| Summary: | 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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