Evaluation of Stability and Cytotoxicity of Self-Doped PEDOT Nanosheets in a Quasi-Biological Environment for Bioelectrodes

PEDOT:PSS is the most extensively used organic conductive material for flexible sensors; however, its long-term use in wet biological environments is difficult due to the swelling of PSS and the leaching of additives introduced by secondary interactions. Self-doped PEDOT (S-PEDOT) containing sulfonic acid groups covalently bonded to the PEDOT backbone has been applied to evaluate the stability of its performance as a conductive nanosheet and cytotoxicity. The S-PEDOT nanosheets and the control PEDOT:PSS nanosheets were prepared with a spin-coating method and immersed in a quasi-biological environment (PBS or culture medium, 37 °C, 5% CO2). Approximately 60 days of immersion increased the resistance of PEDOT:PSS 6-fold, while the resistance of S-PEDOT remained 3-fold. The difference would be caused by the crystal structure of the PEDOT chains being disrupted by the replacement of protons of the sulfonic acid groups in PSS with Na+ ions, swelling of PEDOT:PSS, and elution of the surfactant Zonyl, while the crystal structure of the main chain of S-PEDOT was maintained even after immersion, although the replacement by Na+ ions occurred. In addition, cytotoxicity was observed in the extract of the PEDOT:PSS nanosheet due to the elution of Zonyl, which is highly cytotoxic, while S-PEDOT showed low cytotoxicity because the additive containing Si was covalently bonded to the main chain. Thus, it was suggested that S-PEDOT nanosheets can function as minimally invasive bioelectrodes that can be used in the physiological environment.