Biomedical Engineering Reference
In-Depth Information
processes proceed in an aggregation-based manner and result in the construction of
characteristic fine structures [
88
]. Gebauer et al., using methods similar to those
mentioned above, conducted measurements on calcium phosphate solutions and
have confirmed the presence of clusters in this case as well. Given the various lines
of experimental evidence presented in recent years, it appears almost certain that
clusters are involved in some way in the growth of insoluble biominerals.
3.5
Future Perspectives
The crystal growth of HAP does not qualitatively differ from that of either soluble
inorganic crystals or organic crystals. In other words, spiral growth mediated by
dislocation having spiral components or growth by two-dimensional nucleation
occurs. This is supported by surface observations of growing crystals. Regarding
the rate-limiting process of growth, although incorporation of a growth unit into the
crystal requires the highest energy, whether or not this incorporation proceeds
through surface diffusion is unclear. The diffusion length is probably minimal. The
important point is not in the growth mode but that classical growth models—i.e.,
models in which growth proceeds as a result of ionic bonds—may not be applicable.
This is a problem that is not limited to HAP but also applies to biominerals in
general. In the aforementioned cases where growth occurred with cluster units, how
the driving force of growth is defined is very important. Depending on the definition,
the values for the various physical constants involved in growth, which have been
reported in studies to date, may change substantially. Furthermore, it is likely that
the problem of how to quantify the intermolecular interactions between clusters
and then relate them to growth features will become an issue as well. As mentioned
above, the crystallization process for proteins is qualitatively the same as for soluble
salts but the growth features are complex because intermolecular interactions are
included as a growth factor. With intermolecular interactions, the magnitude of
the Hamaker constant (a constant indicative of the van der Waals attraction force)
of a molecule has a powerful effect. In biomineral growth as well, it will likely
be necessary to conduct tests on the Hamaker constant for each cluster in order
to understand the growth mechanisms completely and to clarify the relationship
between the final product and the growth unit.
References
1. Newman, W.F., Newman, M.W.: The chemical dynamics of bone mineral. University Chicago
Press, Chicago (1958)
2. Aoki, H.: Kyoi no Seitai Busshitsu Apatite (in Japanese). Ishiyaku Publishers Inc., Tokyo
(1999)
3. Elliot, J.C.: Structure and chemistry of the apatites and other calcium orthophosphates.
Elsevier, Amsterdam (1994)
