Geoscience Reference
In-Depth Information
5.8 Hydroxyapatite
Apatite is a common mineral in igneous, sedimentary, and metamorphic rocks
[193]
,
which has the general chemical formula of A
10
(BO
4
)
6
3
2
, where A
Ca, Sr, Ba, Fe,
5
PO
3
2
,VO
3
2
,SiO
4
2
,AsO
3
2
,
Pb, Cd, and contains many rare earth elements: BO
4
5
CO
2
2
;X
OH
2
,C
2
,F
2
,CO
2
2
. Water exists in different forms
[194,195]
.
The presence of the elemental phosphorus, which is a major component of apatite, was
first reported in the seventeenth century and the phosphorus in mineral pyromorphite
(lead apatite) was discovered in 1779
[196,197]
. Since the first synthesis of apatite
done in the middle of nineteenth century
[198]
, a lot of studies in the geochemical,
crystallochemical, biological, and other fields have been carried out. In fact, most of
the geochemical and crystallochemical studies were carried out on natural apatites,
whereas biological or other recent studies were mainly done using synthetic apatites.
However, many efforts have been made to prepare large single crystals of HAp with
only limited success. The interest in the synthesis of HAp is linked with its importance
as a major constituent of the inorganic component in bones and teeth where it occurs
as tiny crystals. Since it has been recognized to be one of the best biocompatible mate-
rials, there have been many reports on the biological aspects
[199
5
201]
. HAp crystals
have selective adsorption ability of proteins depending on crystal planes and are used
as an adsorbent in liquid chromatography
[202]
. Recent interest in the material has
spread to the fields of electronics and surface science (or adsorption chemistry), where
sintered or powder HAp is expected to function as, for example, gas sensors, catalysts,
and chromatographic absorbers. The fabrication processing of HAp has also pro-
gressed with the growing importance of the biological applications of HAp as artificial
bone and teeth
[203]
.
Table 5.16
gives the physicochemical properties of apatites
[204]
. However, a detailed knowledge of chemical and physical properties of HAp,
including those relevant to the behavior in a biological environment, depends on the
availability of sufficiently large single crystals of known chemical composition. Even
today most of the studies on chemical properties, mechanical properties, biological
responses, and the prosthetic application of HAp are based on polycrystalline bodies
with random crystal arrangement including sintered bodies, powders, granules and
coatings.
Daubree (1851) was the first to synthesize apatite by passing phosphorus
trichloride vapor over red-hot rime
[198]
. Since then a number of reports on the
preparation of HAp by various methods have appeared in the literature. Yoshimura
and Suda (1994)
[204]
have reviewed the preparation methods for HAp and
grouped all the methods into three categories, as given in
Table 5.17
. First is the
conventional solid-state reaction method at high temperature, the second is the wet
chemical method at relatively low temperature and the third, which is a very impor-
tant method today, is the hydrothermal method using high-temperature
high-pres-
sure aqueous solutions.
The solid-state reaction method has generally been used for processing ceramic
powders and for studying phase stabilities. The powders prepared by this method,
however, usually have irregular forms with a large grain size and often have
