A Wireless System for Monitoring Polymer Encapsulations

We present a principle for monitoring of the electrical properties of polymers used as insulators for electrically active implants. This method can be used for measurements in vitro and in vivo with incomplex instrumentation. The system is based on the detuning of an oscillating circuit with an interdigital electrode (IDE) structure serving as a capacitive and resistive sensor within the oscillator. This circuit is powered via an inductive link from an external coil. The phase of the external coil´s impedance is used to determine the resonance frequency and quality factor of the sensing part wirelessly. The research objective is to obtain detailed information about processes at the metal/polymer interface such as a change of the capacity due to altering of the dielectric constant (i.e. uptake of water vapor or condensation of water) and lowering of the quality factor because of leakage currents. With this information it is possible to detect if the encapsulation is stable, if degradation and loss of adhesion occurs, and if the metal corrodes. The method can be used to evaluate the long term stability of materials and technologies in vitro. The future application is to monitor the stability of implant encapsulations in situ to predict failures before they occur.