Biomedical Engineering Reference
In-Depth Information
and partially transformed into crystalline HAP although remaining ACP in the
final product. In this process, the induction time is reduced by the amelogenin,
indicating that amelogenin promotes the formation of pre-nucleated CaP and amel-
CaP clusters. The postulated roles of amelogenin are (1) capturing and stabilizing
the pre-nucleated CaP clusters, (2) controlling the aggregation of primary-composite
nanoclusters into the initial building blocks, and (3) controlling the sequential
assembly of the building blocks in the post-nucleation crystal growth stage.
Thus, the size of the initial CaP clusters and the growth unit are determined at
the atomic level by amelogenin and the solution conditions. The spherules formed in
the Ca-PO
4
solution with rP172 were mainly 0.8-1.4 nm, while the rP172 spherules
were larger than 10 nm. This strongly suggests that the initially formed phase is CaP
clusters (pre-nucleated), which are stabilized by rP172 [
108
], enabling them to serve
as building blocks for the nanocluster composites. Note that the size of 0.8-1.4 nm
is comparable to a hydrodynamic radius of aggregates (about 0.5 nm) detected in
pseudo-body fluid [
111
,
112
] (see 3.4.7 in Chap. 3).
5.4.5
Transformation from ACP to HAP and Intermediate OCP
The phase transformation from ACP to HAP was extensively investigated by
thermodynamic analysis in the 1970s [
113
-
115
]. These studies postulated that the
transition involves OCP and the transition usually starts before all the ACP has
transformed into OCP because OCP is a very unstable intermediate of H-Ap. The
lifetime of OCP is remarkably reduced in the presence of a small amount of F
,
in a basic solution, and at a higher Ca/PO
4
molar ratio. This makes it difficult
to detect the intermediate phase. Therefore, OCP has been recognized as a labile
intermediate. However, even when OCP was not detected as an identifiable solid
phase, solution chemistry has shown that when OCP is forming in solution, the
I
AP
of the solution is larger than the solubility product of OCP [
115
]. This is
thermodynamic evidence for the existence of OCP. For detail in this area, review
is available [
116
].
In the transformation from ACP to HAP, OCP is usually observed as the
first crystal in close contact with the surfaces of ACP particles [
117
,
118
]. The
subsequent transformation from OCP to H-Ap appears to be in situ, whereby the
OCP undergoes a solid-state rearrangement into an H-Ap structure [
45
]. The first
crystalline phase detected usually gives an apatitic X-ray diffraction (XRD) pattern
with the a-axial dimension larger than that of HAP. This indicates that there is an
OCP-like structure in the crystal [
119
]. For the transformation from ACP to H-Ap,
review is available [
105
].
