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Powerful Machines Are Coming in Small Packages
A new class of micro-gadgets – some no larger than a pencil eraser – are poised to make military and other equipment easier to power and carry. Scientists presented the latest developments from their micro-engineering labs at the 2004 AAAS Annual Meeting.
Chemical and biological warfare suits worn in warm climates, such as the Iraqi desert, can become unbearably hot. The solution may be a portable cooling system that weighs just several pounds.
The concept that makes this possible is also leading to miniature sensors for detecting chemical and biological toxins, as well as tiny chemical reactors for hydrogen fuel processing or environmental cleanup.
In a nutshell, small is powerful. These devices send large amounts of liquid or gas through thousands of microchannels that stand roughly as tall as a human hair. In each channel, heat transfer or chemical reactions happen more efficiently than they do in larger spaces, permitting better process control, shorter channel lengths and overall system miniaturization.
“The channels are to microfluidic devices what wires are to microelectronics,” said panelist Brian Paul of Oregon State University.
While conventional refrigeration systems in the United States require huge amounts of electricity, the one designed by Ward TeGrotenhuis and his colleagues at Pacific Northwest National Laboratory could run off burning fuel, carried in lightweight, portable canisters. They have demonstrated that miniaturizing each of the components of such as system is possible and are now working on fitting all the pieces together.
The final product would consist of an absorption heat pump, which would fit in a small backpack, connected to a vest threaded with water-filled microchannels. The water would be cooled in the pump then recirculated through the channels to keep the person wearing the vest from overheating.
In his lab, Paul is developing methods for mass-producing arrays of these microchannels to perform various fluidic operations such as heat exchange, mixing, reaction and separation. Combining these operations can require the integrating feature sizes across seven or eight orders of magnitude. (By comparison, assembling a jet airplane using centimeter-sized rivets involves three orders of magnitude.)
Some of the technologies Paul is working on include micromachined gas-liquid contactor membranes for portable heat pumps, high-temperature microchannel arrays for increasing the efficiency of fuel reforming, and microfluidic integration for improving the shelf-life of tissue-based sensors.
“We’re trying to take large amounts of fluids down through microchannels and then back out to affect the macroscale environment,” Paul said. “These technologies are expected to revolutionize the way we process mass and energy.”
TeGrotenhuis, Paul, and their colleagues call their field “MECS,” for “Microtechnology-based Energy and Chemical Systems.” They have taken their cue from the technology known as “MEMS” (Microelectromechanical Systems), which involves the flow of electricity rather than fluids through micromechanical structures.
The carbon-based arrays of MEMs microbatteries made by Marc Madou of the University of California, Irvine, and his colleagues have thousands of anodes and cathodes very close together. Thus, the ions don’t have to travel as far as they do in standard batteries.
“It’s like a traditional battery concept but multiplexed,” said Madou, who calls each battery element a “baxel” (a riff on “pixels,” also arranged in a matrix).
These devices could have applications for “all types of tiny electronic gadgets, from hearing aids, to implants, to cell phones. These arrays of batteries are much smaller and would last longer than conventional batteries,” Madou said.
Taking a different approach to powering small devices, Ashok Patil of the U.S. Army Communications-Electronics Command and his colleagues have produced a fuel cell that runs on methane, which is easier to store and transport than hydrogen.
“Basically the device would be a chemical engine that you could refuel, just like an engine,” Patil said.
“The whole idea is the size and shape would be like a battery. The goal is an energy density of 1,000 watt-hours per kilogram, which is 3-4 times [as efficient] as a conventional battery.”
10 March 2004
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Advancement of Science.
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