A tiny implant the size of a grain of sand has been created that can connect computers to the human body without the need for wires or batteries, opening up a host of futuristic possibilities.

The devices, dubbed "neural dust", could be used to continually monitor organs like the heart in real time and, if they can be made even smaller, implanted into the brain to control robotic devices like prosthetic arms or legs.

It is believed they could help treat conditions like epilepsy by stimulating nerves and muscles, help people with incontinence control their bladder and even suppress appetite. They could also potentially either be used to prompt the immune system into action or reduce inflammation.

One of the inventors, Professor Michel Maharbiz, of University of California, Berkeley, said: “I think the long-term prospects for neural dust are not only within nerves and the brain, but much broader.

“Having access to in-body telemetry has never been possible because there has been no way to put something super-tiny super-deep [in the body].

“But now I can take a speck of nothing and park it next to a nerve or organ, your [gastro-intestinal] tract or a muscle, and read out the data.”

Ultrasound vibrations, which can penetrate almost every part of the body, are used to power the sensors, which are about a millimetre across.

They contain a special crystal that converts ultrasound into electricity to power a tiny transistor.

If there is a voltage spike in a nerve or muscle fibre this alters the vibration of the crystal, changing the way the sound echoes back to an ultrasound receiver.

So far, experiments have been carried out on muscles and the peripheral nervous system of rats, but the researchers believe the dust could also work in the central nervous system and brain to control prosthetics.

This can already be done using brain implants, but these require wires that go through a hole in the skull, potentially allowing in infection or movement of the sensor within the brain. They must also be replaced after about one to two years.

The researchers are currently building neural dust that could last in the body for more than 10 years. And because they are wireless there is no need for holes to remain in the skull.

Professor Jose Carmena, a neuroscientist at Berkeley, said: “The technology is not really there yet to get to the 50-micron target size which we would need for the brain and central nervous system.

“Once it’s clinically proven, however, neural dust will just replace wire electrodes. This time, once you close up the brain, you're done.”

However he added: “The beauty is that now, the sensors are small enough to have a good application in the peripheral nervous system, for bladder control or appetite suppression, for example.”