It’s an unusual aesthetic, but it works: Spray-on tattoos that dot the head can collect brain activity without the consternation caused by typical tests. The electronic tattoos, described December 2 in Cell Biomaterials, go on with a microjet printer, last for hours and wipe off with soapy water.

Electroencephalography, or EEG, is a common medical test that can help diagnose epilepsy, brain injuries and sleep disorders. The test relies on electrodes glued to the scalp, where they pick up signals generated within the brain. But pitfalls abound: Hair can stymie electrode contact, wet gels can dry out and wires and caps can form a Medusa-esque snarl, making it hard to stay comfortable.

Electrodes that are sprayed directly onto the scalp may get around a lot of these EEG problems, interdisciplinary engineer Nanshu Lu of the University of Texas at Austin and colleagues report. The newly described e-tattoos go on wet. The liquid, full of polymers that can carry electrical signals, oozes around hair to settle directly on the skin of the scalp before drying, much like writing with a pen. A printer sprays the liquid onto precise spots on a person’s head.

The team even printed “wires” — lines made of a different sort of ink that carried signals down volunteers’ heads to the top of their necks, where they were then hooked into standard wires.

Once dry, the spray-on tattoos can pick up the brain’s electrical activity about as well as traditional electrodes, the researchers report. What’s more, the e-tattoos last longer. In tests, most of the standard electrodes’ data had worsened after six hours, but the printed electrodes were still chugging.

Common EEG electrodes don’t always work well for people with thick or curly hair (SN: 3/11/20). The new system should be tested on people with all sorts of hair types, the researchers write.

Similar temporary e-tattoos could be used on other body parts, including the heart, bones and muscles. The new system also raises the possibility of seamless connections between bodies and computers.