Now, wireless heart implant powered by radio waves

The implanted device is contained in a cube just eight-tenths of a millimeter in radius and could fit on the head of pin.

The Stanford researchers demonstrated wireless power transfer to a millimeter-sized device implanted five centimeters inside the chest on the surface of the heart, a depth once thought out of reach for wireless power transmission.

The study was led by Ada Poon, professor of electrical engineering at Stanford.

The engineers say the research is a major step toward a day when all implants are driven wirelessly.

Beyond the heart, they believe such devices might include swallowable endoscopes -- so-called "pillcams" that travel the digestive tract -- permanent pacemakers and precision brain stimulators; virtually any medical applications where device size and power matter.

Implantable medical devices in the human body have revolutionised medicine, but these devices are not without engineering challenges.

First off, they require power, which means batteries, and batteries are bulky. In a device like a pacemaker, the battery alone accounts for as much as half the volume of the device it drives. Second, batteries have finite lives. New surgery is needed when they wane.

"Wireless power solves both challenges," said Poon.

Her device works by a combination inductive and radiative transmission of power. Both are types of electromagnetic transfer in which a transmitter sends radio waves to a coil of wire inside the body. The radio waves produce an electrical current in the coil sufficient to operate a small device.

There is an indirect relationship between the frequency of the transmitted radio waves and the size of the receiver antenna.

"For implantable medical devices, therefore, the goal is a high-frequency transmitter and a small receiver, but there is one big hurdle," said Sanghoek Kim, doctoral candidate in Poon's lab.

The findings were published in the journal Applied Physics Letters.