Assessing the RF Performance of Medical Implant Antennas

Active implantable medical devices are becoming increasingly more popular as new applications are developed to improve patient treatment and care. Many implant applications are utilising RF techniques to provide communication and control for electronic devices within the human body, especially at frequencies between 400 MHz and 2.45 GHz. Physical size is an obvious and important concern in the design of implant devices and this means that there is a need for innovation in the design of miniaturised antenna solutions that have adequate performance in the challenging human body environment. While there is a role for simulation and modelling, the test and characterisation of these innovative implant antennas is another crucial factor in the development of these medical systems. There are many issues to take into consideration that can affect the results obtained when testing miniaturized implant antennas. For example, the local environment surrounding the antenna affects both impedance matching and radiation performance and so the test environment, typically a tissue equivalent phantom, is extremely important. Additionally, in practice the close proximity of the body tissues affects the antenna’s radiation performance and introduces significant losses into the reactive near fields that are associated with resonant antennas. This highlights not only the importance of accurate phantom modelling and the effect of antenna coating material, but also the source feeding techniques and characterizing its sensitivity inside the phantom. 
This paper will specifically illustrate the challenges faced when conducting radiation efficiency measurements for implantable antennas when they are inserted inside a tissue representative human body phantom. Our objective is to highlight the disturbance caused by the necessary feed cable and demonstrate techniques to minimize it. Only standardized antennas were used and the measurements were conducted using a novel reverberation chamber approach to obtain highly accurate results at extremely low radiation efficiencies associated with these applications. In order to gain a better understanding, the experiment was repeated with different antennas and phantom set ups and the results clearly illustrate the factors that need to be considered when assessing the RF performance of medical implant antennas.