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Electrochemical Bubble-Based Bidirectional Microfluidic Transport Hirotaka Obata, Tomoaki Kuji, Kenichi Kojima, Fumihiro Sassa, Masatoshi Yokokawa, Kazuhiro Takekoshi, and Hiroaki Suzuki ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.5b00059 • Publication Date (Web): 11 Dec 2015 Downloaded from http://pubs.acs.org on December 22, 2015
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Electrochemical Bubble-Based Bidirectional Microfluidic Transport Hirotaka Obata†, Tomoaki Kuji†, Kenichi Kojima†, Fumihiro Sassa‡, Masatoshi Yokokawa†, Kazuhiro Takekoshi§, and Hiroaki Suzuki*† †
Graduate School of Pure and Applied Sciences, ‡Graduate School of Life and Environmental Sciences, §Graduate School of Comprehensive Human Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan
ABSTRACT: With the aim of application to biochemical analyses, efficient bidirectional microfluidic transport was achieved through the reversible electrochemical production and shrinkage of hydrogen bubbles. A three-electrode system with a platinum black working electrode, a Ag/AgCl reference electrode, and a platinum auxiliary electrode was incorporated into a poly(dimethylsiloxane) structure containing the necessary flow channels and compartments. The influence of the electrode and flow channel structures on the operation of the system was investigated. The production and shrinkage of bubbles was achieved by applying appropriate potentials to the working electrode, which minimized the influence of spontaneous shrinkage resulting from the oxidizing effect of dissolved oxygen. Device performance depended on the structure of the working electrode, meaning that further optimization will be necessary. The device was shown to withdraw solution through a minimally invasive needle and to process liquid plugs in a microfluidic system. KEYWORDS: micropump, platinum black, bubbles, point-of-care testing, whole blood, plug, microfluidics
In the long history of the development of micropumps, a variety of mechanisms and devices have been proposed that could lead to the development of integrated microfluidic systems.1-3 However, despite many theoretical and technological advances, there are still very few micropumps that can be practically employed in this way. These limitations have in turn affected the development of point-of-care testing (POCT) technology, a field that has shown increasing promise in the last few years.4-6 Processing of solutions always poses a problem for POCT, since processes like solution exchange may be required for certain analytes. One approach for addressing this issue is to incorporate pumps directly into the measuring instrument in which the signal from the analytical chip is processed, though this might make compact instruments prohibitively expensive. In this case, it will be necessary to develop new ways to simplify the structure, fabrication, and function of micropumps for use in microanalytical devices, a goal that might be achievable through the use of bubble-based micropumps. In older bubble-based micropumps, gas bubbles were produced to displace a liquid column in a microfabricated flow channel. These systems relied on oxygen and/or hydrogen bubbles produced by the electrolysis of water 7-14 or the catalytic decomposition of H2O2.15-16 Other systems that have used carbon dioxide produced by electrolysis 17 and nitrogen gas produced by thermolysis 18 relied on the bubbles to apply pressure for unidirectional solution transport. However, these systems allow only for forward movement, and do not address situations in which backward movement is required 19 – for example, when withdrawing solutions for direct sampling. Some bidirectional systems do exist – for example, several use water vapor bubbles from boiling water that can be shrunk through cooling.8 However, the generation of heat in this system poses a challenge in integrating it with biological components. As an alternative, we have proposed micropumps that rely on the growth and shrinkage of hydrogen bubbles on platinum black electrodes.20-21. This electrode material was selected because it effectively catalyzes the H+/H2 reversible redox reaction, while its
very large surface area allows for significant control in either direction.22 Unlike with many other materials, shrinking the produced bubbles is unchallenging, and can be accomplished by applying a potential to the electrode.20-21,23 However, realizing efficient, reproducibile pumping with this system can be problematic because of the insulating and bouyant nature of the bubbles, in addition to other effects that may influence bubble volume change. This study addresses these problems to increase the functionality of this system with the aim of potentially integrating the micropump with chemical sensors. ■ EXPERIMENTAL SECTION Reagents and Materials. Reagents and materials used for the fabrication of the devices are as follows: glass wafers (#7740, 3 inch, 500 µm thick), from Corning Japan (Tokyo, Japan); polyimide prepolymer solution (Semicofine SP-341), from Toray Industries (Tokyo, Japan); a thick-film photoresist (SU-8 25), from Microchem (Newton, MA, USA); polydimethylsiloxane (PDMS) prepolymer solution (KE-1300T), from Shin-Etsu Chemical (Tokyo, Japan); and a micro-needle (Nanopass needle II, Nanopass 34G), from Terumo (Tokyo, Japan). Other reagents were purchased from Wako Pure Chemical Industries (Osaka, Japan) unless otherwise noted. All solutions were prepared with ultrapure water produced by a water purification system (Direct-Q UV 3, Merck, Darmstadt, Germany). Human whole blood samples were obtained from Tsukuba iLaboratory LLP. The Institutional Review Board at the University of Tsukuba reviewed and approved this study (No. 963). Micropump Structure and Fabrication. The micropump was constructed by stacking a glass substrate with a thin-film threeelectrode system and a PDMS substrate with microfluidic structures (Figure 1A). Details about the electrodes and compartments are summarized in Table S1. The system contained a working electrode used for actuation, a Ag/AgCl reference electrode, and a platinum auxiliary electrode. A base layer for the electrodes was formed with platinum. Furthermore, a silver pattern was formed,
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yet only on the reference electrode area. The active areas of the electrodes were delineated with a polyimide insulating layer. Figure 1B shows the working electrode structure. The electrode did not extend to the vicinity of the flow channel connecting it with the reference electrodes, with the edge of the working electrode on that side equidistant from the exit of the connecting flow channel. Straight thin walls with a width of 50 µm and a height of 40 µm, formed using the SU-8, extended from the connecting flow channel exit to the lower stream. Additionally, the working electrodes included five walls, though one was also tested without the wall structure (Figure 1B). Chromium and platinum were deposited on this structure. The polyimide insulating layer covered the entire silver layer of the reference electrode, leaving 600 pinholes of 20 µm in diameter. To complete the reference electrode, AgCl was grown from the pinholes.24 Flow channels, compartments, and an injection port were formed with PDMS by replica molding using an SU-8 template. Compartments for the electrodes were separated and each compartment was connected with a flow channel that played the role of a liquid junction. The width of the flow channels was 400 µm. The height of the compartments and flow channels was 150 µm, unless otherwise noted. In addition, a through-hole was formed at the solution inlet by punching, at which point a silicone tube (inner diameter: 500 µm) was connected and fixed with a silicone adhesive. To measure the displacement of the front meniscus, a long serpentine flow channel was formed at the end of the compartment of the working electrode. The PDMS substrate was placed on the glass one after carefully aligning the patterns on both. The substrates were then intercalated between two plastic plates and fixed with a clip to ensure hermetic sealing. In the following experiments, the influx of oxygen into the working electrode compartment affected the pumping action. Aluminum foil was therefore embedded to investigate the extent of this effect (Figure 1C). For the experiment in which solution was withdrawn into the device, a needle was inserted into a through-hole on the flow channel and fixed with a silicone adhesive. For this purpose, the height of the flow channel was 200 µm. Details regarding the fabrication of the device are provided in the supporting information. Principle of Operation, Resulting Problems, and Attempted Solutions. The bidirectional pump operated through the reversible production and shrinkage of hydrogen bubbles based on the following redox reactions. 2H+ + 2e- → ← H2
(1)
In a flow channel with a closed upper stream end, the increase in bubble volume applies pressure to the solution, moving it downstream to an open outlet. When the bubbles shrink, that solution is then withdrawn. However, in practice, because of the Ohmic drop along the current pathway, the production of bubbles starts near the reference electrode. As the hydrogen bubbles grow, they merge, increasing the magnitude of the Ohmic drop due to their insulating nature and impeding the diffusion of protons to the electrode surface. This effects is particularly large near the liquid junction between the compartments. If taken too far, this can terminate the circuit. The thin-walled working electrode structure was used to solve this problem and ensure that the electrode is sufficiently wet, even after substantial bubble accumulation. In our preliminary study, we tested electrodes with various microstructures, such as pillars and dents, and found that they significantly extend the redox reactions. However, we concluded that an array of thin walls would work better, as they keep larger bubbles away from the electrode surface (Figure S2A). Even when a substantial volume of the gas occupies the compartment and the solution between the walls is excluded, some solution is always left in the bottom corners be-
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Figure 1. Structure of the bubble-based micropump. (A) Separated view of the individual layers. (B) Working electrode structure. (C) Cross-section along the X-X’ line in (B).
