistribution Effect of Demineralized Dentin on Hydrophobic Monomers

Inadequate dentin bonding persists as a major cause of secondary caries and restoration failure, accounting for >60% of global dental restorative costs. Prevailing adhesion theory attributes this to challenges in displacing water from the demineralized dentin matrix (DDM) to infiltrate hydrophobic monomers. However, techniques such as ethanol-wet bonding, which eliminate water, yield only partial improvement, suggesting limitations in this water-centric view. Using chromatography equivalence modeling and Nile red molecular tracing, we identified a solvent-independent redistribution effect exerted by the DDM itself on BisGMA, which was selected as a representative hydrophobic multifunctional monomer. This effect causes BisGMA to concentrate preferentially in the adhesive layer rather than infiltrating the 3-dimensional DDM uniformly, resulting in a sharply attenuated penetration profile and a defective hybrid layer. We demonstrate that interface-confined water exacerbates this effect by swelling the DDM, amplifying its polarity, and increasing viscous resistance, whereas free water has minimal influence. Crucially, DDM reconstruction mitigates the effect and improves infiltration. Our findings establish that the DDM's inherent properties, not merely the water that it contains, fundamentally limit hydrophobic monomer infiltration, challenging the existing paradigm and advocating for a new interface-focused framework in dentin bonding.