Scientists invent ‘hair-friendly’ solution for measuring brainwaves

By Stephen Beech

A temporary head tattoo printed directly on the scalp has provided a "hair-friendly" solution for measuring brainwaves.

A liquid ink that doctors can print onto a patient’s scalp to measure brain activity has been invented by American scientists.

They say the new technology offers a "promising" alternative to the cumbersome process currently used for monitoring brainwaves and diagnosing neurological conditions.

It also has the potential to enhance non-invasive brain-computer interface applications, according to a study published in the journal Cell Biomaterials.

Co-corresponding author Professor Nanshu Lu said: “Our innovations in sensor design, biocompatible ink, and high-speed printing pave the way for future on-body manufacturing of electronic tattoo sensors, with broad applications both within and beyond clinical settings.”

She explained that electroencephalography (EEG) is an important tool for diagnosing neurological conditions - including seizures, brain tumors, epilepsy, and brain injuries.

During a traditional EEG test, technicians measure the patient’s scalp with rulers and pencils, marking over a dozen spots where they will glue on electrodes, which are connected to a data-collection machine via wires to monitor the patient’s brain activity.

However, the set-up is time-consuming and can be uncomfortable for many patients, who must sit through the EEG test for hours.

= Lu and her team at the University of Texas, Austin, have been pioneering the development of tiny sensors that track bodily signals from the surface of human skin- a technology known as electronic tattoos, or e-tattoos.

Scientists have applied e-tattoos to the chest to measure heart activities, on muscles to measure how fatigued they are, and even under the armpit to measure components of sweat.

Previously, e-tattoos were usually printed on a thin layer of adhesive material before being transferred onto the skin.

But that method was only effective on hairless areas.

Lu said: “Designing materials that are compatible with hairy skin has been a persistent challenge in e-tattoo technology."

To overcome the issue, the Texas team designed a type of liquid ink made of conductive polymers.

The ink can flow through the hair to reach the scalp, and once dried, it works as a thin-film sensor, picking up brain activity through the scalp.

Using a computer algorithm, Lu says researchers can design the spots for EEG electrodes on the patient’s scalp.

Then, they use a digitally controlled inkjet printer to spray a thin layer of the e-tattoo ink onto the spots.

Lu says the process is quick, requires no contact, and causes no discomfort in patients.

The team printed e-tattoo electrodes onto the scalps of five participants with short hair.

They also attached conventional EEG electrodes next to the e-tattoos.

The researchers found that the e-tattoos performed comparably well at detecting brainwaves with minimal noise.

After six hours, the gel on the conventional electrodes started to dry out.

More than a third of the electrodes failed to pick up any signal, although most of the remaining electrodes had reduced contact with the skin, resulting in less accurate signal detection.

However, the e-tattoo electrodes showed stable connectivity for at least 24 hours.

The research team also tweaked the ink’s formula and printed e-tattoo lines that run down to the base of the head from the electrodes to replace the wires used in a standard EEG test.

Co-corresponding author Professor Ximin He, of the University of California, Los Angeles, said: “This tweak allowed the printed wires to conduct signals without picking up new signals along the way."

The researchers then attached much shorter physical wires between the tattoos to a small device that collects brainwave data.

They plan in future to embed wireless data transmitters in the e-tattoos to achieve a fully wireless EEG process.

Co-corresponding author Professor José Millán, also of the University of Texas, Austin, said: “Our study can potentially revolutionize the way non-invasive brain-computer interface devices are designed."

He explained that brain-computer interface devices work by recording brain activities associated with a function, such as speech or movement, and use them to control an external device without having to move a muscle.

Such devices currently often involve a large headset that is cumbersome to use.

But Millán says e-tattoos have the potential to replace the external device and print the electronics directly onto a patient’s head, making brain-computer interface technology more accessible.