Highly stable carbon nanotube doped poly(3,4-ethylenedioxythiophene) for chronic neural stimulation

Abstract

The function and longevity of implantable microelectrodes for chronic neural stimulation depends heavily on the electrode materials, which need to present high charge injection capability and high stability. While conducting polymers have been coated on neural microelectrodes and shown promising properties for chronic stimulation, their practical applications have been limited due to unsatisfying stability. Here, poly(3,4-ethylenedioxythiophene) (PEDOT) doped with pure carbon nanotubes (CNTs) was electrochemically deposited on Pt microelectrodes to evaluate its properties for chronic stimulation. The PEDOT/CNT coated microelectrodes demonstrated much lower impedance than the bare Pt, and the PEDOT/CNT film exhibited excellent stability. For both acute and chronic stimulation tests, there is no significant increase in the impedance of the PEDOT/CNT coated microelectrodes, and none of the PEDOT/CNT films show any cracks or delamination, which have been the limitation for many conducting polymer coatings on neural electrodes. The charge injection limit of the Pt microelectrode was significantly increased to 2.5 mC/cm(2) with the PEDOT/CNT coating. Further in vitro experiments also showed that the PEDOT/CNT coatings are non-toxic and support the growth of neurons. It is expected that this highly stable PEDOT/CNT composite may serve as excellent new material for neural electrodes.

Figure 1

SEM images of PEDOT/CNT coatings on Pt microelectrodes. The films were electrodeposited at a constant current of 100 nA for (a) 100, (b) 200, (c) 300 and (d) 400 seconds.

Figure 2

(a) Cyclic voltammograms of bare Pt electrode and PEDOT/CNT coated electrodes before and after long-term stimulation. (b) The charge capacity of coated electrodes as a function of the PEDOT/CNT electrodeposition charge.

Figure 3

Electrochemical impedance spectroscopy of Pt electrodes coated with PEDOT/CNT for 50, 100, 200, 300 and 400 seconds in comparison to the bare Pt electrode.

Figure 4

Voltage excursions of the PEDOT/CNT coated and bare Pt electrodes under the biphasic current stimulation. The electrodes were stimulated in PBS under a charge-balanced, cathodic-first, biphasic pulse current at 1 mC/cm² at 50 Hz (the top curve).

Figure 5

Monitored electrode impedance changes at 1K Hz over time during the three-month soaking in PBS. Plots 1–4 in red are bare Pt electrodes; plots 5–6 in blue are ultrathin PEDOT/CNT (deposition charge less than 5 μC) coated Pt electrodes with stimulation; plots 7–10 in black and plots 11–14 in green are normal PEDOT/CNT (deposition charge more than 10 μC) coated Pt electrodes with (7–10) and without (11–14) stimulation.

Figure 6

SEM images of PEDOT/CNT coating on Pt microelectrode after electrical stimulation with (a) lower and (b) higher magnifications.

Figure 7

Fluorescent (a) and SEM (b) images of neurons cultured on PEDOT/CNT surfaces. For the fluorescent image, the scale bar represents 100μm. The inset in (b) shows the SEM of neurites grown on the PEDOT/CNT surface with high magnification.