Mapping water and monomer gradients in the adhesive/dentin interface with confocal micro-Raman imaging

Abstract

Objectives

Water in the adhesive/dentin (a/d) interface plays a crucial role in the quality of the hybrid layer (HL). This study aims to directly measure depth profiles of water content and adhesive monomers within the HL and explore the relationship between adhesive hydrophilicity and water content under wet bonding conditions using two model adhesives.

Methods

The occlusal one-third of the crown was removed from six unerupted human third molars. The exposed dentin surfaces were etched with 35 % phosphoric acid for 15 s, followed by the application of model adhesives with varying BisGMA/HEMA ratios (40/60 and 70/30) using the wet bonding technique. After light curing and 24 h of storage in water, the specimens were examined using a confocal Raman microscope under a 100x objective. Raman spectral imaging or mapping was performed at 1-micron intervals across the a/d interface in the Z direction. Reference spectra were obtained from model compounds, including type I collagen, BisGMA, HEMA, water, and the model adhesives, to generate calibration curves. These curves were then used to calculate the weight percentages of the components within the HL, which were subjected to statistical analysis (α = 0.05).

Results

Raman imaging shows that the HL is not a uniform structure, exhibiting gradients in both adhesive penetration and dentin demineralization. However, water content consistently remains higher in the HL compared to both the adhesive and underlying dentin. The water content in the HL formed by the model adhesives varies between approximately 9 % and 24 %, depending on location. This water content is strongly influenced by the hydrophobicity of the adhesives, with greater water accumulation at the bottom of the HL when a more hydrophobic adhesive (BisGMA/HEMA = 70/30) is used.

Significance

For the first time, the distribution of water, collagen, and adhesive within the HL has been quantified using confocal Raman microscopy combined with z-mapping. This technique allows for direct, nondestructive detection of the HL’s interfacial structure and composition.