Tissue tether: improved conducting plastic could boost nerve-regeneration success

Science News, May 28, 2005 by Alexandra Goho

Usually touted as materials for cheap, flexible versions of electronic devices such as computer displays and solar panels, conductive polymers could also have roles in emerging medical technologies. In a new investigation, biomedical engineers have chemically modified a conducting polymer so that it can coax nerve tissue to grow.

Several years ago, Christine Schmidt of the University of Texas at Austin discovered that nerve cells on a film of polypyrrole grow faster when the film is exposed to an electric field. Schmidt suspected the discovery was just a beginning.

"There are so many other signals in the body" that also spur tissue growth, says Schmidt. For instance, cells take cues from a variety of growth-promoting biochemicals.

To leverage the cell-growing effects of polypyrrole, Schmidt and her colleagues at the Massachusetts Institute of Technology (MIT) considered decorating the polymer with a biochemical that would cause cells to stick and then grow in one direction. The trouble was that biological molecules usually don't bind to polypyrrole.

To find a biomolecule that would bind, the team sifted through a library of protein fragments, or peptides. The screening technique, developed by Schmidt's MIT collaborator Angela Belcher, was originally designed to screen for peptides that bind to semiconductor particles. The goal of that work was to develop a scheme for assembling miniature circuits (SN: 7/5/03, p. 7).

The library consists of bacteriophages, which are viruses that prey on bacteria. Each of the library's billion phages produces a different peptide on its surface. So when the researchers bathed polypyrrole films in a solution containing the entire library, only those phages with peptides that bind to the polymer hung on.

One peptide, called T59, stuck especially well, the researchers report in the June Nature Materials. After making a batch of T59, the researchers coated a polypyrrole film with it. Next, they chemically attached cell-binding molecules to the T59 layer. When the team placed rat cells on the modified polymer, the cells stuck and proliferated.

"This is a really creative technique," says Xinyan Tracy Cui, a bioengineer at the University of Pittsburgh. Cui speculates that modified polymers like the one Schmidt's team is developing could serve not only as scaffolds for growing nerve tissue but also as coatings for brain implants, such as tremor-controlling devices for patients with Parkinson's disease. Such coatings would make the implants more biocompatible while also enhancing contact between the implant and the patient's brain cells, Cui notes.

Schmidt and her colleagues next aim to include nerve-growth factors in the conducting polymer. The goal, says Schmidt, is to place the resulting polymer scaffolding inside patients to repair damaged nerve tissues, including the spinal cord. An electric field applied to the conductive coating would stimulate nerve cells to migrate along the complex and form new tissue in target areas, she says.