A micropump operating with chemically produced oxygen gas

Sensors and Actuators A 111 (2004) 8–13

A micropump operating with chemically produced oxygen gas夽 Yo Han Choi∗,1 , Sang Uk Son, Seung S. Lee Department of Mechanical Engineering, Pohang University of Science and Technology, San 31 Hyoja-dong Nam-gu, Pohang, Kyungbuk 790-784, South Korea Received 28 May 2003

Abstract A novel oxygen micropump is described in this paper. Pumping power is generated through the chemical production of oxygen gas. Decomposition of hydrogen peroxide by aid of catalyst results in the production of oxygen. Hydrogen peroxide is separated by oily paraffin from catalyst before the start of decomposition reaction. The decomposition of hydrogen peroxide by catalyst is commenced through melting the paraffin by an underlaid microheater. An initial triggering of the micropump by input of electric current is sufficient for the continuous actuation through the decomposition reaction. Liquid sample in reservoir is pushed by the produced oxygen gas and flows through a microchannel. Commercially available thin paraffin layers can replace paraffin for easier fabrication. Moreover, the operation of oxygen micropump can be tightly controlled if hydrogen peroxide is decomposed only by heating. Oxygen micropump will be a preferable component of lab-on-a-chips (LOCs) as well as the source of pure oxygen for chemical systems. © 2003 Elsevier B.V. All rights reserved. Keywords: Micropump; Oxygen; Hydrogen peroxide; Paraffin

1. Introduction Having been introduced, lab-on-a-chip (LOCs) have been favored for the application to the medical diagnoses and many biological experiments. Some LOCs are portable laboratories for experiments which need real-time results such as environmental applications. By the way, many of the LOCs need to be supplied with continuous power for their operation, that is, transportation of liquid samples through microchannels in most cases. Previous transportation methods essentially need exogenous power supply, and this problem results in large size of the chip or demand for accessory devices. High dependency of previous micropumps upon exogenous devices makes LOC impractical for real application. It is also needed to develop a micropump which are cheap enough to be adequate for single usage. Here, we present a new type of micropump using chemically produced gas as the source of propelling power. The 夽

This paper was presented at the 16th IEEE MEMS conference, held in Kyoto, Japan, 19–23 January 2003, and is an expansion and modification of the abstract as printed in the Technical Digest of this meeting. ∗ Corresponding author. Tel.: +82-54-279-8210; fax: +82-54-279-5899. E-mail address: [email protected] (Y.H. Choi). 1 Present address: Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, DaeJeon 305–701, Republic of Korea, Tel.: +82-42-869-3086; fax: +82-42-869-5046. E-mail: [email protected]. 0924-4247/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2003.10.005

use of gases as pumping sources were previously reported [1–3]. Because they electrically hydrolyzed water for the production of oxygen and hydrogen, continuous electric input was essential for pumping operation which resulted in the large system size. The production of explosive hydrogen gas as a result of the hydrolysis of water would be another possible problem. Weng reported another type of micropump which was actuated by nitrogen gas produced through thermolysis [4]. This pump also needed continuous power supply in order to maintain the thermolysis reaction. More recently, Hong et al. described a micropump which was actuated by pressurized gas with single pulsed power input [5]. The short working time and small capacity, however, were the shortcomings of that micropump. Hydrogen peroxide (H2 O2 ) is known to be decomposed into water and gaseous oxygen by some catalysts such as manganese dioxide (MnO2 ) as shown in the following chemical equation: 2H2 O2 (l) → 2H2 O(l) + O2 (g) Substantial amount of gaseous oxygen and negligible heat are produced after the decomposition of H2 O2 . For example, more than 100 ␮l of gaseous oxygen is produced after the decomposition of only 1 ␮l of 30% (w/w) H2 O2 . This reaction is spontaneous and does not need additional constitution for continued reaction. The reaction products, water and oxygen, are totally safe and non-toxic. Therefore, a simple

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Fig. 1. Schematic diagram of the device.

as well as cheap micropump can be realized by use of the decomposition reaction of H2 O2 . By using lower concentrations (<30%) of H2 O2 , the risk of explosion would be eliminated.

