Researchers have fashioned the world's tiniest radio out of a carbon nanotube. The nanotube, placed between two electrodes, combines the roles of all the major electrical components in a radio, including the tuner and amplifier. It can tune in to a radio signal and play the audio through an external speaker.

While the practical application of the radio is uncertain, it could be used in biological and environmental sensors. Researchers are now developing microelectromechanical (MEMS) sensors to measure blood sugar levels or cancer markers in the body. Instead of researchers using a stamp-size radio-frequency identification tag, a nanotube radio could be packaged with the MEMS-based sensor and injected directly into the bloodstream, says Alex Zettl, an experimental physicist at the University of California, Berkeley, who is leading the development of the nanotube radio. Once in the body, the radio could provide wireless communication between the tiny biological sensors and an external monitor. To do that, however, the nanotube radio would have to work as a transmitter. Right now, it is only configured as a receiver, but Zettl says that "the same physics would work as a transmitter."

The nanotube radio works differently than a conventional radio does. Conventional radios have four main functional parts: antenna, tuner, amplifier, and demodulator. Radio waves falling on a radio antenna create electric currents at different frequencies. When someone selects a radio station, the tuner filters out all but one of the frequencies. Transistors amplify the signal, while a demodulator, typically a rectifier or a diode, separates the data--the music or other audio--that has been encoded on a "carrier" electromagnetic wave.

Zettl's team used one carbon nanotube for all these functions. Because of their unique electrical properties, carbon nanotubes have been previously used to make electronic components such as diodes, transistors, and rectifiers. "It was a revelation that all of this could be built into the same [nanotube]," Zettl says.

The nanotube is grown sticking out from a tungsten surface, which acts as a negative electrode. The tip of the carbon nanotube is also negatively charged. A vacuum separates the nanotube from a positive copper electrode. The researchers use an external battery to apply a voltage between the two electrodes. Electrons jump out from the negative nanotube tip to the positive electrode, creating what is called a field emission current.