Moisture-sensing Diaper Made With Flexible RFID Tag (page 2)
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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