Impedance spectroscopy and nanoindentation of conducting poly(3,4-ethylenedioxythiophene) coatings on microfabricated ne
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David C. Martinb) 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 devices facilitate the functional stimulation of and recording 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 biocompatibility of the device. The conducting polymer poly(3,4ethylenedioxythiophene) (PEDOT) has been used for biomedical applications because of its excellent longterm stability and relatively high transparency.2,3 This material exhibits significantly better electrical conductivity and chemical stability than polypyrrole (PPy).4 In our laboratory, we have been investigating the use of conducting PEDOT coatings for improving the long-term performance of microfabricated neural prosthetic devices that are directly implanted into the central nervous a)
Present address: Dow Chemical Company, Freeport, TX 77541 Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0145 b)
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J. Mater. Res., Vol. 21, No. 5, May 2006 Downloaded: 25 Mar 2015
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 techniques.6–8 By correlating measurements of probe electrical properties with their surface morphologies, we have found that maximizing the effective surface area of
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