Scientists have integrated carbon nanotubes in neurons to control growth and restore lost electrical connections between nerve cells.

They have shown that the carbon nanotubes can be used safely and hope they can restore neural function to people with spinal injuries. The integration of carbon nanotubes brought along some unexpected benefits too.

Carbon nanotubes have some remarkable properties: excellent thermal conductivity, mechanical strength, and electrical conductivity. They have been used to make the toughest fibre ever made, computer chips that run twice as fast as silicon chips and they have also been used to create the world's blackest material – Vantablack.

Because they are long, thin and conductive, carbon nanotubes seemed like the ideal candidate for neuronal prostheses, restoring function to damaged neural pathways, and systems that interface with the human body.

"The perfect material to build neural interfaces does not exist, yet the carbon nanotubes we are working on have already proved to have great potentialities," said Laura Ballerini, one of the researchers from the International School for Advanced Studies in Italy.

"After all, nanomaterials currently represent our best hope for developing innovative strategies in the treatment of spinal cord injuries."

So why aren't we already using them?

There have been concerns in the past about the safety of carbon nanotubes. Their fibrous nature puts them in the same class as asbestos and they have been shown to penetrate the cell membrane – a delicate layer made of lipid molecules.

In this study, the researchers chemically modified the surface of carbon nanotubes so that they could be turned into a carbon nanotube ink for easy processing. The ink was dropped onto a flat glass surface and heated to a temperature of 350 degrees Celsius to create a thin mat of pure carbon nanotubes.

The neurons were harvested from the hippocampus of laboratory rats and deposited directly on top of the nanotube mats. After an incubation period at body temperature, the cells were tested for conductivity and compatibility with the carbon nanotube surface.

Ballerini and her team are confident that, this time, they have shown carbon nanotubes can be used safely.

"First of all, we have proved that nanotubes do not interfere with the composition of lipids, of cholesterol in particular, which make up the cellular membrane in neurons," said Ballerini.

Just when the researchers thought it couldn't get any better, their study also found that nerve cells growing on a flat bed of carbon nanotubes reached maturity much quicker than normal.

"[Carbon] nanotubes facilitate the full growth of neurons and the formation of new synapses. Having established the fact that this interaction is stable and efficient is an aspect of fundamental importance," said Ballerini.

These are still early days and there are still a couple of important issues that need to be addressed. Understanding exactly how the integration of carbon nanotubes impacts the creation and structure of neuronal pathways will need to be fleshed out.

"If, for example, the mere contact [with carbon nanotubes] provoked a vertiginous rise in the number of synapses, these materials would be essentially unusable," said Maurizio Prato, another member of the research team.

Despite this concern, the researchers are hopeful that carbon nanotubes can be used safely as neuronal prostheses and are confidently pursuing the next stage of research – animal testing.

"We are proving that carbon nanotubes perform excellently in terms of duration, adaptability and mechanical compatibility with the tissue. Now we know that their interaction with the biological material, too, is efficient."

"Based on this evidence, we are already studying the in vivo application, and preliminary results appear to be quite promising also in terms of recovery of the lost neurological functions."

The study has been reported in Nanomedicine: Nanotechnology, Biology and Medicine.