Biomedical Engineering Reference
In-Depth Information
adhesives. The adhesive is often chemically similar to the composite but
with reduced filler content. Composite uses include filling materials,
21,22
cements,
23
adhesives,
24
laminate veneers,
25,26
direct
and fixed partial
dentures.
27
Dental composites shrink upon set by 0.9 to 3.5 vol%.
28
The percent of
shrinkage is reduced by raising the volume percentage of filler or increasing
the monomer molecular weight. This shrinkage creates stresses
29
as high as
10-25 MPa within the composite and at its interface with the tooth struc-
ture.
30
The shrinkage stresses severely strain the interfacial bond creating
micron dimension gaps at the composite-tooth interface. Subsequently, the
invasion of fluid and bacteria can lead to post-operative sensitivity, pain,
tooth discoloration, caries and, finally, restoration failure. The elastic
properties of the polymer matrix and the ability of the polymer chains to
rearrange and relieve stresses have been shown to influence the final poly-
merization stresses.
30
Water sorption (typically
d
n
3
r
4
n
g
|
1
1.5%)
31
and the sub-
sequent expansion can relieve some polymerization stresses. Water sorption,
however, can be a slow process. These two processes of shrinkage and ex-
pansion can make gaining a strong, resilient bond between dental com-
posites and tooth structure, dentin in particular, dicult.
29
Several strategies have been attempted to reduce shrinkage stresses. These
include modifying the composition (e.g., increasing filler content, using high
molecular weight monomer, or low shrink monomer), placement technique
(curing the composite in thin consecutive layers) or the curing methods (e.g.,
soft start and pulse delay).
29
B
.
7.1.1.3 The Future
Composites may be employed in the future as scaffolds for hard (e.g., poly(e-
caprolactone)-bioactive glass composite
32
) or soft tissue regeneration (e.g.,
poly(2-hydroxyethylmethacrylate)-carbon nanotubes hydrogel compos-
ites
33
). Other future potential applications of composites includes oral (e.g.,
polylactide-co-glycolide-bioactive glass composite
34
) and ophthalmic im-
plants (e.g., silicon polycaprolactone composites
35
). Moreover, composites
may be used as wound dressing (e.g., silk fibroin-hydroxyapatite compos-
ites
36
) and biosensors/electronic devices (e.g., polysaccharide-polyaniline
composites
37
). Furthermore, composites can be used as delivery vehicles for
drugs (e.g., polycarbonate-silica xerogel nano-composites for the release of
rifampicine
38
), proteins (e.g., poly(ethylene glycol) fumarate and gelatin
microparticles for the release of bone morphogenetic protein-2
39
) or cells.
40
7.2 Composite Interfaces
The boundary between any two layers or phases of different chemistry and/or
microstructure can be described as an 'interface'. Unlike the interface,
interphase describes a gradual but not abrupt change in properties from one
phase to another and is three-dimensional in nature.
41
Generally, either the
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