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A study on the development of a continuous peristaltic micropump using magnetic fluids
Sensors and Actuators A 128 (2006) 43–51
A study on the development of a continuous peristaltic micropump using magnetic fluids Eui-Gyu Kim, Jae-geun Oh, Bumkyoo Choi ∗ Department of Mechanical Engineering, Sogang University, #1 Sinsu-Dong, Mapo-ku, Seoul 121-742, Republic of Korea Received 23 March 2005; received in revised form 23 December 2005; accepted 7 January 2006 Available online 24 February 2006
Abstract This paper presents the development of a peristaltic MF (magnetic fluid) micropump for the purpose of applications of ‘lab on a chip’, which is the core technology of the medical & biological fields. In the device, magnetic fluids are gathered in the round-shaped channel by the magnetic force, which is applied from the external permanent magnet controlled by stepping motor. The gathered MF lump deforms the silicone rubber diaphragm and then the deformed diaphragm pushes the sample liquid in the microchannel. The micropump was fabricated through the conventional MEMS (micromachining) technology. It doesn’t have any complicated moving part, but uses magnetic fluids as a driving source. Thus, the proposed micropump can realize more reliability and durability. In addition, the driving method of our ‘continuous peristaltic’ micropump is different from that of other pumps using cavity sequence. Consequently, it brings out significant improvements in the performance. Furthermore, it cannot only pump in forward and backward directions, but also can control flow rate by adjusting the external permanent magnet. The MF lump also serves as a valve when the stepping motor of micropump stands still. The maximum flow rate of the micropump was 2.8 l/min at 4 rpm and 3.8 l/min at 8 rpm. © 2006 Elsevier B.V. All rights reserved. Keywords: Micropump; Magnetic fluid; Micro actuator; MEMS; Micromachining
1. Introduction This paper introduces enhanced experiments to innovate the existing magnetic fluid (hereafter abbreviated as MF) diaphragm actuator [1,2] and proposes a new type of peristaltic MF micropump based on the experimental results. In recent years a variety of microfluidic devices are developed for a wide range of applications, from chemical analysis systems to actuating systems such as micropumps in medicine and biology [3,4]. Many researchers have tried to control the actuating and pumping components of microfluidic devices in the micron or sub-micron unit range by various methods (electrostatic, thermo-pneumatic, piezoelectric, continuous electro wetting, etc.) [5–7]. A few researchers have made efforts to carry the experiments for the delivery systems using the MF [8–13]. Miller proposed a pumping device utilizing a MF composed of a series of electromagnets, one of which generated
∗
Corresponding author. Tel.: +82 2 705 8639; fax: +82 2 712 0799. E-mail address: [email protected] (B. Choi).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.01.021
a gradient magnetic field to continuously move MF through a length of a tube. In order for the electromagnets to constantly pump the MF, the MF must be broken in pieces in the tubes while it was being pumped. This pumping idea was not actually realized. However, Miller’s pump showed the simplicity in that it could move the fluids without any moving parts into a tube of circular cross-section [8]. Lately, Anson Hatch et al. proposed the ferrofluid peristaltic micropump that could be actuated in a chip level [12]. But their MF pumping systems could not get rid of the disadvantages caused by the contamination from the MF. In this study, we present a new microfluidic system to pump and control a tiny quantity of fluid by MF without any contamination by means of the diaphragm structure. In our experiments for diaphragm actuator, we used MF (N304) with highly saturated magnetization (330 G) and low viscosity (10 cP) instead of SMF110 (saturated magnetization: 100 G, viscosity: 1000 cP) in the previous study [1] to improve its functioning. Also it was designed to have a larger volume of reservoir and the optimal corrugated diaphragms for the better performance.
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E.-G. Kim et al. / Sensors and Actuators A 128 (2006) 43–51
We suggested and fabricated the new MF peristaltic micropump based on the driving method of the enhanced MF diaphragm actuator. In the micropump, magnetic fluids are gathered in the loop channel by magnetic force, which is applied from the external permanent magnet controlled by stepping motor. The gathered MF lump deforms the diaphragm and then deformed diaphragm pushes the sample liquid in the microchannel. The driving method of our peristaltic micropump is different from that of other pumps using cavity sequence. Consequently, it brings out significant improvements in the performance. Furthermore, it cannot only pump in forward and backward directions, but also can control flow rate by adjusting the external permanent magnet. The magnetic fluid lumps are also operated as valves. This MF micropump was made by conventional MEMS fabrication technology. The loop microchannel (with outer diameter of 9 mm, inner diameter of 6 mm and depth of 300 m) was etched by ICP RIE process. It doesn’t have any complicated moving part, but uses magnetic fluids as a driving source. Thus, the proposed micropump can realize more reliability and durability than other existing pumps. 2. Magnetic fluid Magnetic fluids or ferrofluids are the stable colloidal dispersion of sub-micron sized single domain magnetic particles in carrier liquids [14]. The carrier liquid can be an organic solvent, water, or a variety of oil bases. The particles, which have an ˚ are coated with stabilizing disperaverage size of about 100 A, sants, which prevent particle agglomeration even when a strong magnetic field gradient is applied to the magnetic fluid. These suspensions are stable and preserve their properties at extreme temperatures and over a long period of time. The exceptional properties of the fluids have charmed many researchers in various fields. A number of applications have been proposed, and some of them, such as rotating shaft seals, vibration damper, tilt or angle sensor, have been materialized. In this study, we chose the synthetic iso-paraffinic magnetite MFs (N304 and EMG901) with a saturation magnetization of 300 or 600 G and a low viscosity (10 cP) made by Sigma-hi Tech. and Ferrotech Company, and thus a good fluidity could be expected in a micro-scaled channel. The physical and magnetic properties of certain kinds of magnetic fluids are indicated in Table 1. This magnetic fluid is composed of base liquid, ferromagnetic particles, and chemically adsorbed surfactant. The vigorous thermal-movement makes particles avoid settling, and the repulsion caused by surfactant
Fig. 1. The exploded view of the MF diaphragm actuator [2].
