10 December 2009—This week at the International Electron Devices Meeting (IEDM), in Baltimore, Md., Cornell University engineers presented research that shows progress in powering cybernetic organisms with a radioactive fuel source.
Electrical engineering associate professor Amit Lal and graduate student Steven Tin presented a prototype microelectromechanical systems (MEMS) transmitter—an RF-emitting device powered by a radioactive source with a half-life of 12 years, meaning that it could operate autonomously for decades. The researchers think the new RFID transmitter, which produces a 5-milliwatt, 10-microsecond-long, 100-megahertz radio-frequency pulse, could lead to the widespread use of radioisotope power sources.
The work is funded by the Defense Advanced Research Projects Agency (DARPA), which also funds Lal and Tin’s work on another project, called Hybrid Insect Micro-Electro-Mechanical Systems (HI-MEMS), whose aim is the creation of hybrid cybernetic organisms. In his presentation, Tin said that part of the goal of the radioisotope transmitter work is to power the insects that the group is developing for DARPA. The HI-MEMS program, which is approaching its fourth year, has already grown several kinds of insects—moths and beetles—with implanted control electronics. With such controls, they can be driven by a remote operator for ”stealth applications” and disaster response.
The insects themselves are powered by their own living tissue, but the onboard electronics (sensors and transmitters) require a separate power source. But the insects are too light to carry batteries, and logistical problems would prevent regular battery changes regardless. Therefore, Lal and his group at Cornell turned to radioactive isotopes to generate the necessary power.
To retain memory state in a typical low-power wireless sensor node, you need between 1 and 10 nanowatts. For periodic sensing and processing, the requirement jumps to between 0.1 and 1 mW. If you want to imbue your sensor with the ability to carry on periodic communications, you’re looking at a power requirement of between 1 and 100 mW. By using just a small amount of radioactive material, the Cornell team’s MEMS-based piezoelectric generator was able to create enough output energy to enable a high-power RF pulse every three minutes. The material in question is nickel-63 (Ni-63), a mildly radioactive isotope having a few extra neutrons in its nucleus. When it decays, Ni-63 emits beta particles—high-energy electrons that are relatively innocuous.
The RFID transmitter converts the energy of radioactive decay into mechanical movement in a MEMS device. A tiny silicon and piezoelectric cantilever—40 micrometers thick and 4 to 8 millimeters long—is suspended on a chip over the radioactive thin film like a diving board over a pool. It is similar to the nuclear microbatteries Lal and University of Wisconsin nuclear engineer James Blanchard described in IEEE Spectrum in 2004.
Here’s how it works: Electrons ejected from the radioisotope accumulate on the cantilever, giving it a negative charge. Now the cantilever is attracted to the (relatively) positive Ni-63 thin film and begins to bend toward it. As soon as it bends enough to touch the Ni-63, the charges jump back onto the thin film, and the cantilever, freed of the accreted electrons, springs back to its starting position. The power is generated when the cantilever snaps back to its original position. It continues this way until the isotope’s radioactivity is depleted, which the authors estimate could take up to 100 years.
Each seesaw motion takes three minutes to complete. Tin explained to IEDM attendees that some trade-offs could be exploited between the amount of output power and the length of electron accumulation. To achieve a burst every 30 seconds, for example, the power of each burst would have to be reduced.
Because of the way the transmitter is engineered, only a small amount of Ni-63 is needed to generate high-power pulses. However, Tin said that because the device is still a prototype, no optimizing had been done, and its efficiency in converting the energy of the radioisotope was just 0.06 percent. The entire device is 1 square centimeter.
The work drew a mixed reaction. NEC Electronics research fellow Yoshihiro Hayashi wondered whether the device would be a stable source of power because of the stochastic nature of radioactivity.
Another engineer questioned the transmitter’s reliability. Would the beta particles emitted by the Ni-63 source degrade the insect’s onboard electronics—or possibly the insect itself? How much would the package need to be radiation hardened? Tin was sanguine on this point, pointing out that Ni-63 beta particles penetrate only about 21 µm into a surface, requiring minimal packaging.
Cyborg insects aren’t the only devices that stand to benefit from the new research. Lal and Tin claim that using radioisotopes as they suggest could revolutionize long-term reliable sensing and monitoring. HP Labs reported in October that it was making progress on such long-term sensors in its Central Nervous System for the Earth (CeNSE) project, whose goal is blanketing Earth with tiny sensors. But at that time, Peter Hartwell, a senior researcher at HP Labs, in Palo Alto, Calif., admitted that power was one of the last remaining fundamental challenges they had yet to work out. HP’s new sensor requires a little under 50 mW, an improvement over the high-performance sensors, which now require about 1 watt.
According to Lal and Tin, radioisotope-powered devices could be built directly into concrete infrastructures, such as bridges and highways, and could remain operational for ”hundreds of years,” they say.