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Sensors and Actuators A 135 (2007) 833–838

A valveless micropump for bidirectional applications

Jae Sung Yoon, Jong Won Choi, Il Hwan Lee, Min Soo Kim ∗School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-744, Republic of Korea

Received 11 March 2006; received in revised form 14 August 2006; accepted 21 August 2006Available online 18 September 2006

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This paper describes a new valveless micropump for bidirectional use. The micropump was fabricated by bonding a fluidic channel and aembrane actuator. The fluidic channel was etched on a glass substrate and the membrane actuator was made on a silicon wafer. The membrane is

ctuated by a piezoelectric ceramic (PZT) plate so that the membrane generates blowing and suction flows successively along an oblique channelhich is connected to the main channel. A blowing flow from an oblique channel into the main channel generates forward flow in downstream,hile a suction flow generates the smaller reverse flow in downstream. Therefore, the overall flow in the main channel has a downstream flow

nd it can have a directional characteristic when two oblique channels are used. The performance of the micropump depends on the geometry

f the fluidic channel, driving frequency and voltage of the actuator. The geometry of the micropump was optimized by numerical analysis andhe performance test was done under various operating conditions. The maximum flow rate was 323 �l/min and the maximum back pressure was94 Pa when the membrane actuator of 10 mm × 10 mm was driven at 130 Hz and 385 V.

2006 Elsevier B.V. All rights reserved.

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eywords: Valveless micropump; Bidirectional micropump; Oblique channel

. Introduction

A number of different micropumps have been developed overhe last decades. Among the various kinds of working principles,he micropump using membrane actuation is known as a promis-ng approach [1]. Therefore, a lot of membrane micropumpsave been studied with various actuators. The valve is anotherssue for the development of micropumps. Since a piezoelectric

icropump with check valves was first presented by van Lintelt al. [2], passive or active check valves have been widely usedor micropumps. The micropump with check valves can preventnintended backflow while the pump is turned off. Moreover,he pumps with check valves have self-priming and bubble-olerance characteristics and they can be used for both liquidnd gas, if the volume stroke ratio is large enough [1]. But theressure drop at the valve is relatively large and the valves areery sensitive to the bubbles or small particles. Therefore, valve-

ess diffuser micropump has been presented to overcome theseroblems [3–5] and theoretical analyses were also conducted6–8]. Since the diffuser and nozzle has no moving part, most

∗ Corresponding author. Tel.: +82 2 880 8362; fax: +82 2 883 0179.E-mail address: minskim@snu.ac.kr (M.S. Kim).

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924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2006.08.017

f the problems caused by the valves are eliminated. The pumptroke efficiency of the diffuser micropump is about 19–25%,hich is the ratio of the pumped volume to the total stroke vol-me [3].

A bidirectional micropump has been a new challenge toroaden the application range of the microfluidic devices.he bidirectional characteristic has been investigated with theicropump with nozzle and diffuser [9], the peristaltic microp-

mp [10,11] and the conventional micropump with check valves12]. The peristaltic micropump can generate forward and back-ard flows by changing working sequence of three actuators.owever, the peristaltic micropump needs at least three pumping

hambers and actuators, which are not suitable for miniaturiza-ion.

In this study, a new valveless micropump for bidirectionalse has been designed, fabricated and tested. Since there are noalves, this pump is expected to be free from wear, fatigue andlogging problems. Moreover, this pump can be made in a sim-le design, which is helpful for miniaturization. A piezoelectriceramic plate (PZT) and silicon membrane were used for the

ctuator, and the membrane and channels were fabricated byicromachining process. The performance characteristics were

nvestigated by the experiments in order to find out the optimumeometry and operating conditions.

834 J.S. Yoon et al. / Sensors and Actua

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ig. 1. Construction of the micropump with fluidic channels, silicon membranend piezoelectric plate.

. Working principle

The micropump in this study consists of a membrane actua-or, a pumping chamber, fluidic channels and two piezoelectriceramic plates (see Fig. 1). The membrane actuator, which islso known as the bimorph actuator, can generate large bendingisplacement even though the deformation of the transducer is

elatively small [13]. The membrane actuator pulsates and gen-rates blowing and suction flows successively along an obliquehannel which is connected to the main channel as seen inig. 2. A discharge stroke through the oblique channel generates

Fig. 2. Working principle of the micropump.

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tors A 135 (2007) 833–838

orward flow in downstream, while a suction stroke producesmaller reverse flow in downstream. The discharged flow fromhe oblique channel behaves like a jet flow, which brings about aow motion toward the downstream in the main channel. On theontrary, the suction flow into the oblique channel is expectedo create a flow motion which is similar to the flow around theink. The flow motion around the sink is less directional thanhe jet flow is, so there exists a flow rectifying effect in the mainhannel. Therefore, the difference between forward and reverseow will be the overall flow rate of this micropump. Besides, theicropump has two actuators and two opposite oblique chan-

els. So the overall flow direction can be determined by selectingn actuator to be powered.

