A VALVE-LESS MICROPUMP DRIVEN BY CONTINUOUS ELECTROWETTING (CEW) ACTUATION

Transcription

1 ISTP-16, 2005, PRAGUE 16 TH INTERNATIONAL SYMPOSIUM ON TRANSPORT PHENOMENA A VALVE-LESS MICROPUMP DRIVEN BY CONTINUOUS ELECTROWETTING (CEW) ACTUATION Yuh-Chung Hu, Xiang-Jun Chen Department of Mechatronic Engineering, Huafan University Corresponding author: ychu@huafan.hfu.edu.tw, #4050, Keywords: micropump, microfludics, electrowetting, MEMS, CEW Abstract According to the comprehensive survey of the present literatures about micropumps, there are still many improvements space to the issues about power consumption, performance, reliability, and process cost. This paper is to develop a valve-less micropump driven by continuous electrowetting actuation for low driving voltage and low power consumption. It is especially suitable for portable devices. Considering the issue of power consumption, the continuous electrowetting (CEW) actuation has the advantage of low driving voltage and low power consumption. Considering the issues of performance and reliability, the diffuser/nozzle elements with direction dependent flow characteristic are used to replace the function of check valve. Thus, the breakdown in virtue of the wear, fatigue, and blocking of check valve can be avoided. Considering the issue of fabrication process and cost, the thick photoresist SU-8 is used to form the structures of pump chamber, diffuser/nozzle elements, channels, and CEW actuator. SU-8 can easily be made to high aspect ratio structures by simply UV light photolithography and are very robust and corrosion endurance after hardening. Moreover, SU-8 is nonconductor, so the dielectric layers such as SiO 2 /Si 3 N 4 by chemical vapor deposition is not required. The fabrication techniques adopted in this paper are only the photolithography to form microstructures and the sputtering to deposit electrodes. So the fabrication process equipments and cost are very cheap. 1 Introduction There is a steadily growing market for Micro-Electro-Mechanical Systems (MEMS) in medical applications. This is driven by continued technological innovation and strong market demand. Small size, low cost, improved performance and efficient power consumption are a few of the benefits that MEMS-based devices have to offer. The next 5 years will see the main growth areas for MEMS-based medical devices in tools for minimally invasive surgery, hearing aids, and DNA chips. Markets for drug delivery systems and neural rehabilitation devices are expected to develop more slowly [1]. This work is to develop a micropump with low operation voltage and low power consumption which is the key component in the biomedical applications of MEMS. In the past, most drugs were small chemical molecules, and were taken orally passing straight through the wall of the stomach into the blood stream. New developed biotech drugs are larger molecule protein and peptide based drugs. As many of these drugs are broken down by the stomach or cannot be absorbed by the stomach wall, they have to be delivered via the lungs, nose or by injection. The drug delivery market is close to US $ 20 billion and is segmented into four categories: oral (53%), inhalation (27%), transdermal (10%), and implanted (8%). Companies that are active in the implantable pumps include Cordis, Tricomed, Pfizer, Therex, Medtronic, Duret, and Debiotech. The biochip business, which includes devices known as DNA chips, micro 1

