UBC Theses and Dissertations

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UBC Theses and Dissertations

A micromachined inductive sensor using folded flex-circuit structures and its wireless telemetry applications Sridhar, Vijayalakshmi


This thesis reports a flexible, passive wireless inductive sensor with micromachined variable inductors for telemetric applications. The variable inductor is formed by folding coplanar dual spiral coil with 5-10 mm size that are 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 between the aligned coils. The sensor can be combined with a variety of hydrogel materials for biomedical and chemical applications. A stimuli-responsive 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/μm. A sensitivity of 71-110 ppm/μm in wireless frequency measurement is obtained using the passive resonant device that combines the variable inductor with a fixed capacitor created on the polyimide substrate. The fabricated devices are coupled with pH- sensitive poly (vinyl alcohol)-poly (acrylic acid) hydrogel and a commercial wound dressing to experimentally demonstrate wireless monitoring of pH and moisture level within the dressing product, respectively. Theoretical inductive responses of the developed device obtained through finite element analysis and their comparison with the measurement result are also presented.

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Attribution-NonCommercial-NoDerivatives 4.0 International