Biomedical Engineering Reference
In-Depth Information
added, its crystallinity and crystal size. Although the effects of seeds
under different conditions are not entirely clarified, it seems that their
main effect is to reduce the setting time of the cement.
∑ The composition of the liquid phase: The primary role of the liquid phase
is to function as a vehicle for dissolution of the reactants and precipitation
of products. The liquid phase in CPCs is always water or an aqueous
solution. Water solutions ranging from plain water to simulated body
fluid have been used. In some cases, some soluble phosphate salts such
as NaH
2
PO
4
and/or na
2
HPO
4
are added as a source of phosphate ions in
solution, because it is known that common ions can have an accelerating
effect on the setting reaction.
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Therefore, neutral salts dissolved in the
liquid phase can be used to shorten the setting time exhibited by the
cement, for instance in apatitic cements.
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in some cases, water-soluble
polymers can be added providing scope to modify the cohesion of the
cement paste or its rheological properties.
∑ The liquid to powder ratio is a factor that increases the initial plasticity
of the paste and consequently its injectability and setting times. The
final strength is also affected by this parameter since the porosity of
the set specimen is directly correlated to the liquid-to-powder ratio
used. Therefore, reducing the liquid-to-powder ratio within the limits
of workability would be a means of improving the strength of CPCs.
Several properties must be taken into account in relation to the applicability
of the CPCs as bone substitutes. Among them, we can mention setting and
cohesion times, the injectability, the hardening rate, the mechanical strength
and pH evolution during setting. All of these depend on the composition and
the processing parameters that are chosen for each formulation.
Setting properties
Setting properties include the setting time, the cohesion time and
injectability.
The setting time
of a CPC can be defined as the time required for the initial
setting of the cement paste, which is reflected in a loss of plasticity. Usually
it is measured following mechanical methods, as a fast way of determining
whether a reaction occurs upon making a paste of the mixture of reactants
with water. The most common methods are based on the assessment of the
ability of the cement paste to resist a mechanical load applied to its surface.
Two examples are the Vicat needle and the use of two Gillmore needles. In
the first, a single needle is applied to the cement surface. The rationale for
the two Gillmore needles, as proposed by Driessens and co-workers
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is that
with the light-and-wide needle one, the initial setting time can be measured,
indicating the end of mouldability without serious damage to the cement
structure, whereas with the heavy-and-fine needle the final setting time can