2 arrays, and Lab-on-a-chip, has advanced significantly in the past few years. In the early 1990 s all the required biochemical reaction were carried out on a macro scale using testtube chemistry. More and more functions are now possible on a micro-scale using the devices made by MEMS. The advantages of using MEMS include faster sample preparation, multiple samples, faster and multiple reactions, built-in electronic and optical detection, small volumes and sealed cartridges for easier disposal. Micropump is one of the key components in the drug delivery systems and the biochips, which plays the role of preciously controlling or supplying tiny fluid flow. There are some commonly used actuating types in micropumps, those include the piezoelectric, the electrostatic, the thermopneumatic, the electromagnetic, the bimetallic, the shape memory alloy (SMA), and the continuous electro wetting (CEW) that is adopted in the present paper. The performances of the aforesaid actuating methods are listed as the Table 1 [2]. It is shown that the driving voltages of the piezoelectric and electrostatic require several hundred volts, while the power consumptions of the thermopneumatic, the electromagnetic, and the SMA are several hundred and even several thousands mw, all of them can not satisfy the demands of low driving voltage and low power consumption for mobile products, such as the embedded medical devices, the remote environment monitoring systems, the portable chemical analyzing instruments,...etc. In recent years, Yun et al. reported the micropump based on continuous electrowetting (CEW) with low voltage and low power consumption [2]. The classic construction of a mechanical micropump consists of a pump chamber closed with a flexible diaphragm on one side and the inlet/outlet check valves. By means of a suitable actuation principle, upward and downward movement of the diaphragm is achieved to generate volume changes and, hence, under- and overpressure transients in the pump chamber. The inlet and outlet check valves are used to control the flow and prevent backward flow. The effort to design and fabricate such valves is considerably high, as a number of properties, like backward flow, pressure drop and switching speed have to be kept under tight control to achieve a working micropump. Moreover, wear and fatigue is a critical issue. There is also the risk of valve blocking by even small particles, which instantly would degrade the pumping performance and therefore limits the application range of most valve-based micropumps to filtered media [3]. The so-called valve-less micropump concept was tailored to avoid these problems. The device was first introduced by Erik Stemme, Gőran Stemme, and Anders Olsson in 1993 [4, 5]. It uses diffuser/nozzle elements with direction-dependent flow characteristics to mimic the function of a check valve. After that, there are many diffuser/nozzle based valve-less micropumps made on silicon substrate by silicon micromachining were introduced [6 10]. In the past, the manufacturing of micropump is very complicated and high-cost [2 10]. The pump chamber, diaphragm, and check valves must be Table 1. Performance comparison of micropumps [2]. Actuation Type Maximum flow rate (µl/min) Operation voltage (V) Power consume (mw) Maximum pump pressure (kpa) Piezoelectric Electrostatic Thermopneumatic Electromagnetic Bimetallic SMA CEW

3 A VALVE-LESS MICROPUMP DRIVEN BY CONTINUOUS ELECTROWETTING (CEW) ACTUATION patterned and made by photolithography and etching. The dielectric layers, such as SiO 2 and Si 3 N 4 are made by chemical vapor deposition. The electrodes must be made by sputtering. The fabrication of pump chamber always needs the high-cost deep reactive ion etching. There is still large improving range no matter on the issues of power consumption, performance, reliability, manufacturing, and cost. Therefore, this paper introduces a new valve-less micropump driven by CEW, which can meet the demands of low driving voltage, low power consumption, and high performance. The whole construction of the proposed micropump, including pump chambers, diffuser/nozzle elements, and fluid channels, is made of the thick photoresist SU-8 entirely and the manufacturing requires only two-mask photolithography. SU-8 is a kind of thick photoresist which can be easily formed into high aspect ratio structure by UV-light source and is very robust and corrosion-endurance after hardening. In this paper we report the design concepts of the proposed micropump followed by its fabrication process and results. 2 Design Concepts Since the thick photoresist SU-8 can be formed into high-aspect-ratio structures by UVlight lithography and is very robust and corrosion-endurance after hardening, therefore, this work is to use it to manufacture the structures of the pump chamber, the diffuser/nozzle elements, the fluid channels, and the CEW actuators. The manufacturing cost can be reduced greatly in virtue of giving up the expensive processes of deep reactive ion etching. Furthermore, the chemical vapor deposition of dielectric layers is not required since that SU-8 itself is electric-isolative. Figure 1 depicts the design of the proposed micropump. The proposed micropump is composed of a stack of three parts. The upper part is the pump chamber with diffuser/nozzle elements, the bottom part is the CEW actuator, and a diaphragm is sandwiched in between the upper and bottom parts. The diaphragm is made of silicon-rubber or Parylene-C. Pump chamber with Diffuser/Nozzle Diaphragm CEW actuator Fig. 1. The exploded schematic of the proposed micropump. 2.1 CEW Electrowetting is an electrically induced change of a material s wettability. The principle of electrowetting is first introduced by Lippman in He found that the surface tension of a mercury drop immersed in electrolyte can be changed by changing the electric potential difference between the electrolyte and the mercury drop. Fig. 2 shows a schematic illustration of the continuous electrowetting. Without bias voltage, the mercury drop in a capillary filled with electrolyte has uniformly distributed surface charge density along the x- direction. If a bias voltage is applied between the two electrodes, the variation of the electric potential difference between the mercury drop Fig. 2. The schematic of CEW [2]. 3

