Miniaturization of Implantable Antennas for Medical Applications (Σμίκρυνση Εμφυτεύσιμων Κεραιών Για Ιατρικές Εφαρμογές)

European Postgraduate Programme on Biomedical Engineering

Miniaturization of Implantable Antennas for Medical Applications (Σμίκρυνση Εμφυτεύσιμων Κεραιών για Ιατρικές Εφαρμογές)

A Dissertation submitted in partial fulfilment of the requirements for the degree of Master of Science in Biomedical Engineering

Supervised by Prof. Nikolaos Uzunoglou

UNIVERSITY OF PATRAS NATIONAL TECHNICAL UNIVERISTY OF ATHENS FACULTY OF MEDICINE SCHOOL OF ELECTRICAL & COMPUTER ENGINEERING

Panagiotis Blanos

June 2013 Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Acknowledgements

I would like to thank Professor Nikolaos Uzunoglou and Senior Researcher Irene Karanasiou for their supervision, help and encouragement throughout the research and project work.

I would also like to thank MediWise Ltd, in particular George Palikaras and Themos Kallos, for their support, guidance and their technical expertise, as well as for giving me the chance through this project suggestion to further my knowledge and expertise.

Last but not least, I would like to thank my respective friends and family for their love and for believing in me.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Abstract

The underlying project was ran in collaboration with MediWise Ltd. The solution proposed in this dissertation is an optimised implantable antenna, for wireless radiation dosimetry for usage within external-beam radiotherapy, which aims to be further developed in the future in order to produce a commercially viable product. The dissertation presents the design of two types of implantable antenna structures that are suitable for miniaturisation, and focuses on the development of an implantable antenna design that is smaller than 5 x 5 mm in size which operates at 402 - 405 MHz MICS band and on the optimization of the chosen implantable antenna for bandwidth, return loss, radiation, etc. and aim to miniaturise further the antenna at 1 x 1 mm in size.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Περίληψη

Η χρήση της προηγμένης τεχνολογίας για την παροχή υγειονομικής περίθαλψης από απόσταση έχει

τη δυνατότητα να είναι μία από τις πιο καθοριστικές ιατρικές επαναστάσεις του 21ου αιώνα. Είναι

κοινώς αποδεκτό ότι η σύγχρονη ασύρματη τεχνολογία θα διαδραματίσει σημαντικό ρόλο στην

εξέλιξη της προηγμένης τηλεϊατρικής. Η ανάπτυξη των εμφυτεύσιμων ιατρικών συσκευών (IMDs)

είναι μία από τις πιο σημαντικές πτυχές για την εγκαθίδρυση ενός τέτοιου προηγμένου συστήματος

υγειονομικής περίθαλψης. Σημαντικό στοιχείο των εμφυτεύσιμων συσκευών είναι κεραίες που

ενσωματώνονται σε τέτοια συστήματα και επιτρέπουν την ανταλλαγή δεδομένων μεταξύ των

εμφυτεύσιμων συσκευών με το εξωτερικό περιβάλλον.

Η εν λόγω εργασία έγινε σε συνεργασία με την ερευνητική ομάδα της MediWise Ltd. Η λύση που

προτείνεται στην παρούσα εργασία είναι μια βελτιστοποιημένη εμφυτεύσιμη κεραία για ασύρματη

δοσιμετρία ακτινοβολίας για χρήση σε ακτινοθεραπεία εξωτερικής δέσμης, που έχει ως στόχο να

αναπτυχθεί περαιτέρω στο μέλλον, προκειμένου να παραχθεί ένα εμπορικά βιώσιμο προϊόν. Η

διατριβή παρουσιάζει το σχεδιασμό των δύο τύπων των εμφυτεύσιμων δομών κεραίας που είναι πιο

κατάλληλα για την ελαχιστοποίηση των διαστάσεων, και επικεντρώνεται στην ανάπτυξη ενός

εμφυτεύσιμου σχεδιασμού της κεραίας που είναι μικρότερο από 5 x 5 χιλιοστά σε μέγεθος το οποίο

λειτουργεί στα 402 - 405 MHz MICS μπάντα και στην βελτιστοποίηση της επιλεγμένης εμφυτεύσιμης

κεραία για εύρος ζώνης, απώλεια επιστροφής, ακτινοβολία, κλπ. και αποσκοπεί στην περαιτέρω

σμίκρυνση της κεραίας σε μέγεθος 1 x 1 χιλιοστά.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Table of Contents Acknowledgements ...... 0 Abstract ...... 2 Chapter 1 ...... 8 Introduction ...... 8 1.1 Introduction to Implantable Devices ...... 8 1.2 Wireless Medicine ...... 11 1.3 Biomedical Telemetry ...... 12 1.4 Frequency Bands [1] ...... 15 1.5 Implantable Antennas ...... 16 1.6 Challenges in the Design of Implantable Antennas ...... 18 1.7 MediWiSe [19] ...... 19 1.8 Aims, Objectives and Overview of the MSc Thesis ...... 19 Chapter 2 ...... 21 Theory and Research Background ...... 21 2.1 Biological Tissues Properties ...... 21 2.1.1 Biocompatibility and Biomaterials ...... 21 2.1.2 Electrical Properties of Human Tissue ...... 22 2.1.3 Electromagnetic Radiation Interaction with biological tissues ...... 22 2.2 Operating Principles of Implantable Antennas ...... 24 2.2.1 Microstrip - Patch Implantable Antenna ...... 24 2.2.2 Planar Inverted F-Antenna (PIFA) [31] ...... 24 2.2.3 Loop Antenna ...... 25 2.3 Antenna Performance Parameters ...... 26 2.4 Miniaturization Techniques...... 29 2.5 Literature Review ...... 30 2.5.1 Microstrip and PIFA Antennas ...... 30 2.5.2 Stacked PIFA Antennas...... 33 2.5.3 Loop Antennas ...... 34 2.6 CST Microwave Studio - 3D EM Simulation Software [72] ...... 36 2.6.1 Antenna Calculations ...... 36 2.6.2 Meshing (Discretization) ...... 37 2.6.3 Simulation ...... 37 2.6.4 Adaptive Meshing...... 38 Chapter 3 ...... 39 Antenna Designs - Simulations - Parametric Studies - Effects ...... 39 3.1 Initial Antenna Designs ...... 39 3.1.1 Meandered PIFA ...... 40 3.1.2 Spiral PIFA ...... 42

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

3.2 Parametric Studies of Meandered PIFA – Effects ...... 44 3.2.1 Effect of the Electrical Length ...... 44 3.2.2 Effect of the Dielectric Constant (Substrate - Superstrate) ...... 46 3.2.3 Effect of background material and size (Phantoms) ...... 48 3.2.4 Effect of Rectangular Strip Width ...... 51 3.2.5 Effect of Feeding Point Position ...... 52 3.2.6 Effect of Port Impedance ...... 55 3.2.7 Effect of Antenna Size ...... 57 3.3 Proposed Meandered PIFA design ...... 58 3.4 Parametric Studies of Spiral PIFA - Effects...... 61 3.4.1 Effect of Electrical Length ...... 61 Chapter 4 ...... 63 Optimised Square Spiral Antenna Design ...... 63 4.1 Effect of Antenna Size ...... 64 4.2 Effect of Port Impedance...... 65 4.3 Effect of Superstrate ...... 68 4.4 Effect of Antenna Implanted Depth in Fat Phantom ...... 69 4.5 Proposed Spiral Antenna ...... 70 Chapter 5 ...... 73 Future Work ...... 73 5.1 Antenna Design (1 x 1 mm) ...... 73 Chapter 6 ...... 76 Conclusions ...... 76 Chapter 7 ...... 77 References ...... 77

