As neural-implant use grows, so does concern over damage the devices do to neural tissue. Placing rigid-metal electrodes into soft tissue generates inflammatory reactions, damaging or killing neurons, and triggering the formation of scar tissue around the metal.

Neural interfaces coated with an electrically conductive polymer have outperformed conventional metal counterparts in early studies. U. of Michigan scientists hope the material's properties will help lessen tissue damage caused by medical implants and improve long-term function. Devices such as cochlear implants approved by the FDA use an implanted electrode to stimulate nerve cells.

Material scientist and U. of Michigan Ann Arbor researcher David Martin has been coating electrodes with an electrically conductive polymer that increases the surface area of the metal-biological interface, which boosts electrode performance. "Lots of surface area lets us inject current more efficiently," says Douglas McCreery, director of the Neural Engineering Program at the Huntington Medical Research Institute, Pasadena, Calif. "That means less demand on batteries and more importantly, you're not stimulating the nasty electrochemical reactions that might be hazardous to surrounding tissue."

Adding small amounts of another polymer to the first lets the conductive one form a hairy texture along the electrode. The idea is that the polymer's hair-like fingers would reach into the tissue, extending beyond the dead zone surrounding the metal electrodes. Martin's goal is to get the electrodes to combine with tissue by growing the coating hairs after implanting the electrode. Scientists can also tack on nanofibers loaded with controlled-release drugs to inhibit an inflammatory reaction.

Tests of cortical and cochlear implants in guinea pigs suggest that coated electrodes perform better than bare metal versions, particularly over short periods. The long term is another matter. "Recording quality deteriorates over time with all existing electrodes," says U. of Pittsburgh neuroscientist Andrew Schwartz.

In the accompanying image, a slice of cortical mouse tissue shows the polymer (in blue) 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.