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Carbon nanotubes are perhaps not the first thing that spring to mind when you think of neuroprosthetics. Rather, electrode arrays spring into the mind's eye. Yet, nanotube scaffolds are definitely advancing, as brain interfaces in their own right, both to repair brain function, and to augment it.
At the end of 2008, a group of Italian scientists first proved that it was possible to repair damaged brain tissue. A study conducted by Laura Ballerini, a professor of physiology, together with Maurizio Prato, professor of organic chemistry, both at the University of Trieste, Italy, succeeded in depositing a mixture of carbon nanotubes onto a glass substrate, which was then heated to encourage the nanotubes to spread and layer themselves.
Next, the cultured nanotubes on substrate were compared to rat neurons cultured on glass in the same way, and tested for electrical conductivity in the same way, for a period of two weeks.
The findings were fairly surprising. There were negligible differences in electrical conductivity of the two networks. Both were highly efficient signal propagators, with electrical signals keeping to discrete networks formed of either neurons or nanotubes.
Nanotubes thus became a serious contender for neural work. Given the similarity for propagating the same electrical impulses that natural neurons propagate, it becomes feasible to deposit them into a brain, with the assurance they will continue to propagate signals, in a very similar fashion to ordinary neurons. To complete the work, a propagated layer of neurons was overlaid on a propagated layer of carbon nanotubes. The two functioned together as if the layer was composed solely of neurons.
cultured rat hippocampal neurons grown on a layer of purified carbon nanotubes.
Credit: Laura Ballerini, University of Trieste
The team published their findings in the December 21, 2008 online edition of Nature Nanotechnology, entitled 'Carbon nanotubes might improve neuronal performance by favouring electrical shortcuts'. A link is in the references at the end of this article.
Fast forward one month, to the beginning of 2009, and the first of two groups attempting to take the work further. Scientists at the University of Southern California are trying to replicate the functions of brain neurons using carbon nanotubes. This work directly feeds on from the Italians. The end goal is to build an interconnected artificial brain that can do basic functions in the same way that the brains of small animals can.
"At this point we still don't know if building a synthetic brain is feasible," said Alice Parker, professor of electrical engineering. "It may take decades to realise anything close to the human brain but emulating pieces of the brain, such as a synthetic vision system or synthetic cochlea that interface successfully with a real brain may be available quite soon, and synthetic parts of the brain's cortex within decades."
The researchers have shown that portions of a neuron can be modelled electronically using carbon nanotube circuit models and have performed detailed simulations of the circuit models. A single archetypal neuron, including excitatory and inhibitory synapses, has been modelled electronically and simulated. Parker and her co-researcher, Chongwu Zhou, are in the process of combining these circuit models of neurons to create a functional carbon nanotube circuit model of a small network of neurons.
At time of writing, the University of Southern California's work has been underway a scant two months. However, the work produced has already resulted in a second paper, 'Shorting neurons with nanotubes' which describes the advances in interfacing carbon nanotubes and rat neurons directly, trying to form completed circuits out of a mix of both. With multiple, competing teams now racing to perfect this form of neuroprosthetic, it may well be just scant decades before such interfaces are realised, for human use.
Carbon nanotubes might improve neuronal performance by favouring electrical shortcuts
Nanomedicine Shorting neurons with nanotubes
National Science Foundation: Synthetic Brains
Biomimetic Cortical Nanocircuits