High Density Carbon Fiber Array Assembly Machine for Brain Electrophysiological Recordings

Abstract

Carbon fiber electrodes (CFEs) are ideal for chronic electrophysiological recordings because of their small diameter (5 to 10 μm), high Young’s modulus (241 GPa), and low electrical resistivity. To minimize buckling during CFE array implantation, electrodes are sharpened to reduce the insertion force and attached to a support structure to reduce the unsupported length. The thin carbon fiber (CF) requires great hand stability and is labor-intensive to cut, sharpen, and assemble the array. The aim of this research is to develop a laser CF cutting and sharpening method enabling custom electrode lengths and to build a high density carbon fiber (HDCF) array assembly machine that can precisely place the CFEs along with the thin silicon support structure (SSS). The first study established a CFE cutting, sharpening, and insulation removal method using a moving laser beam. Custom lengths of CFEs in an array were achieved in one cut by designing the laser moving path. The combination of high laser input voltage (5 DCV) and large moving speed (25 μm/s) resulted in a sharp tip (27.9°) and short insulation stripped length (122.5 μm). Custom electrode lengths of 1.2, 1.2, 1.6, and 1.4 mm were realized in a CFE array with 200 μm pitch. The laser sharpened CFEs had an electrochemical impedance spectroscopy (EIS) measurement of 460 ± 225 kΩ at 1 kHz, which was significantly lower than that of scissor cut or torch burned CFEs. The in-vivo recording performance in rat motor cortex was comparable with CFEs sharpened with torch burning. The second study developed two image-based algorithms for aligning the CF with other parts of the HDCF array as the foundation for the HDCF assembly machine. The HDCF array has a printed circuit board (PCB) with an Omnetics connector and a SSS with gold pads and shanks. The Algorithm I measured the difference between the orientation of the CF and the shanks and corrected the deviation by rotating the CFE. The deviation angle was reduced from 1.16 ± 0.94 to 0.54 ± 0.51 degrees after rotation compensation. Algorithm II calculated the distance between the CF tip and the target position on gold pads and translated the PCB to get them aligned. The error was less than 40 μm for 91.7% of the placement tests. The third study constructed an HDCF array assembly machine for automatic precise placement of CFEs, integrating the algorithms developed in the second study. CFEs were placed along shanks of SSS using the machine, which had four subsystems: 1) a roller-based extruder adapted from an open-source system, 2) a motion system, 3) an imaging system, and 4) a laser cutter. The CF was extruded from the extruder, rotated parallel to and translated to place along a trench in the shank of SSS by the motion system, and cut off by the laser. With the Parylene C coating on its tube, the extruder achieved 4.009 ± 0.073 mm for a target of 4 mm in extrusion length. The delicate and stable adjustment of CF orientation and SSS position ensured their great alignment. Two HDCF arrays of 16 and 8 CFEs with 80 μm pitch were assembled without manual manipulation of CF after the initial loading. Future work of this research includes automating the water dispensing and expanding the application to arrays with other electrode materials and configurations.