A Micropump Driven by Continuous Electrowetting Actuation for Low Voltage and Low Power Operations

454 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 11, NO. 5, OCTOBER 2002

A Surface-Tension Driven Micropump for

Low-Voltage and Low-Power Operations

Kwang-Seok Yun, Il-Joo Cho, Jong-Uk Bu, Chang-Jin (CJ) Kim, and Euisik Yoon

Abstract—In this paper, we first report a micropump actuated

by surface tension based on continuous electrowetting (CEW). We

have used the surface-tension-induced motion of a mercury drop

in a microchannel filled with an electrolyte as actuation energy

for the micropump. This allows low voltage operation as well as

low-power consumption. The micropump is composed of a stack

of three wafers bonded together. The microchannel is formed on

a glass wafer using SU-8 and is filled with electrolyte where the

mercury drop is inserted. The movement of the mercury pushes

or drags the electrolyte, resulting in the deflection of a membrane

that is formed on the second silicon wafer. Another silicon wafer,

which has passive check valves and holes, is stacked on the mem-

brane wafer, forming inlet and outlet chambers. Finally, these two

chambers are connected through a silicone tube forming the com-

plete micropump. The performance of the fabricated micropump

has been tested for various operation voltages and frequencies. We

have demonstrated actual liquid pumping up to 70

l/min with a

driving voltage of 2.3 V and a power consumption of 170

W. The

maximum pump pressure is about 800 Pa at the applied voltage of

2.3 V with an operation frequency of 25 Hz. [713]

Index Terms—Continuous electrowetting, liquid metal, low

power, low voltage, microfluidics, micropump, surface tension.

I. INTRODUCTION

T

HERE is strong demand for miniaturized flow con-

trol devices, including micropumps, microvalves, and

micromixers in the fields of miniaturized chemical anal-

ysis systems such as micro total analysis systems (

TAS)

or lab-on-a-chip, as well as embedded medical devices,

micro-dosing systems, and miniaturized production sys-

tems. Micromachining and microelectromechanical systems

(MEMS) technology have increased the performance and

functionality of such microfluidic devices with cost effective

miniaturization and has made it possible to integrate them with

microsensors and control circuitry, which has opened the new

application fields mentioned above.

The micropump is a key component that exerts a force on a

liquid to sample blood or water, dose a drug, and allow liquid

to flow. Though some micropumps are nonmechanical types

utilizing electrohydrodynamic, electroosmotic, ultrasonic or

Manuscript received June 27, 2001; revised March 23, 2002. This work was

supported in part by the Korea Science and Engineering Foundation through the

MICROS center at KAIST and the National Research Laboratory program from

the Ministry of Science and Technology of Korea. Subject Editor K. D. Wise.

K.-S. Yun, I.-J. Cho, and E. Yoon are with the Department of Electrical Engi-

neering and Computer Science, Korea Advanced Institute of Science and Tech-

J.-U. Bu is with LG Electronics Institute of Technology (LG Elite), Seoul

137-724, Korea.

C.-J. Kim is with the Mechanical and Aerospace Engineering Department,

University of California, Los Angeles, CA 90095-1597 USA.

Digital Object Identifier 10.1109/JMEMS.2002.803286.

thermocapillary forces, most micropumps are mechanical types

composed of mechanically moving membranes. This is because

mechanical micropumps can be used for wide applications

without any restrictions in pumping liquid [1]. Previously,

a number of mechanical micropumps have been developed

using various mechanisms, including piezoelectric [2]–[4],

electrostatic [5], [6], thermopneumatic [7]–[9], electromag-

netic [10], bimetallic [11] and shape memory alloy (SMA)

actuations [12]. However, most of them require either high

operation voltages (piezoelectric and electrostatic actuations)

or high operation powers (thermopneumatic, electromagnetic

and SMA actuations). These devices are difficult to apply in

certain application fields such as portable embedded medical

devices, remote environmental monitoring systems, handheld

chemical analyzers, etc., where both low voltage and low power

operation are crucial factors.

At microscale, surface tension is a relatively large force com-

pared to other forces such as gravity or structural stiffness, and

has been a serious hindrance to the successful fabrication and

operation of microdevices. Electrocapillary and electrowetting,

including continuous electrowetting (CEW), are interesting

phenomena which show active use of strong surface tension at

microscale. Beni et al. demonstrated the surface-tension-driven

motion of a mercury drop in a glass capillary and applied this

principle to an optical switch [13], [14]. A micropump using the

electrocapillarity of mercury was first theorized by Matsumoto

and Colgate [15], and was recently realized by Ni et al., who

fabricated the pump using traditional manufacturing methods

[16]. In recent years, Lee et al. reported MEMS devices,

which employ the CEW phenomenon for generating linear and

circular motions of a mercury drop with a small driving voltage

of about 3 V and low power consumption (10–100

W) [17].

We have applied this actuation mechanism for a micropump in

which the pumping membranes are deflected according to the

pressure induced by the mercury movement in a microchannel.

In this paper, we report the working principle of the proposed

micropump followed by its fabrication process and experi-

mental results.

II. P

RINCIPLE OF ACTUATION MECHANISM

Electrowetting is an electrically-induced change of a ma-

terial's wettability. Fig. 1 shows a schematic illustration of

continuous electrowetting. Initially, without bias voltage, a

liquid metal (typically mercury; gallium was tested for CEW

in previous report [13] as well as low-melting-temperature

alloys [18], but not for the devices in this work). in a capillary

filled with an electrolyte has a uniformly distributed charge

density along the

-direction. If a voltage is applied between

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