Researchers have developed a solar-powered sensor system that is just nine cubic millimeters in size. It is 1,000 times smaller than comparable commercial counterparts and can harvest energy from its surroundings to operate nearly perpetually. The system could enable new biomedical implants as well as building and bridge-monitoring devices. It could also vastly improve the efficiency and cost of current environmental sensor networks designed to detect movement or track air and water quality.

Developed at the University of Michigan the system’s industry-standard ARM Cortex-M3 processor, solar cells and battery are all contained on its tiny frame, which measures 2.5 x 3.5 x 1mm. The engineers say successful use of an ARM processor - the industry’s most popular 32-bit processor architecture - is an important step toward commercial adoption of this technology.

“The ARM Cortex-M3 processor has been widely adopted throughout the microcontroller industry for its low-power, energy efficient features such as deep sleep mode and wake-upinterrupt controller, which enables the core to be placed in ultra-low leakage mode, returning to fully active mode almost instantaneously,” said Eric Schorn, vice president, marketing, processor division, ARM. “This implementation of the processor exploits all of those features to the maximum to achieve an ultra-low-power operation.”

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 one nanowatt, which is one-billionth of a watt.

“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 developers say the key innovation is their method for managing power. The processor only needs about 0.5V to operate, but its low-voltage, thin-film Cymbet battery puts out close to 4V. The voltage, which is essentially the pressure of the electric current, 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.