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Objective. Neural interfaces are an essential tool to enable the human body to directly communicate with machines such as computers or prosthetic robotic arms. Since invasive electrodes can be located closer to target neurons, they have advantages such as precision in stimulation and high signal-to-noise ratio (SNR) in recording, while they often exhibit unstable performance in long-term in-vivo implantation because of the tissue damage caused by the electrodes insertion. In the present study, we investigated the electrical functionality of flexible penetrating microelectrode arrays (FPMAs) up to 3 months in in-vivo conditions. Approach. The in-vivo experiment was performed by implanting FPMAs in five rats. The in-vivo impedance as well as the action potential (AP) amplitude and SNR were analyzed over weeks. Additionally, APs were tracked over time to investigate the possibility of single neuron recording. Main results. It was observed that the FPMAs exhibited dramatic increases in impedance for the first 4 weeks after implantation, accompanied by decreases in AP amplitude. However, the increase/decrease in AP amplitude was always accompanied by the increase/decrease in background noise, resulting in quite consistently maintained SNRs. After 4 weeks of implantation, we observed two distinctive issues regarding long-term implantation, each caused by chronic tissue responses or by the delamination of insulation layer. The results demonstrate that the FPMAs successfully recorded neuronal signals up to 12 weeks, with very stably maintained SNRs, reduced by only 16.1% on average compared to the first recordings, although biological tissue reactions or physical degradation of the FPMA were present. Significance. The fabricated FPMAs successfully recorded intracortical signals for 3 months. The SNR was maintained up to 3 months and the chronic function of FPMA was comparable with other silicon based implantable electrodes.

For implantable devices, Parylene C (hereafter referred to as Parylene) has shown promising properties such as flexibility, biocompatibility, biostability, and good barrier properties. Parylene-based flexible interconnection cable (FIC) was previously developed to connect a flexible penetrating microelectrode array (FPMA) with a recording system. However, Parylene-based FIC was difficult to handle and prone to damage during the implantation surgery because of its low mechanical strength. To improve the mechanical properties of the FIC, we suggest a mechanically enhanced flexible interconnection cable (enhanced FIC) obtained using a combination of Parylene and polyimide. To investigate the long-term stability of the enhanced FIC, Parylene-only FIC, and enhanced FIC were tested and their mechanical properties were compared under an accelerated aging condition. During the course of six months of soaking, the maximum strength of the enhanced FIC remained twice as high as that of the Parylene-only FIC throughout the experiment, although the mechanical strength of both FICs decreased over time. To show the capability of the enhanced FIC in the context of nerve signal recording as a part of a neural interfacing device, it was assembled together with the FPMA and custom-made wireless recording electronics. We demonstrated the feasibility of the enhanced FIC in an in vivo application by recording acute nerve signals from canine sciatic nerves.