Biomedical Engineering Reference
In-Depth Information
skeleton is reacted with dominium hydro-
gen phosphate and, by means of a hydrothermal exchange of carbonate and phosphate,
is converted to HA. Moreover, the calcium carbonate can also be transformed into HA in
the presence of the appropriate amounts of the dicalcium phosphate anhydrous (DCPA,
CaHPO
4
), as shown in reactions 6.11 and 6.12:
materials [155]. The calcium carbonate (CaCO
3
) skeleton is reacted with dominium hydro-
10CaCO
3
+ 6(NH
4
)
2
HPO
4
+ 2H
2
O → Ca
10
(PO
4
)
6
(OH)
2
+ 6(NH
2
)CO
3
+ 4H
2
CO
3
(6.11)
4CaCO
3
+ 6CaHPO
4
→ Ca
10
(PO
4
)
6
(OH)
2
+ 6H
2
O + 4CO
2
(6.12)
Under stable temperature and pressure conditions, the exchange results in a nearly pure
HA. The HA structure is an exact replicate of the porous marine skeleton. Ito et al. [156,157]
have done the study on the single crystal growth of HA. Single crystals of carbonate-
containing HA have been grown hydrothermally by gradually heating with a temperature
gradient applied to the pressure vessel, using the DCPA. Reaction 6.13 also shows this
hydrothermal reaction for the formation of HA single crystals.
10CaHPO
4
+ 2H
2
O → Ca
10
(PO
4
)
6
(OH)
2
+ 4H
3
PO
4
(6.13)
In addition, TCP and TP can be easily converted to HA under the hydrothermal conditions
[59], and the chemical reactions as shown in 6.14 and 6.15 can be carried out hydrother-
can be carried out hydrother-
mally at 275°C, under a steam pressure of about 82.76 MPa [158,159].
and the chemical reactions as shown in 6.14 and 6.15 can be carried out hydrother-
he chemical reactions as shown in 6.14 and 6.15 can be carried out hydrother-
6.14 and 6.15 can be carried out hydrother-
.14 and 6.15 can be carried out hydrother-
14 and 6.15 can be carried out hydrother-
6.15 can be carried out hydrother-
.15 can be carried out hydrother-
15 can be carried out hydrother-
3Ca
4
P
2
O
9
+ 3H
2
O → Ca
10
(PO
4
)
6
(OH)
2
+ 2Ca(OH)
2
(6.14)
3Ca
3
(PO
4
)
2
+ Ca(OH)
2
→ Ca
10
(PO
4
)
6
(OH)
2
+ H
2
O
(6.15)
Considering the biomedical applications, plasma-sprayed HACs deposited on metallic
implants, which is an extensively used process in commercial products, tend to avoid the
inherent mechanical property limitations of HA without any significant loss in biocom-
patibility. It has been generally recognized that the crystallization of hydroxyl-deicient
plasma-sprayed HACs requires at least 600°C [45,64,116] for postheat treatments in vacuum
or in the air. But high heating temperatures tend to cause phase decomposition (Figures
6.7 and 6.8) and undermined the microstructural integrity (Figures 6.11 and 6.12) of crys-
talline HA. Research results have indicated that HACs with a higher impurity content
of TCP, TP, CaO, OHA, and ACP will result in a higher dissolution rate than crystalline
HA in aqueous solutions or in human body fluids. The higher dissolution rate will lead
to a decrease in microstructural homogeneity, poorer mechanical properties, and lack of
coating adherence, which will undermine the long-term fixation between the implants
and the surrounding bone tissue. To solve these problems, it has been reported that sur-
To solve these problems, it has been reported that sur-
face modifications for forming bonelike apatite can induce high bioactivity of bioinert
metals in simulated body fluid [160]. Since HA is a stable phase under a partial steam
pressure of 500 mmHg [70], therefore a hydrothermal crystallization process conducted
with a water vapor atmosphere has been developed to minimize impurity phases and
promote HA crystallization at relatively lower temperatures. Performing a hydrothermal
treatment on plasma-sprayed HACs is an effective method to promote HA crystallization,
increase the crystallinity, and significantly improve the extent of new bone apposition in
vivo [59,63,121].
To solve these problems, it has been reported that sur-
Search WWH ::
Custom Search