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

LIST OF FIGURES

Figure 1.1 Pacemaker [10] ...... 9 Figure 1.2 Cochlear Implant [11] ...... 10 Figure 1.3 Retinal Implant [12] ...... 11 Figure 1.4 A wireless digital biotelemetry system that monitors vital signs using a smartphone [13] ...... 12 Figure 1.5 Wireless Implantable System for Data Telemetry [15] ...... 13 Figure 1.6 Illustration of a biomedical antenna application based on an implantable RF system [17] ...... 17 Figure 2.1 A patch antenna in its basic form [30] ...... 24 Figure 2.2 The Planar Inverted-F Antenna (PIFA)[31] ...... 25 Figure 3.1 Simpliﬁed body model for the design of planar antennas implanted in a human body [42] ...... 40 Figure 3.2 Meandered PIFA configuration [42] ...... 41 Figure 3.3 3D illustration of the Meandered PIFA in CST Microwave Studio environment ...... 41 Figure 3.4 Spiral PIFA configuration [42] ...... 42 Figure 3.5 3D illustration of the Spiral PIFA in CST Microwave Studio environment ...... 43 Figure 3.6 Top view of the Meandered PIFA with (a) 4 strips and (b) 8 strips ...... 45 Figure 3.7 Front view and side view of Meandered PIFA configuration parameters ...... 45 Figure 3.8 Position of feeding point along different strip arms ...... 54 Figure 3.9 3D farfield radiation pattern of the proposed meandered PIFA at 403 MHz ...... 59 Figure 4.1 Front view and side view of 9-turn square spiral PIFA (10 x 10 mm in size) configuration parameters ...... 63 Figure 4.2 Schematic of different antenna implanted depth in Fat Phantom (50 x 50 x 30 mm,εr=4.6023) ...... 69 Figure 4.3 3D farfield radiation pattern of the proposed spiral PIFA at 402 MHz ...... 71 Figure 5.1 Miniature implantable square spiral chip antenna on high resistivity silicon wafer [64]. (a) Top view: Square spiral conductors. (b) Bottom view: serrated ring ground plane ...... 73 Figure 5.2 3D illustration of spiral chip antenna in CST Microwave Studio environment ...... 74

LIST OF TABLES

Table 2.1 Dielectric Properties of Diﬀerent Body Tissues at 403.5 MHz [15] ...... 22 Table 3.1 Number of Meandered strips with the corresponding resonant frequency ...... 44 Table 3.2 Parameters of Meandered PIFA configuration with 8 strips ...... 45 Table 3.3 Effect of dielectric material on the resonance frequency, and reflection coefficient at 350 MHz ...... 47 Table 3.4 Table Parameters of Meandered PIFA configuration ...... 55 Table 3.5 Effect of antenna size on the resonance frequency, reflection coefficient at 403 MHz and antenna bandwidth ...... 58 Table 3.6 Table Parameters of proposed Meandered PIFA configuration ...... 58 Table 3.7 Table of Performance Parameters of proposed Meandered PIFA ...... 60 Table 3.8 Number of spiral turns with the corresponding resonant frequency ...... 61 Table 4.1 Parameters of 9-turn square spiral PIFA (10 x 10 mm in size) configuration ...... 63 Table 4.2 Effect of antenna size on the resonance frequency and reflection coefficient at 403 MHz 65 Table 4.3 Effect of antenna size on the resonance frequency, reflection coefficient at 402 MHz and antenna bandwidth ...... 68 Table 4.4 Table Parameters of proposed Meandered PIFA configuration ...... 70 Table 4.5 Table of Performance Parameters of proposed Spiral PIFA ...... 72

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

LIST OF GRAPHS

Graph 3.1 Simulated and measured return-loss characteristics of meandered PIFA [42] ...... 41 Graph 3.2 Simulated return-loss characteristics of the meandered PIFA in CST Microwave Studio environment ...... 42 Graph 3.3 Simulated and measured return-loss characteristics of spiral PIFA [42] ...... 43 Graph 3.4 Simulated return-loss characteristics of spiral PIFA in CST Microwave Studio environment ...... 43 Graph 3.5 Number of Meandered strips with the corresponding resonant frequency ...... 44 Graph 3.6 Reflection Coefficient of Meandered PIFA configuration with 8 strips ...... 46 Graph 3.7 Reflection coefficient responses for different dielectric constant materials ...... 46 Graph 3.8 Reflection coefficient responses for different fat phantom sizes ...... 48 Graph 3.9 Zoomed in view of reflection coefficient responses for different dielectric constant materials ...... 49 Graph 3.10 Reflection coefficient response for the Meandered PIFA with 8 strips in free space ..... 50 Graph 3.11 Reflection coefficient responses for different Skin phantom sizes ...... 51 Graph 3.12 Reflection coefficient responses for different strip widths ...... 52 Graph 3.13 Reflection coefficient responses for different feeding point positions in the same strip 53 Graph 3.14 Zoomed in view of reflection coefficient responses for different feeding point positions in the same strip ...... 53 Graph 3.15 Reflection coefficient responses for different feeding point positions in different strip arms ...... 54 Graph 3.16 S-parameter impedance view (smith chart) of fres 348 MHz at 50 Ohms...... 56 Graph 3.17 S-parameter impedance (smith chart) view of fres 341 MHz at 100 Ohms ...... 56 Graph 3.18 Reflection coefficient responses for different feeding point impedances ...... 56 Graph 3.19 Reflection coefficient responses for different antenna sizes ...... 57 Graph 3.20 Reflection coefficient of the proposed meandered antenna resonating at 403 MHz ...... 59 Graph 3.21 H-plane radiation pattern of the proposed meandered PIFA (θ = 0°) at 403MHz ...... 60 Graph 3.22 E-plane radiation pattern of the proposed meandered PIFA (θ = 90°) at 403 MHz ...... 60 Graph 3.23 Resonant frequency versus number of spiral turns ...... 61 Graph 3.24 Return loss for 3 and 4 turns Spiral PIFA ...... 62 Graph 4.1 Reflection coefficient responses for different antenna sizes ...... 64 Graph 4.2 S-parameter impedance view (smith chart) of fres 403 MHz at 50 Ohms...... 66 Graph 4.3 S-parameter impedance (smith chart) view of fres 347 MHz at 130.8 Ohms ...... 66 Graph 4.4 Reflection coefficient responses for different feeding point impedances ...... 66 Graph 4.5 Reflection coefficient responses for different antenna sizes ...... 67 Graph 4.6 Reflection coefficient responses for the antenna with and without superstrate ...... 68 Graph 4.7 Reflection coefficient responses for different antenna implanted depth ...... 69 Graph 4.8 Reflection coefficient of the proposed spiral antenna resonating at 402 MHz ...... 70 Graph 4.9 H-plane radiation pattern of the proposed spiral PIFA (θ = 0°) at 403MHz ...... 71 Graph 4.10 E-plane radiation pattern of the proposed spiral PIFA (θ = 90°) at 403 MHz ...... 71 Graph 5.1 Measured inductance and quality factor for inductor on SOG/HR-Si [64] ...... 74 Graph 5.2 Simulated return-loss characteristics of the square spiral chip antenna in CST Microwave Studio environment ...... 75

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Chapter 1

Introduction

The first chapter is introductory and clearly shows the current state-of-the-art in implantable medical devices for medical telemetry. Essential element of implantable devices are antennas embedded in such systems, which enable the exchange of data between implantable devices and external environment. Challenges faced by the designer of implantable antennas are considered and the subject of this thesis is presented, which is the design of an optimised implantable antenna, for wireless radiation dosimetry, for usage within external-beam radiotherapy.

