Geoscience Reference
In-Depth Information
HAp can directly bond to the bone. Unfortunately, due to low reliability, especially
in wet environments, the HAp ceramics cannot be used for heavy load-bearing
applications, like artificial teeth or bones. Nevertheless, there has been a lot of
research aiming to fabricate more mechanically reliable bioactive ceramics includ-
ing, of course, the HAp materials. Suchanek and Yoshimura
[265]
have reviewed
in detail the past, present, and future of the HAp-based biomaterials from the point
of view of the preparation of hard tissue replacement implants. The chemical com-
ponents of the mineral constituents of teeth and bones are very important in the
synthesis of HAp-based biomaterials. The inorganic phases present in the hard tis-
sues contain mostly Ca
2
1
and P, considerable amounts of Na
1
,Mg
2
1
,K
1
, also
CO
2
3
F
2
,Cl
2
, and H
2
O. All these species, if applied in appropriate quantities,
should be well tolerated in the implant by the surrounding tissues.
Presently, the HAp-bioceramics are at the pinnacle stage of their development.
Powder processing, formation, and densification have been understood quite well,
allowing the control of chemical composition and microstructures of both dense and
porous HAp ceramics. Any new developments concerning powder preparation/shap-
ing/densification may affect only the price of the products but are not expected to
affect their medical applications, which are restricted, due to the nature of HAp
[265]
.
Several
;
techniques have been used for
the preparation of HAp powders
[265
268]
. Two main methods of preparation of the HAp powders are wet meth-
ods and solid-state reactions. In the case of HAp fabrication, the wet methods can
be divided into three groups: precipitation, hydrothermal technique, and hydrolysis
of other calcium phosphates. Depending upon the technique, materials with various
morphology, stoichiometry, and level of crystallinity can be obtained. Solid-state
reactions usually give a stoichiometric and well-crystallized product, but they
require relatively high temperatures and long heat-treatment times. Moreover, the
sinterability of such powders is usually low. In the case of precipitation,
nanometer-sized crystals can be prepared, and they have the shapes of blades, nee-
dles, rods, or equiaxed particles. Their crystallinity and Ca/P ratio depend mainly
upon the preparation conditions which are, in many cases, lower than for well-
crystallized stoichiometric HAp. The hydrothermal technique usually gives HAp
materials, with a high degree of crystallinity and with a Ca/P ratio close to the
stoichiometric value, a better outcome. Their crystal size is in the range of nan-
ometers to millimeters. Hydrolysis of tricalcium phosphate, monetite, brushite, or
octacalcium phosphate requires low temperatures (usually below 100
C) and results
in HAp needles or blades the size of microns.
The authors have extensively studied the hydrothermal synthesis of HAp by
adapting the intelligent engineering approach based on thermodynamic principles
[28,269
271]
. Experimental conditions were planned based on calculated phase
boundaries in the system CaO
200
C. HAp powders
were then hydrothermally synthesized in stirred autoclaves at 50
P
2
O
5
NH
4
NO
3
H
2
Oat25
200
C and by
the mechanochemical
hydrothermal method in a multiring media mill at room
temperature. The synthesized powders were characterized using X-ray diffraction,
infrared spectroscopy, thermogravimetry, chemical analysis, and electron micros-
copy. Hydrothermally synthesized HAp particle morphologies and sizes were