. Performance parameters

The performance of this micropump depends on severalarameters, which are channel angle, actuation frequency, volt-ge and so on. The channel angle is a critical parameter becauset influences flow interaction at the junction of two channels. Ifhe angle becomes smaller, the discharged fluid from the obliquehannel can deliver larger momentum to the main stream, whicheans that a larger downstream flow is expected. But on the con-

rary, when the actuator generates suction flow, small angle is notelpful for attracting the upstream fluid into the oblique channel,nd vice versa. Therefore, the channel angle should be optimizedefore designing the micropump. A simple simulation was con-ucted in order to investigate the influence of the channel anglen the flow motion. A commercial software package FLUENTas used for the simulation [14].The downstream flow (Qdis,down) which is caused by the dis-

harge flow from the oblique channel was calculated and thenompared with the reverse flow (Qsuc,down) which is caused byhe suction flow. Pumping occurs when Qdis,down > Qsuc,down andhe net flow rate can be described as:

net = Qdis,down − Qsuc,down. (1)

The membrane motion was not considered here, but the flowelocity in the oblique channel (±Vo) was given instead. Sohe simulation was done on the discharge stroke (+Vo) and theuction stroke (−Vo) separately. The optimum channel angleould be found by comparing these two flow rates (Qdis,downnd Qsuc,down) under various channel angles (θ). Fig. 3 showshe variation of flow rate ratio (Qdis,down/Qsuc,down) under varioushannel angles from 5◦ to 25◦. It should be noted that the flowate ratio is the largest when the channel angle is 10◦.

The actuation frequency is another parameter that affectshe pump performance. High frequency will generate largeow velocity at the oblique channel (Vo), so the flow rate isxpected to increase with the frequency. But if the frequencyets too high, the flow in the channel may not be synchro-ized with the actuator motion because of the mass inertia of the

uid.

The driving voltage is related to the magnitude of the mem-rane motion. If the driving voltage is higher, the membraneotion will be larger and the flow rate will be increased. But an

J.S. Yoon et al. / Sensors and Actuators A 135 (2007) 833–838 835

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Fig. 3. Variation of the flow rate ratio under various channel angles.

xcessively high voltage may cause a mechanical damage on theembrane actuator. In this study, a preliminary test was done

n the membrane actuators and all the experiments were donender 400 Vpp (peak to peak). The deflection of the membraneas also measured as seen in Fig. 4. The deflection is slightlyore than linear to the driving voltage and the largest deflectionas 8.16 �m at 385 Vpp.

. Fabrication and test apparatus

The membrane was fabricated by etching the silicon wafer

ith deep reactive ion etch (DRIE) process. The etched area

s 10 mm × 10 mm and the etching depth is 400 �m so thathe thickness of the membrane is 100 �m (using 500 �m thickafer). The fluidic channel and pumping chamber were made

ig. 4. Measured membrane deflection under the various driving voltage.

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Fig. 5. Photograph of the micropumps after bonding process.

n the glass wafer by sand-blast process. The area of pump-ng chamber is 10 mm × 10 mm and the depth is 200 �m. Theblique channel and the main channel are 0.5 and 1.0 mm wide,espectively, and 200 �m deep. Two port holes (inlet and outlet)ere made at both ends of the main channel. Then, the silicon

nd glass substrates were bonded together by anodic bondingrocess. The piezoelectric ceramic plates (200 �m thick) werelued on the membrane. Fig. 5 shows the pictures of the microp-mps after the anodic bonding process.

A schematic overview of the test apparatus is shown in Fig. 6.t consists of a reservoir, an amplifier, a function generator andn open-loop tubing circuit. The driving signal was generatedy a function generator, amplified by an amplifier and then sento the piezoelectric actuator. The signal was sinusoidal wave ofarious frequencies. The micropump was connected to the testig with flexible teflon tube of 3.175 mm (1/8 in.) outer diametersee Fig. 7). Before the test, the priming job was done using theacuum pump. Especially, the pumping chamber of the microp-mp should be free from gas bubbles for a proper operation. Theiezoelectric actuator was turned on while the priming job waserformed, so that the bubbles were scattered and released fromhe pumping chamber. By repeating the priming jobs of vacuumurge and filling, the test rig and the micropumps were filledith the working fluid. The working fluid was water of room

emperature and the test rig was adjusted horizontally.The flow rate was measured by reading the moving distance

f the water column in the teflon tube per unit time. The exper-ments were also conducted under various back pressures. Theack pressure was changed by lifting up the downstream tube.