prevents aggregation between particles. As a result, it is so stable that no separation occurs. 3. MF actuator and peristaltic MF micropump Before we developed MF micropump, we carried out the preliminary experiments of a diaphragm actuator to get the properties of diaphragm and to confirm the moving ability of the MF. Fig. 1 displays the view of the MF diaphragm actuator. The MF actuator consists of a Parylene diaphragm, two or four reservoirs and an inductor. Two kinds of diaphragms were fabricated for experiments: the planar diaphragm and the corrugated one. The wrinkled diaphragm has four corrugations and its thickness is 3 m. The width and depth of the corrugations are 80 and 20 m, respectively. The first prototype of micropump was a square-type as shown in Fig. 2, which was fabricated by using a wet etching technology. However, when the round-shape micropump was fabricated, the dry etching (deep ICP RIE) process was applied. Fig. 3 shows the pumping movement of the fluids in the channel of the square-type pump. MF flows through the connected channel and pushes the Parylene diaphragm and makes it deformed so that the pressure of the deformed diaphragm drives the fluids into other microchannels. Based on the experimental results of diaphragm actuator and prototype micropump, we proposed a new type of peristaltic MF micropump. The final design of this loop microchannel had the structure as shown in Fig. 4. The diaphragm of micropump was simply spin-coated with silicon rubber (ShinEtsu KE-1206). Accordingly, it had excellent ductility compared with Parylene
Table 1 The physical and magnetic properties of some kinds of magnetic fluids Properties
SMF110
N304
EMG900
EMG901
EMG909
Density, ρ, at 25 ◦ C (g/cm3 ) Saturation of magnetization, Ms , (G) Viscosity, µ, at 27 ◦ C (cP) Volume, % particle concentration ˚ = 10−10 m) Particle size (A
1.04 100 1000 15.4 100
1.14 330 10 12.3 100
1.74 900 60 16.3 100
1.53 600 10 10.7 100
1.02 200 6 3.6 100
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Fig. 3. The peristaltic actuation of a square-type MF micropump [2].
Fig. 2. The exploded view of the square-type micropump.
diaphragm of the actuator. Our micropump consists of the microchannels in both sides of silicon wafer, permanent magnet (3 mm diameter, 2 mm thick, surface Gauss 3400) and stepping motor. It can generate enough pressure for pumping fluids through a strong eternal magnetic field derived from the permanent mag-
net attached to the stepping motor. Therefore, the stepping motor is utilized for controlling the position, the speed, and the direction of the magnetic fluids. In this device, magnetic fluids are collected in the loop microchannel by magnetic force emanated from the external permanent magnet. The collected MF lump deforms the silicone rubber diaphragm as shown in Fig. 5 and then the deformed diaphragm pushes the sample liquid in the microchannel. The peristaltic MF micropump adopted a loop microchannel structure (connected cavity) because fluid was the driving source and it was the most effective structure to get the biggest pressure. Since magnetic fluids flowed in the connected cavity channel we designed the structure of channel as shown in Fig. 6. We named this driving method ‘continuous peristaltic’.
Fig. 4. The schematic of proposed micropump.
Fig. 5. Schematic view of the principle of pump actuation and 3D modeling of proposed micropump [2].
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Fig. 6. The principle of ‘continuous peristaltic’ actuation of MF micropump.
It was different from other micropumps using cavity (or chamber) sequence and brought out significant improvements in the performance.
4. Fabrication process Fig. 7 shows the completely fabricated diaphragm actuator before injecting magnetic fluids and the corrugated Parylene diaphragm. These devices were produced by conventional MEMS technology. Our fabrication process is as follows: first of all, the design of a micropump was printed on a transparent film and was attached on a glass plate to function as a mask against UV light. On ˚ by the other hand, a 4 in. silicon wafer was oxidized to 8000 A thermal oxidation. Then AZ 9260, a thick positive photoresist, was coated on the wafer as the masks for the later ICP RIE process. The photoresist pattern was made by the following procedure: first, silicon substrate was spun at 2000 rpm for 20 s to achieve maximum thickness of photoresist. Then, the substrate was baked for 120 s at 125 ◦ C, exposed to UV light for 60 s followed by development for 2 min to get the complete photoresist pattern. And silicon dioxide was also patterned using a hydrofluoric acid etchant (BOE). The fabrication process of the MF micropump is shown in Fig. 8. Deep inductively-coupled plasma reactive ion etching (ICP RIE) is widely used for etching material in micromachining
applications. We used this process to obtain the 300 m-deep microchannels on both sides of the silicon substrate. Fig. 9 shows dry etched silicon wafer. On the upper side of the silicon wafer, silicon rubber was coated by spin coating processes (300 rpm for 5 s, 800 rpm for 20 s and 1300 rpm for 30 s) and Pyrex was stuck on the etched silicon wafer by the adhesive strength of silicon rubber. Through the transparent Pyrex, we observed how the micropump worked and also bonded Pyrex on the other side of the silicon wafer. Finally, the bonded wafer was diced to produce each piece of the packaged micropump. Fig. 10(a) is the photo of silicon rubber diaphragm fabricated in the middle of two microchannels taken by microscope and Fig. 10(b) is the photo of aligned microchannels. The completely fabricated micropump device is shown in Fig. 11.
5. Experiment and discussion 5.1. Experiments of diaphragm actuator The experiment aims to test the improved performance of the newly designed diaphragm actuator using MF (N304), and to measure the range of diaphragm deflections and to confirm the feasibility of a micropump. Fig. 12(a) shows setups for the magnetic flux density measurements by Gauss meter with an inductor. A Gauss meter
Fig. 7. (a)–(c) The assembled actuator before injecting magnetic fluids [2]. (d) The corrugated Parylene diaphragm [2].
