One of the main ways to save power, says Otis, was to reduce how often the sensor measured electrical signals produced by neurons. The researchers programmed the microcontroller to "wake up" when an electrical spike occurred, and record only the signals that were above a certain threshold. "Neuroscientists are interested in the spike rate," says Otis. "We don't digitize the entire brain waveform."

In addition to a handful of low-power circuit design considerations, researchers built a small signal amplifier that boosts the electrical signal from neurons while minimizing electrical noise. For this, they split the incoming signal into two parts. The amount of incoming electricity from neural activity is the same, but by splitting it between a pair of transistors within the circuit, the amount of noise is cut in half.

In the moth experiment, researchers tested the battery-free system by collecting data on electrical signals from the moth's wing muscles. The tests showed the frequency with which the moth flapped its wings. The results are published in the journal IEEE Transactions on Biomedical Circuits and Systems; the researchers also discussed the work at a summit on Wireless Identification and Sensing Platforms (WISP) in Berkeley, CA, on Tuesday. The current system is too large to allow the moth to fly freely, but an upcoming chip, which will be presented in February, is small enough to enable unencumbered flight, says Otis.

"Most implantable devices have used lower frequencies," says Josh Smith, principal engineer at Intel, and organizer of the WISP Summit. A lower frequency also means that the devices must be read at close range, he adds. Using commercial RFID readers, says Smith, allows the device to be powered and data to be read from further away. However, he says it's still an open question whether the antenna will maintain the long range once implanted in animal tissue, because the signal might be absorbed. "Measuring moths is a good fit for this approach, since the antenna does not have to go inside the animal's tissue," he says.

Arto Nurmikko, professor of engineering at Brown, agrees, saying that it's useful to measure neural activity in moths, "but the real challenges and application potential emerge in work with primates."