A valveless switch for microparticle sorting with laminar flowstreams and electrophoresis perpendicular to the direction of

fluid stream

Toru Takahashi a,*, Sachiko Ogata b, Matsuhiko Nishizawa b, Tomokazu Matsue b,*

a Center for Interdisciplinary Research, Tohoku University, Aoba, Aramaki, Aoba-ku,

Sendai 980-8578, Japanb Department of Biomolecular Engineering, Graduated School of Engineering, Tohoku University, Aoba,

Aramaki, Aoba-ku, Sendai 980-8579, Japan

Received 26 November 2002; received in revised form 6 January 2003; accepted 6 January 2003

Abstract

A simply designed valveless switch for microparticle sorting was fabricated on a glass chip. A successful sorting of 10 lm di-

ameter polystyrene latex beads was performed by the microfluidic system consisted of a unique electrophoretic switch and pair of

parallel laminar flow streams. In applying the voltage to the electrodes placed on the banks of the flow through channel, micro-

particles were electrophoretically diverted across the boundary between two distinct laminar flows.

� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Microfluidic device; Microparticle sorting; Polystyrene latex beads; Laminar flow streams; Electrophoresis

1. Introduction

There has been great interest in separation and sort-

ing of microparticle including living cells using micro-

fluidic devices [1–3]. The design criterion for sorting

system [1–3] is quite different from that of fractionation

system [4–6] because of their structural differences. In a

typical microsorting device, an ‘‘unblanched’’ singlechannel for fractionation is connected to a branched

channel with several inlets and outlets. The critical

technique in a microsorting device is to control the be-

haviour of the flowing microparticles in the blanched

micro channels. Several techniques, dielectrophoresis

[1], pressure switch [2], electrokinetic flow [3], had been

employed to control the movement of microparticles in

the micro channels. For sorting living cells, electroki-netic flow [3] or pressure switch [2] alone do not meet the

demands for immediate treatments of cells [7]. In di-

electrophoretic devices [3], poorly conductive media

should be employed because dielectrophoretic force is

decreasing when the concentration of electrolytes in the

medium is increasing [8]. However, the use of highly

conductive isotonic solution is necessary with respect to

the better physiological environment for living cells. The

conductive media is advantageous also for the electro-phoretic manipulation of charged species [9]. In addi-

tion, a simply designed system has the advantage in

construction of a microfabricated device integrated

more functionalities.

In this paper, a simply designed valveless switch for

microparticle sorting with a unique electrophoretic

switch technique and a laminar microfluidic system was

investigated. A pair of parallel laminar flows was em-ployed to transport the microparticles toward the waste

and the collection outlets, respectively. The d.c. voltage

was temporarily applied to a pair of the electrodes

placed on both sides of the flow through channel to

divert the particles to the collection flow. The design

allows microparticles to be divided into two parts and to

be retrieved.

Electrochemistry Communications 5 (2003) 175–177

www.elsevier.com/locate/elecom

*Corresponding authors. Tel.: +81-0-22-217-7222; fax: +81-0-22-

217-7223 (T. Takahashi); Tel.: +81-0-22-217-7209; fax: +81-0-22-217-

6167 (T. Matsue).

E-mail addresses: toru@analchem.che.tohoku.ac.jp (T. Takahashi),

matsue@bioinfo.che.tohoku.ac.jp (T. Matsue).

1388-2481/03/$ - see front matter � 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S1388-2481(03)00002-X

2. Experimental

All chemicals used were the guaranteed grade re-

agents. Fluorescein (Wako Chemical, Japan) were dis-

solved in minute volume of ethanol at first, and then

diluted to 5 lM with double distilled water (DDW). A

commercially available standard polystyrene latex

sphere suspension solution (Polyscience, USA, 10 lmdiameter) was diluted in an aqueous solution containing0.002 wt% of Tween 20 and 5 mM of potassium chlo-

ride.

A schematic representation of the microfluidic device

fabricated on a glass chip was shown in Fig. 1. A pair of

Ti/Pt thin film electrodes was first fabricated on a grass

substrate by photolithography, followed by casting the

flow through channel in polydimethylsiloxane material

(Shin-etsu Chemical Industry, Japan) [10]. The channeldepth was 20 lm. Fused silica capillary tubes (GL Sci-ence, Japan, 50 lm inner diameter) served as the con-

nection between syringes and the inlets of the flow

through channel. A KDS 200 dual syringe pump system

(Muromachi Kikai, Japan) with two MS-GLL 025 sy-

ringes (Ito, Japan) was used to inject two independent

continuous flows into the channel through the dual inlet.

