A Novel Electromechanical Interrogation Scheme for Implantable Passive Transponders

This paper presents design, fabrication, and implementation of a novel electromechanical energy scavenging and wireless interrogation scheme using low frequency components of musical vibrations to overcome challenges associated with previously reported passive transponders such as: short transmission range, misalignment sensitivity, and complicated receiver circuitry. The transponder has two phases of operation: 1) mechanical vibration phase, in which an acoustic receiver (a piezoelectric cantilever) converts the sound vibration into electrical power and charges a capacitor; and 2) electrical radiation phase, in which the stored charge is dumped into an LC tank, forcing it to oscillate at its natural resonance frequency and emitting the energy to an outside receiver. In a pressure sensing configuration, the distance between a planar coil and a ferrite core is modulated by the pressure, thus changing the inductance and in turn inducing a change in the frequency of the emitted signal. A prototype transponder was built and tested using a PZT cantilever with a mechanical resonant frequency of 435 Hz encapsulated in a glass capsule (length=40 mm, diameter=8 mm) along with a rectifier circuitry and a storage capacitor. The inductive pressure sensor located outside the capsule had a sensitivity of 2.5k Hz/kPa. We were able to easily pick up the transmitted RF pulses at distances of up to 7 cm without the tight requirement on alignment between the receiver and the transponder coils.