Biomedical Engineering Reference
In-Depth Information
5.7
Reinforced Formulations and Concretes
Being aware on the excellent bioresorbability of DCPD and CDHA,
researchers are focused on attempts to overcome the mechanical
weakness of the self-setting calcium orthophosphate formulations
by using different fillers, fibers and reinforcing additives that give
rise to formation of various multiphasic biocomposites [104, 105,
109, 199, 259, 274, 409, 412-418]. Even carbon nanotubes have
been successfully tested to reinforce the self-setting formulations
[419]. Although the biomaterials community does not use this term
(just 1 paper has been published [420]), a substantial amount of such
formulations might be defined as calcium orthophosphate concretes
[421]. The idea behind the concretes is simple: if a strong filler is
present in the matrix, it might stop crack propagation. However,
adding fillers always reduced the porosity that negatively influenced
the ability of the concretes to allow bone ingrowth into the pores.
Hence, a denser formulation has a slower resorption rate and thus
a slower bone substitution [141]. Moreover, due to the presence
of fillers, injectability and other rheological properties of calcium
orthophosphate concretes frequently appear to be worse than
those ones of calcium orthophosphate cements. Thus, it is difficult
to increase strength of the cement formulations without having a
negative influence on the other properties.
Calcium orthophosphate concretes can be prepared from both
apatite and brushite cement formulations. For example, in an attempt
to improve the mechanical properties of calcium orthophosphate
cements, a group of investigators prepared concretes by adding
human cadaveric femur bone chips in amounts of 25, 50 and 75%
(w/w) to α-BSM
®
cement [414]. The mechanical tests revealed that
the specimens of pure cement exhibited a relatively high stiffness
but a low ductility. However, for the cement-bone concretes an
increasing of bone content was found to result in the elastic
modulus decreasing and the ductility increasing; however, the
ultimate strength showed only small changes with no apparent
trend [414]. A concrete of Biopex
®
cement with allografts taken
from femurs and tibiae of rabbits is also available. Unfortunately,
nothing is written on the mechanical properties improvement but,
surprisingly, by the addition of allografts, the hydrolysis process of
Biopex
®
was significantly changed [259]. By adding polymers, other
researchers succeeded in improving the mechanical strength of the
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