A silicon chip the size of a match tip is placed in a contaminated environment. Bioluminescent bacteria intentionally placed on the chip begin to "eat" the pollutant. As the living cells enjoy their feast, they light up. Their photons of blue-green visible light are absorbed by the silicon, creating electrical charges that are fed into processing circuitry on the chip. Signal-processing microelectronics measure the tiny electrical current and sound an alarm. The "critters on a chip," which were genetically engineered to emit light as they eat and digest environmental pollutants, have indeed detected naphthalene. The chip electronics linked to these living sensors reveal the concentration of the pollutant, which is related to the amount of electric current produced.

The critters-on-a-chip technology developed at ORNL in 1996 could be used to map soil contamination (by sprinkling a suspect area with the tiny chips). A prototype device was made at ORNL by coupling Pseudomonas fluorescens HK44, a novel naphthalene bioreporter microorganism developed by the University of Tennessee Center for Environmental Biotechnology (UT-CEB), to an optical application-specific integrated circuit (OASIC) developed at ORNL. A measured electrical signal was obtained when the device was exposed to moth balls, which are made of naphthalene. A second prototype used the toluene-sensitive Pseudomonas Putida TVA8, also developed at UT-CEB.

The combination of the OASIC chip and bacteria engineered to be sensitive to a specific biological or chemical agent could have many applications. The technology could be used to detect specific chemicals in groundwater or soil, including liquid pollutants from fuel spills, toxic metals such as mercury, and explosives such as TNT that may have leaked from land mines. Oil exploration companies might want to use it to detect hydrocarbons that indicate the presence of nearby oil and gas deposits.

Because of the technology's exciting uses and because of the catchy nickname "critters on a chip" coined by chip developer Mike Simpson of ORNL to better promote the concept, the news media has had a field day with it. It has been featured on television and National Public Radio, and articles on the technology have appeared in Christian Science Monitor, New Scientist, Chemical and Engineering News, National Geographic, and Business Week. As a result of the Business Week article, a large environmental instrumentation company is negotiating for an exclusive license to manufacture and market living sensor chips.

The concept sprouted in the summer of 1996 after Simpson heard a seminar on bioremediation given at ORNL by Gary Sayler, UT-CEB director and UT professor of microbiology and ecology. Sayler mentioned successes in incorporating a gene into the bacterial genome that makes the one-celled organism emit visible light during metabolism of its favorite nutrient. One gene (luciferase) comes from fireflies, and the other gene comes from the bacterial genus Vibrio, whose members get protection on a species of deep-sea fish in exchange for providing bioluminescence needed by the fish to attract prey or a mate.

After the talk, Simpson approached Sayler saying something like, "You engineer bacteria to emit low levels of light, and I develop optical sensor chips that detect low levels of light. Perhaps we should do a project together."

The rest is history. Thanks to a project supported by ORNL's internally funded Laboratory Directed Research and Development Program, it is now possible to make living sensors using integrated-circuit chips that are small, rugged, and require little power. Such wireless chips can be deployed where other devices can't (e.g., groundwater, industrial process vessels, and the battlefield). The key is to place the bioreporter bacteria on a transparent silicon nitride film that protects the etched silicon chip from damage in the presence of hazardous chemicals. To increase the shelf life of the bioreporter chip, the bacteria could be freeze dried, and a micromachine on the chip could activate the living sensors when needed by "dumping" water and nutrients on the dormant bacteria. The chip can be configured to transmit a signal to a receiver linked to a computer. The technical people on the project now call the hybrid, half-living, half-silicon devices "bioluminescent bioreporter integrated circuits," but most folks prefer to call them "critters on a chip."—Carolyn Krause