cause of the rounding effects of interfacial tension (Figure S2B). Compartment geometry is also important in this regard. Therefore, in this device, a fan-shaped structure was used that expanded from the exit of the connecting flow channel between the working and reference electrodes; this ensured that accumulating and growing bubbles moved downstream as a result of the difference in Laplace pressure 25 (Figure 1B). Procedure. A 0.5 M HCl solution was introduced into the compartments and flow channels through a silicone tube connected to the end of the auxiliary electrode compartment, after which the tube was closed with a clip. Although a more concentrated HCl solution may be used to promote the production of hydrogen bubbles, we chose this concentration considering the potential damage to the Ag/AgCl reference electrode.24,26 The volume of the solution was adjusted so that the front meniscus settled in the middle of the flow channel. The movement of the meniscus was observed under a microscope (VB-G25, Keyence, Tokyo, Japan) and was recorded using a video camera (VB-7010, Keyence, Tokyo, Japan). The electrode reactions were controlled using a potentiostat (HA-151, Hokuto Denko, Tokyo, Japan) by applying a constant potential to the working electrode with respect to the on-chip Ag/AgCl electrode. The applied potentials were −1.2 V and 0 V for the production and shrinkage of the hydrogen bubbles, respectively, unless otherwise noted. AgCl in the reference electrode was regrown when the experiments were repeated using the same device. The experiments were conducted at room temperature. ■ RESULTS AND DISCUSSION Effect of the working electrode structure. Figure S3 shows SEM images of the platinum black surface, which consisted of numerous larger protrusions ranging from 300 to 600 nm in size and a larger number of smaller ones ranging from 30 to 100 nm, yielding a significantly enhanced surface area. When hydrogen bubbles were produced on a platinum electrode, a lag time accompanied by a substantial current was always observed before an
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increase in gas volume; however, this lag phase was notably longer when platinum black was used. Although it is evident that reduction was taking place, it is not currently clear what role the lag time plays. The new device geometry successfully prevented insulation between the working and reference electrodes, keeping the circuit from terminating. Without the wall structure, the production of the bubbles stopped when the compartment was only partially covered, yet with the wall structure, bubble production continued until the entire compartment had been filled. Figure 2A shows the dependence of liquid displacement on applied potential. Bubble production increased as the potential dropped from −0.6 to −1.6 V. Cyclic voltammograms taken for samples in which only the electrodes were submerged in 50 mL of the electrolyte solution showed a rapid current increase corresponding to the evolution of hydrogen at potentials more negative than 0 V (Figure 2B). Considering the pH of the solution, the potential values in Figure 2A are very negative. Of course, the proton depletion that accompanies hydrogen evolution cannot be neglected for the small amounts of solution found in the thin compartment. Because the bubble production rate actually changes monotonicaly with respect to applied potential, both the electrode reaction and proton diffusion are rate limiting. We also examined changes at constant currents. In this case, the Ohmic drop is compensated to maintain the constant current. However, no significant difference was observed in the tendencies, including the lag time before bubble production. In this case, the accumula-
tion of bubbles in the auxiliary compartment was more significant and the electrical connection was terminated more often with the same rate of change, as shown in Figure 2A. Thus, structural modification, including that required for releasing the bubbles, will be necessary for stable operation of the device based on this mode of operation. To investigate the Ohmic drop, we replaced the platinum black working electrode with a silver electrode, which actually worked as a Ag/AgCl electrode during potential cycling. Because of the nonpolarized nature of the Ag/AgCl electrode, the curve obtained for large solution volume was almost vertical in the presented potential range (Figure 2C). On the other hand, within the PDMS compartments and flow channels, the current changes accompanying the potential change was small, even though the same working electrode was used; this indicates that the voltage applied between the working electrode and reference electrode was used for the Ohmic drop – or, in other words, that the Ohmic drop depends on the current. Note that the Ohmic drop at a current can be estimated from the potential difference between the two curves. Since the current recorded during the operation was on the order of several hundred µA, the Ohmic drop must have been on the order of several hundred mV. Inherent Problems in