Biomedical Engineering Reference
In-Depth Information
1.4
Biological Hard Tissues of Calcium
Orthophosphates
Biological mineralization (or biomineralization) is the process of
in
vivo
formation of inorganic minerals (so-called, biominerals). One
should stress, that the term “biomineral” refers not only to a mineral
produced by organisms but also to the fact that almost all of these
mineralized products are composite materials comprised of both
inorganic and bioorganic components. Furthermore, having formed
in vivo
under well-controlled conditions, the biomineral phases
often have properties, such as shape, size, crystallinity, isotopic and
trace element compositions, quite unlike its inorganically formed
counterpart (please, compare Figs. 1.2, 1.8, 1.10 and 1.14). Thus, the
term “biomineral” reflects all this complexity [454, 455].
As shown in Table 1.2 and discussed above, in the body of
mammals the vast majority of both normal and pathological
calcifications consist of non-stoichiometric and ion-substituted
calcium orthophosphates, mainly of apatitic structure [87, 533]. At
the element scale, nano-sized crystals bone apatite exhibit a variety
of substitutions and vacancies that make the Ca/P molar ratio
distinct from the stoichiometric HA ratio of 1.67. Their chemical
composition is complicated and varies in relatively wide ranges.
This depends on what the animal has ingested [534]. Occasionally,
attempts are performed to compose chemical formulas of biological
apatites. For example, the following formula Ca
Mg
Na
8.856
0.088
0.292
K
was
proposed to describe the chemical composition of the inorganic part
of dental enamel [535].
The impurities in biological apatite of bones and teeth introduce
significant stresses into the crystal structure, which make it less stable
and more reactive. Among all substituting ions, the presence of 4-8%
of carbonates instead of orthophosphate anions (so called, B-type
substitution [27-29, 516]) and of 0.5-1.5% of mg is of the special
importance because it leads to large lattice strain and significantly
increases the solubility [533, 535, 536]. Higher concentrations of
magnesium and carbonates in bone or dentine compared to those
in enamel (Table 1.2) may explain a higher solubility and a lower
crystallinity (smaller crystal size) of bone or dentine compared to
enamel.
(PO
)
(HPO
)
(CO
)
(OH)
Cl
(CO
)
0.010
4
5.312
4
0.280
3
0.407
0.702
0.078
3
0.050
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