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MEMS project shrinks bio agent detector

Posted: 14 Mar 2002 ?? ?Print Version ?Bookmark and Share

Keywords:microelectromechanical systems? mems? sandia national laboratories? bio agent detector? sensor?

MEMS technology is the key component in a bid by Sandia National Laboratories researchers to reduce a biological detector the size of a lab bench to desktop dimensions.

"Right now we are shooting for a prototype size of about two footballs?one to collect the sample and the other to do the analysis," said lead researcher Curtis Mowry of Sandia's microchemistry lab. He plans to use one of the many commercially available collectors, focusing all his own attention on the second "football"?the analysis unit.

"There is a mass spectrometer in the literature that does the same kind of reaction we do, but in a suitcase-size device that uses a lot more power?we are trying to make a smaller version of that which will hopefully be cheaper, because we are using microfabricated parts," Mowry said.

The eventual application, Mowry envisions, will not be a calibrated lab instrument, but an inexpensive early-warning device that can quickly set off an alarm that calls in more sophisticated analysis methods.

"Other, slower methods are going to be more specific than ours, but the idea is to have something fast that tells you, 'hey, bring in something more expensive, because this could be anthrax,' " Mowry said.

The subsystems successfully tested for the prototype include several that had already been fabricated at Sandia's Intelligent Micromachine Initiative. The labs have a proprietary Sandia Ultraplanar, Multilevel MEMS Technology?or Summit?fabrication process for MEMS, a five-level polycrystalline silicon-surface micromachining process. Sandia has also built a catalog of standard subcomponents, from stepper motors to a gas chromatograph.

"We have a microfabricated chemical-agent sniffer program here that's been around for about five years, and that originally developed some of the components we used for this project, which is only two and a half years old," Mowry said.

Threat detector

Now that all the subcomponents have been tested, the next step will be to integrate the unit into a single device enclosure and characterize its performance with real-world tests. Mowry said he wants to test the device on all biological agents that could be used as weapons, as well as make sure that it does not trigger false alarms for normal bacteria.

"We need to tell the difference between things that are already in the atmosphere vs. something that is a threat," he said, "to tune our sensitivity and characterize just how well it can tell different agents apart. We hope to have a field prototype to demonstrate in one to two years."

The analysis unit performs the tasks traditionally done in a lab, but in an automated assembly line fashion that requires no human assistance. Basically, a collected sample of air is mixed with a reagent that enables any bacteria present to give off distinctive vapors when they are flash-burned. Then they are analyzed by an on-chip gas chromatograph.

An air sample enters the analysis device and first receives a dose of methylation reagent to enable the fatty acids to give off methyl ester gases when heated. A flash-quick heater then initiates the pyrolization reaction. "It's often called a Fame detector, since it is the gas chromatography of the fatty acid methyl esters produced by the bacteria that distinguishes anthrax from other, harmless bacteria," Mowry said.

Heat wave

In the laboratory, any number of instruments could quickly vaporize a sample of air, but for the MEMS-based device air was floated over an integrated flash heater, capable of jumping hundreds of degrees in temperature.

A thin-membrane resistive heater about the size of a fingernail produces a 500C temperature jump in 30ms. Because the heater is so small, it requires only 150mW; a macroscopic heater with the same performance would consume 130W of power.

Once the Fame vapors have been created by heating, a microfabricated gas chromatograph separates the components, since their different weights affect their flying time down the separation column. On the output of the column is an array of SAW devices, which creates a solid-state gas sensor.

"When our gases come out of the column they interact with the coatings, which will effectively change the mass of that coating, which in turn changes the resonant frequency of the quartz. We have an array of these SAW sensors with different coatings, so we get a different pattern for each different molecule," Mowry said.

The next milestone on the Sandia National Laboratories project will be when it integrates each of the MEMS-based components into a single, field-tested, football-size unit.

"So far we have only tested each subcomponent of the system to see if it could do the job of its larger, normal-size relative. We've shown that each microfabricated component can do the job of its bigger relative, but now we need to do the system integration," Mowry said.

? R. Colin Johnson

EE Times

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