Development of Permeable and Transparent Intracranial Electrode Embedded in Hydrogel Substrate
Authors: Ogihara Yuka , Yoshida Shotaro , Abe Hiroya , Abe Yuina , Oribe Shuntaro , Osawa Shin-ichiro , Nakagawa Atsuhiro , Iwasaki Masaki , Tominaga Teiji , Nishizawa Matsuhiko 
Published: December 2020

In surgery of epilepsy, a patient's cranium is opened for intracranial electroencephalogram (iEEG) measurement or electrical stimulation to treat a part of a brain causing seizures [1]. Conventionally, intracranial metal electrodes with a silicone rubber substrate have been widely used in iEEG measurements during surgery. However, the substrate of the electrode is stiff and needs to be pressed down to fit on the surface of the brain, which increases the tissue damage during the insertion into in a deep sulcus called a longitudinal cerebral fissure or a Silvian fissure. Besides, the low transparency of the electrode substrate decreases the immediate recognition of the diseased area, and the lack of permeability of that also cause damage to the brain in long term treatment including an implantation of the electrode. Herein, a flexible intracranial electrode using a hydrogel as a substrate was developed [2]. A soft carbon fabric electrode was embedded in a hydrogel composed of poly (vinyl alcohol) (PVA). The PVA hydrogel has excellent flexibility, biocompatibility, water content, and hydrophilicity, which leads to well adhesion and conformability to the brain. Moreover, the transparency of PVA hydrogel is expected to improve the visibility of the brain under the substrate and its permeability ensure adequate fluid delivery.
Carbon fabric electrodes were fabricated by cutting the electrodes and lead wires out of the fabric and applying an insulation coating. The non-insulated electrode surface was coated with conductive particle polymer poly (3,4-ethylenedioxythiophene) (PEDOT) to increase the surface area and increase the electrical capacity. This process is to prevent electrolysis from occurring during electrical stimulation[3]. A gel precursor solution was prepared by dissolving PVA in distilled water and adding dimethyl sulfoxide (DMSO). The electrodes were embedded in the precursor solution and gelated by repeated freezing and thawing cycles. DMSO provided an uniform and clear gel, and to avoid its irritation, the obtained gel was soaked and washed well in saline.
Figures 1 shows the electrodes placed on the brain model. While the conventional electrodes with silicone substrate hardly followed the shape of the object (Figure 1a), the electrodes with hydrogel substrate followed the object well (Figure 1b). The object was not clear to observe through the conventional silicone substrate, and that substrate did not swell the water droplet placed on the surface (Figure 1c). In contrast, the hydrogel-based substrate was clear after the adhesion onto the brain model, and the water droplets placed on the surface were spread over the entire surface and adsorbed to the hydrogel-based substrate. These results indicated that electrodes with a hydrogel substrate could adhere well to the brain surface while minimizing damage to the tissue, and allowing observation of the affected area during surgery.
Our developed electrode with soft, hydrophilic, and soft hydrogel substrate were promising device for the treatment of epilepsy, overcoming the conventional problems and leading to new medical approach.
References
[1] S. Noachtar, I. Borggraefe, “Epilepsy surgery: A critical review”, Epilepsy Behav., 15, 66-72, (2009)
[2] S. Oribe, S. Yoshida, S. Kusama, S. Osawa, A. Nakagawa, M. Iwasaki, T. Tominaga, and M. Nishizawa, “Hydrogel-Based Organic Subdural Electrode with High Conformability to Brain Surface”, Sci. Rep., 9, 13379, (2019).
[3] S. Yoshida, K. Sumomozawa, K. Nagamine, and M. Nishizawa, “Hydrogel Microchambers Integrated with Organic Electrode for Efficient Electrical Stimulation of Human iPSC-Derived Cardiomyocytes”, Macromol. Biosci., 19, e1900060, (2019)

Figure 1