Biomedical Engineering Reference
In-Depth Information
Yokoi et al. (2010) used polyacrylamide (PAAm) hydrogel as the template to
synthesize calcium phosphate crystals. Their study showed that the concentra-
tions of calcium and phosphate ions played an important role on the crystalline
phases and morphology of the products. Several kinds of calcium phosphate
crystals were precipitated at various Ca(NO 3 ) 2 concentrations (0.5~4.0 mol·dm −3 )
or (NH 4 ) 2 HPO 4 contents (3.6~21.6 mmol) in the gels. The crystalline phases
were mainly determined by the (NH 4 ) 2 HPO 4 content in the gels. When the
(NH 4 ) 2 HPO 4 content was ≥10.8mmol, HAp formed near the interfaces between
Ca(NO 3 ) 2 solution and the gels, whereas OCP formed in gels with ≤10.8mmol
(NH 4 ) 2 HPO 4 . HAp crystals were granular in form and about 200 nm in diam-
eter, and OCP crystals were spherulitic with diameter 10~70 µm.
Simulated body fluid (SBF) is a popular solution applied to synthesize HAp
material. Usually the two key parameters of pH value and media tempera-
ture are used to adjust the morphologies of HAp crystals (Kobayashi et al.
2012). When the pH value of SBF was adjusted to 6.5, the phosphate-rich HAp
nanocrystals with needlelike shape precipitated at 38°C. With the increase of
the pH value to 7.0, the sheetlike nanostructures with (1 1 0) surfaces were
obtained. With the increase of the SBF temperature to 160°C, HAp crystals
with rodlike and platelike shapes in micrometer size were prepared at pH
value of 7.0 and 7.4, respectively. It is assumed that the adsorption of phos-
phates to the specific faces inhibits the growth of HAp crystals and changes
the morphology to low-dimensional forms. Bouyer et al. (2000) also sug-
gested that the morphology and size of HAp crystals were sensitive to the
reaction temperature. A critical temperature of 60°C could be used to define
the HAp crystals in monocrystalline or polycrystalline in the synthesis pro-
cess. The anisotropic growth of HAp crystals on the template of bombyx
mori silk fibroin (SF) films in 1.5 times SBF at 37°C was also researched by Li
et al. (2008). The result indicated that the HAp crystalline properties would
be controlled effectively by the positions and density of carboxyl groups, C =
O, and amino groups on the surface of SF films, which provided a mimicking
biomineralization process to fabricate the polymer-apatite composites.
The hydrothermal treatment can enhance the apatite crystal growth in the
c-axis and also increases the crystallinity of the products (Lin, Chang, Cheng,
et al. 2007), and the similar phenomenon has also been observed in Ca-Si-based
bioceramic particles (Lin et al. 2006; Lin, Chang, Chen, et al. 2007; Lin, Chang,
and Cheng 2007). The HAp (Lin, Chang, Cheng et al. 2007), tobermorite (Lin,
Chang, and Cheng 2007), nanowires, xonotlite, and wollastonite nanowires (Lin
et al. 2006) can be synthesized via hydrothermal or hydrothermal microemul-
sion processes. Viswanath and Ravishankar (2008) hydrothermal prepared
plate-shaped OCP and HAp crystals at the low pH value of 6. The fundamen-
tal reason behind the plate-shaped morphology control for HAp crystals was
revealed. They believed that the plate-shaped OCP and HAp was mainly due
to the fact that the chemical driving force at which OCP or HAp forms falls
in the layer-by-layer growth zone. Furthermore, the physiological conditions
existing during bone biomineralization (pH and temperature) are associated
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