Biomedical Engineering Reference
In-Depth Information
All these elements favor a model in which nano-sized crystals
of apatites are covered with a rather fragile but structured surface
hydrated layer containing relatively mobile ions (mainly, bivalent
anions and cations: Ca
2+
4
2-
3
2−
) in “non-apatitic” sites
(Fig. 3.2), which is supposed to be of either OCP or DCPD structure.
Unfortunately, both the exact structure and the chemical composition
of this hydrated layer are still uncertain (regrettably, as the
hydrated layer cannot be isolated, it is not possible to standardize
the methods for detailed studies) [203, 205-207]. Nevertheless, it
is known that the surface layer might adsorb considerable amounts
of foreign compounds (molecules and ions) in the percent mass
range [215]. Strictly speaking, all the aforementioned apply to
both biological apatite of calcified tissues [216] and micron-sized
apatites as well [217]; nonetheless, in nano-sized crystals, the
composition of the hydrated surface layer contributes to the global
composition for a non-negligible proportion. The results of electron
states spectroscopy of nanostructural HA bioceramics are available
elsewhere [218, 219].
, HPO
, CO
Figure 3.2
A schematic representation of the “surface hydrated layer
model” for poorly crystalline apatite nanocrystals. Reprinted
from Ref. [203] with permission.
The hydrated surface layer confers unexpected properties
to nano-sized apatite, is responsible for most of the properties
of apatites, and, for example, can help to explain the regulation
by biological apatites of the concentration in mineral ions in
body fluids (homeostasis). These properties are important for
living organisms; therefore, they need to be used in both material
science and biotechnology [202]. The consideration of this type of
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