They have used the plumbing to build a mixing factory for microscopic fluid streams, and future applications may include uses in sensors, labs-on-chips and self-repairing materials.

"Our approach opens up new avenues for device design that are currently inaccessible by conventional methods," said Jennifer Lewis, a professor of materials science and engineering and of chemical engineering at Illinois.

The Illinois team reports its findings in the April 2003 Nature Materials. The research is supported by the Air Force Office of Scientific Research and the National Science Foundation, the independent federal agency that supports basic research in all fields of science and engineering.

The plumbing, built out of 3-D networks of microchannels that can range from 10 microns to 300 microns (millionths of a meter) in diameter, represents several advances over conventional, 2-D networks that are etched into a flat wafer, for example.

Because pipes are stacked on top of one another in the 3-D plumbing, it occupies a much smaller footprint than 2-D channels of comparable length.

At the same time, the Illinois team's 3-D networks are much more effective at mixing fluids than the best 2-D alternatives. "Fluid mixing is critical" for many microfluid applications, Lewis said.

The networks of microchannels may also provide "an analog to the human circulatory system for the next generation of self-healing materials," said Scott White, a professor of aeronautical and astronautical engineering and a researcher at the Beckman Institute for Advanced Science and Technology.

"The embedded network would serve as a circulatory system for transporting repair chemicals to damage sites within the material."

To create the microchannel networks, Lewis, White, and graduate student Daniel Therriault utilize a computer-controlled robotic "pen." The pen's nozzle deposits a line of waxy ink from which a three-dimensional scaffold is constructed.

"The ink exits the nozzle as a continuous, rod-like filament that is deposited onto a moving platform, yielding a two-dimensional pattern," Lewis said.

"After a layer is generated, the stage is raised and rotated, and another layer is deposited. The process is repeated until the desired structure is produced." The waxy ink retains its cylindrical shape, even as it crosses gaps in the level of the scaffold below it.

Next, the scaffold is surrounded with an epoxy resin. After the resin solidifies, the material is heated, and the waxy ink melts away and leaves behind an empty network of crisscrossed pipes and joints.

In the final step, the pipes are flooded with a resin that hardens under exposure to ultraviolet light. The material is selectively exposed to UV rays, sealing off channels to create the desired plumbing pathways. The relatively simple technique permits the Illinois team to construct plumbing with regular geometries, such as square-spiral towers that stair-step their way up through the scaffolding.

The researchers built these square-spiral towers for mixing microfluid streams to demonstrate the effectiveness of their fabrication technique. Each of the integrated tower arrays was made from a 16-layer scaffold. The mixing efficiency of these stair-cased towers was measured by monitoring the mixing of two dyed fluid streams using fluorescent microscopy.

"These three-dimensional towers dramatically improve fluid mixing compared to simple one- and two-dimensional channels," White said. "By forcing the fluids to make right-angle turns as they wind their way up the tower, the fluid interface is made to fold on top of itself repeatedly. This causes the fluids to become well-mixed in a short distance."

The Illinois researchers are considering more sophisticated techniques for creating arbitrary 3-D plumbing structures in the final step. "That would just open up a huge window for what we could do in the future in 3-D," Lewis said. "Full-fledged 'factories-on-a-chip' for any of the long-term applications envisioned may require these more complex structures."

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