E.-G. Kim et al. / Sensors and Actuators A 128 (2006) 43–51
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The experimental setups for measuring the diaphragm deflections due to MF pressure are shown in Fig. 12(b). Both ends of the inductor are connected to the power supply to control current. We estimated the deflection of the diaphragm by MF pressure generated along the flow of input current with a laser deflection measurer (KEYENCE LC-2420). The measured normal magnetic flux density (Bn ) versus input current amplitude (I) is plotted in Fig. 13. The result illustrates that the magnetic flux density is linearly proportional to the input current amplitude. The maximum magnetic field from the inductor is 110 G at 600 mA. Therefore, in this experiment, driving range of magnetic field varies from 0 to 110 G. Fig. 14 shows the deflections of two kinds of diaphragms to various input current amplitudes. The displacements of the corrugated diaphragms with the input current of 500 mA were about 120 m and more than 240 m for the case using old MF (SMF110) and N304 MF, respectively. On the other hand, the maximum displacements were just 22 and 24 m in the case of the flat diaphragms, respectively [1]. These results showed that the enhanced deflection was resulted from both the new MF (N304) and the changed number of corrugations by simulation [15]. As the magnetic flux density was gradually increased, the deflection of diaphragm was also increased remarkably. The response time was reduced compared with the original model. In the test of the previous work [1], it took 1–2 min to return to 0 deflection. But in this experiment, it took just 2 s to bring back due to the difference of viscosity. The prototype (polygonal peristaltic) micropump with 2 mm × 3 mm diaphragm brought about 15 m of deflection even though the maximum magnetic field was amplified without any corrugation on the diaphragm. It implied that corrugation was essential factor for driving the diaphragm and the upper channel was designed by considering the deformation of corrugation. The peristaltic actuation could not acquire enough deflection for pumping due to relatively fewer amounts of MF and small-sized diaphragm. At the same time, it was clearly confirmed that MF had sufficient fluidity for peristaltic actuation and the fluidity would increase considerably in the circle-type peristaltic micropump. 5.2. Experiments of micropump
Fig. 8. The fabrication process of MF micropump.
was used to measure the magnetic flux density in the surface of the inductor placed directly under the actuator. In our experiment, the magnetic field was considered as normal to the actuator because the size of the inductor is larger than that of the diaphragm. It was assumed that the value of magnetic field was measured at 500 m distance from the inductor, which was equal to the width of Pyrex glass.
The experimental setup for valve evaluation of peristaltic micropump is exhibited in Fig. 15. The capillary tubes connected to the micropump were planned to have much wider dimension than the microchannels, which contributed to negligible resistance in system flow. To inject MF into the microchannel, we took the following procedures. As shown in Fig. 15, the micropump has two holes opened for the syringe and atmospheric pressure to help smooth injection of MF. The thin wire was penetrated into the silicon tubes and stuck to the hole for MF injection. Interconnection area was glued with epoxy. After removing the wire, we completed to set the injection tube as shown in Fig. 16. The MF (EMG901) was injected through this tube into the round-shape microchannel with a 10 ml-syringe. Then we manually moved it into the right position with a handheld permanent
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Fig. 9. Etched silicon substrates by ICP RIE process.
Fig. 10. Fabricated silicon rubber diaphragm. (a) Photo of silicon rubber diaphragm fabricated in the middle of two microchannels taken by microscope and (b) photo of aligned microchannels with a diaphragm.
Fig. 11. The fabricated device for the micropump.
magnet. Once the magnetic fluid entered the inlet channel of the pumping loop, it flowed easily by capillary force. The permanent magnets used for pumping operation rapidly forced the magnetic fluid into the desired locations during operation. The magnets were oriented with the same polarity, which was located perpendicular to the plane of the micropump. The permanent magnet was placed on the disk of the stepping-motor, 1.5 mm directly under (the intensity of magnetic field is about 950 G) the pumping loop. Flexible tubes were linked to both the inlet and the outlet holes by epoxy bonding to act as fluid interconnects. After the epoxy was dried, water was injected carefully so as not to trap air bubbles in the microchannel. In the pumping experiment for sampling fluid, we utilized glass capillary tube (1.1 mm inner diameter) for observing the
Fig. 12. The experimental setups [2]. (a) For normal magnetic flux density to input current. (b) For measurements MF pressure-deflection.
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Fig. 13. The magnetic flux density vs. input current amplitude [2].
Fig. 15. Schematic of the experimental setup for valve test.
Fig. 14. The deflection vs. input current amplitude [2].
movement of water. Two 10 ml-syringes were used as reservoirs. And both columns of water were placed at the same height in the syringe reservoirs. The stepping-motor could be controlled at a various speed (2–30 rpm) and both directions by PICBASIC module. To evaluate the performance of micropump on speed of rotation, the mobility of MF in the microchannel was checked as shown in Fig. 17. From the results of experiments, MF could not maintain its shape over 15 rpm. So we chose 4 and 8 rpm as rotational speeds of experiments.
For the measurement of pumping flow rate of the fabricated micropump, it was useful to compare the measured flow rate with the expected one, based on the volume of the pumping microchannel and the speed of revolution of the stepping-motor. With an outer diameter of 9 mm, inner diameter of 6 mm and depth of 200 m, the volume of the pumping microchannel was about 7 l at one rotation (we ignored the volume of concentrated MF lump). Therefore, at the rotational speed of 4 rpm, the expected volumetric flow rate was under 24 l/min and at the speed of 8 rpm, the flow rate must be under 48 l/min. Fig. 18 shows the photos of the pumping flow rate experiments. When we made experiments of pumping flow rate, we maintained the same hydraulic pressure level of both reservoirs after water was injected into the microchannel. As a result, pumping flow rate was 3 mm/min at 4 rpm, and 4 mm/min at 8 rpm. The pumping efficiency was about 10% at 4 rpm and 7% at 8 rpm. The results were not sufficient but we could confirm the feasibility for pumping. For the evaluation of the performance as a valve, we checked the time required to overcome the hydraulic pressure difference
Fig. 16. The assembled micropump after injecting magnetic fluids.
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Fig. 17. Mobility of magnetic fluid during pump operation.
(the height of the water couldn’t be equal due to surface tension). In other words, we measured the time difference between the cases with and without permanent magnet. As the results of the valve test, it took 4 min with permanent magnet that made diaphragm valve according to magnetic fluid lump to overcome initial hydraulic pressure difference of 558 Pa (syringe volume 10 ml, inner diameter 5 mm), whereas it took 30 s without permanent magnet. Namely, the flow rates passing microchannel were 8 and 1 ml/min for each case, so it could be used as a valve.
