To test the device, he rigged it up to a simple accelerometer circuit and found that it was able to convert 30 percent of the available kinetic energy into electricity. Although it's difficult to make direct comparisons with other devices because of the differences in design, energy source, and size, Beeby's group nevertheless carried out a comparison that tried to take these factors into account. According to these calculations, the group's device performed very well and was the most efficient yet.

The work has been published in the Journal of Micromechanics and Engineering and was carried out as part of a wider European project called Vibration Energy Scavenging, or VIBES. The device is designed to mop up energy from vibrations of particular frequencies. (The prototype, for example, was designed to work with vibrations typical of manufacturing equipment.) But by varying the parameters, the same design could be used to work with other frequencies for other applications, says Beeby.

According to Yeatman, however, the most likely commercial applications will be in situations in which low-cost devices can't be easily reached or accessed, such as in wireless sensor networks for bridges and other large structures.

Another example of this is the use of microgenerators to power medical implants, says Beeby. One suggestion is to employ them to power devices like pacemakers, possibly even using the motion of the heart as the energy source. With these kinds of devices, one of the big risks is the need for a battery change, says Andrew Grace, a cardiologist at Cambridge University, in the United Kingdom, and a consultant at Papworth Hospital, also in Cambridge. "The idea that the heart could provide the energy to recharge the pacemaker, almost like a wristwatch, sounds very attractive," he says.

Beeby suspects that there are better parts of the body from which to scavenge energy, such as the limbs. And he feels that there are other types of implants that would use less energy than a pacemaker, such as biomedical sensors or drug-delivery systems.