Device Operation. Meniscus movement resulting from hydrogen bubble production for the device not employing the wall structure is shown in Figure S5A. In this case, a constant voltage of −1.2 V was applied throughout. The expected termination of the circuit due to bubble growth would have resulted in the dotted line shown in the graph, indicating bubble saturation and retention. However, instead, once bubble production stopped, the resulting bubbles then began to shrink, despite the continued voltage application. The applied potential was able to produce the bubbles but was too negative to shrink them. Figure S5B shows the performance of a device with the wall structure under similar conditions. In this case, the production of hydrogen bubbles did not stop even when the entire compartment was filled with the bubbles, moving the meniscus much further. Nevertheless, the bubbles did shrink after the potential was switched off. Overall, the observed shrinkage could be due to both the dissolution of the gas in the electrolyte solution and termination of the circuit on the working electrode, leading to simultaneous redox reactions in the lower stream. Spontaneous shrinkage was observed mainly for bubbles on the working electrode, suggesting that the effect is mostly electrochemical in origin. The reduction of dissolved oxygen through the oxidation of hydrogen bubbles on the working electrode may account for this. O2 + 4H+ +4e- → 2H2O
Figure 2. (A) Meniscus displacement, as a result of hydrogen generation, with respect to time. The portions corresponding to lag times are omitted. (B) Cyclic voltammograms of the prepared devices. The black line corresponds to a platinum black working electrode, while the red line represents a platinum working electrode without platinum black. (C) Cyclic voltammograms obtained with a Ag/AgCl working electrode. The black line shows a voltammogram obtained in 50 mL of 0.5 M HCl solution in a beaker. The red line shows a voltammogram obtained for solution within the chip. In (B) and (C), the potential was measured with respect to the on-chip Ag/AgCl reference electrode.
(2)
To assess the effect of gas dissolution, an empty compartment, otherwise identical to the one containing the reference and auxiliary electrodes, was placed next to the working electrode compartment in the lower stream, such that the produced hydrogen moved there. In this case, bubble shrinkage proceeded slowly, with volumes after 10 min at 87% of the initial volumes, on average (n = 5). Of course, hydrogen could have diffused through the PDMS, but the effect may be negligible on the time scale of the pumping cycles. To determine whether this is the case, hydrogen bubbles produced on the working electrode were introduced to the flow channel by applying pressure from the inlet using a syringe, after which the change in length of the bubbles was measured. Five bubbles were observed, ranging from 9.2 to 22.5 mm in size; overall, they showed a volume shrinkage ratio of between 0 and 11.6% over 10 min. Since the bubbles were mostly in contact with PDMS, the influence of this factor is overall negligible with re-
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spect to the timescale of several seconds under which this device operates. Effects of Potential on Bubble Shrinkage. The potential electrochemical origin of the effect of spontaneous bubble shrinkage prompted testing to determine whether changing the applied potential using a potentiostat could control bubble shrinkage. In conducting this experiment, bubbles were first produced until approximately 80% of the working electrode compartment was occupied, after which the connection to the working electrode was terminated for 10 min to allow for gas dissolution and prevent circuit termination. The bubbles were then restored to their previous volume and a potential was applied. Figure 3A shows meniscus displacement with respect to time for both various applied voltages and a disconnected circuit, while Figure 3B shows the dependence of meniscus velocity on applied potential. Overall, disconnecting the circuit entirely resulted in a slightly greater inverse displacement and bubble velocity than an applied voltage of −0.2 V, and notably greater values than for an applied voltage of −0.4 V. In the latter case, it is likely that hydrogen oxidation was forcefully suppressed. To investigate bubble shrinkage in the case of disconnection in more detail, the open circuit potential of the working electrode was measured with respect to the on-chip Ag/AgCl reference electrode (Figure 4A). The measured mixed potential was initially around −275 mV, though gradually shifted to a more positive value (Figure 4A, solid line). This suggests a change in the oxy-
Figure 3. (A) Meniscus displacement with respect to time for the shrinking of hydrogen bubbles. (B) Meniscus velocity dependence on applied potential. The band in the graph indicates the velocity range in which the electrical connection to the working electrode was terminated. Averages and standard deviations are shown for five measurements.
