A floating 5-µm-diameter needle-electrode on the tissue for damage-reduced chronic neuronal recording in mice*
K. Yamashita, a H. Sawahata, b S. Yamagiwa, a S. Yokoyama, c R. Numano, d,e K. Koida, d,f and T. Kawano a, da.
Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, Japanb. National Institute of Technology, Ibaraki College, Japanc. TechnoPro, Inc., TechnoPro R&D, Company, Japand.
Electronics-Interdisciplinary Research Institute (EIIRIS), Toyohashi University of Technology, Japane.
Department of Applied Chemistry and Life Science, Toyohashi University of Technology, Japanf. Department of Computer Science and Engineering, Toyohashi University of Technology, JapanAbstract Microelectrode technology is essential in electrophysiology and has made contributions to neuroscience as well as to medical applications.

However, it is necessary to minimize tissue damage associated with needle-like electrode into the brain tissue and the implantation surgery, which make thestable chronic recording impossible. Here, we report on an approach to use a 5-μm-diameter needle electrode, which enables the following of tissue motions bythe surgical method. The electrode is placed on the brain tissue of the mouse with a dissolvable material while reducing the physical stress to the tissue; this isfollowed by implantation of the device in the brain without fixing it to the cranium while achieving a floating electrode on the tissue. The electrode shows thestable recording with no significant degradation of the signal-to-noise ratios for 6 months, and minimized tissue damage is confirmed compared to that whenusing the other cranium-fixed electrodes with the same needle geometry.
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ElectrodedeviceWire(Stainless)Comparison of electrode implantation. a) Schematic showing conventional implantation,in which the electrode is fixed to the cranium of the brain. b) Schematic showing theproposed implantation, in which the electrode packaged with a flexible lead is implantedin the brain without fixing to the cranium, achieving a “floating electrode” on the tissue.

d1

d2

d3

(0 min)(2 min)(6 min)An electrode device. a) Schematic of the electrode device, containing a needle electrodewith a diameter of 5-µm and length of 400-µm. b) Assembly of the electrode device on apin connector via the PEG paste. c) SEM images of the overall needle and the needle
tip portion. d) Placement of the floating electrode on the brain tissue of a mouse.

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14 days (2-week)

180 days (6-month)

Chronic in vivo neuronal recording. a) Schematic of the recording with visual stimulation provided by a light-emitting diode b) Photographs of a mouse implanted with a floating
electrode. Each device is implanted into the mouse’s visual cortex. c1–4) Waveforms recorded from the pin-type electrode 1 day after implantation; c1) an average waveform of lowfrequency band signals (filtering=10–80 Hz, n=100 trials), c2) a single high-frequency band signal from a single trial (filtering=500–1,000 Hz), and c3, 4) raster plot diagrams andPSTHs extracted from the high-frequency band signals, respectively (n=100 trials). d1–4) Waveforms recorded from the floating electrode 1 day after implantation. e) SNR of spike
detected on each electrode implanted mouse for 7 days. f) Numbers of electrodes detecting spike from each mouse 7 days after implantation. g) SNR of spike detected for 180 days.

Conclusion In summary, we proposed a method for chronic neuronal recordingin mice in which a 5-μm-diameter microneedle electrode penetrates brain tissue via
dissolvable material-based detachment and remains on it without being fixed to thecranium, resulting in a floating electrode architecture. Although the electrode device
requires further advancements such as miniaturization and wireless recording system,
the proposed recording technology showed clear advantages in terms of the high SNR
during implantation and less tissue damage. These findings show that the proposed
method will enable stable and safe chronic recording in not only the mice demonstrated inthis study but also other animals, including rats and monkeys.

Acknowledgements We acknowledge support from JSPS KAKENHI Grant

Histological comparison of the tissue response to two-week chronically implanted
conventional pin type and floating electrodes. a) Photograph of the brain tissues. b)
Quantitative comparison of damaged areas. c) Tissue responses. Tissues are labeled
for reactive astrocytes (GFAP). d) Quantitative comparisons of astrocytes.

Numbers 17H03250, 26709024, and 20H00244, for the Strategic Advancement of MultiPurpose Ultra-Human Robot and Artificial Intelligence Technologies program from NEDO,
Adaptable and Seamless Technology transfer Program through Target-driven R&D (ASTEP) from JST, and Nagai Foundation for Science & Technology. R. N. was supported
by Takeda Science Foundation. K. K. was supported by JSPS KAKENHI Grant Number
15H05917, 20H00614.

*Journal information
K. Yamashita et al., Lab on a Chip, Issue 2, 2022. in press