Conducting-Polymer Nanotubes Improve Electrical Properties of Neural Electrodes

An in vitro comparison of conducting-polymer nanotubes of poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(pyrrole) (PPy) and to their film counterparts is reported. Impedance, charge-capacity density (CCD), tendency towards delamination, and neurite outgrowth are compared. For the same deposition charge density, PPy films and nanotubes grow relatively faster vertically, while PEDOT films and nanotubes grow more laterally. For the same deposition charge density (1.44 C cm À2), PPy nanotubes and PEDOT nanotubes have lower impedance (19.5 AE 2.1 kV for PPy nano-tubes and 2.5 AE 1.4 kV for PEDOT nanotubes at 1 kHz) and higher CCD (184 AE 5.3 mC cm À2 for PPy nanotubes and 392 AE 6.2 mC cm À2 for PEDOT nanotubes) compared to their film counterparts. However, PEDOT nanotubes decrease the impedance of neural-electrode sites by about two orders of magnitude (bare iridium 468.8 AE 13.3 kV at 1 kHz) and increase capacity of charge density by about three orders of magnitude (bare iridium 0.1 AE 0.5 mC cm À2). During cyclic voltammetry measurements, both PPy and PEDOT nanotubes remain adherent on the surface of the silicon dioxide while PPy and PEDOT films delaminate. In experiments of primary neurons with conducting-polymer nanotubes, cultured dorsal root ganglion explants remain more intact and exhibit longer neurites (1400 AE 95 mm for PPy nanotubes and 2100 AE 150 mm for PEDOT nanotubes) than their film counterparts. These findings suggest that conducting-polymer nanotubes may improve the long-term function of neural microelectrodes.