2. Device and fabrication Because H2 O2 is effectively decomposed even by trace amount of MnO2 , MnO2 must be perfectly separated from H2 O2 before the start of pumping reaction for the tight control. Moreover, easy as well as fast mixing of the two components in the starting is also essential. These two conflicting aims were achieved by using paraffin. Although paraffin has been used as an actuator media [6], it is novel to use paraffin as a depot or a separator of catalyst. There was no detectable decomposition of H2 O2 when dropped over MnO2 caged in hardened paraffin. H2 O2 started to be decomposed just after heating the hardened paraffin. Fig. 1 shows the constitution and action mechanism of oxygen micropump. H2 O2 in a chamber meets MnO2 only after paraffin being melted by the underlaid microheater. By selecting a paraffin of low melting temperature from the homogeneous paraffin family, the triggering by melting paraffin can be performed without deleterious over-heating. Handling small volume of melted MnO2 /paraffin slurry is, in fact, not a simple process. More recently, we revised the constitution of oxygen micropump. Parafilm (Pechiney Plastic Packaging, Chicago), an extensible thin layer of paraffin, was introduced instead of paraffin slurry in this system. Microheater is covered with a layer of extended Parafilm, and MnO2 powder is laid over the film. Finally, another upper layer of extended Parafilm prevent MnO2 from facing H2 O2 until the start of reaction (Fig. 2). Following fabrications

Fig. 3. Fabrication process of an oxygen micropump.

are the same as the previous one. The original thickness of Parafilm is about 100 ␮m, and can be extended to about 20 ␮m. By being thinned, Parafilm could be easily melted as the case of paraffin. H2 O2 decomposed by MnO2 produces oxygen, which moves to the sample chamber pushing the liquid sample to move. Once the paraffin melted, MnO2 particles are released from the paraffin into the aqueous phase. It was not needed to continuously supply electric current, after an initial triggering, to keep paraffin being melted. MnO2 continued to catalyze the decomposition of H2 O2 without returning to paraffin phase even after hardening of paraffin by turning off the input current. MnO2 seemed to be trapped within aqueous phase once escaped from paraffin phase. This phenomenon was more clearly detected when paraffin was substituted with Parafilm membrane. Fabrication process of an oxygen micropump using paraffin is briefly illustrated in Fig. 3: (a) aluminum was thermally

Fig. 2. Schematic diagram of the MnO2 caged by two layers of Parafilms (the thickness of Parafilm layers is exaggerated for clarity).

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evaporated on a 22 mm × 22 mm borosilicate glass plate to 0.25 ␮m thickness; (b) microheater with size of 2 mm×2 mm was patterned by successive lithographic and etching methods; (c) the fabricated microheater showed 150–200  of electrical resistance. For the formation of a chamber of H2 O2 and catalyst, punctured poly(dimethyl siloxane) (PDMS) was layered on the glass; (d) about 1 ␮l of paraffin/MnO2 slurry was poured into the chamber. Patterns of microchannel and chambers were made by SU-8 photoresist (MicroChem) and used as a mold for PDMS upper sheet (Fig. 4). SU-8 was spin-coated on a Si wafer, and patterns were made by lithographic methods. PDMS was poured on this SU-8 mold and peeled off after curing for more than 5 h at 70 ◦ C. An oxygen micropump was completed by being covered with the upper PDMS sheet (Fig. 5). Because the adherence of PDMS to glass plate is almost irreversible and paraffin is very inert, the fabricated micropump is stable when preserved at room temperature.

Fig. 4. Upper PDMS sheet on SU-8 mold.

3. Experiments and results Oxygen micropump was activated by electric input after injection of 5 ␮l of H2 O2 (30%, w/w) and 10 ␮l of sample dye (bromophenol blue with xylene cyanol FF) into the reaction chamber and sample chamber, respectively.

Fig. 5. (a) An example of completed device containing two symmetric oxygen micropumps on a 22 mm × 22 mm borosilicate glass plate. The microheater region and a part of it are magnified in (b).

Fig. 6. Microheater is covered with black MnO2 powder before paraffin being melted (a). After the reaction being triggered, oxygen bubbles of various sizes can be seen as noted by circles and a rectangle (b). Both of the picture were taken at the same position in the reaction chamber.

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Fig. 7. H2 O2 is being decomposed producing oxygen bubbles. Black MnO2 granules can be seen through transparent Parafilm. over the microheater.

