Researchers at the University of California San Diego (UC San Diego) and the Salk Institute for Biological Studies have engineered a tiny flexible neural probe (about one-fifth the width of a human hair) that can be used for examining small and dynamic areas of the nervous system like the spinal cord, according to a press release by UC SanDiego published on Thursday.
Really small, flexible probes that can fit in between vertebrae
"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," said Axel Nimmerjahn, associate professor at the Salk Institute and co-senior author of the study.
The probe is ideal for long-term use as it is more compatible with biological tissue and, therefore, less prone to triggering an immune response.
"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.
This is not the first ultra-thin, flexible probe ever created. But it is unique in that it can both record the electrical activity of neurons and stimulate 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 researchers experimented with implanting the probes in the brains of live mice for up to one month and found that they caused hardly any inflammation in the brain tissue after even a long-term presence.
Recording electrical activity from neurons with high sensitivity
The team also found that the probes could successfully record electrical activity from neurons with high sensitivity and could even be used to target specific neuron types to engender certain physical reactions. To illustrate this fact, the researchers made use of the probes' optical channels to get the mice to move their whiskers.
These initial experiments with mice were conducted as a proof of concept. Now, the researchers hope to perform future studies in the spinal cord where the probe was initially invented to be placed.
"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."
The probes could have many applications in medicine, particularly in the treatment of spinal cord injuries.
The results of the study were published in a paper published on June 7 in Nature Communications.
Central to advancing our understanding of neural circuits is developing minimally invasive, multi-modal interfaces capable of simultaneously recording and modulating neural activity. Recent devices have focused on matching the mechanical compliance of tissue to reduce inflammatory responses. However, reductions in the size of multi-modal interfaces are needed to further improve biocompatibility and long-term recording capabilities. Here a multi-modal coaxial microprobe design with a minimally invasive footprint (8–14 µm diameter over millimeter lengths) that enables efficient electrical and optical interrogation of neural networks is presented. In the brain, the probes allowed robust electrical measurement and optogenetic stimulation. Scalable fabrication strategies can be used with various electrical and optical materials, making the probes highly customizable to experimental requirements, including length, diameter, and mechanical properties. Given their negligible inflammatory response, these probes promise to enable a new generation of readily tunable multi-modal devices for long-term, minimally invasive interfacing with neural circuits.