DIELECTROPHORETIC SORTING OF MICROPARTICLES AND
LYMPHOCYTES USING RAIL-TYPE ELECTRODES
K. Tatsumi1*, H. Shintani1, Y. Katsumoto2 and K. Nakabe1
1Kyoto University, JAPAN and2
Sony Corporation, JAPANABSTRACT
A particle manipulation device using the dielectrophoretic (DEP) force is described in this study. The electrode patterndeveloped in this study (referred to as “rail-type electrodes”) can generate a DEP force distribution that captures theparticles, the DEP force of which is “negative” (repulsion force), in the specific area located at the centerline of theelectrodes with some distance to the channel bottom wall. The particles can be guided accurately along the electrode, andin combination with the “flip-type electrode” the manipulation direction can be controlled, providing a possibility ofparticle sorting with high accuracy, reliability and response.
KEYWORDS: Microchannel, Dielectrophoretic force, Micro-particle, Manipulation, Sorting
INTRODUCTION
The technologies for manipulating and sorting cells and particles in the microchannel flow by applying optical, fluid
dynamic, magnetic, and electric forces have been studied intensively in the last decade [1]. In particular, the DEP force ispowerful because it can produce a driving force to the particles without changing or modifying them or the fluid
properties. Although many types of electrodes and channels have been proposed in other studies [2, 3], substantial issues
remain to be solved for satisfying the demands of the applications, such as accuracy, response, sorting rate, andapplicability under various conditions. One such problem is because the DEP force, FDEP, is mainly produced by theelectric field gradient, it decays markedly with the distance from the electrode. Another problem is the fact that in manycases, a negative FDEP, namely a repulsion force against the electrode works on the particles and cells; therefore, it isdifficult to precisely control the particle position. In order to overcome both these problems, an electrode pattern, which
is not only novel and effective but also fundamental and versatile, is developed in this study; we refer to these electrodes
as “rail-type electrodes.” By using the rail-type electrodes, particles can be guided along the electrode in the streamwiseand spanwise directions of the channel accurately. When the flip-type electrode, which is another electrode presented inthis study, is combined with the rail-type electrodes, then the direction in which the particle should be guided can beselected.
THEORY
Figure 1 (a) shows the schematic of the microchannel, and the rail-type and flip-type electrodes. The top figure in Fig.
1 (b) shows the schematic of the device trapping and sorting the particles using these two electrodes. The bottom figure
depicts the distributions at the streamwise cross-sectional plate (yz plane) of the FDEP working on a polystyrene particle
obtained by the numerical simulation [4]. The left figure shows the y component of the FDEP, and the right one shows the zcomponent.

(a) Microchannel and electrodes(b) Physics of the sorting technology

Figure 1: (a) Schematic of the electrodes and microchannel, (b) The schematic of the physics of the sorting technology using the rail-type and flip-type electrodes, and the y and z components of dielectrophoretic forces FDEP .
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Figure 2. Snapshots of, (a) PS particle trapped and guided by the rail-type electrodes, (b) lymphocytes trapped, guided, and sorted by the rail-type and flip-typeelectrodes (turning the flip electrode “on” and “off”).

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The rail-type electrodes consist of three parallel electrodes attached to the bottom
wall, and a plane electrode covering the top wall which is connected to the electric ground. As shown in the FDEP
distributions in Fig. 1(b), the rail-type electrodes generate negative FDEP in the z direction at the centerline of theelectrodes, and a positive value in adjacent to the channel bottom wall. In the y direction, a symmetric distribution isgenerated for FDEP. These distributions produce a pocket area trapping the particles at the centerline of the electrodes andadjacent but with a certain distance to the bottom wall. The height position of the particles can be controlled by changing
the voltage applied to the center electrode. Thus, the particles or cells will not contact or adhesive to the wall, whichmakes it easier to manipulate the particle smoothly. The particles flowing from the upstream region are trapped in thisregion, and then guided along the rail-type electrodes. If the electrodes are inclined to one side against the main flow
(oblique region), the particles can be guided in the spanwise direction.
The flip-type electrode is located right upstream of the oblique electrodes. Above this electrode, a strong positive FDEP
in the z direction will be generated. If the voltage at the flip-type electrode is turned “off” when the particle passes theflip-type electrode, then the particles will flow along the oblique rail-type electrodes. When the flip-type electrode isturned “on”, then the particles are pushed and excluded from the rail-type electrode region and flow downstream.
EXPERIMENTAL
As shown in Fig. 1(a), the device has three flow inlets by which a sheath flow is generated and the spanwise positionof the particle entering the electrode area is precisely controlled. In the downstream region of the manipulation(electrode) area, the channel is bifurcated where the particles are normally guided to one side excluded from the rail-type
electrodes by the flip-type electrode. The specific particles are guided along the oblique region of the rail-type electrodes
by turning the DEP force at the flip-type electrodes “off”, and is excluded by turning it “on”. The electrodes sputtered andpatterned on the bottom glass plate of the channel was made of platinum, and the top ground electrode was made ofindium tin oxide (ITO). These two plates were bonded by SU-8, on which the channel was etched and patterned. The
channel height and width were 50m and 200m, respectively. The motion of the particles was measured using a
microscope and high-speed digital video camera. The particle positions and velocities were measured from the recorded
images. Experiment was carried out for polystyrene microparticles (Thermofisher scientific; 4212A) with nominal
diameter of 12m, and lymphocytes (ATCC; CRL-2570) suspended in PBS solution. In the case of lymphocytes, thechannel was filled and coated by the bovine serum albumin in advance of the experiment. AC voltage of Vp-p=18V and10MHz was applied to the two side electrodes of the rail-type electrodes, and 8.0V was supplied to the center one. The
total flow rates in the main channel in the cases of PS particles and lymphocytes were 2.2L/min and 0.6L/min,respectively. The mean flow velocities in these cases are 3.7mm/s and 1.0mm/s.
RESULTS AND DISCUSSION
Figure 2 shows the photographs depicting the trajectories of the particles and lymphocytes. Figure 3 shows thequantitative values of the positions of several samples of lymphocytes in the cases when the flip-type electrode is turned “off”
and “on”.

