Biomedical Engineering Reference
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surface state can help understanding and explaining the behavior
of biological apatites in participating in homeostasis due to a very
high specific surface area of bone crystals and in constituting an
important ion reservoir with an availability that depends on the
maturation state. The important consequences are that the surface
of nanodimensional apatites has nothing in common with the bulk
composition and that the chemistry of such materials (e.g
binding
of protein molecules) must be reconsidered [203, 205]. Interestingly,
but, in response to an electrical potential, the surface of nano-sized
HA bioceramics was found to exhibit dynamic changes in interfacial
properties, such as wettability. The wettability modification enabled
both a sharp switching from hydrophilic to hydrophobic states and
a microscopic wettability patterning of the HA surface, which may
be used for fabrication of spatially arrayed HA for biological cells
immobilization or gene transfer [220].
.
,
Figure 3.3
Variation of nanocrystalline apatite dimensions with
maturation time. Reprinted from Ref. [203] with permission.
Furthermore, dry powders of nanodimensional HA were found
to contain an X-ray amorphous portion with an unspecified location
[221]. After mixing of an initial nano-sized HA powder with a
physiological solution (aqueous isotonic 0.9% NaCl solution for
injections), this amorphous portion was fully converted into the
crystalline phase of HA. The initial crystallite average size (~35
nm) was enlarged by a factor of about four within the first 100 min
after mixing the powder with the physiological solution and no
more structural changes were detected during the following period
[221]. In the light of the aforementioned studies, presumably, the
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