Biomedical Engineering Reference
In-Depth Information
Referring to past reports, it has been generally recognized that postheat treatments such
as air or vacuum heat treatments, spark plasma sintering (SPS) technique, and hydrother-
mal treatments, etc., can significantly help to improve the phase composition, crystallinity,
mechanical properties, and biological responsibility of plasma-sprayed HACs [16,45,52-65].
In addition, the steam treatment during the in-flight stage of the plasma spraying can also
result in a significant increase in the crystallinity (from 58% to 79%) of plasma-sprayed
HACs [66]. The mechanism can be recognized that the entrapping of water molecules into
HA droplets and the improvement in crystallinity and phase purity from amorphous cal-
cium phosphate to HA is achieved by reversing the HA decomposition through providing
extra OH
−
. Overall, the degree of crystallinity, phase stability, and postheat treatments of
calcium phosphate and plasma-sprayed HACs—which are closely related to heating tem-
peratures, atmosphere, water molecules, and partial steam pressure—is presented in the
next section. Considering that HA is one of the natural apatite minerals and the phase sta-
bility in an atmosphere with plenty of water molecules and saturated steam pressure, the
hydrothermal synthesis, which is quite similar to mineral formation environment in the
earth, is an important method in the preparation of HA crystals. The hydrothermal tech-
nique and hydrothermal materials processing are becoming a popular field of research
for scientists and technologists of various disciplines. Therefore, Sections 6.3 and 6.4 will
specifically reveal the advantages and effects of hydrothermal crystallization on improv-
ing the microstructural homogeneity, phase purity, biological responses, adhesive bond-
ing strength, and failure mechanism of plasma-sprayed HACs. In addition, the kinetics of
hydrothermal crystallization, which is significantly related to the saturated steam pressure
in a hermetical system, will also be deduced and discussed in Section 6.3. The reliability
and failure behaviors of HA-coated implants should be studied in detail to ensure their
long-term stability in clinical applications, since biological degradation and failure of arti-
ficial joints that result from dissolution and dissociation may occur during the period of
implantation. Thus, knowledge of statistical analysis of the reliability engineering by the
Weibull model will be represented in Section 6.5. Meanwhile, the failure probability den-
sity function, cumulative failure probability, failure rate, and reliability functions, which
correlate with the cohesive strength of coatings and the adhesive strength of a coating to a
metal substrate, will also be reviewed in this section.
CharacteristicsofHACoatings
Phase Stability of the Crystalline HA Powders
Pure HA has a theoretical composition of 39.68 wt.% Ca element, 18.45 wt.% of P element, a
Ca/P weight ratio of 2.151, and a Ca/P molar ratio of 1.667. According to criterion of ASTM
F1185-88, the acceptable composition for commercial HA powder should be a minimum
value of 95 wt.% HA purity, as established by x-ray diffraction analysis. HA can be produced
by using a variety of methods, and the characteristics of raw HA powders have significant
effects on the subsequent products with HA being in the form of dense or porous bulks
and in coatings. In biological and clinical applications, HA bulks and HA-coated implants
are often immersed and applied in solutions or in the body fluids. Thus, the stability of HA
bulks and coating implants is significantly affected by the environmental temperatures
and the pH values.
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