Wearable sensors based on modulated frequency selective surfaces

The aim of this work is to study the feasibility of using the modulated frequency selective surface (FSS) as a semi-passive tag for body sensing applications at 2.45 GHz ISM band. Two prototypes are presented: a breathing sensor and a skin thermometer. The breathing sensor measures the change of the airflow temperature that is produced during respiration. The core of this sensor is a negative temperature coefficient resistor (NTC), which is placed close to the nose. Likewise with the breathing sensor, the skin thermometer uses an NTC resistor, which is mounted directly onto the skin. It detects variations in terms of temperatures less than 0.05 K. In both, the changing in the resistance modifies the oscillation frequency of an oscillator, which is based on a low-power NAND that modulates the bias on the varactor diodes. The wireless communication is performed by a backscattered field technique, which varies the signal received by the transponder. The tag is composed by an array of dipoles, which are loaded with varactor diodes. This backscattering technique saves a large amount of energy in comparison to commercial alternatives and guarantees a long battery lifetime. Consequently, these sensors can be used simultaneously to perform long continuous real time measurements permitting them to collect information on the heath condition of the user. The design, simulation, and characterization of the FSS are discussed, as well as performance of the sensors when place on the body. The final range that has been read is greater than 3 m.