Every year, more than 3000 children in the United States visit the emergency room after swallowing button batteries, which power anything from toys to calculators. Once inside the throat, the flat, round power sources release caustic chemicals that can severely burn the esophagus, causing internal bleeding and even death. Now, scientists have found a simple solution: an armor that renders these batteries harmless inside the gut. When a battery is ingested, it reacts with intestinal fluids to generate an electrical current and release hydroxide, a caustic ion that can severely damage gut tissue within just a few hours. So bioengineers took advantage of the fact that batteries experience a gentle pressure inside their housing, but not so much inside the digestive tract. They coated button batteries with a quantum tunneling composite, a rubberlike material made of silicone embedded with metal particles. Normally, the metal particles are too far apart to conduct electricity. But when the particles are compressed within a few nanometers of one another, the electrons burrow through the silicone through a process called quantum tunneling, turning the composite from an insulator into a conductor. The researchers calibrated the coating such that an armored button battery would function normally inside devices, but lose its conductivity inside the digestive tract. To test their design, the scientists placed a conventional button battery and a coated one inside a living pig’s esophagus. In vitro monitoring revealed that the conventional battery immediately began releasing hydroxide, as shown in the video, whereas the coated battery remained intact, leaving no trace of damage. Because the coating is waterproof, it could also be used to make weather-resistant batteries suited for outdoor use, the researchers report online today in the Proceedings of the National Academy of Sciences.