Continuous Sweat Sensing Wearable Patches - Reliability of Screen-Printed Sweat Rate Electrodes

Multilayer sweat-rate electrodes (SREs) with conductors and resistors were successfully screen-printed using a solvent-based silver ink and a water-based CB ink, respectively, and their environmental and mechanical reliability was comprehensively investigated. Water-based carbon inks provide a cost effective and sustainable alternative to organic-solvent based metal conductive inks, making them attractive for wearable sensor applications. However, poor adhesion on nonporous polymer substrates and susceptibility to temperature and humidity fluctuations raise concerns about printability and reliability, hindering their widespread commercial adoption. This study focuses on screen-printing defect-free multilayer structures on flexible and stretchable polymer substrates using a commercial water-based conductive carbon black (CB) ink and evaluating their reliability. A robust printing process was developed by modifying the fabrication flow, optimizing printing parameters, and maintaining atmospheric relative humidity (RH) between 70% and 75%. The water-based carbon resistors printed on polyimide substrate demonstrated promising results. Ambient-dried resistors on polyimide exhibited satisfactory electrical performance and reliability, while thermal curing further reduced their electrical resistance by 18% without compromising reliability. Moreover, these resistors demonstrated excellent environmental and mechanical reliability by withstanding thermal exposure at 125 ◦C, RH of 15%, and 500 tensile bending cycles at a 1-cm bend radius, suggesting their suitability for wearable sensors. Failure analysis revealed the development of crater-like morphological structures during the drying process, which later acted as stress concentration points. Resistors printed on polyester, high-density polyethylene, and thermoplastic polyurethane (TPU) substrates failed due to cracking, delamination, ink-to-ink interactions, or out-of-plane deformation. Cracking and delamination patterns provided useful insights into failure mechanisms.