The combination of graphene nanoribbons made with a process developed at Rice University and a common polymer could someday be of critical importance to healing damaged spinal cords in people, according to Rice chemist James Tour.
The Tour lab has spent a decade working with graphene nanoribbons, starting with the discovery of a chemical process to ‘unzip’ them from multiwalled carbon nanotubes, as featured in a Nature article in 2009. Since then, the researchers have used them to enhance materials for the likes of deicers for airplane wings, better batteries and less-permeable containers for natural gas storage.
Now their work to develop nanoribbons for medical applications has resulted in a material, dubbed Texas-PEG, that may help knit damaged or even severed spinal cords.
Graphene nanoribbons customised for medical use by William Sikkema, a Rice graduate student and co-lead author of the paper, are highly soluble in polyethylene glycol (PEG), a biocompatible polymer gel used in surgeries, pharmaceutical products and in other biological applications. When the biocompatible nanoribbons have their edges functionalised with PEG chains and are then further mixed with PEG, they form an electrically active network that helps the severed ends of a spinal cord reconnect.
“Neurons grow nicely on graphene because it’s a conductive surface and it stimulates neuronal growth,” said Tour.
In experiments at Rice and elsewhere, neurons have been observed growing along graphene.
“We’re not the only lab that has demonstrated neurons growing on graphene in a petri dish,” said Tour. “The difference is other labs are commonly experimenting with water-soluble graphene oxide, which is far less conductive than graphene, or non-ribbonised structures of graphene.
“We’ve developed a way to add water-solubilising polymer chains to the edges of our nanoribbons that preserves their conductivity while rendering them soluble, and we’re just now starting to see the potential for this in biomedical applications,” he said. He added that ribbonised graphene structures allow for much smaller amounts to be used while preserving a conductive pathway that bridges the damaged spinal cords.
Tour said only one per cent of Texas-PEG consists of nanoribbons, but that’s enough to form a conductive scaffold through which the spinal cord can reconnect.
Texas-PEG succeeded in restoring function in a rodent with a severed spinal cord in a procedure performed at Konkuk University in South Korea by co-authors Bae Hwan Lee and C-Yoon Kim. Tour said the material reliably allowed motor and sensory neuronal signals to cross the gap 24 hours after complete transection of the spinal cord and almost perfect motor control recovery after two weeks.
“This is a major advance over previous work with PEG alone, which gave no recovery of sensory neuronal signals over the same period of time and only 10 per cent motor control over four weeks,” Tour said.