Plugging gadgets into a socket in the wall, or loading them with batteries - or maybe even unfurling a solar panel - is how most of us think of getting electricity. But what about plugging them into your body?
It may sound far fetched, but under the shadow of the Alps, Dr Serge Cosnier and his team at the Joseph Fourier University of Grenoble have built a device to do just that. Their gadget, called a biofuel cell, uses glucose and oxygen at concentrations found in the body to generate electricity.
They are the first group in the world to demonstrate their device working while implanted in a living animal. If all goes to plan, within a decade or two, biofuel cells may be used to power a range of medical implants, from sensors and drug delivery devices to entire artificial organs. All you'll need to do to power them up is eat a candy bar, or drink a coke.
Biofuel cells could kick-start a revolution in artificial organs and prosthetics that would transform tens of thousands of lives every year.
A new range of artificial, electrically-powered organs are now under development, including hearts, kidneys, and bladder sphincter, and work has begun on fully-functioning artificial limbs such as hands, fingers, and even eyes. But they all have one Achilles heel: they need electricity to run.
Batteries are good enough for implants that don't need much power, but they run out fast, and when it comes to implants, that is more than just an inconvenience, it is a fundamental limitation.
Even devices that do not use much power, such as pacemakers, have a fixed lifespan because they rely on batteries.
They usually need their power packs replaced 5 years after implantation. One study in the US found that one in five 70 year-olds implanted with a pacemaker, survived for another 20 years - meaning this group needed around 3 additional operations after the initial implant, just to replace the battery.
Each operation is accompanied by the risk of the complications of surgery, not something anybody should have to face if it is avoidable.
Other devices such as artificial kidneys, limbs or eyes, would have such high energy demands that users would have to change their power source every few weeks to keep them working. It is simply impractical to use batteries in these devices.
That is where biofuel cells come in. Dr Cosnier and his team are one of a growing number of researchers around the world developing the technology in an attempt to side-step this inherent limitation.Bodily fluids
At heart, biofuel cells are incredibly simple. They are made of two special electrodes - one is endowed with the ability to remove electrons from glucose, the other with the ability to donate electrons to molecules of oxygen and hydrogen, producing water.
Pop these electrodes into a solution containing glucose and oxygen, and one will start to rip electrons off the glucose and the other will start dumping electrons onto oxygen. Connect the electrodes to a circuit and they produce a net flow of electrons from one electrode to the other via the circuit - resulting in an electrical current.
Glucose and oxygen are both freely available in the human body, so hypothetically, a biofuel cell could keep working indefinitely. "A battery consumes the energy stored in it, and when it's finished, it's finished. A biofuel cell in theory can work without limits because it consumes substances that come from physiological fluids, and are constantly being replenished," said Dr Cosnier.
The idea of powering fuel cells using glucose and oxygen found in physiological fluids was first suggested in the 1970s, but fell by the wayside because the amount of energy early prototypes produced was too little to be of practical use.
However, in the 2002, advances in biotechnology spurred Itamar Willner, a researcher at the Hebrew University in Jerusalem, to dust down the idea and give it a fresh look.
In a paper published in the prestigious journal Science, he speculated that thanks to advances in biotechnology, the day would come when devices such as artificial limbs and organs would soon be powered by biofuel cells that create electricity from bodily fluids.
"Since then biofuel cells have received a huge amount of attention," said Dr Eileen Yu, a researcher at Newcastle University, who is part of UK-wide multi-university project to develop biofuel cells.Nano technology
The key to the recent breakthroughs has been our understanding of rather special biological molecules called enzymes. Enzymes are naturally occurring molecules that speed up chemical reactions. Researchers studying bio fuel cells have discovered that one particular enzyme, called glucose oxidase, is extremely good at removing electrons from glucose. "It is very efficient at generating electrons," said Prof Willner.
Spurred by new developments in enzyme manipulation, and the growth in availability of carbon nanotubes - which are highly efficient electrical conductors - many groups around the world have developed bio fuel cells capable of producing electricity.
Dr Cosnier and his team decided to take things one step further. "In the last 10 years there has been an exponential increase in research, and some important breakthroughs in enzyme research," he said.
He decided it was time to make the first attempt to take the cumulative knowledge of the last decade of research and engineer it into a device the size of a grain of rice that could generate electricity while implanted inside a rat.
In 2010, they tested their fuel cell in a rat for 40 days and reported that it worked flawlessly, producing a steady electrical current throughout, with no noticeable side effects on the rat's behaviour or physiology.
Their system is surprisingly straightforward. The electrodes are made by compressing a paste of carbon nanotubes mixed with glucose oxidase for one electrode, and glucose and polyphenol oxidase for the other.
The electrodes have a platinum wire inserted in them to carry the current to the circuit. Then the electrodes are wrapped in a special material that prevents any nanotubes or enzymes from escaping into the body.
Finally, the whole package is wrapped in a mesh that protects the electrodes from the body's immune system, while still allowing the free flow of glucose and oxygen to the electrodes. The whole package is then implanted in the rat.
"It is an important step towards demonstrating the translation of basic research into a practical device," said Willner. "It shows the feasibility of making an implantable package."
Implantation in a rat was a good proof of concept, said Dr Cosnier, but it had drawbacks. "Rats are so small that the production of energy is insufficient to power a conventional device."
Next he plans to scale up his fuel cell and implant it in a cow. "There is more space, so a larger fuel cell can be implanted, meaning a greater current will be generated."
Dr Cosnier hopes it will be enough to power a transmitter that will be able to beam out of the cow information about the device and control sensors inside the animal.More power
There is still a long way to go. Prof Willner explains that, while the enzyme glucose oxidase has performed optimally, the efficiency of the electron-donating enzymes could still be dramatically improved. He is optimistic that breakthroughs will be made.
"Based on the current rate of progress, I am confident we will see exciting developments in the next decade," said Prof Willner.
Dr Cosnier agrees that there is a lot of room for improvement. "Today we can generate enough power to supply an artificial urinary sphincter, or pacemaker. We are already working on a system that can produce 50 times that amount of power, then we will have enough to supply much more demanding devices," he said.
Implants aren't the only place you may find bio fuel cells in the future. The electronics giant Sony recently announced that it had created a biofuel cell fuelled with glucose and water that was capable of powering an MP3 player. "In 10 years time you may see bio fuel cells in laptops and mobile phones," said Prof Willner.
Dr Cosnier points out that bio fuel cells would be especially useful in places where there is no electricity supply to recharge your batteries. "If you were in a country without electricity, and needed to re-charge a bio fuel cell, all you would have to do is add sugar and water."