In-vitro evaluation of the long-term stability of PEDOT:PSS coated microelectrodes for chronic recording and electrical stimulation of neurons.

For the chronic application of neural prostheses long-term stable microelectrodes for electrical stimulation are essential. In recent years many developments were done to investigate different appropriate materials for these electrodes. One of these materials is the electrical conductive polymer PEDOT, which has low impedance and high charge injection capacity. However, the long-term stability of this polymer is still unclear. Thus this paper reports on the in-vitro evaluation of the long-term stability of PEDOT coated gold microelectrodes. For this purpose a flexible electrode array is used, which consists of circular gold microelectrodes. The electrodes were coated simultaneously with the polymer PEDOT:PSS using a galvanostatic electropolymerization process. After coating the array is additionally sterilized using a steam sterilization process, which is necessary prior to the implantation of such an electrode array. The long-term measurements were performed in phosphate-buffered saline solution at the constant body temperature of 37°C. For the in-vitro electrical stimulation a single channel bipolar current stimulator is used. The stimulation protocol consists of a bipolar current amplitude of 5 mA, a pulse duration of 100 μs per phase, an interphase gap of 50 μs and a frequency of 1 kHz. The electrical stimulation is performed continuously. The condition of the PEDOT coated electrodes is monitored in between with electrical impedance spectroscopy measurements. The results of this study demonstrate that the PEDOT coated electrodes are stable for at least 7 weeks of continuous stimulation, which corresponds in total to more than 4.2 billion bipolar current pulses. Also the unstimulated electrodes show currently no degradation after the time period of more than 10 months. These current results indicate an appropriate long-term stability of this electrode coating for chronic recording and electrical stimulation.