Evaluating the biostability of polypyrrole microwires

Abstract

The ability to record signals from the brain has wide reaching applications in medicine and the study of the brain. Currently long term neural recording is precluded by the formation of scar tissue around the electrodes inserted into the brain. Conducting polymers present a possible solution to this problem as their biocompatibility and low stiffness could improve the quality of the interface between the electrode and the brain. In order to assess the long term stability of conducting polymers, electrodes are fabricated from polypyrrole using a variety of dopants to improve conductivity. These electrodes are then immersed in artificial cerebrospinal fluid while impedance measurements are taken over a period of days. The impedance of the electrodes increases rapidly for the first 40 hours before leveling off with only a slow increase in impedance being observed over the next 80 hours. When the ends of the electrodes are trimmed the impedance drops and then undergoes an accelerated rise and levels off. An experiment on the dimensional changes of the polypyrrole reveals that the polymer shrinks when placed into the solution. This may affect the integrity of the electrode and contribute to the increasing impedance. Further research will be necessary to understand the mechanism of the impedance increase and the electromechanical behavior of polymers with different biocompatible dopants.