Biomedical Engineering Reference
In-Depth Information
3.4.5
Real-Time Phase-Shift Interferometry for Observation
of Biomineral Growth
Although it is possible in principle to obtain large HAP crystals that can be used for
observation, the rate of growth for each face is not performed using a single HAP
crystal due to the slowness of the growth. The growth rate of a soluble inorganic
crystal depends on the type of crystal and crystal face. It typically ranges from 10
to 100 nm/s even when the supersaturation is a few percent. The growth rate of a
HAP crystal is thought to be 2-4 orders of magnitude lower than this value. The
simplest way to measure the growth rate using a single crystal is to measure the
amount of normal growth of the target face. However, an excessively low growth
rate markedly decreases the time resolution for the growth amount, and precise data
to compare with theory cannot be obtained. Although optical interferometry can
measure growth rates far more rapidly than simply measuring the normal growth
of the crystal face, the technique is extremely sensitive to temperature changes in
the vicinity of the equipment and to external disturbances such as vibration. It is
therefore not well suited to measurements of long duration. For this reason, optical
interferometry has not been applied to the measurement of HAP growth rates.
However, real-time phase-shift interferometry has helped to resolve the problem
of external disturbances [
65
].
Although real-time phase-shift interferometry is essentially a type of optical
interferometry, in contrast to conventional interferometry, which produces only
an interference image of the crystal surface, it simultaneously (or nearly simul-
taneously) obtains multiple interference images that are slightly phase shifted
from each other. This method markedly improves the height resolution because
it includes calculation of the two-dimensional intensity-distribution profile using
multiple interference images. This enables the increments in the height direction
to be observed within a short time. Since low-frequency external disturbances
are the primary factor that causes artifactual changes in interference fringes, the
phase-shift method can greatly reduce the effects of these disturbances. While
the height resolution depends on the number of interference images used, phase-
shift interferometry can increase the height resolution to more than 100 times that
of conventional two-beam interferometry, which is comparable to the resolution
achieved using atomic force microscopy (AFM).
The first application of phase-shift interferometry to biominerals was to calcite
[
66
]. With natural calcite crystals as the seed, the dissolution rate in aqueous solution
was measured in situ. Maruyama et al. elucidated the growth process of calcite [
67
].
Their research included detailed investigation of how the chirality of amino acids
(asparagines), included as an impurity, affected the step velocity. The calcite growth
rate was estimated with good precision by taking measurements over approximately
20 min.
HAP growth rate measurements using the phase-shift method were first reported
for the
c
-face of HAP [
68
]. An example of
c
-face observation using real-time
phase-shift interferometry is shown in Fig.
3.2
. In this research using large HAP
single crystals obtained from hydrothermal synthesis as seeds, a pseudo body fluid