1.1 Introduction to Implantable Devices

The development of Integrated Circuits (ICs), over the last 35 years, has facilitated the evolution of complex and highly integrated small medical devices [1]. The increasing demand for non-invasive surgical operations has made the use of Implantable Medical Devices (IMDs) as part of medical procedures highly attractive. Consequently, current invasive procedures to elicit physiological and biological data, may be avoided by using implantable devices. The great impact of implantable devices was first shown by the introduction of pacemakers in the early 1960s, which enabled monitoring and treatment within the human body. IMDs are used presently to perform an expanding variety of diagnostic and therapeutic procedures enabling the control of human functions as well as data on the patient’s status [2].

Millions of people worldwide depend upon implantable medical devices to support and improve the quality of their lives. IMDs are already in use for a wide variety of applications according to their functions, categorized as following:

 The first category includes all those devices used to diagnose various diseases. These IMDs in addition to their communication system with the external environment include some sensors that interact with the human body to measure the necessary physiological data. This category includes microsystems implanted within the human body (i.e. temperature monitors [3], electrocardiograms ECG [4], blood-glucose sensors [5] etc. ) to monitor important biosignals.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

 The second category includes implantable devices used as stimulators. Stimulators receive information from an external unit (usually managed by doctors) and stimulate (irritating) specific nerves. Such devices are pacemakers and cardioverter defibrillators [6], Functional Electrical Stimulators (FES) [7], cochlear [8] and retinal implants [9].

Those are a few examples of the medical applications that could take advantage of remote monitoring and control of an implantable unit. As technology continues to evolve, new implantable medical devices are being developed, and their use is expected to rapidly increase from a substantial existing market.

Some of the applications of such implantable devices currently used are specifically discussed below.

1. Pacemaker - is an implantable medical device including a small battery and placed beneath the epidermal tissue of the chest. Its purpose is to stimulate the muscles of the heart via electrical pulses to ensure the smooth functioning of the heart.

Figure 1.1 Pacemaker [10]

2. Intracranial Pressure Sensor System, ICP - is a system which is used to monitor (short or long term) the intracranial pressure. The need for monitoring of intracranial pressure is either due to

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

a head injury or a genetic deformity. The implantable device is powered by inductive coupling from an external control unit. Through the same coupling data exchange takes place as well.

3. Cochlear Implant - is an electronic device that converts mechanical sound energy into electrical signals that can reach the cochlear nerve using electrodes and simulate sound waves.

Figure 1.2 Cochlear Implant [11]

4. Retinal Implant - by similar logic to that developed for cochlear implants, retinal implants allow the electrical stimulation of retinal neurons bypassing degenerated photoreceptors, and restoring to some degree the patient’s vision.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Figure 1.3 Retinal Implant [12] 5. In Vivo Dosimeter - is a device or instrument that directly or indirectly measures quantities of ionizing radiation placed within a human body.

1.2 Wireless Medicine

The use of wireless technology in the field of medicine brings together medical information with seamless communication, providing limitless opportunities for improving healthcare. Medical professionals are now able to diagnose, oversee patient’s vital functions in acute and long-term situations and monitor the effectiveness of treatment plans in a superior way through the use of wireless medical devices. It also offers patients and caregivers access to systems that can support them in managing their conditions more easily and safely, thus improving their quality of life.

A few of the most recognizable wireless medical products to initially appear on the market have been; glucometers, insulin pumps, ultrasound units, blood pressure and ECG monitors. More recently these products are extending to ingestible and implantable wireless medical devices that are showing the capacity to improve the accuracy, effectiveness, ease of use and portability of devices because they are self-contained and in vivo. This in conjunction with the emerging wireless protocols, including 4G, Bluetooth, ZigBee and ANT, provides potential for advancement that has been unparalleled until now. It is predicted that these devices will evolve into autonomous units that can be used for; diagnostics, relaying patients records and ultimately to administer appropriate treatments.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

The increasing prevalence of smartphone technology by lay consumers as well as medical professionals makes it possible for large numbers of people to harness the power of handheld devices, which can relay and compute data instantly from any location, for medical applications. The field of mobile health (or mHealth) is gaining popularity with patients and health care staff as more and more smartphone apps become available to help support a wide range of medical conditions. Research has substantiated that mobile devices can provide useful medical applications at the point of care, as well as helping patients to manage their conditions through education and remote monitoring.

Figure 1.4 A wireless digital biotelemetry system that monitors vital signs using a smartphone [13]

1.3 Biomedical Telemetry

The use of advanced technology to deliver healthcare from a distance has the potential to be one of the defining medical revolutions of the 21st century. It is commonly recognized that modern wireless technology will play an important role in making telemedicine possible. The field of IMDs has also been applied to the latest development of Biomedical Telemetry which permits the measurement of physiological signals at a distance, through either wired or wireless communication technologies [14]. The main goal of a healthcare monitoring system, with a wireless implantable device is to provide reliable information from inside of the human body to an external Base Station (BS) or subsequently a smartphone. Physiological signals are obtained by means of appropriate transducers, then post- processed, and eventually transmitted to exterior monitoring/control equipment for analysis by the operator.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Figure 1.5 Wireless Implantable System for Data Telemetry [15] A successful biomedical telemetry system must consider a multitude of separate factors that will come together to deliver the desired results. The following section outlines the components of a typical wireless biomedical telemetry system that must be included:

1. Base Station (or Smartphone) - a typical base station consists of several sub-systems:

a. a controller to drive the entire system and to store the measurements; b. a receiver which includes antennas; c. a connector to the data collecting system.

The sensitivity of the receiver, the performance of its antennas (in terms of directivity, efficiency, polarization, etc.) and its portability are of fundamental importance for the realization of a system that targets real life applications [15].

2. Channel Propagation

The analysis of the Electromagnetic (EM) propagation from the implanted device to the Base Station is another important aspect. As implantable devices mainly target indoor applications, the study of the multi-path propagation of the radiated EM waves and the scattering because of nearby objects is necessary. This analysis, together with the design of antennas for the Base Station, can noticeably improve the performance of the entire system.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

3. Human Body

The complex, dispersive and highly lossy characteristics of the human body inescapably affect the analysis, design, realization and characterization of implantable antennas, thus the wireless performances of the entire system.

4. Insulations

For any implantable device, biocompatible insulation is necessary in order to avoid any undesirable reaction with the living tissues. From the antenna point of view, such insulation is quite important, as the human body is not a hospitable environment for the Radio Frequency (RF) radiation. Additionally placing insulating layers either around the antenna or on the surface of the human skin, can enhance the EM transmission from an implantable radiator to the Base Station.

