July 13, 2004 -- It gave Popeye the power to knock out arch rival Brutus and take the hand of fair Olive Oyl. But someday, spinach could lead to another, more useful form of energy — the electrical kind.
Researchers at the Massachusetts Institute of Technology and the University of Tennessee in Knoxville are working on developing a new type of photovoltaic cell that turns sunlight into electricity using a protein found naturally in the leafy green vegetable.
Marc Baldo, an assistant professor of electrical engineering and one of the project's scientists at MIT, says the research's goal was to come up with a better, more flexible solar cell.
"There are a bunch of ways to make conventional solar cells, most of them are made of silicon and fabricated the same way as silicon chips," says Baldo. "It's a high-energy and high-temperature process that's not really compatible with lightweight [and bendable] plastics."
So, scientists at MIT and the University of Tennessee figured the best way around those limitations was to borrow from Mother Nature.
"We got interested in how plants convert solar energy to help them grow," says Baldo. And while studying the process of photosynthesis, the researchers discovered an interesting fact: the proteins that naturally convert sunlight to plant energy for growth actually produce a tiny, but measurable, amount of electricity.
Tiny Green Power Plants
Baldo and the researchers then isolated these microscopic protein structures, measuring about five or six nanometers in size, from the plant and sandwiched them between a thin gold film attached to a sheet of electrically-conductive, transparent material on one side and an organic layer of conductive material on the other.
When scientists directed sunlight on to the sandwich, the proteins generated electrons which passed from one layer of the sandwich to the other and produced a tiny electrical current.
The team's research work has been published in a recent issue of Nano Letters, a scientific journal.
"We just proved that you can take structures of plant and produce electricity," says Baldo. And more importantly, he says the work could lead to developing tiny power plants that measure not much thicker than a human hair.
"People have been trying nano-fabrication, trying hard to develop electrical circuits in the one-nanometer range and [it's] really difficult," says Baldo. "Nature gives a way to do that with these structures. And that's what we're really excited about."
And the researchers aren't the only one excited by the potential.
The U.S military, which is funding the project through the Defense Advanced Research Projects Agency (DARPA) and the Naval Research Labs, is hoping that one day biological-based solar cells could provide power to tiny robot spy craft or even woven into soldiers' uniforms to lessen their dependency on batteries for radios and other high-tech gear carried into battle.
Working Out the Weeds
But before that happens, Baldo admits that a lot more research needs to be done to solve some difficult hurdles.
For one, since the proteins are organic material, the scientists still need to figure out ways to ensure longevity.
"The big challenge is stability," says Baldo. "These protein complexes get repaired in plants… they have [other] proteins that provide support. But outside of that natural environment, [the photosynthesis proteins] can fall apart and the devices fail.
For now, the scientisst use a soap-like material in the solar cell that protects the energy-producing proteins by providing a similar environment as that found in plants.
"Right now, they work for up to three weeks," says Baldo. "We need to boost that significantly."
Another sticky hurdle: efficiency. Using the proteins from spinach plants, Baldo says the solar cell is roughly converting 12 percent of the sunlight into electricity. Most conventional solar cells, by comparison, are 20 percent to 30 percent efficient.
But Baldo believes that the efficiency problem could be solved with better designs that capture light more effectively. And while the prototypes use spinach proteins — "because they were readily available," says Baldo — other proteins from other plants might provide the solution. The team, for example, is going forward with their research using proteins from peas.
"This is [just] a proof of principle," says Baldo. "What might happen from here, I can't tell."
And whether or not plants might actually one day become a form of literally green power, at least one thing is certain. Our parents and doctors were right when they said green vegetables are good for you.