Nine cubic mm and powered by the sun

A processor, solar cells, and battery all sit within a 2.5 by 3.5 by 1-mm frams that is 1,000 times smaller than comparable commercial counterparts. The system, developed by University of Michigan researchers, could enable new biomedical implants as well as monitoring devices for homes, buildings and bridges.

With an industry-standard ARM Cortex-M3 processor, the system contains the lowest-powered commercial-class microcontroller. It uses about 2,000 times less power in sleep mode than its most energy-efficient counterpart on the market today.

The U of M engineers say successful use of an ARM processor— the industry's most widely used 32-bit processor architecture—is an important step toward commercial adoption of this technology.

"Our system can run nearly perpetually if periodically exposed to reasonable lighting conditions, even indoors," said David Blaauw, an electrical and computer engineering professor. "Its only limiting factor is battery wear-out, but the battery would last many years."

The sensor spends most of its time in sleep mode, waking briefly every few minutes to take measurements. Its total average power consumption is less than 1 nW.

The developers say the key innovation is their method for managing power. The processor only needs about 0.5 V to operate, but its low-voltage, thin-film Cymbet battery puts out close to 4 V. The voltage must be reduced for the system to function most efficiently.

"If we used traditional methods, the voltage conversion process would have consumed many times more power than the processor itself uses," said Dennis Sylvester, an associate professor in electrical and computer engineering.

One way the U-M engineers made the voltage conversion more efficient is by slowing the power management unit's clock when the processor's load is light.

The designers are working with doctors on potential medical applications. The system could enable less-invasive ways to monitor pressure changes in the eyes, brain, and in tumors in patients with glaucoma, head trauma, or cancer. In the body, the sensor could conceivably harvest energy from movement or heat, rather than light, the engineers say.

The researchers described their work in a paper entitled "Millimeter-Scale Nearly Perpetual Sensor System with Stacked Battery and Solar Cells." at the recent International Solid-State Circuits Conference in San Francisco.

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