Microfluidic actuation using electrochemically generated bubbles.

Bubble-based actuation in microfluidic applications is attractive owing to elementary microfabrication requirements. In the present study, the mechanical and chemical characteristics of electrochemically generated bubble valves were studied. By generating electrochemical bubbles as valves directly inside the channel, valves could be closed and opened in milliseconds. Whereas bubble inflation (or valve closing) rate increases with applied voltage, small microfluidic dimensions accelerate bubble deflation rates. It is found that bubbles need not collapse fully to restore full flow, and the channel opens when its hydraulic resistance equals that between the bubble and the wall--a process requiring only milliseconds. Since only picomoles of salt are needed to generate bubbles, pH gradients that are invariably associated with electrochemical reactions were readily suppressed by using a small amount of buffer, as visualized by a pH-sensitive fluorescent dye. A range of common laboratory reagents and electrolytes in varying concentrations, including weak to strong acids and bases, as well as nonaqueous/aqueous mixtures were successfully tested. Using such bubble valves, an eight-way multiplexer was fabricated and tested.