Preparation and Mechanism of Novel Bioceramics with Controllable Morphology and Crystal Growth - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 159

the FHAp product was composed of uniform 3D nanorod aggregates, which
were composed of numerous well-aligned nanorods. With the increase of the
pH to 5.2, the FHAp was composed of dumbbell-like crystals as well as some
particles with irregular shape. With further increase of the pH, the aggre-
gates disappeared and ultimately resulted in morphology of short nanorods
with a mean length of about 50 nm under pH 10.0. One-dimensional FHAp
nanocrystals with hexagonal structure were always produced in the presence
of Na 2 EDTA under different pH values. When the two chelating reagents of
CA and Na 2 EDTA were simultaneously used, the flowerlike crystals with
shuttle-shaped petals were obtained at pH 3.6. With the increase of the pH to
5.2, the FHAp sample was made up of 1D hexagonal nanorods with about 2
µm in length and 300 nm in diameter. The well-aligned nanorods of FHAp
were obtained at pH 6.1. With further increase of the pH to 10.1, the sample is
composed of a wealth of branchlike nanorods. It implied that the combina-
tion of these two chelating reagents induced not only morphology change
but also structural transformation. The possible mechanism for the control
of FHAp crystals with various morphologies might be attributed to the pref-
erential crystal growth habit along the (0 0 0 1) direction for FHAp, possible
anisotropic growth along crystallographically reactive directions under dif-
ferent pH and in the presence of chelating reagent, the molecule structures,
ionization, and surface adsorption of the chelating reagents at different pH
conditions. The kinetic analysis of HAp crystal growth also revealed that
the template depressed the interfacial potential energy ( E ), then enhanced
the roughness on the surface of crystal nucleus and directed HAp crystal to
selectively grow along the (0 0 0 1) direction, and consequently governed the
aperture of porous HAp microspheres (He and Huang 2007).
The HAp microtubes were prepared using CaCl 2 and NaH 2 PO 4 in mixed
solvents of water-N, N-dimethylformamide (DMF) at ratio of 1:2 by a solvo-
thermal method at 160°C for 24 h. The concentration of DMF played an impor-
tant role on the morphologies of the product (Ma et al. 2008). The flowerlike
nanostructured HAp hollow spheres (NHHS) assembled with nanosheets
with a hierarchical morphology were fabricated via a rapid microwave-
assisted hydrothermal route. The presence and concentration of block copo-
lymer poly(lactide)-blockpoly(ethylene glycol) (PLA-PEG) were important
parameters for the self-assembly of the hollow structure (Wang et al. 2010).
The polyelectrolytes have been widely used as templates to synthesize and
modulate the morphologies of the advanced inorganic materials. Wang et al.
(2009) used negatively charged poly(styrene sulfonate) (PSS) to modify the
morphology and particle size of HAp by simply adjusting the PSS concentra-
tion. Within the PSS concentration range of 0 to 9.6 wt%, the HAp crystal-
lites grew from ribbons to microspheres, and the building units of various
microspheres changed from nanofibers to nanorods or nanoplates. Along
with that, the microspheres became smaller and more compact at higher PSS
concentrations. The adsorption of PSS onto certain crystal faces as well as the
complex effect of PSS with Ca 2+ could be considered as the controlling factors