6. Discussion The cause of leakage (back flow) problem was the wrinkled surface of the silicon rubber diaphragm. In spin coating process, there were surface quality problems especially in 300 m-deeppattern. That is, many wrinkles were formed in radial direction. They made diaphragm more ductile but were operated as a resisting power in junction with Pyrex wafer or as the cause of the transformation of magnetic fluids. And in case of stopping up the upper channel due to the deformation of diaphragm, formed wrinkles were used as passages of leakage. To solve these problems, first, surface quality must be improved. Second, diaphragm-coating side of wafer would be changed from magnetic-fluid-moving microchannel to water-moving-microchannel. Third, after the improvement of diaphragm surface quality, precision design of each channel shapes is requested. It can minimize the leakage in upper channel by accurate measurement and analysis of diaphragm deformation behavior. Concentrated magnetic fluid lump acts like a piston. That is, it acts as a pump and also as a valve. So, the improvement of valve efficiency is the way of improvement of transportation. 7. Conclusion
Fig. 18. Photos of the pumping flow rate experiments.
We showed the feasibility of MF microfluidic system that can pump accurately microfluids using the relatively high magnetic pressure. For the fabrication of the micropump, we adopted the MEMS fabrication process. Two microchannels were etched on the both sides of silicon substrate by double side alignment. We acquired the miniaturization and simplicity of fabrication simultaneously by locating diaphragm in the middle of two microchannels. Other materials such as PDMS will be able to be applied to the fabrication of the proposed device due to the simplicity of the design and the process. So the integration with other microfluidic components can be readily realized. For the result of fluid pumping experiment, the capacity of movement was 2.8 l/min at 4 rpm and 4.7 l/min at 8 rpm for the rotational speed of stepping motor. Even though the efficiency of moving was lower than we expected, the fact that the fluid movement could be possible using the proposed micropump was meaningful. In future work, it is necessary to fabricate the micropump using Parylene with creases in order to get strong diaphragm enough to obtain more efficiency.
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Acknowledgement This study was supported by the Korea Research Foundation Grant (KRF-2002-005-D00003). References [1] W.C. Sim, et al., Fabrication, test and simulation of a Parylene diaphragm, in: Transducers ’01 Eurosensors XV, Munich, Germany, 2001, pp. 1382–1385. [2] E.-G. Kim, W.C. Sim, J.-g. Oh, B. Choi, A continuous peristaltic micropump using magnetic fluid, in: 2nd Annual International IEEE-EMB Special Topic Conference on Microtechnologies in Medicine and Biology, 2–4 May, 2002, pp. 509–513. [3] J. Cheng, L.J. Kricka, Biochip Technology, Harwood Academic Publishers, 2001. [4] G.T.A. Kovacs, Micromachined Transducers Sourcebook, The McGrawHill Companies, 1998. [5] P.R. Simmons, K—An Introduction to Piezoelectric Crystals-Sensors Magazine, Helmers Publications, 1994, pp. 26. [6] Z. Cui, A knowledge Base for Microfluidic Devices, http://www.ponttech.it/know/infobase.htm. [7] G.T.A. Kovacs, Micromachined Transducers Sourcebook, McGraw-Hill, pp. 839–840. [8] C.W. Miller, Magnetic Fluids: Magnetic Forces and Pumping Mechanisms, NTSI Final Report AD/A-006323, 1973. The Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithach, New York. [9] V.M. Korovin, A.A. Kubasov, Tangential magnetic flied induced structure in a thin layer of viscous magnetic fluid when developing Rayleigh–Taylor instability, J. Magn. Magn. Mater. 202 (1999) 547–553. [10] J.J. Ahn, et al., The viscosity deviation of magnetic fluids for microactuator due to temperature changes, in: IEEE-EMBS Special Topic Conference on Microtechnologies in Medicine and Biology, May 2–4, 2002. [11] N.E. Grevell, et al., The design of ferrofluid magnetic pipette, IEEE Trans. Biomed. Eng. 44 (3) (1997). [12] A. Hatch, et al., A ferrofluidic magnetic micropump, J. Microelectromech. Syst. 10 (2) (2001). [13] C.W. Miller, E.L. Resler Jr., Magnetic forces and the surface instability in ferromagnetic fluids, Phys. Fluids 18 (9) (1975) 1112–1118.
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[14] R.E. Rosensweig, Ferrohydrodynamics, Cambridge University Press, New York, 1985. [15] W.C. Sim, et al., Thermal and load-deflection FE analyses of the Parylene diaphragms, MSM (2002).
Biographies Eui-gyu Kim received his BS and MS in mechanical engineering from Sogang University, Seoul, Korea in 2001 and 2003, respectively. From 2003, he joined Samsung SDI R&D Center for developing microactuators. Jae-geun Oh received his BS in Electronic engineering and his MS in mechanical engineering from Sogang University, Seoul, Korea in 1995 and 1999, respectively, and PhD in Mechanical Engineering from Sogang University, Seoul, Korea in 2004. From 1986 to 1988, he was a research engineer at R&D Center in Kia Motors, where he worked on design of electronic engine control unit for engine management system. From 2005, he joined MDT R&D Center for developing microsensors. He was in charge of the design and fabrication of the passive radio sensor system. He is currently a principal research engineer in Sensor Valley Co., Ltd. (Kyunggi-do, Korea), where he worked on design of passive radio sensor system and developing of TPMS (Tire Pressure Monitoring System) sensor. His research interest includes microelectromechanical systems (MEMS), microfabrication technologies, RF MEMS and the ubiquitous sensor issues. Bumkyoo Choi received his BS in mechanical engineering and his MS in mechanical design engineering from Seoul National University, Seoul, Korea in 1981 and 1983, respectively, and PhD in engineering mechanics from the University of Wisconsin, Madison, in 1992. From 1984 to 1986 he was a research engineer to the CAD/CAM Laboratory at KAIST (Korea Advanced Institute of Science and Technology), where he worked on the structural analysis and design for machine elements. From 1992 to 1994, he was a technical staff member of CXrL (Center for X-ray Lithography) in the University of Wisconsin where he developed a computer code for thermal modeling of an X-ray mask membrane during synchrotron radiation. From 1994 to 1997, he joined Samsung Electronics for developing microsensors and microactuators. He was in charge of the design and fabrication of the micromirror for display system. He is currently a professor in the Dept. of Mechanical Engineering of Sogang Univ., Seoul, Korea. His research interest includes microelectromechanical systems (MEMS), micromachining and microfabrication technologies, and modeling issues.
