MEMORY MADE EVEN MORE INDELIBLE WITH E-INK TATTOO
Researchers at the University of Texas at Austin and the University of California, Los Angeles have developed temporary conductive tattoos to measure brain activity.
Written by Kayt Sukel
The new approach is quicker and more convenient than a traditional EEG. Photo: University of Texas at Austin
THE FIELDS OF MEDICINE AND NEUROSCIENCE have benefitted from the discovery and use of electroencephalography (EEG), a non-invasive technology that can measure brain activity. Researchers use the tool to better understand the timing and dynamics of cognitive activity. Physicians rely on EEG to diagnose epilepsy, sleep disorders, and tumors. Yet, the standard set-up for EEG relies on an electrode cap and sticky, conductive gel to pick up brain waves. Moreover, despite decades of improvements, EEG systems do not work well on people with thick, curly hair. But e-tattoos, or thin wearable sensors that can be worn temporarily, may provide a fix to this conundrum.
Nanshu Lu, professor at the University of Texas’s Cockrell School of Engineering’s Department of Aerospace Engineering and Engineering Mechanics has spent her career designing novel e-tattoos for a variety of applications. She and her colleagues wanted to find a way to design a conductive interface that would work on a hairy scalp.
“We know that gel-based electrodes provide the lowest contact impedance and most reliable interfacing with the scalp,” she said. “So, we thought for a long time about how to ultimately apply an e-tattoo on hairy skin. We eventually decided to try 3D printing.”
The result is an ink-based temporary tattoo applied to the scalp, which creates sensors and connectors that can be linked via cables to an EEG recorder. While the use of 3D printing in tissue engineering is not new, the development of ink-based electrodes required an entirely new approach. Lu said they faced many obstacles, from the make-up of the conductive ink to the design of the 3D printer.
“We invented our own inks with a materials science professor at UCLA, Ximin He,” she said. “We developed an organic conductor-based ink, which is very fluid in its liquid phase for application. We used two completely different types of ink, one for electrodes, which must maintain very low contact impedance, and then one for the interconnect, which needs high connectivity so it can transmit the EEG signals picked up by the electrodes.”
The base for the inks is a conductive polymer material—poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) or PEDOT:PSS—which is used for bio-interfacing. But the ink used for printing electrodes has a high saline concentration to reduce contact impedance and enhance signal capture. The interconnect ink, on the other hand, also has dimethyl sulfide (DMSO) to enhance connectivity. Coming up with the right recipe required some tinkering, Lu said.
“This is just the beginning of this kind of printed technology. We are improving the ink to make it more abrasion- and friction-resistant, and even, in the future, shower compatible. The future goal is fully ambulatory EEG.”
—Nanshu Lu, professor, Department of Aerospace Engineering and Engineering Mechanics, Cockrell School of Engineering, University of Texas at Austin
The ink must be biocompatible, liquid, and capable of solidifying on the skin’s surface, she continued. The PEDOT:PSS ink is aqueous when dissolved in water, so when the water evaporates, the ink self-deposits into a thin, black conductive film that doesn’t require heat or ultraviolet light, Lu said.
Once the water evaporates and the ink is cured, it’s also highly breathable—and stretchable just like the outer layer of skin. The electrodes also demonstrate contact impedance, comparable to conventional gel electrodes, without needing to bruise or braise the skin.
After developing these inks, the team had to contend with finding an effective way to apply them to the scalp. Today’s 3D printers are very powerful, Lu added. They were able to leverage modern components to create a system with the power to quickly propel ink through the hair and place the electrodes in the appropriate places on the scalp—done after the individual’s head shape has been mapped digitally.
“We used jetting since it is very fast, has a wide tolerance of viscosity, and can have a stand-off distance of 5 to 10 millimeters, which gives us room for the hairs,” she said. “Because of the high-speed jetting process, the liquid ink goes through the hairs and then go directly to the skin, by the roots of the hairs.”
Currently, the open loop process to map the head and then apply the e-tattoos takes more than one hour and only works on “buzz cuts.” But Lu said the team plans to refine the application system, with the addition of 3D cameras to track head movement, to close the loop to allow adaptive smart printing and bring application time down to 30 minutes.
As the researchers improve the system, Lu hopes to create an e-tattoo EEG that can be used to monitor conditions over time—and even to help power brain-machine interfaces.
“This is just the beginning of this kind of printed technology,” she said. “We are pursuing adaptive printing to save time and to work for longer and thicker hair in the future. We are improving the ink to make it more abrasion- and friction-resistant, and even, in the future, shower compatible. The future goal is fully ambulatory EEG.”
Kayt Sukel is a technology writer and author in Houston.
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