Vol. 54, No. 8        April 19, 2002

Vibration powered sensor • Nanotube transistors • Back-support system alleviates pain •

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By John German

Civil engineers assessing the health of a structure following an earthquake, storm, bomb blast, or other insult need to know how severely structural elements have been stressed. A beam or buttress strained beyond its tolerance limits might be dangerous.

In a project initiated by Sandia's Architectural Surety program, Labs researchers have demonstrated the key components of a self-contained microsensor system that powers itself by converting mechanical energy from the subtle vibrations of structures into electrical power that drives the system.

It uses this vibe-power to take simple sensor measurements, then stores the data in a memory device that can be read from outside the structure -- through concrete, steel, and other building materials -- with a commercial radiofrequency (RF) tag reader used by trucking and warehousing operations to track tagged inventories.

No batteries, no wires

Because the sensor system requires no hookups to batteries or wires, it could be embedded into a structure during construction and forgotten until a need arises to assess the health of the structure.

"If you bend a beam to a certain point, the next time you bend it, it is going to break," says Kent Pfeifer (1744), who conceived the device and demonstrated its feasibility along with colleagues Sarah Leming, Art Rumpf (both 1744), and Robert Waldschmidt (former contractor). "This technique could lead to self-powered sensor packs that can take a variety of measurements over a long period of time and store them until needed."

The system's power plant is a swath of piezoelectric material attached to a structural element, such as a beam. (A piezoelectric material produces electricity when subjected to stress or strain. The strain changes the electromagnetic alignments of vibrating crystals in the material -- the same natural vibrations that keep time in a quartz crystal watch -- to produce a net electrical charge.)

Each time the beam bends from a load on the structure (for instance, when a tall building sways in the wind or a truck traverses a bridge), the piezoelectric ceramic generates a tiny parcel of charge -- about 100 microcoulombs is all -- which is stored temporarily in the system's capacitance bank.

This stored charge is sufficient to power the microsensor system for a fraction of a second, long enough to take a simple reading.

Later, if strains on the piezoelectric exceed a predetermined threshold -- as the result of a significant insult on the structure, for instance -- the system's low-power microprocessor could turn on, command the sensor to take a measurement, commit the reading to the RF tag's flash memory, and quickly shut down into its power-conserving sleep mode.

To retrieve the stored data, a structural engineer can point a commercial hand-held tag reader at the structure near the embedded microsensor system. The embedded tag's resonant circuit harvests RF energy from the hand-held reader through changes in the circuit's impedance, making it possible for the hand reader to power the tag's response remotely.

Uses not yet explored

So far the Sandia team has demonstrated a system that powers a microprocessor generated from vibration only and illustrated an approach for storing and retrieving sensor data taken by such a system, says Kent.

"There's still more to be done," he says, including integrating and testing a complete self-powered microsensor system. "But we've shown that this is a viable approach to monitoring critical infrastructures."

"A finished device could provide thermal measurements, stress measurements, deflection and strain measurements, or other information that could be stored over time in a database or captured immediately after an event to help plan an evacuation of a tall building, for instance," says Rudy Matalucci, Architectural Surety program manager. "It would be a great way to monitor performance and health of high rises, bridges, dams, tunnels, and other infrastructures."

Wireless microsystems might initially be employed in structures to predict fatigue or failure of key structural elements, keep track of the number of stress cycles a bridge endures as heavy trucks cross it, or determine the integrity of key structures such as hospitals or emergency command centers following a disaster.

The technique might also be used in any application where a self-contained, no-maintenance monitoring capability is needed over a long period of time, says Kent.

Stacks of piezoelectric generators might make higher-power applications possible, perhaps continuously powering embedded clocks.

"I'm sure we haven't thought of all the possible applications," adds Kent. "This has never been done before."

The system's development was funded through the Laboratory Directed Research and Development program and Sandia's MESA Institute. John German

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By Nancy Garcia

Francois Leonard, a theoretical physicist in Thin-Film and Interface Science Dept. 8721, and his IBM collaborator have discovered one more reason to further develop nanotube transistors.

Over a two-year effort to understand and model these new-material laboratory curiosities, they found these transistors work differently from conventional devices and, in fact, offer an additional way to switch the transistor on and off.

The promise of more functionality at a reduced size, Francois says, "is a further motivation to try to make nanotube transistors even smaller."

Their findings, published in Physical Review Letters, also offer a new approach to modeling that will be of interest for predicting performance of nanoscale devices in general.

Conventional devices can be switched off by raising voltage at a gate between two electrodes. The research team shortened the conventional distance between electrodes some 100 times, to just 10 nanometers. They modeled a device in which the electrodes were linked by a strong, thin filament of graphite-like carbon, rolled into a nanotube no more than two nanometers in diameter.

