Heiko Stemmann

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.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.

Design and fabrication of multi-contact flexible silicon probes for intracortical floating implantation

This paper reports on a novel design and process flow development for the fabrication of multi-contact silicon probes with monolithically integrated highly flexible ribbon cables on wafer level, based on the biocompatible polymer parylene-C. Compared to the state-of-the-art silicon probes, this novel development allows for the first time a floating implantation of these neural probes in the cortex with reduced destructive forces applied to the brain tissue. In-vitro electrical impedance spectroscopy measurements and first in-vivo measurements in the cortex of a rat demonstrate the functionality of these probes.

Encoding of dynamic visual stimuli by primate area MT neurons

Neural stimulus selectivity is thought to be optimized for the representation of real-world stimuli. Neural coding properties, therefore, may adapt to different environments. Here, we address the question if tuning curves depend on the statistics of visual stimuli. This is done by studying the directional tuning of macaque area MT neurons exposed to dynamic motion stimuli of two different direction progression statistics. Despite an apparent difference of tuning curves across stimulus conditions, our results support the view that the underlying encoding system is robust and subject to only restricted malleability by stimulus statistics.

Stimulus representation in rat primary visual cortex: multi-electrode recordings with micro-machined silicon probes and estimation theory

The study of neural population codes relies on massively parallel recordings in combination with theoretically motivated analysis tools. We applied two multi-site recording techniques to

Silicon-Based Microfabrication of Free-Floating Neural Probes and Insertion Tool for Chronic Applications

Bidirectional neural interfaces for multi-channel, high-density recording and electrical stimulation of neural activity in the central nervous system are fundamental tools for neuroscience and medical applications. Especially for clinical use, these electrical interfaces must be stable over several years, which is still a major challenge due to the foreign body response of neural tissue. A feasible solution to reduce this inflammatory response is to enable a free-floating implantation of high-density, silicon-based neural probes to avoid mechanical coupling between the skull and the cortex during brain micromotion. This paper presents our latest development of a reproducible microfabrication process, which allows a monolithic integration of a highly-flexible, polyimide-based cable with a silicon-stiffened neural probe at a high resolution of 1 µm. For a precise and complete insertion of the free-floating probes into the cortex, a new silicon-based, vacuum-actuated insertion tool is presented, which can be attached to commercially available electrode drives. To reduce the electrode impedance and enable safe and stable microstimulation an additional coating with the electrical conductive polymer PEDOT:PSS is used. The long-term stability of the presented free-floating neural probes is demonstrated in vitro and in vivo. The promising results suggest the feasibility of these neural probes for chronic applications.

Implications for a Wireless, External Device System to Study Electrocorticography

Implantable neuronal interfaces to the brain are an important keystone for future medical applications. However, entering this field of research is difficult since such an implant requires components from many different areas of technology. Since the complete avoidance of wires is important due to the risk of infections and other long-term problems, means for wirelessly transmitting data and energy are a necessity which adds to the requirements. In recent literature, many high-tech components for such implants are presented with remarkable properties. However, these components are typically not freely available for such a system. Every group needs to re-develop their own solution. This raises the question if it is possible to create a reusable design for an implant and its external base-station, such that it allows other groups to use it as a starting point. In this article, we try to answer this question by presenting a design based exclusively on commercial off-the-shelf components and studying the properties of the resulting system. Following this idea, we present a fully wireless neuronal implant for simultaneously measuring electrocorticography signals at 128 locations from the surface of the brain. All design files are available as open source.