Design and Implementation of Subcutaneous UHF Band Antennas for Smart Implants using a Novel Characterization Procedure.

Abstract

Smart implants enable the wireless transfer of physiological parameters gathered inside the human body. In this research work two broadband antennas for implanted smart central venous catheters (SCVC) are designed, implemented and characterized using a novel characterization procedure. The design of implanted antennas involves several challenging aspects such as miniaturization because of the very limited space, high efficiency despite the highly lossy environment in the near field of the antenna, adaptability to the given shape of the implant as well as insulation from the surrounding tissue. These constraints in mind, the electromagnetic specifics of body tissues are studied. This knowledge is required for a profound simulation and analysis of antenna topologies suitable for smart implants. According to two different scenarios for SCVC applications, two different antenna topologies are proposed. A planar round-shaped broadband UHF antenna for passive RFID is designed for mounting on the top of a smart CVC reservoir placed in a subcutaneous position in the chest. This printed monopole-strip antenna operated at 868 MHz is suitable for near field applications. Alongside a virtual body phantom of the chest, near field simulations as well as simulations in the close far field up to 1 meter distance are run. Since the actual working range turns out to be narrower than anticipated, another topology is projected answering the purpose of higher performance in the far field. This dual-band CVC antenna is 3-D conformal to a truncated cone and scheduled for 402-405 MHz MICS band and 2.4 GHz ISM band. The corresponding smart CVC is battery powered to provide a wide working range. Measurement environments imitating the properties of the human body are prepared and the antenna prototypes are implemented in a test bed. Measurements inside a body phantom are carried out, yet, the results do not reveal conclusive data. Simulations of the antenna in the test bed detect an influence of the test bed feeding cables on the radiation properties. This observation anticipates the insight that simulation and measurement cannot be regarded separately, but need to be interpreted in common. Only a procedure that comprehends a combination of both is a viable way to accurately characterize antenna properties for a selected application despite all manipulating factors. In order to resolve the observed mismatch, an uncertainty factor is calculated taking into account the measured and the simulated maximum gain. The obtained results, again, are used to adapt the dual-band UHF antenna to the smart implant prototype. Finally, the performance of the system is examined by running functional tests. These prove that the link budget calculation reliably enables the evaluation of possible application scenarios and, in particular, the maximum operating distance of the future system in certain positions even before a working smart implant prototype is manufactured. The results of the study state that the presented novel characterization procedure is suitable to verify obtained property data. Consequently, the limits of measurement set ups can be compensated and realistic and comparable antenna characterization can be assured.