A flexible resistive-type humidity sensor for harsh environments is successfully designed and fabricated by an inkjet printing method using a Dimatix materials printer (DMP-2800 series from Fujifilm). Construction of the sensors is based on inkjet printed interdigitated silver electrodes on a polyimide flexible substrate along with an inkjet printable polyaniline (PANi) as humidity sensitive material. A copolymer of ethylene and vinyl alcohol (EVOH) is used as sensor protective coating. Double strand water- soluble PANi ink is synthesized by polymerization of aniline monomers with poly(4- styrenesulfonic acid) (PSSA) as a template.
Manufactured devices showed high sensitivity (/% @ 45%RH) to humidity with good linearity (R-squared correlation value of 0.99) and fast absorption and desorption responses over a broad range of humidity (/5-95%RH). The response time for the sensors without EVOH coating on top is around 5 and /5 seconds for absorption and desorption, respectively. The response time for the coated sensors is about 40 and //0 seconds for absorption and desorption, respectively. Sensors showed small hysteresis (about 3%) while a protective polymer coating provided a barrier to damage or false signals due to solvents and chemicals.
Fabricated sensors are characterized in order to investigate the structure and morphology of the thin films. UV-Visible spectroscopy is used to obtain information on PANi in solution form and to confirm that PANi in the form of emeraldine salt is obtained. FT-IR spectroscopy is used to verify the incorporation of the PSSA into PANi. SEM microscopy method is used to characterize the PANi-PSSA in powder form and the AFM method is used to show the morphology of the thin films
