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Engineering, 2012, 5, 84-87
doi:10.4236/eng.2012.410B022 Published Online October 2012 (http://www.SciRP.org/journal/eng)
Copyright © 2012 SciRes. ENG
Partial Reflection of Hyal uronan Molecul es Inside the
Taylor-cone During Electrospray
School of Biology, Hunan University, Changsha, Hunan, China
Electrospray (ES) is of great interest in research for its finely controlled delivery of molecules. This study investigated mechanisms
involved in the electrospray of biological macromolecules which may cause spraying instabilities. Hyaluronan (HA) has been studied
for its biological significance. A mixture of ethylene glycol and deionized water with volume ratio of 1:1 is used to dissolve HA mo-
lecules. HA solutions with different concentrations and molecular sizes are investigated. Experimental results demonstrate that the
molecular size and solution concentration of macro-molecules are critical factors affecting the spraying process. A concentration
reduction of HA molecules happened during the ES process, and a hypothesis on partial reflection of HA molecules inside the Tay-
lor-cone is pre sented i n the study.
Keywords: Electro s pray; Macro-molecules; Reflection; Talor-cone
With the emergence of nanotechnology, researchers become
more interested in the electrospray (ES) technique in recent
years due to its versatility and potential for applications in di-
verse fields [1,2]. By applying a suitable voltage to a conduct-
ing liquid supplied into a capillary, the liquid meniscus will
take a form of cone [3,4]. Further increasing the electric field
on the surface of the liquid to overcome the surface tension, a
controlled μm-sized jet will emerge from the tip of the liquid
cone whi ch i s called the st abl e cone-jet mode . Consequently,
by manipulating the microscopic fluid jet and ejected charged
droplets onto a desired place, an e-jet del ivery or d eposit ion can
be achi eved. By using micro-sized fine nozzles, ES can oper ate
at a low flow rate, which accordingly makes t he delivery of tiny
volumes possible .
The electrospray of macromolecules has seen a tremendous
increase in research and commercial attention over the past
decade due to a number of applications such as in fabrication of
enzyme elect ro des [ 6] , micro-cel ls for bi och emical reacti on s [7 ,
8] and thin polymer films . In this study, molecules of Hya-
luronan (HA, i.e. hyaluronic acid) are chosen to be investigated
due to its unique rheological properties and complete biocom-
patibility. As a naturally occurring linear polysaccharide, HA is
popularly found in connective tissues such as umbilical cord,
synovial fluid and vitreous etc. Meanwhile, HA has been
widely used in fields of drug delivery, cell encapsulation and
tissue r egeneration . The chemical stru cture of H A consists
of alternating disaccharide units of D-glucuronic acid and
N-acetyl-D-glucosamine. Molecular weight of HA can vary
from 5 kDa to 2.1 MDa in vivo which will result in varied mo-
lecular s izes .
Increasingly considerable attentions has been received
nowadays for the electrospray of HA solutions into types of
microfibers or membranes [10,12]. However, how to achieve
stable electrospray of HA solutions still remains problematic
due to uncertain reasons. Fluid motions within the Taylor-cone
and the way that carries HA macro-molecules is of some rele-
vance in understanding of HA spraying jet stability controlling
difficulties. These motions during electrospray are driven by
the tan gential electrical stress acting o n the liquid-gas int erface
and the flow rate injected through the electrified emitt er [1 3].
Based on work of Hayati, Barrero and Sheldon for liquid
motions inside the Taylor-cone containing both experimental
results and theoretical modelling, there is a recirculating me-
ridional motion during the ES process, liquid moves towards
the ap ex along th e generatr ix and away fro m it alon g th e axis as
shown in Figure 1 [13-16],. Their results show that only fluid
particles lying close to the surface are ejected through the jet
while th e rest r ecircu lates to ward s th e apex al on g th e generat rix
and away from it along the axis [13,17]. When solutions of HA
macro-molecules are sprayed under stable cone-jet conditions,
the anticipated molecules movement could be as shown in Fig-
ure 1. HA molecules move following liquid towards the apex
and away from it along the axis [15,16].
Figure 1. Streamlines and movements of HA molecules within the
Taylor-cone during the electrospray: Meridional circulation of
liquid with molecules towards the apex along the generatrix and
away fr om th e apex along the axis .
Z. K. TAN
Copyright © 2012 SciRes. E NG
During the process, only that fraction of HA molecules lying
close to the surface are ejected through the jet while the rest
recirculated towards the apex away from it along the axis [13,
17]. This recirculation may cause a back diffusion movement of
HA molecules, and with a higher HA concentration nears the
tip which would result in partial reflection of HA molecules
durin g the electr ospray. It is possible then that this process may
cause in stabilities t o the ES process and a con centration reduc-
tion of HA molecules after t he electrospray process.
2. Materials and Methods
Hyaluronan powders (sodium salt, molecular weight: 2.1 MDa)
were purchased from Genzemy Co. (USA) and diluted in eth-
ylene glycol and water (EG/H2O, volume ratio=1) with differ-
ent concentrations (0.1, 0.2 and 0.4 mg/ml). A sonication
method was used to r educe the mo lecular si ze and chain length
of H A [ 18]. After so nicati on, average molecul ar weights o f H A
were measured (200 kDa, 390 kDa and 1000 kDa) by size-
exclusion high performance liquid chromatography (HPLC).
In order to examine the partial reflection of HA molecules,
experiments were undertaken to investigate whether the elec-
trospray process was acting as a filter of HA molecules or not.
