Manipulation of microparticles by shaping the applied electric field in microelectrodes

1
Manipulation of microparticles by shaping the
applied electric field in microelectrodes Jorge
Quijano, Shalini Prasad Department of
Electrical Engineering, Portland State University
ABSTRACT A microscale platform consisting of a
planar array of microelectrodes has been
optimized for the electrical manipulation of
microparticles. This platform was used to observe
and characterize the effects of dielectrophoresis
and electrorotation in microparticles. By making
use of selected sets of voltage waveforms, the
electric field gradient induced in the
microelectrode array was shaped. Variations in
the voltage waveform were used as the controlling
parameter for positioning of microparticles in
specific areas of the device, without changing
its geometry. This waveform-oriented design of a
particle positioning system resulted in the
development of a multipurpose device as the same
platform was used to accomplish several electric
field micromanipulation functions. The
development of this device is targeted to build a
cost effective platform using simple electronic
components such as resistor-capacitor (RC)
networks and a commercial digital microcontroller
which can be employed to generate excitation
waveforms in a wide range of frequencies. The
eventual goal is the construction of a closed
loop system for tracking/positioning of single
particles.
II) Exponential waveforms in combination with
square waveforms
The distribution of microparticles in the
microelectrode array varied according to the
nature of the input waveform applied to the
array. Figure 4 shows the dielectrophoretic
process when a combination square
wave/exponential wave was applied to the MEA. It
was observed that after 40 s of the application
of the input signal, the average distribution of
the particles remained the same either for
sinusoid input or square/exponential input, but
the speed of the dielectrophoretic process was
improved when the asymmetric square/exponential
wave was applied to the microelectrode array
(approximately two times faster). This state was
identified as a steady state, because after
long periods (gt2 min), the microparticles
distribution did not undergo significant
variations under visual inspection
(B)
(C)
(A)
Figure 2. Experimental set up for positioning of
microparticles using dielectrophoresis forces. By
changing the applied voltage waveform and by
selecting the excited microelectrode, the
electric field distribution can be shaped to
produce different effects in the microparticles,
like translation and rotation.
(A)
RESULTS
MATERIALS AND METHODS
Waveform dependent particle manipulation
I) Sinusoid waveforms
Figure 4. (A). A combination of a bipolar square
wave and a bipolar exponential wave was applied
asymmetrically in four specific points of the
microelectrode array.
The 3x3 microelectrode array (MEA) composed of
platinum electrodes (80 µm diameter, with spatial
separation of 200 µm between elements) on a
silicon/silicon nitride substrate has been used
to generate non-uniform electric fields with
different distributions (Figure 1). Trapping of
microparticles in broad areas of the MEA was
achieved by exciting specific array elements.
A silicon chamber was placed over the MEA to
localize the liquid media over the electrode
platform and to maintain a stable
microenvironment. A function generator was
used to vary the applied input signal strength
and shape. Sinusoid and rectangular pulses were
utilized as excitation sources for electrical
manipulation of particles. A resistor-capacitor
filter was used to generate a third waveform
based on the signal from the generator. This
waveform has an exponential shape and is
associated with a faster decay/rise time compared
to a sinusoid. The combination of sinusoid,
square and exponential signals were used to
demonstrate and characterize particle
manipulation. The movement of the particles was
optically monitored with a CCD camera attached to
a compound microscope.
(E)
(D)
Two sinusoid waveforms with a phase delay were
applied to the microelectrode array as shown in
figure 3. The resulting electric field was a
vector with an angular frequency equal to the
frequency of the applied signal. The
dielectrophoresis process was more evident in the
vicinity of the microelectrodes were the
excitation voltage was applied.
Figure 7 (A) Microelectrode geometry for the
development of a multipurpose platform. The
features in (B), (C), and (D) are intended for
coarse positioning of groups of particles. (E)
The small features allow the manipulation of
particles as small as 10 µm. Once a group of
particles is trapped into the region of interest,
a constant voltage can be applied to the four
region microelectrodes, isolating the particles
for further manipulation with the fine adjustment
microelectrodes.
(B)
Figure 5 (A). Four delayed unipolar exponential
waveforms with 2 Vpp were applied symmetrically
to four specific points of the microelectrode
array. This example proved the potential use of
multiple digital signals for particle
manipulation. (B) The effect of the
dielectrophoretic force after 80 seconds produced
patterns not only around the excited
microelectrode, but also in the vicinity of the
metal track that interconnects the microelectrode
with the pad. The size of the microelectrode is
still too big compared with the particles, which
results in good control of regions but deficient
control of single particles.
Figure 4. (B) Theoretical representation of the
rotating nature of the electric field in two
dimensions by solving the partial differential
equation using Matlab.
SUMMARY AND CONCLUSIONS

