Incorporation, characterization, and application of integrated sensors to assess local humidity changes in fully-enclosed paper-based devices

Full text: In this work, we report for the first time the fabrication and characterization of polypyrrole (PPy) based conductive structures that are integrated into multilayer paper-based microfluidic devices. The fabrication of the PPy sensors is divided into three steps: (1) distribution of oxidation ions in the microfluidic channels, (2) device exposure to pyrrole monomers in the gas-phase, and (3) washing step[1,2]. The last step changes the wettability and the chemical composition of the interface, which was monitored by contact angle experiments and X-ray photoelectron spectroscopy (XPS), respectively. The PPy sensors exhibit excellent mechanical stability, with more than 600 bending cycles achieved. We observed that the electrical resistance increases with relative humidity (RH) increase. Most importantly, the humidity sensor response is reversible. To understand the response mechanism, we mapped the surface of the PPy sensor using three dimensional laser confocal microscope during water exposure. Our results suggest an increase of the fiber volume when RH increases. Furthermore, we successfully combined the PPy sensor with microfluidic channels by folding steps, to produce an integrated system capable of measuring RH variations in a fully enclosed microfluidic device. As a result of such combination, we noticed that water molecules in the microfluidic channel evaporate and diffuse through the wax barriers. This effect was confirmed by locally measuring the variations of RH inside of the fully-enclosed device. Also, we managed to speed up the liquid flow by adding a water reservoir underneath the main channel. Our results suggest that RH variations of 5-10% can affect the flow of extended channels (>5 cm) even when they are fully-enclosed. References: [1] M. Santhiago, et.al. ACS Appl. Mater. Interfaces 2016, 8 (17), 10661-10664. [2] M. M. Hamedi, et.al. Adv. Mater. 2016, 28 (25), 5054-5063. (author)