High-resolution mechanical mapping of the adhesive-dentin interface: The effect of co-monomers in 10-methacryloyloxydecyl dihydrogen phosphate

The presence of 10-methacryloyloxydecyl dihydrogen phosphate (MDP) at the adhesive-dentin interface enables ionic binding to calcium salts, which results in rigid nano-layering within the submicron scale resin-dentin interdiffusion zone. MDP has been used with additional co-monomers, such as hydroxyethyl methacrylate (HEMA) and/or 4-methacryloyloxyethyl-trimellitic acid (4-MET), mainly to enhance the chemical bonding properties. However, the use of co-monomers may compromise the rigidity of the adhesive-dentin interface. In this study, we use high-resolution mechanical mapping across the interface to discern the in situ mechanical properties of each target region at the nanoscale. Visualization by modulus mapping demonstrated that HEMA increases the diffusion properties of MDP into dentin structures. However, the rigidity of the adhesive-dentin interface indicated by the storage modulus was markedly lower in MDP containing HEMA than in MDP containing 4-MET. Dynamic indentation testing revealed that the bonding layer was more deformable in the presence of HEMA. Moreover, the presence of MDP in the bonding layer might also increase the deformability because the polymerization linearity allows a large degree of viscoelasticity. These factors also diminish the rigidity of the adhesive-dentin interface. Within the limitations of this study, our findings demonstrated that 4-MET is a better co-monomer than HEMA in MDP-based dental adhesives. Modulus mapping and nanoindentation are introduced as new tests for the adhesive-dentin interface to address queries about the effectiveness of dental adhesives.