A sweat-powered battery could charge your mobile phone or MP3 player - while you work out!

The device, a temporary skin tattoo that produces an electrical current from the lactate in your perspiration, was unveiled earlier this week by a team of scientists from the University of San Diego.

So how does a sweat-powered battery work?

A sensor, no larger than a stamp, is embedded into a temporary tattoo sheet, before being applied to your skin. The sensor contains an enzyme that strips out electrons from a substance called lactate, a chemical contained in sweat. This generates a weak electrical current, enough to charge a small battery.

The amount of energy a person generates varies according to their fitness levels. The fitter the person the less energy they will produce, since it takes their body longer to produce lactate. Un-fit people who tire quickly produce more lactate, and therefore provide more energy.

The team didn't actually set out to build a bio-battery. Originally, the device was designed to simply monitor lactate levels – something that previously required a blood sample. Analysis of lactate levels can be used to evaluate how hard an athlete’s body is working during training, or as an early indicator of heart or lung disease. The team at the University of San Diego then took the sensor a step further by adding an enzyme to generate a weak electrical current.

Although this is the first time sweat has been used to generate electricity, it is not the first attempt to harness the human body as an energy source. In 2011 Dr Serge Cosnier built a device called a biofuel cell which uses glucose and oxygen found in blood to generate electricity. The biofuel battery could be used to power a range of medical implants, including a series of complex artificial organs that are currently in development.

So as non-renewable energy sources decrease, the answer to a future energy shortage may be very simple… it could be you!

Watch Biofuels to discover another alternative method of producing energy.