The hypothesis is that if the partial reflection takes place then
downstream of the Taylor cone, liquid collect from the jet end
may have a lower concentration of HA molecules than in the
initial fluid solution. Figure 2 shows the experimental sche-
matic of the ES apparatus for the study. The main components
consist of fluid supply system, spraying nozzle, high voltage
power supply, current measuring unit and an imaging system to
monitor the spraying cone and jet.
Nozzles were made from glass tubes. A micropipette puller
P-97 (Sutter Instrument Co.) was used to produce tips of noz-
zles down to 30 μm. Liquid contained in a small reservoir was
forced through a narrow silica tube into the nozzle by pressur-
ized nitrogen gas. All experiments were completed at atmos-
pheric conditions at the emitter tip, with the same initial fixed
pressure. The change in pressure as the reservoir drained was
insignificant, as each spray used less than 0.5 ml, which re-
sulted in a change in heigh t less than 1 mm.
The power supply (F.u.G. Electronik) can provide a voltage
up to 6 kV. The spray distance between the nozzle tip and the
Figure 2. Expe rimen t al schemat ic for el ect rospr a y of HA s olutions.
substrate was set to 3 mm. A voltage ranged in 2.3 ~ 2.5 kV
was applied to produce a stable co ne-jet electrospray. The slight
adjustment of the applied voltage was made to maintain a
spraying due to different physical properties of solutions. The
current was determined by measuring the voltage drop across a
100MΩ resistor. An ISO-TECH IDM 207 voltmeter logged the
voltage to a PC at a freq uency of 2 Hz.
The jet during an electrospray process was monitored by an
optical system based on a microscope. Images were recorded
and captured by a computer. The system consists of a combined
zoom lens and a CCD camera (UEYE). An infinity corrected
objective lens from Mitatoyo and a variable zoom from Thales
Optem were used. The optical resolution can reach to 1μm.
Additionally, a cold light source was used for illumination.
3. Results and Discussion
Figure 3 shows optical micrographs of spraying pure EG/H2O
solvent and HA solutions (molecular weight: 1000 kDa) with
concentrations of 0.1, 0.2 and 0.4 mg/ml. Respectively, stable
spraying jets were observed in HA solutions with concentra-
tions of 0, 0.1, and 0.2 mg/ml. However, further double the
concentration up to 0.4 mg/ml the stable jet was not observed,
which clearl y indicated that the stable electrospray process can
be hampered by adding HA macromolecules.
Varied concentrated HA solutions (Cin) with different mo-
lecular sizes were electrosprayed at stable cone-jet mode for
further investigations. Solutions of HA 1000 kDa molecules
were only be cone-jet electrosprayed when Cin < 0.4 mg/ml
while other smaller HA molecules (200 kDa and 390 kDa)
could reach the stable cone-jet electrospray at the highest con-
centrati on. Molecular si ze and concentration do affect th e elec-
trospray stability of HA solutions.
Samples of liquid from downstream spraying jet were col-
lected every 20 minutes from the plate electrode after cone-jet
electrospray have been reached, and then concentrations of
samples (Cout) were measured. Concentration change R was
calculat ed as following:
Figure 3. Microphotography images showing the electrospray of
solutions: a) electrospray of pure EG/H2O solvent. b)-d) electros-
pray of HA (1000 kDa) solutions with a concentration of 0.1, 0.2
and 0.4 mg/ml, respectively.
Z. K. TAN
Copyright © 2012 SciRes. ENG
Figure 4 gives results of HA concentration changes after 20
minutes cone-jet electrospray under varied initial concentra-
tions. After electrospray, measured concentrations of HA have
lower values, there is an obvious concentration reduction which
is up to 30% (HA 1000 kDa) from Figure 4.
This is consistent with the hypothesis that a partial reflection
of HA macro-molecules takes place during the cone-jet elec-
trospray process. Figure 4 also indicates that varied molecular
sizes of HA result in different concentration reduction values
after spray at the same concentration, a larger HA molecular
size will result in a greater reflection rate. The reduction value
of the largest HA molecule (1000 kDa) is almost three times
greater th an the o ne cau sed by the smallest molecu le (20 0 kDa)
as listed in the figure. The size o f HA molecules do es affect the
reflection process and a higher concentration would also
slightly increase the concentration reduction value after ES for
all HA molecules. Figure 5 shows that the reduction value
decrease with spraying ti me, which indi cated that the reflect ion
of HA molecules is not constant during the electrospray.
Concentration reduction results of HA molecules after elec-
trosp ray demonstrat e the hypo thesis th at HA molecules create a
higher con cen tratio n layer near th e Tayl or-cone tip which cause
Figure 4. Concentration reductions of HA molecules after 20 min-
utes spray under differe nt initial concentrations.
Figure 5. Concentration reductions of HA (0.1 mg/ml) solutions
after elect r o sp ra y as a fu n ct ion o f s p r a ying time.
molecules back diffusion during electrospray process. This HA
partial reflection pro cess is depend ent of molecular sizes, so lu-
tion concent rations and spraying time.
The electrospray of HA molecules is investigated in the study,
and a partial reflection of HA molecules is found during the
stable cone-jet electrospray. A hypothesis which describes the
partial HA reflectio n is develo ped. It indicates the build-up of a
concentrated HA layer near the Taylor-cone tip during elec-
trospray and the development of HA concentration changes
with spraying time. This work elucidates the mechanism may
responsible for instability of electrospray process when using
HA macro-molecules solutions. The concentration reduction
during electrospray of macro-molecules has important implica-
tions for applications of electrospray. For example, this process
may affect t he signal when use E S for mass spectrometry, it can
also affect in the fields of drug delivery, cell en capsulation and
tissue engineering when using the ES technique.
The author acknowledges the financial support of Hunan Uni-
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