Takuya Harashima

Design and evaluation of area-efficient and wide-range impedance analysis circuit for multichannel high-quality brain signal recording system

To enable chronic and stable neural recording, we have been developing an implantable multichannel neural recording system with impedance analysis functions. One of the important things for high-quality neural signal recording is to maintain well interfaces between recording electrodes and tissues. We have proposed an impedance analysis circuit with a very small circuit area, which is implemented in a multichannel neural recording and stimulating system. In this paper, we focused on the design of an impedance analysis circuit configuration and the evaluation of a minimal voltage measurement unit. The proposed circuit has a very small circuit area of 0.23 mm2 designed with 0.18 µm CMOS technology and can measure interface impedances between recording electrodes and tissues in ultrawide ranges from 100 Ω to 10 MΩ. In addition, we also successfully acquired interface impedances using the proposed circuit in agarose gel experiments.

Evaluation of insertion characteristics of less invasive Si optoneural probe with embedded optical fiber

A less invasive Si optoneural probe with an embedded optical fiber was proposed and successfully fabricated. The diameter of the optical fiber was completely controlled by hydrogen fluoride etching, and the thinned optical fiber can propagate light without any leakage. This optical fiber was embedded in a trench formed inside a probe shank, which causes less damage to tissues. In addition, it was confirmed that the optical fiber embedded in the probe shank successfully irradiated light to optically stimulate gene transfected neurons. The electrochemical impedance of the probe did not change despite the light irradiation. Furthermore, probe insertion characteristics were evaluated in detail and less invasive insertion was clearly indicated for the Si optoneural probe with the embedded optical fiber compared with conventional optical neural probes. This neural probe with the embedded optical fiber can be used as a simple and easy tool for optogenetics and brain science.

Development of Si neural probe with piezoresistive force sensor for minimally invasive and precise monitoring of insertion forces

A Si neural probe is one of the most important tools for neurophysiology and brain science because of its various functions such as optical stimulation and drug delivery. However, the Si neural probe is not robust compared with a metal tetrode, and could be broken by mechanical stress caused by insertion to the brain. Therefore, the Si neural probe becomes more useful if it has a stress sensor that can measure mechanical forces applied to the probe so as not to be broken. In this paper, we proposed and fabricated the Si neural probe with a piezoresistive force sensor for minimally invasive and precise monitoring of insertion forces. The fabricated piezoresistive force sensor accurately measured forces and successfully detected insertion events without buckling or bending in the shank of the Si neural probe. This Si neural probe with a piezoresistive force sensor has become one of the most versatile tools for neurophysiology and brain science.