Microwave antennas for biomedical application

Medical diagnosis is one of the key steps to determine the problem of the human body. The current diagnostic tools are expensive, bulky and long exposure to some of these diagnostic tools can be injurious to the human body. Hence, researchers are now exploring through different possibilities to replace current diagnostic tools. Microwave regime is one of the potential candidate to replace current diagnostic system providing with a chip, portable system suitable for the human body. One of the fundamental tool for a microwave diagnostic system is microwave antenna. The current findings on designing microwave antennas for biomedical diagnosis lacks due to low microwave power penetration inside the human body, high specific absorption rate (SAR), low directivity and compactness. This thesis aims on improving the microwave penetration inside the human body and develop antennas that can perform efficiently for biomedical diagnosis application. A multi-layer reflection model is investigated for evaluation of the combined material characteristics of different lossy human tissues, along with the enhanced antenna designs, suitable for biomedical application, operating on-body and as an implant, have been presented within this thesis. The rationale behind this work relates to the early detection of cancerous tissues, internal injuries and other characteristic changes inside the human body with the primary goals being to improve microwave power penetration inside the human body and to provide low SAR and compact microwave antenna system for biomedical diagnosis. The penetration of microwave power inside a human head model is improved by employing calculated permittivity inside a rectangular waveguide used as the microwave transmission source. Firstly, a multi-layer reflection model is created from various human tissue material. The wave impedance of the multi-layer is then extracted from the overall reflection coefficient found at the edge of the multi-layer tissue model. Furthermore, a rectangular waveguide is constructed and an L-probe rectangular waveguide feeding technique is presented. The measured results validate the approach with an increment in power penetration inside the human head 1.33 dB at 2.45 GHz.  Antennas are characterized in-front of homogeneous and a frequency-dependent inhomogeneous human head and shown that inhomogeneous phantom provides with real-life scenario for the measuring antenna whereas the homogeneous phantom only resembles the scenario. The effect of superstrate at the boresight of an on-head matched antenna for biomedical applications is analysed and shown that superstrate layer at the boresight direction of the antenna provides with ~8 dB increased directivity towards the human head with 0.0731 W/kg reduction of SAR compared to the antenna without the superstrate. The design of a 3-D on-body antenna and a coplanar waveguide (CPW) fade antenna matched with an inhomogeneous human head provides the second investigation area. Specific focus has been given to make the designs compact, increase the front to back ratio (FBR) of the radiation pattern and decrease the SAR of the antenna. The 3-D antenna is realized combining a folded inverted F-like structure and a slot-loaded ground plane and backed by a rectangular cavity to minimize side and back lobe radiation. An FBR of 17 dB with SAR less than 0.0147 W/kg is achieved throughout the operating frequency ranging from 1 - 1.7 GHz by the designed antenna while acquiring a compact dimension of 0.23 × 0.23 × 0.04 λ in size with respect to the lowest operating frequency.  An inhomogeneous human head phantom is constructed and used to analyze the antennas performance in real-life scenario. Moreover, the choice of operating frequency for on-head antennas and effect of a superstrate on on-head matched antennas is investigated. An FBR of 20 dB with SAR less than 0.037 W/kg is achieved throughout the operating frequency ranging from 0.788 - 2.5 GHz by the designed antenna while acquiring a compact dimension of 0.1 × 0.1 × 0.008 λ in size with respect to the lowest operating frequency. Finally, the design of an implantable coil antenna is investigated for wireless power transmission inside the human body. The biocompatibility of the building material is analyzed. Polydimethylsiloxane (PDMS) and gold (Au) is utilized as the biocompatible building material to realize the designed implantable antenna. Furthermore, the antenna is characterized in a "complete medium" composed of fetal bovine serum (FBS), penicillin-streptomycin and dulbecco's modified eagle's medium (DMEM) which is used as the cell culture media to resemble designed antennas operation environment. The antenna is impedance matched at 5 MHz frequency with a maximum received voltage of 35 mV is recorded by utilizing the designed implantable antenna