The new model revealed that increasing voltage at the gate first turns off the transistor as in a conventional device, then switches it back on again. This occurs because some electrons tunnel through a quantum state and move across the gap via gated resonant tunneling -- which becomes the added functionality possible at this smaller scale. Increasing the voltage even more then creates a negative differential resistance, turning the circuit off again.

"The reduced dimension actually gives you additional functionality," Francois says, comparing the phenomenon to a quantum dot. "By controlling the gate voltage, we can make electrons pass through just one (quantum) level." Furthermore, unlike silicon, devices made with nanotubes would not need to be "doped" with impurities. The work involved new ideas in how to calculate current in a device in which electrons move in single file. "How do you calculate current when devices are governed by single electrons?" Francois asked.

Existing modeling tools were not adapted to the problem because they were more statistical in nature, essentially allowing behavior to be predicted by averaging the flow of many electrons.

Francois' co-author is Jerry Tersoff of IBM's T.J. Watson Research Center, a fellow theoretical physicist. The calculations, which took a couple of years to develop, were presented last spring at the San Francisco meeting of the Materials Research Society and are the subject of an invited talk at the International Conference on Computational Nanoscience and Nanotechnology this month.

Carbon nanotubes were first discovered in the laboratory about 10 years ago and are novel materials for several reasons. They are very strong and can be made in long filaments. Depending on their atomic structure the material can behave either electronically like a metal or a semiconducting material. The first nano- tube transistor was demonstrated in the lab more than three years ago. However, creating just one transistor takes some time, so potentially packing more circuits into small spaces with these materials is still very exploratory.

On the other hand, Francois says, since nanotube transistors have been demonstrated, using them in practical devices "is not that far-fetched -- it is a matter of time and effort." -- Nancy Garcia

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By Neal Singer

A unique cushion designed to relieve the lower back pains of office workers, motorists, and truck drivers -- as well as quadrapalegics and others immobilized by reason of occupation or health -- is under development at Sandia.

The already-patented device does not support the spine with pressure achieved by pressing the back against a pre-formed semi-rigid foam shape -- the commonly used method.

Rather, in a process that resembles assisted power steering in a car, 16 pre-formed inflatable bladders aid muscles in the back intended by nature to support the spine. There is no direct contact between chair back and spine.

"A prototype enabled a man with degenerative joint disease who couldn't drive five hours to drive across the US and back," says Robert Felton, president of the Los Angeles medical company Numotech that built the prototype and will market the finished device.

The electronic work at Sandia is intended to improve reliability of the prototype device. A second goal is to shrink its pumps, batteries, and circuits from an auxiliary box currently a foot square and four inches deep to one-third that size.

"We want to integrate the electronics to make them flush with the chair back for office workers," says Sandia project lead Mark Vaughn (15252).

The work is being done in conjunction with a Russian manufacturing corporation, Spektr-Conversion, and New York investment banking house M.R. Beal.

The device, says Felton, "should significantly reduce the amount of drugs needed for pain management." The back cushion is expected to be on the market in little more than a year and should be available at a price in the range of $500-$700, he says.

How it works

Almost all back-support systems attempt to relieve the intradisk pressure believed to be the source of lower back pain. The method commonly used is to superimpose order on the disks by means of semi-rigid molding in a support backing.

The patented system under development aligns the spine in a more comfortable way. The bladders can be inflated and deflated in groupings to achieve levels of support that vary with the needs of individuals. The entire apparatus adjusts forward and back from the action of a single large bladder. A concave depression, achieved by side bladders, holds the back straight regardless of side movements of the vehicle or chair. A series of rocker switches adjust inner contours to each individual's geometry.

The device is the second product arranged for manufacture in Russia by Numotech. A two-year design for manufacturability with Spektr-Conversion was signed a year ago for a wheelchair seat cushion designed to improve blood circulation for paraplegics and the formerly bedridden in hospitals.

Numotech cooperates with DOE through a program called the Russian Transition Initiative to achieve non-proliferation of nuclear arms. The project provides work for competent scientists who otherwise could be hired by unfriendly countries to make nuclear weapons. Spektr-Conversion consists of former workers from Russian nuclear lab Chelyabinsk-70.

Though national defense goals are involved, the projects are no sham. Commercially viable products and profits are expected. The cheapness of the labor market in Russia is also a factor. Labor costs are reduced by a factor of 10 in the former USSR, says Mark.

The back-support device is also the third medical assistive device on which Sandia has worked jointly with Numotech. The first, called the Numobag, uses a slight increase in oxygen concentration to heal potentially lethal sores faster than they would on their own, thus decreasing mortality and drains on medical resources. The process, called topical hyperbaric oxygen therapy, uses inexpensive plastic bags and an inexpensive metering system to contain and monitor concentrations of oxygen around the patient's affected part. The Numobag is now being tested in Veterans Administration hospitals and has been FDA-approved. Jan. 1, 2003, is the target date for that product's general market availability. -- Neal Singer

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Last modified: April 17, 2002

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