Passive Moisture Sensor Based on Conductive and Water-Soluble Yarns

In this paper, we establish a new type of passive ultra-high-frequency (UHF) radio frequency identification (RFID) technology-based moisture sensor. The sensor system consists of a sensor tag and a reference tag to indicate exposure to moisture. The sensor tag is fabricated on a cotton substrate from conductive silver yarn and water-soluble polyvinyl alcohol (PVA) yarn. When the sensor is exposed to water, the PVA yarn will melt, which causes a permanent change to the tag antenna body and to the wireless performance of the sensor tag. By measuring the backscattered power strength difference, ΔP, the presence of moisture can be detected and recorded. When ΔP is below 0, the sensor system shows that it has been in a highly moist condition. Our PVA-based moisture-sensing system can provide cost-effective and maintenance-free monitoring of moisture exposure for versatile application fields.