Reliability of Screen-Printed Water-Based Carbon Resistors for Sustainable Wearable Sensors

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 non-porous polymer substrates and susceptibility to temperature and humidity fluctuations raise concerns on 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 ink and evaluating their reliability. A robust printing process was developed by modifying the fabrication flow, optimizing printing parameters, and maintaining atmospheric relative humidity between 70% and 75%. Multilayer sweat-rate electrodes (SREs) with conductors and resistors were successfully screen-printed using a solvent-based silver ink and a water-based carbon black ink, respectively, and their environmental and mechanical reliability was comprehensively investigated. 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, relative humidity 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 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.