Metal electrodes are increasingly being used in brain implants that help treat depression and the tremors of Parkinson’s disease, and in ever more sophisticated prosthetic devices. In spite of these successes, conventional metal electrodes have major limitations: performance deteriorates over time, and it’s difficult to design electrodes that are efficient at both sending and receiving electrical signals. Now researchers at the University of Texas are developing electrodes that are more efficient at both sending and receiving electrical stimuli. These electrodes, which are coated with carbon nanotubes, could lead to neural implants that monitor how they affect the neurons that they stimulate, conserving battery life and reducing side effects.
Neural nanotubes: In these scanning electron microscope images, electrodes coated with carbon nanotubes, like the one on the right, are more conductive and better at interfacing with nervous tissue. The electrode on the left is bare.
Researchers led by Edward Keefer at the University of Texas Southwestern Medical Center developed a simple method for coating electrodes with carbon nanotubes. The coated electrodes were better at recording neural activity than were bare electrodes when implanted in mice and in a monkey. Importantly, the coated electrodes provided less-noisy recordings than bare ones did. They also required less power to operate.
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And the nanotubes enhanced the electrodes’ ability to both record and stimulate neural activity more than any other coating previously reported. Today’s neural prosthetics are good at sending electrical signals but not at receiving them, says Ravi Bellamkonda, director of the Neurological Biomaterials and Therapeutics group at Georgia Tech. Thus, the batteries in deep-brain stimulators–implanted devices used to treat Parkinson’s–last only three years because the devices are constantly on. “You want to see if the neuron is quiet,” says Bellamkonda. A feedback-enabled device that powered off when not needed could potentially use the same battery for a few more years.
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The University of Texas researchers’ technique for modifying electrodes is simple. Electrodes are placed in a water-based solution of carbon nanotubes; when a small voltage is applied to sites on the electrodes, carbon nanotubes localize there and can be fixed. Joseph Pancrazio, a neuroscientist at the National Institute of Neurological Disorders and Stroke, says that Keefer’s electrode modification “is something that can be done readily.” This means that other labs experimenting with neural prosthetics are likely to adopt the technique. By contrast, Pancrazio says, other methods for interfacing carbon nanotubes with neurons have required the use of special substrates and must be done at very high temperatures.
Pancrazio says that the nanotube coating might enable researchers to make smaller electrodes that cause fewer side effects. Using conventional electrodes for deep-brain stimulation, Pancrazio says, “you end up stimulating not only the area of interest but also other regions, leading to speech dysfunction and other problems.” The ideal electrode would be small enough to interact with only a single neuron. But when electrodes are miniaturized, their impedance increases and their performance decreases. Electrodes coated in carbon nanotubes might be more amenable to miniaturization.
Keefer began working on the electrode coatings to advance his work on prosthetic devices that give sensory feedback. Advanced prosthetic arms mimic the movements of real joints and even have realistic skinlike coatings. But when the lights are out, the person wearing such a prosthetic has no way of knowing where his hand is. Prosthetics capable of providing this kind of feedback will require high-performing electrodes whose electrical properties don’t decay rapidly like those of conventional electrodes, says Keefer.
However, it remains to be seen how the nanotube-coated electrodes will perform over the long term and whether there will be problems with biocompatibility; so far, the longest Keefer has tested the implants in animals is 60 days. One reason for the deterioration in the performance of conventional implanted electrodes over time is the formation of scar tissue between the electrode and the neuron. Keefer says that he will study whether nanotubes also induce this scarring.
One advantage of carbon nanotubes is the relative ease of making chemical modifications to their surfaces, such as by attaching proteins, that could make them more tissue-friendly in the long term. By contrast, chemically modifying bare metal electrodes is impracticable. “We want to modify the carbon-nanotube electrodes in order to make cells happier when sitting next to them,” says Keefer.
