Biomedical Engineering Reference
In-Depth Information
stages, with the first papers published in 2004 [307, 525]. Due to
this reason, the mechanical property data for such biocomposites
have been reported only in few papers; however, these results are
encouraging. For example, Chen et al. performed nano-indentation
tests on biocomposite coatings to give hardness and Young's modulus
values [1079]. They found that the higher the loading of the nanotubes,
the better the mechanical properties. Namely, at 20 wt.% loading,
hardness was increased by ~43% and Young's modulus by ~21% over
a single-phase HA coating [1079]. Scratching test results indicated
that as alloyed HA biocomposite coatings exhibited improved wear
resistance and lower friction coefficient with increasing the amount
of carbon nanotubes in the precursor material powders [1078].
Additionally, measurements of the elastic modulus and hardness of
the biocomposite coatings indicated that the mechanical properties
were also affected by the amount of carbon nanotubes [1077].
Another research group performed compression tests on bulk HA/
carbon nanotubes biocomposites and found an increase in strength
over single-phase HA [307]. However, the highest compressive
strength they achieved for any material was only 102 MPa, which
is similar to that of cortical bone but much lower than the typical
values for dense HA [217]. More complex formulations, such as poly-
L-lysine/HA/carbon nanotube hybrid biocomposites, have been also
developed [1083]. Furthermore, calcium orthophosphate/carbon
nanotube biocomposites might be immobilized by hemoglobin
[1084]. Unfortunately, carbon nanotubes are very stable substances;
they are neither bioresorbable nor biodegradable. Therefore, during
in vivo
bioresorption, the nanotubes will get into the human body
from the biocomposite matrix and might cause uncertain health
problems. Certainly, this problem must be solved. To conclude the
carbon subject, one should mention an application of carbon fibers of
microscopic dimensions [1085-1087], nanodimensional diamonds
[1088] and C
[847] to reinforce HA bioceramics.
As clearly seen from the amount of the references, apatite/
zirconia biocomposites are most popular ones among the researchers.
The main disadvantage of HA reinforced by PSZ is degradation of
zirconia in wet environments [923, 932, 933, 955]. Transformation
of the tetragonal ZrO
60
to the monoclinic phase on the surface results
in formation of microcracks and consequently lowers the strength
2
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