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Growing Neural Implants

New approaches could more seamlessly integrate medical devices into the body.
July 16, 2008

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.

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.

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.

Animal tests of cortical implants in rodents and cochlear implants–in which an electrode array is implanted into the auditory portion of the inner ear–in guinea pigs suggest that coated electrodes perform better than bare metal versions, particularly in the short term. However, it’s not yet clear how they’ll fare in the long term, which is one of the biggest problems facing chronic implants–especially devices that record neural activity. “Recording quality deteriorates over time with all existing electrodes,” says Andrew Schwartz, a neuroscientist at the University of Pittsburgh.

Martin’s most ambitious goal is to get the electrodes to fully integrate with tissue by growing the coating after the electrode is implanted. The idea is that the polymer’s hairlike fingers would reach into the tissue, extending beyond the dead zone surrounding the metal electrodes. “Imagine the cells are like M&Ms suspended in Jell-O,” says Martin. “We’re growing the polymer around the M&Ms and through the Jell-O.” So far, the scientists have succeeded in growing the polymer in a piece of muscle tissue and a piece of mouse cortex.

Scientists developing new implants are excited about the possibility. But they also see serious hurdles. “It’s a very interesting concept,” says Ravi Bellamkonda, a biomedical engineer at the Georgia Institute of Technology, in Atlanta. “But the challenge is, will they actually penetrate the scar tissue and grow through or not?” McCreery, whose work has centered on acoustic prostheses, says that such an approach would be useful for cochlear implants. However, he warns that “you’d need to make sure it doesn’t grow into a frizzy mess that shorts everything out.”

Along with former lab members, Martin founded a company, Massachusetts-based Biotectix, to commercialize the materials developed in his lab. He says that he is already in talks with a cochlear-implant technology company about using his lab’s materials in their devices.

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