Biomedical Engineering Reference
In-Depth Information
around the splats' interface, could act as an important factor contributing to improved
mechanical properties.
It is evident that partially melted state of HA powders (low MFP values) is beneficial
toward obtaining a nanostructured high crystalline HA coating. Furthermore, phase
composition analyses indicate that the melted portion of the particles contained predomi-
nantly ACP and TCP. Therefore, in order to effectively inhibit HA decomposition, limited
melting of the HA powders must be ensured. It was believed that the high levels in crys-
tallinity of HA coatings were beneficial for long-term survivability, and proper functional
life in service with regard to the resolvability of different phases in bony tissues; that is,
ACP has a far higher resolvability in the bony tissues than crystalline HA (Ducheyne et
al. 1993). However, the high resolvability of the amorphous phase is beneficial for acceler-
ated fixation of the implant, and it was also believed that ACP was good for facilitating
mechanical mismatch, improving fatigue behavior, and promoting faster bone remodeling
and hard tissue attachment (Gross et al. 1998b). In consideration of the above points, HA
coatings with a small amorphous content (<15%) would hence be preferred. It should be
noted that, during the present HVOF spray study, the particles that have a large diameter
(i.e., >30 μm) are only partially melted. Since the thermal transformation of HA generally
occurred at the temperatures beyond 1000°C (Aoki 1994), the melted part of HA particle
should contribute strongly to the transformation (relatively low temperature transforma-
tion by loss of water is also involved to a lesser extent). The present study indicates that
during the HVOF spray, the heating of HA powders is rather limited, which further sug-
gests the suitability of HVOF technique for HA coating deposition.
It is well known that a thermal-sprayed coating shows a layered structure, which has an
accumulated character composed of individual splats. In order to better understand the
coatings, researchers have extensively concentrated on the study of thermal sprayed splats
(Bianchi et al. 1997; Montavon et al. 1995; Gougeon et al. 2001). Generally, in most cases, as
the substrate temperature is low (e.g., <500°C), splat formation is an isolated procedure,
which means minor influence of the subsequent splat on the phases of the former one.
Therefore, the overall structure/phases and in vitro behavior of a bulk HA coating can
arguably be intimately related to that of individual HA splats. A good understanding of
a single HA splat would significantly contribute to the knowledge on structure and dis-
solution/precipitation mechanism of HA coatings. Therefore, a study on splats is essen-
tially crucial toward establishing an understanding of individual splats' contribution to
the phase composition of the thermal-sprayed coating and satisfactory control of phase
composition of the coating through elucidation of phases' response within a splat.
In order to characterize the CP phases, which only differentiate slightly in structures,
within a splat, a structure-sensitive and localized technique is required. Raman spectros-
copy technique could provide information on the short- and intermediate-range ordering
in the solids. It allows a direct and nondestructive detection from the sample surface with
spatial resolution (micrometric) 100 times higher than the infrared resolution. Since the
biological performances of the CP deposits, both in vitro and in vivo, are significantly
dependent on their phases (Yang et al. 1997; Cleries et al. 2000), and the local phases play an
extremely important role in determining their behaviors (Suominen et al. 1996), the study
on the detailed structure information using the Raman spectroscopy technique could be
essentially important. It indeed has been successful in using the technique for studying
individual HA splats within minor zones (<50 μm) (Li et al. 2004c).
During the in vitro testing, the splats samples were incubated in the SBF solution for var-
ious durations to reveal their dissolution behavior. The Kokubo SBF (pH = 7.40) (Kokubo et
al. 1990) was used for the in vitro
incubation. The solution is composed of 142.0 mM Na
+
,
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