Originally Published MDDI January 2005

R&D DIGEST

Maria Fontanazza

Mechanical engineers and neurobiologists are collaborating to develop a neural prosthetic that may help people who are suffering from severe paralysis. The movable electrode system has the potential to detect neural signals, translating them to a computer cursor, robot, or even a prosthetic limb, offering hope to patients who cannot communicate or who have body parts that cannot function.

While research on the device is intended for the recording of neurons, it also can be used as a stimulation device. "We're thinking about how this might be a useful technology as a deep brain stimulator for Parkinson's disease," says Joel Burdick, professor of mechanical engineering and bioengineering at the California Institute of Technology (Pasadena, CA). It could allow doctors to reposition electrodes without having to perform surgery, or it could be used for locomotion in spinal cord injuries to pinpoint and manage stimulation. But, he says, right now these are just theories.

The system is designed to move an implanted smart electrode through the brain. The autonomous quality of the electrode gives it the ability to continually readjust while following a neuron. "It's been a challenge for a long time to get a good interface between a neuron and an electrode," Burdick says.

When implanting an electrode inside the brain, the targeted region is usually so small that an error of a millimeter can have a negative effect. And even if it is implanted in the right area, it might not be sitting next to the best neuron. "If the electrode is really smart, we can figure out if there's another neuron that is better suited for the neural prosthetic task," says Burdick. "We've demonstrated that the electrode can continually adjust itself, but we're still working on how to hunt for the right neuron."

The second challenge is to figure out a way to make the technology small enough and safe enough for use in the brain. The prototype uses a piezoelectric motor, which measures in millimeters and inches, to drive electrodes. Burdick wants to downsize to the micron scale and use microminiature motors. His team also needs to develop a method that can generate high forces without high voltage that will allow the electrode to travel. Excess heat generation can destroy delicate brain cells.

The technique currently uses a bellows-like device to push the electrode around. Once fluid is sealed inside the bellows, a current is pumped through to transform the water molecules into hydrogen and oxygen. This process, called electrolysis, causes the bellows to expand and propel the electrode forward. The reverse current pulls the gas back and turns it into water again. "Basically, we can generate and remove gas with just a small amount of electrical current," says Burdick. "Once you turn water into gas, it stays there until you reverse it. There's no power loss, and it doesn't generate much heat."

The next step involves studying a safe surgical technique. Implants introduce the potential for inflammation in the brain. "Just our device won't solve this problem," says Burdick. As technology is refined, it must be combined with advances that address the biochemical aspect.

"There are plenty of challenges ahead for [developing] human uses, but right now we're on a path that suggests we're going to get there," Burdick says.

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