Biomedical Engineering Reference
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1075 cm
−1
; and u
4
: 575, 589, and 608 cm
−1
). The main u
1
peak is also observed to have nar-
rowed and shifted from center of ~950 to ~960 cm
−1
, which is assigned to a typical feature
of crystalline HA (Wen et al. 2000; Cuscó et al. 1998; Silva et al. 2002; Penel et al. 1997). The
results from the Raman technique tallies with the postspray heat-treatment studies (Li et
al. 2002a), as it was found that the heat treatment bought about full crystallization from
ACP to HA. Likewise, it is proposed that for small HVOF splats, HA powders were near
fully melted and decomposed during deposition. There is a uniform distribution of ACP,
α-TCP, β-TCP, CaCO
3
, and very small amount of unmelted HA. Overall, the Raman spectra
of HVOF splats suggested that the HA powders are only partially decomposed, which
must be attributed to their partially melted state during deposition. The highest degree of
crystallinity exists at the center of the big splat, with HA and α-TCP being the two main
phases. A small HVOF HA splat has a uniform spread of phases, with higher content of
α-TCP and CaCO
3
and overall lower crystallinity, and TEM analysis has confirmed the
presence of α- and β-TCP at the fringes of the splat (Figure 4.19) (Li et al. 2004c). The pres-
ent Raman results correspond very well with the other findings on the influence of HA
particle size on phase composition of resultant HVOF coating (Li et al. 2000).
The in vitro dissolution rates (Figure 4.20) together with the microstructural changes of
the splats (Figure 4.21) confirm the possible phases within different locations revealed by
the Raman detection (Li et al. 2004c). It was revealed that surrounding parts of the splats
preferably dissolved into the SBF. It has been determined that the dissolution rate of differ-
ent CP phases in the SBF is in the order HA < CDHA < OHA < β-TCP < α-TCP < TTCP <
ACP (Ducheyne et al. 1993). The dissolution results of the splats fit very well with their
Raman spectra at different zones. It is noted that even under full melt state of the sprayed
HA powders, plasma spray (45-75 μm) and HVOF (20-45 μm) showed different effects on
the dissolution behaviors of the splats (Li et al. 2004c). The results revealed that under the
same full melt state, the dissolution rates of the resultant splats are different depending
on the phases present within the splats. On the other hand, the obvious dissolution of the
splats has indicated their remarkable biocompatibility. For the plasma-sprayed HA splats,
2 h resulted in full dissolution of the phases, while for HVOF-sprayed HA splats (fully
melted), 4 h resulted in full dissolution of the phases (Li et al. 2004c). For the partially
melted HVOF splats, a precipitation appeared after 24 h of incubation, and the dissolution
reached a proximately stable state after 14 h, which indicates extremely low dissolvability
0.5 µm
FIGURE 4.19
TEM microstructure of a single HA splat at its fringe showing presence of (a) α-TCP indicated by the selected
diffraction pattern from its (001) zone axis, and (b) β-TCP indicated by the selected diffraction pattern from its
(
111
) zone axis. (From Li, H., et al.
Biomaterials
, 25, 3463-3471, 2004a. With permission.)
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