Biocompatible ink could transform diagnosis of neurological conditions

Detailed in Cell Biomaterials, the technology also has the potential to enhance non-invasive brain-computer interface applications.

“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,” said Nanshu Lu, the paper’s co-corresponding author at the University of Texas at Austin.

Electroencephalography (EEG) is currently used to diagnose neurological conditions, including seizures, brain tumours, epilepsy, and brain injuries.

During an EEG test, technicians measure the patient’s scalp, marking over a dozen spots where they will glue on electrodes that are connected to a data-collection machine via long wires to monitor the patient’s brain activity. This setup is time consuming and cumbersome, and it can be uncomfortable for patients undergoing the EEG test for hours.

Lu and her team have been pioneering the development electronic tattoos (e-tattoos), that track bodily signals from the surface of human skin. Scientists have applied e-tattoos to the chest to measure heart activities, on muscles to measure how fatigued they are, and 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 this was only effective on hairless areas.

“Designing materials that are compatible with hairy skin has been a persistent challenge in e-tattoo technology,” Lu said in a statement. To overcome this, the team designed a type of liquid ink made of conductive polymers. The ink can flow through 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, the 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 on to the spots. The process is quick, requires no contact, and causes no discomfort in patients, the researchers said.

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 team 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. Over a third of these electrodes failed to pick up any signal, although most the remaining electrodes had reduced contact with the skin, resulting in less accurate signal detection. In contrast, the e-tattoo electrodes are said to have showed stable connectivity for at least 24 hours.

Additionally, the researchers adjusted 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.

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

The team then attached much shorter physical wires between the tattoos to a small device that collects brainwave data. The team said that in the future, they plan to embed wireless data transmitters in the e-tattoos to achieve a fully wireless EEG process.

“Our study can potentially revolutionise the way non-invasive brain-computer interface devices are designed,” said co-corresponding author José Millán of the University of Texas at Austin.

Brain-computer interfaces record brain activities associated with a function, such as speech or movement, and use them to control an external device. Currently, these devices often involve a large headset that is cumbersome to use. 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, Millán said.