Biomedical Engineering Reference
In-Depth Information
11
.
4 Nano-SizedCalcium-Deficient
Apatite Crystals
Bioactive ceramic materials, such as hydroxyapatite [Ca
10
(PO
4
)
6
(OH)
2
, HA] were widely used as bone substitutes for several decades
[14]. Stoichiometric hydroxyapatite has a hexagonal structure.
On the other hand, the calcium-deicient hydroxyapatites [Ca
10 -
x
(PO
4
)
6 -
x
(HPO
4
)
x
(OH)
2 -
x
, 0 ≤ x ≤ 1, CDHA] are of greater biological
interest than stoichiometric HA since the Ca/P ratio in bone is
nearer to 1.5 [14]. It has been suggested that calcium-deicient
apatitic (CDHA) plays important roles in several processes such
as bone remodeling and bone formation [13]. Furthermore, both
compositions are chemically and structurally similar to the mineral
constituent of human hard tissue. However, bone mineral (natural
biocrystal) essentially has a CDHA structure with a Ca/P ratio of
about 1.5 which, strictly speaking, is a Ca/P ratio similar to tricalcium
phosphates (TCP), Ca
3
(PO
4
)
2
, (Ca/P = 1.5) but structurally and
chemically compositionally similar to stoichiometric hydroxyapatite,
Ca
10
(PO
4
)
6
(OH)
2
, (Ca/P = 1.67). It was also reported that the
chemical properties such as catalysis, ion-exchange, and degradation
in solution are strongly dependent on Ca/P ratios [19].
Recently, various processes have been developed to prepare
nano-sized apatite powders [13, 20, 21, 24]. The synthesis of single-
phased needle-like CDHA nano-crystals with Ca/P ratios from 1.5 to
1.67 can be performed by a simple coprecipitation method with a
good reproducibility [13].
Figure 11.3 shows the TEM bright-ield (BF) image of the
precipitated CDHA powders with different Ca/P ratios from 1.5 to
1.67. The powder morphology presents a needle-like shape and the
needle-like particles are easily developed to form aggregates when
higher concentration and fast mixing are applied during the solution
preparation [11]. The dimension of these CDHA crystals in both
radial (diameter) and axial (length) directions is 5-10 nm in
diameter and 40-50 nm in length in terms of different Ca/P ratios.
An axial zone in some needle-like CDHA particles can be clearly
observed in the high-resolution TEM [13]. The lattice fringes were
seen predominantly in long needle-like crystallites and parallel to
(1 0 0) CDHA with a width of 0.82 nm. It was suggested that the HA
crystals tend to grow along the [0 0 0 1] direction and the fastest
growth direction results in needle-like morphology.
