Preparation and Mechanism of Novel Bioceramics with Controllable Morphology and Crystal Growth - Advanced Bioactive Inorganic Materials for Bone Regeneration and Drug Delivery - page 151

the Eu ion inhibited the crystal growth in the c-axis direction and decreased
the ratio of width to length of HAp crystals. The possible reason was that the
replacement of Ca by Eu inhibited the crystal growth along the active plane.
Apart from the influence of ion additives on the resultant morphologies,
the organics, including the surfactants, polymers, and proteins, are another
choice to be used as templates in solution systems to control the shape and
size of HAp crystals. The previous studies revealed that organic molecules
could be absorbed on the crystal surfaces and regulated the morphologies
of the products (Chander and Fuerstenau 1984; Burke et al. 2000; Spanos et
al. 2001). Bose and Saha (2003) synthesized spherical-like nanocrystalline
HAp powder with particle diameters of 30 and 50 nm using the reverse
microemulsion method, cyclohexane as the oil phase, and a mixture of
poly(oxyethylene) nonylphenol ether and poly(oxyethylene) nonylphenol
ether (NP-12) as the surfactant phase. Cai et al. (2007) using hexadecyl (cetyl)
trimethylammonium bromide (CTAB) as the efficient reagent to regulate the
morphologies and sizes of HAp nanocrystals. The particle sizes of the HAp
spheres can be facilely regulated via changing the concentration of CTAB in
the calcium phosphate supersaturated solutions. Three different spherical-
like HAp nanoparticles with uniform and average diameters of 20±5, 40±10,
and 80±12 nm, respectively, were obtained under different concentrations
of CTAB. In contrast, HAp nanoparticles grown in the absence of organic
additives are typically rodlike particles with lengths of hundreds of nano-
meters and widths of tens of nanometers. The glutamic acid was used as
the additive to hydrothermally synthesize large-sized HAp whiskers with
length of 50 to 100 µm and width of 0.5 µm in a dilute reaction solution,
where the supersaturation degree with respect to HAp precipitation was
low. It was found that the different adsorption and desorption behaviors of
Glu occurred on (1 0 0) and (0 0 1) planes of HAp and resulted in the crystal
growth along the c-axis (Li et al. 2010). The peptides possess well affinity
with HAp, which has been developed to modulate the morphology and size
of the HAp crystals. The research of Diegmueller et al. (2009) showed the
type of charged peptide, peptidic molecular weight, and concentration on
the morphology and size of the HAp crystals. In their study, the poly-L-argi-
nine (Arg), poly-L-aspartate (Asp), poly-L-glutamate (Glu), and poly-L-lysine
(Lys) with different molecular weight were synthesized, and then were used
as the biomimetic analogs of noncollagenous proteins to investigate their
influence on mineral nucleation and growth kinetics, and crystal morphol-
ogy of HAp. The results showed that the negatively charged polymers (Asp,
Glu) and higher molecular weight (HMW) had greater affinity for HAp than
positively charged polymers (Arg, Lys) and lower molecular weight (LMW).
The poly-L-Asp LMW, poly-L-Glu LMW, and poly-L-Lys HMW were found
to significantly increase the aspect ratio in comparison to the control. Such
knowledge could be useful for identifying the peptides ideal for in vitro or
in vivo administration to customize crystal growth or for coating of implant
surfaces for improving anchorage to neighboring bone tissue.