Development of Water-Resistant Au-PVA/WBPU Nanomesh Electrodes for Stable Monitoring of Skin Impedance

Electrodermal activity, which is known to vary due to various factors such as psychological states like stress level [1], and environmental factors like temperature [2] and humidity [3] is one of the important biological signals for early detection of disease and preventive healthcare. Previously, ultrathin and lightweight Au-PVA nanomesh electrode which does not cause discomfort when they were attached on skin has been developed [4] and continuous measurement of skin resistance and skin impedance were achieved by using the nanomesh electrodes [5, 6]. However, these electrodes were vulnerable to water and prone to damage, requiring careful handling during attaching to the skin and being susceptible to breakage with slight movements or sweat.<br/>To address this issue, we developed nanomesh electrodes with excellent water resistance and stable measurement capabilities by mixing hydrophobic WBPU (waterborne polyurethane) with the conventional material PVA (polyvinyl alcohol). The Au-PVA/WBPU nanomesh electrodes were fabricated by following method. First, a mixture of a 15 wt% PVA aqueous solution and a 30 wt% WBPU solution in 2:1 ratio was made into a nanofiber sheet by electrospinning method. And then, a heat-press treatment was applied to the nanofiber sheet to improve the inner connect between fibers. After a heat-press treatment at 150°C for 1 minute, a 100 nm thick layer of gold was deposited by vacuum evaporation method. To check the water resistivity of the nanomesh electrode, we conducted a water immersing test. After a 1-minute stirring test in the pure water, the conventional Au-PVA nanomesh electrodes completely disappeared from the artificial skin, whereas Au-PVA/WBPU nanomesh electrodes remained more than 95% area on the artificial skin. In addition, we checked the stability of the nanomesh electrodes on the skin. After attaching the electrodes on the skin for 2 hours, the resistance of the Au-PVA/WBPU nanomesh electrodes were almost constant while that of conventional nanomesh electrodes showed a deterioration, increasing by 3 to 4 times.<br/><br/>[1] R. B. Malmo, Activation: a neurophysiological dimension.,<i> Psychol. Rev.,</i> 66, 1959<br/>[2] R. Gudivaka <i>et al., </i>Effect of skin temperature on multifrequency bioelectrical impedance analysis.,<i> Journal of Applied Physiology, </i>81(2), 1996<br/>[3] Y. Yamamoto, T. Yamamoto, Characteristics of skin admittance for dry electrodes and the measurement of skin moisturisation., <i>Med. Biol. Eng. Comput.,</i> 24, 1986<br/>[4] A. Miyamoto<i> et al.,</i> Inflammation-free, gas-permeable, lightweight, stretchable on-skin electronics with nanomeshes.,<i> Nat. Nanotech., </i>12, 2017<br/>[5] A. Miyamoto<i> et al.,</i> Highly Precise, Continuous, Long-Term Monitoring of Skin Electrical Resistance by Nanomesh Electrodes.,<i> Adv. Healthcare Mater., </i>11, 2022<br/>[6] R. Matsukawa <i>et al.,</i> Skin Impedance Measurements with Nanomesh Electrodes for Monitoring Skin Hydration.,<i> Adv. Healthcare Mater., </i>9, 2020