A study on the development of a continuous peristaltic micropump using magnetic fluids Eui-Gyu Kim, Jae-geun Oh, Bumkyoo Choi ∗ Department of Mechanical Engineering, Sogang University, #1 Sinsu-Dong, Mapo-ku, Seoul 121-742, Republic of Korea Received 23 March 2005; received in revised form 23 December 2005; accepted 7 January 2006 Available online 24 February 2006
Abstract This paper presents the development of a peristaltic MF (magnetic fluid) micropump for the purpose of applications of ‘lab on a chip’, which is the core technology of the medical & biological fields. In the device, magnetic fluids are gathered in the round-shaped channel by the magnetic force, which is applied from the external permanent magnet controlled by stepping motor. The gathered MF lump deforms the silicone rubber diaphragm and then the deformed diaphragm pushes the sample liquid in the microchannel. The micropump was fabricated through the conventional MEMS (micromachining) technology. It doesn’t have any complicated moving part, but uses magnetic fluids as a driving source. Thus, the proposed micropump can realize more reliability and durability. In addition, the driving method of our ‘continuous peristaltic’ micropump is different from that of other pumps using cavity sequence. Consequently, it brings out significant improvements in the performance. Furthermore, it cannot only pump in forward and backward directions, but also can control flow rate by adjusting the external permanent magnet. The MF lump also serves as a valve when the stepping motor of micropump stands still. The maximum flow rate of the micropump was 2.8 l/min at 4 rpm and 3.8 l/min at 8 rpm. © 2006 Elsevier B.V. All rights reserved. Keywords: Micropump; Magnetic fluid; Micro actuator; MEMS; Micromachining
1. Introduction This paper introduces enhanced experiments to innovate the existing magnetic fluid (hereafter abbreviated as MF) diaphragm actuator [1,2] and proposes a new type of peristaltic MF micropump based on the experimental results. In recent years a variety of microfluidic devices are developed for a wide range of applications, from chemical analysis systems to actuating systems such as micropumps in medicine and biology [3,4]. Many researchers have tried to control the actuating and pumping components of microfluidic devices in the micron or sub-micron unit range by various methods (electrostatic, thermo-pneumatic, piezoelectric, continuous electro wetting, etc.) [5–7]. A few researchers have made efforts to carry the experiments for the delivery systems using the MF [8–13]. Miller proposed a pumping device utilizing a MF composed of a series of electromagnets, one of which generated
∗
Corresponding author. Tel.: +82 2 705 8639; fax: +82 2 712 0799. E-mail address: [email protected] (B. Choi).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.01.021
a gradient magnetic field to continuously move MF through a length of a tube. In order for the electromagnets to constantly pump the MF, the MF must be broken in pieces in the tubes while it was being pumped. This pumping idea was not actually realized. However, Miller’s pump showed the simplicity in that it could move the fluids without any moving parts into a tube of circular cross-section [8]. Lately, Anson Hatch et al. proposed the ferrofluid peristaltic micropump that could be actuated in a chip level [12]. But their MF pumping systems could not get rid of the disadvantages caused by the contamination from the MF. In this study, we present a new microfluidic system to pump and control a tiny quantity of fluid by MF without any contamination by means of the diaphragm structure. In our experiments for diaphragm actuator, we used MF (N304) with highly saturated magnetization (330 G) and low viscosity (10 cP) instead of SMF110 (saturated magnetization: 100 G, viscosity: 1000 cP) in the previous study [1] to improve its functioning. Also it was designed to have a larger volume of reservoir and the optimal corrugated diaphragms for the better performance.
44
E.-G. Kim et al. / Sensors and Actuators A 128 (2006) 43–51
We suggested and fabricated the new MF peristaltic micropump based on the driving method of the enhanced MF diaphragm actuator. In the micropump, magnetic fluids are gathered in the loop channel by magnetic force, which is applied from the external permanent magnet controlled by stepping motor. The gathered MF lump deforms the diaphragm and then deformed diaphragm pushes the sample liquid in the microchannel. The driving method of our peristaltic micropump is different from that of other pumps using cavity sequence. Consequently, it brings out significant improvements in the performance. Furthermore, it cannot only pump in forward and backward directions, but also can control flow rate by adjusting the external permanent magnet. The magnetic fluid lumps are also operated as valves. This MF micropump was made by conventional MEMS fabrication technology. The loop microchannel (with outer diameter of 9 mm, inner diameter of 6 mm and depth of 300 m) was etched by ICP RIE process. It doesn’t have any complicated moving part, but uses magnetic fluids as a driving source. Thus, the proposed micropump can realize more reliability and durability than other existing pumps. 2. Magnetic fluid Magnetic fluids or ferrofluids are the stable colloidal dispersion of sub-micron sized single domain magnetic particles in carrier liquids [14]. The carrier liquid can be an organic solvent, water, or a variety of oil bases. The particles, which have an ˚ are coated with stabilizing disperaverage size of about 100 A, sants, which prevent particle agglomeration even when a strong magnetic field gradient is applied to the magnetic fluid. These suspensions are stable and preserve their properties at extreme temperatures and over a long period of time. The exceptional properties of the fluids have charmed many researchers in various fields. A number of applications have been proposed, and some of them, such as rotating shaft seals, vibration damper, tilt or angle sensor, have been materialized. In this study, we chose the synthetic iso-paraffinic magnetite MFs (N304 and EMG901) with a saturation magnetization of 300 or 600 G and a low viscosity (10 cP) made by Sigma-hi Tech. and Ferrotech Company, and thus a good fluidity could be expected in a micro-scaled channel. The physical and magnetic properties of certain kinds of magnetic fluids are indicated in Table 1. This magnetic fluid is composed of base liquid, ferromagnetic particles, and chemically adsorbed surfactant. The vigorous thermal-movement makes particles avoid settling, and the repulsion caused by surfactant
Fig. 1. The exploded view of the MF diaphragm actuator [2].
