Researchers at Stanford University have used electrodes made of bundles of multiwalled carbon nanotubes to stimulate rat neurons. In a Nano Letters paper published online this week, the researchers describe making arrays of the 50-micrometer electrodes on a silicon substrate and growing the neurons on the arrays. The neurons responded consistently to the electrical signals from the electrodes.
The experiment is an advance toward the long-term goal of using neural prosthetics, such as cochlear and retinal implants, to address individual neurons. Neural prosthetics that restore vision or hearing typically use implanted microelectrode arrays to send electrical signals to nerve cells or directly to the brain. While cochlear implants are already in use, scientists are in the process of developing artificial retinas. A retinal prosthetic, for instance, would have an array implanted near the retina, stimulating the nerve cells that send signals to the optic nerve.
The problem so far with making safe, effective implants has been the electrodes, which are made of metals such as platinum and iridium. To effectively transfer electrical current to the neurons, electrodes made of platinum, for example, have to be larger–by hundreds of micrometers–than the cells they are trying to stimulate. This makes it hard to address specific cells. Other metals can be used for smaller electrodes, but they can sometimes dissolve or cause chemical reactions that harm the surrounding tissue.
Besides, the human body sometimes treats long-term implants that use metals as foreign objects. What’s more, the relatively large metal electrodes are rigid and can damage the soft tissue in which they are implanted. “Often, what happens in standard electrical prosthetics is you get an inflammatory reaction that can basically form a scar tissue around the implant,” says Harvey Fishman, an ophthalmologist and neuroscientist who led the work at Stanford University, along with Ke Wang, a graduate student in applied physics.
Fishman, who is now at the Plager Vision Center in Santa Cruz, CA, says that the human body would not treat implants containing carbon nanotube electrodes as foreign objects. “It’s probably one of the safest materials to use, because carbon naturally occurs in the body,” he says. “We’re mostly made of carbon and water.” In addition, the Stanford team could make very small electrodes, 50 or 100 micrometers in diameter, out of the carbon nanotubes, which are tough, flexible, and good electrical conductors.
Other researchers are also seeking ways to include carbon in the electrical interface of neural prosthetics. Todd Pappas at the University of Texas Medical Branch and his colleagues at Rice University have successfully stimulated rat neurons grown on carbon nanotube mats made of horizontally grown nanotubes. The advantage of having an electrode that juts out vertically from a surface is that you can place the charge where you want it, Pappas says. But he thinks that the small electrodes could be a problem when implanted in a living organ–they will not be easy to couple to individual nerve cells. For that, he says the researchers will have to take additional steps–guiding the neurons to grow towards the electrodes, for example, or covering the surface with a more cell-friendly material.
Meanwhile, other researchers are adapting the microelectrode technology specifically for retinal implants. Orlando Auciello and his colleagues at Argonne National Lab are working on making silicon microchips bio-inert by incorporating carbon, albeit in another form–a material they have developed called ultrananocrystalline diamond. While silicon cannot be placed directly in the eye, a chip encapsulated with this material could be, Auciello says. The device is now in clinical trials. Since the diamond-coating technology has not been fully developed yet, each of the six patients in the clinical trial had a microchip implanted on the side of the head that is connected to a platinum electrode array in the retina. The electrodes are 500 micrometers in diameter.
With the smaller carbon nanotube electrodes, Auciello says, “You can address much lower number of cells with one electrode, even if it doesn’t go to a single cell.” The Stanford work opens up the possibility of integrating carbon nanotubes on the silicon microchip, he says. “This new development is very exciting if it can be reproduced.”