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In vivo neuronal action potential recordings via three-dimensional microscale needle-electrode arrays

Very fine needle-electrode arrays potentially offer both low invasiveness and high spatial resolution of electrophysiological neuronal recordings in vivo. Herein we report the penetrating and recording capabilities of silicon-growth-based three-dimensional microscale-diameter needle-electrodes arrays. The fabricated needles exhibit a circular-cone shape with a 3-μm-diameter tip and a 210-μm length. Due to the microscale diameter, our silicon needles are more flexible than other microfabricated silicon needles with larger diameters. Coating the microscale-needle-tip with platinum black results in an impedance of ~600 kΩ in saline with output/input signal amplitude ratios of more than 90% at 40 Hz–10 kHz. The needles can penetrate into the whisker barrel area of a rat's cerebral cortex, and the action potentials recorded from some neurons exhibit peak-to-peak amplitudes of ~300 μVpp. These results demonstrate the feasibility of in vivo neuronal action potential recordings with a microscale needle-electrode array fabricated using silicon growth technology.


Supplementary Text
Modeling Electrical Characteristics of the Needle-electrode Device.
The electrical impedance of the recording-site is an important electrode characteristic for neural recordings.
Because the electrode and embedded parasitic impedances in the recording system construct the voltage divide configuration, both detected neural and phase delayed signals are attenuated (1). The parasitic impedances in this work are explained by considering the parasitic capacitances of the needle-electrode chip and the cable line between the needle-electrode chip and the external buffer amplifier.
Supplementary Figure S1a depicts an equivalent circuit of the recording system in extracellular neural recordings. Z e is the metal/electrolyte (electrode/saline) interfacial impedance at the needle tip, and consists of the spread resistance (R sp ), charge transfer resistance (R ct ), and constant phase element (CPE) (2) shown in where Q is the measured magnitude of CPE, n is constant parameter that depends on the inhomogeneous surface of the electrode. C pa is the parasitic capacitance of the needle electrode chip between the metal interconnection and the electrolyte, and C oxide is another parasitic capacitance between the interconnection and the silicon substrate.
C line is due to recording cable from the chip-bonding pad to the external buffer amplifier. In our previous report (1), the detected signals during neural recording can be calculated with the voltage divide configuration of the , where V Ein (jω) is the input signal, which is the voltage due to extracellular neural activity. V Eout (jω) is the output signal obtained as the input voltage of the buffer amplifier through the needle-electrode chip, the flexible printed circuit (FPC), and the cable line. Z p is the shunt impedance, configuring parasitic capacitances (C pa , C oxide , and C line ), and input impedances of the buffer amplifier (R a , C a ) as defined in equation (3). The transfer function in a neural recording θ argH jω .
The absolute value of the transfer function H(jω) depicted in equation (5) indicates that higher output/input (O/I) signal amplitude ratios are obtained as the needle-electrode impedance (Z e ) decreases. In our previous study (3), low O/I signal ratios (less than 57%) were observed due to more than 2-MΩ impedance microneedle electrodes (electrode material: Au, tip diameter: 2 µm). Such high impedance characteristics make the neuronal spikes (the center of frequency ≈ 1 kHz) difficult to record (1). In addition, equation (6) Fig. S1b).

Protocol for Immunohistochemical Staining.
Sections (20-µm thick) were immunohistochemically stained by a rabbit polyclonal antibody (Iba1 antibody; BIOCARE Medical LLC, Concord, USA). The protocol was arranged from those used in the previous literature (4,5). Briefly, sections mounted on glass slides were rinsed for five min in deionized water (DW), and then (ImmPRESS Reagent, Anti-Rabbit IgG, MP-7401; Vector Laboratories, Inc.). The sections were subsequently exposed to a diaminobenzidine substrate solution (ImmPACT DAB, SK-4105; Vector Laboratories, Inc.) for five min. The DAB reaction was terminated by rinsing the sections thrice for five min each time in DW. After counterstaining with hematoxylin (Hematoxylin QS, H-3404; Vector Laboratories, Inc.), the slides were protected with a coverslip. As the negative control, normal rabbit immunoglobulin G (IgG; AB-105-C; R&D Systems, Inc., Minneapolis, USA), was applied in the primary-antibody-reaction process instead of the Iba1 antibody.