Characterization of miniaturized RLC resonators made of biodegradable materials for wireless implant applications

As a first step toward the ultimate goal of making fully biodegradable implants for in vivo operation, miniaturized radio frequency (RF)-driven resistor-inductor-capacitor (RLC) resonators made of biodegradable materials are fabricated and characterized. The resonators made of biodegradable metals (magnesium, iron, Mg- and Fe-alloys) are fabricated by electric discharge machining, while the resonators made of biodegradable conducting polymer composites (polylactide-polypyrrole, PLLA-PPy and poly(epsilon-caprolactone)-polypyrrole, PCL-PPy) are fabricated by compression molding combined with laser-cutting. The unloaded resonant frequency f(unloaded) and unloaded quality factor Q(unloaded) of the RLC resonators are obtained from capacitive coupling and inductive coupling measurements. f(unloaded) and Q(unloaded) are found to be 0.5-1 GHz and 8-410 for the metal resonators, and 2.0-3.4 GHz and 6-19 for the polymer resonators, respectively. The RF conductivity and RF relative permeability of the materials used to fabricate the RLC resonators are evaluated from capacitive coupling measurements combined with finite element simulations. Finally, the performance in a human body-like environment is evaluated. Even if largely attenuated, the resonance peak remained detectable with 6 mm of muscle/fat tissue placed between the RLC resonator and the measurements coil, for both metal and polymer resonators. As a conclusion, all the investigated biodegradable materials can be suitable to fabricate passive wireless communication devices for biosensors implanted inside the body. At this state of the research, magnesium is the most promising candidate to fabricate RLC resonators, since it combines high conductivity (high Q) and promising biodegradation properties. Alternatively, the Mg-alloy could be of interest if faster biodegradation and higher mechanical performance of the implant is required. On the other hand, the biodegradable conducting polymers PCL-PPy and PLLA-PPy are also of interest because of their light weight, higher flexibility in the fabrication process, compatibility with magnetic resonance imaging and transparency to X-rays. (C) 2012 Elsevier B.V. All rights reserved.