Tiny pumps drive liquid circuits

By Kimberly Patch, Technology Research News

Many research teams are working to make labs-on-a-chip that manipulate tiny amounts of fluids. A second major research effort is geared toward making cheap and flexible organic, or plastic, electric components. And other researchers are looking for better ways to control, or tune, compact optical devices like fibers, waveguides and photonic crystals.

Researchers from the University of Illinois at Urbana-Champaign and Lucent Technologies' Bell Laboratories have combined microfluidics and organic electronics to make a tunable plastic transistor that could enable low-cost methods to drive, control and monitor labs-on-a-chip. The device can also use tiny amounts of fluid to adjust optical devices.

The idea is to use the microfluidic organic transistors to drive fluid motion, said John Rogers, a professor of materials science and engineering at the University of Illinois at Urbana-Champaign. Such motion can be harnessed to change circuit properties in order to drive pumps, detect fluid motion, and control microvalves, he said. "It is the marriage and direct integration of electronics with microfluidics," he said.

Electric transistors turn on when electricity flows from a source electrode through a central channel to a drain electrode, and off when the flow of electricity is blocked. Transistors usually use a gate electrode to generate an electric field that controls the flow of electricity through the central channel.

The researchers' microfluidic transistor contains channels filled with short plugs of a conducting liquid like mercury that act as source and drain electrodes. The transistor's electrical properties depend on geometries like the amount the electrodes overlap with one another and with the semiconductor.

And the position of the mercury in the channels adjusts the electrical response of the transistor, said Rogers. "As the plugs move through these channels, the geometries of these electrodes change, thereby altering the electrical properties of the transistor," he said.

This type of transistor provides a flexible way to drive a device and to sense its operation and amplify the output, said Rogers. This opens the way to building active power supplies and control circuitry for microfluidic pumps directly into the chip structure of a device in a low-cost and convenient way, he said.

Key to the concept is the use of an elastomeric, or rubbery, material to form the channels, and surface chemistries that allow the material to instantly bond to an organic semiconductor once they come in contact, said Rogers. "Because they're soft and rubbery it is possible to achieve leakage-free seals... against a range of organic semiconductors without application of external pressure... heating [or] adhesives," he said.

The process doesn't affect the organic semiconductor, said Rogers. This is important because "nearly all organic semiconductors are very fragile from a chemical and mechanical standpoint -- it's easy to degrade them by doing any kind of processing on top of them," he said.

The elastomeric material is inexpensive, cheap and quick to process, and structures can be built over large areas, said Rogers. These characteristics make the material practical for new types of active, low-cost plastic microfluidic devices in addition to the tunable transistors, he said.

The next step is demonstrating microfluidic pumps that are capable of reconfiguring tunable microfluidic photonic systems, said Rogers.

The researchers are working on ways to use tunable microfluidic transistors or transistor arrays to build self-starting microfluidic pumps that can move liquids around in microfluidic systems, said Rogers. They are also looking to use the method to make active components like valves and fluid motion sensors, he said.

The researchers' test device shows that it may be possible to make tunable organic transistors, said Chang-Jin Kim, a professor of mechanical aerospace engineering at the University of California at Los Angeles. The device is "an important step" toward the goal of demonstrating such transistors. The approach is novel because it uses mercury as a part of a transistor, said Kim.

Tunable microfluidic transistors could be used practically in three to six years, according to Rogers.

Rogers's research colleagues were George Maltezos, Robert Nortrup and Jana Zaumseil of Lucent Technologies' Bell Laboratories, and Seokwoo Jeon of the University of Illinois at Urbana-Champaign. The work appeared in the September 8, 2003 issue of Applied Physics Letters. The research was funded by Lucent Technologies and the University of Illinois.

Timeline:   3-6 years
Funding:   Corporate, University
TRN Categories:  Microfluidics and BioMEMS; Integrated Circuits
Story Type:   News
Related Elements:  Technical paper, "Tunable Organic Transistors that use Microfluidic Source and Drain Electrodes," Applied Physics Letters, September 8, 2003


March 10/17, 2004

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