Implantable antennas for biomedical applications

Recently, the interest in implantable devices for biomedical telemetry has significantly increased. Amongst the different components of the implantable device, the antenna plays the most significant role in the wireless data transmission. However, the human body around the antenna alters its overall characteristics and absorbs most of its radiation. Therefore, this thesis is mainly focused on improving the antenna characteristics (bandwidth and radiation efficiency) to overcome the human body effect and investigating new structures that reduce the power absorption by the human body tissues. A novel antenna design methodology is developed and used to design new flexible implantable antennas of much lighter weight, larger radiation efficiency, and wider bandwidth than existing embedded antennas. These antennas work for multiple ((401-406 MHz) MedRadio, 433 MHz and 2.45 GHz ISM) bands which satisfy the requirements of low power consumption and wireless power transfer. This has been combined with thorough investigations of the antenna performance in the anatomical human body. New effective evaluation parameters such as the antenna orientation are investigated for the first time. New structures inspired by complementary and multiple split ring resonators (CSRRs and MSRRs) are designed. The structures are found to reduce the electric near field and hence the absorbed power which increases the radiated power accordingly. This new promising function of metamaterial based structures for implantable applications is investigated for the first time. The path loss (between pacemaker and glucose monitoring implantable antennas inside the anatomical body model) and (between an implantable and external antennas for a wireless power channel at 433 MHz) are estimated. Moreover, the optimum antenna type for on-in body communication is investigated. Loop antennas are found to outperform patch antennas in close proximity to the human body

Implantable antennas for bio-medical applications

Biomedical telemetry has gained a lot of attention with the development in the healthcare industry. This technology has made it feasible to monitor the physiological signs of patient remotely without traditional hospital appointments and follow up routine check-ups. Implantable Medical Devices(IMDs) play an important role to monitor the patients through wireless telemetry. IMDs consist of nodes and implantable sensors in which antenna is a major component. The implantable sensors suffer a lot of limitations. Various factors need to be considered for the implantable sensors such as miniaturization, patient safety, bio-compatibility, low power consumption, lower frequency band of operation and dual-band operation to have a robust and continuous operation. The selection of the antenna is a challenging task in implantable sensor design as it dictates performance of the whole implant. In this paper a critical review on implantable antennas for biomedical applications is presented

Implanted Antennas for Biomedical Applications

Body-Centric Wireless Communication (BCWC) is a central topic in the development of healthcare and biomedical technologies. Increasing healthcare quality, in addition to the continuous miniaturisation of sensors and the advancement in wearable electronics, embedded software, digital signal processing and biomedical technologies, has led to a new era of biomedical devices and increases possibility of continuous monitoring, diagnostic and/or treatment of many diseases. However, the major difference between BCWC, particularly implantable devices, and conventional wireless systems is the radio channel over which the communication takes place. The human body is a hostile environment from a radio propagation perspective. This environment is a highly lossy and has a high effect on the antenna elements, the radio channel parameters and, hence a dramatic drop in the implanted antenna performance. This thesis focuses on how to improve the gain of implanted antennas. In order to improve the gain and performance of implanted antennas, this thesis uses a combination of experimental and electromagnetic numerical investigations. Extensive simulation and experimental investigations are carried out to study the effects of various external elements on the performance improvement of implanted antennas. The thesis also shows the design, characterisation, simulation and measurements of four different antennas to work at ISM band and seventeen different scenarios for body wireless communication. A 3- layer (skin, fat and muscle) and a liquid homogenise phantom were used for human body modelling in both simulation and measurements. The results shows that a length of printed line and a grid can be used on top of the human skin in order enhance the performance of the implanted antennas. Moreover, a ring and a hemispherical lens can be used externally in order to enhance the performance of the implanted antenna. This approach yields a significant improvement in the antenna gain and reduces the specific absorption rate (SAR) in most cases and the obtained gain varies between 2 dB and 8 dB

