Biomedical Engineering Reference
In-Depth Information
2+
4
3−
Ca
ions in a certain ratio to the solution and after that
induce the reaction [39]. Biomimetically, one can achieve an oriented
growth of CDHA crystals onto dissolved collagen fibrils in aqueous
solutions via a self-organization mechanism [747]. A number of
authors produced calcium orthophosphate/collagen biocomposites
by mixing preformed ceramic particles with a collagen suspension
[765-767]. However, in all blended composites, the crystallite sizes
of calcium orthophosphates were not uniform and the crystals were
often aggregated and randomly distributed within a fibrous matrix
of collagen. Therefore, no structural similarity to natural bone was
obtained and only a compositional similarity to that of natural bone
was achieved. Besides, CDHA crystallization from aqueous solutions
might be performed in the presence of a previously dispersed
collagen [33, 597]. More to the point, collagen might be first
dispersed in an acidic solution, followed by addition of calcium and
orthophosphate ions and then co-precipitation of collagen and CDHA
might be induced by either increasing the solution pH or adding
mixing agents [41]. Although it resulted in biocomposites with poor
mechanical properties, pressing of the apatite/collagen mixtures at
40 ºC under 200 MPa for several days is also known [768]. Attempts
have been performed for a computer simulation of apatite/collagen
composite formation process [769]. It is interesting to note, that such
biocomposites were found to possess some piezoelectric properties
[770].
As the majority of the collagen/HA biocomposites are
conventionally processed by anchoring micron-sized HA particles
into collagen matrix, it makes quite difficult to obtain a uniform
and homogeneous composite graft. Besides, such biocomposites
have inadequate mechanical properties; over and above, the proper
pore sizes have not been achieved either. Further, microcrystalline
HA, which is in contrast to nanocrystalline bone apatite, might
take a longer time to be remodeled into a new bone tissue upon
the implantation. In addition, some of the biocomposites exhibited
very poor mechanical properties, probably due to a lack of strong
interfacial bonding between the constituents. The aforementioned
data clearly demonstrate that the chemical composition similar to
bone is insufficient for manufacturing the proper grafts; both the
mechanical properties and mimetic of the bone nanostructure are
necessary to function as bone in recipient sites. There is a chance for
improving osteointegration by reducing the grain size of HA crystals
and PO
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