Today, implanted medical devices continuously monitor the body to give doctors and researchers more information than ever about the state of a patient's disease and the effects of certain treatments. Engineers have also been making devices that a patient can swallow, which lets doctors and patients avoid risky surgical and other invasive procedures when able. Ideally, these devices would continuously monitor conditions inside the body or release drugs, and, once they've done their job, they would pass through the body without harming it. While some devices are on the market today, one of the biggest technological impediments to these devices is finding a non-toxic power source. The best solution might be to create a battery that uses the unique chemical environment inside the body to function and to construct the battery using metals that the body already needs, according to a review published today in Trends in Biotechnology by Christopher Bettinger, a researcher from Carnegie Mellon University.