Transparent brain implant reads deep neural activity

24 Jan 2024

UC San Diego combines microelectrodes with two-photon microscopy to study neurons from implant on brain's surface.

A project at the University of California, San Diego has developed a transparent neural implant intended to sit on the surface of the brain and record neural activity deeper inside.

Published in Nature Nanotechnology, the findings are a step towards minimally invasive brain-computer interfaces (BCIs) that provide high-resolution data about neural activity.

The science of brain implants and the combination of electrical and optical processes employed to record the activity of functioning neurons or stimulate them into action has been a hot research topic for some time, with the biggest challenge being reaching deep areas of the brain.

One logical approach has been to use transparency to simplify the problems, as in the transparent implant developed by the University of California, Riverside in 2016 which allowed light from waveguides and optical fibers to pass with less hinderence into the brain.

Recent studies using optically transparent microelectrodes have shown that electrophysical recordings from the brain surface can be achieved alongside simultaneous optical imaging or stimulation of the neural activity, but the challenge has been to scale down the electrode dimensions and increase their density to capture high-resolution data, according to the UC San Diego team.

The project's solution is a thin, transparent, flexible polymer strip that conforms to the brain's surface. The strip is embedded with a high-density array of circular graphene electrodes each measuring 20 micrometers in diameter, with platinum nanoparticles incorporated to overcome the quantum capacitance limit of graphene and allow the microelectrode diameter to be scaled down.

When placed on the surface of the brain, the implant records electrical signals from neurons in the outer layers. At the same time two-photon microscopy can be carried out by shining laser light through the implant, to image calcium spikes from neurons located as deep as 250 microns below the surface.

Optical imaging and electrical recording integrated seamlessly

In trials on mice this approach allowed researchers to capture high-resolution information about electrical activity and calcium activity in neurons at the same time, and see a correlation between surface electrical signals and calcium spikes in deeper layers. This correlation let researchers use the electrical signals as training data for neural networks, which could then predict the corresponding calcium activity at various depths for both individual neurons and large populations.

An ability to predict calcium activity from electrical signals could help overcome some of practical limitations involved in imaging that activity microscopically, which usually involves physically restraining the subject's head.

"Seamless integration of recording electrical signals and optical imaging of the neural activity at the same time is only possible with this technology," said UC San Diego's Duygu Kuzum. "Being able to conduct both experiments at the same time gives us more relevant data because we can see how the imaging experiments are time-coupled to the electrical recordings."

With a view to future clinical translation the project will next focus on testing the technology in different animal models, but the same approach could also help to advance fundamental neuroscience research or study how vascular activity is coupled to electrical activity in the brain.

"This technology can be used for so many different fundamental neuroscience investigations, and we are eager to do our part to accelerate progress in better understanding the human brain," said Duygu Kuzum.