prevents aggregation between particles. As a result, it is so stable that no separation occurs. 3. MF actuator and peristaltic MF micropump Before we developed MF micropump, we carried out the preliminary experiments of a diaphragm actuator to get the properties of diaphragm and to confirm the moving ability of the MF. Fig. 1 displays the view of the MF diaphragm actuator. The MF actuator consists of a Parylene diaphragm, two or four reservoirs and an inductor. Two kinds of diaphragms were fabricated for experiments: the planar diaphragm and the corrugated one. The wrinkled diaphragm has four corrugations and its thickness is 3 m. The width and depth of the corrugations are 80 and 20 m, respectively. The first prototype of micropump was a square-type as shown in Fig. 2, which was fabricated by using a wet etching technology. However, when the round-shape micropump was fabricated, the dry etching (deep ICP RIE) process was applied. Fig. 3 shows the pumping movement of the fluids in the channel of the square-type pump. MF flows through the connected channel and pushes the Parylene diaphragm and makes it deformed so that the pressure of the deformed diaphragm drives the fluids into other microchannels. Based on the experimental results of diaphragm actuator and prototype micropump, we proposed a new type of peristaltic MF micropump. The final design of this loop microchannel had the structure as shown in Fig. 4. The diaphragm of micropump was simply spin-coated with silicon rubber (ShinEtsu KE-1206). Accordingly, it had excellent ductility compared with Parylene
Table 1 The physical and magnetic properties of some kinds of magnetic fluids Properties
SMF110
N304
EMG900
EMG901
EMG909
Density, ρ, at 25 ◦ C (g/cm3 ) Saturation of magnetization, Ms , (G) Viscosity, µ, at 27 ◦ C (cP) Volume, % particle concentration ˚ = 10−10 m) Particle size (A
1.04 100 1000 15.4 100
1.14 330 10 12.3 100
1.74 900 60 16.3 100
1.53 600 10 10.7 100
1.02 200 6 3.6 100
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Fig. 3. The peristaltic actuation of a square-type MF micropump [2].
Fig. 2. The exploded view of the square-type micropump.
diaphragm of the actuator. Our micropump consists of the microchannels in both sides of silicon wafer, permanent magnet (3 mm diameter, 2 mm thick, surface Gauss 3400) and stepping motor. It can generate enough pressure for pumping fluids through a strong eternal magnetic field derived from the permanent mag-
net attached to the stepping motor. Therefore, the stepping motor is utilized for controlling the position, the speed, and the direction of the magnetic fluids. In this device, magnetic fluids are collected in the loop microchannel by magnetic force emanated from the external permanent magnet. The collected MF lump deforms the silicone rubber diaphragm as shown in Fig. 5 and then the deformed diaphragm pushes the sample liquid in the microchannel. The peristaltic MF micropump adopted a loop microchannel structure (connected cavity) because fluid was the driving source and it was the most effective structure to get the biggest pressure. Since magnetic fluids flowed in the connected cavity channel we designed the structure of channel as shown in Fig. 6. We named this driving method ‘continuous peristaltic’.
Fig. 4. The schematic of proposed micropump.
Fig. 5. Schematic view of the principle of pump actuation and 3D modeling of proposed micropump [2].
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Fig. 6. The principle of ‘continuous peristaltic’ actuation of MF micropump.
It was different from other micropumps using cavity (or chamber) sequence and brought out significant improvements in the performance.
4. Fabrication process Fig. 7 shows the completely fabricated diaphragm actuator before injecting magnetic fluids and the corrugated Parylene diaphragm. These devices were produced by conventional MEMS technology. Our fabrication process is as follows: first of all, the design of a micropump was printed on a transparent film and was attached on a glass plate to function as a mask against UV light. On ˚ by the other hand, a 4 in. silicon wafer was oxidized to 8000 A thermal oxidation. Then AZ 9260, a thick positive photoresist, was coated on the wafer as the masks for the later ICP RIE process. The photoresist pattern was made by the following procedure: first, silicon substrate was spun at 2000 rpm for 20 s to achieve maximum thickness of photoresist. Then, the substrate was baked for 120 s at 125 ◦ C, exposed to UV light for 60 s followed by development for 2 min to get the complete photoresist pattern. And silicon dioxide was also patterned using a hydrofluoric acid etchant (BOE). The fabrication process of the MF micropump is shown in Fig. 8. Deep inductively-coupled plasma reactive ion etching (ICP RIE) is widely used for etching material in micromachining
applications. We used this process to obtain the 300 m-deep microchannels on both sides of the silicon substrate. Fig. 9 shows dry etched silicon wafer. On the upper side of the silicon wafer, silicon rubber was coated by spin coating processes (300 rpm for 5 s, 800 rpm for 20 s and 1300 rpm for 30 s) and Pyrex was stuck on the etched silicon wafer by the adhesive strength of silicon rubber. Through the transparent Pyrex, we observed how the micropump worked and also bonded Pyrex on the other side of the silicon wafer. Finally, the bonded wafer was diced to produce each piece of the packaged micropump. Fig. 10(a) is the photo of silicon rubber diaphragm fabricated in the middle of two microchannels taken by microscope and Fig. 10(b) is the photo of aligned microchannels. The completely fabricated micropump device is shown in Fig. 11.
5. Experiment and discussion 5.1. Experiments of diaphragm actuator The experiment aims to test the improved performance of the newly designed diaphragm actuator using MF (N304), and to measure the range of diaphragm deflections and to confirm the feasibility of a micropump. Fig. 12(a) shows setups for the magnetic flux density measurements by Gauss meter with an inductor. A Gauss meter
Fig. 7. (a)–(c) The assembled actuator before injecting magnetic fluids [2]. (d) The corrugated Parylene diaphragm [2].
