A new kind of ink, made from living, genetically programmed cells, has been developed by researchers at the Massachusetts Institute of Technology. The cells were engineered to respond to various stimuli by lighting up. They can be mixed with a slurry of hydrogel and nutrients and then printed layer by layer, to form three-dimensional, interactive structures and devices. The team has tested the concept by printing a living tattoo, a thin, transparent patch of these cells. The "device" could be programmed to detect environmental pollutants, report pH, measure temperature and other factors but be used as a truly wearable technology.