Preparation and Mechanism of Novel Bioceramics with Controllable Morphology and Crystal Growth - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 150

habit of HAp nanocrystals along the (0 0 0 1) face was observed by real-time
phase shift interferometry and atomic force microscopy (AFM) technology
in simulated body fluid (SBF) condition (Onuma et al. 1998).
Furthermore, the physicochemical properties of the bioceramic particles
are also remarkably influenced by the reaction conditions, such as the type
of raw materials and mixing types, molar ratio, concentrations, molecule
and ion additives, pH value, temperatures, and aging time. Zhang and Lu
(2007) found that the modes of reaction, namely, quick mixing and drop-
let adding, was in favor of spherical and rodlike morphologies, respectively.
When the (NH 4 ) 2 HPO 4 water solution was quickly added into the Ca(NO 3 ) 2
ethanol solution, the spherical HAp particles were obtained. This might be
attributed to the fast mixing process, which enables homogenous nucleation
of HAp particles and hence restrains the preferential growth of HAp, and
subsequently a uniform chemical reaction occurred. The temperatures can
also significantly influence the morphology and size of the bioceramic mate-
rials (Kumar et al. 2004). In the synthesis of HAp materials via wet chemical
precipitation route using Ca(OH) 2 and H 3 PO 4 as raw materials, needlelike
nanoparticles with a high aspect ratio were obtained at 40°C, while spherical
particles were obtained with the increase of the precipitation temperature to
100°C. The analysis indicated that the supersaturating level of the reactants,
especially the concentration of Ca 2+ ions, played a major role in the precipi-
tate morphology in the acid-base reaction system.
Iijima et al. (1996) investigated the role of F − ions on apatite/OCP/apatite
structure formation mechanism. The following conclusion was obtained:
When F − ions existed in solution where OCP precipitated, F − ions enabled
sequential precipitation of a low-substituted FHAp on an OCP template,
which incorporated a small amount of F − . A small amount of F − -containing
OCP, or a surface reaction layer of OCP that has accumulated a small amount
of F − , was suggested to be formed. The roles of F − in the HAp/OCP/HAp
structure formation were hypothesized as the reduction of the growth rate
and the critical thickness in the a-axis direction of OCP, the enhancement of
hydrolysis of OCP, and the activation of the growth of FHAp, which resulted
in thinner OCP lamella and thicker apatite lamella in the a-axis direction with
an increase in the F − concentration. The study showed that the carbonate sub-
stitution caused a reduction in crystallinity and changed in the shape from
needlelike to rod-shaped to platelike (equiaxed) apatite crystals (Robinson
1952; Bocciarelli 1970; Weiner et al. 1991). The presence of chloride ions in the
crystallizing medium favored the formation of platelike HAp crystals when
a large amount of precipitation took place (Koutsoukos and Nancollas 1981).
Kanchana and Sekar (2010) also found that the addition of NaF significantly
reduced the growth rate and the yield of HAp in the diffusion gel method
using sodium meta silicate (SMS) gel. At the same time, the microstructural
morphology of the synthesized HAp changed from fibrous to granular struc-
ture due to fluoride substitution, and the crystallinity of the HAp increased
with fluoride substitution. The study of Huang et al. (2012) suggested that