Preparation and Mechanism of Novel Bioceramics with Controllable Morphology and Crystal Growth - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 155

promoting particle repulsion that prevented particle agglomeration, thereby
contributing to maintaining the nanometric size of the particles. Recently,
the HAp microwhiskers more than 100 µm diameter were prepared using
acetamide as an additive. The ultralong crystal product was attributed to
the low hydrolysis rate of acetamide under hydrothermal conditions, the
pH rising slowly, giving smaller numbers of nuclei, and facilitating whis-
ker growth at a low supersaturation (Zhang and Darvell 2010a,b; Shen et al.
2012). The well-crystallized platelike β-TCP whiskers with width of 0.5 to 1
µm and length of 5 to 10 µm were synthesized from an acid solution (initial
pH = 4.5) through hydrolysis of urea at 96°C for 96 h in the triangular flask.
The pH value was the critical factor affecting the crystal growth process after
fixing the Ca/P ratio condition (Kang et al. 2008).
The phase transformation using solid materials as precursors was newly
developed to synthesize inorganic crystals with special morphologies. After
soaking the flat-plate shapes of brushite (CaHPO 4 ·2H 2 O) in Dulbecco's modi-
fied eagle medium (DMEM) solutions at 36.5°C, the OCP with similar shapes
were obtained (Mandel and Tas 2010). Nanoscale needles, fibers, and sheets
of HAp were selectively hydrothermal prepared through the hydrolysis of a
solid precursor crystal of monetite (CaHPO 4 , DCP) in an alkali solution by
simply varying the pH, ion concentrations, and phosphate ions. Long HAp
fibers were observed under a relatively mild basic condition at pH 9-10. The
fibrous morphology evolved from the nanoneedles produced by the solid-
solid transformation with the elongation of the c-axis through a dissolution-
precipitation route. Flaky HAp nanosheets consisting of a parallel assembly
of nanoneedles were observed with an excess amount of phosphate ions
under mild basic conditions. The presence of phosphate ions suppressed the
solid-solid transformation and promoted the formation of a 2D structure
through dissolution-precipitation process. Moreover, the oriented array of
bundled nanoneedles of HAp elongated in the c-axis was obtained under
a highly basic condition at pH 11-13. The ordered architecture originated
from the spatially periodic nucleation of HAp seeds on the DCP surface was
through topotactic solid-solid transformation (Ito et al. 2008). Furthermore,
the functional ions could be incorporated into the final products via addition
of the ions into the solutions (Pieters et al. 2010).
The α-triclcium phosphate [α-Ca 3 (PO 4 ) 2 , α-TCP] and calcium silicate with
good dissolution ability provide other kinds of precursors to modulate the
morphology of HAp crystals. After hydrothermal treatment of the α-TCP
powders, granules, and scaffolds in aqueous, the precursors usually trans-
formed to whisker-like HAp crystals with nano- or microstructured mor-
phologies (Ioku et al. 2006; Wakae et al. 2008; Liu et al. 2011). Through simply
adjusting the reaction temperature and the concentration of Ca 2+ ions under
hydrothermal treatment of the α-TCP powders in aqueous without using
any surfactants or additives, the well-developed HAp crystals with differ-
ent structures and morphologies (chrysanthemum-like HAp microflowers,
enamel-like HAp microparticles, rectangle-shaped HAp microplates, and