4 Fig. 3. The schematic of valve-less pump [4]. and the electrolyte will induce the redistribution of the surface charge on the mercury drop. The surface tension in the region of higher charge density is lower than in the lower charge density region. This surface tension gradient induces motion of the mercury drop. 2.2 Valve-less Pump The pump action, illustrated in Fig. 3, where fluid directing diffusers are used instead of passive check valves in a diaphragm pump arrangement [4 8]. Two identical fluid directing elements are connected to the inlet and outlet of the pump. Each element acts as a diffuser in the main pump flow direction with a lower flow resistance in the diffuser direction than in the opposite, nozzle direction. This means that during the supply mode a larger amount of fluid flows into the pump chamber through the inlet element, which acts as a diffuser, than through the outlet element, which acts as a nozzle. The opposite is true during the pump mode. Thus, during a complete pump cycle a net fluid volume is transported from the inlet to the outlet side. 3 Fabrication Process As shown in Fig. 1, the proposed micropump is composed of a stack of three parts. The upper part is the pump chamber with diffuser/nozzle elements, the bottom part is the CEW actuator, and a diaphragm is sandwiched in between the upper and bottom parts. The pump chamber and diffuser/nozzle elements are made of SU-8 by photolithography on a glass substrate. The structure of the CEW actuator is also made of SU-8 by photolithography on a silicon substrate. The diaphragm is made of silicon-rubber or Parylene-C. The dimensions and fabrication processes are detailed in the following. 3.1 Pump Chamber and Diffuser/Nozzle Two parallel pump chambers working in anti-phase are used to reduce the inlet and outlet pressure pulses and to increase pump flow performance, see Fig. 4. The 500 µm thick Pyrex-7740 glass wafer is used as substrate and as the upper cover of the micropump. The pump Fig. 4. The schematic of pump chamber and diffuser/nozzle elements and the finished component. 4

5 A VALVE-LESS MICROPUMP DRIVEN BY CONTINUOUS ELECTROWETTING (CEW) ACTUATION Table 2. The process parameters of pump chamber and diffuser/nozzle elements. Process Parameters Substrate pre-bake 120 o C, 5 min. SU-8 spin-on 800 rpm, 30 sec 1400 rpm, 50 sec Soft bake 65 o C, 30 min. 95 o C, 100 min. Exposure 895 mj/cm 2 Post-bake 65 o C, 10 min. 95 o C, 25 min. Development Immerse 15 min. chamber and diffuser/nozzle elements are made of SU-8 by photolithography on the glass substrate. The entire process needs only one mask. The process parameters are listed in Table 2. The finished element is shown in Fig CEW Actuator The structure of the CEW actuator is also made of SU-8 by photolithography on a silicon substrate. The platinum electrodes are made by e-beam evaporation deposition and wet etching on silicon substrate firstly, as shown in Fig. 5. One mask is needed to pattern the electrodes. Then, a second mask is used to form the SU-8 structure of actuator over the electrodes, as shown in Fig. 6. The process parameters of the electrodes are listed in Table 3 and the process Table 3. The process parameters of electrodes. Process Parameters Pt deposition 2000 µm AZ P4620 spin-on 1000 rpm, 15 sec 3800 rpm, 25 sec Soft bake 90 o C, 2 min. Exposure 410 mj/cm 2 Development AZ 400K 210 sec Hard bake 120 o C, 2 min. Etching H2O:HCl:HNO3 = 4 :3:1 80 o C, 2 hr parameters of the actuator structure is the same as the ones of pump chamber, see Table 2. The finished element is shown in Fig Diaphragm The diaphragm is made of silicon-rubber or Parylene-C. The thin film of Parylene-C can be fabricated by chemical vapor deposition (CVD). The CVD process consists of three steps: vaporization of the Parylene at 150 o C, pyrolysis of the Parylene into monomer at 690 o C, and then the deposition of Parylene-C on substrate at room temperature. Fig. 7. The finished structure of CEW actuator. Fig. 5. The schematic of electrodes. Fig. 6. The schematic of CEW actuator. Fig. 8. The schematic of Parylene-C diaphragm. 5

