Dentin/Adhesive Interface in Teeth - Structural Interfaces and Attachments in Biology - page 133

adhesive infiltration. However, HEMA, a primary component in many single bottle
commercial dentin adhesives, can dramatically reduce the evaporation of water
[ 69 ]. Hydrophobic monomers, such as 2,2-bis[4(2-hydroxy-3-methacryloyloxy-
propyloxy)-phenyl] propane (BisGMA), would resist diffusing into these sites
where there is residual water [ 25 , 50 , 70 - 72 ]. Under in vivo conditions, there is
little control over the amount of water left on the tooth. As a result, it is possible to
leave the dentin surface so wet that the adhesive undergoes physical separation into
hydrophobic and hydrophilic-rich phases [ 71 ].
Results from our laboratory indicated that excess moisture prohibited the forma-
tion of an impervious, structurally integrated d/a bond at the gingival margin of
Class II composite restorations [ 25 , 26 ]. Clinicians must routinely attempt to bond
to naturally wet substrates such as caries-affected dentin [ 73 ] or deep dentin [ 36 , 37 ,
74 , 75 ]. The water content of caries-affected dentin has been reported to be 2.7
times greater than that of normal dentin [ 73 ]. In deep dentin, 22% of the surface
area is exposed tubules while exposed tubules account for 1% of the surface area of
dentin close to the DEJ [ 76 ]. The large increase in surface area attributable to
tubules means that in deep dentin, pulpal fluid will contribute additional moisture to
that already present within the demineralized dentin matrix. Since our current
adhesives are very sensitive to excess moisture, bonding to these clinically relevant
substrates is a formidable challenge [ 26 , 77 - 79 ].

7.4.2 Extrinsic and Intrinsic Water Absorption

Absorption of extrinsic water leads to plasticization of the adhesive and loss of
interfacial d/a bond strength as a result of water attack. One example of the effect of
water absorption on chemically cured poly-HEMA specimens is the dramatic
decrease in physical properties after 24 h aqueous storage; the tensile properties
were reduced to an almost gum-like quality [ 80 ]. The mean values for tensile
strength of dry and wet poly-HEMA specimens were ~18 and 1 MPa, respectively.
This reduction was attributed to water sorption after polymerization and/or extrac-
tion of water-soluble unreacted monomers or oligomers. As a result of water uptake
into the poly-HEMA specimens, the percent elongation increased from ~20 to
220%. The authors suggested that since there is no cross-linking in the poly-
HEMA, the water allowed the linear chains to slide over one another, thus resulting
in a tenfold increase in percent elongation. In this investigation, intrinsic water at
concentrations

5 vol% inhibited the light polymerization of HEMA, even with a
tenfold increase in the initiators camphorquinone (CQ)/dimethylaminoethyl meth-
acrylate (DMAEMA).
A study from our laboratory showed that at water concentrations 25 vol%,
BisGMA-based adhesive/water solutions mimicked oil and water mixtures in that
they separated into distinct phases immediately following sonication [ 71 ]. At 25 vol
% water the adhesive separates into particles whose composition is primarily
BisGMA; the composition of the surrounding matrix material is primarily HEMA

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