Development of Biocompatible Electrode with Self-Doped PEDOT

Biocompatible electrodes, which are flexible, gas permeable, and self-adhesive, can be applied to long-term monitoring of biosignal on the skin. The gas permeable electrodes are less likely to cause inflammation of the skin, and self-adhesiveness is important for accurate measurements under human motion. Nanosheet electrodes are good candidates for the biocompatible electrode, and our group has developed a nanosheet electrode using polydimethylsiloxane (PDMS) and polyurethane (PU) nanofiber with Au deposition, which is referred to as Au/PU-PDMS nanosheet electrode, here [1]. In Au/PU-PDMS, PU nanofiber is used as a framework in the PDMS nanosheet, and conductivity is acquired by Au deposition. However, the self-adhesiveness is drastically decreased by Au deposition, and the conductive region is only the Au part. As a result, an electrical wire should be inserted between the skin and the Au to extract the signal, and it causes damage to the nanosheet during human motion. In this study, we have developed a PU:S-PEDOT nanosheet electrode with an intrinsically conductive elastomer instead of PDMS, and it does not require wire insertion between the skin and electrode.<br/><br/>We prepared the conductive elastomer by mixing self-doped poly (3,4-ethylene dioxythiophene) (S-PEDOT) and waterborne PUs (WBPUs), which is referred to as PU:S-PEDOT, here. S-PEDOT is a conductive polymer and is well soluble in water. Furthermore, it does not require secondary doping, unlike PEDOT: poly (styrenesulfonic acid) (PEDOT:PSS) [2]. The PU nanofiber was dipped into the PU:S-PEDOT, and it was dried in an oven.<br/><br/>First, we evaluated the sheet resistance of the PU:S-PEDOT nanosheet electrodes prepared with different S-PEDOT solution concentrations. As a result, the sheet resistance decreased with an increase of S-PEDOT solution concentration, and it achieved about a few kilo-ohm at 67 wt% of S-PEDOT solution concentration, which is sufficiently low to be applicable to on-skin measurements such as electrocardiogram (ECG) and skin impedance. Next, we evaluated the self-adhesiveness of the electrodes, and it has about 50 μJ/cm² of adhesion energy even at high S-PEDOT solution concentration. Then, gas permeability was estimated by comparing the loss of water in bottles sealed with the electrodes and in bottles not sealed, and the water loss was about 70% of that in the unsealed bottle. Finally, as a demonstration, we applied our PU:S-PEDOT electrode to ECG monitoring, and a clear signal was obtained. These results shows that our PU:S-PEDOT electrode is a biocompatible electrode for long-term monitoring.<br/><br/>References<br/>[1] Y. Wang et al., Proc. Natl. Acad. Sci. 118, e2111904118 (2021). [2] H. Yano et al. Sci. Adv.5,eaav9492 (2019).