Electroencephalography (EEG) is a critical diagnostic tool for various neurological conditions, such as seizures, epilepsy, brain tumors, and injuries. Traditionally, the EEG procedure involves technicians marking over a dozen spots on the patient’s scalp using rulers and pencils, where they then glue electrodes to monitor brain activity. These electrodes are connected to a data collection machine by long wires. This method is not only time-consuming but also cumbersome, often causing discomfort for patients who must remain still for hours. Now, scientists have developed a groundbreaking liquid ink that can be directly printed onto the scalp, enabling doctors to measure brain activity without the need for traditional electrode setups. This new technology, described in the journal Cell Biomaterials, offers a promising alternative to current methods for monitoring brain activity and could also significantly improve brain-computer interface applications.

A team of scientists from the University of Texas (Austin, TX, USA) has been advancing a technology called electronic tattoos (e-tattoos), which are small sensors designed to track bodily signals from the skin’s surface. E-tattoos have previously been applied to measure heart activity on the chest, muscle fatigue, and even components of sweat from under the armpit. Historically, e-tattoos were printed on adhesive layers that could be transferred onto hairless areas of the body. However, a major challenge in e-tattoo technology has been designing materials that work effectively on areas with hair, such as the scalp. To overcome this, the team developed a liquid ink made from conductive polymers, which can flow through hair and form a thin-film sensor that detects brain activity once it dries.

The researchers used a computer algorithm to design the placement of EEG electrode spots on the scalp. Then, using a digitally controlled inkjet printer, they applied a thin layer of the e-tattoo ink to these spots. The process is fast, non-contact, and painless for the patient. In their experiment, the team printed e-tattoo electrodes onto the scalps of five participants with short hair, alongside conventional EEG electrodes for comparison. They found that the e-tattoos performed nearly as well as the traditional electrodes in detecting brainwaves, with minimal noise interference. After six hours, the gel in the conventional electrodes began to dry, and over a third of these electrodes failed to detect any signals. In contrast, the e-tattoo electrodes maintained stable connectivity for up to 24 hours.

Further modifications to the ink’s formula allowed the researchers to print conductive lines from the electrodes to the base of the head, replacing the wires used in standard EEG setups. This adjustment allowed the printed lines to transmit signals without picking up interference from surrounding areas. The team then connected shorter wires between the e-tattoos and a device that collected the brainwave data. Going forward, the researchers plan to incorporate wireless data transmitters directly into the e-tattoos, eliminating the need for any external wiring. This advancement could transform non-invasive brain-computer interface devices, making them more efficient and accessible. These devices currently use large, cumbersome headsets to capture brain activity for functions like controlling external devices with thoughts. By replacing external hardware with printed electronics on the scalp, e-tattoos could make brain-computer interfaces far more practical and accessible for patients.

“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.

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