Biomedical Engineering Reference
In-Depth Information
rediscovered again. Such type of α-TCP is called “silicon stabilized
α-TCP” [252-257].
Although α-TCP and β-TCP have exactly the same chemical
composition, they differ by the crystal structure (Table 1.3) and
solubility (Table 1.1). In addition, β-TCP is more stable than the
α-phase [217]. Therefore, of them, α-TCP is more reactive in aqueous
systems, has a higher specific energy and it can be hydrolyzed to CDHA
[258-260]. milling was found to increase the α-TCP reactivity even
more [261]. Although, α-TCP never occurs in biological calcifications,
in medicine, it is used as a component of calcium orthophosphate
cements [131, 148-151, 166-168, 262-265]. On the other hand,
the chemically pure α-TCP has received not much interest in the
biomedical field [244]. The disadvantage for using α-TCP is its quick
resorption rate, which limits its application in this area. However,
the silicon stabilized α-TCP (more precisely as a biphasic composite
with HA) has been commercialized as a starting material to produce
bioresorbable porous ceramic scaffolds to be used as artificial
bone grafts [239, 252-256]. Theoretical insights into bone grafting
properties of the silicon-stabilized α-TCP might be found in ref.
[266]. The structure of α-TCP is well described in literature [217,
218, 267], while the surface and adsorption properties are available
in ref. [268]. Similar to β-TCP, α-TCP of a technical grade might be
used slow release fertilizer for acidic soils [134].
1.3.8
ACP
Amorphous calcium phosphates (ACPs) represent a special class of
calcium orthophosphate salts, having variable chemical but rather
identical glass-like physical properties, in which there are neither
translational nor orientational long-range orders (LRO) of the
atomic positions. until recently [269], ACP has been considered as an
individual chemical compound; however, this is just an amorphous
state of other calcium orthophosphates. Therefore, in principle, all
compounds mentioned in Table 1.1 might be somehow fabricated
in an amorphous state but, currently, only few of them (e.g., an
amorphous TCP) are known [269]. Thus, strictly speaking, ACP
should be excluded from Table 1.1.
Depending on the production temperatures, ACPs are divided
into two major groups: low-temperature ACPs (prepared in aqueous
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