Flexible Sensor Wraps Around Brain's Wrinkles
A new high-resolution, flexible, sensor could lead to new ways of monitoring the brain.
CREDIT: Travis Ross and Yun Soung Kim, University of Illinois at Urbana-Champaign
A surgically implanted, cellophane-like device that wraps snuggly around the brain's wrinkles and captures high-resolution videos of neuronal activity promises to change the way scientists regard the brain. It already has upended common views about seizures and about sleep.
Unlike conventional brain measurement devices, which require a wire for each measuring point, the new brain sensor combines the signals from thousands of electrodes right at the source and eventually will operate wirelessly. Its researchers say the implant could open the door to years-long studies of the brain and could even lead to brain prosthetics that restore vision.
The 1-centimeter-square device is an electrode array shielded in biocompatible plastic that is thinner than a human hair. The plastic shields the array from fluids as it bends with the brain's contours to densely map surface activity.
The device employs technology similar to the way digital cameras work to produce real-time color videos of brain activity in a screen of 360 channels, each equivalent to one clinical electrode.
The tissue-electronic interface hasn't changed in 40 years. All the devices used in clinical research are passive contacts made of different metals [that] have individual wires. In this case, the electronics are right next to the tissue and they are active, explained researcher Brian Litt at the University of Pennsylvania's Perelman School of Medicine.
You have electrodes sitting on the surface of the brain. And underneath each one is its own circuit, Litt said.
The device contains the same electronics found in cellphones, but it can bend without breaking because of the extreme thinness of its 100-nanometer-sensor array. Just as a piece of timber and piece of paper are the same material, but paper can bend and wood cannot, this silicon can fold to explore rarely observed regions in the brain such as fissures and the space between the two brain hemispheres, said co-researcher Jonathan Viventi at the Polytechnic Institute in New York.
Currently researchers studying electrical activity in the brain have to puncture a hole in the brain's membrane for each electrode. Not only does this damage the brain and cause swelling, it severely limits the number of sensors that can be used. Fifty sensors require fifty wires coming out of the head, Litt said.
Even with direct contact with the brain, the resolution of the setup is not very good, because each electrode is responsible for monitoring about 12 million neurons, Viventi said. In contrast, the flexible device records brain activity with a resolution of one piece of metal per estimated 30,000 neurons.
We're getting closer to a functional scale of where the brain actually operates, where it's processing information, in much smaller groups of neurons, Viventi said.
Tests of the sensors' usage on the brains of cats during a seizure showed the spread of electrical brain signals occur as a spiral wave. The swirl of neuron activity surprised researchers accustomed to the two-dimensional spikes traditionally indicative of a seizure.
Recordings of sleep spindles, a brain activity that occurs in deep sleep, similarly uncovered something new. Videos showed electrical flickers occurring simultaneously in different areas and then disappearing, in contrast to the common knowledge that the signals move across the brain in waves, Viventi said.
The new sensor's helicopter view" of brain activity, as well as its ability to see very-small-scale local activity, will ultimately change our understanding of functional networks, Litt said. Distant-future versions of the implant include mentally controllable prosthetics and vision-restoring stimulations for the blind.
Viventi said he thinks the new sensor could be ready for human testing in about two years.
The research is detailed in this week's issue of the journal Nature Neuroscience.