It's a bird! It's a plane! It's ... a piece of cellophane! Researchers have discovered that cellulose, the ubiquitous building block of the plant kingdom, will flap when exposed to an electric field. Delicate sheets of cellulose with electrodes attached could be used to make microrobots, biodegradable sensors, and paper airplanes that flap like birds.
Grass, weeds, shrubs, and trees all rely on cellulose for structure and support. People use it in everything from pharmaceutical coatings to textiles, but most importantly for paper, extracting the fiber from wood pulp. Since the 1950s, researchers have known that wood is piezoelectric--it bends slightly when put in an electric field. But very little follow-up was done after those early observations, and no one thought to check whether plain cellulose had the same bendy tendency.
In the 27 June issue of Macromolecules, mechanical engineer Jaehwan Kim and colleagues at Inha University in Korea and Texas A&M University in College Station report that cellulose is indeed piezoelectric. They took cellophane, a lightweight paper made of cellulose, and deposited a very thin layer of gold on each side. The gold layers were connected to a voltage and acted as electrodes, with one side of the paper positive and the other negative. When the voltage was turned on, the cellophane curved toward the positive side; when the voltage was switched, the paper bent to the other side. If switched rapidly, the paper could "flap" like a wing. Wires aren't necessary, because the cellulose is sensitive enough to be controlled by microwaves (an antenna converts them into dc current).
Why the cellophane curves is not entirely clear. Kim and colleagues suggest that positively charged ions in the paper migrate towards the negative electrode and drag water molecules with them, creating a higher pressure on that side that causes the cellophane to bend towards the positive electrode.
"It's completely novel--everyone just assumes that cellulose is something that just sits there," says Mike Jarvis, a chemist at the University of Glasgow in the United Kingdom. The flapping cellulose is "ingenious," he says. But he's not entirely convinced by the team's explanation of how it works. Cellulose may float like a butterfly and flap like a bird, but until researchers figure out exactly what is going on, it may be a while before those paper airplanes go up, up, and away.