Circularly Polarized Antennas For Implantable Medical Devices

With the aging of the world's population and the high intensity of work,the consciousness of health is growing,causing the researchers' interests in the design of the antennas for implantable medical devices(IMDs).As the one of key components in IMDs,implantable antennas can transmit the physiological information to the outside receivers wirelessly.Not only can circularly polarized antennas solve the issue of maintaining a fixed orientation of linearly polarized in-body antenna with respect to the external linearly polarized receiver,but also improve the ability of anti-interference,caused by multipath propagations.This dissertation was supported in part by the National Natural Science Foundation of China under Grant 61372008,and in part by the Science and Technology Planning Project of Guangdong Province under Grant 2014A010103014,2015B010101006,tackling with the investigation of circularly polarized implantable antennas.The main works include as follows:(1)A single-band miniaturized wide-band circularly polarized(CP)implantable antenna: Due to the independence of orientation and immune to multipath interference,a miniaturized CP antenna operating at 915-MHz Industrial,Scientific,and Medical(ISM)band is investigated.The miniaturization of the proposed antenna is achieved by adding crossed slot to the square patch.Two shorting pins are introduced,enhancing the impedance bandwidth.Meanwhile,arrow-shaped slots are loaded to realize the performance of circular polarization.Finally,a low-profile antenna with a dimension of 13 mm × 13 mm × 0.635 mm is achieved.The impedance bandwidth is 0.743–1.07 GHz(35.4%)and the 3-dB axial-ratio(AR)bandwidth is 0.863–1 GHz(14.7%).Due to the restricted operating environment of implantable antennas,the simple human model and the high spatial resolution human phantom are built.In order to reduce the power consumption of IMDs and extend its life cycle,a dual-band implantable antenna is further studied.(2)A differentially-fed dual-band dual circularly polarized implantable antenna: Through loading triangle-shaped patches to the double rings,the proposed antenna achieves a compact dimension in principle of slow wave effect.Meanwhile,915-MHz ISM band is covered for controlling mode.Moreover,the loading patches contribute to the resonance in the 2.45-GHz ISM band for data communication.The proposed design allows IMDs to operate in dual bands corresponding two modes,thus prolonging the battery lifetime.By adjusting the sizes and shapes of double rings and loadings,dual circular polarizations(CPs)are obtained.Besides,differentially-fed structure is adopted,making the proposed antenna conveniently connected with the differential circuits in IMDs and improving the systematic integration level and the ability to suppress interference.The proposed antenna features low profile with the dimensions of 18 mm × 9 mm × 0.635 mm.The impedance bandwidth of 795–1110 MHz(33%)and the AR bandwidth of 859–960 MHz(11.1%)for the lower band are obtained,respectively.The impedance bandwidth is 2.36–2.58 GHz(8.9%)and the AR bandwidth is 2.29–2.5 GHz(8.7%)for the higher band.The principle of the antenna,critical parameters,radiation performance,SAR(Specific Absorption Rate),and link budgets are analyzed.Finally,the proposed antenna was fabricated and measured.The measured results agree well with simulated ones,validating the theoretical design.This work has been presented in IEEE International Conference on Computational Electromagnetics(ICCEM).(3)A circularly polarized antenna for wireless capsule endoscope application: Based on the structure and principle of the dual-band dual circularly polarized antenna as designed in Part(2),the proposed planar antenna is folded to conform outer wall in a capsule by using a new three-layer transformation approach.The three-layer transformation approach and the design process of capsule antenna are described.The size of the capsule is 11.4 mm × 26 mm(Diameter × Length)and the thickness of the proposed antenna is 0.6 mm.The final capsule antenna has the characteristic of self-packaged structure,acting as a good radiating unit and as mechanically packaging and EMI protecting layer.Lastly,the radiation performance of the proposed capsule antenna is analyzed.This works show that the circularly polarized antennas play critical role in improving the ability to restrain interference for IMDs.With the advantages of miniaturization,bandwidth enhancement,and low power consumption,the proposed circularly polarized antenna can fit for complicated environment,ideally suiting to biomedical telemetry.The designing method-ology of the CP antennas would supply the reference guideline for similar researches.