Comparison of the Carbon Nanofiber-/Fiber- and Silicone-Based Electrodes for Bioimpedance Measurements

Electrodes for bioimpedance measurements remain a challenge. In practice, commercially available nonpolarizable silver/silver chloride (Ag/AgCl) gel electrodes prove to be the best option for bioimpedance-based testing. The hydrogel layer reduces the problematic electrode-skin interface impedance, which results in a highly reliable signal. Our workgroup is developing a wearable device that estimates aortic blood pressure from the bioimpedance taken from the wrist area. Understandably, wet electrodes do not suit for a wearable, and there is a need for dry, soft, reusable, and stable electrodes. This study proposes stretchable carbon nanofiber/carbon fiber silicone electrodes as an alternative for cardiac signal measurements with bioimpedance. Five different electrode materials are tested and analyzed: Ag/AgCl gel, Ag/AgCl dry, two carbon and silicone composite materials, and carbon textile. To compare the electrodes' characteristics, the current through the tissue under the changing pressure, frequency, contact duration, and skin preparation is registered, and the electrode-skin impedance is calculated. The soft and stretchable carbon fiber silicone electrodes proved to have similar response as rigid nonpolarizable Ag/AgCl dry electrodes. Selected methods and proposed instrumentation ensure acceptable reproducibility of the base value and the modulation depth of the measured impedance despite the different electrode materials, large variation of the electrode-skin impedance, and the actual measurement current.