A flexible protruding microelectrode array for neural interfacing in bioelectronic medicine.

Recording neural signals from delicate autonomic nerves is a challenging task that requires the development of a low-invasive neural interface with highly selective, micrometer-sized electrodes. This paper reports on the development of a three-dimensional (3D) protruding thin-film microelectrode array (MEA), which is intended to be used for recording low-amplitude neural signals from pelvic nervous structures by penetrating the nerves transversely to reduce the distance to the axons. Cylindrical gold pillars (Ø 20 or 50 µm, ~60 µm height) were fabricated on a micromachined polyimide substrate in an electroplating process. Their sidewalls were insulated with parylene C, and their tips were optionally modified by wet etching and/or the application of a titanium nitride (TiN) coating. The microelectrodes modified by these combined techniques exhibited low impedances (~7 kΩ at 1 kHz for Ø 50 µm microelectrode with the exposed surface area of ~5000 µm²) and low intrinsic noise levels. Their functionalities were evaluated in an ex vivo pilot study with mouse retinae, in which spontaneous neuronal spikes were recorded with amplitudes of up to 66 µV. This novel process strategy for fabricating flexible, 3D neural interfaces with low-impedance microelectrodes has the potential to selectively record neural signals from not only delicate structures such as retinal cells but also autonomic nerves with improved signal quality to study neural circuits and develop stimulation strategies in bioelectronic medicine, e.g., for the control of vital digestive functions.

The effects of annealing on mechanical, chemical, and physical properties and structural stability of Parylene C.

Parylene C is one of the established encapsulation polymers for chronic implants. We investigated the influence of annealing Parylene C on its mechanical properties, chemical structure, and on the stability of Parylene C - platinum - Parylene C sandwich structures as a model of flexible neural interfaces in 0.9 % saline solution. Films of Parylene C were annealed at 200 °C, 300 °C, 350 °C, and 400 °C in nitrogen atmosphere. Temperatures of 350 °C and higher as well as annealing in air destroyed the Parylene C layers; films annealed at lower temperatures showed identical infrared spectra. Higher anneal temperatures produced increased values of elongation at break, tensile and yield strength, and yield strain while at the same time Young's modulus was shown to decrease. Crystallinity increased with annealing temperature. The structural stability of sandwich structures benefitted remarkably from annealing. Sandwich structures annealed at 300 °C maintained their structural integrity during 320 days in saline solution at 37 °C and the insulation capability stayed consistently high.

Characterization of parylene C as an encapsulation material for implanted neural prostheses.

The applicability of parylene C as an encapsulation material for implanted neural prostheses was characterized and optimized. The adhesion of parylene C was tested on different substrate materials, which were commonly used in neural prostheses and the efficiency of different adhesion promotion methods was investigated. On Si(3)N(4), platinum, and on a first film of parylene C, a satisfactory adhesion was achieved with Silane A-174, which even withstood standard steam sterilization. The adhesion to gold and polyimide could not be improved sufficiently with the tested methods. Furthermore, tensile tests and measurements of the degree of crystallinity were performed on untreated, on steam sterilized, and on annealed parylene C layers to investigate the influence of thermal treatment. This led to more brittle and stiffer films due to an increase in the crystalline portion in the parylene layers. Finally, an electrochemical impedance spectroscopy was used to test if a parylene C layer was able to protect a metallic structure against corrosion on a Si(3)N(4) substrate. The results indicated that this could be only possible by treating the substrate with Silane A-174. To receive parylene C layers with a good encapsulation performance, it is important to consider the materials, which are used in the neural prosthesis, to find the best suited process parameters.

A wireless system for monitoring polymer encapsulations.

We present a principle for monitoring of the electrical properties of polymers used as insulators for electrically active implants. This method can be used for measurements in vitro and in vivo with incomplex instrumentation. The system is based on the detuning of an oscillating circuit with an interdigital electrode (IDE) structure serving as a capacitive and resistive sensor within the oscillator. This circuit is powered via an inductive link from an external coil. The phase of the external coil's impedance is used to determine the resonance frequency and quality factor of the sensing part wirelessly. The research objective is to obtain detailed information about processes at the metal/polymer interface such as a change of the capacity due to altering of the dielectric constant (i.e. uptake of water vapor or condensation of water) and lowering of the quality factor because of leakage currents. With this information it is possible to detect if the encapsulation is stable, if degradation and loss of adhesion occurs, and if the metal corrodes. The method can be used to evaluate the long term stability of materials and technologies in vitro. The future application is to monitor the stability of implant encapsulations in situ to predict failures before they occur.