Biomedical Engineering Reference
In-Depth Information
calcium orthophosphate cements containing 2 wt. % gelatin were
found to harden in an acceptable time and were recommended
for clinical applications [345]. In general, the use of gelling agents
widened a possible application of calcium orthophosphate cements
because these cements can be used even when complete homeostasis
is difficult. In some cases addition of a gelling agent might cause an
increase in hardening time [349] but this was remedied by the use of
a sodium orthophosphate solution as the cement liquid [143, 144].
Most polysaccharide solutions are thixotropic, i.e., the viscosity
of the solution decreases as the shear rate increases. Certain
polysaccharides, such as sodium alginate, pectize in contact with
calcium ions. This property can be used to make putty-like cement
pastes [22]. However, only few polysaccharides are accepted for
parenteral use [197, 198].
Of two families of calcium orthophosphate cements, brushite
cements react generally much faster than apatite ones. As a result,
to satisfy the clinical requirements (Fig. 5.2), the setting time of
brushite cements has to be prolonged, whereas that of apatite
cements has to be shortened [197, 198]. In general, setting reactions
of any calcium orthophosphate cements consist of three successive
stages: (1) dissolution of reactants to saturate the mixing liquid in
calcium and orthophosphate ions; (2) nucleation of crystals; and (3)
growth of crystals. Therefore, experimental approaches to modify
the setting reaction of calcium orthophosphate cements are to be
targeted to these three stages. The available approaches have been
summarized in Table 5.3 [207]. Furthermore, seven strategies have
been described to control the setting time of calcium orthophosphate
cements [208]. They are as follows: (i) mean particle size decreasing
of the initial powders; (ii) the P/L ratio increasing; (iii) pH drop of
the mixing liquid to increase calcium orthophosphate solubility and
hence accelerate the chemical transformations; (iv) a nucleating
phase addition, such as a nano-sized HA powder; (v) adding
orthophosphate and/or calcium ions into the mixing liquid to
accelerate the setting reaction according to the common-ion effect;
(vi) solubility reducing of the reaction end-product, for example, by
adding fluoride ions into the mixing liquid; (vii) solubility increasing
of the starting material by amorphization, e.g.,
by prolonged milling.
For further details on these strategies and approaches, as well as
for application examples, the interested readers are referred to the
original papers [207, 208].
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