Biomedical Engineering Reference
In-Depth Information
remineralized, were investigated by a nanoscratch technique. The nanoscratch
technique is an experimental method for assessing the mechanical properties of
surface layers and the wear resistance of materials [
48
]. Generally, the scratch depth
of materials via nanoscratch ranges from nanometers to micrometers.
4.5.1
Effect of Erosion Time on the Tribo-Erosive
Properties of Enamel
In vitro erosion tests of different times were performed on human tooth enamel in
citric acid solution to explore the effect of erosion time on the tribo-erosive proper-
ties of enamel [
49
].
Six freshly extracted human teeth were used in this study. Each tooth was cut,
embedded, ground, and polished to obtain an enamel specimen with the exposed
occlusal surface [
49
]. Specimens were individually placed in 0.001 M citric acid
solution (pH = 3.2) and agitated with an automatic stirrer. After 1 min, the speci-
mens were taken out from the citric acid solution and then carefully washed using
deionized water to remove any residual acid and dried. Next, the surface morphol-
ogy, profi le, composition, and hardness of three specimens, which were randomly
selected from all six specimens, were measured to study the surface loss of exposed
enamel. In order to investigate the infl uence of enamel erosion on its friction and
wear behavior, the other three specimens were used to do nanoscratch tests.
Subsequently, all six specimens were immersed in the citric acid solution again.
Erosion tests lasting up to 1, 3, 5, 10, 15, and 30 min were gradually performed for
each specimen.
Figure
4.15
gives the micrographs of the enamel surface after different erosion
times. Surface profi les and erosion depths are shown in Fig.
4.16
. Compared with
the original enamel before erosion, no remarkable difference in surface morphology
was observed after 1 min (Fig.
4.15a, b
). The profi le lines appeared relatively
smooth (Fig.
4.16a
), and the erosion depth was near zero (Fig.
4.16b
). Even after
3 min, only a few indistinct keyhole-like rods appeared on the enamel surface
(Fig.
4.15c
), and the erosion depth was still very shallow (Fig.
4.16b
). Obviously, no
signifi cant substance loss occurred at the initial stage of erosion. Tooth enamel
uniquely consists of aligned keyhole-like rods (6-8
m in diameter) that are embed-
ded in interrod enamel [
8
,
9
]. Each rod consists of tightly packed hexagonal carbon-
ated hydroxyapatite crystals with a mean width of 68.3 nm and a mean thickness
of 26.3 nm. The hydroxyapatite crystals are covered by a nanometer-thin layer of
enamelin. The interrod enamel between rods, mostly a result of the incoherence of
combining crystals of different orientations, is rich in protein. A natural coupling
agent locates where
enamel rods
meet the
interrod enamel
, which is known as the
rod sheath. The rod sheath consists of more
protein
than the interrod enamel. Such
a compact alternate arrangement of mineral and organic phases endows enamel with
a considerable resistance against erosion. Therefore, a short-time acid attack could
cause only a little substance loss.
μ