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The experimental setups for measuring the diaphragm deflections due to MF pressure are shown in Fig. 12(b). Both ends of the inductor are connected to the power supply to control current. We estimated the deflection of the diaphragm by MF pressure generated along the flow of input current with a laser deflection measurer (KEYENCE LC-2420). The measured normal magnetic flux density (Bn ) versus input current amplitude (I) is plotted in Fig. 13. The result illustrates that the magnetic flux density is linearly proportional to the input current amplitude. The maximum magnetic field from the inductor is 110 G at 600 mA. Therefore, in this experiment, driving range of magnetic field varies from 0 to 110 G. Fig. 14 shows the deflections of two kinds of diaphragms to various input current amplitudes. The displacements of the corrugated diaphragms with the input current of 500 mA were about 120 m and more than 240 m for the case using old MF (SMF110) and N304 MF, respectively. On the other hand, the maximum displacements were just 22 and 24 m in the case of the flat diaphragms, respectively [1]. These results showed that the enhanced deflection was resulted from both the new MF (N304) and the changed number of corrugations by simulation [15]. As the magnetic flux density was gradually increased, the deflection of diaphragm was also increased remarkably. The response time was reduced compared with the original model. In the test of the previous work [1], it took 1–2 min to return to 0 deflection. But in this experiment, it took just 2 s to bring back due to the difference of viscosity. The prototype (polygonal peristaltic) micropump with 2 mm × 3 mm diaphragm brought about 15 m of deflection even though the maximum magnetic field was amplified without any corrugation on the diaphragm. It implied that corrugation was essential factor for driving the diaphragm and the upper channel was designed by considering the deformation of corrugation. The peristaltic actuation could not acquire enough deflection for pumping due to relatively fewer amounts of MF and small-sized diaphragm. At the same time, it was clearly confirmed that MF had sufficient fluidity for peristaltic actuation and the fluidity would increase considerably in the circle-type peristaltic micropump. 5.2. Experiments of micropump
Fig. 8. The fabrication process of MF micropump.
was used to measure the magnetic flux density in the surface of the inductor placed directly under the actuator. In our experiment, the magnetic field was considered as normal to the actuator because the size of the inductor is larger than that of the diaphragm. It was assumed that the value of magnetic field was measured at 500 m distance from the inductor, which was equal to the width of Pyrex glass.
The experimental setup for valve evaluation of peristaltic micropump is exhibited in Fig. 15. The capillary tubes connected to the micropump were planned to have much wider dimension than the microchannels, which contributed to negligible resistance in system flow. To inject MF into the microchannel, we took the following procedures. As shown in Fig. 15, the micropump has two holes opened for the syringe and atmospheric pressure to help smooth injection of MF. The thin wire was penetrated into the silicon tubes and stuck to the hole for MF injection. Interconnection area was glued with epoxy. After removing the wire, we completed to set the injection tube as shown in Fig. 16. The MF (EMG901) was injected through this tube into the round-shape microchannel with a 10 ml-syringe. Then we manually moved it into the right position with a handheld permanent
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Fig. 9. Etched silicon substrates by ICP RIE process.
Fig. 10. Fabricated silicon rubber diaphragm. (a) Photo of silicon rubber diaphragm fabricated in the middle of two microchannels taken by microscope and (b) photo of aligned microchannels with a diaphragm.
Fig. 11. The fabricated device for the micropump.
magnet. Once the magnetic fluid entered the inlet channel of the pumping loop, it flowed easily by capillary force. The permanent magnets used for pumping operation rapidly forced the magnetic fluid into the desired locations during operation. The magnets were oriented with the same polarity, which was located perpendicular to the plane of the micropump. The permanent magnet was placed on the disk of the stepping-motor, 1.5 mm directly under (the intensity of magnetic field is about 950 G) the pumping loop. Flexible tubes were linked to both the inlet and the outlet holes by epoxy bonding to act as fluid interconnects. After the epoxy was dried, water was injected carefully so as not to trap air bubbles in the microchannel. In the pumping experiment for sampling fluid, we utilized glass capillary tube (1.1 mm inner diameter) for observing the
Fig. 12. The experimental setups [2]. (a) For normal magnetic flux density to input current. (b) For measurements MF pressure-deflection.
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Fig. 13. The magnetic flux density vs. input current amplitude [2].
Fig. 15. Schematic of the experimental setup for valve test.
Fig. 14. The deflection vs. input current amplitude [2].
movement of water. Two 10 ml-syringes were used as reservoirs. And both columns of water were placed at the same height in the syringe reservoirs. The stepping-motor could be controlled at a various speed (2–30 rpm) and both directions by PICBASIC module. To evaluate the performance of micropump on speed of rotation, the mobility of MF in the microchannel was checked as shown in Fig. 17. From the results of experiments, MF could not maintain its shape over 15 rpm. So we chose 4 and 8 rpm as rotational speeds of experiments.
For the measurement of pumping flow rate of the fabricated micropump, it was useful to compare the measured flow rate with the expected one, based on the volume of the pumping microchannel and the speed of revolution of the stepping-motor. With an outer diameter of 9 mm, inner diameter of 6 mm and depth of 200 m, the volume of the pumping microchannel was about 7 l at one rotation (we ignored the volume of concentrated MF lump). Therefore, at the rotational speed of 4 rpm, the expected volumetric flow rate was under 24 l/min and at the speed of 8 rpm, the flow rate must be under 48 l/min. Fig. 18 shows the photos of the pumping flow rate experiments. When we made experiments of pumping flow rate, we maintained the same hydraulic pressure level of both reservoirs after water was injected into the microchannel. As a result, pumping flow rate was 3 mm/min at 4 rpm, and 4 mm/min at 8 rpm. The pumping efficiency was about 10% at 4 rpm and 7% at 8 rpm. The results were not sufficient but we could confirm the feasibility for pumping. For the evaluation of the performance as a valve, we checked the time required to overcome the hydraulic pressure difference
Fig. 16. The assembled micropump after injecting magnetic fluids.
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Fig. 17. Mobility of magnetic fluid during pump operation.
(the height of the water couldn’t be equal due to surface tension). In other words, we measured the time difference between the cases with and without permanent magnet. As the results of the valve test, it took 4 min with permanent magnet that made diaphragm valve according to magnetic fluid lump to overcome initial hydraulic pressure difference of 558 Pa (syringe volume 10 ml, inner diameter 5 mm), whereas it took 30 s without permanent magnet. Namely, the flow rates passing microchannel were 8 and 1 ml/min for each case, so it could be used as a valve.
