Using cheap materials that anyone can order by mail, scientists built several color-changing electronic displays on sheets of ordinary paper, using thermochromic ink, found in mood rings and disposable thermometers (when its temperature changes, so does its color).

By coating one side of the paper with ink, and affixing metal heating elements to the other, a Harvard scientist built a makeshift monitor. These simple devices could provide the readout for equally inexpensive tests that can tell if someone has an infection, or if water is safe to drink.