Miniaturized and Highly Sensitive Epidermal RFID Sensor for Reliable Skin Temperature Monitoring at a Distance

An electrically small (ES), epidermal radio frequency identification (RFID) tag is developed, which enables reliable human body temperature monitoring at a distance. It is based on the Huygens dipole antenna (HDA) technology. The inherent unidirectional radiation advantage of a Huygens source along with the tag being designed to be circularly polarized (CP) enhances the read distance of the reader and reduces the polarization loss. The CP HDA is then seamlessly integrated directly with a commercial temperature sensor to facilitate both minimal insertion loss and overall size. The simulation results of this passive tag show that ka of the antenna is 0.52 and the gain at 915 MHz reaches 2.32 dBic. A complete RFID system was assembled to validate the performance of the epidermal tag. Measurements demonstrate that it achieves stable body temperature monitoring with a measurement resolution of 0.25 °C and a maximum read distance of 170 cm, which is a factor of more than 14 times larger than a commercial dipole-based product.