• We have experimentally demonstrated the
  hypothesis that by selecting a set of waveforms
  and by controlling the timing and the terminals
  of the MEA where the voltage signals are applied,
  the electric field gradient can be shaped and
  manipulated. This results in a versatile platform
  that performs several functions like
  centrifugation, sorting, linear movement and
  positioning of particles in selected areas
  without any variations to the MEA geometry. The
  use of exponential waveforms instead of the
  traditional sinusoids has in general two
  advantages
• Since the voltage decay of the exponential
  waveforms is faster than that of sinusoids, the
  resulting electric field for a given
  configuration presents steeper gradients, which
  translates in faster dielectrophoresis processes.
  
• Generation of exponential waveforms with
  controllable frequency and phase can be easily
  achieved by using the digital outputs of any
  commercial microcontroller in combination with
  RC.
• The performance of such a device with
  multifunctional capabilities can be improved by
• Optimizing the design of the microelectrode array
• Variation in the MEA geometry for coarse
  positioning, fine movements, electrorotation and
  trapping.

Figure 3 (A). Two delayed sinusoidal waveforms
were applied asymmetrically in four specific
points of the microelectrode array.
Prototype system for the positioning of
bioparticles over microelectrodes
(A) Particle experiencing zero dielectrophoretic
force. In this case, the density of electric
field lines in both sides of the particle is
equal the net force experienced is zero.
Microelectrode arrays have proved to be a
convenient method for the recording of electrical
activity in cells to be used in biosensors, but a
reliable technique for positioning the
bioparticles onto the recording microelectrodes
has to be implemented. The effects of
dielectrophoresis and electrorotation can be
applied to achieve electrical self assembly of
microparticles in the device. By controlling the
temporization of a set of input signals applied
to the microelectrode array, the induced electric
field gradient can be shaped for coarse and fine
positioning of bioparticles. This
waveform-oriented design of particle positioning
systems results in a multipurpose device, as the
same platform can be used to accomplish different
effects. The development of this device is
targeted to build a cost effective platform using
simple electronic components such as RC networks
and a commercial digital microcontroller which
can be used to generate excitation waveforms in a
wide range of frequencies and phases. The
eventual goal is the construction of a closed
loop system for tracking/positioning of single
particles.
Figure 4. (C) The pattern followed by the
polystyrene beads is similar to the pattern shown
in figure 3, but the dielectrophoretic force is
clearly more intense, yielding a more defined
pattern.
Figure 3 (B) Theoretical representation of the
rotating nature of the electric field in two
dimensions by solving the partial differential
equation using Matlab.
(B) Particle experiencing positive
dielectrophoretic force. Due to asymmetry of the
density of electric field lines in both sides of
the particle a net force is produced. The
strength of this force is given by
III) Delayed exponential waveforms
The advantage of using delayed exponential
waveforms for the manipulation of microparticles
by dielectrophoresis is that multiple signals can
be generated by a combination of digital pulses
and a resistor-capacitor network. In this
experiment, four exponential signals were
generated with a 90 phase between each other.
The dielectrophoresis process was slower because
of the smaller amplitude used in the excitation
signals(2 Vpp), but this could be an advantage
when dealing with temperature sensitive
microparticles such as cells, because the heat
transfer to the liquid media is reduced by the
limited power of the excitation signals.
ACKNOWLEDGEMENTS
We would like to thank Portland State
University Faculty Enhancement Grant and ONAMI.
The author would like to thank the Fulbright
scholarships program for the sponsorship and
support given.
Figure 3 (C) The effect of the dielectrophoretic
force after 40 seconds produced patterns, which
evidence the regions were the electric field
gradient is more intense. The prototype model
comprised of polystyrene particles of 15 µm
diameter (permittivity of 2.56 and conductivity
of 10-16 S/m) suspended in de-ionized water.
Figure 1. The net force experienced by spherical
particles in suspension due to the gradient of
the electric field is known as dielectrophoretic
force, which is proportional to the radius (r) of
the particle, its permittivity, the
Clausius-Mosoti factor (Re(a)) and the gradient
of the squared magnitude of the electric
field(?E2).
REFERENCES
1 Madou, Marc. Fundamental of microfabrication.
pp570.