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gen/hydrogen ratio in the working electrode compartment – specifically, a relative increase in oxygen content. It is possible that this effect was due to continued diffusion of oxygen from the surrounding environment and the resulting oxidation of hydrogen present in the compartment. To confirm this, we embedded an aluminum layer in the working compartment to suppress oxygen diffusion (Figure 1C, dashed line). The change in the mixed potential recorded with the aluminum layer is shown as a broken line in Figure 4A. Although the initial potential was almost the same, the change observed with the aluminum layer was significantly slower. The difference in dissolved oxygen concentration was also checked using the working electrode as an oxygen sensor; in this case, the working electrode was polarized at −0.8 V with respect to the on-chip Ag/AgCl reference electrode, and no hydrogen bubbles existed in the working electrode compartment. Figure 4B shows changes in the current originating from the reduction of oxygen. The effect of using the aluminum layer was remarkable, with a significant difference observed between the two cases: without the aluminum layer, the current stabilized at about 11 µA, though with it, the current continued to decrease and settled at about 3 µA. The fact that oxygen is involved in the oxidation of hydrogen bubbles is not necessarily a disadvantage, since the spontaneous shrinkage of the bubbles does not require that the potential be switched and in fact simplifies the operation of the pump. In other words, once the hydrogen bubbles are produced using a potentiostat, the reverse change occurs automatically. Applications. Processing liquid plugs requires being able to switch repeatedly between forward and backward solution movement. Figure S8 shows this kind of repeated movement within the flow channel for alternating potentials of −1.2 and 0 V. The potentials were applied for 40 and 100 s, respectively. Under these conditions, the produced bubbles did not completely shrink within a realistic timeframe. Therefore, the plug gradually moved downstream for fixed time intervals. As shown in Figure 5C, it is not easy to move a solution plug precisely by only controlling the length of time during which potential is applied. To realize and facilitate reproducible pumping, we additionally used a flow channel structure with an air vent (Figure 5A). An operating water plug of appropriate length was placed to close the air vent along with the electrolyte solution for the pump and the solution to be ejected or withdrawn. When hydrogen bubbles are produced, the solutions move. However, when the upper stream end of the controlling plug reaches the air vent, the pressure is released and the controlling plug does not move anymore. The flow channel at the air vent has an expanding structure (Figure 5B). The entrance of
Figure 4. (A) Changes in mixed potential after the working electrode was disconnected from the potentiostat. The potential was measured with respect to the on-chip Ag/AgCl reference electrode. (B) Changes in current due to oxygen reduction. In both graphs, the black solid lines and red dashed lines show changes in the compartment without and with aluminum foil, respectively. The same electrodes were used for comparison in each experiment.
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Figure 5. Regulator for determining the range of displacement. (A) Structure of the regulator. (B) Magnified view of the structure. (C) Repetitive bidirectional movement of a liquid plug achieved by applying the potentials for fixed lengths of time. (D) Repetitive bidirectional movement of a liquid plug achieved using the regulator.
