Biomedical Engineering Reference
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and tissue response of bone remodeling. These examples emphasize
that nanophase materials deserve more attention in improving
orthopedic implant failure rates. However, to reduce surface energy,
all nano-sized materials tend to agglomerate, and to avoid self-
aggregation of calcium orthophosphate nano-sized particles [167-
169], special precautions might be necessary [102, 170-172].
Finally yet importantly, nano-sized crystals of CDHA obtained by
precipitation methods in aqueous solutions were shown to exhibit
physico-chemical characteristics rather similar to those of bone
apatite [173]. In particular, their chemical composition departs from
stoichiometry by calcium and hydroxide ions deficiency, leading to
an increased solubility, and in turn bioresorption rate
[123,
174-176]. The nano-sized crystals of CDHA have also a property to
evolve in solution (maturation) like bone crystals. Namely, freshly
precipitated CDHA has been shown to be analogous to embryonic
bone mineral crystals whereas aged precipitates resemble bone
crystals of old vertebrates [173].
in vivo
3.4
Nanodimensional and Nanocrystalline
Calcium Orthophosphates in Calcified
Tissues of Mammals
3.4.1
Bones
Bone is the most typical calcified tissue of mammals and comes in
all sorts of shapes and sizes in order to achieve various functions
of protection and mechanical support for the body. The major
inorganic component of bone mineral is a biological apatite, which
might be defined as a poorly crystalline, non-stoichiometric, and
ion-substituted CDHA [2-5, 174-177]. From the material point of
view, bone can be considered as an assembly of distinct levels of
seven hierarchical structural units from macro- to micro- and to
nano-scale (Fig. 1.11) to meet numerous functions [2, 5, 109, 178-
180]. Furthermore, all these levels of bones permanently interact
with cells and biological macromolecules. At the nanostructural
level, tiny plate-like crystals of biological apatite in bone occur
within the discrete spaces within the collagen fibrils and grow with
specific crystalline orientation along the
c
-axes, which are roughly
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