Microfluidic Devices Using Electrochemical Micropumps and Microvalves for Autonomous Solution Processing
Authors: Ma Chengrui , Suzuki Hiroaki 
Published: December 2020

In realizing sophisticated micro analytical devices, controlled solution transport in microfluidic channels is indispensable. The procedures have predominantly been conducted using external pumps (or pressure source) and valves. However, because of this, the entire setups become very bulky, although the chips may be small. If the control of solution transport is realized using integrated microfluidic components, it will accelerate automation and realization of user-friendly devices. Apart from chemical analysis, realization of artificial lives 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.
Doped polypyrrole (PPy) was used for the valve. A platinum electrode was formed on a glass substrate, and PPy was grown on the electrode. For this purpose, 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 of 10 μA. In as-deposited PPy, the alkyl chains of NaDBS stand upward and the surface is hydrophobic. When PPy is reduced, polar groups come to the surface and becomes hydrophilic.
First, a two-channel test device with a control flow channel and a main flow channel was fabricated (Figure 1). The valve was formed in the main flow channel and a zinc electrode was formed in the control flow channel. The electrodes were connected each other. Also, solutions in the solution reservoirs of the two flow channels were connected with an iridium wire with oxides on both ends as an alternative for a liquid junction. When a solution was injected into the main flow channel, it stopped at the valve. When another solution was injected into the control channel and wetted the zinc electrode, PPy on the valve was reduced. As a result, and the valve opened.
As the next step, a three-channel device was fabricated (Figure 2). Interdigitated platinum electrodes were formed in flow channel 2 in the figure. One of them was connected to a zinc electrode in flow channel 1 and the other was connected to a platinum electrode in flow channel 3. Solutions in the reservoirs of the flow channels were connected with Ag/AgCl wires. As a preliminary experiment, a pH indicator (thymol blue) was first injected into flow channel 2. When a KCl solution was injected into flow channel 1, the color of the area around the interdigitated electrodes of flow channel 2 changed to blue by the electrolysis of water. When a 0.1 M AgNO3 solution was injected into flow channel 3, the color changed to yellow again because of the oxidization of hydrogen bubbles.
Another device with the same basic structure was used to demonstrate autonomous bidirectional transport of a solution (Figure 3). In addition to the structures shown in Figure 2, the PPy valve used in the device shown in Figure 1 was formed in flow channel 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. Furthermore, solution reservoirs were closed with a polyimide tape. Following the same procedure for the previous device, hydrogen bubbles were produced on one of the interdigitate electrodes in flow channel 2 by the reduction of protons. When the solution in flow channel 2 reached the zinc electrode at the end, the valve in flow channel 3 opened and a AgNO3 solution flowed through the platinum electrode. Following this, Ag+ ions were reduced to silver there and the hydrogen bubbles in flow channel 2 were oxidized on one of the interdigitate electrodes. As a result, the bubbles shrank and the solution in the flow channel moved backward.
For the device shown in Figure 3, the injection of the solution in flow channel 1 can be carried out by forming an additional valve at the solution reservoir and open it using another zinc electrode in another control flow channel. Many units like the one shown in Figure 3 can be connected to the same control flow channel and operated sequentially. The technique will enable autonomous multiplexed and/or sequential processing of solutions within microfabricated analytical devices and autonomous actuation of soft robots.

Figure 1