Droplet μTAS using electro-conjugate fluid — Feedback position control of multiple droplets in flow channel matrix
Introduction
In the last decade, micro total analysis system or μTAS widely attract attentions in the fields of chemistry, biochemistry, bioengineering, etc. It is generally considered as a tiny system which may carry out chemical synthesis, analysis and cell analysis on a chip. By miniaturizing the system, there could be a number of advantages, such as decreasing the usage amount of sample and reagent, reducing cost, increasing the rate of chemical reaction and improving the accuracy in analysis [1], [2].
One of the key issues for the robotic researchers like us to contribute on μTAS is to propose a new concept to transport samples and reagent on tiny chips. There are two ways reported in the previous studies to transport samples and reagent. The first is called “closed-channel μTAS,” which transports fluid (sample/reagent) in a micro channel fully filled with the sample and the reagent. The second is “droplet-based μTAS,” which transports the sample/reagent in the form of droplet on a chip by using, for example, interface tension [3].
Closed-channel μTAS has a high transporting capability and general versatility, because it mechanically transports fluids with a micro pump. However, the micro pump has difficulty in miniaturization and integration with a microchip, because it requires mechanically moving elements to generate flow. Furthermore, closed-channel μTAS needs to fill the channel with the sample or reagent, and requires continuous flow. As a result, it is difficult to reduce the usage amount of them dramatically.
On the other hand, droplet-based μTAS has advantages in miniaturization, smaller usage amount of sample/reagent and fast chemical reaction. In droplet-based μTAS, electrostatic force is mainly used as the driving force, thus it requires only electrodes on the substrate, which has no mechanically moving elements to drive droplets. Thus, droplet-based μTAS could be miniaturized easier compared with closed-channel μTAS. Besides, in droplet-based μTAS, since the transportation target is a droplet, we could decrease the usage amount of sample and reagent and chemical reaction time [4]. In addition, some chemical materials and microbes could be dangerous in high-volume use. According to these reasons, droplet-based μTAS can be concluded as an efficient system for analysis and examination of biological samples, which may not obtain an adequate amount.
In droplet-based μTAS, velocity and bidirectional movement of a droplet are important capabilities. In order to reduce the total chemical reaction time, the droplet is required to move as fast as possible to reach the reaction area or other droplet position. For further miniaturization, the droplet requires bidirectional movement; forward–reverse movement. Without this capability, a number of reaction spaces need to be located in series, in order to complete a complicated analysis. In consequence, size of the chip will increase.
The existing transportation method has been performed with various driving mechanism, for example, electrowetting [5], [6], [7], electrostatic force [8], [9], surface acoustic wave pumping [10] and thermocapillary [11]. However, the velocity of the droplet is low and it might be unidirectional movement. Thus, if it is possible to improve the droplet's velocity and to achieve bidirectional movement of a droplet, droplet-based μTAS may become a promising candidate for the next generation μTAS platform.
Considering the current situation of μTAS mentioned above, in this study, we introduce an electro-conjugate fluid (ECF) to droplet-based μTAS in order to improve the moving velocity and to achieve bidirectional movement of a droplet. The ECF is a kind of functional fluid, which generates a powerful flow when subjected a high DC voltage across the electrodes. The flow velocity may reach up to several m/s as shown in Ref. [12]. Since a tiny electrode pair is only required to generate the flow of ECF, it is easy to produce a flow along a micro channel located on a chip. When we placed a droplet of sample or reagent into the channel, the droplet may move along the channel flow.
