A hydrogel-based wireless sensor using micromachined variable inductors with folded flex-circuit structures for biomedical applications

This paper reports a flexible, inductive wireless sensor that can be combined with a variety of hydrogel materials for biomedical applications. The variable inductor is formed by folding coplanar dual spiral coil with 5-10 mm size that are microfabricated using 50-mum-thick copper-clad polyimide film so that when folded, the two coils are aligned to each other and the mutual inductance depends on the gap 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/deswells depending on the target parameter. The response of a variable inductor to the displacement of the coils is measured to be 0.40 nH/mum. A sensitivity of 113 ppm/mum in wireless frequency measurement is obtained using the passive resonant device that combines the variable inductor with a fixed capacitor. The resonant devices are coupled with a commercial hydrogel wound dressing as well as pH-sensitive poly(vinyl alcohol)-poly(acrylic acid) hydrogel. Wireless monitoring of the swelling of the wound dressing is experimentally demonstrated.