Biomedical Engineering Reference
In-Depth Information
stated that the chemical composition of the formed HAP was constant throughout,
and it was necessary to carefully analyze errors in the rate data since the amount
of growth was small. Nonetheless, this study showed the possibility of investigating
growth mechanisms even for insoluble salts using a method similar to that used for
soluble salts.
Development of the constant composition (CC) method [
47
] ultimately resolved
the technical problems of the experiment discussed above. In this method, real-
time measurements of the pH and chemical composition of the growth solution
are taken with a glass electrode. The composition of the mixture is maintained
by automatically supplementing the solution with drops containing calcium or
phosphate ions. In order to maintain the lowered pH at its original value during
growth, a solution containing hydroxide ions is added. The growth rate is calculated
from the surface area of the seed crystal and the drip rate. The CC method has three
particular advantages: (1) more accurate growth rate data can be obtained since it is
possible to sufficiently grow the crystals with respect to the seeds, (2) the amount
of growth can be measured even in solutions with a relatively low supersaturation
since the measurement of the pH and chemical composition with the glass electrode
is highly sensitive, and (3) the metastable phase that forms during HAP growth can
be estimated by tracking changes in the solution composition. The CC method has
been used to not only measure the growth rate but also the dissolution rate and
nucleation induction time.
In the CC method, the relationship between the growth rate and supersaturation
is approximated by the following simple equation, and the growth mechanisms are
discussed in terms of
n
:
R
D
C
n
.C
is the reaction constant
W
n
is the reaction order
:/
(3.10)
Koutsoukos et al. grew HAP to an amount several times that of the seed crystal
using the CC method and obtained the relationship
[
48
]. From this
result they concluded that spiral growth is involved in HAP growth. A similar
conclusion was reached in other studies [
49
]. In contrast, Arends et al. obtained
the relationship
R
R
2:93:2
, and postulated that growth involved multiple two-
dimensional nucleation [
50
]. Christoffersen and Christoffersen used various growth
theory equations to analyze HAP growth rate data obtained with the CC method.
They concluded that while growth rate equation that takes multiple two-dimensional
nucleation into account is optimal, spiral growth occurring simultaneously with
two-dimensional nucleation would be possible [
51
,
52
]. Although debates on growth
mechanisms are common, with the CC method, there are also debates ranging from
the reaction orders to growth path (via surface diffusion or not).
3.4.4
Synthesis of HAP Single Crystal
Although it is desirable to conduct growth rate measurements for each crystal face
using a single crystal when investigating crystal growth mechanisms in relation to
