Biomedical Engineering Reference
In-Depth Information
bulk HA without remarkable loss in biocompatibility (Oonishi et al. 1989; Hardy et al.
1999). It was found that the macro- and microstructure of HA was particularly impor-
tant for the apposition of bone (Hing et al. 1999). The biological performance of the cal-
cium phosphate coatings was essentially phase-dependent, and biological behavior of
the phases in the calcium phosphate family has been largely elucidated (Yang et al. 1997;
Cleries et al. 2000). It is therefore clear that optimization of the phase composition of as-
sprayed calcium phosphate coatings is a prerequisite toward their competitive biomedical
applications.
Due to the obvious influence of the starting powder on coating performances, a favor-
able method for HA powder preparation was actively sought. The spheroidization pro-
cess by using plasma spray was found as one sound method for HA powder preparation
(Liu et al. 1994). This process has proven to be the most favorable method to prepare
HA feedstock for plasma spraying due to the limited phase changes (due to the short-
term stay in the torch) and good flowability of the powders (Cheang et al. 1994; Wang
et al. 1999; Oonishi et al. 1987). Different starting HA powder, calcined HA, spray-dried
HA, and spheroidized HA (Cheang et al. 1996b) resulted in remarkably different phase
composition and microstructure of the coatings. Nearly all the studies involved in the
influence of starting HA feedstock revealed that the shape, geometry, and structure of
HA powders exhibited significant effects on coating characteristics (Wang et al. 1999; Vu
et al. 1996) apart from the spray parameters (Yang et al. 1995; Wang et al. 1993a). Various
calcium phosphates with their respective Ca/P ratios in the HA material system are listed
in Table 4.2.
Basically, there are two approaches for fabricating nanostructures in thermal sprayed
coatings, retaining nanostructures from starting nanophase/nanostructured feedstock
(Gel et al. 2001; Bansal et al. 2003; Lima et al. 2001), and spray-forming nanostructures
owing to the rapid cooling of the droplets upon their impingement (Tjong et al. 2004; Gang
et al. 2003). The use of nanostructured feedstock is a promising way for deposition of
nanostructured coatings (Li et al. SCT, 2006). Generally, the spray powder can be fabri-
cated through agglomerating nanosized particles to form big ones (Tjong et al. 2004; Gel
TABLE 4.2
Various Calcium Phosphates with Their Respective Ca/P Atomic Ratios
Ca/P
Formula
Name
Abbreviation
2.0
Ca
4
O(PO
4
)
2
Tetracalcium phosphate
(Hilgenstockite)
TCPM
(TTCP)
1.67
Ca
10
(PO
4
)
6
(OH)
2
Hydroxyapatite
HA
Ca
10-x
H
2x
(PO
4
)
6
(OH)
2
Amorphous calcium phosphate
ACP
1.50
Ca
3
(PO
4
)
2
Tricalcium phosphate (α, β, γ)
TCP
1.33
Ca
8
H
2
(PO
4
)
6
×5H
2
O
Octacalcium phosphate
OCP
1.0
CaHPO
4
×2H
2
O
Dicalcium phosphate dihydrate (Brushite)
DCPD
1.0
CaHPO
4
Dicalcium phosphate (Monetite)
DCP
1.0
Ca
2
P
2
O
7
Calcium pyrophosphate (α, β, γ)
CPP
1.0
Ca
2
P
2
O
7
×2H
2
O
Calcium pyrophosphate dehydrate
CPPD
0.7
Ca
7
(P
5
O
16
)
2
Heptacalcium phosphate (Tromelite)
HCP
0.67
Ca
4
H
2
P
6
O
20
Tetracalcium dihydrogen phosphate
TDHP
0.5
Ca(H
2
PO
4
)
2
×H
2
O
Monocalcium phosphate monohydrate
MCPM
0.5
Ca(PO
3
)
2
Calcium metaphosphate (α, β, γ)
CMP
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