Seif-Anchoring of Transformable Electrode-Hydrogel Hybrid Sheet for Intraoperative Physiological Monitoring
Authors: Okabe Daiki , Nagamine Kuniaki , Kai Hiroyuki , Kaji Hirokazu , Nishizawa Matsuhiko 
Published: February 2020

Introduction
Intraoperative physiological monitoring is crucially important to avoid surgical wounding of the normal organs and tissues such as brain and neurons. Conventionally, silicone-based electrode arrays are used to detect or stimulate electrophysiological activity on the surface of organ and tissues, while the silicone was so stiff as to cause deformation or injury to the soft organs and tissues in some cases. Besides molecular-impermeable silicone substrate hinder feeding of the tissue fluid including nutrients and growth factors to the organs and tissues through the substrate. Soft, flexible, and molecular-permeable substrate-based electrodes will enable biocompatible intraoperative monitoring.
We have developed hydrogel-based electrode arrays in which stretchable conducting polymer electrode was tightly bound to the hydrogel surface by means of electrochemically growing poly(3,4-ethylenedioxythiophene) (PEDOT) adhesive into the hydrogel [1]. Their stiffness, flexibility, and molecular-permeability are the characteristics required for conformable contact with biological surfaces, while the problem of stable immobilization to the surface is still remained. In this study, we developed self-transformability electrode/hydrogel composite to be immobilized to specific geometry of the organs and tissues. Two types of immobilization methods were examined. For monitoring of brain activity, the dried electrode/gel composite was inserted into a cerebral sulcus and swollen on site to fill its groove with fitting to the complex brain surface geometry. For monitoring of nerve bundle activity, self-rolling cuff-type electrode/hydrogel was developed.
Experiments
Preparation of the PEDOT/PU electrode/hydrogel composite sheet
A composite film of PEDOT and polyurethane (PEDOT/PU) was prepared on the glass substrate by spin coating of a precursor solution and its thermal polymerization at 100 °C for 10 min. The precursor solution used here was a mixture of butanol, EDOT, pTSFe(III) and 10 wt% PU/tetrahydrofuran solution. The thickness of the PEDOT/PU film was approximately 3 μm. The PEDOT/PU film on the glass plate was immersed in distilled water overnight to remove residual cytotoxic monomer and dopant, and cut out to a desired pattern by a cutting plotter. A double netwowk hydrogel sheet was then placed on the patterned PEDOT/PU film. After permeation of 50 mM EDOT monomer solution containing 0.1 M LiClO4 as a dopant into the hydrogel sheet, PEDOT was electropolymerized to anchor the electrode to the hydrogel film. Finally, the resulting electrode/hydrogel composite film was gently peeled from the glass substrate and further immersed in water overnight to remove a residual EDOT monomer.
Preparation of self-rollable PEDOT/PU electrode/hydrogel composite sheet [2]
The electrode / hydrogel composite sheet was put on the pool of the NaAMPS prepolymer solution, and immediately irradiated with UV light. The resulting surface region-modified electrode / hydrogel composite and the additional polymer formed in the chamber can be easily divided by tweezers because of the large difference in their swelling ratios. The hydrogel was washed with distilled water overnight to remove residual cytotoxic monomers.
Results and discussion
Immobilization into the groove 0.6 wt% agarose gel was used as a brain model. The dried electrode/gel composite sheet quickly swelled within 10 min on site in a groove created in the agarose (1 mm width) to be the original size of 2 cm x 2 cm x 1 mm. The swollen composite was stably fixed in the groove even when 1.2 g of maximum load was applied to the composite. Stiffness of the hydrated electrode detected by the mechanical compression test was ca. 8 kPa which value was less than that of brain (less than 10 kPa). The swelling pressure of the composite that will be applied to the sidewall of a cerebral sulcus was detected by using force gauge and found ca. 0.17 kPa which value was much less than the pressure constantly applied to the brain (intracranial pressure: 0.5-2 kPa).
Cuff-type electrode/hydrogel composite The dried electrode/gel composite the surface of which was modified with PAMPS polymer were self-rolled around the glass rod upon hydration (within 5 min) with inner diameter of 2-5 mm which value can be controlled depending on the degree of modification.
References
[1] M. Sasaki et al., Adv. Healthcare Mater., 2014, 3, 1919.
[2] K. Nagamine et al., RSC Adv., 2015, 5, 66334.