Thursday, October 26, 2006

Tiny Pumps for Diagnostic Chips

A new way to move fluids around on a "lab on a chip" could make sophisticated portable blood tests more practical.

By Jennifer Chu

The original version of this article contained several errors. The corrected version appears below. Technology Review regrets the errors.

The potential of "lab on a chip" technology is immense: it could yield fast, cheap, and portable devices to test soldiers for biological or chemical poisoning within minutes, or a handheld device that takes a drop of blood and scans it for diseases such as HIV. But one problem in developing these microfluidic devices is how to precisely pump fluids through a chip without using a significant amount of power. As a result, existing labs on a chip are weighed down by large, bench-top power sources.

Now Martin Bazant, a professor of applied mathematics at MIT, and his colleagues at MIT's Institute for Soldier Nanotechnologies have designed a tiny, battery-powered pumping system for existing microchips that the researchers hope to build into a handheld diagnostic device. "In terms of the platform for labs on chips," says Bazant, "we can integrate the power source right into the chip and miniaturize it, and I think that's a major advantage."

An average lab on a chip can theoretically perform hundreds of different tests on a single drop of blood. A pumping system directs fluids, such as blood, through a complex system of microchannels, each no wider than a hair, pumping blood to this chamber or that. Once in the chamber, the fluid can be analyzed by different sensors, depending on the kind of test that needs to be done. But fluids at such a microscale act very differently from those on an everyday macroscale. For example, it takes a much stronger force to push a drop of blood through a tiny channel than it does to move, say, a liter of blood through a column. So, Bazant and others are exploring electro-osmosis--that is, creating an electric field along a chip via electrodes to move a fluid from one end to the other.

Most labs on a chip are made of glass or silicon, and micro- or nanosize channels can now easily be etched in, and then plated with a second layer of the material to create a complex system of canals. Still, to electrically pump fluids through this canal system requires a tremendous amount of power. Many researchers have explored the phenomenon of capillary electro-osmosis, using large DC electric fields of up to 1000 volts to power electrodes on either end of a chip. In this way, labs on a chip are able to, for example, analyze DNA from blood samples, but are often tethered to a large power source. "The system requires all this extra stuff," says Bazant. "And you're manipulating things on a small scale by using a large-scale device."

Bazant's goal is to eliminate such large-scale devices by integrating a power supply within a chip to make it fully portable and free to operate in the field. To that end, he has designed a new approach to AC electro-osmosis (ACEO), using a battery-powered pumping system that's proven much faster than previous ACEO pumps. The system requires only one volt of electricity to power a set of evenly spaced tiny electrodes built within a chip, versus a pair of large electrodes at either end of a chip. Going one step further, he has made the system three-dimensional, using electrodes with tiny steps. This stepwise electric field creates what Bazant calls a "conveyer-belt effect," pulling fluids across the chip depending on theorientation of the steps.