Biomedical Engineering Reference
In-Depth Information
(a)
20 kV
×
7,500
1
µ
m
000019
(b)
20 kV
×
7,500
1
µ
m 000031
Figure 2.2.
Effect of incorporation of fl uoride (F
−
) ions on crystal size of apatite: causes larger
and thicker apatite crystals [59,64].
biologic apatite causes growth of larger and thicker apatite crystals as shown in
Figure 2.2 [59,64,79] and are less soluble than the F-free apatites [45,59,64,100].
Full or partial Cl-for-OH substitution in the apatite structure depends on the
method of preparation. Partial Cl-for-OH substitution, Ca
10
(PO
4
)
6
(OH,Cl)
2
, is
obtained from aqueous systems at low temperatures (37 ° C to 95 ° C [58,64,109]
while full substitution, Ca
10
(PO
4
)
6
Cl
2
, can only be obtained from non-aqueous
systems at temperatures 1000 °C or above [22,64]. The forming apatite from
solution highly discriminates against the incorporation of the Cl ions. This
explains why biologic apatite contains very low Cl concentration in spite of the
high Cl concentration in the biologic fl uid.
Thermal stability of apatites decreases in the order: FA
>
H A
>
ClA [124]
while solubility decreases in the following order: ClA
>
HA
>
FA [52] .
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