Electrochemical Stability of Electrodeposited PEDOT:PSS Neural Stimulation Electrodes

Microelectrode arrays are used for neural stimulation and recording; however, their performance is limited by the high impedance and low charge-injection capacity of conventional metallic interfaces. To overcome these challenges, a variety of coatings, including metal oxides, carbon nanotubes, and conductive polymers, are applied to improve the electrochemical performance of microelectrode arrays. Among these materials, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has gained significant attention due to its low impedance, high charge-injection capacity, and compatibility with microfabrication processes. Despite these advantages, the long-term electrochemical stability of PEDOT:PSS at high levels of stimulation charge injection remains insufficiently understood. In this study, we investigated the stability of PEDOT:PSS films electrodeposited on gold microelectrodes for prolonged electrical stimulation. Voltage transient (VT) analysis revealed that PEDOT:PSS degradation occurs when the maximum cathodic excursion (E_mc) exceeds -0.6 V versus Ag|AgCl. The degradation is accompanied by the appearance of asymmetric access voltages and a voltage inversion in the anodal phase of the VT. Electrochemical impedance spectroscopy (EIS) revealed a sharp increase in PEDOT:PSS impedance at negative DC bias potentials, beginning at -0.5 V and increasing as the potential became more negative. These results identified -0.6 V as an electrochemical limit for PEDOT:PSS stability on gold and implicate the reduction and de-doping of PEDOT:PSS, and sequalae, in the observed PEDOT:PSS degradation. Nonetheless, PEDOT:PSS electrodes had stable charge-injection capacities of 3.6 mC/cm² (200 µs pulse width) over the course of 200 million pulses.