Stanford researchers have developed a cardiac device that fits on a pin head and derives power from radio waves from outside the body. The research is a breakthrough in terms of how small medical devices can be powered in the future.

Researchers say that the wireless power technology used in this device can also be used to power other devices like endoscopes and cochlear implants. This technology can remove the excess weight of batteries in implants plus batteries die out after some time needing another surgery to replace them. The new technology helps avoid these extra procedures.

"Wireless power solves both challenges," said Ada Poon, an assistant professor of electrical engineering at Stanford.

Last year, Poon had demonstrated a device that can propel itself through the blood stream.

The latest device works by "combination inductive and radiative transmission of power". A transmitter outside the body sends radio waves to a coil inside the device that in turn produces enough current to power the tiny device.

The frequency of radio waves and the size of coil required to generate enough power is inversely related, meaning that if higher frequencies are used then smaller coils can generate the required power.

However, there is a catch. Existing mathematical models say that higher frequency radio waves can be absorbed by the body. And according to these models, lower frequency/bigger coil can be used to power the device, something that would defeat Poon's idea of making tiny devices.

Poon and colleagues overcame these problems by proving that the mathematical models were wrong. Tissues can dissipate electric current but radio waves can travel in another way - alternating magnetic and electrical field and this can penetrate deep into the body.

 "In fact, to achieve greater power efficiency, it is actually advantageous that human tissue is a very poor electrical conductor. If it were a good conductor, it would absorb energy, create heating and prevent sufficient power from reaching the implant," said Sanghoek Kim, doctoral candidate in Poon's lab and first author of the study.

The researchers found that the maximum power transfer through human tissue occurs at about 1.7 billion cycles per second, according to a press release by Stanford University.

Researchers were also able to solve the heating and orientation problem by designing the antennae in such a way that the device can generate power regardless of the orientation of the antennae.

The research paper is published in the journal Applied Physics Letters.