Biomedical Engineering Reference
In-Depth Information
2.3.5
Nanotechnology in Dental Adhesives
In order to increase the cohesive strength of dental adhesives it is common to use filler particles
which have been treated with a polymerizable silane. However, since the adhesive liquids are not very
viscous the filler particles tend to settle out during storage which may lead to inconsistency in their
performance. To overcome this disadvantage discrete silane-treated nanoparticles of silica or zirco-
nia in the size range of 5-7 nm have been added to dental adhesives. These particles are too small to
be affected by gravity or to be visible by the bare eye and can provide transparent adhesive liquids.
Examples of such adhesives are 3M™ ESPE™ Singlebond Plus and 3M™ ESPE™ Scotchbond™
SE adhesive. The latter adhesive contains zirconia nanoparticles
[42]
to impart radiopacity to the
adhesive. Weakly agglomerated nanoparticles of tantalum oxide and silica with primary particle size
of 10 nm have been prepared by flame pyrolysis and incorporated into experimental dental adhesives.
Stable suspensions were obtained with no decrease in bond strength
[43]
.
2.4
SELECTED PROPERTIES OF DENTAL MATERIALS CONTAINING
NANOPARTICLES
2.4.1
Optical Properties
A primary reason for using nanoparticles in dental materials is to improve their optical properties
without sacrificing mechanical strength and wear resistance. Since the size of individual nanoparticles
are smaller than the wavelength of visible light nanocomposites made from these particles exhibit
extremely low visual opacity—a property highly desirable for making aesthetic dental restorations.
A wide variety of shades and opacities can be constructed increasing the clinician's latitude to accu-
rately match the natural dentition. As shown in
Figure 2.5
disks of composites made from hybrid and
microfill fillers are quite opaque whereas that made from nanofillers is almost glass-like in translu-
cency
[22]
. In addition, when placed on a black background, the nanoparticles preferentially scatter
blue light, giving the composite a desirable opalescent effect. The ability to create a nanocomposite
with a very low opacity provides the ability to formulate a vast range of shades and opacity options
from the very translucent shades needed for the incisal edge and for the final layer in multilayered
restorations to the more opaque shades desired in the enamel, body, and dentin shades.
Although the average size of the clusters in nanofill composite is similar to that in conventional
hybrid or microhybrid fillers, the nanoclusters are fundamentally different from hybrid filler particles.
Typically, hybrid fillers are large dense particles of an average size of about 1 μm. These particles
cannot be further subdivided under normal abrasive forces in the mouth. Similar remarks also apply
to microhybrids, which are only slightly smaller than hybrids in average particle size (0.4-0.6 μm).
By contrast, the nanoclusters consist of loose agglomerates of primary particles clearly seen in the
cluster domain in
Figures 2.1 and 2.2B
. The addition of nanomers reduces the interstitial spacing
between the nanoclusters thus providing a very smooth surface in a nanofilled composite. During nor-
mal abrasion, the wear rate and pattern of the nanoclusters closely match those of the surrounding
matrix, resulting in increased polish retention and surface gloss when compared to traditional hybrid
types of composites. In contrast, the resin matrix of hybrids and microhybrids wear away by abrasive
force over time exposing the large filler particles. The average particle size of these fillers is typi-
cally between 0.4 and 0.6 μm. As these materials are abraded further, loss of the large filler particles