The rate of the flow [11] was within the range 0.24–2.4ll/min. Application of d.c. voltage between a pair of Ti/Pt thin film electrodes was accomplished with a model

248 d.c. high voltage power supplier (Keithley Instru-

ments, USA). The particle behavior was monitored via a

ME 600 microscope (Nikon, Japan) with a COOLPIX

990 charge-coupled device camera (Nikon, Japan).

3. Results and discussion

Firstly, the behaviour of the flows injected into the

flow through channel was investigated. As shown in Fig.

1, the flow system of the device consisted of two inlets,

two outlets and the flow through channel. The pair of

distinct flows injected from the upper and lower inlet

meet on the flow through channel and then drain away

from the two outlets. Fig. 2 shows fluorescent micro-graphs of the chip device with flowing fluorescein solu-

tion and DDW introduced into the channel through the

upper and lower inlet, respectively. As shown in Fig. 2,

at the flow rate [11] above 1 ll/min, the boundary be-tween the DDW and the aqueous fluorescein solution in

the flow through channel can be recognized clearly, in-

dicating that these two distinct flows from the upper and

lower inlet brought together in the channel and cameout through the two respective outlets of the channel

without significant mixing.

The electrophoretic switching of polystyrene latex

beads was performed with the microfabricated device.

Fig. 3 shows the behaviours of microparticles injected

into the device at 0 V or 30 V applications between the

pair of ‘‘switch’’ electrodes placed on the upper and the

bottom side of the flow through channel. The polysty-rene latex spheres suspension and the latex sphere-free

carrier solution both containing 0.002 wt% of Tween 20

and 5 mM of potassium chloride were injected into the

lower and upper channel, respectively. Without the ap-

plication of voltage, all of the latex particles introduced

into from the lower inlet were carried and drained away

from the lower outlet by the lower phase of the parallel

laminar flows simply according to the hydrodynamiccondition (Fig. 3(a)). On the other hand, as shown in

Fig. 3(b), applying the voltage to the ‘‘switch’’ elec-

trodes, the latex particles were perfectly switched from

lower phase to the upper phase across the boundary

between the two streams in the flow through channel.

The results satisfactory demonstrate the sorting of

polystyrene latex beads by temporarily applying d.c.

voltage perpendicular to the direction of the fluidicstreams in the channel.

Fig. 1. A schematic illustration of the microfluidic device fabricated on

a glass substrate.

Fig. 2. Fluorescent micrographs of the flow through channel. An

aqueous fluorecein solution (5 lM) and a DDW were injected into the

channel from the upper and lower inlet, respectively. The flow rate [11]

was 2.4 ll/min.

176 T. Takahashi et al. / Electrochemistry Communications 5 (2003) 175–177

A valveless switch for microparticle sorting with theelectrophoretic switch technique and the two phases

laminar flow streams had been presented. To the best of

our knowledge, the electrophoretic phenomena of

polystyrene latex beads vertically to the flow axis over

the two parallel laminar flow streams have never been

reported. While the direction of the electrophoreticmovement of the polystyrene latex particles indicate that

particles were negatively charged under the present

conditions, one can change the direction opposite in

addition of the cationic surfactants, such as cetyltrime-

thylammonium bromide, to both the beads suspension

solution and the carrier solution [12]. Since the surface

of living cells are thought to be negatively charged in

their physiological conditions, it is expected that livingcells can be also driven by electrophoresis, as demon-

strated for the latex beads. Therefore, the application of

perpendicular electrophoretic sorting system for living

cells is now under investigation. The combination of

some diagnostic method for cells with the sorting tech-

nique presented here will bring about an inexpensive

microfabricated flow cytometric cell sorter [7] with eas-

ier mechanical setup.

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[11] The flow rate in the micro fluidic channel was evaluated with the

product of the cross section of the syringe multiplied by the stroke

of the pump in a unit time. The stroke of the syringe had been

measured separately.

[12] T. Takahashi, unpublished data, 2002.

Fig. 3. Optical micrographs of the electrophoretic transfer of micro-

particles in the flow through channel with (a) 0 V and (b) 30 V ap-

plication to the upper electrode. Arrows depict the directions of

transported polystyrene latex beads. The flow rate [11] was 1.2 ll/min.

T. Takahashi et al. / Electrochemistry Communications 5 (2003) 175–177 177