Fig. 1: Representative schematic and SEM images of fabricated bipolar electrode array, microscopic images of packaged electrodes integrated with printed circuit board, and impedance-frequency characteristic.

a A cross-sectional schematic (ai) and a SEM image (aii) after the formation of the microneedle array. b Same as a but after selective exposure of the upper insulation layer. c Same as a but after the final fabrication step. d A magnified view of a single microneedle highlighting the microtip electrode, local ground electrode, and insulation layers (di). Two white dashed lines indicate the microtip electrode and local ground electrode, with two insulation layers are positioned between two electrodes and beneath the local ground electrode. Yellow dashed boxes indicate the locations of cross-sectional images for (dii) and (diii) obtained with FIB etching process. e A microscope image of the fabricated bipolar electrodes showing through-glass vias. f A microscope image of metal pads and bonding wires. See Figures S2 and S3 for additional details. g Averaged impedance-frequency curve from randomly chosen bipolar electrodes (n = 10). Red dot indicates the impedance at 1 kHz which is considered to be crucial for neural signal recordings. Error bars indicate standard deviation