the air vent also has an expanding structure to avoid penetration of the controlling plug. The meniscus of the controlling plug is pushed forward before the produced bubbles return to the narrowest portion of the structure when the pressure decreases, which minimizes the free energy and closes the air vent. Therefore, when the bubbles are shrunk, pressure is applied and the controlling plug withdraws into the interior. In this case, the solution in the lower stream to be processed also moves to the upper stream, but stops because air is released when the lower stream end of the controlling plug reaches the air vent. When bubbles are produced again, pressure is not applied to the solution to be processed until the lower stream end of the controlling plug reaches the air vent. After the air vent is closed, pressure is applied to the solution to be processed. Simultaneously, the solution in the lower stream displaces only a limited range, which is determined by the length of the controlling plug at the T-junction. This device shows particular promise in POCT applications, given that it can be used to withdraw a very small volume of external solution through a minimally invasive needle. This premise was tested with human whole blood samples obtained from healthy volunteers, with results shown in Figure 6. A blood sample was placed onto the chip such that the end of the needle was immersed within it. Hydrogen bubbles were grown as much as possible and then shrunk by applying the corresponding potentials. The experiment was carried out using the thinnest needles we could obtain. Detailed information is provided in Table S2. With a 34G needle (inner diameter: 70 µm) and a 33G needle (inner diameter: 110 µm), the micropump could not withdraw the whole blood samples. However, with a 27G needle (inner diameter: 210 µm), the whole blood sample could successfully be withdrawn (Figure 6B). In a separate preliminary experiment, we could withdraw whole blood samples by directly inserting a 34G needle into a human vein and manually applying negative pressure using a commercial syringe. Therefore, we believe that our
Figure 6. Sampling of whole blood through a needle. (A) Cross section including the flow channel, needle, and whole blood. (B) Top views showing the flow channel of the micropump, before and after withdrawal of a whole blood sample, respectively.
micropump can be used to withdraw whole blood samples even using 34G needles through improvements in the application of pressure to the solution. The micropump can also be used to process solutions in microfluidic systems. In the device shown in Figure 7, a T-junction was formed and two integrated pumps were connected, one at one end of the main flow channel and the other at the end of an auxiliary flow channel. Two colored liquid plugs were formed and sorted at the liquid junction. Unit operations carried out at a T-junction can be combined to process solutions for biochemical analyses.19 Specifically, the micropumps can be integrated easily with other components such as microfluidic channels and sensors, and can therefore serve as a building block for sophisticated multiplexed analytical systems. ■ SUMMARY AND CONCLUSIONS A device was constructed that relied on volume changes resulting from the production and shrinkage of hydrogen bubbles to apply pressure to solution, causing forward and backward movement of a liquid column or plug. Working electrode structures with thinwalls and fan-shaped containers allowed for continuous bubble growth, even when they accumulated in the working electrode area. In addition, the repetitive bidirectional movement of a liquid plug can be achieved using a regulator with an air vent. Furthermore, the device was able to withdraw an external solution and sort plugs in a microfluidic system. Overall, the geometry and dimensions of the working electrode compartment and liquid junctions are critical, and will require adjustment for optimal performance in each application. As a next step, we will attempt the integration of sensors for the analysis of blood components.
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Figure 7. On-chip processing of liquid plugs. (A) Device schematic, with two integrated pumps connected to a flow channel structure with a T-junction. The red rectangle indicates the area of the pictures in (B). (B) Sorting of two plugs at the T-junction using the two integrated pumps.
■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:
Device component details; details of used mirconeedles; dimensions of critical parts of the micropump; electrolyte solution around the wall structure; SEM images of the surface of the platinum black electrode; meniscus displacement with respect to time recorded at constant currents; and meniscus displacement with respect to time.
■ AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. Tel: +81- 29-853-5598
Notes The authors declare no competing financial interest.
■ ACKNOWLEDGMENT This study was supported by Grants-in-Aid for Scientific Research under the Japan Society for the Promotion of Science (JSPS).
■ REFERENCES (1) Gravesen, P.; Branebjerg, J.; Jensen, O. S. Microfluidics - a review. J. Micromech. Microeng. 1993, 3, 168-182.
