HIGH CONSTRICTION RATIO CONTINUOUS INSULATOR BASED
DIELECTROPHORETIC PARTICLE SORTING
Q. Wang and C.R. Buie
Massachusetts Institute of Technology, USA
ABSTRACT
Low frequency insulator based dielectrophoresis (iDEP) is a promising technique to study cell
envelope electrical properties. Although cell discrimination based on their size and species has been
reported, high sensitivity isolation of cells with diverse surface phenotypes is still challenging. Previously
we have demonstrated that iDEP can achieve selective immobilization of Pseudomonas aeruginosa at
strain level. In this paper, we investigate the potential for iDEP to achieve continuous cell separation
based on cell surface dielectric properties. Successful continuous phenotypic-based cell separation would
facilitate rapid analysis of clinically relevant microbial properties for point of care diagnostics.
KEYWORDS: Insulator based dielectrophoresis, Particle separation, Surface polarizability
INTRODUCTION
The integration of dielectrophoresis (DEP) into microfluidic systems opens the possibility of portable,
rapid, and label-free cell separation with small sample volumes, while avoiding the need for laborintensive operations. However, increasing separation sensitivity, i.e. to increase the electrical field
gradient, without inducing strong joule heating effects is challenging. High purities have been achieved
for sized based particle separation using conducting sidewall PDMS electrodes[1] and ridged insulating
microstructures[2]. Additionally, live/dead cell sorting has been achieved utilizing contactless DEP[3],
and gram positive/negative bacterial trapping demonstrated using insulating post arrays[4]. The ability to
discriminate cells with diverse surface properties but similar sizes would offer a more significant impact
on accurate biochemical analysis and clinical treatment. However, such systems would require higher
separation sensitivity than has been achieved previously with iDEP devices. In our recent work, we have
demonstrated that three dimensional insulator based dielectrophoresis (3DiDEP) can be a promising
technique for strain level bacteria separation[5, 6]. In this paper, we evaluate the potential for cell
envelope phenotyping using a 3DiDEP based continuous cell separation platform.
THEORY
A
Reservoirs
B
C
F
Fluid Flow
F
Top Chip
F
Bottom Chip
F
DEP
F
a
D
b
DEP
y
x
D
F
D
DEP
D
c
Figure 1: a) Microfabrication process. b)Micrograph of the 3DiDEP channel geometry. c) Schematic
force diagram. Also illustrated are the distribution of the y component of the gradient of electric field
squared, y(EE) (light colors indicate high intensity) and streamlines of the fluid flow originating from
the sheath flow inlet A (yellow) and the particle inlet B (red). Large (red) and small (green) particles
experience a DEP force (FDEP) and a Stokes drag force (FD) in the vicinity of the constriction. Scale bar,
(a) 1 cm, (b) 150 m.
The 3DiDEP device is fabricated on a PMMA chip with two inlets and two outlets bridged by a three
dimensional insulating constriction (Figure 1a and c). The cross-sectional areas of the constriction and the
lower outlet are 50 × 80 m2 and 600 × 500 m2, respectively, resulting in a high electric field gradient
978-0-9798064-7-6/µTAS 2014/$20©14CBMS-0001 2465
18th International Conference on Miniaturized
Systems for Chemistry and Life Sciences
October 26-30, 2014, San Antonio, Texas, USA
perpendicular to the streamlines in the fluid flow. Particles moving in the microchannel mainly
experience two forces, the DEP force (FDEP) and Stokes drag force (FD) due to background fluid flow. In
the vicinity of the constriction (Figure 1c), the negative DEP force experienced by more polarizable
particles can exceed the drag force, repelling the particles towards the upper streamlines. For less
polarizable particles, the drag force dominates particle motion and drives them into the lower outlet.
EXPERIMENTAL
Inlet
Outlet
a
b
c
d
e
f
g
h
i
Figure 2: Fluorescence time-lapse image sequences for motion of (a-c) 6 m beads, (d-f) 1 m beads,
and (g-i) both 6 m (red) and 1 m (green) beads near the constriction region. Each experiment was
conducted in triplicate under a 500 Hz AC signal with RMS voltages of (a, d, and g) 64 V, (b, e, and h) 81
V, and (c, f, and i) 99V. (Scale bar, 100 m).
