Biomedical Engineering Reference
In-Depth Information
be governed by the interactions with the amino acid additives,
which restricted a growth of the primary crystallites [422, 423].
Furthermore, nanodimensional apatites might be precipitated from
aqueous solutions of gelatin [59, 424]. The development of nano-
sized apatite in aqueous gelatin solutions was highly influenced by
the concentration of gelatin: namely, a higher concentration of gelatin
induced formation of tiny (4 × 9 nm) nano-sized crystals, while a
lower concentration of gelatin contributed to the development of
bigger (30 × 70 nm) nano-sized crystals. In this experiment, a higher
concentration of gelatin supplied abundant reaction sites containing
groups such as carboxyl, which could bind with calcium ions. This
leads to formation of a very large number of nuclei and creation of a
large number of tiny nano-sized crystals [59].
Although each of the reported approaches to produce
nanodimensional apatites has both a scientific and a practical
relevance, a little attention has been dedicated to the physicochemical
details involved in the careful control of the particle size distribution
and particle shape. Indeed, in the case of particle size distribution,
most of the reported ways to synthesize nanodimensional apatites
really produced a particle mixture with a wide size distribution from
tens to hundreds of nanometers. Moreover, the control of particle
shape is another problem for these methods, which commonly result
in pin-like or irregular particles. It is well known that bone consists
of homogeneous plate-like crystals of biological apatite of 15-30 nm
wide and 30-50 nm long, while enamel consists of rod-like crystals
of biological apatite of 25-100 nm thick and lengths of 100 nm to
microns (Fig. 1.14) [2, 5, 177, 178, 180, 187, 195, 197]. The study
of higher-level biomineralization and biomimetic assembly involves
a search for advanced methods so that the synthesis of nano-
sized apatite can be accurately controlled [425]. Namely, the size-
controlled synthesis of materials can be achieved by using limited
reaction spaces. For example, microemulsions have been shown
to be one of the few techniques, which is able to produce particle
sizes in the range of nanometers and with minimum agglomeration
[426]. Thus, microemulsions [309, 363-371], micelles [427], and
reverse micelles [299, 428, 429] have been successfully applied to
synthesize nanodimensional apatites with minimal agglomeration.
It was found that experimental conditions, such as aqueous/organic
phase volume ratio, pH, aging time, aging temperature, and ion
concentration in the aqueous phase can affect the crystalline phase,
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