Scientists continue to unveil impressive innovations at the American Chemical Society's annual conference, currently being held in San Francisco. The latest is a removable tattoo that doubles as a miniature battery -- turning human sweat into storable electricity.

The device is meant to be worn during a trip to the gym. It can monitor a person's progress during exercise routines while simultaneously powering a small electronic device, like an iPod.

The mini tattoo tracks athletic performance by measuring levels of lactate in sweat secreted by the exerciser.

"Lactate is a very important indicator of how you are doing during exercise," explained researcher Wenzhao Jia, in an ACS press release. Jia is one of the engineers from UC San Diego who helped developed the biobattery.

Lactate is a byproduct of a process called glycolysis; the body instigates glycolysis to shore up extra energy when it is overexerted. As a general rule, higher levels of lactate are produced and excreted when the body undergoes more intense levels of exercise. Doctors and physical therapists often measure lactate as a way to test physical fitness. A fit person will likely not produce as high levels of lactate as a less-in-shape person performing the same exercise. Physicians sometimes look for abnormally high lactate levels as a way to identify heart or lung disease.

Currently, lactate testing is done via blood samples. But by installing a lactate sensor in a temporary tattoo, researchers found a way to track performance in a much less evasive way. They also found a way to produce electricity. As the sensor processes the lactate in the sweat, it strips the lactate of electrons.

Engineers designed the sensor so it could pass the stripped electrons from an anode to a cathode, just like a battery.

UC nanoengineering professor Dr. Joseph Wang said the device is "the first example of a biofuel cell that harvests energy from body fluid."

"The current produced is not that high," Jia admitted, "but we are working on enhancing it so that eventually we could power some small electronic devices."

Ultimately engineers could have a device that is powered by -- more or less -- the same thing it measures.

The biobattery presentation by UC San Diego engineers is one of 2,000 projects, studies, products and theories being shared at the ACS conference.