A hydrogel-based passive wireless sensor using a flex-circuit inductive transducer

This paper reports a passive wireless sensor that can be combined with a variety of hydrogel materials for biomedical and chemical sensing applications. The sensor utilizes the inductive transducer that is formed by folding coplanar dual spiral coil with 5- or 10-mm size, which is microfabricated using 50 μm thick copper-clad polyimide film commonly used for flex-circuit manufacturing. When folded, the two coils are aligned to each other where the mutual inductance depends on the gap separation between the aligned coils. A hydrogel element is sandwiched by the folded substrate to modulate the gap, or inductance of the device, as it swells/de-swells depending on the target parameter. A highly linear inductive response of 0.40 nH/μm to the displacement of the coils is observed with the developed device. A frequency sensitivity of 71–110 ppm/μm is measured using the resonant device with the inductive transducer by monitoring the resonant frequency of the device in a wireless measurement set-up. The fabricated devices are coupled with pH-sensitive poly(vinyl alcohol)–poly(acrylic acid) hydrogel as well as a commercial wound dressing to experimentally demonstrate wireless tracking of pH and moisture level, respectively. Finite element analysis of the sensor's responses and their comparison with the measurement result are also presented.