Andreas Schander

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.

Current driver with read-out HV protection for neural stimulation

This paper presents a current driver with a novel high voltage (HV) switch schematic for the use as a protective switch for recording circuits during the stimulation sequence. The current driver can source and sink currents of amplitudes up to ±8.2 mA with HV tolerance from 30 V up to 120 V. The mismatch between the sourced and sinked current does not exceed 20 μA. The inter pulse current is no more than 60 pA. The output HV compliance depends on the HV supply voltage with maximum value of the 120 V. The proposed HV switch also tolerates the voltage difference up to 120V between its terminals. The chip was fabricated with AMS HV 0.35 μm CMOS technology.

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.

Fabrication of parylene channels embedded in silicon using a single parylene deposition step

In-situ integration of microfluidic channels into the microfabrication process flow of implantable microsystems is desirable, for example to enable efficient drug delivery. We propose a fabrication method for such microfluidic channels using parylene C, a biocompatible material whose inert nature favours water flow. A single deposition of parylene C enabled monolithical integration of fully-sealed micro-channels in a silicon substrate. The channel geometry was predefined by etching 100 μm-deep grooves into a silicon substrate. A PVC foil was fixed manually on the wafer and served as a top-cover for the grooves. The wafers were coated with the adhesion promoter AdPro Poly® and a 15 μm-thick parylene C film was deposited conformally into the grooves-foil enclosed space. The outgasing nature of the PVC foil hindered the adhesion of parylene C, allowing the foil to be peeled off easily from the parylene surface. The functionality of the fully-sealed parylene channels, embedded in the silicon wafer, was verified by injecting DI water with dispersed polystyrene microbeads (diameter 6 μm): the polystyrene beads were successfully transported along the channel. Further, a fully-sealed parylene chamber remained leak-tight throughout a stepwise application of hydrostatic pressures from 0.2 to 3.0 bar (15 s step-interval). In short, our parylene channels are: (1) suitable for microsystem drug-delivery; (2) in-situ enclosed hollow spaces embedded in the silicon substrate, realized with a single parylene deposition; (3) intact at hydrostatic pressures up to 3 bar.