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.