Engineering boffins in the States have announced details of a new method of wireless power transmission which can reach inside a human body to power tiny implanted devices, so removing the need for repeated surgery in order to change batteries - or movie-style options such as the chest-socket "arc reactor" or hand-carried car battery as favoured by Tony Stark in Iron Man.

According to Professor Ada Poon of Stanford uni's engineering faculty, wireless power hasn't so far been a practical option for use inside the body due to a range of different issues. One of these is that low-frequency radio trasmissions which can penetrate human tissue easily require a prohibitively large receiving antenna.

Poon and her colleagues overcame this using high frequency electrical fields, which - it had been thought - couldn't penetrate the not very conductive human body.

“In fact, to achieve greater power efficiency, it is actually advantageous that human tissue is a very poor electrical conductor,” explains PhD student Sanghoek Kim, who worked on the kit in Poon's lab. “If it were a good conductor, it would absorb energy, create heating and prevent sufficient power from reaching the implant.”

According to the Stanford announcement of the new gear, the discovery of an optimum electrical frequency of 1.7 Gigahertz meant that power transmission - even to a tiny, sub-millimetre sized device - could be cranked up by a factor of 10 over what had been thought possible:

The discovery meant that the team could shrink the receive antenna by a factor of 10 as well, to a scale that makes wireless implantable devices feasible. At that the optimal frequency, a millimeter-radius coil is capable of harvesting more than 50 microwatts of power, well in excess of the needs of a recently demonstrated eight-microwatt pacemaker.

Other technical hurdles included limiting power leakage so as to get the devices within IEEE standards on how much heat an implant can produce inside the body, and working round reception problems caused when receiving and transmitting antennae altered the direction they were pointed in - even by as little as a few degrees. With the rectenna often needing to be mounted on the surface of a beating heart, it naturally had to be able to move about without suffering power losses.

We learn courtesy of the Stanford communications office:

The team responded by designing an innovative transmit antenna structure that delivers power efficiency regardless of orientation of the two antennas. The new design serves additionally to focus the radio waves precisely at the point inside the body where the device rests on the surface of the heart, increasing the electric field where it is most needed, but canceling it elsewhere. This helps reduce tissue heating to levels well within the IEEE standards.

Poon and her colleagues think the new kit could be big stuff in the medical world, and have applied to patent it with a view to manufacture. There would also, presumably, be other applications outside the medical sphere, though in such cases it's common to use ultrasound rather than electrical waves (this was formerly a secret spook technology, but it has since been independently reinvented in academia).

The Stanford pacemaker research is published in the journal Applied Physics Letters. ®