Biomedical Engineering Reference
In-Depth Information
not affect the edge free energy. This implies that both impurities have almost no
effect on the nucleation frequency (at least for the range of concentrations tested).
This conclusion is consistent with the morphology of HAP seen in bone tissue.
If zinc and magnesium do not affect the nucleation frequency of HAP crystals
but rather only suppress the growth of precipitated crystals, likely result is the
formation of numerous nano-sized HAP crystals, as seen in human bone. If zinc and
magnesium had no evident impurity effect for growth, HAP crystals would easily
become enlarged, and the combined strength and flexibility originally possessed by
bone might be lost.
In growth rate measurement experiments in the presence of impurities using the
phase-shift method, investigations have been carried out on adsorption mechanism
using Langmuir and Temkin isotherms as well as on the chemical morphologies of
adsorbed zinc and magnesium [ 71 ]. A Langmuir-type adsorption model for kinks
was determined to be appropriate on the basis of model fitting in which impurities
adsorb onto the kink sites in the step front and onto the terraces between steps.
In contrast to detailed growth rate measurements for the c -face, there are no
reports on the growth rate of the a -face. As mentioned previously, the reason for this
is the lack of reliable data due the fact that the growth rate of the a -face is more than
1 order of magnitude lower than that of the c -face. Observations based on AFM have
been reported for the growth mode of the a -face [ 77 ]. Under similar experimental
conditions as those for the c -face (growth in pseudo body fluid), growth by step
movement occurs on the a -face. Although it has not been directly proven that this
step originates from spiral dislocation, spiral growth probably occurs because the
growth proceeds with the filling of etch pits on the surface. The dependence of the
step velocity on the supersaturation, as well as changes in velocity associated with
changes in the step spacing, has been measured, and the surface diffusion length
was concluded to be only a few nanometers.
3.4.7
Discussion of HAP Growth Unit
In discussing crystal growth mechanisms, the growth unit (that which serves as
the minimal unit of growth) is generally considered to be an atom or molecule.
In this model, growth of soluble inorganic salts is assumed to proceed as a result
of positive or negative ions being incorporated into the crystal lattice. However,
depending on the type and growth environment of the crystals, there are instances in
which the experimental data are better explained by considering the growth unit as
a far larger assembly than an atom or molecule [ 78 - 80 ]. Such a cluster is composed
of aggregates of atoms or molecules and has a defined structure, unlike random
aggregates.
The concept of the cluster itself is very old. For example, even when the term
cluster was not in widespread use, there had been attempts to explain the mosaic
structure displayed by NaCl crystals growing in aqueous solution with a growth
model incorporating units called block nuclei. Biominerals, including HAP, are
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