Effect of solvent content on micro-scale mechanics/chemistry of adhesive/dentin interface

Objective: Solvents such as acetone or ethanol could improve adhesive penetration into the wet demineralized dentin layer. The presence of solvents with other adhesive ingredients must have an optimum concentration. This study investigated the effect of different ethanol content on interfacial micromechanics/chemistry in the adhesive/dentin interface using scanning acoustic microscopy (SAM) and confocal Raman microscopy. Methods: The occlusal 1/3 of the crown was removed from 18 unerupted human third molars. Using the wet bonding technique, the dentin surfaces were etched and then treated with SingleBond with ethanol content ranging from 0-50 vol%. SAM images of the adhesive/dentin interface were acquired at 1 GHz. Chemical analysis of the same specimens was completed using micro-Raman mapping/imaging. Both measurements were performed under wet conditions at same spatial resolution (1 micron). Gray level index (GI) value of the SAM image was used to correlate with elastic modulus (E) derived from a series of calibration curves. Results: Superimposed mechanical (GI/E) and chemical images (adhesive/collagen/mineral distribution) were generated from the same region of the interface at a resolution of 1 micron, which allowed direct correlation. Differences in GI values within the interfaces reflect differences in mechanical properties. The interfacial micromechanical property was highly sensitive to the initial solvent concentration. The results showed that the GI values of the interfacial layer were lower than those of both dentin and adhesive layer, were also different among adhesives containing various concentrations of ethanol. These values were the highest for the interface formed with adhesive containing 30% ethanol, which correlated well with adhesive penetration as recorded with micro-Raman. Conclusions: The amounts of solvents are essential for achieving effective bonding to dentin. Using noninvasive, complementary imaging techniques, this study provides the first direct correlation of mechanics/chemistry relationship of the adhesive/dentin interface at micrometer scale. Supported by NIH K25 DE015281, R03 DE15735