Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
**
⎡ − εε ⎤
mp
f = ⎢ ⎥
(2)
CM
**
⎢⎣
+ 2εε
mp
⎥⎦
*
*
where ε and
p
ε are the complex permittivity of the
m
particle and the medium, respectively. The complex
permittivity is related to the conductivity σ and angular
frequency ω, which relates to ω = 2πf
, through the
formula:
σ
* εε j−≡
ω
(3)
where j equals − 1 . Therefore, the DEP force is
dependent mainly on difference between the dielectric
properties of particles and the suspending medium solution.
It can be either a positive DEP, which pulls the particle
toward the location of the high-electric field gradient, or a
negative DEP, that repels the particle away from the region
of high-electric-field gradient.
The schematic diagram of the cDEP microfluidic chip is
depicted in Fig. 1. Four insulating structures which formed
an X-pattern in the microchannel, as shown, were employed
to squeeze the electric field in a conducting solution, thereby
generating the regions high-electric-field gradient. The inlet
flow field and the electric field were applied vertically. The
negative-dielectrophoretic particles were repelled from the
high- electric-field region, moving to the center of the
microchannel where the flow velocity was higher. The
insulator was designed 60 μm in width and 200 μm in length.
The distance of the insulators along the direction of flow, as
well as along the direction of the electric field, was 120 μm.
The inclined angle of the insulator was 45 degrees. The
particles were introduced into the microchannel and
pre-confined hydrodynamically by the funnel-shaped
insulating structures close to the inlet. The particles with a
negative dielectrophoretic response were repelled toward the
center of the constricting region. The electric field in the
main microchannel was generated by using electrodes
inserted into two conductive micro-reservoirs, which were
separated from the main microchannel by thin insulating
barriers made of 20 μm-width of PDMS [7].
III.
EXPERIMENTAL SECTION
A. Chip Fabrication
A biocompatible material of polydimethylsiloxane
(PDMS) was adopted for cDEP microfluidic chip. The mold
master was fabricated by using the inductively coupled
plasma (ICP) dry etching technique on the silicon wafer
(around 100 μm in height) to define the micropatterns, as
showed in Fig. 2a. The PDMS prepolymer mixture was
poured and cured on the mold master to replicate the
structures (Fig. 2b). After the PDMS replica had been peeled
off, the replica was bonded with the glass substrate after
treatment of the oxygen plasma in the O 2 plasma cleaner
(Model PDC-32G, Harrick Plasma Corp. Ithaca, NY, USA).
The image of the fabricated microchip for particle focusing
taken by the optical microscope was revealed in Fig. 2c.
B. Apparatus and Materials
A function/arbitrary waveform generator (Agilent
33220A, Agilent Technology, Palo Alto, CA, USA) was
employed as the AC signal source and connected to an RF
amplifier (HSA-4011, NF corporation, Japan) to apply
electric fields required for dielectrophoretic manipulation in
the microchannel. Polystyrene particles, 10 mm in diameter,
(G1000, Thermo Scientific Inc., USA) were used to
investigate the efficiency of focusing. A sample of
polystyrene particles with a concentration of 10 6
particles/mL was injected using a syringe pump (Model KDS
101, KD Scientific Inc., Holliston, MA, USA). The
dielectric permittivity and conductivity of polystyrene
particles at the frequency of 1 MHz are about ε r =2.6; σ=10 -16
S/m, respectively [9]. The particles are suspended in a
sucrose solution with an 8.62 wt% and 2.74×10 -2 wt% of
K 2 HPO 4 (ε r =78; σ=4.50×10 -2 S/m). The dielectrophoretic
focusing of particles was observed and recorded by an
inverted fluorescence microscope (model CKX41, Olympus,
Tokyo, Japan) mounting a CCD camera (DP71, Olympus,
Tokyo, Japan) and a computer with Olympus DP controller
image software.
IV. RESULTS AND DISCUSSION
To investigate the capacitive behavior of the PDMS barrier,
the Wayne Kerr precision impedance analyzer 6420 was
used to measure the impedance of the PDMS in DMEM
medium (Gibco, Grand Island, NY, USA), which was
commonly used in cell culture. The micro-reservoirs were
filled with DMEM medium with an electric conductivity of
0.8 S/m; besides, this medium in light red color could be used
for confirm that there was no leakage from insulating barriers.
The experimental set up was depicted in Fig. 3. The
measured impedance of the PDMS barrier under different
frequencies and the fitting curve to experimental data using
the least-squares method were depicted in Fig. 4. The results
revealed the capacitive behavior of the PDMS barrier. The
impedance decreased with the frequency. The relative
dielectric permittivity and conductivity of PDMS at a
frequency of 1 MHz were 10.46 and 7.6×10 -4 S/m,
respectively. The numerical simulations of the electric and
flow fields, as well as the particle trajectory, were performed
using the commercial software CFDRC-ACE + (ESI Group,
France). Fig. 5 showed the transient simulation of the tracks
of latex particles (ε r =2.6; σ=10 -16 S/m) [9] under varying
electric field strengths and inlet velocity. The increase in the
applied electric field significantly enhances the performance
of focusing. Furthermore, decreasing inlet velocity increases
the efficiency of focusing because the higher velocity results
in more lateral expansion. Experimental results of focusing
of fluorescent latex particles at different inlet flow rates and
under varying electric field strengths of a frequency of 1
MHz were demonstrated the performance of focusing, as
showed in Fig. 6. The latex particles of 10 μm in diameter
suspended in the sucrose medium with 8.62 wt% and
2.74×10 -2 wt% of K 2 HPO 4 (ε r =78; σ=4.50×10 -2 S/m) were
used to investigate the efficiency of focusing. The sample of
latex particles was injected using a syringe pump. The
experimental results showed that the performance of
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