According to Li, the brain is just another electrical system, albeit one with incredibly sophisticated defense mechanisms designed for self-healing that defeat the electrode technologies available today. As soon
as a traditional metal electrode is placed in the brain, according to Li, the body's immune system begins attacking it for two reasons: shape and texture.
"Today's metal electrodes are degenerative for two reasons," said Li. "First, the neural networks of the brain are three-dimensional, whereas most electrodes are basically planar. Secondly, neural tissues are very
sensitive to the stiffness of metal electrodes, which are a million times stiffer than a neuron."
Consequently, the very neurons that electrodes are supposed to monitor and stimulate start moving away from metal electrodes as soon as they are implanted. To fill the void, the immune system begins surrounding
the electrode with scar tissue (glial cells), which are insulators.
Consequently, electrodes implanted in the brain continually degrade in performance until they eventually just stop working altogether.
To solve the first problem, Li grew groups of parallel nanotubes vertically from a substrate, resulting in an electrode that extends into a third dimension. Since the entire surface of the nanotube can act as a sensor or as a stimulator, their non-planar shape more easily
interfaces with the three-dimensional neural network structures of the brain.
Second, because he grows long flexible groups of very thin nanotubes, the resulting electrodes are almost as soft as the surrounding tissue into which they are implanted. The body accepts the electrodes, and the
neurons grow into the implant instead of insulating it with scar tissue, allowing the electrodes to be embedded into the brain.
"Our nanotube electrodes are vertically aligned into a structure that resembles a soft brush, so that it is not rejected by the body," said Li.
Finally, the nanofibers are coated with a polymer to prevent them from sticking to each other. In addition, the polymer carries neurotransmitter-like ions, which are released when it is electrically stimulated and replenished when the body is idle. Consequently, Li
predicts that NASA's nanofiber electrodes will eventually be able to be implanted into astronauts' brains for indefinite periods.