23rd April 2020
1,000-electrode implant can survive in the brain for six years
Researchers have demonstrated the ability to implant an ultra-thin, flexible neural interface with more than 1,000 electrodes into the brain with a lifetime of at least six years.
Protected from the ravaging environment of internal biological processes by less than a micrometre of material, the achievement is an important step toward high-resolution neural interfaces that could persist inside the human body for an entire lifetime.
"Trying to get these sensors to work in the brain is like tossing your foldable, flexible smartphone in the ocean and expecting it to work for 70 years," said Jonathan Viventi, assistant professor of biomedical engineering at Duke University. "Except we're making devices that are much thinner and much more flexible than the phones currently on the market. That's the challenge."
The human body is an unforgiving place if you're an uninvited guest – especially if you're made of polymers or metal. Besides attacks from the surrounding tissues and immune system, foreign objects must withstand a corrosive, salty environment.
Creating electrical devices that can survive this assault is an even more daunting prospect. Current long-term implantable devices are almost universally hermetically sealed within a laser-welded titanium casing. Think of a pacemaker, for example.
"Building water-tight, bulk enclosures for such types of implants represents one level of engineering challenge," said John Rogers, Professor of Materials Science and Bioengineering at Northwestern University. "We're reporting here the successful development of materials that provide similar levels of isolation, but with thin, flexible membranes that are 100 times thinner than a sheet of paper."
When it comes to the human brain, space and flexibility are of the essence. There is no room for rigid devices with thick walls. These challenges mean that existing neural interfaces can sample only about a hundred sites, which pales in comparison to the tens of billions of neurons that make up the human brain. Any attempt to make these devices larger invariably runs into the hurdle of wiring logistics – because each sensor requires its own wire, size constraints quickly become an issue. Viventi and his colleagues have been working on a different approach.
"You need to move the electronics to the sensors themselves and develop local intelligence that can handle multiple incoming signals," said Viventi. "This is how digital cameras work. You can have tens of millions of pixels without tens of millions of wires, because many pixels share the same data channels."
The researchers had already developed flexible neural devices with 360 electrodes just 25 micrometres thick. But previous attempts to keep them safe from harm inside the body failed, as even the tiniest defect thwarted the entire effort.
"We tried a bunch of strategies before," said Viventi. "Depositing polymers as thin as is required resulted in defects that caused them to fail, and thicker polymers didn't have the flexibility required. But we finally found a strategy that outlasts them all, and have now made it work in the brain."
In their new paper – published by the journal Science Translational Medicine – Viventi and colleagues demonstrate that a thermally grown layer of silicon dioxide less than a micrometre thick can last for extended periods in the hostile environment within the brain, degrading at a rate of only 0.46 nanometres (nm) per day. And because this material is biocompatible, any trace amount that dissolves into the body should not create any problems of its own.
The device's electrodes can detect neural activity through capacitive sensing – the same sort of technology that can detect the movements of a finger on a smartphone's touchscreen. To test their work, they implanted a 64-electrode neural interface into a rat for over a year and a 1,008-electrode neural interface into the motor cortex of a monkey using a touchscreen.
"Successfully deploying the device in monkeys doing human-like tasks is a huge leap forward," said Bijan Pesaran, Professor of Neural Science at New York University. "Now we can refine our technology to help people suffering brain disorders."
Based on their results, and experiments to heat the devices to simulate longer periods of time, the researchers believe their devices could withstand implantation for more than six years. A student in Viventi's lab is now working to scale up this prototype, from 1,000 electrodes to more than 65,000.
"One of our goals is to create a new type of visual prosthetic that interacts directly with the brain that can restore at least some sight capacity for people with damaged optic nerves," concludes Viventi. "But we can also use these types of devices to control other types of prosthetics, or in a wide range of neuroscience research projects."
For their next device, the team will use commercial foundries to make the electrodes, which are far superior to their own capabilities and could further increase the survival time within the human body, as well as boosting the signal quality.