We’re all familiar with using a splint to provide support so a broken limb can heal itself over time – but how about a splint that fits between the nerve cells in your brain? A new groundbreaking study in the journal Science Advances reveals that small “bridges” of multiple carbon nanotubes formed together to make a "sponge" support the growth of nerve fibers and can even connect individual nerve networks that have previously been severed.
These neural bridges do not appear to cause any major scarring, nor do they provoke a damaging and self-destructive immune response. The hybrid between neurons and nano-materials could eventually culminate in their use as implants for those suffering from neurodegenerative diseases, potentially helping to restore some motor function to those afflicted by them.
“These materials could be useful for covering electrodes used for treating movement disorders like Parkinson's because they are well accepted by tissue, while the implants being used today become less effective over time because of scar tissue,” Laura Ballerini, a professor at the Italy-based International School for Advanced Studies (SISSA) and chief coordinator of the study, said in a statement. “We hope this encourages other research teams with multidisciplinary expertise to expand this type of study even further.”
Carbon nanotubes are cylindrical carbon-based nanostructures, with diameters being on average 4 nanometers – just slightly thicker than a strand of human DNA. They are formed by rolling up 1-atom-thick sheets of carbon known as graphene, a veritable wonder material that is highly conductive to both heat and electricity.
It is this extraordinary ability to conduct electricity along with their flexible, strong, minuscule form that makes them the perfect material to connect neurons together, which are essentially biochemical structures that communicate with each other using electrical signals. These nanotubes have previously been used to connect someone’s nervous system with a touch-sensitive artificial skin, but this team wanted to implant the tubes into a neuronal network itself.
Comparing two 14-day-long cultured neural networks - the control (left), where the gap was left unfilled, and the far more connected version (right) featuring nanotube sponges. Usmani et al./Science Advances
First off, they wanted to see what would happen when a carbon nanotube bridge was added to nerve tissue in vitro, meaning that it was not still part of the original host organism.
Specifically, two spinal cord segments were grown in a laboratory setting, but they were separated by a distance of 0.3 millimeters (0.012 inches). When allowed to grow on their own, they formed nerve fibers that extended in straight bundles in multiple directions, but not necessarily towards each other.
When several carbon nanotubes, taking the shape of a “sponge” structure, were inserted within the small gap, "bridging it", most of the outgrowing fibers used it as a connecting pathway and the two spinal cord segments were interlinked before long.
In order to demonstrate that both populations of neurons could transmit signals to each other, the team applied an electrical signal to one of the cords, which was successfully transferred across the nanotube sponge along to the second. However, this wasn’t enough to prove their viability in living organisms.
Implanting carbon nanotube sponges within the brains of healthy, live rodents, the team noticed that, after four weeks, there was an insignificant immune system response, low levels of scarring, and, most importantly, a progressive “invasion” of neurons within the sponge.
At the very least, this shows that these nanotubes are easily accepted as part of their brains’ neural networks. Perhaps it won’t be long before the same can be said of humans.
Gif in text: Nanotube sponge in 3-D animation. SISSA/UNITS
