Biomedical Engineering Reference
In-Depth Information
Interestingly, but collagen, which is the principal matrix protein
in skeletal tissues, has no effect on ACP stability [239, 244], while
presence of gelatin promotes transformation of ACP into crystalline
phases [49]. One should note, that in wet atmosphere solid ACP with
Ca/P ratio of 1.33 (“amorphous OCP”) was found to re-crystallize
into a mixture of CDHA + DCPA [142].
Conversion of ACPs to solid phases with X-ray diffraction
characteristics of CDHA has been studied largely in aqueous
suspensions [67, 68, 72, 116, 119, 121, 130, 132, 136, 154, 185,
190, 196, 218, 248-253]. Transmission electron micrographs of this
transformation are shown in Fig. 2.3. Kinetics of this process can be
described by an empirical equation: dC/dt = k
C, where C is the
fraction of ACP converted into the crystalline phase by time t, k
+ k
1
2
is
a rate constant associated with the nucleation of the first crystals
and k
1
is an autocatalytic rate constant indicative of the observation
that the transition rate is proportional to the mass fraction already
crystallized and not to the fraction of remaining ACP [134, 196].
Numerical values for k
2
appeared to be much larger than those for
2
k
, reflective of the exponential rapidity of the transition once started
[196].
The amorphous-to-crystalline transformation mechanisms
of ACPs have not been well elucidated. Chemically, this process is
described by Eqs. (2.1) and (2.2); while in the reality it might proceed
along several pathways. Namely, it might occur as dissolution of ACP
and re-precipitation of crystalline phases (e.g., CDHA — see Fig. 2.12
[72]) [20, 72, 118, 121, 128, 130, 154, 190, 248], internal structure
rearrangements [136, 190, 218, 229, 254, 255], development of a
LRO without changing the immediate environment of Ca [116],
formation of the crystalline phases either directly within the ACP
phases [149, 236] or on the surface of ACP particles [253], as well as
by self-aggregation and surface-mediated transformations [31]. For
example: “… When the density reached a critical value, the random
arrangement of growth units became disadvantageous in terms of
total free energy, resulting in a sudden regularization of the structure,
which was deposited as HA.” (Ref. [229], page 241). Furthermore, Yin
and Stott suggested that, in the transformation from ACP to CDHA,
ACP needed only to dissociate into clusters rather than undergo
complete ionic salvation [256].
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