Hydrogel-Based Transparent Subdural Electrode with Salt Bridge As Interface to Brain Surface
Authors: Nishimura Ayaka , Suwabe Ryota , Ogihara Yuka , Abe Yuina , Abe Hiroya , Yoshida Shotaro , Nishizawa Matsuhiko 
Published: December 2020

Intraoperative biomedical devices are used in many medical fields such as monitoring, diagnosis and treatment of diseases. Subdural electrodes used in monitoring cortical activity contribute to an identification of pathological lesions in an epilepsy surgery and also the advance of elucidation of brain function. These electrodes as an interface with living systems should be flexible, biocompatible, and conformable to the curved brain surface. However, conventional electrodes consisting of metals such as Pt electrodes and a silicone-based substrate have not fully resolved mismatches between electronics circuit of a machine and biological wet systems, which causes a decrease of measurement accuracy and damage to bio-tissue[1]. In a previous study, we have developed a totally organic hydrogel-based subdural electrode by embedding a carbon fabric (CF) modified with poly(3,4-ethylenedioxythiophene) (PEDOT) into a poly(vinyl alcohol) (PVA) hydrogel substrate[2]. This wet electrode showed a good softness similar to the living tissue, a high conformability to a curved surface such as brains, and easy handling. Moreover, the totally organic nature would contribute to obtain clear MRI images without image artifacts.
In this study, we have developed a new type of the hydrogel-based transparent subdural electrode in which the salt bridge acts as interface to bio-tissue (Fig. 1A). The present electrode consisted of three different parts, a salt bridge made of PVA hydrogel insulated by the microchannel made of Poly(dimethylpolysiloxane) (PDMS) membrane embedded into the PVA hydrogel substrate. The unique feature of the hydrogel-based electrode with the salt bridge system was aqueous-based interface to bio-tissue on which can be decreased an effect of the electrochemical reaction. The aqueous-based electrode similar to bio-tissue enable to be flexible, biocompatible, and conformable due to the use of highly soft and wet biomaterial. In addition, the device has an optical transparency which can contribute to obtaining detailed visual information under the electrode during surgical operations and would be valuable to the neuroscience in optogenetics.
The PDMS membrane-based channel (thickness: 600 µm, width: 500 µm, height 100 µm) was fabricated by spin-coating at 700 rpm for 15 seconds on the SU-8 mold and bonding with an oxygen plasma treatment. PVA (Mw~145000) hydrogels for both a salt bridge (10 wt%) and substrate (15 wt%) were dissolved in a mixture of dimethyl sulfoxide (DMSO) and saturated KCl solution (mass ratio = 4:1). The PDMS membrane-based channel connected to the silicone tube for extension was filled with PVA hydrogel as a salt bridge, sandwiched by glass slides with 1.5 mm-thickness spacer which was filled with the PVA hydrogel, and crosslinked by freeze-thaw cycles (as -30 °C for 10 min and 4 °C for 10 min, each repeated three times). The fabricated electrode was rinsed twice in Ringer’s solution for one hour and overnight. The conformability was evaluated by the rate of contact area between electrodes and the surface of the curved sheet with various curvatures. In vivo recording was conducted on a porcine which was anaesthetized with the head fixed in a stereotaxic apparatus, and the dura mater was partially removed by a neurosurgeon to expose the surface of the cerebral cortex. Then, electrodes were placed on the exposed cortex on each hemisphere for simultaneous electrocorticography (ECoG) recording (2019MdA-324).
Figure 1B shows the conformability of the developed and conventional electrode to the curved surface. The contact area of the hydrogel electrode showed more than 80 %, which was higher conformable than that of the silicone-based conventional electrode (~ 50 %). Figure 1C shows that the hydrogel electrode consisting of PVA hydrogels and PDMS as a highly transparent and biocompatible material, showed a sufficiently transparent to recognize the condition under the device. In contrast, the commercially available electrode was opaque due to thick silicone-based substrate and metal. As shown in Figure 1D, we successfully measured brain waves by the hydrogel electrode with the salt bridge system in vivo measurements on porcine. From this result, the salt bridge acted as the resistance for ionical connection and ionically connected from bio-tissue to electronic measurement machine. We successfully developed a new hydrogel-based transparent subdural electrode with the salt bridge as a bio-friendly interface.
References
[1] H. Yuk et al., “Hydrogel bioelectronics,” Chemical Society Reviews, 48, 1642–1667 (2019).
[2] S. Oribe et al., “Hydrogel-Based Organic Subdural Electrode with High Conformability to Brain Surface,” Scientific Reports, 9, 1–10 (2019).

Figure 1