Conformal antenna-based wireless telemetry system for capsule endoscopy

Capsule endoscopy for imaging the gastrointestinal tract is an innovative tool for carrying out medical diagnosis and therapy. Additional modalities beyond optical imaging would enhance current capabilities at the expense of denser integration, due to the limited space available within the capsule. We therefore need new designs and technologies to increase the smartness of the capsules for a given volume. This thesis presents the design, manufacture and performance characterisation of a helical antenna placed conformally outside an endoscopic capsule, and the characterisation in-silico, in-vitro and in-vivo of the telemetry system in alive and euthanised pigs. This method does not use the internal volume of the capsule, but does use an extra coating to protect the antenna from the surrounding tissue and maintain biocompatibility for safe use inside the human body. The helical antenna, radiating at 433 MHz with a bandwidth of 20 MHz within a muscle-type tissue, presents a low gain and efficiency, which is typical for implantable and ingestible medical devices. Telemetry capsule prototypes were simulated, manufactured and assembled with the necessary internal electronics, including a commercially available transceiver unit. Thermistors were embedded into each capsule shell, to record any temperature increase in the tissue surrounding the antenna during the experiments. A temperature increase of less than 1°C was detected for the tissue surrounding the antenna. The process of coating the biocompatible insulation layer over the full length of the capsule is described in detail. Data transmission programmes were established to send programmed data packets to an external receiver. The prototypes radiated at different power levels ranging from -10 to 10 dBm, and all capsules demonstrated a satisfactory performance at a data rate of 16 kbps during phantom and in-vivo experiments. Data transmission was achieved with low bit-error rates below 10-5. A low signal strength of only -54 dBm still provided effective data transfer, irrespective of the orientation and location of the capsule, and this successfully demonstrated the feasibility of the system

Development of multi-material phantoms and implanted monopole antennas for bone fracture monitoring

This thesis presents a novel method for monitoring the healing of severe bone fractures. This would be particularly useful during the first two to four weeks after trauma where x-ray and computerised tomography scanning cannot provide an accurate indication regarding the healing status of the fractured bone. The technique involves measuring the radiofrequency transmission from one bone-implanted monopole to another, each one located on either side of the bone fracture. Throughout this thesis, it is envisaged that the monopoles will also act as the screws of an external fixation implanted into patients for the stabilization and alignment of the bone fragments. To replicate a simplified version of a human limb, several multi-material semi-solid phantoms were developed to represent bone marrow, bone cortical, blood and muscle. Medical literature indicates that the amount of blood found at the initial stage of a bone fracture decreases as bone regeneration takes place towards the healed state. The rate of change of the 21 of the implanted monopoles over time was shown to provide a tool that allowed the estimation of the amount of blood (hematoma) inside any bone fracture. In this thesis it has been shown that as the effective dielectric properties of the investigated fractured area shifted from the dielectric properties of blood towards the properties of bone, the 21 of the monopoles increased, thus, this technique can be used to indicate bone healing. The simulated results were validated in measurements using several multi-material phantoms and a real lamb joint. Finally, an analytical model on the approximation of the 21 of the monopoles in the near field inside the multi-material phantoms was developed. The results showed good agreement over the frequency spectrum of 1 to 4GHz and reasonable agreement over the parametric investigation of separation distance between them for the range of 1 to 7cm. This will potentially allow the application of the proposed technique for special types of fractures where the screws of the external fixation are separated by different distances

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance

Beam scanning by liquid-crystal biasing in a modified SIW structure

A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium

Link budget investigations for ingestible antenna in MedRadio band

Ingestible Medical Devices (IMDs) have given a strong boost to the health sector by creating new horizons in the prevention, monitoring and treatment of diseases in the gastrointestinal (GI) tract. In this study, we numerically assess the telemetry link between an ingestible antenna and an on-body antenna which acts as a repeater. Both antennas can be incorporated in a complete Wireless Capsule Endoscopy (WCE) system. Two different scenarios are examined to assess the performance of the ingestible antenna inside the small intestine of an anatomical human model and the link with the on-body antenna. The ingestible antenna is placed in vertical and horizontal position inside the small intestine. It is found that a reliable communication link can be established, regardless the antenna orientation inside the small intestine, for a net input power of the ingestible antenna significantly lower than the maximum allowable power recommended by the IEEE safety guideline