5. Implantable Antennas

One of the most important design aspects of an IMD is the transmitter as the specifications determine if it will be able to operate in a wireless body area network (WBAN) which is typically within a range of few meters to the human body. The factors that must be taken into consideration are; the radiation efficiency, bandwidth characteristics, compatibility between it and the lossy biological material and optimal use of accessible volume. In general the aim of an implantable antenna is to occupy the least amount of space possible, promote unification of the other parts of the device and to achieve the necessary EM performance.

6. Electronics and Power supply

The electronic elements of an IMD drive the system by enabling the exchange of data and providing signal processing, they also define the power demand necessary to make the unit operational. In essence this component determines how powerful or efficient the device will be, which is dependent on the application. Furthermore the power source takes up the most space within the device and due to the fact that it degrades will determine the longevity of the device. To extend the life span of IMDs several options are being explored including; the use of internal power supplies; transferring power

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

wirelessly from an external source; and blood sugar energy harvesting which uses the body’s natural process to power biofuel cells.

7. Bio-sensors and Bio-actuators

Depending on the purpose of the IMD it will include biosensors, which utilise biological material to monitor physiological process, or bio-actuators to stimulate a physiological reaction to perform a bodily function and in some cases both. New sensors are being explored for use in IMDs to measure a range of internal biological properties such as blood glucose levels, temperature and the blood acidic/alkaline levels. Actuators are used in active systems such as those in the field of pharmaceuticals for automatic drug delivery equipment.

8. Characterization and Experiments

All equipment or devices (and their respective components) that are intended for use in the health care system must conform to strict regulations to ensure the patient’s and operator’s safety, it must also undergo extensive testing to validate the accuracy and reliability of the required results. In order to accomplish this in vitro and in vivo testing on animals is required before human testing can commence.

1.4 Frequency Bands [1]

Until recently, no globally accepted frequency band had been dedicated to biotelemetry for implantable medical devices. In order to overcome range limitations in the mid-1990s Medtronic, the world’s largest implanted medical device manufacturer, petitioned the U.S. Federal Communications Commission (FCC) for spectrum dedicated to medical implant communication. The 402-405 MHz Medical Implant Communication Service (MICS) band was recommended for allocation by ITU-R Recommendation SA1346 in 1998. The FCC established the band in 1999 with similar standards following in Europe. The allocation of this band supports the use of larger range (typically two meters to five meters), relatively high-speed wireless links.

A further development of the MICS band was finalized in March 2009, with the FCC announcing the establishment of the MedRadio Service under Part 95 of the Commission’s rules. This followed earlier European regulatory approval in 2007. This new service incorporates the existing MICS “core” band

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

at 402-405 MHz, and also includes two megahertz of newly designated spectrum in the adjacent “wing” bands at 401-402 MHz and 405-406 MHz. The core MICS band is only available for fully implanted devices whilst the wing bands may be used for both implant and body worn medical devices. The MICS band channels have a 300 kHz wide maximum emission bandwidth whilst the wing band channels may be up to 100 kHz wide. In both cases, the device must be provided to the patient by a duly authorized health care professional.

The MedRadio band overcomes the limitations of dated inductive systems and facilitates the development of next generation implanted medical devices delivering improved patient care. This is especially important as escalating health costs drive adoption of home-based patient monitoring. The 401-406 MHz band is well suited for this service, due to the signal propagation characteristics in the human body, compatibility with the incumbent users of the band (meteorological aids such as weather balloons), and, most importantly, its international availability. Note that higher frequencies suffer from greater body attenuation, although this is somewhat compensated for by improved antenna gain. The industrial, scientific and medical bands (ISM) are heavily used by other systems and generally less favourable for the communication of medical data.

1.5 Implantable Antennas

Implantable antennas are electrically small antennas similar to typical antennas used for common wireless applications such as mobile phones, but with the additional complication that the implant will be located in a complex lossy medium[16]. Most of the research on implantable antennas for medical purposes has focused on therapeutic applications such as hyperthermia, balloon angioplasty, etc. or on sensing applications. In both cases, the antennas works in its near field and propagation over a certain distance is not an issue. In Biomedical Telemetry applications on the other hand, the system is unlikely to be in the near field therefore it should have the capacity to transmit data over a longer distance. In this case, features like the radiation efficiency and the bandwidth are essential in order to provide transmission over a large enough range with a high enough data rate [16] to be able to operate in wider environments like those experienced in the day-to-day life of the user.

Currently, the application of the implantable antenna for building a communication link between the implanted devices and outside the human body is receiving more attention. As already mentioned above, the integrated implantable antenna is a key and critical component of RF-linked implantable

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

medical devices, which enables bidirectional communication with the exterior monitoring/control equipment [14].

Figure 1.6 Illustration of a biomedical antenna application based on an implantable RF system [17]

Bio-implantable antennas are in many ways similar to small antennas for wireless communications in general. There are, however some major differences, which have to be taken into account to design an efficient antenna [18]:

• The antenna of a bio-implant radiates into a complex lossy environment (the body) rather than into free space.

• The environment surrounding the implant is known only in a statistical sense, as it can vary from individual to individual.

• The implant has to be coated by a biocompatible layer. The list of materials that can be used to this aim is limited, especially since conducting materials are not permitted for obvious reasons.

• The required data rate to be transmitted is usually lower than what is used for standard mobile communication devices.

• The distance between the implant and the base station is much lower than distances targeted in mobile communications: state of the art is around 1-2 meters; 10 to 15 meters would be more than adequate for IMD applications.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

These differences have a major impact on the design strategy of implantable antennas when compared to the design of small antennas for mobile communications. The key point in the design of an implanted antenna is the optimization of the energy transmitted by the implanted antenna out of the host body and to the base station; or conversely, the maximization of the power received by the implanted antenna from the base station taking into consideration the host body and the purpose of the entire implanted structure. The antenna is insulated by a layer of low loss biocompatible material, and implanted more or less deeply in an inhomogeneous lossy medium. The hosting body is usually a multi-layered structure, which depending on the desired position of the device, can be made of several layers as skin, fat and muscles (e.g. torso and membranes) or skin, bone, white matter and grey matter (e.g. skull).

1.6 Challenges in the Design of Implantable Antennas

In order to perform a successful design, it is necessary to go systematically from the simple to the complex taking into account all the following requirements and constraints:  Biocompatibility, in order to preserve patient safety and prevent rejection of the implant  Miniaturization, since the dimensions of the traditional antennas in MICS make these antennas unfeasible for implantable applications  Restricted power incident, on the implantable antenna due to patient safety related issue;  Far-Field Gain, which indicates the desired receiver sensitivity for achieving reliable biotelemetry communication  Low Power Consumption, to extend the lifetime of the implantable medical device

Miniaturization becomes one of the greatest challenges in implantable-antenna design, with the aim of new technological developments in IMD electronics, leading to ultra-small antennas. Antennas used in retinal prosthesis implantable medical devices, for instance, are small enough to be inserted inside the human ocular orbit (a radius of ~12.5 mm). The dimensions of the traditional half-wavelength (λ/2) or quarter-wavelength (λ/4) antennas at the frequency bands allocated for medical implants – and especially at the low-frequency MICS band – make them impractical for implantable applications [14].

