PORTLAND, Ore. — Trees can be used to power circuits, but their voltage is too small to charge conventional batteries. University of Washington (Seattle) researchers recently demonstrated a nanoscale "boost converter" that integrates the ultra-low-voltage potentials generated by trees.

"The nanoscale is not just in size, but also in energy and power consumption," said engineering professor Babak Parviz, who specializes in what he calls "research at the interface between biology and electrical engineering."

"Normal electronics were not going to run on the types of voltages and currents that we got out of a tree," said Parviz.

A tree's output voltage can be as low as 20 millivolts, according to the researchers. They designed a circuit to nevertheless accumulate enough power over time to produce a 1.1-volt output--enough to power wireless sensors. The tiny nanoscale-sized power harvester consumed just 10 nanowatts when operating, but spent most of its time in sleep mode.

A special ultra-low power clock circuit also was designed to run on a single nanowatt during sleep mode. It requires only 350 millivolts to accumulate before awakening.

While designing the energy harvester, the researchers also attempted to gauge tree health by monitoring the "pulse" of its power output. "When you go to the doctor, the first thing that they measure is your pulse," Parviz said. "There seems to be some signaling in trees, similar to what happens in the human body, but at a slower speed."

Last year, the theory behind "tree power" was demonstrated by MIT researchers in an attempt to harness the pH difference between a tree's inner trunk and the ground. MIT professor Andreas Mershin demonstrated that a tree's systems work to maintain a constant pH differential, thus supplying the work that powers any energy-harvesting circuitry.

Mershin founded a company called Voltree Power (Canton, Mass.) which recently won a contract from the U.S. Forest Service to supply a wireless mesh sensor network powered by trees. "We've designed a custom product for the U.S. Forest Service that will integrate with their existing weather stations," said Voltree CEO Stella Karavas.

The Forest Service operates 28,000 U.S. weather stations, each transmiting conditions on the ground to a satellite. Voltree's energy harvester gathers information from the surrounding forest and transmits it wirelessly to existing weather stations. The architecture extends the stations' reach under the forest canopy for the first time. Pilot sites will be tested this fall, with the first weather station upgrades scheduled for the first quarter of 2010.