| Over the past few decades, there has been growing interest and increased research on bio-implantable devices using RF telemetry links, enabling the continuous monitoring and recording of physiological data. However the dispersive properties of tissues make this a formidable task. In the present work, a novel technique for tissue characterization using an LC sensor is developed which allows for the extraction of the relative permittivity, and the conductivity of dispersive tissues. The resonant frequency of the sensor is monitored by measuring the input impedance of an external antenna, and correlated to the desired quantities. The impact of multi-layered dispersive tissues on the setup of the telemetry link is demonstrated where the role of the capacitor is analyzed. The sensor consists of a planar inductor, and an interdigital capacitor. Using an equivalent circuit model of the sensor that accounts for the properties of the encapsulating tissue, analytical expressions have been developed for the extraction of the tissue permittivity and conductivity. In addition, the effect of the thicknesses of the tissue layers on the sensor resonant frequency is studied. It is seen that the resonant frequency is strongly affected by the properties of the first layer, whereas additional layers prove to have little to no effect. A saturation thickness is defined that allows for the sensor to be implanted at a depth where it is only affected by the properties of the layer in which it is embedded. In order to analyze the telemetry system, a single loop antenna is evaluated in proximity to the biological tissue layer and the interaction of the electromagnetic field with the body is assessed in terms of specific absorption rate (SAR). It is studied through different multi-layered models composed of skin, fat and muscle, with typical values of tissue thickness. The introduction of multiple tissue layers as well as the misalignment effect is investigated from the SAR distribution. Finally, experimental validation has been performed with a telemetry link that consists of a loop antenna and a fabricated LC sensor immersed in single and multiple dispersive regions. |