The performance of the 3DiDEP microfluidic device was tested by separating polystyrene beads with
diameters of 6 m and 1 m. The polystyrene beads were suspended in a pH 7 diluted PBS buffer with a
conductivity of 500 S/cm. We first tested the DEP behavior of the large and small beads in the 3DiDEP
channel separately at a low frequency of 500 Hz under three applied RMS voltage levels, 64 V (low),
81 V (medium), and 99 V (high). Then with the same electric field conditions, the DEP response of a
mixture of the two-sized beads was observed. The densities of the 6 m and 1 m bead suspensions were
3.54 × 106 and 3.6 × 106 per ml, respectively. Each test was repeated three times with a total flow rate of
72 l/hr. To evaluate the separation purity of the device, time-lapse image sequences were recorded and
the number of particles passing the inlet and outlet regions (illustrated by a dashed red box in Figure 2a)
was counted for three minutes. A separation parameter, , is defined as the average number of particles
counted at the outlet region divided by the number counted at the inlet region. Experimentally observed
particle motion is shown in Figure 2.
RESULTS AND DISCUSSION
Figure 3 shows the calculated for both of the large and small particles tested separately (Figure 3a)
and the particle mixture (Figure 3b) under different voltage conditions. We observed that with increasing
2466
100
80
60
40
20
0
6 μm particles
1 μm paricles
75%
63%
26%
22%
𝛼 (%)
𝛼 (%)
applied voltage, more particles can be deflected across the streamlines towards the upper outlet, leading to
a higher However, larger particles are more polarizable, corresponding to a higher than the smaller
particles. Ultimately, we obtain an enriched large particle suspension at the upper outlet and an enriched
small particle suspension at the lower outlet.
26%
0%
Low
Voltage
Medium
Voltage
High
Voltage
a
100
80
60
40
20
0
6 μm paricles
1 μm paricles
77%
52%
19%
19%
15%
3%
Low
Voltage
Medium
Voltage
High
Voltage
b
Figure 3: Sorting efficiency parameter  calculated from (a) experiments conducted with 6 m beads
(red) and 1m beads (green) separately, and (b) experiments conducted with a mixture of large (red) and
small (green) beads.
CONCLUSION
In this paper, a 3DiDEP sorter using low frequency AC signals is developed to separate polystyrene
beads based on their size. Particles can be effectively separated at a low average electric field (~100
V/cm). With future optimization, this 3DiDEP device can be used to isolate bacteria and cells based on
surface polarizabilities.
ACKNOWLEDGEMENTS
This work was supported by the Institute for Collaborative Biotechnologies through grant W911NF09-0001 from the U.S. Army Research Office. The content of the information does not necessarily reflect
the position or the policy of the Government, and no official endorsement should be inferred.
REFERENCES
[1] N. Lewpiriyawong, C. Yang, and Y.C. Lam, "Continuous Sorting and Separation of Microparticles
by Size Using AC Dielectrophoresis in a PDMS Microfluidic Device with 3-D Conducting PDMS
Composite Electrodes," Electrophoresis, 31, 2622-2631, 2010.
[2] B.G. Hawkins, A.E. Smith, Y.A. Syed, and B.J. Kirby, "Continuous-flow Particle Separation by 3D
Insulative Dielectrophoresis Using Coherently Shaped, DC-biased, AC Electric Fields," Anal. Chem.,
79, 7291-300, 2007.
[3] H. Shafiee, M.B. Sano, E.A. Henslee, J.L. Caldwell, and R.V. Davalos, "Selective Isolation of
Live/dead Cells Using Contactless Dielectrophoresis (cDEP)," Lab. Chip., 10, 438-445, 2010.
[4] B.H. Lapizco-Encinas, B.A. Simmons, E.B. Cummings, and Y. Fintschenko, "Insulator-based
Dielectrophoresis for the Selective Concentration and Separation of Live Bacteria in Water,"
Electrophoresis, 25, 1695-1704, 2004.
[5] W.A. Braff, A. Pignier, and C.R. Buie, "High Sensitivity Three-dimensional Insulator-based
Dielectrophoresis," Lab. Chip., 12, 1327-31, 2012.
[6] W.A. Braff, D. Willner, P. Hugenholtz, K. Rabaey, and C.R. Buie, "Dielectrophoresis-Based
Discrimination of Bacteria at the Strain Level Based on Their Surface Properties," PLoS ONE, 8,
e76751, 2013.
CONTACT
* C. R. Buie; phone: +1 617-253-9379; crb@mit.edu
2467