The recent spate of poisonings at chain restaurants in the United States has once again put food contamination in the spotlight.

While food that's fresh is seen as healthier, it also has the potential to carry pathogens unless it has been thoroughly cleaned. Unfortunately, tests to determine whether foodstuffs have been contaminated with harmful bacteria such as salmonella and E. coli take as much as a day to complete—meaning the items that were tested have been passed into the food stream before the results are in. What's more, such testing is too expensive to be routine. Often, the first indication of a problem is a report of sickened diners.

That situation may soon change, thanks to a technology developed at Drexel University in Philadelphia. The heart of the system is a series of vibrating cantilevers that can detect as few as four cells per milliliter. And unlike other cantilever-based systems, the Drexel cantilevers could be mass produced cheaply.

A new MEMS-based device may one day be used instead of Petri dishes for measuring bacterial contamination.

At present, anyone who wants to test a food processing or preparation area for bacteria such as E. coli must collect a sample and place it in a Petri dish filled with a growth medium such as agar. The sample is then incubated to enable whatever bacteria are present to grow and multiply. After 12 to 24 hours, the dish is removed from the incubator and examined for signs of bacteria.

The cantilevers developed by engineering professor Raj Mutharasan detect the same pathogens in minutes, not hours. Compared to the microscopic cantilevers used by other researchers, Mutharasan's antibody-coated cantilevers are huge—2 millimeters long and 1 millimeter wide.

"We wanted to optimize this for use in a liquid environment, and micron-scale cantilevers get highly damped in a liquid environment," Mutharasan said. "We also use a piezoelectric material ceramic rather than a crystal, but we can measure its impedance very accurately. Our discovery is that we can detect extremely tiny quantities of change in mass—on the order of sub-centigram per hertz with this system."

Since the cantilevers operate in liquids, the samples don't have to be preprocessed. The fluids pass through the chamber where the cantilevers are oscillating; as microbes glom onto the antibody coating, they change the mass—and thus the vibrating frequency—of the arms. Mutharasan said the system can examine as much as 1 ml per minute, a rate fast enough to make even daily testing in food processing plants possible.

And the cantilevers should be inexpensive enough to produce that the system may one day find itself used in Third World nations, or in private homes in the West.

Stress fractures are easy to predict in aggregate: Out of a given number of adults engaging in aggressive, physical activity, some will develop cracks in their foot and leg bones that will need medical attention. But catching these fractures before they become debilitating is a much harder task, one that takes almost constant attention.

"The cracks that lead up to these fractures are typically so small that by the time you can diagnose them radiologically, it's too late," said Ozan Akkus, a biomedical engineering professor at Purdue University in West Lafayette, Ind.

But a sensor system developed by Akkus and his colleagues may help prevent stress fractures by listening to the infinitesimal pops and cracks made by the bones themselves.

An ultrasonic sensoring system may be able to warn athletes that their bones are undergoing microcracks that could lead to stress fractures.

Bones are subjected to severe, repetitive stress by such activity as running long distances on unforgiving surfaces. While the forces applied to the feet and legs of dancers and marathoners are relatively small in themselves, when these forces are repeated over and over, they can create microscopic fractures that can weaken the bone and eventually lead to a larger, spontaneous fracture that may appear to occur without warning.

"It's like a car axle breaking after years of functioning perfectly," Akkus said. "There's an accumulation of microcracks that no one notices."

As many as 20 percent of soldiers in basic training, for instance, experience stress fractures in the bone of the lower leg, a condition commonly known as shin splints.

Akkus and Brent Cameron, a bioengineering professor at the University of Toledo in Ohio, found that these smaller fractures created a telltale sign: a high-frequency acoustic signal. In many ways, the acoustic signal in these bone microfractures is similar to the seismic signal created by an earthquake. The bioengineers found that the shape of the signal wave had a specific characteristic. Focusing on the envelope of the waves rather than on the high-frequency waves themselves made the microcracks easier to detect. All that was needed was a device to listen for the signal and interpret the data.

"The kind of people who experience these fractures are young: athletes, military recruits, people who don't have the chance to undergo X-rays or other clinical tool," Akkus said. "We saw that there's a need for an 'on-board' detection system. Something that's portable, something that's wearable. They can put it on and do their normal training activity. If there's some cracking, they would be issued a warning so they could adjust their activity." Akkus said the device could be similar to an iPod—small and light enough to be unobtrusive.

The bioengineers have developed lightweight prototype piezoelectric ultrasound sensors capable of detecting a microcrack up to five inches away. Such sensors could be attached to the ankles of a runner with medical adhesive and feed signals to a small device in his pocket.

But what do you do when you find out your bones are cracking? The easiest way to head off stress fractures is simple: rest. But the very people most prone to stress fractures are the ones least inclined to take a preventive break from training.

"These cracks are microscopic, so the pain they induce isn't debilitating," Akkus said. "And in a very competitive slice of society, athletes keep pushing themselves. If this sensor becomes a tangible and reliable indicator of impending stress fractures, they would have an objective sign that it's time to back off and get some rest."

home | features | breaking news | marketplace | departments | about ME back issues | ASME | site search

© 2007 by The American Society of Mechanical Engineers