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Figure 4. Probability density functions of the particle and cell positions at the inlet and outlet of the electrode region
in the cases of the flip-type electrodes turned “on” and “off”.
Fig. 2 (a) shows how the particles are guided along the electrode when the rail-type electrode only are used in themicrochannel. One can see that the particle is trapped and guided precisely along the centerline. In Figures 2(b-1) and 3(a),which show the results when the flip-type electrode is “off”, the lymphocytes are first trapped and guided along the rail-type
electrode, pass the flip-type electrode, and then are guided along the oblique part. The position distributions show that thelymphocytes approaching the electrode region from various spanwise positions are all trapped and guided along the electrodeat an identical position. In the case when the flip-type electrode is “on”, then the lymphocytes are pushed upwards and areexcluded from the rail-type electrodes as shown in Fig. 2(b-2). In Fig. 3(b), the lymphocytes are all excluded, and are notcaptured by the oblique rail-type electrodes and flow downstream of the straight region of the rail-type electrode. In this case,there is some dispersion in the position after the lymphocytes are excluded. This is due to the fact that the cell size and shape
are not completely identical and the difference of these factors influences the amplitude and direction of the FDEP working on
the lymphocytes at the flip-type electrode.
Finally, Fig. 4 shows the probability density functions of the positions of the particles and lymphocytes. The black bars
show the distribution at the inlet region of the straight rail-type electrodes. The red bars show the distribution at the outlet,
which is the downstream of the electrode region (the region where the oblique part ends). In the figures, a dispersion isobserved in the distribution for both spanwise positions of the particles and lymphocytes at the inlet. At the outlet, when theflip-type electrode is turned “on” (Figures (a) and (c)), the particles and lymphocytes are excluded from the electrodes andflow downstream indicted by the results that the spanwise positions do not differ much from the inlet. However, when theflip-type electrode is turned “off” the particle positions move in the spanwise direction by approximately 100m at the outletof the sorting region. Further, the dispersion is small owing to the trapping effect of the rail-type electrodes. These results
confirm that, the particles and cells can be guided and sorted accurately in the spanwise direction of the channel by using therail-type and flip-type electrodes.
CONCLUSION
Rail-type and flip-type electrodes were proposed and the performances were evaluated experimentally using
polystyrene particles and lymphocytes. The results showed that the rail-type electrodes can trap the particles flowing fromthe upstream of the electrode region in spite of their inlet positions. They can, then, be guided along the centerline of theelectrode in the streamwise and spanwise direction accurately. When the flip-type electrode was turned on, the particles
were pushed away from the electrode region. The probability density function of the particle position at the outlet regionof the electrodes showed that the spanwise position of the particles can be precisely controlled and selected by using
these electrodes.
ACKNOWLEDGEMENTS
This work is supported by Japan Agency of Science and Technology (Development of Systems and Technology for
Advanced Measurement and Analysis).
REFERENCES[1] P. Chen, X. Feng, W. Du, and B.-F.. Liu, “Microfluidic Chips for Cell Sorting”, Frontiers in Bioscience, Vol. 13, pp.
2464, (2008).
[2] P. R. C. Gascoyne and J. Vykoukal, “Particle Separation by Dielectrophoresis”, Electrophoresis, Vol. 23, pp. 1973,(2002).
[3] Z. R. Gagnon, “Cellular Dielectrophoresis: Applications to the Characterization, Manipulation, Separation and Patterning of Cells”, Electrophoresis, Vol. 32, pp. 2466, (2011).
[4] K. Tatsumi, K. Imajou, H. Shintani, Y. Katsumoto, and K. Nakabe, “Numerical Model for Microparticle and Lymphocyte Motions in Dielectrophoretic Manipulation Device”, micro-TAS2011, pp. 2466, (2011).
CONTACT*K. Tatsumi, tel: +81-75-3833606; tatsumi@me.kyoto-u.ac.jp1360