June 2007 Mechanical Engineering Departments: Tech Focus, Pt. 2

This section was edited by Associate Editor Jeffrey Winters.

Technology Focus part 2:
Instrumentation and Control

Stretching Sensors

Flexible electronics is poised to be the next big thing over the next two years. If things go according to some plans, future electronic displays will roll up like sheets of paper. But the upcoming generation of flexible electronics, which are to be embedded on a layer of plastic, do have some severe limitations. For all their ability to bend, vital connections can break if the plastic sheet is stretched too far or wrapped too tightly around a corner.

But research at the University of Illinois at Urbana-Champaign and Argonne National Laboratory may change all that. Engineers there have developed a new type of substrate for electronic sensors that would enable them to be as flexible as cloth.

Illinois material science professor John Rogers said that he has been working on flexible electronics, which are built on a thin plastic substrate, for about 10 years, almost from the beginning of their development. But Rogers said there were many sensor applications for which these new plastic semiconductors were unsuitable. One example would be monitors on the skin on the wings and fuselage of an aircraft, which is subject to small but substantial strain. Another, Rogers said, is medical monitoring devices that could be embedded in surgical gloves.

Thin ribbons of silicon undulate across the rubbery surface of this test device. As the device stretches, the ribbons unfold.

"Biologically inspired electronic devices require the electronics to be not on a flat wafer, but on a curved surface," Rogers said. A stretchable sheet of light sensors could wrap across the inside of a sphere to mimic the retina of a human eye.

To get there, however, Rogers's team would have to figure out how to make the material stretchy. After looking at placing the circuits directly on materials that are elastic like rubber, Rogers's team found a more elegant solution. They created an undulating field of microscale ribbons made of common semiconducting material, then fixed the ribbons to a stretchy substrate. As the substrate stretched and compressed, the ribbons folded and unfolded much like accordion bellows.

"The key to our strategy is not to throw out well-developed electronics and replace them with rubber-based semiconductors," Rogers said. "Instead, we wanted to shape silicon into geometries that enabled the kind of mechanical characteristics you're after at the device level.

"It's not just easier," Rogers added, "but it's probably a more realistic technology approach."

Although it's taken a decade to bring flexible electronics to the market, Rogers believes products using his stretchable semiconductors are not so far away—only three to five years.

Littlest Fingers

Perhaps the ultimate dream of nanotechnology is the nanoscale fabricator. Minute robots would grab molecule-size parts and assemble them into functioning nanoscale machines. Once you have the nanoscale factory, the thinking goes, you can make anything.

Laxman Saggere, a mechanical engineering professor at the University of Illinois at Chicago, hasn't made one of those—not yet. But he and his colleagues have built a centimeter-square micromachine with fingers that can grip and manipulate micrometer-scale particles.

"The idea is to have three or four fingers that can be programmed to reach any point in the workspace, grab a particle, and work with it," Saggere said. "Once we have a tool that can easily handle 3-D assembly, then it's a question of imagination as to what we can assemble."

The work was reported in the March issue of the Journal of Micromachines and Microengineering.

At present, workers who want to handle minuscule objects must work with cumbersome tweezers while looking through a microscope. As anyone who has eaten with chopsticks knows, a set of several fingers provides a better grip and finer control than two stiff tines.

Using common photolithographic techniques, the UIC team created a MEMS-type device with four actuators, each coming to a point with a finger just a couple dozen micrometers wide. When the researchers applied a force to the actuators, they bent toward one another. In experiments with polystyrene spheres, the fingers could grab and move the spheres through three dimensions.

While such fingers can work well in stable media, Saggere said that objects such as biological cells floating in a fluid would likely be beyond the grasp of this sort of manipulator. To tackle that problem, he and his colleagues are also developing a clasp-type mechanism, which would center itself around the floating object and slowly bring a pair of walls in on it.

"Right now, this is accomplished with lasers," Saggere said, "but the heat from the lasers causes problems with the cells. We think our approach will work better."

Shower Power Switch

It's a problem that parents of teenagers know all too well: One or more family members are tying up the bathroom taking extraordinarily long showers. Since bathroom doors lock from the inside, there have been few ways to turn off the taps on an amphibian son or daughter.

Don Brunkhardt knew the problem all too well. And he has developed a solution, too. He is marketing a device that automatically cuts water flow through a shower head to subtly—or not so subtly—prod the water hog out of the shower.

An electric motor cuts the flow of water after five, eight, or 11 minutes.

Turning the idea into a saleable product was far from straightforward. After working on various iterations of the idea for some five years, Brunkhardt thought he had a finished product in 2004. Unlike a low-flow showerhead, which may save water but, as Brunkhardt said, "doesn't motivate you to actually get out," his Shower Manager has a battery-operated microprocessor that is activated as soon as the water begins flowing. Once one of three preset time limits is reached, the Shower Manager cuts the water flow by two-thirds, a level that Brunkhardt said is enough to allow some last-minute rinsing, but takes most of the fun out of showering.

The initial model used a solenoid to cut the flow. But the range of pressures that the solenoid operated—between 40 and 70 pounds per inch—was too small. "That covered about 75 percent of potential customers," Brunkhardt said, "but there were a lot of people who had lower pressure than that, and some greater, so we had to redesign the device to accommodate them."

After considering the problem a little more, Brunkhardt switched from a solenoid to a small electric motor, which he said can operate across a range three times greater. He believes this wider range will enable his device to operate in most parts of the United States and elsewhere. Brunkhardt said he has received inquiries from Australia, where a multiyear drought has pushed water supplies to the limit. He also has had some interest from the American Southwest, which has water issues of its own, but one natural purchaser, Las Vegas Casinos, may pass on the device for now. High rollers, they discovered, love their long showers.