Nanotube implant guides spinal nerve growth

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Scanning electron micrograph detailing the structure of the carbon nanotube sponge. Cells are visible invading the artificial skeleton.
Credit: Denis Scaini

Nerve cells sprouting from cut spinal cord followed a tangle of tiny tubes made from carbon and formed webs of “wires”, bridging the gap in 3-D.

Electrical signals were able to pass between spinal segments again, and – perhaps most importantly – the nanotube sponges are safe to implant in brain tissue. The work, by biologists in Italy and Spain, was published in Scientific Reports.

“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,” says senior author and Italian International School for Advanced Studies neuroscientist Laura Ballerini.

Carbon-based nanostructures – such as tubes made from graphene, a chicken-wire-like sheet of carbon atoms – are particularly attractive materials for neuronal prostheses, especially for injury or illnesses affecting the central nervous system (brain and spinal cord).

Studies have shown multi-walled carbon nanotubes, which consist of multiple layers of rolled graphene tubes, nested like a retractable telescope, can guide nerve cell growth in two dimensions. But spinal cord injuries tend to happen in 3-D.

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Scanning electron microscopy showing the tight interactions between the carbon nanotube 3-D sponge and the living brain tissue, two weeks after implantation.
Credit: Denis Scaini

Sadaf Usmani, also from the International School for Advanced Studies, and colleagues used a sponge-like mesh of multi-walled carbon nanotubes (originally designed to clean oil from seawater) and popped it between severed sections of mouse spinal cord, around 300 microns apart, which they grew in a dish.

They compared it to another severed cord, but this one was grown without any nanotubes.

After around two weeks, they saw nerve growth in both dishes. But the directions of growth were completely different.

Where nerves in the second dish grew in straight lines in all directions like spokes on a wheel, cells in first used the mesh as scaffolding and grew towards each other.

Could any type of 3-D mesh to the same job? To test this, they repeated the experiment using a 3-D porous structure made from a silicon-based organic polymer. But while regenerating cells successfully infiltrated the polymer bridge, they didn’t grow as far or as well as those in the carbon nanotubes.

Cultured cells also transmitted electrical impulses between the spinal cord segments, but because the experiment was carried out in a dish, they couldn’t tell if the signals were getting where they needed.

Finally, to see the long-term effects of such implants, they embedded tiny bundles of multi-walled carbon nanotubes into the brain of four rats for a month. 

Post-surgery, there was a little inflammation as expected. But once it settled, the rats remained “vital and healthy during the entire four weeks”, Usmani says.

So why are carbon nanostructures so well suited to guiding nerve cells? Exactly how they interact, the researchers don’t know. It might be because they form tight junctions with living cells, and perhaps help things along thanks to graphene’s electrical conductivity.

And while there’s still a long way to go before these technologies are suitable for human use, Ballerini says she hopes the work will encourage “other research teams with multidisciplinary expertise to expand this type of study even further”.

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