Geoscience Reference
In-Depth Information
natural fibers, asbestos has been widely used, due to its high tensile strength, chem-
ical stability, and low cost. Unfortunately, its application has a great health risk as
it causes many serious diseases, including lung cancer. The toxicity of widely used
silicon carbide whiskers is stated to be only slightly lower. The carcinogenic effect
of the fibrous materials was restricted to long and thin fibers (diameter
1
μ
m,
,
length
m). Many commercially available whiskers and fibers may be safe
from this point of view; however, their chemical compositions must also be taken
into account. Alumina, zirconia, titania, silicon carbide, and silicon nitride are
known as bioinert materials
[259]
. If the material is toxic, the surrounding tissue
dies. If the material is nontoxic and biologically inactive (bioinert), a fibrous tissue
of variable thickness is formed. If the material is nontoxic and biologically active,
an interfacial bond forms. In this respect, HAp is known as the most biocompatible
material. It has been used in medicine for many years in the form of small
unloaded implants, powders, and coatings.
Fibrous HAp can be used as insulating agents, packing media for column
chromatography, etc. It is a very promising material for preparation of compo-
sites. In recent years, because of their excellent biocompatibility, HAp whiskers
are the most studied whiskers. HAp contains nontoxic species, such as Ca, P,
OH
2
, usually CO
2
3
10
μ
.
;
and, even if some of the crystals have a dangerous shape,
they should dissolve in a human body without causing any health problems. It
has been reported that different kinds of proteins are absorbed on different
planes of HAp crystals. Therefore, if HAp crystals, whose particular planes
grow selectively, are obtained, they can separate various kinds of proteins.
Nagata et al.
[333]
has obtained plate-like HAp crystals synthesized hydrother-
mally with methanol.
In recent years, several reports concerning the fabrication of HAp fibers and
whiskers appeared in literature
[257,266,268,334
337]
. All preparation techniques
for HAp whiskers can be divided into two main groups: (1) the homogeneous pre-
cipitation method, using urea, and (2) the decomposition of chelating agents. The
first method utilizes a continuous increase of pH in a solution containing calcium
and phosphate ions at a high temperature. In the second case, chelating agents like
EDTA, lactic acid, or citric acid are used. During the heat treatment, which is usu-
ally carried out under hydrothermal conditions, Ca-complexes with chelating agents
decompose, followed by the precipitation of HAp whiskers.
In the preparation of HAp whiskers by the first method, the resultant product
is always contaminated with large quantities of carbonate ions and they have
not been verified, as yet, to be single crystals (i.e., whiskers)
[257]
. Yoshimura
and coworkers prepared, under hydrothermal conditions, HAp materials, such as
fine needle-like crystals and whiskers
[265,266,268]
. They prepared HAp whiskers
of diameters 0.1
m, using the decomposition of
chelating agents. Christiansen and Riman
[257]
also adopted a similar procedure
to prepare HAp whiskers
[335
5
μ
m and length 30
100
μ
338]
. The most significant feature of these whis-
kers is the absence of the CO
2
3
species in larger quantities, but their Ca/P
molar ratio was in the range of 1.59
1.63, deviating from the stoichiometric value
of 1.67.