Section snippets
Electro-conjugate fluid
The electro-conjugate fluid is a kind of dielectric and functional fluid, which generates a powerful flow (ECF flow) by applying a high voltage across a pair of electrodes inserted into the fluid. We can observe the ECF flow between a pentagonal prism-slit electrode pair as shown in Fig. 1, and this phenomenon is called ECF effect [13]. The ECF flow may be observed especially when it is under a non-uniform electric field. According to the previous studies [14], the ECF flow becomes more
Concept
This study introduces the ECF to droplet-based μTAS to develop a novel droplet transportation method. Fig. 2 shows a conceptual diagram of our idea, which we call an active flow channel matrix. The active flow channel matrix is mainly composed of a lattice-shaped flow channels filled with the ECF, and the electrode pairs appropriately located in the channels. By applying a high voltage across the electrode pair, we can generate the ECF flow in the desired direction. When we placed a droplet
Design and fabrication of active flow channel matrix
Fig. 4 shows the schematic illustration of the designed active flow channel matrix. Fig. 4(a)–(c) shows an exploded view of the active flow channel matrix, top view and dimensions of electrode pair, respectively. As shown in Fig. 4(a) and (b), the active flow channel matrix is mainly composed of a substrate with flow channels, a cover and electrode pairs. Each flow channel is 2.0 mm wide and 2.5 mm deep, which is fabricated with a milling machine. As shown in Fig. 4(a), the substrate and the
Droplet position control using image processing
Before we carry out the experiment of droplet position control, we have confirmed the behavior of a droplet in the ECF flow as follows. We placed blue-colored water (0.3 mL) in drops into the ECF and the experimental result is as shown in Fig. 7. Fig. 7 shows that the droplet was completely separated from the ECF and it kept its spherical shape.
Fig. 8 shows an experimental setup to control the position of a droplet. As shown in Fig. 8, a USB camera and two amplifiers via DA converter,
Conclusion
This study proposed a novel active flow channel matrix using electro-conjugate fluid. First, we proposed the concept of active flow channel matrix. Second, we developed the prototype of the active flow channel matrix. Finally, we conducted the position control of the droplet and the merging experiment of the multiple droplets using the developed prototype. As a result, the droplet successfully stopped at the arbitrary target position and the multiple droplets reached the intersection at the
Yoko Abe received her B.E. degree in Mechanical Engineering from Keio University, Yokohama, Japan in 2011. She is currently working for her M.E. degree from the School of Science for Open and Environmental Systems, Keio University.
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Yoko Abe received her B.E. degree in Mechanical Engineering from Keio University, Yokohama, Japan in 2011. She is currently working for her M.E. degree from the School of Science for Open and Environmental Systems, Keio University.
Kenjiro Takemura received his B.E. degree in Mechanical Engineering and M.E. degree in Biomedical Engineering from Keio University, Yokohama, Japan in 1998 and 2000, respectively. He also received his Ph.D. degree in Integrated Design Engineering from Keio University in 2002. He was a research associate in the Department of Mechanical Engineering at Keio University in 2002. Then, he was an assistant professor in the Precision and Intelligence Laboratory at Tokyo Institute of Technology from 2003 to 2008. Currently, he is an associate professor in the Department of Mechanical Engineering at Keio University. He received JSME Young Engineers Award in 2002 for a research on Multi-DOF ultrasonic motor, Best Research Paper Award at the 10th International Conference on Mechatronics Technology in 2006, etc. His research is focused on functional fluids, new actuators/sensors, and mechatronics.
Katsunari Sato received his B.S. degree in engineering from Tohoku University, Sendai, Japan in 2006. He also received his M.S. and Ph.D. degree in Information Science and Technology from the University of Tokyo, Tokyo, Japan in 2008. He has been a JSPS Research Fellow (PD) in the Graduate School of System Design and Management at Keio University since 2011. His research interest includes haptic interfaces for telexistence.
Shinichi Yokota received his Ph.D. degree in Mechanical Engineering from Tokyo Institute of Technology in 1982. He was a research associate in the Research Laboratory of Precision Machinery and Electronics at Tokyo Institute of Technology from 1975 to 1986. Since 1986, he has been with the Precision and Intelligent Laboratory also at Tokyo Institute of Technology, where he is currently a full professor. His research interests are on micro actuators using functional fluids, micro machines, and fluid power mechatronics.
Kazuya Edamura received his M.S. degree in Chemical Engineering from Tokyo Institute of Technology in 1978. After his career as a chemist in the Research Lab. at Kyowa Hakko Kogyo Co., Ltd. and a Product Manager in Diagnostics Dept. Boehringer Mannheim Japan, he started up a new enterprise named New Technology Management Co., Ltd. His main works are on ECF, MRF, ERF technologies, etc.