Biomedical Engineering Reference
In-Depth Information
mineral precursors and a universal strategy for calcium carbonate-
based and calcium orthophosphate-based biomineralization in
both vertebrates and invertebrates [21-29]. See Section 1.4.1 for
additional details on this topic.
An interesting study on a potential role of ACP in facilitating
assembly of nano-sized particles of HA into highly ordered structures
has been published recently [30]. Higher order HA architectures
were detected only when the starting particles were aggregates of
nanodimensional spheres with HA cores and ACP shells. Surface
ACP initially linked HA nano-sized particles in a way that allowed a
parallel orientation of the nano-sized particles of HA and then was
incorporated into HA by phase transformation to produce more
ordered architectures with the characteristic features of apatites in
biologic structures. Further, it was demonstrated that enamel- and
bone-like apatites could be prepared by using nanodimensional HA
and ACP under the controls of different biological additives [30].
This study points to an important role of ACP might play in bone
formation. However, due to a lack of undeniable proofs, the question
on the occurrence of ACP phases in newly mineralized tissues of
vertebrates still remains unanswered [31].
The complete list of the available calcium orthophosphates
combined with their standard abbreviations, chemical formulae,
and major properties is given in Table 1.1 [32, 33]. As the majority
of calcium orthophosphates are crystalline, this chapter is devoted
to the detailed description of ACP, which is of a great biomedical
importance due to its chemical and structural similarities to the
calcified tissues of mammals. Furthermore, with a few important
exceptions, neither ion-substituted forms of ACP [34-47], nor ACP-
containing biocomposites [48-75] are considered and discussed.
The readers interested in either these topics and/or other types
of amorphous calcium phosphates (e.g., amorphous calcium
polyphosphate [76-80] and amorphous calcium metaphosphate
[81]) are referred to the original publications.
2.2
Basic Definitions and Knowledge on the
Amorphous State of Solids
According to the thermodynamic laws, the perfect infinite crystals
cannot exist in the real world. Various disorders in the forms
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