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

1.7 MediWiSe [19]

MediWiSe is a medtech knowledge-driven company that empowers individuals to take control of their health. Founded by a team of entrepreneurial individuals in London in 2010, MediWiSe is developing breakthrough solutions targeted at the big three: diabetes, cardiovascular disease and cancer. The team includes award winning scientists with proven track record in wireless devices, implantable and wearable sensors, physics, nanotechnology, imaging and intelligent algorithms. It also includes award- wining entrepreneurs and experienced executives specializing in medical devices commercialization, business and product development, as well as corporate and startup governance. MediWiSe has established strategic partnerships with medical institutions and received interest from industry leaders and it is the owner of three patents in the medical diagnostics field.

MediWiSe's core technology is metamaterials, i.e. manmade nanocomposite materials that enable non- invasive precision diagnostics. This technology has allowed MediWiSe to develop three wireless products for the medical diagnostics market: the first accurate, non-invasive on-ear glucose sensor and mobile platform for continuous diabetes management; the first radio-wave imaging scanner for early stage screening of breast cancer aimed at younger women; and the first real-time wearable radiation sensor for cancer patients undergoing radiotherapy treatments.

Medical wireless sensing is a rapidly developing scientific field which has the potential to revolutionise the medical diagnostics and monitoring devices markets. MediWise's mission is to lead this revolution, through the development of innovative technologies and solutions for industry and the wider research community.

1.8 Aims, Objectives and Overview of the MSc Thesis

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Following a requirements analysis with representatives from MediWise Ltd a list of aims were identified. These aims specified key features and functionalities which were required within the proposed solution, and were considered at every stage of the overall design.

1) Design two types of implantable antenna structures that are suitable for miniaturisation.

2) Develop an implantable antenna design that is smaller than 5 x 5 mm in size which operates at 402 - 405 MHz MICS band.

3) Optimise the chosen implantable antenna for bandwidth, return loss, radiation, etc. and aim to miniaturise further the antenna at 1 x 1 mm in size.

The organisation of the MSc dissertation is as follows:

Chapter Two summarizes the theoretical knowledge and research background required to design an implantable antenna for medical purposes. Finally, a brief description about the Simulation Software CST that is used in this paper is reported.

In Chapter Three analytical parametric studies of two small low-profile implantable Planar Inverted F Antennas are shown that are suitable for miniaturisation.

Chapter Four presents an optimised Square Spiral Antenna Design that is smaller than 5 x 5 mm in size which operates at 402 - 405 MHz MICS band.

Chapter Five focuses on a 1 x 1 mm miniature implantable square spiral chip antenna proposed as future work

In the Final Chapter, the conclusions from the work that has been carried out throughout the project are presented and the results are discussed. Further improvements of the proposed miniaturized antennas are considered and future developments are proposed.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Chapter 2 Theory and Research Background

This chapter summarizes the theoretical knowledge and research background required to design an implantable antenna for medical purposes. First, the Biological Tissue Properties are presented since the biological environment is crucial to the operation an implantable medical device. Then the basic operating principles of microstrip, PIFA and loop antennas as well as the Antenna Performance Parameters are described following by standard Miniaturization techniques. Finally, several antenna designs in Literature Review are reported as well as a brief description about the Simulation Software CST that is used in this paper.

2.1 Biological Tissues Properties

One of the most critical parameters to be taken into account in this thesis is the implantation of the antenna in human tissue and the detuning issues that appear on implantable antennas. Implantable antennas are required to operate within a complex biological environment and this feature significantly differentiates the design of the antennas, which are not intended to operate in free space as already mentioned in Chapter 1.When detuning occurs, one or more characteristics of the antenna (such as resonant frequency, gain, bandwidth, etc.) can be changed to the point where the original design specifications are no longer met. It is necessary, therefore, the analysis of complex biological environment in advance, in order to evaluate all those parameters that will lead to a correct design giving the antenna the desired characteristics [20]. To avoid detuning problems great care in the design of the implantable antenna is required to the greatest possible accuracy in the simulation of the biological tissue in which the antenna is placed.

Biocompatibility and Biomaterials

The design and construction of an implantable antenna must make provision for avoiding different side effects after the implantation in the human body which comes into contact with a foreign object. Biocompatibility is defined as the property of some materials do not cause toxic reactions or effects or injuries in the human body. This means that the host, the human body and its immune system, is not directed "against" this material. With regard to IMDs, to improve the biocompatibility of devices, and

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

to avoid undesirable side effects use of specific biocompatible coatings is made [21], [22], [23]. The materials used today as coatings or as base materials for the construction of other provisions are metals, polymers or ceramics [24], [25], [26].

Electrical Properties of Human Tissue

The medium in which the antenna is radiating influences the performance of the antenna. Therefore, when the antenna is inserted into human or animal skin (tissue), the radiation field and the impedance of the antenna will change according to the electrical properties of the tissue and the implementation of the antenna. When we place the antenna inside the tissue, we are radiating in a high permittivity and conductivity media. The effect of the material on the antenna radiation depends on the dielectric constant or the complex permittivity of the material.

Table 2.1 Dielectric Properties of Diﬀerent Body Tissues at 403.5 MHz [15]

Body Tissue ε’e σ tan δ ε”e 2/3 muscle 38.10 0.530 0.620 23.61 muscle 57.10 0.797 0.622 35.51 fat 5.58 0.042 0.328 1.83 skin 43.50 0.87 0.799 34.75 head 43.50 0.87 0.799 34.75

It can be seen that both the dielectric constant and conductivity of biological tissues depend significantly on the type of tissue and the frequency of the electromagnetic wave. It is worth noting that changes in the properties of biological tissues can be observed from organization to organization, even for the same tissue. Also in some cases, changes in the characteristics of the tissues are detected even in the same body depending on its age.

Electromagnetic Radiation Interaction with biological tissues

When an electromagnetic wave encounters an obstacle or a dividing surface, part of it may be reflected, refracted, and to propagate through the material or even be absorbed by the material or barrier. Which of these procedures will prevail and to what extent depends on many factors such as the frequency of the wave, the angle of incidence, and the electromagnetic characteristics of the obstruction material [20].

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

The Specific Absorption Rate (SAR) is the size associated with the absorption of electromagnetic radiation by biological tissues and based on which safe exposure limits for electromagnetic radiation frequencies higher than 1 MHz have been established [20]. Nevertheless, many times because the SAR is not directly measurable the power density is used instead i.e. the power of the radiation incident on the unit surface.

SAR is the largest size to quantify the effects of electromagnetic radiation on the human body and can be determined theoretically or measured in simulated biological tissue by exposure to electromagnetic radiation. It can be calculated from the electric field within the tissue as:

Where: σ is the electrical conductivity of the tissue (S/m) Ε is the RMS electric field ρ is the sample density (Kg/m3)

It depends on various factors such as:  The radiation characteristics (frequency, polarization, intensity),  The characteristics of the biological object, geometry (size and shape) and the internal structure,  The distance of the emission source of radiation and biological objects (near or far field) and  The properties of the surrounding area.

