Differentially Fed Fractal Antennas For Implantable Medical Devices

Due to their inherent capabilities to detect the human vital signs more accurately and conveniently than the sensors outside the body, the implantable medical devices(IMDs) are getting increasingly popular in the fields of diagnostics, biomonitoring, and therapeutics. Implantable antennas are the crucial component for the wireless communication of IMDs. It is used to wirelessly transmit the physiological medical data that are collected by medical sensor to the receiving device outside body. On account of the special working environment, the implantable antennas should have compact size, high anti-interference ability, long service life that bring challenges in antenna designing and fabricating.This dissertation was supported in part by the National Natural Science Foundation of China(61372008), in part by the Science and Technology Planning Project of Guangdong Province(2014A010103014, 2015B010101006), and the differentially fed implantable antennas are designed and researched.The special operating environment of implantable antenna requires us to take the effect of the human body into account at the beginning of antenna design. Hence, the simple human model, and the high spatial resolution human phantom have been built, sequently.(1) A differentially fed implantable fractal antenna operating at MICS band: A differentially-fed structure is adopted for the proposed antenna as it allows for an easily direct connection with the differential circuits of IMDs, meanwhile enabling an enhanced common-mode noise suppression. The high permittivity substrate, Hilbert fractal curves are employed to miniaturize the antenna. The volume of the antenna is 9.3 × 9.3 × 0.635 mm3 and the bandwidth is 67 MHz(374–441 MHz). The process of human modeling is described, and the far-field gain and SAR value associating with human safety are analyzed.In order to further reduce the power consumption of IMDs, and extend its life cycle, based on the above single band antenna, a dual band antenna is designed and analyzed.(2) A differentially fed dual-band implantable fractal antenna: The antenna covers MICS band and 2.44-GHz ISM band. The dual-band operation allows IMDs to operate in two modes, thus saving energy and extending working time of IMDs. The principle of the antenna is analyzed. The parameter, which is used to quantitative analyze the noise suppression ability, is given and the principle of high noise suppression ability is studied. The critical parameters, radiation performance, SAR, and link budgets are analyzed. The antenna prototype is fabricated and measured. The measured results approximately agree with the simulated results that verify the correctness of theoretical design.(3) A differentially fed dual-broadband antenna: In order to improve the robustness of the antenna, and withstand the frequency shift when the antenna is implanted in tissue with different dielectric properties. This work expands the bandwidth of the antenna in the way of creating two nearly resonant frequencies. The bandwidths are 89 MHz(22.1%) and 1 GHz(41%) at MICS and ISM bands, respectively. The evolution process of proposed antenna is introduced. The critical parameters, radiation performance, and SAR are analyzed. For evaluating the communication performance, the relationship between input power and communication rate is studied. In consideration of human safety, the temperature distribution of the body is given while antenna operates.This article shows that the differentially-fed antenna plays an important role in enhancing system integration, suppressing common-mode noise, and reducing power dissipation. The fractal structure has obvious advantage of miniaturizing the size of antenna. The differentially fed fractal antenna can further develop the system integration and noise suppression, reduce power dissipation, and miniaturize antenna size.