Biomedical Engineering Reference
In-Depth Information
the nanometer grain size and surface wettability. On top of this consider-
able impact on osteoblast function, the nanometric dispersion has also been
shown to induce absorption of specific proteins important for osteoblast
adhesion. 7-10
nHA can be used in two configurations, as a coating material or mixed in
a composite material, and both approaches improve the osteoconductivity of
implant materials. When used as a coating material, nHA can be applied onto
implant materials by electrostatic spray deposition (ESD) of a nHA precursor
solution, direct precipitation using a biomimetic depositing technique, or an
electrocrystallization technique from dilute electrolytes. 6,11-14
4.1.1 Preparation of Nanohydroxyapatite
Several techniques for preparing HA nanocrystals are known, all based on
precipitation reactions. In the following sections, all techniques for the pro-
duction of nHA powders as well as nHA coatings are described.
One of the methods to create nHA powders is wet chemical deposition. For
this method, the characteristics of the nHA created depend on the reaction
formula, temperature, time, and addition rate of the reactant. The most com-
monly used reaction formula is the one defined by Bouyer et al. 15
10Ca(OH) 2 + 6H 3 PO 4 → Ca 10 (PO 4 ) 6 (OH) 2 + 18H 2 O
The purity can be controlled by the reactant addition rate, while the crys-
tallinity is determined by the reaction temperature, where low temperatures
( T ≤ 60°C) result in monocrystallinity and high temperatures ( T > 60°C)
result in polycrystallinity. The Ca 2+ ion concentration was found to play a
dominant role on the final morphology of the synthesized nHA, as described
by Kumar et al. 16 Another wet chemical deposition formula is based on the
reaction between calcium nitrate (Ca(NO 3 ) 2 × 4H 2 O) and ammonium hydrox-
ide (NH 4 HPO 4 ). 5 Here, the nHA grain size can be controlled by changing the
reaction time and temperature. 8,9 Further, Janackovic et al. 17 reported on a
wet chemical precipitation reaction according to the following formula:
2
2
Ca EDTA
(
)
+
35
/HPO
+
25
/HO/Ca
110
(
PO
) (
OH
)
4
2
10
46
2
3
+
HEDTA/OH
+
15
Urea is added instead of NaOH, which leads to a more homogeneous mon-
etite precipitation, followed by transformation to HA due to pH changes.
Crystal growth can be modified by changing the reaction temperature
between 125°C and 160°C, changing the reaction time, or changing the
precursor concentration. A final method to produce nHA by wet chemical
deposition is by mixing H 3 PO 4 with Ca(OH) 2 and C 3 H 6 O 3 at pH 10 under
continuous stirring at room temperature until a Ca/P ratio of 1.67 is reached.
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