Wednesday, July 16, 2008

Growing Neural Implants

New approaches could more seamlessly integrate medical devices into the body.

By Emily Singer

Implanted network: Scientists are developing new ways to coax electrodes to integrate with brain tissue. One approach is to grow PEDOT, an electrically conductive polymer, onto an electrode after it is surgically implanted into the body. Shown here is a slice of cortical tissue from a mouse in which the polymer (shown in blue) was deposited after insertion of the metal electrode. The polymer surrounds the cells, forming a diffuse, conductive network that follows the white-matter tracts of the cortex. Credit: Sarah Richardson-Burns

Conductive polymer coatings that weave their way into implanted tissue might one day improve the performance of medical implants, such as cochlear implants and brain stimulators used to treat Parkinson's disease. In early studies, neural interfaces coated with an electrically conductive polymer outperformed conventional metal counterparts. Scientists at the University of Michigan hope that the material's novel properties will help lessen the tissue damage caused by medical implants and boost long-term function.

Use of devices that are surgically implanted into the brain or other parts of the nervous system is growing rapidly. Cochlear implants, which help deaf people hear, and deep brain stimulation, which relieves symptoms of Parkinson's disease, for example, are approved by the Food and Drug Administration. Both work by stimulating nerve cells via an implanted electrode. Devices that record and translate neural activity are also under development for people with severe paralysis.

But as use of neural implants grows, so does concern over the damage that those devices can impose on neural tissue. Insertion of the rigid metal electrode into soft tissue triggers a cascade of inflammatory signals, damaging or killing neurons and triggering a scar to form around the metal. "We hope to come up with a way to communicate across the scar layer and send information to and from the device in a way that is as friendly as possible," says David Martin, a materials scientists at the University of Michigan, in Ann Arbor, who is leading the research into the polymer coatings.

Martin and his collaborators coat the electrodes with an electrically conductive polymer originally developed for electronic devices, such as organic LEDs and photovoltaics for solar cells. The polymer coating increases the surface area of the metal-biological interface, which in turn boosts performance of the electrode. "If you have lots of surface area, you can inject current more efficiently," says Douglas McCreery, director of the Neural Engineering Program at the Huntington Medical Research Institute, in Pasadena, CA. "That means less demand on batteries, but, probably more importantly, you're not recruiting the nasty electrochemical reactions that might be hazardous to surrounding tissue."

The Michigan scientists electrochemically deposit the polymer onto the electrode, much like chroming a car bumper. By peppering the material with small amounts of another polymer, they can coax the conductive polymer to form a hairy texture along the metal shaft. Martin says that the approach mimics nature: the numerous tiny alveoli of the lungs, for example, increase the surface area available for the oxygen exchange between air and blood. Scientists can also tack on nanofibers loaded with controlled-release drugs to inhibit the inflammatory reaction.