Skin-Conformable Electrophysiological Sensor Devices with Tailored Negative Poisson’s Ratio

Electrophysiological sensing is a crucial functionality that provides valuable information in clinical diagnosis and treatment. Recent advances in wearable electronics enabled tracking electrophysiological information such as electrocardiogram (ECG) in a daily basis.[1] However, conventional elastomer substrate-based devices suffer from the lack of long-term adhesion and conformability to human skin due to Poisson’s ratio mismatch. To address these limitations, the Poisson’s ratio (PR) of the devices should match with human skin, especially in the vicinity of joint areas which have negative Poisson’s ratio. One approach to achieve negative PR is to utilize auxetic mechanical metamaterials with negative PR as device substrates.<br/>In this study, we demonstrate electrophysiology devices with enhanced skin conformability, consisting of seamless auxetic substrates and printed liquid metal electrodes. To achieve a highly conformal contact, we analyze the PR of the human wrist skin directly, using 3D digital image correlation (DIC) method. Through this information, the devices are tailored to match the PR of the human skin. The negative PR of the device is achieved by the auxetic structured glass-fabric reinforced substrate with optimized geometric parameters. Also, a soft elastomer with extreme modulus difference with the glass-fabric reinforced film fills the perforations of the auxetic structure to form a continuous surface. [2] Utilizing the advantage of the seamless surface, we employ high precision 3D printing technique to fabricate liquid metal-based electrocardiogram (ECG) sensors and electrotactile stimulators with simple configurations.<br/>The liquid metal electrodes maintain their conductivity during up to 30% of stretching and 100 times of repeated stretching. The resulting devices exhibit increased sensitivity and stability even under dynamic motion of the wrist thanks to their enhanced adhesion and conformability compared to pristine elastomer substrates. Precisely, the ECG sensors maintain signal to noise ratio of over 20 dB after 30 times of wrist bending, while pristine PDMS-based sensors showed degradation to 13 dB. Moreover, the electrotactile stimulators showed high consistency of perceived sensation for 6 stimulated perception levels, even stable under bending.<br/><br/>[1] Ha M. et al., <i>J. Mater. Chem. B</i>, 2018, 6.24: 4043-4064.<br/>[2] Kim M. S. et al., <i>Adv. Funct. Mater.</i> 2023, 33, 2208792<br/>[3] Park Y.-G. et al., <i>Sci. Adv.</i> 5, eaaw2844 (2019)