6 Fig. 9. The finished CEW actuator filled with the electrolyte, sodium sulphate, and the mercury drop. 3.4 Assembly Adding a mercury drop into the channel in the actuator and filled with the electrolyte, sodium sulphate, see the Fig. 9. Three components, the pump chamber with diffuser/nozzle, the diaphragm, and the CEW actuator, are stacked together by AB-glue, see the Fig Conclusions This paper proposes a valve-less micropump driven by continuous electrowetting actuation for low driving voltage and low power consumption. The breakdown in virtue of the wear, fatigue, and blocking of check valve can be avoided by the use of diffuser/nozzle elements to act as the function of check valves. The thick photoresist SU-8 is used to form the whole structures of pump chamber, diffuser/nozzle elements, channels, and CEW actuator. SU-8 can easily be made to high aspect ratio structures by simply UV light photolithography and are very robust and corrosion endurance after hardening. Moreover, SU-8 is nonconductor, so the dielectric layers such as SiO 2 /Si 3 N 4 by chemical vapor deposition is not required. The fabrication techniques adopted in this paper are only the photolithography to form microstructures and the sputtering to deposit electrodes. So the fabrication process equipments and cost are very cheap. The driving voltage of the proposed micrupump is below 10 V. Fig. 10. The schematic of the assembled micropump and the finished physical micropump. References [1] Malcolm Wilkinson, Medical market for microsystems, NEXUS-mstnews, Vol. 4, No. 2, pp 37-38, [2] C. J. Kim et al., A surface-tension driven micropump for Low-Voltage and Low-Power Operations, Journal of Microelectromechanical Systems, Vol. 11, No. 5, pp , [3] Peter Woias, Micropumps Summarizing the first two decades, Microfluidics and BioMEMS, Carlos H. Mastrangelo, Holger Becker (eds.), Proceedings of SPIE, 4560, pp 39-52, [4] Erik Stemme and Gőran Stemme, A valve-less diffuser/nozzle-based fluid pump, Sensors and Actuators A, Vol. 39, pp , [5] Anders Olsson, Gőran Stemme, and Erik Stemme, A valve-less planar fluid pump with two pump chambers, Sensors and Actuators A, Vol , pp , [6] Anders Olsson, Gőran Stemme, and Erik Stemme, Micromachined diffuser/nozzle elements for valveless pumps, Proceedings of the IEEE MEMS 96 Workshop, pp , [7] Anders Olsson et al., An improved valve-less pump fabricated using deep reactive ion etching Proceedings of the IEEE MEMS 96 Workshop, pp , 1996 [8] Anders Olsson et al., Valve-less diffuser micropumps fabricated using thermoplastic replication Proceedings of the IEEE MEMS 97 Workshop, pp , [9] Jr-Hung Tsai and Liwei Lin, A thermal bubble actuated micro nozzle/diffuser pump, Proceedings of the IEEE MEMS 2001 Conference, pp , [10] Shunichi Hayamizu et al., New bi-directional valveless silicon micropump controlled by driving waveform, Proceedings of the IEEE MEMS 2002 Conference, pp ,