Biomedical Engineering Reference
In-Depth Information
FA easily forms solid solutions with HA with any desired
F/OH molar ratio. Such compounds are called fluorhydroxyapatites
(FHA) or hydroxyfluorapatites (HFA) and described with a chemical
formula Ca
< 2. If the F/OH ratio is
either uncertain or not important, the chemical formula of FHA and
HFA is often written as Ca
(PO
)
(OH)
F
, where 0 <
x
10
4
6
2−
x
x
. The lattice parameters,
crystal structure, solubility and other properties of FHA and HFA lay
in between of those for the chemically pure FA and HA [447-451].
Similar to pure HA, pure FA never occurs in biological systems.
Obviously, a lack of the necessary amount of toxic fluorides (the
acute toxic dose of fluoride is ~5 mg/kg of body weight) in living
organisms is the main reason of this fact (pure FA contains 3.7%
mass. F). Enameloid of shark teeth [107, 452-457] and some
exoskeletons of mollusks [458] seem to be the only exclusions
because they contain substantial amounts of FA. Among all normal
calcified tissues of humans, the highest concentration of fluorides is
found in bones and the lowest — in dental enamel [459]. However,
even in bones, the total amount of fluorides is not enough to form FA;
it is generally considered that the inorganic part of bones consists of
ion-substituted CDHA. Due to the lowest solubility, good chemical
stability and toxicity of high amounts of fluorides, chemically pure
FA is rarely used as a bone substituting material [466]. However,
various FA-containing composites [467-469], FHA [470, 471] and
porous FA bioceramics [472] seem to be better candidates for
biomedical applications. Furthermore, due to the ability to form
FHA and/or HFA, minor amounts of fluorides might be intentionally
added to calcium orthophosphate biomaterials [473-479]. The
effect of fluoride contents in FHA on both osteoblast behavior [480,
481] and leukemia cells proliferation [482] has been described. Non-
biomedical applications of FA include its application as a catalyst
[483].
(PO
)
(F,OH)
10
4
6
2
1.3.12
OA (or OAp, or OXA)
Oxyapatite (Ca
O; the chemically correct name is
decacalcium oxide hexakis(phosphate)) is the least studied calcium
orthophosphate. To the best of my knowledge, pure OA has never
been prepared; therefore, its properties are not well established.
Furthermore, still there are doubts that OA exists. However, a
mixture of OA and HA (oxy-HA) might be prepared by dehydration
(PO
)
10
4
6
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