Biomedical Engineering Reference
In-Depth Information
chemically correct name is pentacalcium fluoride tris(phosphate))
is the only ion-substituted calcium orthophosphate, considered
in this review. It is the hardest (5 according to the Mohs' scale of
mineral hardness), most stable, and least soluble compound among
all calcium orthophosphates (Table 1.1). Perhaps, such “extreme”
properties of FA are related to the specific position of F
−
ions in
the center of Ca(2) triangles of the crystal structure [115]. Due to
its properties, FA is the only calcium orthophosphate that naturally
forms large deposits suitable for the commercial use [35-38] (see
also Fig. 1.2). Preparation techniques of the chemically pure FA are
similar to the aforementioned ones for HA but the synthesis must be
performed in presence of the necessary amount of F
−
ions (usually,
F is added). unlike that for HA (see CDHA), no data are
available on existence of calcium-deficient FA. under some special
crystallization conditions (e.g., in presence of gelatin or citric acid),
FA might form unusual dumbbell-like fractal morphology that finally
are closed to spheres (Fig. 1.8) [436-442]. A hierarchical structure
for FA was proposed [443]. The crystal structure of FA for the first
time was studied in 1930 [444, 445] and is well described elsewhere
[28, 114-116, 446]. The detailed analysis of the electronic structure,
bonding, charge transfer and optical properties is available as well
[348]. In addition, there are reviews on FA solubility [397] and the
dissolution mechanism (ref. [398] and Chapter 7 of this topic).
NaF or NH
4
Figure 1.8
A biomimetically grown aggregate of FA that was crystallized
in a gelatin matrix. Its shape can be explained and simulated by
a fractal growth mechanism. Scale bar: 10 μm. reprinted from
Ref. [437] with permission.
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