On the tattoo, an ATiny85 microcontroller stores and receives data from temperature sensors via electroconductive paint. There’s also room on there for an ambient light sensor that illuminates LEDs when it’s dark. And unlike a fitness tracker, you can mount it on places other than your wrist. The tech tattoos could be used to monitor a child’s vitals after a surgery, or provide a less public (or more, if you choose) display of fitness tracking.