6. Discussion The cause of leakage (back flow) problem was the wrinkled surface of the silicon rubber diaphragm. In spin coating process, there were surface quality problems especially in 300 m-deeppattern. That is, many wrinkles were formed in radial direction. They made diaphragm more ductile but were operated as a resisting power in junction with Pyrex wafer or as the cause of the transformation of magnetic fluids. And in case of stopping up the upper channel due to the deformation of diaphragm, formed wrinkles were used as passages of leakage. To solve these problems, first, surface quality must be improved. Second, diaphragm-coating side of wafer would be changed from magnetic-fluid-moving microchannel to water-moving-microchannel. Third, after the improvement of diaphragm surface quality, precision design of each channel shapes is requested. It can minimize the leakage in upper channel by accurate measurement and analysis of diaphragm deformation behavior. Concentrated magnetic fluid lump acts like a piston. That is, it acts as a pump and also as a valve. So, the improvement of valve efficiency is the way of improvement of transportation. 7. Conclusion
Fig. 18. Photos of the pumping flow rate experiments.
We showed the feasibility of MF microfluidic system that can pump accurately microfluids using the relatively high magnetic pressure. For the fabrication of the micropump, we adopted the MEMS fabrication process. Two microchannels were etched on the both sides of silicon substrate by double side alignment. We acquired the miniaturization and simplicity of fabrication simultaneously by locating diaphragm in the middle of two microchannels. Other materials such as PDMS will be able to be applied to the fabrication of the proposed device due to the simplicity of the design and the process. So the integration with other microfluidic components can be readily realized. For the result of fluid pumping experiment, the capacity of movement was 2.8 l/min at 4 rpm and 4.7 l/min at 8 rpm for the rotational speed of stepping motor. Even though the efficiency of moving was lower than we expected, the fact that the fluid movement could be possible using the proposed micropump was meaningful. In future work, it is necessary to fabricate the micropump using Parylene with creases in order to get strong diaphragm enough to obtain more efficiency.
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Acknowledgement This study was supported by the Korea Research Foundation Grant (KRF-2002-005-D00003). References [1] W.C. Sim, et al., Fabrication, test and simulation of a Parylene diaphragm, in: Transducers ’01 Eurosensors XV, Munich, Germany, 2001, pp. 1382–1385. [2] E.-G. Kim, W.C. Sim, J.-g. Oh, B. Choi, A continuous peristaltic micropump using magnetic fluid, in: 2nd Annual International IEEE-EMB Special Topic Conference on Microtechnologies in Medicine and Biology, 2–4 May, 2002, pp. 509–513. [3] J. Cheng, L.J. Kricka, Biochip Technology, Harwood Academic Publishers, 2001. [4] G.T.A. Kovacs, Micromachined Transducers Sourcebook, The McGrawHill Companies, 1998. [5] P.R. Simmons, K—An Introduction to Piezoelectric Crystals-Sensors Magazine, Helmers Publications, 1994, pp. 26. [6] Z. Cui, A knowledge Base for Microfluidic Devices, http://www.ponttech.it/know/infobase.htm. [7] G.T.A. Kovacs, Micromachined Transducers Sourcebook, McGraw-Hill, pp. 839–840. [8] C.W. Miller, Magnetic Fluids: Magnetic Forces and Pumping Mechanisms, NTSI Final Report AD/A-006323, 1973. The Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithach, New York. [9] V.M. Korovin, A.A. Kubasov, Tangential magnetic flied induced structure in a thin layer of viscous magnetic fluid when developing Rayleigh–Taylor instability, J. Magn. Magn. Mater. 202 (1999) 547–553. [10] J.J. Ahn, et al., The viscosity deviation of magnetic fluids for microactuator due to temperature changes, in: IEEE-EMBS Special Topic Conference on Microtechnologies in Medicine and Biology, May 2–4, 2002. [11] N.E. Grevell, et al., The design of ferrofluid magnetic pipette, IEEE Trans. Biomed. Eng. 44 (3) (1997). [12] A. Hatch, et al., A ferrofluidic magnetic micropump, J. Microelectromech. Syst. 10 (2) (2001). [13] C.W. Miller, E.L. Resler Jr., Magnetic forces and the surface instability in ferromagnetic fluids, Phys. Fluids 18 (9) (1975) 1112–1118.
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[14] R.E. Rosensweig, Ferrohydrodynamics, Cambridge University Press, New York, 1985. [15] W.C. Sim, et al., Thermal and load-deflection FE analyses of the Parylene diaphragms, MSM (2002).
Biographies Eui-gyu Kim received his BS and MS in mechanical engineering from Sogang University, Seoul, Korea in 2001 and 2003, respectively. From 2003, he joined Samsung SDI R&D Center for developing microactuators. Jae-geun Oh received his BS in Electronic engineering and his MS in mechanical engineering from Sogang University, Seoul, Korea in 1995 and 1999, respectively, and PhD in Mechanical Engineering from Sogang University, Seoul, Korea in 2004. From 1986 to 1988, he was a research engineer at R&D Center in Kia Motors, where he worked on design of electronic engine control unit for engine management system. From 2005, he joined MDT R&D Center for developing microsensors. He was in charge of the design and fabrication of the passive radio sensor system. He is currently a principal research engineer in Sensor Valley Co., Ltd. (Kyunggi-do, Korea), where he worked on design of passive radio sensor system and developing of TPMS (Tire Pressure Monitoring System) sensor. His research interest includes microelectromechanical systems (MEMS), microfabrication technologies, RF MEMS and the ubiquitous sensor issues. Bumkyoo Choi received his BS in mechanical engineering and his MS in mechanical design engineering from Seoul National University, Seoul, Korea in 1981 and 1983, respectively, and PhD in engineering mechanics from the University of Wisconsin, Madison, in 1992. From 1984 to 1986 he was a research engineer to the CAD/CAM Laboratory at KAIST (Korea Advanced Institute of Science and Technology), where he worked on the structural analysis and design for machine elements. From 1992 to 1994, he was a technical staff member of CXrL (Center for X-ray Lithography) in the University of Wisconsin where he developed a computer code for thermal modeling of an X-ray mask membrane during synchrotron radiation. From 1994 to 1997, he joined Samsung Electronics for developing microsensors and microactuators. He was in charge of the design and fabrication of the micromirror for display system. He is currently a professor in the Dept. of Mechanical Engineering of Sogang Univ., Seoul, Korea. His research interest includes microelectromechanical systems (MEMS), micromachining and microfabrication technologies, and modeling issues.