Although not approved by FDA for direct application to the skin, henna is commonly used for temporary tattoos. Allergic reactions from use of pure henna on the skin have rarely been reported.

Right now, such tattoos don’t exist, but the key technology is being worked on in labs around the world, including my lab at ...

temporary "e-tattoos."… Brain activity is measured with electroencephalograms (EEGs), common tests used to help diagnose conditions such as epilepsy, sleep disorders, and brain injuries.