Interfaces in Composite Materials - Biointerfaces: Where Material Meets Biology - page 170

hybrid layer, an ionic interaction between carboxylate or phosphate groups
on the bonding monomer and remaining hydroxyapatite crystals can also
occur. 169
The oral environment with associated chemical and mechanical stresses
continually challenges the integrity of the tooth-restoration interfaces. 175 For
example, the durability of this interface is highly vulnerable to fatigue failure
by progressive localized damage under cyclic loading. 176 The durability of
this interface could also be jeopardized by the endogenous matrix metallo-
proteinases (MMPs), e.g., MMP-2, MMP-8 and MMP-9 177,178 and cysteine
cathepsins (B, K and L) 179,180 that attack the demineralized collagen network
left after acid-etching. The activation of MMPs could occur upon exposure to
acid produced by bacteria in biofilms 181,182 or by acid-etch reagents. 183
Preserving the integrity of the collagen matrix via controlling the proteo-
lytic enzyme activity is therefore a key issue in optimizing the durability of
the tooth/restoration interface. 179 Several attempts include application of
MMPs inhibitor as a pre-conditioning step of dentin or by its inclusion into
dentin adhesives (etchant or primer or bonding). Among these inhibitors
are: benzalkonium chloride, 174 galardin, 184 BB94 and GM6001, 185 chlor-
hexidine digluconate 186,187 and ZnO particles. 188 Pre-conditioning of dentin
with ethylenediaminetetraacetic acid (EDTA), 189 sodium oxalate, 190 titanium
tetrafluoride and caffeic acid phenethyl ester, 191 a phosphoric acid-sodium
hypochlorite (NaOCl) mixture 192 can inhibit MMPs activity and accordingly
hybrid-layer degradation nano-leakage at the interface without comprom-
ising the bond strength of the adhesives.
Protecting the exposed collagen network from the action of MMPs is an-
other strategy used to improve the durability of the resin-bond interface.
Incorporation of nanofillers or nanogels could reinforce the hybrid layer and
maintain its stability through their protective and/or therapeutic effects as-
sociated with mineral deposition and/or possible interference with
MMPs. 193 Examples of these fillers are: zirconia (20-50 nm), 194 biomimetic
or thermally produced hydroxyapatite nanoparticles (20-70 nm), 195 colloidal
silica (5-40 nm) or barium aluminosilicate nanofillers (400 nm), 196 hydro-
xyapatite nanorods, 197 reactive nanogel 198 and bioactive calcium/sodium
phosphosilicate. 193 Cross-linking of collagen networks enhances its stability
by increasing the stiffness 199,200 and nano-hardness of collagen fibrils
thereby reducing its degradation by MMPs. 200 Examples of cross-linking
agents include: tannic acid, 201 combination of riboflavin and ultraviolet
radiation, 199 riboflavin and/or chitosane, 202 glutaraldehyde and grape seed
extract 200 and proanthocyanidins. 203

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7.4.3.1 Methodology of Resin-Dentin Interface Examination
The interaction between treated dentin surface and resin can be measured
through morphology, chemical changes, bond strength or nanoleakage
across the interface. This section deals with different methodological tech-
niques used to study this interface.