A flexible protruding microelectrode array for neural interfacing in bioelectronic medicine

Abstract

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.

Keywords: Engineering; Materials science.

Fig. 1. Schematic principle of bioelectronic medicine.

Implanted electrodes address a variety of peripheral nerves to record neurophysiological signals. The data are decoded and processed to specify appropriate stimulation patterns and accurate dosage to modulate pathological signals. Moreover, metabolic biomarkers are obtained from the body to provide feedback, creating a closed-loop system for the automatically controlled regulation of organ function. In our approach, we aim to address pelvic neural structures with a flexible, protruding microelectrode array

Fig. 2. Design and fabrication process of the 3D neural interface.

a Sketch of the neural interface with protruding microelectrodes decreasing the distance to the neurons. b Layout of neural interface design with dimensions and compartment descriptions. c Process flow for the microfabrication of a flexible polyimide-based 3D MEA. d Schematic view of the MFI technique connecting a flexible MEA with a rigid printed circuit board (PCB). A tiny gold ball (~Ø 90 µm) is formed at the capillary by the flame-off electrode. The gold ball is pushed against the hole of the MEA contact pad onto the heated PCB bond pad with a defined force and ultrasonic energy connecting the components mechanically and electrically. Finally, the capillary is lifted, and the gold wire is cut off, leaving a tiny gold stud that acts as a microrivet between the MEA and the PCB

Fig. 3. Different gold pillar modification methods.

Type A: Gold pillar side-insulated with parylene C and wet etched. Type B: Gold pillar side-insulated with parylene C, wet etched, and coated with TiN

Fig. 4. Fabricated 3D neural interface.

a Photograph of the fabricated neural device showing the flexible MEA with 3D microelectrodes connected to a PCB with a soldered Omnetics connector. b Assembling of the neural interface. b-i Macroscopic image of the MEA connector with the corresponding PCB. b-ii Scanning electron microscopy (SEM) image of an assembled neural device with finger structures electrically connected by microrivets to the underlying PCB. b-iii SEM image of a single gold bump stud connecting the neural interface contact pad to the PCB bond pad. The entire hole is filled and covered by the gold stud, which acts as a microrivet. c Macroscopic images of the neural interface with compartment descriptions and functions of the entire measurement field (c-i) and of the area with electrodes arranged in a chessboard pattern (c-ii). The electrode heads were coated with TiN, which appears black in the images. d Macroscopic images (d-i, d-ii) and SEM images (d-iii, d-iv) of an MEA and of a single microelectrode (Ø 50 µm, height ~50 µm). The pillars exhibit parylene C side insulation, and their heads were wet etched and coated with TiN. The microelectrode shown in d-iv has a TiN coating on its head of 4–5 µm height. e SEM images of single modified microelectrodes, which were either wet etched (e-i, e-ii) or wet etched and TiN-coated (e-iii, e-iv) (both Ø 20 µm with a height of ~60 µm). The pillars were etched by RIE (Fig. 2c, step i) until the parylene C was removed from the exposed tops and sides of the pillar heads. Scale bars: b-i: 2 mm, c-ii: 1 mm; b-ii: 500 µm; d-iii: 200 µm; b-iii, d-iv, e-i, e-iii: 20 µm; e-ii, e-iv: 1 µm

Fig. 5. Electrical characterization of the fabricated 3D microelectrodes and performance of the developed neural interface in ex vivo experiments with mouse retinae.

a Schematic of a three-electrode electrochemical system with the neural interface as the working electrode (WE), a platinum net as the counter electrode (CE) and an Ag/AgCl (3 M KCl) electrode as the reference electrode (RE). b Ex vivo experimental description of spike activity recordings on mouse retinae using the fabricated neural device. c, d Electrical in vitro characterization of different electrodes with a pillar diameter of 50 µm and a pillar height of 60 µm. For each sample, the electrode electrical impedance magnitude |Z| and phase were measured by applying a voltage of 100 mV and sweeping the frequency from 1 Hz to 100 kHz. The noise was calculated from the real part Re(Z) of the impedance Z. The impedance magnitude, phase and noise were averaged for each sample over the electrode number n. c-i, c-ii, c-iii Electrical characterization of gold electrodes encapsulated with parylene C, opened by RIE to a specific height and wet etched (Fig. 3, type A). The averaged impedance magnitude, phase and noise of electrodes with head heights of 9.3 µm (black), 13.3 µm (orange), 22.8 µm (blue), and 60.0 µm (green) are shown in individual diagrams, with the respective standard deviation indicated as the shaded area. d-i, d-ii, d-iii Electrical characterization of gold electrodes encapsulated with parylene C, opened by RIE, wet etched and coated with TiN to a specific height (Fig. 3, type B). The averaged impedance magnitude, phase and noise of electrodes with head heights of 8.2 µm (black), 12.5 µm (orange), and 19.4 µm (blue) are shown in individual diagrams, with the respective standard deviation indicated as the shaded area. e Electrical characterization of type B electrodes with a diameter of 20 µm and a 20.7 µm high head (with an overall height of 60 µm). The averaged impedance magnitude (black) and noise (light blue) are shown in a single diagram, with the averaged phase displayed in the right-hand corner. The corresponding values for |Z|_1kHz and f_cutoff are given in the box on the right side of the diagram. f Recording of retinae spike activity with a type B electrode of diameter 20 µm with an exposed head of 23.8 µm height. A magnification of a single spike is shown in the top right corner of the diagram