Breathable Electrode based on Hydrophobic-Hydrophilic Membranes with Water Stability for Stable Biosignal Monitoring on Wet/Sweaty Skin

Realizing a breathable electrode, which can reliably monitor biosignal after being exposed to water or sweat is challenging. Depending on the stability of its polymer chain backbone towards water or sweat, some of the electrodes could be damaged when exposed to water or sweat. Besides, sweat could also introduce undesired noise artifacts, which lowers the quality of the acquired biosignal. Here, a water- and sweat-stable breathable electrode was demonstrated by utilizing a stack layer of ultrathin hydrophobic microporous Au/polyurethane-polydimethylsiloxane (Au/PU-PDMS) membrane and water-stable hydrophilic poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) nanofibers membrane. The hydrophobic membrane was fabricated by dip-coating the PU nanofibers into PDMS-hexane solution. By controlling the dip-coating withdrawal speed, PU-PDMS membrane with pore density of 39% was achieved. Despite having pores on its surface, the PU-PDMS membrane exhibited hydrophobicity with a water contact angle (WCA) of ~108^o. Meanwhile, the hydrophilic nanofibers membrane was fabricated using electrospinning followed by a partial crosslinking process to achieve the water stability yet maintain the hydrophilicity. The total thickness of the stack layer was below 15 µm. Along with its porous structure, the electrode exhibited an excellent breathability equal to the open bottle with a water vapor transmission rate (WVTR) of 0.58 kg/m²day.<br/><br/>Moreover, the stack of these two membranes facilitated a spontaneous water/sweat pulling, of which the water/sweat could be pulled up vertically in antigravity direction. As a result, when the sweat was secreted to the skin surface, the sweat was spontaneously pulled away from the skin surface. Consequently, the skin-electrode interface could be kept dry, ensuring reliable biosignal monitoring. In addition, the hydrophilic membrane showcased an excellent stability in water that it did not break even after being continuously immersed in deionized (DI) water or phosphate-buffered saline (PBS) solution for up to 1 month. PBS solution was used in this study to simulate the ions contained in the sweat. More importantly, even after continuous immersion in both liquids, the hydrophilic membrane could maintain its hydrophilicity. From the WCA measurement at 37 s, the WCA of the membrane after the immersion in DI water remained stable at ~23.5^o while after the immersion in PBS solution the WCA decreased to 14.4^o, indicating that the membrane became more hydrophilic. Furthermore, electrocardiogram (ECG) signal was measured at rest (dry skin) and after doing a physical exercise (sweaty skin) to evaluate the biosignal acquisition stability against sweat. Exhibiting a sweat pulling property, the electrode was able to maintain a high signal-to-noise ratio (SNR) of 30 dB on the sweaty skin. This result could potentially provide more insight for the medical doctors, especially when evaluating cardiovascular health of a subject that may require the subject to do physical exercise or under a dynamic ambient (temperature/humidity) conditions.