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Bidirectional field-flow
particle separation method
in a dielectrophoretic chip
with 3D electrodes
Date
：2012/12/24
Name ： Po Yuna Cheng(鄭博元)
Teacher：Professor Hsu
Outline
Introduction
Separation methods
Fabrication of the DEP device
Experiment
Conclusions
Introduction
This paper proposes a bidirectional field-flow separation method in a
dielectrophoretic chip with 3D electrodes.
The 3D electrodes structure that is used in this device is not only used for
generating an uniform DEP force across the microfluidic channel but also
for achieving a gradient of the velocity (and in this way a variable
hydrodynamic force) in the microfluidic device.
Separation methods
Bidirectional separation in a DEP chip with 3D electrode array
The separation method using DEP
chip with 3D electrode array can be
described in four steps (Fig. 2).
First, the solution with the mixture of two
particle populations is injected into the
microfluidic chamber. Secondly, by
applying an electric field the two
populations are separated into different
locations according to their electrical
properties.
In the third step, a fresh buffer solution flows
through in one direction, and one of the
population is collected at one of the outlets.
Finally, the second population is collected at
the second outlet by flowing the fresh buffer
in the perpendicular direction.
Fig. 2. Main steps of the separation technique.
Fig. 3. The electric field distribution in
the DEP structure.
As a result, these geometries lead to a huge
gradient in the flow velocity. The zone with higher
fluid velocity experiences larger hydrodynamic
forces (Zone A) increases and the particles that
are trapped using positive DEP force can be
removed.
While, in the low velocity regions (Zone B) the
hydrodynamic force is too weak to remove the
particles that experience negative DEP.
Fig. 4 shows an ANSYS simulation of
the flow through the electrode-pillars.
As a result, these geometries lead to
a huge gradient in the flow velocity.
Fabrication of the DEP device
Four holes were first etched through in a glass wafer
with a thickness of 500 um for the inlet–outlet access of
fluid using a low-stress amorphous si/si
carbide/photoresist mask (Fig.5a).
After that a 4 in. silicon wafer with a thickness of 100m
was wafer-to-wafer anodically bonded to a 4 in.Pyrex
glass wafer with a thickness of 500 um at 305◦C with an
applied voltage of 800V for 20 min (Fig. 5b).
Next, the microchannel walls and electrode array were
defined in the silicon wafer using deep RIE technology
through a silicon oxide mask (Fig. 5c).
Next, the top glass wafer with two inlet/outlets was
bonded to the patterned silicon wafer using second
anodic bonding at 400◦C with an applied voltage of
1200V and an applied force of 1500N (Fig. 5d).
Subsequently, the bottom glass wafer was thinned from
500um to around 100m using an optimized wet etching
process in a HF 49%/HCl 37% solution (10/1) (Fig. 5e) .
The via holes were wet etched through the thinned
bottom glass wafer in the same solution using a lowstress amorphous silicon mask, to provide a path to
connect the outside metal electrodes and silicon
electrodes (Fig. 5f).
Fig. 5. Fabrication processes of DEP chip.
Experiment
The feasibility of the separation mechanism in the DEP chip with 3D electrode array was
proved using populations of viable and non-viable yeast cells were used.
100 mg of yeast, 100 mg of sugar and 2ml DI water were incubated in an Eppendorf
tube at 37 ◦C for 2 h. Next, the cell culture was divided into two with one population
being boiled for several minutes in 5ml boiling DI water (dead cells).
Both viable and non-viable populations were mixed and resuspended in the separation
buffer, which was a mixture of phosphate buffered saline (PBS) and DI water. The
conductivity of the separation buffer was around 20 uS cm.-1.
The final concentration of the cells was 107 cells/ml. A function generator and a linear
amplifier were used for the drive signal generation of the dielectrophoretic chip.
The drive signal was increased from 0 to 25V peak to peak gradually with the signal
frequency being in the range of 20–100 kHz.
Fig. 7 showed the bidirectional separation
processes for viable and non-viable yeast cells in
a DEP chip with triangular electrode array.
The mixture of viable and non-viable yeast cells
was injected into the microchannel (Fig. 7a);
non-viable yeast cells experiencing positive DEP
move to the highest electric field regions and
viable yeast cells experiencing negative DEP
move to the lowest electric field regions (Fig. 7b);
next, non-viable yeast cells were collected from
the other one outlet by flushing a fresh buffer
solution (Fig. 7c).
Finally the other population is collected at the
opposite outlet using same method.
Fig. 7. Optical image with the main steps of the separation technique: (a) insertion
of dead and live cell, (b) trapping of the cells by positive and negative DEP
and (c) collection of one cell population by flowing a fresh buffer solution.
Conclusions
This paper proposed a bidirectional separation method in a DEP chip with 3D electrode
array, which also functions as microfluidic channels.
These electrodes serve a double function. The first function is to generate positive and
negative dielectrophoretic force, trapping two populations of cells in different locations.
The second function is to produce a gradient of fluid velocity.
As a consequence, the resulted hydrodynamic force will drag out the population trapped
by positive dielectrophoresis. After the removal of the electric field, the remaining
population can then be collected at the outlet.
Thank you for your attention