Tatoo Battery
This battery uses lactate, a chemical in sweat, to produce electricity.
Another use could be to monitor lactate levels in professional athletes during their training programs. This process requires today to collect blood from athletes different times during the exercise, but researches have built a flexible sensor with an enzyme that is able to strip electrons from lactate and generate a weak electrical current. The sensor is imprinted onto a tattoo paper.
As yet, the amount of energy produced is to weak to power other devices, but researchers envision to power some day watches or other small electronic devices.
Original sources:
http://news.sciencemag.org/chemistry/2014/08/tattoo-battery-may-one-day-power-your-smart-phone
