Biomedical Engineering Reference
In-Depth Information
of citric acid and its salts on the rheological properties of CPCs.
151, 152
Citric
acid and sodium citrate have been shown to make the surface charge of the
particles more negative, acting as a dispersant of the paste and acting as a
liquefying agent. This allows the amount of water used in the cement to be
reduced, and therefore decreases significantly the porosity and improves the
mechanical properties. another approach is based on the addition of soluble
polymers, such as polysaccharides, that is sodium alginate, sodium hyaluronate
or chondroitin sulphate
13, 146, 153
or even some polymeric drugs.
154
10.4.3 Properties of set calcium phosphate
bone cements
Microstructure and porosity
The setting reaction of a CPC consists of the dissolution of one or more
constituents of the cement powder and the precipitation of a different
calcium phosphate. Physically, it takes place by the entanglement of the
crystals of the precipitating calcium phosphate. A precipitation reaction will
only lead to a considerable strength in these materials under the following
two conditions: (i) the precipitate grows in the form of clusters of crystals
which have a fair degree of rigidity, (ii) the morphology of the crystals of
the precipitate enables the entanglement of the clusters.
121
The evolution of
the microstructure of a CPC during the setting reaction is shown in Fig. 10.4.
This cement consists of a-TCP as starting product. Once mixed with water,
the a-TCP particles dissolve and CDHA crystals start to precipitate. The
entanglement of these precipitated crystals is responsible for the progressive
hardening of the CPC.
Interestingly, as previously mentioned, many apatite cements involve a
reaction in water between acidic dicalcium phosphates (DCP or DCPD) and
basic TTCP or a-TCP. No water is consumed during the setting of these CPCs,
as it can be seen in equations [10.2] and [10.3], and liquid is required only
to make the reactants workable and to allow homogeneous reaction. In other
cases, when a hydration reaction takes place (see for instance equation [10.1],
some water is consumed, but much less than the total amount added to make
a workable paste. Hence, water is a major contributor to the origin of porosity
in this system and therefore CPCs are intrinsically porous materials. Figure
10.4(d) shows the microstructure of an apatitic cement after setting and it
can be clearly seen that CPCs develop a highly micro/nanoporous structure.
The porosity of the set CPC is closely related to the liquid-to-powder ratio
used and it normally varies between 30% and 50%, although even higher
values can be reached. The pores are normally micro or nanometric in size
and the particle size distribution of the starting powder can modify the size
of the precipitated crystals and also the pore size distribution.
126
