Biomedical Engineering Reference
In-Depth Information
known about their presence in CDHA. It is considered that a portion
of PO
4
3−
4
2−
ions is either protonated (as HPO
) or substituted by
other ions (e.g., CO
) [335]. Theoretical investigations of the defect
formation mechanism relevant to non-stoichiometry in CDHA are
available elsewhere [336].
unsubstituted CDHA (i.e., that containing ions of Ca
3
2−
2+
4
3−
, PO
,
4
2−
−
HPO
only) does not exist in biological systems. However,
the ion substituted CDHA: Na
and OH
+
, K
+
, Mg
2+
, Sr
2+
for Ca
2+
; CO
3
2−
for PO
4
3−
4
2−
−
−
3
2−
−
, plus some water forms biological
apatite — the main inorganic part of animal and human normal and
pathological calcifications [27, 82, 83]. Therefore, CDHA is a very
promising compound for industrial manufacturing of artificial bone
substitutes [337], including drug delivery applications [338]. Non-
biomedical applications of CDHA are similar to those of ACP and HA.
Interestingly, but CDHA was found to possess a catalytic activity to
produce biogasoline [339].
or HPO
; F
, Cl
, CO
for OH
1.3.10
HA (or HAp, or OHAp)
Hydroxyapatite [340] (Ca
(PO
)
(OH), but is usually written as
5
4
3
Ca
to denote that the crystal unit cell comprises two
molecules; the chemically correct name is pentacalcium hydroxide
tris(phosphate)) is the second most stable and least soluble calcium
orthophosphate after FA. Chemically pure HA crystallizes in the
monoclinic space group
(PO
)
(OH)
10
4
6
2
/b [341]. However, at temperatures
above ~250°C, there is a monoclinic to hexagonal phase transition
in HA (space group
P
2
1
/m) [28, 115, 277, 342, 343]. The detailed
description of the HA structure was first reported in 1964 [344] and
its interpretation in terms of aggregation of Ca
P
6
3
clusters, the
so-called Posner's clusters, has been widely used since publication
of the article by Posner and Betts [284]. The Ca
(PO
)
9
4
6
clusters
appeared to be energetically favored in comparison to alternative
candidates including Ca
(PO
)
9
4
6
clusters [345]. In
hexagonal HA, the hydroxide ions are more disordered within each
row, when compared with the monoclinic form, pointing either
upward or downward in the structure. This induces strains that are
compensated for by substitutions or ion vacancies. Some impurities,
like partial substitution of hydroxide by fluoride or chloride, stabilize
the hexagonal structure of HA at ambient temperature. Due to this
reason, hexagonal HA is seldom the stoichiometric phase and very
(PO
)
and Ca
(PO
)
3
4
2
6
4
4
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