Geoscience Reference
In-Depth Information
a sufficiently low solubility such that the initial precipitate does not completely dis-
solve under the experimental conditions, it is likely that the ACP particles acted as
templates for HAp crystallization via interface reaction rate control. The formation
of anisotropic particles may be due to reaction conditions that promote a greater
degree of dissolution
precipitation that competes with interface reaction rate con-
trol. Greater flexibility in tailoring HAp crystal size and morphology by the hydro-
thermal technique could be achieved through precipitation from homogeneous
solutions containing both Ca and P. Use of chelating agents for Ca, such as lactic
acid or EDTA, prevents the formation of ACP upon mixing sources of Ca and P at
room temperature. However, published work with these chelating agents shows that
anisotropic fibers are also formed
[273,274]
. Thus, future work will need to identify
additives that inhibit anisotropic particle growth.
There are also alternative methods of HAp powders preparation, like sol
gel,
flux method, electrocrystallization, spray-pyrolysis, freeze-drying, biomimetic, micro-
wave irradiation, mechanochemical method, or emulsion processing
[275
281]
.
Many HAp powders can be sintered up to theoretical density, without pressure, at
moderate temperatures (1000
1200
C). Processing at higher temperatures may lead
to exaggerated grain growth and decomposition of HAp and, subsequently, to strength
degradation HP, hot isostatic pressing (HIP), or HIP—postsintering makes it possible
to decrease the temperature of the densification process, decrease the grain size, and
achieve higher densities. This leads to finer microstructures, higher thermal stability
of HAp, and subsequently, better mechanical properties of the prepared HAp ceramics
[265]
. HAp ceramics, in a porous form, have been widely applied as bone substitutes.
Porous HAp exhibits strong bonding to bone. The classical way to fabricate porous
HAp ceramics (pore size of 100
m) is through hydrothermal sintering of the
HAp powder with appropriate pore-creating additives, like naphthalene, paraffin, and
hydrogen peroxide, which evolve gases at elevated temperatures. Natural porous mate-
rials, like coral skeletons made of CaCO
3
, can be converted into HAp under hydro-
thermal conditions (250
C, 24
600
μ
48 h)
[283]
. Microstructure, undamaged porous HAp
structures can also be obtained by HHP
[286,288]
. This technique allows solidification
of HAp powder at 100
300
C, 30 MPa, for 2 h.
Calcium phosphate bone cements find extensive applications as important bioma-
terials. These are mixtures of various calcium phosphate powders, such as
CaHPO
4
5H
2
O, Ca(H
2
PO
4
)
2
H
2
O, or
tri calcium phosphate (TCP), and water, or another liquid (e.g., H
3
PO
4
or Na
2
HPO
4
).
The mixture transforms into HAp during setting, forming a porous body, even at
37
C
[286
2H
2
O, Ca
4
(PO
4
)
2
O, CaHPO
4
,Ca
8
H
2
(PO
4
)
6
288]
. The setting time of calcium phosphate cements can be reduced to
a few minutes. Similarly, the decay of cements, when in contact with blood, can be
prevented. Several types of cements like HAp clays, consisting of HAp granules in a
saline solution of calcium alginate, bioactive glass bone cements, HAp-, TCp-, or
bioactive glass-reinforced polymeric bone cements, have also been developed.
The advantages of the calcium phosphate bone cements are high biocompatibil-
ity, bioactivity, and osteoconductivity. The only serious disadvantage is their rela-
tively poor mechanical strength. Easy shaping of bone cements enables using them
to fill bone defects much better than HAp solid blocks, which are difficult to shape,