The unit of measurement of specific absorption rate (SAR) in the international system is W/kg. In 1999 and 2005 adopted by the IEEE the limits ΙΕΕΕ C95.1-1999 [27] και ΙΕΕΕ C95.1-2005 [28]. According to these:  1-g-avg SAR < 1.6 W/kg  10-g-avg SAR < 2W/kg

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

2.2 Operating Principles of Implantable Antennas

Microstrip - Patch Implantable Antenna

Microstrip antennas, also referred to as patch antennas, are low profile, comfortable to planar and non-planar surfaces mechanically robust when mounted on rigid surfaces, compatible with MMIC (Monolithic Microwave Integrated Circuits) designs. Microstrip antennas have a number of advantages over other antennas; they are inexpensive, lightweight and easy to integrate with accompanying electronics. In the wireless communication area, microstrip antennas are of interest for implantable applications because of their flexibility in design, conformability and shapes. In addition, when the particular patch shape and mode are selected they are very versatile in terms of resonant frequency, polarization, pattern and impedance [29].

Figure 2.1 A patch antenna in its basic form [30] At the centre of the patch, the electric field is zero, maximum (positive) at one side, and minimum (negative) on the opposite side. It should be mentioned that the minimum and maximum continuously change side according to the instantaneous phase of the applied signal. The electric field does not stop abruptly at the patch’s outside edge as in a cavity; rather, the fields extend the outer periphery to some degree [30]. As a result, these field extensions, known as fringing fields, cause the patch to radiate. However, there are methods, such by increasing the height of the substrate, which can be used to extend the efficiency and bandwidth [29].

Planar Inverted F-Antenna (PIFA) [31]

Antenna designers are always looking for creative ways to improve performance. One method used in patch antenna design is to introduce shorting pins (from the patch to the ground plane) at various locations which leads into the Planar Inverted-F Antenna (PIFA).

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

The PIFA is increasingly used in the Implantable Devices applications. The antenna is resonant at a quarter-wavelength (thus reducing the required space needed in the implant), and also typically has good SAR properties. This antenna resembles an inverted F, which explains the PIFA name. The Planar Inverted-F Antenna is popular because it has a low profile and an omnidirectional pattern. The PIFA is shown from a side view in Figure 2.2.

Figure 2.2 The Planar Inverted-F Antenna (PIFA)[31] The fringing fields which are responsible for radiation are shorted on the far end, so only the fields nearest the transmission line radiate. Consequently, the gain is reduced, but the patch antenna maintains the same basic properties as a half-wavelength patch, but is reduced in size 50%.

The PIFA is resonant at a quarter-wavelength due to the shorting pin at the end. The feed is placed between the open and shorted end, and the position controls the input impedance. The closer the feed is to the shorting pin, the impedance will decrease; the impedance can be increased by moving it farther from the short edge.

In addition, the shorting pin can become capacitive if instead of extending all the way to the ground plane, it is left floating a small amount above. This introduces another design parameter to optimize performance.

Loop Antenna

Another simple, inexpensive and very versatile antenna type is the loop antenna which refers to a radiating element made of a coil of one or more turns [29] . Both ferrite and air-core loops are commonly used in RF applications. In the current times, loop antennas have also been adopted for wireless communications [32].

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

The main characteristic of the loop antenna which makes it interesting as an implantable antenna is the fact that loops have a dominantly magnetic near field and so the important parameter of the surrounding material would be the permeability instead of the permittivity. This could help to decrease the effect of the biological tissues around the antenna.

2.3 Antenna Performance Parameters

A brief overview of the parameters used to evaluate antenna performance is given below. The definitions in quotations are taken from IEEE Standard Definitions of Terms for Antennas, IEEE Std 145-1983.  Antenna directivity: The directivity of an antenna is given by the ratio of the maximum radiation intensity (power per unit solid angle) to the average radiation intensity (averaged over a sphere). The directivity of any source; other than isotropic, is always greater than unity.  Antenna efficiency: The total antenna efficiency accounts for the two following losses: Reflection because of mismatch between the feeding transmission line and the antenna and secondly the conductor and dielectric losses.  Antenna gain: The maximum gain of an antenna is simply defines as the product of the directivity by efficiency. If the efficiency is not 100 percent, the gain is less than the directivity. When the reference is a lossless isotropic antenna, the gain is expressed in dBi, the efficiency is e  1 and is the same as the directivity of the antenna. When the reference is a half wave dipole antenna, the gain is expressed in dBd (1 dBd = 2.15 dBi).  Antenna pattern: The antenna pattern is a graphical representation in three dimensions of the radiation pattern of the antenna as a function of angular direction. Antenna radiation performance is usually measured and recorded in two orthogonal principal planes (such as E- plane and H-plane or vertical and horizontal planes). The pattern is usually plotted either in polar or rectangular coordinates. The pattern of most base stations contains a main lobe and several minor lobes, termed side lobes. A side occurring in space in the direction opposite to the main lobe is called back lobe.  Antenna Polarisation: “In a specified direction form an antenna and at a point in its far field, is the polarisation of the (locally) plane wave which is used to represent the radiated wave at that point”.” At any point in the far-field of an antenna the radiated wave can be represented by a plane wave whose electric field strength is the same as that of the wave and whose direction of propagation is in the radial direction form the antenna. As the radial distance

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

approaches infinity, the radius of curvature of the radiated wave’s phase front also approaches infinity and thus in any specified direction the wave appears locally a lane wave”. In practice, polarisation of the radiated energy varies with the direction form the centre of the antenna so that different parts of the pattern and different side lobes sometimes have different polarisation. The polarisation of a radiated wave can be linear or elliptical (with circular being a special case).  Coaxial antenna: “An antenna comprised of an extension to the inner conductor of a coaxial line and radiating sleeve which in effect is formed by folding back the outer conductor of the coaxial line”.  Co-polarisation: “The polarisation which the antenna is intended to radiate”.  Cross-polarisation: “In a specified plane containing the reference polarisation ellipse, the polarisation orthogonal to a specified reference polarisation”. The reference polarisation is usually the co-polarisation.  Directional antenna: “An antenna having the property of radiating or receiving electromagnetic waves more effectively in some directions than others”.  Effective radiated power (EPR): “In a given direction, the relative gain of a transmitting antenna with respect to the maximum directivity of a half-wave dipole multiplied by the net power accepted by the antenna from the connected transmitter”.  E-plane: “For a linearly polarised antenna, the plane containing the magnetic field vector and the direction of maximum radiation”. For base station antennas, the E-plane usually coincides with vertical plane.  Far-field region: “that region if the field of the antenna where the angular field distribution is essentially independent of the distance from specified point in the antenna region”. The radiation pattern is measured in the far field.  Frequency Bandwidth: “The range of the frequencies within which the performance of the antenna, with respect to some characteristics, conforms to a specified standard”. VSWR of an antenna is the main bandwidth limiting factor.  Front-to-back ratio: “The ratio of the maximum directivity of an antenna to its directivity in specified rearward direction”. Sometimes the directivity in the rearward direction is taken as the average over an angular region.  Half-power beamwidth: “In a radiation pattern cut containing the direction of the maximum of a lobe, the angle between the two directions in which the radiation intensity is one-half the

