Biomedical Engineering Reference
In-Depth Information
In addition, the crystals of biological apatite are always very
small which also increases its solubility when compared with that
for the chemically pure HA and even CDHA [82]. However, biologic
apatites of enamel have considerably larger crystal size (about 2000
nm) compared to that of either bone or dentine apatite, as indicated
by the well-defined diffraction peaks in the X-ray diffraction profile
of enamel apatite and much broader diffraction peaks of either
bone or dentine apatites (Fig. 1.9, center). Small dimensions and
a low crystallinity are two distinct features of biological apatites,
which, combined with their non-stoichiometric composition, inner
crystalline disorder and presence of other ions in the crystal lattice,
allow explaining their special behavior. For example, the small
crystal size means that a large percentage of the atoms are on the
surface of the crystals, providing a large specific surface area for
sorption of ions, proteins and drugs [536, 537]. The major physical
properties of biological apatite are summarized in Fig. 1.9. It is
interesting to note, that the solubility and equilibrium phenomena
of calcium orthophosphates related to the calcification process have
been studied, at least, since 1925 [538, 539].
Figure 1.9
left: crystal structure of a biological apatite. Powder X-ray
diffraction patterns (center) and infrared spectra (right) of
human enamel, dentine, and bone. Reprinted from Ref. [537]
with permission.
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