Biomedical Engineering Reference
In-Depth Information
The second type of the setting reaction might be defined as
hydrolysis of a metastable calcium orthophosphate in aqueous
media. As the result, both the initial and final compounds have
the same Ca/P ionic ratio. Due to the fact, that only one calcium
orthophosphate is used; the solid part of such formulations might
be called as a single-phase (or single-component) cement powder
[168]. Cements made of ACP + an aqueous solution [169, 170], α-TCP
+ an aqueous solution [171-178], β-TCP + an aqueous solution [175,
179], DCPA + an aqueous solution [24], CDHA + an aqueous solution
[25], TTCP + an aqueous solution [26, 180, 181], or γ-radiated TTCP
+ an aqueous solution [182-184] are the typical examples; the
majority of them are re-crystallized to CDHA during setting:
Ca
H
(PO
)
·nH
O + H
O
Æ
x
y
4
z
2
2
Ca
(HPO
)
(PO
)
(OH)
+ nH
O (5.4)
10−
x
4
x
4
6−
x
2−
x
2
3(α- or β-)Ca
(PO
)
+ H
O
Æ
Ca
(HPO
)(PO
)
(OH) (5.5)
3
4
2
2
9
4
4
5
3Ca
(PO
)
O + 3H
O
Æ
Ca
(HPO
)(PO
)
(OH) + 3Ca(OH)
(5.6)
4
4
2
2
9
4
4
5
2
The experimental details on TTCP hydrolysis under a near-
constant composition condition might be found elsewhere [185]. The
details on α-TCP hydrolysis are also available. The results indicated
that setting of α-TCP was initially controlled by surface dissolution;
therefore, it depended on the surface area of the reactants [186-189].
Hydrolysis of DCPD to CDHA was studied as well [190]. Addition of
~2 wt. % of a precipitated poorly crystalline HA (i.e., CDHA) as a seed
to α-TCP powder phase might be useful to accelerate the kinetics of
reaction (5.5) [191].
Further, there is a single-phase cement powder consisting of
K- and Na-containing CDHA (with the Ca/P ionic ratio of 1.64 ±
0.02) that sets and hardens after mixing with an aqueous solution
of sodium citrate and sodium orthophosphate [17]. After setting,
this formulation gives rise to formation of a weak cement (the
compressive strength of 15 ± 3 MPa) consisting of the ion-substituted
CDHA again (presumably, with another Ca/P ionic ratio), mimicking
the bone mineral. Unfortunately, neither the setting reaction nor the
setting mechanism of this cement has been disclosed in literature
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