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

maximum value”. The half-power beamwidth is also commonly referred to as the 3 dB beamwidth.  Half-wave dipole: “A wire antenna consisting of two straight collinear conductors of equal length separated by a small feeding gap with each conductor approximately a quarter-wave length-long”.  H-plane: “For linearly polarised antenna, the plane containing the magnetic field vector and the direction of maximum radiation”. For base station antennas, the H-plane usually coincides with the horizontal plane.  Input Impedance: “The impedance presented by an antenna at its terminals”. The input impedance is a complex function of frequency with real and imaginary parts. The input impedance is graphically displayed using a Smith chart.  Major/main lobe: “The radiation lobe containing the direction of maximum radiation”. For most practical antenna there is only one main beam.  Normalised pattern: Normalising the power/field with respect to its maximum value yields a normalised power/field pattern with a maximum value of unity (or 0 dB).  Omnidirectional antenna: “An antenna having an essentially non-directional pattern in a given plane of the antenna and a directional pattern in any orthogonal plane”.  Radiation efficiency: “The ratio of the total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter”.  Radiation pattern: The variation of the field intensity of an antenna as an angular function with respect to the axis. A radiation pattern is usually represented graphically for the far-field conditions in either horizontal or vertical plane.  Reflection coefficient: The radio of the voltages corresponding to the reflected and incident

waves at the antenna’s input terminal (normalized to some impedance Z0). The return loss is

related to the input impedance Zin and the characteristics impedance Z0 of the connecting feed

(Zin  Z0 ) line by: Gin  . (Zin  Z0 )  Scattering parameters: The reflection and transmission coefficients between the incident and reflection waves. They describe completely the behaviour of a device under linear conditions at microwave frequency range. Each parameter is typically characterized by magnitude, decibel and phase. The expression in decibel is 20log(Sij) because s-parameters are voltage ratios of the waves. S11: input reflection coefficient of 50W terminated output.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

S21: forward transmission coefficient of 50W terminated output. S12: reverse transmission coefficient of 50W terminated input. S22: output reflection coefficient of 50W terminated input.  Side lobe level: Is the ratio, in decibels (dB), of the amplitude at the peak of the main lobe to the amplitude at the peak of a side lobe.  Side lobe suppression: “Any process, action or adjustment to reduce the level of the side lobes or to reduce the degradation of the intended antenna system performance resulting form the presence of side lobes”. For base station antennas, the first side lobe above the horizon is preferred to be low in order to reduce interference to adjacent cell sites. On the other hand, the side lobes below the horizon are preferred to be high for better coverage.  Voltage Standing Wave Ratio (VSWR): The radio of the maximum/minimum values of standing wave pattern along a transmission line to which a load is connected. VSWR value ranges from 1 (matched load) to infinity for a short or open load. For most base station antennas the maximum acceptable value of VSWR is 1.5. VSWR is related to the reflection coefficient

(1 Gin ) Gin by: VSWR  . (1 Gin )

2.4 Miniaturization Techniques

Taking the advantage that human tissue exhibits relatively high permittivity, or equivalently, reduced wave-propagation velocity, miniaturization of the physical size of the antenna is feasible. However, it should be noted that when a low-permittivity biocompatible layer is inserted around the antenna, the value of the effective permittivity decreases, and miniaturization achieved by the high-permittivity tissue material is degraded. It is out of importance to reduce the size of the antenna at a given operating frequency, while still maintaining adequate electromagnetic performance.

Miniaturization techniques for implantable antennas include:

1. The use of high-permittivity dielectric materials: high-permittivity dielectrics, used within thin superstrate layers, are selected for implantable patch antennas because they shorten the effective wavelength and result in lower resonance frequencies, thus assisting in antenna miniaturization [33].

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

2. Lengthening of the current-flow path on the patch surface: longer effective current-flow paths excited on the radiating patch can reduce the resonance frequency, and achieve a more-compact size for the implantable antenna.

3. The addition of shorting pins: inserting a shorting pin between the ground and patch planes increases the effective size of the antenna, and, in turn, reduces the required physical dimensions, given a specific operating-frequency scenario [34].

4. Patch-stacking: vertically stacking two radiating patches reduces antenna size by increasing (nearly doubling) the length of the current-flow path [35], [36].

2.5 Literature Review

Microstrip and PIFA Antennas

In 1980 [37] one of the first medical applications using microstrip antennas was presented for hyperthermia uses. As already mentioned previously there are numerous methods available to miniaturise an antenna such as the addition of ground pins (therefore converting the microstrip antenna to a PIFA), using relatively huge dielectric constant substrate materials or transforming the conductor shape into a spiral, in more particular a planar helix, as in 2000 [38]. In this example a microstrip antenna was designed and evaluated numerically for an implantable pacemaker.

In 2004 a modified microstrip antenna was evaluated into a numerical model of human chest, using FDTD, and the frequency shift of the antenna resonance was calculated when implanted. The effect of different locations for the feed and the ground point, different materials and thicknesses for substrate and superstrate and different lengths for the antenna were also studied. Additionally, two microstrip antennas with the same size and different trace shaped were compared both having the size of 26.6mm x 16.8mm x 6mm and operating in 402 - 405 MHz frequency band.

 Patch Antenna Designs Operating in MICS Band

Patch designs are preferred for implant-integrated antennas because of their ﬂexibility in design, conformability, and shape. Design of implantable patch antennas operating in the low-frequency MICS band draws high-scientiﬁc interest to deal with miniaturization according to [39].

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

The wavelength in the tissues is shorter, since the wave propagation velocity is lowered. Therefore, the implanted antennas typically have in general geometry less than 10 % of wavelength in the free space, which gives the result of very low transmission efficiency. However, the characteristics of the biological tissues in the MICS frequency band make a small antenna an efficient radiator as stated in [40]. The aim is to decrease the antenna physical size, while increasing its electrical size.

The main issue with these antennas, operating at 403 MHz and thus corresponding to a wavelength of 744.4 mm in free space, is that it is not practical to put them into a living human body without performing thorough miniaturization. Indeed, taking the effect of the body into account during the design, reducing the antenna size by around seven times, without miniaturization it still remains too big to be implanted [41].

 Antenna Design Typology

The shape of implant will dictate the type of antenna used. Normally the antenna form is defined by the implant case. A flat antenna could be appropriate for a pacemaker device, while a helix antenna is required for urinary implant application [40]. Single layer spiral and meander planar typologies are the most common implantable radiators used in the MedRadio band. Indeed, their geometries with and without grounding pin facilitate miniaturization. Focusing on designs operating within the MedRadio band, these two typologies are investigated in [2], [42] and compared in [43] where the frequency shift and the maximum impedance increase indicate that the lossy and dielectric effects of biological tissue on the PIFAs diminish as the skin tissue becomes thinner.

In [2] (2005) two spiral microstrip antennas and two PIFAs were simulated where the characteristics of the implanted antennas in terms of return loss and radiation efficiency were evaluated. In particular, rectangular 10240 and 5760 mm3 chest-implantable antennas were reported, using the high- permittivity (εr = 10.2) Rogers 3210 dielectric and apply a spiral radiator for size reduction. Addition of a shorting pin, thus, conversion to a planar inverted-F antenna (PIFA) acts like a ground plane on a monopole, and has been found to shrink the volume of the aforementioned antennas by 40% and 60%, respectively. It was noted that when placed inside the human chest, the radiation efficiency for the microstrip antennas is 0.16% and for the PIFA is 0.25%.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Other MICS implantable antennas applying similar miniaturization techniques include a 3457 mm3 3 PIFA with a serpentine radiator built on MACOR substrate (εr = 6.1) [34], and a 6480 mm antenna with a wafﬂe-type radiator built on silicon substrate (εr = 3.1) [44], both intended for generic body implantation (2/3 human muscle properties used to represent average body properties).

