Biomedical Engineering Reference
In-Depth Information
biological apatite. This can be explained by the following paramount
differences between the
in vivo
biological and
in vitro
chemical
crystallization conditions [860]:
crystallization normally occurs at permanently
depleting concentrations of calcium and orthophosphate ions,
while the concentrations of all ions and molecules are kept
strictly constant during biological mineralization (the same is
valid for the solution pH);
(ii) Chemical crystallization is a fast process (time scale of minutes
to days), while the biological process is a slow one (time scale
of weeks to years);
(iii) Many inorganic, bioorganic, biological and polymeric
compounds are present in biological liquids (blood plasma,
serum, saliva). Each of these compounds might act as an
inhibitor, promoter, nucleator or even as a template for the
growth of biological apatite [537]. In addition, each of them
somehow influences the crystallization kinetics and might be
either incorporated into the solid structure or co-precipitated
with calcium orthophosphates.
(iv) Chemical crystallization is, by all means, a “passive” process,
while the biological mineralization is strongly influenced by
cells and occurs by the self-organization mechanisms [578,
603, 604]. Still there are no good ways to overcome this
difference.
The first and the second differences might be overcome by using
the appropriate crystallization techniques. The details are available
elsewhere [860] but, briefly, the first problem might be overcome by
either a continuous flow of a supersaturated solution [861, 862] or
using a constant-composition (CC) technique [198, 863, 864]. The
second difference might be surpassed by a restrained diffusion of
calcium and orthophosphate ions from the opposite directions in, for
example, a double-diffusion (DD) crystallization device or in viscous
gels [436-438, 440, 441, 865-868]. The CC and DD techniques have
been combined into a single constant-composition double-diffusion
(CCDD) device, which currently seems to be the most advanced
experimental tool to perform biomimetic crystallization [860, 869-
873]. However, in no case the CCDD device should be considered as
the final construction; it still has much room for further improvement,
e.g.,
(i)
In vitro
by upgrading the design of the crystallization chamber [874].
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