Reaction chamber regions before and under reaction are shown in Fig. 6. The formation of oxygen bubbles in the reaction chamber made of Parafilm rather than paraffin is also shown in Fig. 7. The supply of electric current was stopped once the decomposition reaction of H2 O2 started. In fact, Figs. 6 and 7 were taken after the electric input was stopped. Only about 10–20 mA of current was sufficient to melt paraffin or Parafilm. Alternatively, oxygen micropump could be turned on even by using a 9 V battery (6F22) for about 10 s. The melting temperature of the used paraffin was about 60 ◦ C. Dye in the sample reservoir was pushed through a microchannel by the produced oxygen gas (Fig. 8). The amount of H2 O2 and MnO2 affected working time and pumping volume. The working time was varied by changing the amount of MnO2 . Moving speed increased as more MnO2 is used. Oxygen could be produced for more than 30 min by using very small amount of MnO2 . In addition, short heating resulted in slow decomposition of H2 O2 because of the less exposure of MnO2 . The total volume of produced oxygen is directly proportional to the amount of H2 O2 . The use of lower concentration of H2 O2 resulted in the slower reaction and short reaction time.

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Fig. 8. Dye is flowing through a microchannel after being ejected from the chamber (lower right). Table 1 Calculations of the moving speeds

Maximum Minimum Overalla a

Length (␮m)

Time (s)

Speed (mm/s)

470 225 10028

0.03 1.53 7.51

13.82 0.15 1.34

Measured in the middle of movement.

Dye movement through a microchannel is shown in Fig. 9. Sample dye started to move through a microchannel in Fig. 9(a). Note that all of the dye was pushed out of the sample chamber and still being pushed by the oxygen in Fig. 9(b). All of the dye was pushed out rapidly from the microchannel, and oxygen continued to be produced for more than 10 min. There were some fluctuations of the moving speed reaching over 1 cm/s, and the average flow rate was about 5 ␮l/s (Table 1). According to the Bernoulli’s equation, the maximum pressure exerted by this oxygen micropump was calculated to 0.1 Pa in this example.

4. Discussion Oxygen micropumps described in this paper can be fabricated from cheap materials with ease, are highly

Fig. 9. Movement of dye viewed in the front (a) and rear (b) regions, respectively. Each time interval (T1 to T2 and T2 to T3 ) is 1 s.

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Fig. 10. Flowing of liquid sample through a microchannel. H2 O2 within reaction chamber was decomposed by heating using the underlaid microheater. Reaction was stopped by turning off the electric current in this figure.

Hong et al. reported another micropump simultaneously with our group which used nitrogen gas as the pumping source [7,8]. They used microheater to decompose azobis-isobutyronitrile for the production of nitrogen gas. Therefore, that micropump may correspond to our oxygen micropump controlled by heating only. Our oxygen micropump, however, can be operated by chemical catalysts also, which does not need continuous activation. Therefore, we think that the oxygen micropump is more flexible for broader applications. More recently, we have developed carbon dioxide (CO2 ) generators which can be used as gas suppliers as well as micropumps [9]. Gaseous CO2 is generated through chemical reactions or simple pyrolysis. Importantly, because CO2 is an essential factor for maintaining pH of animal cell culture media, CO2 generator would be an inevitable component in cell chips including portable cell incubators. Application of CO2 generator to microcell incubator is being performed in our group.

Acknowledgements This work was supported by POSCO grant “Development of Micro Device for Cell Manipulation using Bio-MEMS Technology”. Fig. 11. Formation of oxygen bubbles on microheater through the decomposition of H2 O2 by heating only.

References independent upon extra devices, and generate long lasting pumping power. Working time of an oxygen micropump can be prolonged to 30 min, and the total pumping volume also can be expanded by using higher concentration of H2 O2 . It is notable that products of oxygen micropump—water and oxygen—are totally bio-compatible. This micropump will be useful in many microchips including biomedical applications and relieve much of the integration problems in microsystems. In addition, because oxygen micropump produces very pure oxygen gas, it would be the preferable candidate for oxygen source in chemical microchips. It was hard to stop the reaction once turned on, which is the possible shortcoming of oxygen micropump. Fluctuation of pumping rate during long-term working should also be resolved. These could be overcome by using heating system, because H2 O2 is easily decomposed by heating. We have fabricated oxygen micropump which was controlled only by simple heating (Fig. 10). H2 O2 was poured over a microheater without any following addition of catalyst. This type of oxygen micropump showed shortened response times. When heated, H2 O2 was decomposed within two seconds to produce oxygen bubbles as shown in Fig. 11. H2 O2 stopped being decomposed just after withdrawal of the electric current. We are now improving this type of oxygen micropump, and it would be the alternative choice for suitable application.

