Biomedical Engineering Reference
In-Depth Information
DCPA (slightly acidic) in an aqueous suspension to form a poorly
crystalline precipitated HA (slightly basic) [14, 15]:
2Ca
(PO
)
O + 2CaHPO
Æ
Ca
(PO
)
(OH)
(5.1)
4
4
2
4
10
4
6
2
Earlier, it was believed that DCPA and TTCP reacted upon mixing
with water to form the stoichiometric HA [13-16]. However, further
investigations have shown that only the first nuclei consist of a nearly
stoichiometric HA, whereas further growth of these nuclei occurs
in the form of CDHA [145]. Besides, there is a study demonstrating
that the initially formed stoichiometric HA further interacts with
remaining DCPD to form CDHA [146].
According to Eq. (5.1), formation of HA releases neither acidic
nor basic by-products. Thus, the liquid phase of the cement remains
at a near constant pH of ~7.5 for the TTCP + DCPD and ~8.0 for
the TTCP + DCPA formulations, respectively [142-144]. Various
deviations from the stoichiometry of Eq. (5.1) were studied in details
and various apatitic calcium orthophosphates with Ca/P ionic ratio
within 1.5-1.67 were found as the end product [147]. The effect of
mixing ratio and pH on the reaction between TTCP and DCPA is well
described elsewhere [148]. Furthermore, the influence of Ca/P ionic
ratio of TTCP on the properties of the TTCP + DCPD cement was
studied as well [149].
A blend proposed by Lemaître et al. [150, 151] is another example
of the acid-base interaction where β-TCP (almost neutral) reacts
with MCPM (acidic) to form DCPD (slightly acidic):
β-Ca
(PO
)
+ Ca(H
PO
)
·H
O + 7H
O
Æ
4CaHPO
·2H
O (5.2)
3
4
2
2
4
2
2
2
4
2
In Eq. (5.2) MCPM might easily be substituted by orthophosphoric
acid [152-155] or MCPA, while β-TCP might be replaced by either
α-TCP [156, 157], CDHA [158, 159], or HA [160]. For example:
Ca
(HPO
)(PO
)
(OH) + 3H
PO
+ 17H
O
Æ
9CaHPO
·2H
O (5.3)
9
4
4
5
3
4
2
4
2
Furthermore, self-setting formulations based on mixtures of
ACP + α-TCP [161], ACP + DCPD [162, 163], DCPA + α-TCP [157],
OCP + TTCP [164], OCP + α-TCP [165, 166], and partially crystallized
calcium orthophosphate + DCPA [167] as the initial reagents, are
also available.
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