Moisture-sensing Diaper Made With Flexible RFID Tag
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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The rectifier circuit and electrostatic protection circuit consist of Schottky-type organic diodes. For the semiconductor of the organic diodes, a phthalocyanine-based p-type organic semiconductor material is used. The electrostatic protection circuit is necessary because the tag is attached to human bodies.
"Static electricity around human body usually has a voltage of 1kV to 1.5kV," Sakurai said. "So, 2kV is almost enough."
For the organic transistor used for the ring oscillation circuit, the researchers employed a thiophene-based p-type organic semiconductor material. Its carrier mobility is about 1cm2/Vs, which is relatively high. But its oscillating frequency as a ring oscillator is 3 to 10Hz.
Part of the ring oscillation circuit is cut by electrodes that are exposed and look like films. When there is moisture on them, the electric resistance between the electrodes lowers, starting oscillation. So, when there is no oscillation, there is no moisture - and vice versa.
When oscillation occurs, the oscillating frequency is superimposed on the 13.56MHz electromagnetic field sent from the reader unit and is sent back to the unit. In the wireless tag, 13.56MHz signals are not generated.
"It is like reflecting the electromagnetic field sent from the reader unit," said Hiroshi Fuketa, assistant professor at the Institute of Industrial Science, the University of Tokyo.
By using electromagnetic resonance for wireless power transmission, transmission range increased to 4cm or longer. With an electromagnetic induction method, the transmission range is up to 5-10mm. The wireless tag can be bent to a curvature radius of up to 2mm. When the tag is bent and the inductance of the power-receiving coil changes, the reader unit detects the change and adjusts transmission voltage.
"With this function, we reduced power consumption by up to 92%," Someya said.
Commenting on wireless tags that are currently used for physical distribution, etc and have a transmission range from one to several meters (distance between the reader and writer units), Fuketa said, "In those cases, a much higher radio wave frequency is used, making it necessary to drastically improve transistor performance. This time, it was difficult for us."
The latest wireless tag having a transmission range of several centimeters is expected to be used for diapers, adhesive plasters, etc. When it is used for a diaper, the reader unit is attached to the outside of the diaper, and data on the state of the diaper is transmitted to a mobile device via Bluetooth, wireless LAN, etc.
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