Biomedical Engineering Reference
In-Depth Information
soluble than CHA because of the stabilizing effect of F
ions but more soluble than
FA because of the de-stabilizing effect of CO 2− in CFA [73]. In addition the
crystallite size of CFA (from precipitation or hydrolysis preparation) is larger
than that of CHA but smaller than that of FA since CO 3 incorporation causes
the growth of smaller apatite crystals while F incorporation causes the growth of
larger crystals.
Indirect methods of preparing CFA are by suspension of CHA in F-
containing solutions and heating the F - treated CHA at 700 ° C [114] ; treatment
of bovine bone in F-solutions and heating at temperatures 600 °C or above
[28], hydrolysis of calcium carbonate, CaCO 3 . (calcite or aragonite forms) in F-
containing solutions, hydrolysis of CaF 2 in CO 3 - containing solutions, hydrolysis
of non-apatitic calcium phosphates (e.g., DCPD, DCPA, OCP,
- TCP,
or calcium monophosphate, CaH 4 (PO 4 ) 2 .H 2 O) in solutions containing both F
and CO 3 ions [64,78]. In these cases, the effi cacy of conversion of the calcium
phosphate, calcium carbonate or calcium fl uoride depends on reaction pH and
temperature [78] .
α
- TCP,
β
2.3.2.4 Incorporation of Cations Substituting for Calcium Ions. Sev-
eral cations can substitute for the calcium (Ca) ions in the apatite structure,
Ca 10 (PO 4 ) 6 (OH) 2 . For example, strontium (Sr), barium (Ba) and lead (Pb) can
completely substitute for the Ca 2+ ions to yield Sr 10 (PO 4 ) 6 (OH) 2 , Ba 10 (PO 4 ) 6 (OH) 2
or Pb 10 (PO 4 ) 6 (OH) 2 , respectively [27,57,64,80,91]. Because Sr 2+ , Ba 2+ and Pb 2+
have larger ionic radii compared to Ca 2+ , such substitutions result in expanded a -
and c -axes compared to unsubstituted calcium hydroxyapatite (Table 2.2). Colors
(purple, blue, green, pink, brown, etc.) of mineral or synthetic apatites are caused
by incorporation of small amounts of some cations such as manganese (Mn 2+ ),
copper (Cu + , Cu 2+ ), cobalt iron (Fe, Fe) [91,125]. Limited incorporation of
sodium (Na + ) ions Na-for Ca can be obtained when precipitation or hydrolysis or
hydrothermal reactions are performed in the presence of Na-containing solution
[26,91,125]. Because of the similarity in ionic radius between Na + and Ca 2+ , no
signifi cant effect on lattice parameters are observed.
Mg, a minor but important constituent in biologic apatites, is incorporated
only to a very limited extent in synthetic apatites obtained by precipitation or
hydrolysis methods [57,59,60,64,77,89]. Limited amount of zinc (Zn) can be incor-
porated in the apatite [64,76].
Incorporation of Sr 2+ or Mg 2+ in apatite increases its solubility [64,80,89].
Simultaneous incorporation of Mg and CO 3 in apatite has additive effects in
decreasing crystal size and increasing the extent of dissolution in acidic buffer
[89,103] .
2.3.2.5 Incorporation of Anions Substituting for the Phosphate
Ions. Besides the CO 3 - for - PO 4 substitution discussed above, other anions can
substitute for the PO 4 ions in the apatite structure. These anions include sulfate,
manganate, borate, silicate, etc. [31,64,91]. Like any other substitutions in the
apatite structure, these substitutions will affect the properties of the apatite.
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