Biomedical Engineering Reference
In-Depth Information
The sol-gel route is based on controlled hydrolysis and condensation of alkoxides to form a suspen-
sion of colloidal particles (sol), which upon polycondensation forms an interconnected network
structure (gel). However, the particle size of the traditional sol-gel-derived bioactive glasses was
larger than 1
m.
The synthesis of bioactive glass uses typical precursors like tetraethyl orthosilicate (TEOS), calcium
nitrate (CN), and triethylphosphate (TEP). Particle diameter can be controlled by fine-tuning variable
parameters, such as reagent concentration and reaction temperature. According to Bogush and Zukoski
(1991), five parameters play an important role in the size and distribution of silica nanoparticles:
(i) concentration of TEOS, (ii) concentration of ammonia, (iii) concentration of water, (iv) alcohol
effect, and (v) reaction temperature [11,12] . Hong et al. (2009) developed bioactive glass nanoparticles
by combining the sol
μ
gel method and co-precipitation. In that study, the mixture of precursor was
hydrolyzed in acidic medium and condensed under alkaline conditions separately [13,14] .
St¨ber et al. (1968) reported on a pioneering method for the synthesis of spherical and monodisperse
silica nanoparticles from aqueous alcohol solution of silicon alkoxides in the presence of ammonia as a
catalyst from which different sizes of silica particles were obtained [15] .TheSt ¨ ber method can be
modified to produce particles with composition different from those of pure silica. Oliveira (2011)
synthesized bioactive glass nanoparticles based on the St ¨ ber method [16] ( Figure 15.1 ).
Chen et al. (2009) investigated the effects of different morphologies on the in vitro bioactivity
of nanosized bioactive glass particles in the system CaO
SiO 2 by using lactic acid in the
sol gel method and concluded that not only the surface area but also the surface morphology play
an important role in bioactivity of the material [17] .
Nanosized particles contain a large surface area that results in high interfacial energy and
thermodynamic instability. In general, these particles tend to agglomerate during the synthesis of the
particles in an attempt to minimize the free energy of the system. To maintain the stability of the parti-
cles, some studies focused on obtaining spherical bioactive glass particles to create a bioactive material
with dispersion capability under dilute aqueous and alcoholic conditions [18] . Spherical bioactive glass
nanoparticles developed and characterized by Oliveira (2011) are shown in Figure 15.2 [16] .
Several techniques were developed in which SiO 2
P 2 O 5
P 2 O 5 bioactive glass nanoparticles
were prepared. The best bioactivity results were from ternary systems with spherical particles
below 50 nm produced with a pH of 11.5. El-Kady et al. (2010) prepared particles, comprised of
60 wt% SiO 2 , 36 wt% CaO, and 4 wt% P 2 O 5 , with sizes less than 100 nm using alkali-mediation
with the application of moderate ultrasound dispersion combined with mechanical agitation as well
as the addition of ethanol as a dispersant [19] . Previous studies have shown that low concentrations
of TEOS and water provide monodispersed uniform-sized nanoparticles [11,12,15,20
CaO
25] . The size
of the nanoparticles was found to increase with TEOS and with increasing concentrations of water
and ammonia of up to 7 M and 2 M, respectively [20] . Particles prepared in methanol solutions are
the smallest, and the particle sizes increase with the increasing chain length of alcohol. The particle
size distribution also becomes broader when longer chain alcohols are used as solvents. Particles
prepared in methanol and ethanol-glycerol solutions resulted in a stable sol. However, when butanol
and ethanol are used, precipitation could easily be observed [11,12,20,24,26] .
The ammonia-catalyzed reactions of TEOS in ethanol with water (St ¨ ber method) can be used
for the preparation of monodispersed spherical nanoparticles bioactive glass. Alcohol is added to
prevent the liquid
liquid phase separation during the initial phase of the hydrolysis reaction [27] .
Methanol is chosen as the solvent medium to act as a surfactant and promote the production of
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