Research On Implantable Antennas For RF Energy Harvesting

With the development of biotelemetry and microelectronics,implantable medical devices（IMDs）play an increasingly important role in the diagnosis and treatment of various disease.Most conventional IMDs are powered by batteries,but expensive surgical operations are required to change the life-limited batteries.Therefore,wireless power transfer（WPT）provides a convenient way to supply power,extending the lifetime of IMDs.The advantages of WPT through microwave radiation are that it has long transmission distance,and the size of transmission antennas is much smaller than the coils in near-field coupling.As the key component in RF energy harvesting systems,the implantable antennas have a significant effect on the efficiency of WPT systems based on microwave radiation.This dissertation was supported in part by the National Natural Science Foundation of China（61372008）,and in part by the Science and Technology Planning Projects of Guangdong Province（2014A010103014,2015B010101006）,the following work is carried out mainly about the investigation of implantable antennas for RF energy harvesting:（1）A miniaturized implantable antenna based on reactive loading:Considering that in the RF energy harvesting systems of IMDs,the antennas and other electronic components should have a compact size to avoid the rejection reaction caused by the human body,a miniaturized implantable antenna based on reactive loading is proposed.The proposed antenna is miniaturized by means of meandering structure,reactive loading and the use of dielectric substrate with high dielectric constant,and the sizes of the optimized antenna are reduced toπ×4²×1.27 mm³.The impedance bandwidth of the proposed antenna is 0.78–1.13 GHz（36.6%）with|S₁₁|below-10 d B,covering the Industrial,Scientific and Medical（ISM）band of 902–928 MHz.The maximum realized gain of the proposed antenna at 915 MHz is–29.5 d Bi.In addition,to be applied in RF energy harvesting,the rectifier circuit matched in the working frequency of the proposed antenna is presented,and the integration scheme of the implantable rectenna is proposed.In the WPT systems of IMDs,the linearly polarized antennas are prone to polarization mismatch loss due to the motion of the human body,resulting in the reduction of the received RF power.Therefore,a wideband circularly polarized implantable antenna for RF energy harvesting is proposed.（2）A wideband circularly polarized implantable antenna:The sizes of the proposed antenna are reduced to 7.5×7.5×1.27 mm³ by etching four C-shaped slits in the radiation patch,and the resulting multiple resonant frequencies enable the antenna to achieve wide band.In addition,the introduction of rectangular slot truncated diagonally makes the phase difference of the degenerate modes reach 90 degrees for improving the purity of circular polarization.The impedance bandwidth of the proposed antenna is 30.4%（1.9–2.58 GHz）,and the 3-d B axial-ratio bandwidth is 16.9%（2.17–2.57 GHz）,covering the ISM band of 2.4–2.48 GHz.Furthermore,a rectifier circuit is presented and integrated under the proposed antenna,and the performance of RF energy harvesting is verified and analyzed.However,the above antennas are single-frequency antennas.To achieve the multiband operating modes of RF energy harvesting and data transmission,multiband antennas are required.And considering that the wideband antennas working in human tissues have better robustness,we propose a wideband dual-band implantable antenna for RF energy harvesting and data transmission.（3）A dual-band implantable antenna for RF energy harvesting and data transmission:By introducing multiple radiating branches and etching C-shaped slot,multiple resonant frequencies are generated to achieve dual-band and wideband performance.The proposed antenna has the footprint of 7.9×7.7×1.27 mm³.The impedance bandwidths of the proposed antenna are 0.67–1.05 GHz（44.2%）and 2.11–2.96 GHz（33.5%）,covering the ISM band of902–928 GHz and 2.4–2.48 GHz,with peak gains of–28.9 d Bi and–29.5 d Bi,respectively.In addition,the integrated structure of the implantable rectenna is presented for RF energy harvesting at 915 MHz,and the performance of the proposed rectenna is measured and evaluated.The implantable antennas proposed in this dissertation have the characteristics of miniaturization,wide bandwidth,high robustness and easy integration with the rectifier circuit,which can be applied in the RF energy harvesting systems of IMDs.The designed implantable antennas and technical methods would give some guideline for the relevant researches.