Flexible printed RTD sensors for wearable temperature monitoring

This work presents the development of printed resistance temperature detectors (RTD) using silver-based conductive ink for wearable applications and the first systematic comparison of rigid, flexible, and textile substrates across key performance metrics. Initial sensors printed on glass demonstrated a linear and stable response to temperature variations from −10 °C to 50 °C, matching a commercial Pt100 with a resolution of 0.1 °C. Thickness and linewidth mapping further confirmed uniform printed features, supporting consistent sensor behaviour. Optimized sensors were printed on flexible substrates including polyimide (PI) and textile with basecoat. A fabrication process was developed to encapsulate PI-based sensors between textile and thermoplastic polyurethane (TPU) foil for seamless wearable integration. All flexible sensors exhibited consistent performance, with a temperature coefficient of resistance (TCR) of 2.69 × 10⁻³ °C -1 , negligible hysteresis, and strong agreement with reference measurements. Mechanical bending tests, together with additional fatigue tests showing stable operation over 1000 bending cycles across different radii of curvature, confirmed the robustness of PI-based sensors, with negligible resistance variation and rapid recovery. In contrast, textile-based sensors showed higher sensitivity to deformation due to their stretchable nature. The encapsulated PI sensor emerged as the most promising candidate for wearable applications, offering environmental protection, mechanical stability, and a fast thermal response (2.7 s). Exposure to artificial sweat revealed that acidic conditions irreversibly affected non-encapsulated sensors, while the encapsulated maintained stable performance. The encapsulated sensor was successfully tested on the forearm, confirming its effectiveness for skin surface temperature monitoring. These results highlight the potential of printed RTD sensors for reliable and accurate temperature monitoring in flexible and wearable electronic applications. Furthermore, we demonstrate a low-cost and rapid fabrication process suitable for scalable production of wearable temperature sensors.