PEDOT: PSS coating on gold microelectrodes with excellent stability and high charge injection capacity for chronic neural interfaces

PEDOT: PSS has become one of the best choices as coating material of neural interfaces electrodes for its excellent biocompatibility, chemical steadiness and extremely inhomogeneous topography that facilitate reversible faradaic charge injection. However, chronic application of this material is still a matter of challenge for its poor adhesion to metallic electrodes. This study presents two new methods that increases stability and stimulation capacity of PEDOT: PSS (poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate)) coated neural interfaces electrodes. These improvements in the characteristics make PEDOT: PSS as a promising coating material for the long-term stable bidirectional neural application. The first process ensures long-term stability of PEDOT: PSS coating. The stability improvement is done by creating a rough and porous surface with selective iodine etching of Au electrodes. The pores act as anchors and increases mechanical bonding between PEDOT: PSS coating and electrode surface, hence the stability of the coating increases. PEDOT: PSS coating on iodine etched gold electrode showed 100% stability under strong ultrasonic stability test compared to non-etched electrodes. Long-term in vitro continuous stimulation for 7 days was performed to evaluate the stability of the coating. 1 μm thick PEDOT:PSS coating on iodine etched Au electrode completely survived under 604 million continuous current pulses that was evaluated by SEM image and electrochemical impedance spectroscopy (EIS). The second process improves the characteristics of PEDOT: PSS coated electrodes in terms of stimulation capacity. This is done by electrochemical modification of PEDOT: PSS coating surface by applying a cyclic voltammetry method. A perfect and effective method is developed by comparative analysis and optimizing the parameters of cyclic voltammetry. The PEDOT: PSS coating presented here (1 μm thick) reduces the impedance of bare Au electrode by around 99%. This electrochemical modification method further reduces the impedance of this PEDOT: PSS coated electrode by 33%. Moreover, this method reduces electrode polarization under stimulation by 30.5%. Reduced electrode impedance and polarization under stimulation are beneficial for increasing safe charge injection limit of the electrode for stimulation. Both processes facilitate the use of PEDOT: PSS coating as a true long-term stable and promising material on gold microelectrodes of neural interfaces.