A wireless medical implant that could replace pacemakers with a gadget small enough to fit on the head of a pin has been shown off.

A team of engineers at Stanford has demonstrated the feasibility of a super-small, implantable cardiac device that gets its power not from batteries, but from radio waves transmitted from outside the body.

The breakthrough could lead to a plethora of new medical sensors and even 'smart pill' that can be swallowed without having to include batteries.

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

It is hoped they could revolutionise medicine by allowing devices such as pacemakers to be implanted without the need for large battery packs which need surgery to replace them.

Instead, wearers would wear wireless battery packs which transmit power to the implant.

"Wireless power solves both challenges," said Ada Poon, a professor of electrical engineering at Stanford, who led the research.

Last year, Poon made headlines when she demonstrated a wirelessly powered, self-propelled device capable of swimming through the bloodstream.

The findings were published in the journal Applied Physics Letters.

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.

In their paper, the 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 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.

'For implantable medical devices, therefore, the goal is a high-frequency transmitter and a small receiver, but there is one big hurdle,' said Kim.

Existing mathematical models have held that high frequency radio waves do not penetrate far enough into human tissue, necessitating the use of low-frequency transmitters and large antennas too large to be practical for implantable devices.

However, this turned out to be untrue.

'In fact, to achieve greater power efficiency, it is actually advantageous that human tissue is a very poor electrical conductor,' said Kim.

'If it were a good conductor, it would absorb energy, create heating and prevent sufficient power from reaching the implant.'

According to their revised models, the researchers found that the maximum power transfer through human tissue occurs at about 1.7 billion cycles per second.

'In this high-frequency range, we can increase power transfer by about ten times over earlier devices.'

The discovery meant that the team could shrink the receive antenna by a factor of ten 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.