[tt] Growing Neural Implants

[Hughes, James J.]

http://www.technologyreview.com/printer_friendly_article.aspx?id=21087

Wednesday, July 16, 2008

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

By Emily Singer

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.

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.