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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.

Recordings of epidural field potentials (EFPs) allow neuronal activity to be acquired over a large region of cortical tissue with minimal invasiveness. Because electrodes are placed on top of the dura and do not enter the neuronal tissue, EFPs offer intriguing options for both clinical and basic science research. On the other hand, EFPs represent the integrated activity of larger neuronal populations and possess a higher trial-by-trial variability and a reduced signal-to-noise ratio due the additional barrier of the dura. It is thus unclear whether and to what extent EFPs have sufficient spatial selectivity to allow for conclusions about the underlying functional cortical architecture, and whether single EFP trials provide enough information on the short timescales relevant for many clinical and basic neuroscience purposes. We used the high spatial resolution of primary visual cortex to address these issues and investigated the extent to which very short EFP traces allow reliable decoding of spatial information. We briefly presented different visual objects at one of nine closely adjacent locations and recorded neuronal activity with a high-density epidural multielectrode array in three macaque monkeys. With the use of receiver operating characteristics (ROC) to identify the most informative data, machine-learning algorithms provided close-to-perfect classification rates for all 27 stimulus conditions. A binary classifier applying a simple max function on ROC-selected data further showed that single trials might be classified with 100% performance even without advanced offline classifiers. Thus, although highly variable, EFPs constitute an extremely valuable source of information and offer new perspectives for minimally invasive recording of large-scale networks. NEW & NOTEWORTHY Epidural field potential (EFP) recordings provide a minimally invasive approach to investigate large-scale neural networks, but little is known about whether they possess the required specificity for basic and clinical neuroscience. By making use of the spatial selectivity of primary visual cortex, we show that single-trial information can be decoded with close-to-perfect performance, even without using advanced classifiers and based on very few data. This labels EFPs as a highly attractive and widely usable signal.