Dentin/Adhesive Interface in Teeth - Structural Interfaces and Attachments in Biology - page 134

that exhibited limited monomer/polymer conversion. The limited conversion of the
HEMA-rich phase suggests that either the photoinitiator is localized to the hydro-
phobic phase or it is incompatible with the hydrophilic HEMA [ 81 - 83 ].
In the absence of water, HEMA is a good solvent for BisGMA, so a relatively
homogeneous solution can be formed. Water is also a good solvent for HEMA but a
nonsolvent for BisGMA. With increasing water concentration, the adhesive may
experience phase separation. Based on our previous work [ 84 ], a water concentra-
tion of at least 10% is required for visible macro-phase separation in HEMA/
BisGMA formulations with a mass ratio of 45/55. A related study from our
laboratory has provided direct evidence that with phase separation, there is minimal
distribution of BisGMA and the hydrophobic photoinitiators camphorquinone (CQ)
and ethyl 4-(dimethylamino)benzoate (EDMAB) in the aqueous phase [ 85 ].
Studies from our laboratory have shown spectral evidence of phase separation in
a commercial total-etch BisGMA/HEMA adhesive bonded to wet, demineralized
dentin matrices [ 35 , 52 , 72 ]. Ethanol is the solvent in this commercial adhesive. The
primary function of the solvent is to displace the water from the wet, demineralized
dentin matrix, but the spectroscopic results indicate that there is enough water
present to promote detrimental adhesive phase separation. In this study, the major-
ity of the intertubular d/a interface was characterized by collagen fibrils from the
demineralized dentin matrix with limited spectral contribution from the critical
dimethacrylate component (BisGMA). Thus, the demineralized dentin matrix is
primarily infiltrated by HEMA. HEMA has a low cross-link density and thus, it
will tend to absorb extraneous water, leading to plasticization and breakdown of
the adhesive. In this study, the HEMA exhibited limited monomer/polymer conver-
sion and it is expected that the unreacted components would be released in the
mouth [ 86 ].
The sensitivity of our current adhesives to excess moisture is also reflected in
the water-blisters that form in adhesives placed on over-wet surfaces [ 87 - 89 ].
The optimum amount of wetness varies as a function of the adhesive system [ 90 ].
Additionally, it is impossible to simultaneously achieve uniform wetness on all of
the walls of the cavity preparation [ 91 ]. Wet bonding is, in short, a very technique-
sensitive procedure and optimum bonding with our current commercial adhesives
occurs over a very narrow range of conditions, e.g., water content [ 74 ].
One suggested approach to solve these problems is “ethanol-wet bonding” [ 92 ,
93 ]. A concern with this method is that, in the clinical setting, this solvent may be
diluted because of repeated exposure of the material to the atmosphere or
concentrated because of separation of the bonding liquids into layers within the
bottle. Results from our laboratory have shown an inverse relationship between
mechanical and thermal properties and the concentration of ethanol that is present
during photopolymerization of model BisGMA-based adhesives [ 82 ]. In addition,
the hybridization process is very sensitive to the ethanol content in the adhesive
system [ 79 ]. Although the effect of “ethanol-wet bonding” on durability is not
known, results from our laboratory suggest that this approach will not overcome the
clinical challenges associated with forming a durable bond at the dentin/adhesive
interface [ 94 , 95 ].