UHF-band RFID tag sensor that works underwater (1)


We have been conducting research and development of a wearable UHF-band RFID tags that can communicate even when worn on the human body, which is a high-k dielectric material, and have found that such RFID tags even work underwater. In Movie 1, a UHF-band  RFID tag set in a glass container is gradually filled with warm water, and when the entire RFID tag is immersed in warm water, communication with the RFID reader/writer becomes possible and wireless temperature measurement begins. Fig. 1 shows a comparison of wireless temperature measurements by a RFID tag temperature sensors and a K-thermocouple, and the results show good agreement.



For such wireless temperature measurement, a RFID tag IC with temperature sensing functions is used. Magnus®-S series ICs  (AXIZON) are typical UHF-based RFID tag IC chips. In addition to the temperature sensing function, the Magnus®-S3 has an impedance auto-tuning function that outputs the tuning amount as a  sensor code S corresponding to environmental changes such as ambient dielectric constant. It also has a function to output on-chip RSSI and reader RSSI, which are received signal strength indicators (RSSI) on RFID tags.



The Magnus®-S series ICs were developed by the former RFMicron in collaboration with Smartrac and released in 2016 as "Passive RFID Temperature Sensor Technology. Unfortunately, despite its superior functionality, it is currently not widely used. However, it is expected to be one of the elemental technologies in the future ICT society consisting of trillion sensors from the point that WPT (Wireless Power Transfer) is possible in the sensing.


【Movie1】wireless measurement of temperature change  underwater using a UHF-band RFID tag sensor (4x speed).

Fig. 1 Temperature measurement in water; wireless measurement with RFID tag temperature sensor compared to thermocouple measurement. Changes in the temperature of an RFID tag temperature sensor and a K thermocouple when hot water of about 80°C is added step by step.

UHF-band RFID tag sensor that works underwater (2)


In recent years, diaper sensors have been developed to detect when a patient's diaper is wetted by excrement and when it is time to change the diaper, with the aim of reducing the labor burden at nursing care facilities.  For example, OMUTSU-TECH Inc has developed such a diaper sensor.


In conventional diaper sensors using UHF-band RFID tag-type sensors, the timing of diaper change is determined by the communication failures between the RFID tag and the reader due to wetting by excrement.  It is difficult to observe the wet condition of the diaper during the process because the communication of conventional RFID tags is inhibited underwater. The UHF-band RFID tag sensor we developed for high-dielectric materials can communicate underwater, and the Magnus®-S3 IC chip has the advantage of simultaneously detecting the temperature of the person being cared for.


Fig. 2 shows the changes of temperature of a diaper and sensor code S measured using with a UHF RFID tag sensor (IC chip Magnus®-S3) attached to a diaper that can function even underwater, where 5 ml of warm water (ca. 37°C) was dropped stepwise on the diaper. The sensor code S of the RFID tag sensor shows a smaller value in an environment with a high dielectric constant, i.e., in an environment with a lot of water. The sensor coar S was rapidly decreased by dropping the warm water on the diaper including the RFID tag inside. After that, the value of sensor code S gradually recovered to a constant value as the water diffuses into the surrounding polymer water absorbent.  As the number of drops of warm water increased (as the water content of the diaper increased), the value of sensor code S decreased. Thus, the UHF-band RFID tag sensor, which functions even in water, can sense the wetness of a patient's diaper while simultaneously measuring the temperature.


Fig.2  Changes of temperature of a diaper and sensor code S measured using with a UHF RFID tag sensor (IC chip Magnus®-S3) attached to a diaper that can function even underwater, where 5 ml of warm water (ca. 37°C) was dropped  stepwise on the diaper.  In the measurement, the RFID tag sensor was attached to the inside of the diaper's polymer water absorbent.

The production of UHF-band RFID tags usually requires large-scale mass-production equipment and a great deal of expense, making it difficult to produce custom products in small lots. We can custom-manufacture and prototype UHF-band RFID tags with various unique characteristics at low cost and in small lots by laser processing (JP2022-109329).