Biomedical Engineering Reference
In-Depth Information
TABLE 2.3. Comparative data on F - apatite, OH - apatite and C l - apatite
F - Ap
OH - Ap
Cl - Ap
Ionic radius (A)
F = 1.36
OH = 1.53
Cl = 1.81
Ca - F, - OH, - Cl distance (A)
2.29
2.89
2.80
Unit cell volume (A 3 )
523
530
545
Lattice parameters ( ± 0.003A)
a - axis
9.375
9.422
9.647
c - axis
6.880
6.882
6.771
[124]
2.3.2.2 Carbonate Incorporation. Early studies on carbonate - containing
apatite were prompted by the need to understand the nature of carbonate incor-
poration in biologic apatite (particularly bone and tooth mineral). Such studies
demonstrated that carbonate (CO 3 ) 2− can substitute in the apatite structure either
for the (OH) or the (PO 4 ) 3− group, referred to as Type A or Type B substitu-
tion, respectively [7]. The fi rst synthetic carbonate-substituted apatite with Type
A substitution prepared at high temperatures (1000 °C) was fi rst reported by
Elliott [21] and Bonel [7]. Apatite with Type B carbonate substitution prepared
at low temperatures (25 ° C to 95 ° C) was fi rst reported by LeGeros [57,87,126]
and was demonstrated to be more similar to the carbonate substitution in bio-
logic apatite [57,59,83]. The type of substitution depends on the method of prepa-
ration of the carbonate apatite. Type B substitution (CO 3 - for - PO 4 ) is obtained
when prepared from solution (either by precipitation or hydrolysis method at
25 ° C to 95 ° C [57,81,87,126] , or by hydrothermal reactions at 200 ° C and 200 psi
[41,57,117,118,121] . When prepared at high temperature (1000 ° C), Type A
substitution (CO 3 - for - OH) is obtained [7,21] . Simultaneous Type A and Type B
carbonate substitutions have also been reported when prepared by hydrothermal
conversion of fl ux growth (Table 2.4) [24-26,115,116,118,121]. In the presence of
Na + ions in solution, a coupled substitution Na-for-Ca and CO 3 - for - PO 4 occurs
[26,57,59,64,126]. For this report, apatite with CO 3 - for - PO 4 substitution (Type B)
will be referred to as CHA; with CO 3 - for - OH (Type A) substitution, as CA.
The two types of substitution have opposite effects on the lattice parameters.
Type A substitution, in which larger planar CO 3 group substitutes for OH group,
causes an expansion in the a -axis and contraction in the c - axis dimensions. Type B
substitution in which a smaller planar CO 3 group substitutes for the larger tetra-
hedral PO 4 group causes a contraction in the a -axis and expansion in the c - axis
dimensions (Figure 2.3 ). CO 3 -apatite prepared by hydrothermal reactions appear
to allow simultaneous CO 3 - for - OH and CO 3 - for - PO 4 substitution, as evidenced
by the larger a -axis for similar amount of CO 3 incorporation compared to the
a -axis dimension of CHA. Changes in infrared (IR) spectral characteristics are
specifi c for the two types of CO 3 substitution (Figure 2.4 ) [22,83] .
In terms of amount of CO 3 substitution, Type A substitution allows for
the complete substitution of OH (1 mole CO 3 for 1 mole OH), while Type B
Search WWH ::




Custom Search