Impedance spectroscopy and nanoindentation of conducting poly(3,4-ethylenedioxythiophene) coatings on microfabricated neural prosthetic devices Junyan Yang a) Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136 David C. Martin b) Departments of Materials Science and Engineering, and Biomedical Engineering, and Macromolecular Science and Engineering Center, University of Michigan, Ann Arbor, Michigan 48109-2136 (Received 21 September 2005; accepted 16 December 2005) The electrical and mechanical properties of conducting polymer poly(3,4-ethylenedioxythiophene) coatings on microfabricated neural probes have been evaluated by electrochemical impedance spectroscopy and nanoindentation techniques. Our results reveal that for poly(3,4-ethylenedioxythiophene) coatings, the minimum impedance correlates well with the mechanical properties. The lowest impedance films are also those that are the softest. This is consistent with microstructural observations by atomic force microscopy and scanning electron microscopy showing an increase in the effective surface area (“fuzziness”) of the coatings. The presence of these conducting polymer coatings provides an intermediate step along the interface between the devices and brain tissue. This information provides clues for the design of strategies for improving the long-term performance of these electrodes in vivo. I. INTRODUCTION Microfabricated silicon-based neural prosthetic de- vices facilitate the functional stimulation of and record- ing from neurons of the central nervous system. The bulk modulus of silicon is ∼170 GPa, whereas a value of ∼0.1 MPa for the modulus of human brain was obtained. 1 This corresponds to a 7-order of magnitude difference between the modulus of devices and brain tissue. This may lead to local strains at the sample surface during chronic implantations in living tissue that could enhance glial cell inflammation and thus reduce the biocompat- ibility of the device. The conducting polymer poly(3,4- ethylenedioxythiophene) (PEDOT) has been used for biomedical applications because of its excellent long- term stability and relatively high transparency. 2,3 This material exhibits significantly better electrical conductiv- ity and chemical stability than polypyrrole (PPy). 4 In our laboratory, we have been investigating the use of con- ducting PEDOT coatings for improving the long-term performance of microfabricated neural prosthetic devices that are directly implanted into the central nervous system. We have found that soft, low impedance, and biologically active conducting PEDOT coatings can be prepared by electrochemical deposition on the electrode sites. 5 More recently, we also have explored a number of methods to create features of well-defined size and high surface area in nanostructured conducting PEDOT thin films using templating and surfactant-mediated tech- niques. 6–8 By correlating measurements of probe electri- cal properties with their surface morphologies, we have found that maximizing the effective surface area of the electrode coating makes it possible to minimize the elec- trical impedance. This is consistent with the interpreta- tion that the high surface area of the films promotes the most facile charge transport. The presence of conducting polymer thin films on neu- ral probes can provide an intermediate step along the interface between the devices and brain tissues. How- ever, the conducting polymer thin films may have differ- ent mechanical properties from the bulk. To monitor the coating property changes with polymerization condition, methods of mechanical testing with the ability to char- acterize surface properties on a micron to nanometer scale spatial resolution are required. There are a number of methods for measuring the mechanical properties (hardness, stiffness, and modulus) of polymer thin films or coatings on substrates, such as peeling, scratching, a) Present address: Dow Chemical Company, Freeport, TX 77541 b) Address all correspondence to this author. e-mail: milty@umich.edu DOI: 10.1557/JMR.2006.0145 J. Mater. Res., Vol. 21, No. 5, May 2006 © 2006 Materials Research Society 1124