Fabrication and Performance of Liquid Crystal Polymer (LCP)-based Neural Probes: Four-sided Probe and Optrode

Abstract

A novel liquid-crystal polymer (LCP)-based neural probe with four-sided electrode sites is developed. Ideally, neural probes should have channels with a three-dimensional (3-D) configuration to record 3-D neural circuits. Many types of three-dimensional neural probes have been developed

however, most of them were formulated as an array of multiple shanks with electrode sites located along one side of the shanks. The proposed LCP-based neural probe has electrode sites on four sides of the shank, i.e., the front, back and two side walls. To generate the suggested configuration of the electrode sites, a thermal lamination process involving LCP films and laser micromachining are used. Using the proposed novel four-sided neural probe, in vivo multichannel neural recording is successfully performed in the mouse primary somatosensory cortex. The multichannel neural recording shows that the proposed four-sided neural probe can record spiking activities from a diverse neuronal population compared to neural probes with single-sided electrodes. This is confirmed by a pair-wise Pearson correlation coefficient (Pearson's r) analysis and a cross- correlation analysis. This study also presents the development of LCP-based depth-type stimulation electrodes with a high charge storage capacity using electrodeposited iridium oxide film (EIROF). On the electrode sites, iridium oxide is electrodeposited to increase the charge storage capacity for facilitating neural stimulation. After electrodeposition using different numbers of rectangular voltage pulses and triangular waveforms, the iridium oxide electrodes are characterized in terms of charge storage capacity and electrochemical impedance. And the surfaces of EIROFs are examined using atomic force microscopy (AFM) and scanning electron microscopy (SEM). In addition, the elementary composition of the EIROF surfaces is quantitatively determined using X-ray photoelectron spectroscopy (XPS). The in vivo neural experiments verified the feasibility of the proposed LCP-based depth-type stimulation electrode. Additionally, LCP-based optrode is suggested for optical stimulation and electrical recording. The suggested neural probes have four contacts at the tip of the electrode shank. After thermally laminating the LCP films, the four tip electrodes are made by cut-exposing the thickness of the electroplated metals. The four tip electrodes have enough contact areas and electrochemical impedance to ensure good quality of neural signal recordings. After the laser cutting process, an optic fiber is integrated to the neural probes. To demonstrate optical stimulation and electrical recording capability of the fabricated LCP-based optrode, in vivo experiments are done. Spontaneous activity and light-evoked activity are successfully recorded from the cortex and the deep brain area.