Adhesive phase separation at the dentin interface under wet bonding conditions

Abstract

Under in vivo conditions, there is little control over the amount of water left on the tooth and, thus, there is the danger of leaving the dentin surface so wet that the bonding resin undergoes physical separation into hydrophobic and hydrophilic-rich phases. The purpose of this study was to investigate phase separation in 2,2-bis[4(2-hydroxy-3-methacryloyloxy-propyloxy)-phenyl] propane (BisGMA)-based adhesive using molecular microanalysis and to examine the effect of phase separation on the structural characteristics of the hybrid layer. Model BisGMA/HEMA (hydroxyethl methacrylate) mixtures with/without ethanol and commercial BisGMA-based adhesive (Single Bond) were combined with water at concentrations from 0 to 50 vol%. Macrophase separation in the BisGMA/HEMA/water mixtures was detected using cloud point measurements. In parallel with these measurements, the BisGMA/HEMA and adhesive/water mixtures were cast as films and polymerized. Molecular structure was recorded from the distinct features in the phase-separated adhesive using confocal Raman microspectroscopy (CRM). Human dentin specimens treated with Single Bond were analyzed with scanning electron microscopy (SEM) and CRM mapping across the dentin/adhesive interface. The model BisGMA/HEMA mixtures with ethanol and the commercial BisGMA-based adhesive experienced phase separation at approximately 25 vol% water. Raman spectra collected from the phase-separated adhesive indicated that the composition of the particles and surrounding matrix material was primarily BisGMA and HEMA, respectively. Based on SEM analysis, there was substantial porosity at the adhesive interface with dentin. Micro-Raman spectral analysis of the dentin/adhesive interface indicates that the contribution from the BisGMA component decreases by nearly 50% within the first micrometer. The morphologic results in corroboration with the spectroscopic data suggest that as a result of adhesive phase separation the hybrid layer is not an impervious 3-dimensional collagen/polymer network but a porous web characterized by hydrophobic BisGMA-rich particles distributed in a hydrophilic HEMA-rich matrix.

Effects of a solubility enhancer on penetration of hydrophobic component in model adhesives into wet demineralized dentin

“…The unpredictable water content on the etched dentin surface makes the bonding process very complex. In BisGMA/HEMA based adhesives, the presence of water may cause the phase separation between hydrophilic and hydrophobic monomers [4], and inhibit the infiltration of the relative hydrophobic BisGMA monomer. This will affect the integrity of the adhesive and dentin interface.…”

“…BisGMA (Bisphenol-A-glycerolate (1-glycerol/ phenol) dimethacrylate), a widely used component in dentin adhesives has very good mechanical properties after curing, but this component is relatively hydrophobic and thus, does not adequately infiltrate the wet demineralized dentin collagen [4][5][6][7][8]. Complications such as phase separation have been associated with BisGMA based adhesives in the presence of wet, demineralized dentin.…”

“…Complications such as phase separation have been associated with BisGMA based adhesives in the presence of wet, demineralized dentin. Collapse or aggregation of the demineralized dentin collagen as a result of drying has been frequently reported [4,[9][10][11]. The heterogeneous distribution of adhesives and/or the collapse of the collagen network compromise the integrity of the a/d hybrid layer (HL) as well as the marginal stability and strength of composite restorations.…”

“…the water supporting the collagen matrix is not removed [21]. However, the relatively hydrophobic BisGMA cannot adequately infiltrate the demineralized dentin layer under wet bonding conditions [4][5][6][7][8]. So developing adhesive formulations which can decrease the phase separation and promote enhanced penetration of the hydrophobic component into the wet demineralized dentin matrix is very important.…”

“…The micro-Raman technique has been shown to be a powerful tool for detecting and quantifying the molecular structure of HLs at a spatial resolution comparable to optical microscopy [4][5][6][7][8]. SEM observations of HLs exposed by acid-bleach methods and optical observation results of stained interfaces of dentin and model adhesives have been employed to evaluate the quality of HLs [26][27][28][29][30][31][32].…”