Microfluidics harnesses liquid wires and glass electrodes

by R. Colin Johnson
Contributing Editor, MEMS Investor Journal

Today micro-electro-mechanical systems are enabling lab-on-chip devices that pump picoliters of liquid reagents around on a substrate the same way that electrons are shuttled around on an ordinary microchip. These microfluidic devices promise to downsize medical tests in a manner reminiscent of Dr. McCoy's handheld medical scanner in the Star Trek franchise.

One outstanding problem in microfluidics, however, is how to supply electrical control signals to MEMS pumps on chips that are riddled with micron-sized channels filled with fluid. The obvious method would be to embed wires into the devices, but metallic wires must be insulated to prevent shorts plus they need electrodes that will not deteriorate in the presence of fluids. Now researchers at the University of Michigan (Ann Arbor) believe they have the answer -- liquid wires that use waterproof glass electrodes.

Using a femtosecond laser emitting ultra-fast light pulses the researchers were able to hollow-out tiny channels to contain conductive fluids that act as wires. Next they began experimenting with methods of fabricating electrodes that could transfer the electrical signals to the MEMS pumps on a chip from the tiny channels containing the conductive fluid.

Surprisingly, they discovered that if the conductive channels were cast in glass, and the end of the glass channel was made very thin, that the glass itself could act as an electrode to pass electrical signals. As a result, the researchers were able to recently demonstrate the world's smallest microfluidic pump, enabled by liquid wires and glass electrodes.

While the mechanism explaining how glass can be turned into an electrode that conducts electricity (when in its macroscopic state it is an insulator) is still being investigated, the inventor -- Alan Hunt, an associate professor in the Department of Biomedical Engineering at University of Michigan -- believes it is the small scale that enables the change. At large scales, putting enough voltage across glass to force it to conduct electricity would cause excessive heat that would damage the glass, but by making the glass very thin its limits are not exceeded.