Researchers at the University of Tokyo developed a flexible RFID (wireless) tag system that functions as a moisture sensor.
The tag is equipped with about 30 organic transistors, organic diodes, etc. Because power is transmitted from a reader unit by using electromagnetic resonance wireless power transmission technology, the tag can be used several centimeters away from the reader unit. The details of the system were announced at International Solid-State Circuits Conference (ISSCC) 2014, an academic conference on semiconductor technologies, which runs from Feb 9 to 13, 2014, in San Francisco, the US.
The system was developed by a research team led by Takayasu Sakurai (the Institute of Industrial Science, the University of Tokyo) and Takao Someya (professor at the School of Engineering, the University of Tokyo).
The system consists of a wireless tag and reader unit. The reader unit wirelessly transmits power to the wireless tag and reads data by receiving responses from the tag. For the wireless transmission, a frequency of 13.56MHz is used.
"Wireless tags and sensors have been attached to goods," Sakurai said. "But they will be increasingly attached to humans. For that purpose, tags need to be flexible."
The wireless tag was made by forming power-receiving coils and circuits on a 12.5μm-thick polyimide film measuring 78 x 53mm by using a vacuum deposition method. The circuits are (1) rectifier circuit for power reception, (2) electrostatic protection circuit (up to 2kV) and (3) ring oscillation circuit that functions as a moisture sensor. Copper (Cu) is used for coils and long wiring, and gold (Au) is used for electrodes, etc. The interconnection/circuits on the film consists of roughly three layers, and each layer is connected by using Au vias.
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