Implantable medical devices are becoming more common everyday. The problem is that no matter how sophisticated the devices are, most still depend on batteries for power. One solution to this is for the power source to remain outside the body and to beam the power to the device. However, that has its own difficulties because wireless power can’t penetrate very far through human tissue ... until now.

Ada Poon, an assistant professor of electrical engineering at Stanford, and doctoral candidates Sanghoek Kim and John Ho have demonstrated that it’s possible to construct a super-small implantable cardiac device the size of a 1.6 millimeter-wide cube. The device uses gigahertz-frequency radio waves that can power extremely small devices five centimeters (1.96 in) inside the chest on the surface of the heart – a depth once thought impossible.

Existing models indicated that radio waves don’t penetrate very far into human tissues without low frequencies and large antennae. Poon demonstrated that high frequency waves penetrate deeper than expected when she recently demonstrated a wirelessly-powered device capable of swimming in the bloodstream.

Poon’s current device is powered by a combination of inductive and radiative transmission. There’s an indirect relationship between frequency and size of the antenna needed to receive a signal or, in this case, power. The longer or shorter the transmitted frequency, the longer or shorter the antenna. One example of this is AC power wires. The wires snaking from tower to tower act as long antennae that transmit a low frequency radio signal. A small metal object like a pen knife is too small to pick up that signal, but an old-fashioned steel fishing pole might and has on occasion resulted in fishermen getting a nasty shock.

The same principle works in reverse. A shorter wavelength means smaller power receiving coils and that means smaller devices. Poon found that a 1.7 gigahertz signal penetrates living tissue much deeper than low frequency ones and allowed a tenfold increase in power transfer while making the antenna ten times smaller. A millimeter coil was able to handle 50 microwatts, which is a whole order of magnitude greater than previous devices.

Other problems that Poon's team had to face were making sure that the device met the health standards set by the Institute of Electrical and Electronics Engineers (IEEE). This dovetailed with the need for a receiving antenna design that did not have to be pointed in a particular direction, so that the device could be implanted on a heart, and a transmitting antenna with precise focus that would power the device without heating neighboring tissue.

Poon foresees broad applications for the technology, including swallowable endoscopes, permanent pacemakers, precision brain stimulators or any other implantable medical device.

The team's findings appear in the journal Applied Physics Letters