http://www.sciencemag.org/news/2018/05/bacteria-pill-may-one-day-track-your-body-s-chemistry

In the latest twist on an edible sensor that could one day monitor disease, scientists have created a pill-size device that can detect bleeding deep inside a pig’s digestive tract—and relay that information via a wireless signal to a cellphone. If researchers can modify the sensor to pick up other chemicals—and shrink the pill—they could one day create a multipurpose readout of gut health.

To make their sensor, engineers and biologists turned to a bacterium commonly sold as a probiotic in Europe. They genetically engineered it to detect the blood chemical heme by injecting several genes: one that triggers in the presence of heme, and another that makes the cell glow when triggered—enough to light up a detector and produce a wireless signal.