A Fully Self-Healable Sweat Cortisol Sensor For Stress Monitoring

Stress has been identified as one of the most critical causes of various diseases, leading to an increased interest in continuous stress management. However, most stress assessments rely on detecting and measuring physiological changes caused by stress, such as heart rate variability and changes in skin conductivity. Consequently, accurate management of stress using only bio-signals remains challenging. Cortisol, referred to as the stress hormone, directly reflects the stress response of human body so that monitoring cortisol levels in sweat allows for the evaluation of both acute and chronic stress. Due to the softness of the skin-attachable sweat sensor, it is vulnerable to damages by unintentional impacts, raising the issue of developing self-healing functionality.<br/>In this work, we report on the fabrication of a fully self-healable sweat cortisol sensor consisting of molecularly imprinted polymer (MIP) for cortisol, laser induced graphene (LIG) electrode, microfluidic channel and bio-inspired adhesive. For the reliable detection of stress over extended periods, we synthesize self-healing cortisol MIP based on oxime-carbamate bond-based polyurethane (OC-PU). Our synthesized OC-PU can be self-healed by heating at 65 °C for 6 hours under contact of damaged interfaces. Also, graphene formed by laser irradiation on a composite of OC-PU and lignin is used as electrodes and remains conductive up to 15% strain. Such fabricated cortisol sensor exhibits a very low detection limit down to 0.1 nM and sensitivity comparable to previous works. Furthermore, various biomarkers in sweat can be also detected by simply changing the target molecules. Here, we also develop self-healing microfluidic channel for continuous collection of sweat and bio-inspired adhesive to vertically integrate with self-healing sensor layer. Owing to the oxime-carbamate bonding and hydrogen bonding between the common OC-PU layers, a fully self-healing sweat sensor system can continuously monitor the stress even after self-healing, thereby expanding their potential application to wearable electronics with longevity.