Sandia LabNews

Sandians seek tiny chemistry lab on a chip

CHEM LAB IN A SNOW-PEA POD? – The three principal components of Sandia’s micro chem lab for gas-phase detection and analysis are small enough to nestle easily inside a snow-pea pod. The left-most component is the surface acoustic wave sensor array, the lab’s detection mechanism. The center one is a preconcentrator that absorbs or adsorbs chemical vapors. The one on the right that looks like a tiny CD is a miniature gas chromatograph column. Together they collect, concentrate, and analyze a minute chemical sample weighing less than a single bacterium. Other Sandia micro chem labs analyze liquids. (Photo by Randy Montoya)

New chemical sensing approaches are being researched under the first "grand challenge" Laboratory Directed Research and Development (LDRD) proposal funded last year. After three years, the approximately 40 Sandia scientists and engineers working on the challenge hope to demonstrate a palm-top-computer-sized device that can sniff explosives and chemical warfare agents. In five to 10 years, devices should be able to address wide applications and simultaneously identify hundreds of liquids and gases.

Arrays of these chemistry labs on a chip could be sent into battlefields or mounted near factories to provide chemical reconnaissance. The same technology might also provide diagnostic screening in about a minute at a patient’s bedside.

"There’s a huge amount of information in chemical signatures that the world is not making use of," says David Rakestraw (8358), "because it’s too costly. It’s also very difficult to extract out all of this information using traditional analytical chemistry in a laboratory."

Shrinking chemical analysis channels and integrating them onto mass-fabricated chips, as David and fellow researchers are attempting, could overcome the cost-effectiveness hurdle. Gains would not only be the creation of devices that are inexpensive, covert, and portable; speed and reliability would also increase. Sensitivity and selectivity could also be improved through use of multiple parallel separations channels, something only practical in microfabricated devices.

Science in the micro domain

Trying to integrate systems to collect, separate, and identify liquids and gases draws upon many existing capabilities. The devices can incorporate micro heat pipes to distill samples, new laser diodes and microoptics for detection, and microfabrication approaches to make extremely small, precise structures. The effort also capitalizes on Sandia’s expertise in systems integration itself. In Dept. 8120, mechanical engineer Karl Wally and electrical engineer Scott Carichner are charged with combining capabilities in a working prototype.

The "grand challenge" to create an autonomous microchemistry lab includes funding to explore science at the micro domain, where properties sometimes run counter to intuition. For instance, liquids experience no turbulence as they move along channels smaller than a hair, because viscosity dominates. Liquids can be pumped by delivering an electronic "nudge" along glass microcapillaries. There, the slight charge of the walls causes opposite charges in the solution to line up facing the walls. Applying a voltage that induces a current causes the liquid to travel along en masse. An advantage for separations is that the leading edge of the liquid does not blur as it would if the liquid were moved by pressure. Compounds in the solution can thus be shuttled through microchannels in discrete batches, enabling sharp separations.

Sensing samples in parallel

With this separation technique, an unknown sample could be "parallel processed" in multiple channels that contain beads or microfabricated posts to increase surface area. Subjecting the sample to different solvents and particle coatings would alter the retention time for components of different charge and mass, thus providing a signature for compounds of interest, David says.

Sandia’s vertical-cavity surface-emitting lasers would detect samples labeled with fluorescent dye, which would be excited by the laser when the dye is present. These dot-sized micro lasers can be constructed in arrays in various materials allowing different wavelengths to be produced; each wavelength can provide chemically specific information. Paul Gourley (1112) has also used the laser cavity to examine optical properties of substances such as diseased cells (Lab News, Aug. 15, 1997).

Gas detection research for the "lab on a chip" is being led by Greg Frye (1715). The gas detection scheme involves using microporous films to collect and concentrate a minute chemical sample weighing less than a single bacterium. The small sample would be "flash desorbed" under low power, taking just one- thousandth the conventional heating time. The tiny pulse of gas would then flow into a narrow separation column of up to a meter long (since length aids separation) that has been coiled into less than a centimeter square to conserve space.

From there the separated gas pulses would flow over an array of coated acoustic wave sensors. Different absorptions by the different coatings (signaled by a shift in frequency) build up a fingerprint characteristic of each chemical. As an added advantage, surface acoustic wave sensors are among the potential lab-on-a-chip components whose sensitivity increases as their size shrinks, Greg says.

The gas phase analysis system initially will use commercially available pumps and valves, although the researchers are examining the possibility of using novel designs for miniature components that take advantage of phenomena such as surface tension that become more dominant in the micro domain.

"The best designs for microfabricated components will take advantage of the fact that interactions are driven by surface behavior," says Terry Michalske (1114), one of four managers overseeing the grand challenge project.

The microfabrication and packaging of the gas and liquid sample collection, separation, and chemical-detection components used in the "lab on a chip" are carried out at Sandia’s Compound Semiconductor Research Laboratory under the direction of Stan Kravitz, Steve Casalnuovo (both 1713), and Mial Warren (1712). This cleanroom facility provides the versatile, rapid turn-around prototyping capability required to produce parts in gallium arsenide, silicon, quartz, or glass, depending upon the component’s function.

Smart, small sensing machines

A third grand challenge component entails work to develop cooperative distributed sensing and behavior in which swarms of small vehicles, known as robugs, or fixed sensors can communicate and map the location and movement of suspicious or threatening chemicals.

In three years, Barry Spletzer (9611) hopes to have an architecture for what a distributed intelligence system should look like. Nearly two years ago, his group developed its first so-called "small, smart machine," a Miniature Autonomous Robotic Vehicle known as MARV. Built of inexpensive parts and measuring only 1 cubic inch, it drew power from a pair of camera batteries to follow an electrified wire around a tabletop.

They are now working on other vehicles with enhanced mobility, greater intelligence, and the ability to cooperate with each other. The current research simulates a swarm of vehicles acting cooperatively. The simulation will be verified using a small swarm of actual vehicles based on the RATLER concept developed by Advanced Vehicle Development Dept. 9652. The final goal of this component of the grand challenge is to show that a cooperative swarm using autonomous microchemistry labs can quickly and autonomously locate chemical sources.

With this capability, Barry envisions relatively "dumb" (and thus expendable) robugs performing humanitarian demining by moving slowly through fields with energy from lightweight, low-power photovoltaic units. Meanwhile, autonomous units able to sniff chemical warfare agents could replace sensors now mounted on jeeps. Phil Bennett (9651) and co-workers will first simulate this capability in a computer model before it is developed for a field trial.

Inexpensive mass fabrication, based on existing microchip manufacture techniques, could spur widespread use of micro chem labs. Potential national security applications range from detecting weapons of mass destruction to monitoring the state of health of the stockpile. Everyday items employing similar technology might also become available one day at neighborhood stores, to test water and food or perhaps monitor the course of an illness or determine the safety of the immediate environment.