When a semi full of bananas gets a little warm, it makes a detour from its path to the grocery store and drops off the load at the garbage dump instead. But in all probability, not every piece of fruit in the shipment turned bad the moment the thermometer hit, say, 41.

A group of University of Florida engineering students, working with Bruce Welt, an assistant professor of agricultural and biological engineering, has found a more accurate and convenient way of monitoring freshness that will help separate the good bananas from the bad.

Their sensor, a half-dollar-size device they call the Time-Temperature Integrator, collects data and transmits it wirelessly. Current freshness sensors show spoilage or near-spoilage with a tab that changes color. They're not specific to the product; the interpretation of the hue is up to the eye that beholds them, and to collect the data in the first place someone has to actually find the product itself.

"If you're going to go look at a tab to see if the tomatoes are going to spoil, you might as well look at the tomato," Welt said. The TTI, though, can dump its data anywhere there's radio frequency identification—in the supermarket, or when a truck passes the doors of a warehouse.

When temperatures climb too high in the back of that truck, the whole batch of bananas needn't be thrown out. Palettes toward the center that have stayed cool can continue on their journey. And the sensors can be made to model the precise way that different products head toward spoilage. "You can't just say, 'A banana behaves like this.' It might be a certain variety from a certain location," Welt said. "There are a lot of variables."

The "enzyme kinetics" of most fruits and vegetables are well known and those variables can be programmed by the supplier or manufacturer or by whoever ends up producing the Time-Temperature Integrator.

"A fairly substantial amount of food gets wasted each year," Welt said. "Ultimately, if we can cut out a lot of that waste, all of that cost gets built into that commodity. Ideally, the savings get passed on to the consumer."

Heat management on Earth is pretty flexible. You can introduce cool fluids, for instance, or you can vent away hot gases. In Earth orbit, on the other hand, the choices are fairly limited.

"Space is a unique environment," said Ann Darrin, an aerospace engineer who is a program manager with the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. "There is no convective cooling. You don't have an option of just blowing air over something.

"The better you balance your heat," Darrin added, "the longer your life in orbit."

The conventional means of controlling heat in a satellite has been complicated— active cooling systems; heat shields, which simply block the sun; or large louvers, similar in size and function to Venetian blinds. When exposed to searing sunlight, the blinds close to create a reflective shield. In the shadow of the Earth, the blinds open up.

This MEMS device (above) manages heat aboard satellites by use of minuscule shutters (below) that open and close to reflect incoming sunlight.

But on the Space Technology 5 mission launched in March, there was no room for anything so bulky, let alone a more active circulating fluid cooling system. The ST5 consists of three satellites, each no larger than a typical television set, designed to study the Earth's magnetosphere. Given so little volume to work with, Darrin and her colleagues, together with researchers at NASA Goddard Space Flight Center, had to come up with a new way of shedding heat.

Their solution was to take the standard Venetian blinds and shrink them radically. The system, called variable emittance coatings for thermal control, is a microelectromechanical device that opens and closes comb-like shutters each smaller than a human hair. Darrin said they are the first MEMS devices to go through full space qualification before launch. Tiny motors move the combs a fraction of an inch to change the reflectivity of the device, meaning that the changes can be made in just microseconds—an important feature for a system that can experience a temperature change of hundreds of degrees in a second.

"You don't want a system that requires a lot of power," Darrin said, "because it's a self-inflicted wound. You generate heat internally as you try to shed it. You're fighting against yourself."

Also, due to its small mass and number of moving parts, Darrin said the MEMS device ought to be more robust than systems produced at a more familiar scale. "Conventional wisdom in engineering is big is strong and small is weak," Darrin said. "We're here to say small is really strong. We've put a million cycles on these things. Because there's no weight and no friction, they'll go forever."

It's the stuff of Tom Clancy thrillers that has recently become a deadly serious matter: Agents intercept a shipment of radioactive material destined for international terrorists. The U.S. government accuses a certain country of providing the material—a claim that the country in question denies.

Stalemate? Not if a new radiation detector developed at the National Institute of Standards and Technology in Boulder, Colo., gets wide deployment. With a sensitivity some 10 times greater than that of the best radiation detectors now in use, the device can actually come up with a radiation fingerprint that can be used to distinguish one collection of isotopes from another.

"Speaking hypothetically, if material is caught being smuggled at the border, you could measure the fingerprint of the radioactive isotopes," said Joel Ullom, a NIST staff physicist. "You could compare it to a library of isotope signatures from facilities around the world. That could be potentially useful."

Each of the 16 detectors atop this silicon chip can measure the energy of a single gamma ray.

Within a sufficiently large sample of radioactive material, there are at least a few nuclei decaying at any given time. The result of such a decay is a pair of daughter particles and a gamma ray with an energy characteristic of the decay. (The energy of the gamma ray is analogous to the wavelength of light.) If you can measure the energy of the gamma rays coming from a source, you can easily determine what the source is made of—without destroying the material as is necessary for mass spectrometry.

"Mass spectrometry—sorting the sample atom by atom—is expensive, slow, and destructive," Ullom said.

Detecting gamma rays is no easy feat. They are too energetic to focus and so penetrating that conventional charge couple devices don't stop them. The only hope, in fact, is trying to catch the gamma rays in a relatively large, inert block and trying to measure the resulting change in temperature. To do that, the NIST team needed a way to control temperatures very precisely and a detector that was extraordinarily sensitive to changes in temperature.

Controlling temperature was straightforward, if not exactly easy. The device is housed in a cryogenic cooler running at a frigid -273°C, or a tenth of a degree above absolute zero. To detect the minuscule changes in temperature, the team deployed a thin film of superconducting molybdenum that responds to heat by changing its resistance. The thin film was created through photolithography; atop that layer, little squares of tin that absorb incoming gamma rays were placed en masse by means of a device similar to a waffle iron.

Ullom said the pinhead-size prototype worked well enough for the project team to begin building a larger device at Los Alamos National Laboratory before the end of the year. They are also looking at materials that may be more sensitive to gamma radiation than tin.

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

© 2006 by The American Society of Mechanical Engineers