Monday, May 1, 2006

Tiny Electrodes for the Brain

Nanowires could make brain-machine interfaces safer and cochlear implants more effective.

By Kevin Bullis

NYU and MIT researchers have developed a flexible electrode that can send or receive signals from brain cells. The electrode can be inserted into the brain through blood vessels, eliminating the need to open a patient's skull for some neurological treatments. (Courtesy of Zina Deretsky, National Science Foundation.) An electron microscope image showing the size of the nanowire (red dot) as it would appear inside a capillary embedded in brain tissue. Image courtesy of Rudolfo Llinas, New York University

A new type of polymer nano electrode could make brain implants, including those used to treat severe cases of Parkinson's, far safer, and it could also make attempts to restore vision and movement with direct brain-machine interfaces more feasible. Rodolfo Llinas, professor of neuroscience at New York University, and researchers at MIT have developed a nanowire electrode just 600 nanometers wide that can send and receive signals to the brain.

The electrode developed by Llinas and coworkers is so small that it could be inserted through an artery, perhaps in the arm or groin, and threaded up to the brain. Because the electrode is a small fraction of the size of a red-blood cell and flexible, it can be snaked through the smallest blood vessels, getting close enough to neurons deep in the brain to detect and deliver electrical signals.

One current treatment for severe cases of Parkinson's, called deep brain stimulation, involves implanting electrodes that deliver high-frequency electrical pulses which shut down parts of the brain responsible for the disease's symptoms Such treatments, however, are risky and expensive, in part, because they require that a patient's skull be opened to surgically insert electrodes into brain tissue.

The conventional electrodes, which now measure in millimeters, can also damage blood vessels in the brain, says Joseph Pancrazio, program director for neural engineering projects at the National Institute of Neurological Disorders and Stroke (NINDS), one of the National Institutes of Health. "By taking advantage of the nanodimensions to thread the electrodes through the vasculature, you may reduce the risk of stroke," he says. "This is a completely out-of-the box way to think about enabling deep-brain stimulation. I think there may be payoffs in terms of safety, efficacy, robustness, and biocompatibility. It certainly is an area that we need to look at seriously."

"Not having to open the skull would be a clear benefit over what we're now doing," says Jeff Bronstein, neurologist at the UCLA Medical School, who says thousands of Parkinson's patients have undergone the deep brain stimulation procedures.

John Heiss, a neurosurgeon at NINDS, cautions that it will first be necessary to demonstrate that the nanowires do not cause complications, such as blood clots. He also notes that, although the head would not need to be opened, such a procedure would still require some invasive surgery. Heiss says, however, that if the procedure proves to be safe, it could make deep brain stimulation a more attractive alternative at earlier stages of Parkinson's.

Beyond use in deep brain stimulation, Llinas says his electrodes could detect signals, say, in the area of a person's brain responsible for directing arm movement. These signals could then be used to drive a robotic arm, restoring some abilities to people paralyzed by brain and spinal-cord injuries. Llinas says the first application of the nanowire electrodes may be to route nerve impulses around damaged areas of the spinal cord, either to other nerves or directly to muscles, possibly restoring function to paralyzed limbs.

The nano electrodes could also play a role in improving the cochlear implants used to restore hearing. Because the electrodes are so small, it could be possible to increase the number of electrodes used in a cochlear implant, "to stimulate a broader region and give more color to sound," says Patrick Anquetil, a mechanical engineering postdoctoral fellow at MIT and one of the researchers on the project. He says the first commercial uses of the nanowire electrodes are probably still five years away.

In the future, the researchers plan to build steerable electrodes. To do this, they will use a polymer that contracts in response to electricity. A bundle of such nanowires could be directed, by causing selected nanowires to contract.

The researchers think that, eventually, the bundle of nanowires could partly steer itself. Anquetil says they have made polymers that act as pressure sensors, and they see the possibility of using semiconducting polymers as the basis for simple electric switches. "One thing that really excites us about this is, in principle, there's no reason why, with the same material, you cannot build a whole system in which you have contraction, measurement, sensing, and computation."

While the first bundles would use relatively few electrodes, thousands could eventually be grouped together to form a package no wider than the 1-2 millimeter probes Llinas says are used today in the brain. Once near the targeted area, the nanowires would be allowed to separate. The wires would then spread out, pushed into a branching network of capillaries. This would allow researchers to monitor and deliver impulses to individual neurons deep inside the brain in a distributed area, an ability that could prove a boon to brain researchers now limited to using relatively small arrays of electrodes.