New neural probe could further understanding of spinal cord

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Researchers in California have developed a neural probe that can be implanted for longer to record and stimulate neural activity while minimising injury to the surrounding tissue.

Photograph of a small capsule supporting the neural probe, with a closeup of the microfiber tip
Photograph of a small capsule supporting the neural probe, with a closeup of the microfiber tip - Spencer Ward

Developed by researchers at the University of California San Diego and the Salk Institute for Biological Studies, the new neural probe is flexible and about one-fifth the width of a human hair. The team’s neural probe is detailed in Nature Communications.

The team said that this type of neural probe would be suitable for studying small and dynamic areas of the nervous system like peripheral nerves or the spinal cord.

In a statement, Axel Nimmerjahn, associate professor at the Salk Institute and co-senior author of the study, said: “This is where you’d need a really small, flexible probe that can fit in between vertebrae to interface with neurons and can bend as the spinal cord moves.”

According to the team, these features also make it more compatible with biological tissue and less prone to triggering an immune response, making it suitable for long-term use.

“For chronic neural interfacing, you want a probe that’s stealthy, something that the body doesn’t even know is there but can still communicate with neurons,” said study co-senior author Donald Sirbuly, professor of nanoengineering at the UC San Diego Jacobs School of Engineering.

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Similar probes exist but the new advance records the electrical activity of neurons and stimulates specific sets of neurons using light.

“Having this dual modality—electrical recording and optical stimulation—in such a small footprint is a unique combination,” said Sirbuly.

The probe consists of an electrical channel and an optical channel. The electrical channel contains an ultra-thin polymer electrode and the optical channel contains an optical fibre that is also ultra-thin.

To put these two channels together the researchers had to insulate them to keep them from interfering with each other. They also had to have them both fit into a probe measuring 8 to 14 micrometres in diameter, all while making sure that the device was mechanically flexible, robust, biocompatible and able to perform on par with current neural probes. This involved finding the right combination of materials to build the probe and optimising the fabrication of the electrical channel.

The team implanted the probes in the brains of live mice for up to one month, which caused hardly any inflammation after prolonged implantation. As the mice moved about in a controlled environment, the probes were able to record electrical activity from neurons with high sensitivity. The probes were also used to target specific neuron types to produce certain physical responses. Using the probes’ optical channels, the researchers stimulated neurons in the cortex of the mice to move their whiskers.

These tests in brain tissue were done as a proof of concept and the team hopes to perform future studies in the spinal cord using their probe.

“Currently, we know relatively little about how the spinal cord works, how it processes information, and how its neural activity might be disrupted or impaired in certain disease conditions,” said Nimmerjahn. “It has been a technical challenge to record from this dynamic and tiny structure, and we think that our probes and future probe arrays have the unique potential to help us study the spinal cord—not just understand it on a fundamental level, but also have the ability to modulate its activity.”

UC San Diego and the Salk Institute have submitted a patent application on the neural probe technology described in this work.