Electrochemical Circuits for Autonomous Microfluidic Solution Processing
Authors: Ma Chengrui , Satoh Yusei , Suzuki Hiroaki 
Published: July 2021

Introduction

In realizing smart micro analytical devices, controlled solution transport in microfluidic channels is indispensable. The procedures have predominantly been conducted using external pumps (or a pressure source) and valves. However, because of this, the entire setups become very bulky, although the chips themselves may be small. If solutions can be processed using integrated microfluidic components, it will accelerate automation and realization of user-friendly devices. Apart from chemical analysis, realization of soft robots that move by direct conversion of chemical energy to mechanical energy is becoming a hot topic. Independent autonomous microfluidic systems that can control chemical reactions in a coordinated manner can also be critical components in these devices. In this study, we attempted to realize such autonomous microfluidic devices by integrating simple switchable hydrophobic microvalves that employ a conducting polymer and pressure changes by electrochemical production and shrinkage of hydrogen bubbles. The components were connected electrically to form a network of electrochemical components that may be called electrochemical circuits.

Method

Figure 1 shows the structure and function of the valve [1]. Doped polypyrrole (PPy) was used to create the valve. First, patterns of platinum electrodes were formed on a glass substrate. To form the PPy layer, the electrode was immersed in a solution containing 0.2 M pyrrole and 0.2 M sodium dodecylbenzenesulfonate (NaDBS), and electropolymerization was carried out at a constant current (10 μA). In as-deposited PPy, the alkyl chains of NaDBS stand upward and the surface is hydrophobic. When PPy is reduced, polar groups of NaDBS come to the surface to make it hydrophilic. The valve was placed in a flow channel structure formed with hydrophilic glass and hydrophobic polydimethylsiloxane (PDMS). A solution injected into the flow channel moves by capillary action but stops at the valve which is initially hydrophobic. The valve can be opened by applying an appropriate potential to the valve electrode. When the valve is opened, the solution passes through the valve area and moves forward again.
Although sequential switching of the valves is possible by using an external potentiostat, autonomous switching was realized by using a zinc electrode formed in a control flow channel and connected to the valve [2]. Figure 2 shows how the autonomous switching is conducted. The device consists of a main flow channel (MFC) with the valve and a control flow channel (CF). A zinc electrode is formed in the CF and is connected to the valve electrically using a lead pattern. Also, solutions in the reservoirs of the two flow channels are connected with an iridium wire with oxides on both ends as an alternative for a liquid junction. When a solution is injected into the MFC, it stops at the valve. When another solution is injected into the CF and wets the zinc electrode, PPy on the valve is reduced. As a result, the valve opens and the solution moves again in the flow channel.

Results and Conclusions

Based on the technique shown in Figure 2, autonomous bidirectional transport of a solution was achieved by using a circuit shown in Figure 3. The PPy valve was formed in flow channel 3 (FC 3) to control the injection of the solution in the reservoir. The valve was connected to a zinc electrode formed at the end of flow channel 2 (FC 2). Solution reservoirs were connected with metal patterns and were closed with a polyimide tape. When a KCl solution was introduced into flow channel 1 (FC 1) and wets a zinc electrode in the flow channel, hydrogen bubbles were produced on one of the interdigitated electrodes in FC 2 by the reduction of protons. When the solution in FC2 reached the zinc electrode at the end, the PPy valve in FC3 connected to the zinc electrode opened and a AgNO3 solution was injected. Following this, Ag+ ions were reduced to silver there and the hydrogen bubbles in FC2 were oxidized on one of the interdigitated electrodes. As a result, the bubbles shrank and the solution in FC2 moved backward. Figure 4A-C show the movement of solutions.
Multiple devices shown in Figure 3 can be integrated and operated sequentially and autonomously by additionally forming a zinc electrode at the end of FC3 of each device and connect it to a PPy valve additionally formed at the solution reservoir of FC1 of the device that is operated next (Figure 4D). To operate the integrated device, all we need to do is to inject a KCl solution to FC1 of the first unit as a trigger. All the following procedures are conducted autonomously to inject and remove solutions into and out of the reaction chamber sequentially. In addition to these, we are developing other autonomous devices for other purposes by designing other electrochemical circuits. This technique will enable autonomous multiplexed and/or sequential processing of solutions within microfabricated analytical devices and autonomous actuation of soft robots.

References

[1] S.K. Pramanik, H. Suzuki, Switchable microvalves employing a conducting polymer and their automatic operation in conjunction with micropumps with a superabsorbent polymer, ACS Appl. Mater. Interfaces. 12 (2020) 37741−37749. doi: 10.1021/acsami.0c09419
[2] T. Watanabe, G. C. Biswas, E. T. Carlen, H. Suzuki, Autonomous electrochemically-actuated microvalve for controlled transport in stand-alone microfluidic systems, RSC Adv. 7 (2017) 39018–39023. doi: 10.1039/C7RA07335F

Figure 1