Biomedical Engineering Reference
In-Depth Information
macropores suitable for bone ingrowth. One interesting advantage
of long fibers over particulates and short fibers is the fact that once
resorbed they can form interconnected pores inside the cement
structure facilitating bone tissue regeneration [327].
One more approach consists in adding solid NaHCO
to the
starting cement powder and using two different liquids: first, a basic
liquid to form the paste and later an acid liquid to obtain CO
3
bubbles
2
to create porosity [328]. Besides, pore forming CO
bubbles appear
at hardening of an apatite cement, consisting of an acidic calcium
orthophosphate, such as MCPM or DCPD, and either CaCO
2
[33, 53-
3
55] or NaHCO
[329-331]. Furthermore, addition of an effervescent
porogen formulation comprised from NaHCO
3
(54.52%) and citric
acid monohydrate (45.48%) has been suggested [332]. More to the
point, the liquid phase of a cement formulation might be foamed
and subsequently mixed with the powder phase. Further, the setting
reactions of a calcium orthophosphate cement transforms the liquid
foam into a solid, which ideally maintains the geometry, size, and
shape of the bubbles of the liquid foam. Thus, the liquid foam acts
as a template for the macroporosity of the solid foam [333, 334].
In addition, several other porosity creation techniques in calcium
orthophosphate cements are known and, for further details on the
subject, the interested readers are referred to an excellent review
[327].
The major examples of workability improvement agents, which
are added to the cement powders, include water-soluble polymers.
Specifically, polysaccharides [97, 108, 335-338], gelatin [278,
339-345], and polyacrylic acid [346-348] are of an interest due to
their biocompatibility and good rheological properties. Only small
amounts (a few weight %) are needed to dramatically increase the
viscosity of the cement pastes. Besides, the cement paste becomes
more cohesive and highly resistant to washout immediately after
mixing. For example, a 5 wt. % sodium chondroitin sulfate solution
is used as mixing liquid in Biopex
3
[197, 198]. In the case of gelatin,
more than a 50% improvement of the compressive strength was
detected [341]. The gelatin-cement after setting was found to exhibit
reduced crystallinity, much smaller CDHA crystals and a more
compact microstructure; all these phenomena might be accounted
for the improved mechanical properties [342]. The presence of
gelatin improved mechanical properties of the cements; in particular,
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