Versatile and Robust Integrated Sensors To Locally Assess Humidity Changes in Fully Enclosed Paper-Based Devices

The synergic combination of materials and interfaces to create novel functional devices is a crucial approach for various applications, including low-cost paper-based point-of-care systems. In this work, we demonstrate the implementation of surface-modified polypyrrole (PPy) structures, monolithically integrated into a three-dimensional multilayered paper-based microfluidic device, to locally assess humidity changes. The fabrication and integration of the system include the deterministic incorporation of PPy into the paper-based structure by gas-phase polymerization and the modification of the polymer properties to allow local humidity monitoring. The functionalization of PPy changes both the wettability and the chemical composition of the interface, what is of fundamental importance for the sensor's operation. The PPy structure has excellent mechanical stability, enduring at least 600 bending cycles, what is of relevance on flexible electronics. The electrical resistance correlates with the local relative humidity (RH) inside of the sealed microfluidic system, and the sensor response is fully reversible. The integrated system capable of locally monitoring the RH allowed us to verify that inside the microfluidic channel, water molecules can diffuse across the wax barriers-a possibility disregarded so far. Our results attest that RH variations of 5-10% can affect the flow of extended channels (>5 cm) even when they are fully enclosed.