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(2) Shoji, S.; Esashi, M. Microflow devices and systems. J. Micromech. Microeng. 1994, 4, 157-171. (3) Laser, D. J.; Santiago, J. G. A review of micropumps. J. Micromech. Microeng. 2004, 14, R35-R64. (4) Su, W.; Gao, X.; Jiang, L.; Qin, J. Microfluidic platform towards point-of-care diagnostics in infectious diseases. J. Chromatogr. A 2015, 1377, 13-26 (5) Jung, W.; Han, J.; Choi, J.-W.; Ahn, C. H. Point-of-care testing (POCT) diagnostic systems using microfluidic lab-on-a-chip technologies. Microelectron. Eng. 2015, 132, 46–57. (6) Yetisen, A. K.; Akram, M. S.; Lowe, C. R. Paper-based microfluidic point-of-care diagnostic devices. Lab Chip 2013, 13, 2210–2251. (7) Böhm, S.; Timmer, B.; Olthuis, W.; Bergveld, P. A closed-loop controlled electrochemically actuated micro-dosing system. J. Micromech. Microeng. 2000, 10, 498-504. (8) Lee, S. W.; Sim, W. Y.; Yang, S. S. Fabrication and in vitro test of a microsyringe. Sens. Actuators A 2000, 83, 17-23. (9) Satoh, W.; Shimizu, Y.; Kaneto, T.; Suzuki, H. On-chip microfluidic transport and bio/chemical sensing based on electrochemical bubble formation. Sens. Actuators B 2007, 123, 1153-1160. (10) Shimizu, Y.; Takashima, A.; Satoh, W.; Sassa, F.; Fukuda, J.; Suzuki, H. Biochip with integrated pumps for plug-based sequential exchange of solutions. Sens. Actuators B 2009, 140, 649-655. (11) Blanco-Gomez, G.; Glidle, A.; Flendrig, L. M.; Cooper, J. M. Integration of low-power microfluidic pumps with biosensors within a laboratory-on-a-chip device. Anal. Chem. 2009, 81, 1365-1370. (12) Chan, S.-C.; Chen, C.-R.; Liu, C.-H. A bubble-activated micropump with high-frequency flow reversal. Sens. Actuators A 2010, 163, 501-509. (13) Pagonis, D. N.; Kaltsas, A. P. G. A pumping actuator implemented on a PCB substrate by employing water electrolysis. Microelectron. Eng. 2012, 95, 65-70. (14) Sheybani, R.; Gensler, H.; Meng, E. A MEMS electrochemical bellows actuator for fluid metering applications. Biomed. Microdevices 2013, 15, 37-48. (15) Choi, Y. H.; Son, S. U.; Lee, S. S. A micropump operating with chemically produced oxygen gas. Sens. Actuators A 2004, 111, 8-13. (16) Takashima, A.; Kojima, K.; Suzuki, H. Autonomous microfluidic control by chemically actuated micropumps and its application to chemical analyses. Anal. Chem. 2010, 82, 6870–6876. (17) Hur, J. I.; Meng, D. D.; Kim, C.-J. Self-pumping membraneless miniature fuel cell with an air-breathing cathode. J. Microelectromech. Syst. 2012, 21, 476-483. (18) Hong, C.-C.; Murugesan, S.; Kim, S.; Beaucage, G.; Choi, J.-W.; Ahn, C. H. A functional on-chip pressure generator using solid chemical propellant for disposable lab-on-a-chip. Lab Chip 2003, 3, 281-286. (19) Sassa, F.; Fukuda, J.; Suzuki, H. Microprocessing of liquid plugs for bio / chemical analyses. Anal. Chem. 2008, 80, 6206-6213. (20) Suzuki, H.; Yoneyama, R. A reversible electrochemical nanosyringe pump and some considerations to realize low-power consumption. Sens. Actuators B 2002, 86, 242-250. (21) Suzuki, H.; Yoneyama, R. Integrated microfluidic system with electrochemically actuated on-chip pumps and valves. Sens. Actuators B 2003, 96, 38-45. (22) Ives, D. J. G.; Janz, G. J. Reference electrodes. Theory and practice.; Academic Press: New York, 1961. (23) Yoshimi, Y.; Shinoda, K.; Mishima, M.; Nakao, K.; Munekane, K. Development of an artificial synapse using an electrochemical micropump. J. Artif. Organs 2004, 7, 210–215. (24) Suzuki, H.; Taura, T. Thin-film Ag/AgCl structure and operational modes to realize long-term storage. J. Electrochem. Soc 2001, 148, E468E474. (25) Geng, X.; Yuan, H.; Oguz, H. N.; Prosperetti, A. Bubble-based micropump for electrically conducting liquids. J. Micromech. Microeng. 2001, 11, 270-276. (26) Suzuki, H.; Shiroishi, H.; Sasaki, S.; Karube, I. Microfabricated liquid junction Ag/AgCl reference electrode and its application to a onechip potentiometric sensor. Anal. Chem. 1999, 71, 5069-5075.
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