Biomedical Engineering Reference
In-Depth Information
cements [14, 25, 442, 443, 501-505]. However, there is a study in
which an increase of particle and crystallite sizes of TCP did not
prolong but shortened the induction time until the cement setting
reaction started [445], which was against the common physical
rules (generally, smaller particles or crystallites should enhance
reactivity). Nevertheless, two general directions of the biomedical
application of nanodimensional and nanocrystalline calcium
orthophosphates can be outlined: (i) using them in powder form
as filling materials to impart bioactivity to various biocomposites
and hybrid biomaterials [53-87, 153, 506] and Chapter6; (ii)
manufacturing of either dense compacts or porous scaffolds,
possessing the sufficient mechanical properties [63, 80, 261, 262,
492, 463, 507, 508]. As the nanodimensional and nanocrystalline
calcium orthophosphates tend to agglomerate at heating (Fig. 3.6)
[278, 509-511], normally a low-temperature [143, 323] and/or a
rapid consolidation [143, 225, 287, 512-517] techniques must be
employed. The low-temperature approach comprises gel hardening
(at 4°C) [323] and uni-axial pressing at 150-200°C [143]. The rapid
consolidation techniques comprise spark plasma sintering [143, 225,
287, 512-514], pressure sintering [513], and microwave sintering
over the temperature range 1000-1300°C, using a rapid sintering
schedule [515-517]. Furthermore, nanodimensional crystals of
calcined HA might be fabricated by calcination at 800°C for 1 h with
an anti-sintering agent surrounding the original nano-sized CDHA
particles and the agent is subsequently removed by washing after
the calcination [518-520]. These consolidation approaches provided
a limited alteration of the initial nano-sized crystals, while the final
bioceramics possessed the mechanical properties similar to those
reached with sintered stoichiometric HA.
Already in 1990s, implants prepared from nanodimensional
apatites, as well as biocomposites of nanodimensional apatite with
organic compounds were tested
[521-523]. Cylinders made
of both pure nanodimensional apatite and organoapatite containing
a synthetic peptide were analyzed 28 days after implantation into
spongy bones of Chinchilla rabbits. Both implant types were well
incorporated and interface events were found to be similar to those
observed on human bone surfaces with regard to resorption by
osteoclast-like cells and bone formation by osteoblasts. That study
revealed a suitability of such materials for both bone replacement
and drug release purposes [521]. Similar results were obtained in
other studies [522, 523].
in vivo
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