In the study, patients underwent a procedure called transcranial direct current stimulation (tDCS) which uses electrodes placed on the head to stimulate targeted brain regions.

It is hardly the first time this method has been tested in a scientific study.

In fact, scores of studies have looked at making personal tDCS headsets for people to have at home, giving themselves occasional 'zaps' to boost their brain function.

But its use for coma patients is a new frontier.

In the study, a team led by neurologist Aurore Thibaut placed the headsets over the patients' prefrontal cortex, the area which controls consciousness.

Within two hours of receiving one 'zap', two of the patients showed signs of responsiveness by moving parts of their bodies.

Wanting to test it further, the team then started a second trial with 16 new brain damaged patients, who had not been able to communicate for at least three months.

They stimulated half of the patients' brains for 20 minutes a day, five days in a row. The other half were given a 'dummy' treatment - stimulation at such a low frequency that it had no effect.

By the end of the week, two patients were able to answer questions by moving parts of their bodies. Another nine showed signs of awareness, while the group without brain stimulation saw no changes.

Dr Thibaut told the New Scientist that they are pushing ahead with tests - but cautiously, given the lack of research on the long-term repercussions.

'You can find similar devices online, but we don't know the long-term effects yet,' she says. 

'We need to see what happens when we use it for perhaps five hours a day, or what happens if we apply it daily for three months. We need to be really careful.' 

It is not yet known whether the electrical currents, when applied to the scalp, reach further than the parts of the brain being targeted.

If the currents do reach further they may alter brain functions that did not need to be changed.

'We don't know how the stimulation of one brain region affects the surrounding, unstimulated regions,' Roy Hamilton, MD, MS, an assistant professor of Neurology and director of the Laboratory for Cognition and Neural Stimulation at University of Pennsylvania told Daily Mail Online last year.