Preparation and Mechanism of Novel Bioceramics with Controllable Morphology and Crystal Growth - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 149

methods can be mainly divided into several kinds of strategies, such as solid-
state reaction, wet chemical process, precursor transformation method (also
regarded as “hard template” method), and physical approach. The solid-state
reactions usually result in agglomerate granulars in micro or submicro size
level even after ball milling because of the overly high reaction temperatures.
Moreover, the sintering ability of such powders is usually low and ultimately
results in lower mechanical properties of the sintered matrixes (Byrappa
2001). The wet technique includes chemical precipitation (Lin, Pan et al.
2009), sol-gel (Liu et al. 2001), hydrothermal (Byrappa and Adschiri 2007),
microemulsion (Lim et al. 1997), and solution reaction with the assistance of
the microwave (Li et al. 2011), ultrasound (Bang and Suslick 2010), and ultra-
violet (Nishikawa 2003) irradiation. As for the wet chemical process, there
are various surfactants, organic solvent, or molecular template-directing
reagents, such as ethylene diamine tetraacetic acid (EDTA), sodium ethylene
diamine tetraacetic acid (Na 2 EDTA), N, N-dimethylformamide (DMF), hexa-
decyltrimethylammonium bromide (CTAB), bis(2-ethylhexyl) sulfosuccinate
(AOT), sodium dodecyl sulfate (SDS), isopropyl alcohol, hexane, glycine,
formamide, examethylenetetramine, amino acids, protein, and monosaccha-
ride, which can be regarded as “soft template,” and are widely used to con-
trol the morphologies, crystal growth direction, crystal sizes, stoichiometry,
ion substitution, and the crystallinity of the bioceramic particles required
for specific applications in the wet chemical process. The transformation of
the solid precursors into products with designed morphologies and chemi-
cal compositions through oxidation-reduction, replacement reaction, hydro-
lysis and hydrothermal treatment, is another choice. This approach can be
regarded as the “hard template” method. The freeze-drying, mechanochem-
ical method, electrospinning, and spray pyrolysis are widely applied as the
physical approaches to fabricate bioceramics with specific morphologies.
In general, natural bioinorganic apatite crystals in bone and tooth, and
the chemical precipitated hydroxyapatite (HAp) particles are always needle-,
rod-, fiber-, or platelike in thin thickness because of the preferred orienta-
tion growth of HAp crystal along the c-axis (Su et al. 2003; Dorozhkin 2007).
Another explanation is that the plate-shaped OCP is the precursor of HAp
crystals, which grow along the OCP transition phase (Weiner and Wagner
1998). The energy barrier for nucleating HAp is higher than that of OCP since
the surface energy of OCP is lower than that of HAp. Therefore, the HAp pre-
fers to grow along the OCP layer and results in a platelike shape. Viswanath
and Ravishankar (2008) developed a general methodology to illustrate the
evidence for the formation of the platelike HAp. Their results showed that
the lowest energy surface is the prism plane (1 0 0). They strongly recom-
mend that the platelike shape of OCP and HAp is mainly due to the chemical
driving force at which OCP or HAp form falls in the layer-by-layer growth
zone. The relatively low temperature and neutral pH value favor the growth
of two-dimensional nanostructures associated with a low chemical driving
force (Viswanath and Ravishankar 2008). The preferential crystal growth