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Gabriela(Ruphuy(et(al.((
1(
ICBMC’14,)International)Conference)on)Biobased)Materials)and)Composites,)May)13;16)May)2014,)Montreal,)Canada)
(
Preparation of hydroxyapatite nanodispersions in the presence of chitosan by
ultrasonication
Gabriela RUPHUY1*, José Carlos B. LOPES1, Madalena M. DIAS1, Maria Filomena BARREIRO2
1 Laboratory of Separation and Reaction Engineering (LSRE) – Associate Laboratory LSRE/LCM, Faculty of Engineering,
University of Porto, Porto, Portugal
2!Laboratory of Separation and Reaction Engineering (LSRE) – Associate Laboratory LSRE/LCM, Bragança Polytechnic Institute,
Bragança, Portugal.
*Corresponding author: gabriela.ruphuy@fe.up.pt
Abstract. In this work, aqueous nanodispersions of hydroxyapatite (HAp) alone or in the presence of
chitosan (CS) were produced by ultrasonication. The produced samples were analyzed in what concerns
particle size and zeta potential, then related with the process parameters, ultrasonication power and time,
variables related with the energy delivered to the sample. Special emphasis was given to the production of
HAp/CS dispersions using a ratio of 70/30 (w/w) since it represents the typical bone composition. HAp
concentrations of 20, 6.4 and 1.6 g/l were tested.
Introduction
Nowadays, many commercialized products
contain micro- and nanoparticles that need to be
homogenously dispersed in aqueous solutions.
However, nanoparticles tend to form strong
agglomerates due to interaction forces, usually of
van der Waals type, which, in relation to
gravitational forces, increase as the particle size
decreases [1].
Ultrasound has become a common laboratory
method, used either for particle’s size reduction or
agglomeration disruption. As the market for nano-
sized materials grow, the demand for these
technologies increases, not only at laboratory
scale but also at production level. At this stage,
energy efficiency becomes important; hence, the
optimization of the process is fundamental to
reduce operational costs.
The application of ultrasound to disperse
inorganic nanoparticles into organic components
has been widely used. One of the main challenges
of designing hybrid inorganic–organic systems is
controlling the mixing between the two dissimilar
phases, which affects the mechanical, optical and
electrical properties of the final material [2]. A
common hybrid system suitable for bone repair
consists of biodegradable polymers with
ceramics. Chitosan has attracted great attention
for orthopedic applications thanks to its
osteoconductive property. It has been studied in
combination with hydroxyapatite, resulting in
materials with enhanced mechanical strength.
Advances in nanotechnology have increased the
interest on the use of micro- and nanoparticles to
prepare the hybrids. It is expected that with
smaller particle size, more homogenous
dispersions in CS solutions can be obtained and
with it, a better mechanical stability [3,4].
In this work, the effect of the energy applied by
ultrasonication on the particle size and zeta
potential of HAp dispersions was studied under
controlled conditions.
Experimental
Chitosan 90/200/A1, corresponding to flakes with
size <200 µm, deacetylation degree of 93.1%, and
dynamic viscosity of 135 mPa·s (1% at 20 C in
1% acetic acid solution) was acquired to Biolog-
Biotechnologie GmbH (Germany).
Hydroxyapatite aqueous paste (15% wt.,
Ca10(PO4)6(OH)2) from Fluidinova S.A. was used
to prepare the dispersions.
Preparation of the dispersions. Aqueous
nanodispersions of hydroxyapatite (HAp) alone or
in the presence of chitosan (CS) were prepared at
the concentrations of 20, 6.4 and 1.6 g/l, based on
the free mean paths (∆x/L) calculated [5] (0.70,
1.5, and 3.0 respectively).
In the case of the HAp/CS dispersions a mass
ratio of 70/30 was considered since it represents
the typical bone composition. Special care was
taken to assure that both, the geometry of the
vessel and the position of the probe’s tip, were
fixed. Geometric factor affects the amount of
energy absorbed by the particles due to their
Gabriela(Ruphuy(et(al.((
2(
ICBMC’14,)International)Conference)on)Biobased)Materials)and)Composites,)May)13;16)May)2014,)Montreal,)Canada)
(
relative position. Samples were subjected to
ultrasound during different periods of times of at
50% amplitude, using an Ultrasonic Processor
(Sonicator) QSonica Q700.
Characterizations. Mean particle size and zeta
potential were measured by dynamic and
electrophoretic light scattering respectively, using
a Zetasizer Nano ZS from Malvern Instruments,
model ZEN 3600 with a 633 nm ‘red’ laser.
Temperature and pH were measured before and
after ultrasonication.
Results and discussion
Effect of ultrasonication time. Dispersions of
HAp alone (Fig. 1) subjected to ultrasound
showed that at a relatively low energy input
(processing times of 10 to 30 s), large
agglomerates were completely broken down into
smaller components. Zeta potential data exhibited
low variability for all samples, with values
between 28 and 34 mV.
Fig. 1. Mean particle size of HAp dispersions at
50% amplitude versus time of ultrasonication.
The tendency of mean particle size to decrease
with ultrasonication time is also observed for the
HAp/CS dispersions (Fig. 2). However, results
suggest that HAp nanoparticles remain as clusters
when dispersed in CS, but with a good stability
since zeta potential remained nearly constant at
values between 30 to 32 mV. The mean size of
the agglomerates is higher as the concentration of
HAp dispersed in CS increases. Measurements of
HAp/CS dispersions with a concentration of 20
g/l were unsuitable for DLS.
Energy balance. The results obtained from the
energy balance implied that the energy lost as
sound and heat to the environment takes
significant values (higher than 100 J/g) after 50 s
of ultrasonication (50% amplitude). This confirms
that applying ultrasound (50% amplitude) to HAp
dispersions for higher times does not result in
significant differences in terms of particle size
and zeta potential.
Fig. 2. Mean particle size of HAp and HAp/CS
dispersions with concentrations of 1,6 g/l and 6.4
g/l (50% amplitude) versus time of
ultrasonication.
Conclusions
Hydroxyapatite nanodispersions in the presence
of chitosan were prepared by ultrasonication.
Such dispersions presented good stability even
when nanoparticles were proven to remain as
clusters. The study indicated an optimum energy
input being that, for all cases, mean particle size
presented low variability at ultrasonication times
of 50 s and higher, showing that the break up of
clusters does not improve significantly, and even
there was a risk of reagglomeration during a long
ultrasonication (processing times > 100 s). This
was confirmed by the energy balance study.
Acknowledgments
This work was funded by Fundação para a
Ciência e a Tecnologia of Portugal (FCT),
Universidad de Costa Rica (UCR) and Ministerio
de Ciencia, Tecnología y Telecomunicaciones de
Costa Rica (MICITT).
References
1. Sauter, C., Emin, M. A., Schuchmann, H. P. &
Tavman, S., Ultrason. Sonochem. 15, 517–23
(2008).
2. Kickelbick, G., Prog. Polym. Sci. 28, 83–114
(2003).
3. Dorozhkin, S. V., Biomatter 1, 3–56 (2011).
4. Hein, S., Wang, K., Stevens, W. F. & Kjems,
J., Mater. Sci. Technol. 24, 1053–1061 (2008).
5. Fonte, C. M. [PhD] 197-198 (2013).
