Biomedical Engineering Reference
In-Depth Information
Generally, high-purity HA powder is soluble in acid solution, insoluble in alkaline solu-
tion, and slightly soluble in water. Also, the solubility of crystalline HA is varied with the
presence of amino acids, proteins, enzymes, and other organic compounds. The dissolu-
tion rate also depends on the particle size and the shape of HA granules and the poros-
ity, crystal size, and crystallinity of HA implants. As indicated by Driessens [67], there
are only two calcium phosphate compounds that are stable at room temperature when in
contact with aqueous solutions. It is the pH value of the solution that determines which
one is stable. At a pH value lower than 4.2, the component CaHPO
4
⋅2H
2
O (dicalcium phos-
phate) is the most stable phase, while at a pH value higher than 4.2, well-crystallized HA
is a stable phase. In addition, Adam et al. found that the surface of tricalcium phosphate
(Ca
3
(PO
4
)
2
, TCP) and tetracalcium phosphate (Ca
4
P
2
O
9
, TP) compounds will be coated with
thin HA layers through the phase transformation at a suitable pH value [68], and these
reactions can be represented as follows [69].
4Ca
3
(PO
4
)
2
→ Ca
10
(PO
4
)
6
(OH)
2
+ 2Ca
2+
+ 2HPO
4−
(6.1)
3Ca
4
P
2
O
9
+ 3H
2
O → Ca
10
(PO
4
)
6
(OH)
2
+ 2Ca
2+
+ 4OH
−
(6.2)
The in vitro dissolution properties of crystalline HA depend on several factors, such as
the type and concentration of the buffered or unbuffered solutions, pH of the solutions,
degree of saturation, solid/solution ratio, the length of suspension in solutions, and the
crystallinity of the HA [70-74]. In the case of ceramic HA bulks, the degree of porosities,
defect structure, the amount, and the type of other calcium phosphate phases present also
display significant influences. The extent of dissolution of the ceramic HA bulk is less in
lactic acid buffer compared to that in acetic acid buffer [75]. For the crystalline HA pow-
ders containing other calcium phosphate phases, the extent of dissolution will be affected
by the type and the amount of non-HA phases. According to previous studies made by
Ducheyne et al. [76] and Radin and Ducheyne [77], the evaluation of dissolution rate for
the nonwell-crystallized HA, α-TCP, β-TCP, T,P and crystallized HA were measured in 0.05
mol Tris(hydroxy)methylaminomethane-HCl buffered solution at pH 7.3, 37°C for immer-
sion time periods ranging from 15 min to 72 h. The results indicated that the concentration
of dissolved Ca
2+
reaches saturation (about 1.5 mM) for TP in a few minutes immersion,
and the value significantly exceeds that of β-TCP ([Ca
2+
] is about 1 × 10
−1
mM) and crystal-
line HA ([Ca
2+
] is less than 1 × 10
−1
mM) after 24 h immersion. Comparing the dissolution
rate of β-TCP with α-TCP, the dissolution rate of β-TCP is about four times larger than that
of α-TCP. Therefore, it can be recognized that the dissolution rate of various monophasic
calcium phosphate compounds decreased in the following order [76-78]:
Amorphous HA > TP > α-TCP > β-TCP > crystalline HA
In addition, the results of previous studies also showed that the values of solubility product
(K) for β-TCP and crystalline HA powder are 1.2 × 10
−29
mol
5
l
−5
and 3.04 × 10
−59
mol
9
l
−9
at 25°C, respectively [79,80]. Thus, when HA bulks or HA-coated implants, which have a
low crystallinity and high impurity phase content, are implanted, it may result in the dis-
solution, degradation of mechanical properties, and the dissociation of implants [47,48,76].
Therefore, the degree of crystallinity, phases, and chemical compositions of ceramic HA
bulks and HA-coated implants must be controlled in order to maintain long-term stability
in the body fluid after implantation.
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