[1] D.B. Young, T.E. Jackson, D.H. Pearce, A.C. Guyton, A portable infusion pump for use on large laboratory animals, IEEE Trans. Biomed. Eng. BME-24 (1997) 543–545. [2] Y.H. Bae, I.C. Kwon, Insutech, Inc., Gas pressure driven infusion system by hydrogel electrolysis, US Patent 5,354,264 (1994). [3] H.C. Kim, Y.H. Bae, S.W. Kim, Innovative ambulatory drug delivery system using an electrolytic hydrogel infusion pump, IEEE Trans. Biomed. Eng. 46 (6) (1999) 663–669. [4] K.-Y. Weng, Thermolysis reaction actuating pumps (TRAP), in: Proceedings of the Micro Total Analysis Systems 2001 Symposium, Monterey, USA, 21–25 October 2001, pp. 409–410. [5] C.-C. Hong, J.-W. Choi, C.H. Ahn, A disposable on-chip air detonator for driving fluids on point-of-care systems, in: Proceedings of the Micro Total Analysis Systems 2002 Symposium, Nara, Japan, 3–7 November 2002, pp. 949–951. [6] E.T. Carlen, C.H. Mastrangelo, Electrothermally activated paraffin microactuators, J. Microelectromech. Syst. 11 (3) (2002) 165–174. [7] Y.H. Choi, S. Son, S.S. Lee, Novel micropump using oxygen as pumping source, in: Technical Digest, IEEE 6th International Conference on Micro Electro Mechanical Systems, Kyoto, Japan, 19–23 January 2003, pp. 116–119. [8] C.-C. Hong, S. Murugesan, S. Kim, G. Beaucage, J.-W. Choi, C.H. Ahn, A functional on-chip pressure generator using solid chemical propellant for disposable lab-on-a-chip, in: Technical Digest, IEEE 6th International Conference on Micro Electro Mechanical Systems, Kyoto, Japan, 19–23 January 2003, pp. 16–19. [9] Y.H. Choi, S.S. Lee, Novel micro gas generator of carbon dioxide for actuation and gas source, in: Micro Total Analysis Systems 2003 Symposium, California, USA, 5–9 October 2003, pp. 611– 614.

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Biographies Yo Han Choi is currently working as a research Professor in the Department of Mechanical Engineering in Korea Advanced Institute of Science and Technology. He received his PhD degree from the Department of Life Science in Pohang University of Science and Technology, Korea in 2001. He has researched the development of anti-HIV drugs, cell biology, and protein chemistry using biochemical and molecular biological methods. He is currently working as a postdoctoral in the Department of Mechanical Engineering in Pohang University of Science and Technology. His research interests are actuation systems, biochips including protein chips and cell chips, and manipulation of biomolecules using MEMS technology. Sang Uk Son is a candidate for PhD in Korea Advanced Institute of Science and Technology. He received his BS in mechanical engineering and MS in mechanical design engineering from Pusan National Univer-

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sity, Korea in 1997 and 1999, respectively. He is a candidate for PhD in Pohang University of Science and Technology, Korea. His main interest is development of cell manipulation, counting, and sorting system using Bio-MEMS technology. Seung Seob Lee moved to Korea Advanced Institute of Science and Technology as an associate Professor in the Department of Mechanical Engineering in 2003. He was born in Seoul, Korea in 1962. He received the BS degree from Seoul National University, Korea in 1984 and the MS degree in mechanical engineering from University of California, Berkeley, CA in 1989. After his MS degree, he joined the Berkeley Sensor and Actuator Center (BSAC) and got PhD there in 1995. After 1 year at Samsung Advanced Institute of Technology (SAIT) in Kiheung, Korea, he joined the faculty of the Department of Mechanical Engineering in Pohang University of Science and Technology, Korea in 1997. He is an associate professor now.