Tunable nanostructured conducting polymers for neural interface applications

Abstract

Advancement in the development of traditional metallic-based implantable electrodes for neural interfacing has reached a plateau in recent years in terms of their ability to provide safe, long-term, and high resolution stimulation and/or recording. The reduction of electrode size enables higher selectivity through increased electrodes per implant device; however, it also results in lower sensitivity at electrode-tissue interfaces. This limitation can be addressed through the utilization of conducting polymer (CP) coatings, which increase the effective surface area. In this work, we investigate the surface roughness of two common conducting polymers; poly(pyrrole) (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) in the form of films deposited using both potentiostatic (PSTAT) and galvanostatic (GSTAT) methods. We found that the surface roughness of both CP films can be increased by over 90% through control of both deposition time and applied electrical deposition (current for GSTAT and voltage for PSTAT). The impedance of PPy-modified electrodes was found to decrease by up to 88%. This study shows that the surface roughness of CPs can be modulated to control electrical properties of neural electrodes and may improve the cellular response of neurons.

Figure 1

Fabrication of CP modified electrodes. A) Schematic of electrochemical deposition of CP on electrodes. B) Optical micrograph of PPy samples prepared using GSTAT mode for 1.0mA/cm² at 1, 2, 5, and 10 min deposition time respectively. C) Delamination of PPy film from Au surface when deposited at 0.7V for 10min. Scale bars = 5mm.

Figure 2

Scanning electron micrographs of a) bare gold, and PPy-modified electrodes fabricated in GSTAT mode at b) 0.1mA/cm² for 1min, c) 0.5mA/cm² for 2min, d) 1.0mA/cm² for 5min, and e) 1.5mA/cm² for 10 min. Scale bars = 1μm. Atomic force micrographs of the same f) bare gold electrode and PPy film at g) 0.1mA/cm² for 1min, h) 0.5mA/cm² for 2min, i) 1.0mA/cm² for 5 min, and j) 1.5mA/cm² for 10 min. Roughness increases with both deposition time and current density.

Figure 3

Impedance spectroscopy of PPy-coated electrodes deposited in GSTAT mode at A) 1min B) 2min, C) 5min, and D) 10min. Bare gold electrodes (black upsidedown triangles) are compared to 0.1mA/cm² (red squares), 0.5mA/cm² (blue circles), 1.0mA/cm² (magenta diamonds), and 1.5mA/cm² (green triangles) at each depsition time.