On-chip creation and elimination of microbubbles for a micro-object manipulator

This paper describes on-chip microbubble creation and elimination using an electrolytic process integrated with electrowetting on dielectrics (EWOD) actuations. These bubble operation units are developed for the microbubble-based micro-object manipulator in which EWOD-actuated microbubbles can be used not only as micro-object carriers but also as flow amplifiers using ultrasonic excitations. By applying voltages between the anode and cathode, microbubbles of oxygen, hydrogen and their mixture ranging from several microns to several hundred microns in diameter are on-chip created electrolytically. EWOD actuation is used to detach and transport the bubbles from the creation site. On-chip bubble elimination is accomplished by using the reverse electrolytic reaction. A platinum electrode, which acts as a catalyst in the reverse electrolytic reaction, substantially increases the elimination rate of microbubbles, by an order of magnitude, as compared to a Teflon-covered Si surface. The catalytic reaction is effective in eliminating hydrogen, oxygen and mixture bubbles. Interestingly, the Teflon-coated Pt electrodes show a higher elimination rate than the bare Pt electrode. This is attributed to a larger contact area with the Teflon-covered electrode (hydrophobic surface) than with the bare Pt electrode (hydrophilic surface). For the Teflon-covered electrodes, the bubble elimination rate increases with thinner Teflon coating. Integrated bubble operations-creation, transportation and elimination-are realized on a single chip in which coating of the Pt electrode with a Teflon layer allows transporting of bubbles into the elimination site. Finally, as a proof of concept for the micro-object manipulator, it is demonstrated that EWOD-actuated bubbles can push micron- and millimeter-sized objects and release them at a different place.