Biomedical Engineering Reference
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[32, 58, 255, 293, 294, 302, 347-355], hydrolysis of other calcium
orthophosphates [356-358], double step stirring [359], emulsion-
based [275, 309, 360-372], steam-assistant [373], and solvothermal
[374] syntheses. Besides, still other preparation techniques are
known [31, 43, 122, 129, 242, 375-387]. Continuous preparation
procedures are also available [171, 388]. Application of both
ultrasound [389-391] and viscous systems [392] might be helpful.
Furthermore, nanodimensional HA might be manufactured by a
laser-induced fragmentation of HA microparticles in water [393-
395] and in solvent-containing aqueous solutions [304, 323, 396],
while dense nanocrystalline HA films might be produced by radio
frequency magnetron sputtering [397, 398]. An interesting approach
using sitting drop vapor diffusion technique should be mentioned
as well [399]. A comparison between the sol-gel synthesis and wet
chemical precipitation technique was performed and both methods
appeared to be suitable for synthesis of nanodimensional apatite
[292]. By means of these methods, a variety of nanodimensional
calcium orthophosphate building blocks with various structures
and morphologies have been synthesized, including needle-like,
spherical, fibrous, and mesoporous nano-sized crystals, as well as
nano-sized rods, hollow spheres, layered structures, and flowers as
shown in Fig. 3.5 [387].
Table 3.1 presents some data on the chronological development
of synthesis of nanodimensional apatites for the period of 1995-
2004 [122]. Among the methods described, the thinnest crystals
of apatite (60 × 15 × 0.69 or 0.84 nm) have been prepared by
Melikhov et al
; they have been called “two dimensional crystalline
HA” [263], while the smallest ones (size between 2.1 and 2.3 nm,
i.e. around two times the HA unit cell parameters) have been
found by Biggemann et al
.
.
[169], Liu et al
.
[405, 406], and Han
et al
[404] synthesized nano-sized HA via a template-mediated
and a non-template-mediated sol-gel techniques, respectively.
Both triethylphosphate [405, 406] and other alkylphosphates
[104] might be used to produce nanocrystalline apatites. Besides,
nanodimensional ion-substituted CDHA might be precipitated
from both a synthetic [328] and a simulated [407] body fluids.
A relatively simple sol-gel process using ethanol and/or water
as a solvent has also been reported to obtain the stoichiometric,
nanocrystalline single phase HA [323].
.
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