Water-Triggered Self-Wrapping Cuff Electrodes for Nerve Stimulation, Oral Presentation
Authors: Imamura Fumiya , Nozaki Natsuki , Tottori Soichiro , Osawa Shin-ichiro , Anzai Yuki , Wang Gaobo , Nakagawa Atsuhiro , Nishizawa Matsuhiko 
Published: December 2024

In recent years, there has been an increasing interest in implantable stimulation electrodes for supporting and restoring neural functions, with ongoing development efforts aimed at their application in various areas such as vagus nerve stimulation (VNS), spinal cord stimulation (SCS), and gastric electrical stimulation (GES). The development of flexible and thin electrodes is crucial due to the delicate and uneven surfaces of tissues within the body. This is particularly important for thin nerves, which require full coverage and secure adhesion. However, soft and thin electrodes are difficult to handle in the limited space inside the body during the operation. Currently, such electrodes can only be manually attached by skilled surgeons, and the process tends to be time-consuming. Here, we develop multi-layered flexible electrodes that self-wrap around the nerves by exploiting the difference in the compression-expansion rate of each layer upon wetting.
The electrodes are based on a polyvinyl alcohol (PVA) hydrogel substrate with flexible carbon fibers sandwiched between them. The self-wrapping mechanism of the electrode is designed using two layers of hydrogel substrates with different compression-expansion rates upon wetting (Fig.1b). When a stretched hydrogel is adhered to an unstretched one, they deform due to the difference in stress. Additionally, the elasticity of the hydrogel decreases when it dries but is restored upon swelling. Drying the stretched hydrogel preserves the temporarily applied elasticity, enabling the hydrogel-based electrode to become round when it swells. The stretched hydrogel was created by mixing PVA with dimethyl sulfoxide (DMSO) and water to form a gel precursor solution, which was then gelatinized using the freeze-thaw method. This gel was soaked in the solution to replace the solvent within the gel. Afterward, the gel was stretched and dried. The unstressed PVA hydrogel was prepared by dissolving PVA in water, spin-coating the solution onto glass, and gelatinizing it using the cast-dry method. Carbon fibers were placed between the two sheets of dried hydrogel and laminated with a PVA and water solution. The electrodes are flat when dry, making it easy to place under the nerve. Additionally, it automatically deforms when water is applied, making installation easier than traditional electrodes (Figure 1c). It takes only about 30 seconds to completely encase the nerve. This functionality is expected to reduce the time required for surgery compared to traditional electrodes. The deformed hydrogel electrode is securely fixed due to the hydrogel's elasticity and does not move in a direction parallel to the nerve or dislocate vertically.
Animal experiments were conducted by electrically stimulating the vagus nerve of pigs. During electrical stimulation, a bradycardia, which is a side effect of vagus nerve stimulation, was observed. This demonstrates the electrode's capability for stimulation.
In conclusions, we developed the flexible electrodes that self-wraps around the nerves by exploiting the difference of the internal compression-expansion rate upon wetting. We confirmed that the wrapping force was large enough to stably fix the electrodes on the nerves while being small enough not to damage them. Finally, we tested the developed electrodes to stimulate the vagus nerve of pig and confirmed the effectiveness of the electrical stimulation. We hope that this electrode will be a significant step towards the practical use of hydrogel electrodes for stable fixation to the nerves and simplifying surgical procedures. In the future, we aim to further develop this technology and develop more advanced electrode designs that can be applied to various nerve stimulation treatments.
Reference: Terutsuki. D, Yoroizuka. H, Osawa. S, Ogihara. Y, Abe. H, Nakagawa. A, Iwasaki. M, Nishizawa. M, “Totally Organic Hydrogel-Based Self-Closing Cuff Electrode for Vagus Nerve Stimulation.” Adv. Healthcare Mater. 2022, 11, 2201627.
Fig.1 (a) Illustration of self-wrapping electrode. (b) Schematic diagram of electrode fabrication and self-wrapping mechanism. Light blue is freeze-thawed PVA hydrogel with DMSO pre-stretched before drying. Dark blue is cast-dried PVA hydrogel. (c) Image of wrapping an electrode around a nerve. The fabricated hydrogel electrode completely covers the nerve in about 30 seconds after water is applied.

Figure 1