Preparation and Mechanism of Novel Bioceramics with Controllable Morphology and Crystal Growth - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 157

applied to illuminate the effect of the different phosphorus sources on HAp
morphology development (Posner and Betts 1975). The Ca 9 (PO 4 ) 6 clusters are
considered the growth unit of HAp crystals. The Ca 2+ ions gradually dis-
solved from the CS precursors into the phosphate solution to form Ca 9 (PO 4 ) 6
clusters. It is well known that hexagonal HAp crystal has two types of crys-
tal surfaces with different charges, positive on a- and b-surfaces and nega-
tive on c-surfaces (Kawasaki 1991). Usually, hexagonal HAp crystals, which
grow along the c-axis, are easily obtained because of a strong bond site for
the Ca 9 (PO 4 ) 6 cluster in the (0 0 0 1) direction but not in the (1 0 1 0) direction.
The crystal growth is thus easier in the (0 0 0 1) direction than in the (1 0 1
0) direction. In the Na 3 PO 4 solution, a large quantity of PO 4 3- ions was ion-
ized into the solution. Therefore, there would be enough PO 4 3- ions to form
Posner clusters with Ca 2+ ions dissolved from CS particles. The released Ca 2+
ions were rapidly consumed, which resulted in quite a few Ca 2+ ions remain-
ing near the surfaces of the CS particles. According to the Cluster growth
model for HAp, Posner clusters would attach to c-surfaces preferentially
and the direction along the c-axis quickly developed. Ultimately, the HAp
nanowires were obtained. However, in the NaH 2 PO 4 solution, the hydroly-
zation of NaH 2 PO 4 salts was greater than ionization. The major ions in the
solution were H 2 PO 4 and HPO 4 2- ions, and there were only a small amount of
PO 4 3- ions formed from the secondary ionization. Therefore, a large amount
of released Ca 2+ ions were attached to the c-surfaces with negative charges,
which resulted in fewer Posner clusters incorporated onto the c-surfaces.
The growth of c-surfaces was limited, whereas the growth of a, b-surfaces
was enhanced, leading to the aggregation of a, b planes. Ultimately, the HAp
nanosheets were obtained. However, the morphology of the synthesized
HAp was nanoparticles after hydrothermal treatment of the CSH powders
in Na 3 PO 4 solution. This might be attributed to the crystal structure of CSH
itself. It is well known that the CSH is a poorly ordered phase with layered
structures. The layer consists of a central Ca-O part sandwiched between
parallel silicate chains (Taylor 1986). With hydrothermal treatment in phos-
phate solution, the HAp crystal formed accompanied with the split of the
sandwiched layers and silicate chains of the CSH into the short fragmenta-
tions. In this situation, HAp crystals were formed on these fragmentations,
resulting in the particle-like products. At the same time, through regulating
the chemical compositions of the precursors and the reaction ratio of the pre-
cursor/solution, the HAp crystals substituted by different kinds and amount
of elements (such as Si, Na, Mg, and Sr) could be easily obtained.
6.2.2 Fabrication and Morphology Control of the Nanostructured
Bioceramics with Novel 3D Architectures
The three-dimensional (3D) architectured biomaterials with nanostructures
have been attracting intense interests due to their high specific surface area
and novel 3D hierarchical architectures, which make it possible to incorporate