Abstract
Research, design, fabrication and results of various screen printed capacitive humidity sensors is presented in this paper. Two types of capacitive humidity sensors have been designed and fabricated via screen printing on recycled paper and cardboard, obtained from the regional paper and cardboard industry. As printing ink, commercially available silver nanoparticle-based conductive ink was used. A considerable amount of work has been devoted to the humidity measurement methods using paper as a dielectric material. Performances of different structures have been tested in a humidity chamber. Relative humidity in the chamber was varied in the range of 35%–80% relative humidity (RH) at a constant temperature of 23 °C. Parameters of interest were capacitance and conductance of each sensor material, as well as long term behaviour. Process reversibility has also been considered. The results obtained show a mainly logarithmic response of the paper sensors, with the only exception being cardboard-based sensors. Recycled paper-based sensors exhibit a change in value of three orders of magnitude, whereas cardboard-based sensors have a change in value of few 10s over the entire scope of relative humidity range (RH 35%–90%). Two different types of capacitor sensors have been investigated: lateral (comb) type sensors and modified, perforated flat plate type sensors. The objective of the present work was to identify the most important factors affecting the material performances with humidity, and to contribute to the development of a sensor system supported with a Radio Frequency Identification (RFID) chip directly on the material, for use in smart packaging applications. Therefore, the authors built a passive and a battery-supported wireless module based on SL900A smart sensory tag's IC to achieve UHF-RFID functionality with data logging capability.
Highlights
Printed electronics using electrically functional inks and traditional printing technologies are set to revolutionize the fabrication of various electronic devices on flexible substrate materials such as plastic foils, paper and textiles
Recycled paper-based sensors exhibit a change in value of three orders of magnitude, whereas cardboard-based sensors have a change in value of few 10s over the entire scope of relative humidity range
Printed electronics provide a means for low cost manufacture of humidity sensor systems, which can be directly integrated into the materials [11]
Summary
Printed electronics using electrically functional inks and traditional printing technologies (flexo, screen, offset and gravure) are set to revolutionize the fabrication of various electronic devices on flexible substrate materials such as plastic foils, paper and textiles. The possibility of printing UHF RFID antennas on various low-cost and low-quality materials was analyzed in [2]. Some research was done on electronics printed on recycled papers [3,4]. Many studies have reported various applications in printing electronic devices such as sensors, strain gauges, displays and radio frequency identification (RFID) tags [5,6,7,8]. The development of sensors for low cost and accurate humidity measurements is essential for applications in the environmental, agriculture, medical and semiconductor industries [9,10]. Printed electronics provide a means for low cost manufacture of humidity sensor systems, which can be directly integrated into the materials [11]
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