Design of implantable antennas for biomedical applications

Abstract

Biomedical telemetry has gained a lot of attention with recent developments in the healthcare industry. This technology has made it feasible to monitor the physiological signs of a patient remotely without traditional hospital appointments and follow up check-ups. Implantable Medical Devices (IMDs) play an important role in monitoring patients through wireless telemetry. IMDs have a wide range of applications which includes wireless endoscopy, blood pressure monitoring, wireless drug delivery, cardiac defibrillation, pacemakers and blood sugar level monitoring etc. IMDs consist of nodes and sensors in which the antenna is a major component. The selection of the antenna is a challenging task in IMD design as it dictates performance of the whole implant. Various factors need to be considered for implantable antennas such as miniaturization, patient safety, biocompatibility, low power consumption and providing robust and continuous operation within a harsh environment. The human body is a very lossy medium and affects the working of the antenna significantly. Therefore, designing an antenna to operate from inside the body is a very challenging task. Three novel implantable antennas are designed using a simple methodology. Computer Simulation Technology (CST) Microwave Studio software is used to design and simulate the antennas. The antennas are compact in size, light weight and show good performance in implantable conditions. A circular patch antenna is designed for operating in Industrial, Scientific and Medical (ISM) band at 915 MHz using coaxial probe feed. The overall volume of the antenna is (π×42×0.38) mm3. At 915 MHz the antenna has a peak gain of -28.8 dBi and has a bandwidth of 90 MHz when simulated in simplified skin layer phantom of the human body. The radiation efficiency of the antenna is -31.6 dB at resonant frequency. The 1-gram(g) and 10-gram(g) average(avg) SAR values for this antenna are 1218 and 125.2 W/Kg when the input power of the antenna was 0.5W. The antenna satisfies the requirements for implantable applications. A microstrip rectangular patch antenna is designed operating in Medical Implantable Communication Service (MICS) band (402-405) MHz and ISM bands of (902-928) MHz and (2.4-2.45) GHz. The antenna resonates at 402 MHz, 915 MHz and 2.4 GHz when simulated in simplified fat layer phantom of the human. The size of the antenna is (6×5×0.5) mm3. At resonant frequencies the peak gain of the antenna is (-47.7, -37.2, -25.5) dBi. This antenna offers a bandwidth of (108, 170, 250) MHz with a radiation efficiency of (-52, -42, -32) dB at operating frequencies. The 1g avg. SAR values of rectangular patch antenna at operating frequencies are (122, 184, 863) W/Kg and 10g avg. SAR values of rectangular patch antenna at operating frequencies are (12.25, 18.42, 86.42) W/Kg when the antenna was excited with an input power of 0.5W. Finally, design of a compact size antenna operating at 915 MHz is presented. The antenna has a size of (4×4×0.26) mm3. When simulated in simplified skin layer phantom the antenna offers a bandwidth of 170 MHz with a peak gain of -34.7 dBi at resonant frequency. The radiation efficiency of the antenna is -36.5 dB. SAR values of this antenna are 1069 W/Kg for 1g avg. and 108 W/Kg for 10g avg. with 0.5W input power. All of the designed antennas are simulated in simplified human body phantom model and multilayer tissues. After that the antennas are subjected to different implant depths to investigate their performance with varying implant depths. Different thicknesses of insulation layer are used to analyse the effects on antenna resonance. To check the antenna integration with sensors, dummy electronic components are used, and antennas are simulated which shows the diversity of designed antennas. The designed antennas are simulated in anatomical body model and the results showed a good match between anatomical body model and phantom body model. A size reduction of 15%, 29% and 47% and overall performance improvement of 9%, 15% and 12% is achieved for the designed circular patch antenna, rectangular patch antenna and compact size antenna which proves that the designed antennas are best match for the implantable applications.