The sensor, which is smaller than a dime and paper thin, is based on the same theory behind plastic security tags used in stores to prevent shoplifting. The device, which will be described in the July 15 edition of Analytical Chemistry, a peer-reviewed journal of the
American Chemical Society, the world's largest scientific society, also shows promise for monitoring environmental toxins and terrorism agents like ricin.
"The vision of our work is a passive sensor of virtually unlimited lifetime that could be placed in the tissue of the skin," says Craig Grimes, Ph.D., a professor of electrical engineering at Penn State University and lead author of the paper.
Designed as an inexpensive device to continually monitor the blood glucose levels of people with diabetes, the passive sensor requires no internal power supply and no connections outside the body, according to Grimes and his associates. "Whenever a reading is needed, a person can wave their hand or arm in front of a reader that will automatically detect the sensor," Grimes says.
The sensor is based on "magnetoelastic" technology, just like the plastic security tags on store merchandise, which are sensed wirelessly as they pass through an exit.
"The cost of these anti-theft markers is a penny," Grimes says. "We've leveraged off that same premise, so the material cost associated with the sensors is effectively zero." The electronics used in the reader, which is about the size of a wristwatch, cost approximately $50, the researcher estimates.
"Magnetoelastic sensors can be considered the magnetic analog of an acoustic bell," Grimes explains. "If you hit a bell with a hammer, the bell rings at a characteristic frequency. If you coat the bell with a layer of paint, the frequency changes." Likewise, the molecules in a magnetoelastic sensor vibrate in the presence of a magnetic field, and the frequency varies with different chemical coatings.
Grimes' glucose sensor is coated with a polymer that responds to changes in acidity, and then coated with the chemical glucose oxidase. The glucose oxidase reacts with blood glucose to produce an acid, which causes the polymer to swell and changes the frequency of the sensor. The reader then interprets these changes as blood glucose levels.
"Generally speaking, we can monitor a sensor that's 6 millimeters long from about 6 inches away," Grimes says.
Grimes is working toward developing sensors that detect multiple chemicals at the same time. To achieve this, he fabricates a tree of sensors of varying lengths (and therefore varying frequencies) attached to a central "I-beam." The sensors are all coated with a different chemical, resulting in a harp-like platform about a quarter-inch high for a 10-chemical sensor.
"There's a large variety of biological things, from protein toxins to pathogens, that we're interested in detecting," Grimes says. "We've basically been working our way through a shopping list of these." So far, he and his colleagues have developed systems for monitoring E. coli and Staphylococcal enterotoxin B � two causes of foodborne illness � as well as the terrorism agent ricin.
Grimes is not yet partnering with a developer to commercialize the glucose sensor, but he expects that the system could go straight into animal testing after a minimum of "tweaking." "We've been developing the technology for years now on a real shoestring budget," he says. "If somebody came along and said, 'Let's do it,' it could be a very quick process."