The performance of chronically implanted neural electrodes is dependent on the mechanical and electrical properties of the electrode as well as those of the electrode-tissue interface. We report on the electrochemical fabrication of PEDOT:heparin-coated gold nano-electrodes (hereafter called composite electrodes) and focus on optimizing the electrode-parameters of size, Young's modulus, electrical conductivity, and electrode-medium capacitance. The diameter and the Young's Modulus of neural electrodes determine the buckling threshold during penetration, and the size of the "kill zone" and the degree of immunological response (gliosis) once the electrode has been implanted. The composite electrodes reported here have diameters of ~500 nm. Their lengths, determined by the grower, can be hundreds of microns. The Young's modulus of the polymeric coating is 2.0 GPa. We demonstrate the insertion of ~100 micron long electrodes into single living cells as well as into dense aggregates of ~hundreds of cells (approximating brain tissue). This feat is possible because the Euler stress of the electrode exceeds the critical rupture stress of the cellular aggregate. Additionally, the "kill zone" is minimized by the electrodes' ~500 nm bore, which is much smaller than a cell diameter. The reduced size of the "kill zone" and the Young's modulus of the PEDOT:heparin coating (relative to conventional metals) are expected to reduce the degree of gliosis after implantation. The electrical conductivity and the capacitance of the electrode-medium interface strongly affect the bandwidth, noise, and signal attenuation of neural electrodes. By introducing a single crystalline gold core, these composite electrodes attain conductivities in excess of 103 S cm-1, significantly larger than the PEDOT nanofibers that we have previously grown.1 The capacitive properties of these electrodes in PBS buffer solutions, as determined by measurements of their galvanostatic voltage-transients, will be discussed and related to their effective RC time constants and their ability to deliver high fidelity signals to an external amplifier. Taken together, these results show that these electrodes are reasonably successful at optimizing the contradictory set of requirements (as described above) for neural probes and, thereby, are interesting candidates for next-generation neural implants.
1. PS Thapa, DJ Yu, JP Wicksted, JA Hadwiger, JN Barisci, R Baughman & BN Flanders, "Directional growth of polypyrrole and polythiophene nanowires", Appl. Phys. Lett. 94, 033104, (2009).
*Supported by the NIH BRAIN Initiative (1 R21EY026392).