Biomedical Engineering Reference
In-Depth Information
in the preparation of HA. The hydrothermal technique and hydrothermal materials pro-
cessing are becoming a popular field of research for scientists and technologists of various
disciplines, particularly after the successful development of ceramic processing technol-
ogy during the 1970s. The fabrication of HA under hydrothermal conditions usually gives
well-crystallized, compositionally homogeneous, and uniform HA powders with a high
degree of crystallinity, and large, perfect, single crystals of HA with a Ca/P ratio close to
the stoichiometric value of 1.67 [132]. The understanding of minerals formation in nature
under elevated pressure and temperature in the presence of water (hydrothermal condi-
tions are similar to the environment of mineral formation within the earth's crust) has led
to the development of the hydrothermal technique of today. In the following sections, the
principles and applications of the hydrothermal technique for preparing HA are intro-
duced. Next, the effects of hydrothermal crystallization and significantly self-healing on
the microstructural homogeneity of plasma-sprayed HACs are discussed in Section 6.3.2.
In addition, the kinetics, reaction rates, and activation energy of hydrothermal crystalliza-
tion, which are related to the saturated steam pressure within a hermetical system, are
deduced and evaluated in Section 6.3.3.
Hydrothermal Technology for the HA: Principles and Applications
Apatite is a common mineral in igneous, sedimentary, and metamorphic rocks, which has
a general chemical formula of A
10
(BO
4
)
6
X
2
, where A is Ca, Sr, Ba, Fe, Pd, Cd, and rare earth
elements, BO
4
can be PO
4
3−
, VO
4
3−
, SiO
4
4−
, AsO
4
3−
, CO
3
2−
groups, and X can be OH
−
, F
−
, Cl
−
,
CO
3
2−
groups. The inorganic phases of HA with a chemical formula Ca
10
(PO
4
)
6
(OH)
2
pres-
pres-
ent in the hard tissues contain mostly Ca, P, and H
2
O, also a small amount of Na
+
, Mg
2+
, K
+
,
as well as F
−
, Cl
−
, and CO
2
3−
. Therefore, the chemical component of the mineral constitu-
he chemical component of the mineral constitu-
ents of teeth and bones is very important in the synthesis of HA-based biomaterials. In
the present biomedical and clinical applications, HA powders, bulk implants, and surface
coatings on metallic or ceramic substrates prepared under hydrothermal conditions have
been effectively carried out by many research groups [133-144]. The hydrothermal method
has been found to be a suitable method to prepare well-crystallized and nonagglomerated
crystals that are homogeneous in size, shape, and composition that can be achieved even
at low temperatures [145,146]. HA crystals with different morphologies can also be grown
by hydrothermal method using various starting materials such as calcite, brushite, and
monetite [135,147,148].
In the hydrothermal process, water is the most important solvent and a necessary com-
Therefore, the chemical component of the mineral constitu-
a necessary com-
ponent of the system, and indeed it is often the principal component with regard to the
relative content of salts, acids, alkalis, and so forth, which may serve as mineralizers. It
also exhibits unique properties, especially under supercritical conditions. In any system
containing water, a certain concentration of the components occurs in the dissolved state,
changing the pH of the medium, the composition of the original liquid phase, and the
properties of the solution. Several previous publications have made definitions for the term
“hydrothermal” [125,149-154], and a concluded definition of the hydrothermal reaction
can be proposed as follows: any heterogeneous chemical reactions in the presence of a
solvent (whether aqueous or nonaqueous) or mineralizers above room temperature and at
pressure greater than 1 atm in a closed system to dissolve and recrystallize materials that
are relatively insoluble under ordinary conditions.
During the early 1970s in the Material Research Laboratory at Pennsylvania State
University, a process was developed that utilized the skeletal structure of marine inver-
tebrates, especially reef building corals, as a template to make porous structures of other
the most important solvent and a necessary com-
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