Neurophilosophy

Researchers at Stanford University have used microelectrode arrays consisting of multi-walled carbon nanotubes to electrically stimulate rat retinal neurons.  

The small diameter of the nanotubes - 50 micrometres - enabled the researchers to stimulate individual ganglion cells growing on the microelectrode arrays. 

The work, published in Nano Letters, was carried out by neuroscientists and opthalmologists at Stanford’s Opthalmic Tissue Engineering Laboratory, who are  developing bio-microelectromechanical systems (bioMEMS) devices for the treatment of age-related macular degeneration and glaucoma.

One of these devices is the Artificial Synapse Vision Chip, which the researchers hope will eventually be incorporated into retinal implants. The chip would connect retinal neurons to an electronic camera, so that light falling onto the retina would be would be encoded and transmitted to a microelectrode array. The electrodes  would then stimulate retinal ganglion cells to send impulses along the optic nerve to the brain’s visual centres.

Using carbon nanotubes for implants could potentially lead to vastly improved  neuroprostheses. The platinum electrodes currently being used for neural implants are much larger than the cells they stimulate and, because of their rigidity, often damage the surrounding tissue when implanted. An inflammatory reaction  sometimes occurs, leaving a scar around the implant, which can also be rejected by the immune system.

As well as being far smaller than current devices, implants made using carbon nanotubes would not be regarded as foreign objects, and therefore would not induce an immune response. Furthermore, because nanotubes are flexible, implants made from them would not damage the delicate retinal tissue upon implantation.

“It’s probably one of the safest materials to use, because carbon naturally occurs in the body,” says Harvey Fishman, who led the research, and is now at the Plager Vision Center. 

However, coupling the nanotube microelectrodes in an implant to individual cells in the retina would be problematic. This might be overcome by coating the nanotubes with coating the nanotubes with substances which the cells find attractive, or by using guidance molecules to coax the cells into growing towards the microelectrodes. 

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