. Results and discussion

From the simulation, the micropump was expected to have thereatest flow rate when the channel angle was 10◦. To verify this,hree micropumps with different oblique channel angles (5◦, 10◦,5◦) were made and tested. Fig. 8 shows the flow rates measured

836 J.S. Yoon et al. / Sensors and Actuators A 135 (2007) 833–838

of the test apparatus.

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Fig. 6. Schematic

nder different frequencies at the same voltage (320 Vpp). Thisest shows that the micropump with 10◦ channel produced thereatest flow rate, the 5◦ channel was the next and the 15◦ chan-el generated the smallest flow rate, which are consistent withhe simulation result. This result also shows that the flow rateariations were similar to one another, but each pump reachedhe peak flow rate at different frequencies.

With the micropump of 10◦ oblique channel angle, perfor-ance was further tested under various frequencies and voltages.s seen in Fig. 9, the flow rate increased greatly as the volt-

ge became higher. The deflection of the membrane, or thetroke volume became larger at the higher voltage, and con-equently the flow rate through the oblique channel increasedlso. According to the simulation result (see Fig. 3), the flow rate

atio (Qdis,down/Qsuc,down) was increased by increasing the flowelocity through the oblique channel (Vo). This means that theow rectifying efficiency can be improved by supplying higher

Fig. 7. Photograph of the micropump connected to the test rig.

Fig. 8. Flow rates of the micropumps with different channel angles (5◦, 10◦ and15◦) under various driving frequencies.

Fig. 9. Flow rates of the micropumps with 10◦ channel angle under variousdriving frequencies and voltages.

J.S. Yoon et al. / Sensors and Actuators A 135 (2007) 833–838 837

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ig. 10. Flow rates of the micropumps with 10◦ channel angle under variousack pressures.

oltage, which resulted in the increase of the flow velocity inhe oblique channel. The flow rectifying efficiency can be alsonhanced by increasing the frequency, which is another parame-er related to the flow velocity in the oblique channel. However,he test results show that the overall flow rate increased at first,eached the peak value and then decreased. Since the fluid hasts own mass momentum, it cannot follow the vibration of the

embrane when its motion is too fast. So the flow rate or theow rectifying efficiency cannot be improved further beyond

he optimum frequency, where the maximum flow rate occurs.herefore, the effective working frequency of the micropump in

his study should be between 0 and 130 Hz. The largest flow rateas 323 �l/min at 385 Vpp and 130 Hz without back pressure.The back pressure dependence was tested by measuring the

ow rate under various back pressures, so that the performanceurves were obtained as seen in Fig. 10. The flow rate decreaseds the back pressure increased at the fixed frequency and voltage.nd the performance curves were more deteriorated when theoltage dropped than when the frequency changed, which meanshat the large stroke of the membrane plays a key role especiallyhen the back pressure is applied. The maximum back pressureas achieved when the flow rate became zero. In this test, the

argest back pressure was 294 Pa at 385 Vpp and 130 Hz. Thebove results (Figs. 9 and 10) show the influences of the fre-uency and voltage on the pumping performance. There existsn optimum frequency which produces the peak flow rate, sohe frequency higher than the optimum value is not efficient forhe pump operation. However, the pumping performance cane greatly improved by increasing the voltage, as long as theiezoelectric actuator and the silicon membrane can endure theechanical deflection.The bidirectional pumping characteristic was also tested.

ince there are two actuators and two opposite oblique chan-els, this micropump can produce reverse flow by supplyingower to the other actuator. The result shows a good agreementetween the forward and reverse flows (see Fig. 11).

ig. 11. Flow rates of the micropump with 10◦ channel angle for both directionstested at 385 Vpp).

. Conclusions

A bidirectionally driven valveless micropump was designednd made by microfabrication process. Among the parametershat affect pumping performance, the channel angle was a criti-al one. The simulation showed that the angle of 10◦ producedhe largest flow rate, so three pumps with different channel angleere made and tested to back up the simulation. The test showed

hat the optimum channel angle was also 10◦, and that the per-ormance varied with respect to the voltage and frequency. Thenfluence of the back pressure and the bidirectional pumpingharacteristic were also tested. The major advantage of the pre-ented micropump is that it can work bidirectionally withoutalves, although the pump is made in a simple design. Besides,he performance characteristics of both directions were in goodgreement, which will be suitable for bidirectional applications.owever, further improvements are needed to have higher backressure operation characteristics.

cknowledgement

This work was supported by the Micro Thermal Systemesearch Center (MTSRC) of Seoul National University.

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