Extensive analysis of these antenna typologies is also reported in [34] and [44], where the eﬀects of diﬀerent dimensions and materials, and optimization with genetic algorithms techniques are also included, respectively. Both antennas are suited for implantable applications, however:

 The spiral design turns out to have a higher radiation eﬃciency.  The serpentine antenna has a higher resonant frequency for the same physical length  The spiral antenna has strong coupling only at the center of the antenna, but the serpentine antenna also has coupling to adjacent arms.  The spiral antenna radiates slightly better than the serpentine antenna, probably because of the more distributed current distribution.

These antenna designs had great impact on other designs as well, for instance, the spiral radiator presented in [2] is also considered in [45] (2009), [46] (2010). Additionally, a 19.5 x 10.8 x 2.5 mm3 meandered PIFA is discussed in [47] (2010) but for applications at 868 MHz. More complicated planar designs are discussed in [48] (2008) and in [49] (2010) with and without superstrate, respectively. The metallization schemes of these solutions are optimized to reduce the size of the radiator and to obtain the desired performances.

In [50] (2008), the designed antenna has a cylinder height of 22.72 mm and an external radius of 10.5 mm, and so it is a small size antenna. However, the reﬂection coefﬁcient value shown in the paper is only simulated and not measured and the simulated MICS bandwidth is only partially covered.

In 2009 [51], the designed PIFA antenna is also small, i.e., 22.5 mm x 18.5 mm x 1.9 mm, but its fractional bandwidth is 20% lower than the one of the antenna we propose and it is not embedded in any insulating biocompatible material during measurements.

The Inverted-F typology (either Planar or Folded) is the most used and effective way to reduce the size of the radiator. Indeed, the dimensions the Spiral PIFA [34] (17.0 x 27.0 x 6.0 mm3) and of the meandered PIFA presented in [17] (2008) (22.5 x 22.5 x 2.5 mm3) for continuous glucose-monitoring

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

applications, are much smaller than the ones of the spiral inspired from [2] (38.0 x 42.0 x 4.0 mm3). Further miniaturization is then achievable combing stacked planar metallization, as in [52] (2010). However, if dimensions are reduced till less than 30 mm3, a drastic reduction of the radiation performance occurs (gain = - 55.6 dBi). Stacked PIFA Antennas

In literature, most of the implantable antennas have been designed by adapting the stacked planar inverted-F antenna (PIFA) structure in order to reduce the antenna size as well as enhance the impedance bandwidth. These proposed antennas include the rectangle stacked PIFA [53], the circular stacked PIFA [54], the spiral PIFAs [2], [34] and the meandered PIFA [43]. However, among these antennas, even though the antenna reported in [53] has the most compact size with only 10 x 10 x 1.9 mm3 and that presented in [54] is as small as π x (7.5)2 x 1.9 mm3, the two antennas are still either large in antenna size or can only provide a narrow bandwidth to become a stumbling block to the development of the realizably implantable devices.

Multi-layered stacked design has proven to be useful to reduce the radiators size. Multi-layer structures including vertically stacked radiating patches increase the length of the current ﬂow and further assist in miniaturization. Dimensions smaller than 10 x 10 x 1.9 mm3 are indeed achievable. These radiators have a wide bandwidth behaviour because of the strong interaction of the EM near ﬁeld terms with the human tissues.

In the article [55] (2008), a new three-layer stacked PIFA antenna with the merits of wide impedance bandwidth, compact size, and good radiation characteristics suitable for use in the biotelemetry communication when operating at the 402 - 405 MHz frequency band was proposed. The design has an overall size of only π × 52 × 1.9 mm3, which reaches a size reduction of 22% and 56%, comparing to the reported work [53] and [54], respectively. With insertion of a circular hook-shaped slot to each radiating patch, not only reduction in antenna size but also enhancement on impedance bandwidth was obtained.

The design proposed in [56] (2009) has an overall size of only 8 x 8 x 1.9 mm3, which reaches a size reduction of 36% and 64%, comparing with the reported work [53] and [54], respectively. The skill of embedment of shaped slots into the radiating patches was applied to lengthen the effective current path and thus achieving wider bandwidth and more compact size than those reported in [53], [54] making this antenna more suitable for implantable biometry devices.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

As the antenna dimensions shrink, the effect of fabrication issues becomes even more critical. Other reported skin-implantable stacked PIFAs occupy miniaturized volumes of 598 [53] (2008), 416 [57] (2012), 383 [56] (2009), 337 [54] (2006) , 273.6 [58] (2011), 254 [59] (2011) and 32.7mm3 [35], [60] (2011).

Loop Antennas

The design of an antenna for wireless medical implant communications system, operating in the frequency band of 402-405 MHz is presented in [40] (2007) where the theoretical formulation on a small implanted multi-turn printed loop antenna was carried out from [61] (1963), [62] (1976). The size of the proposed antenna is 8.2 x 8.1 x 1 mm, which is quite appropriate for wireless system. The antenna dimensions are small enough to meet the requirement and fit in a deep sited medical implant device. Moreover, the results from these two above mentioned sources did not apply to loops in an insulating cavity, and assume that the antenna is in direct contact with a dispersive medium. However, the loop antenna in an insulating cavity is discussed in [63] (1973).

In paper [64] (2004), the radiation characteristics of a miniature square spiral chip antenna were presented. The antenna was fabricated on high resistivity silicon (HR-Si) for contact-less powering and RF telemetry from implantable bio-micro-electro-mechanical systems (bio-MEMS) based capacitive pressure sensors. In prior publications [65], [66] by the same authors, the wireless RF telemetry scheme, the notional bio-MEMS sensor, and the validation of the telemetry concepts, using biological tissue-like phantom media, have been reported.

Several researchers in the past ([67], [68], [69], [70], [71]) have demonstrated RF antennas for inductive powering and data communications with implantable biosensors. However, the dimensions of the antenna (1 x 1 x 0.4 mm3) described in [64] are significantly smaller which results in a very compact design for the hand-held unit used for powering and data communications.

A detailed table of the majority of the literature from 2008 until today is presented as follows, where the ratio is the antenna wavelength over its size. The bigger the ration the more effective, comparing to its size, the antenna is.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Miniaturization Techniques Implantation Vol. Bands Ref Year Substrate Shape Application Dimensions Dielectric Ratio Tissue [mm3] [MHz] Patch Shape Shorting Pin Patch Stacking Material 2/3 Human - MACOR (sub) - Spiral [34] 2004 Rectangular Cardiac Pacemaker 19.6 x 29.4 x 6 3457.4 402 - 405 YES - 21 Muscle - Silicon (sup) - Serpentine - Contact-less 2/3 Human [64] 2005 Square Powering 1 x 1 x 0.4 0.4 235 Silicon HR-Si Spiral YES - 1185 Muscle - RF Telemetry Wireless - Meandered [43] 2005 Rectangular Skin Tissue Communication 20 x 24 x 2.5 1200 402 - 405 Roger 3210 YES - 23.8 - Spiral Links [54] 2006 Circular Skin Tissue Biotelemetry π x (7.5)2 x 1.9 335.8 402 - 405 Roger 3210 Spiral YES YES 99.2 Medical Implant 2/3 Human - RH-5 Multi-turn printed [40] 2007 Rectangular Communications 8.2 x 8.1 x 1 66.42 402 - 405 YES - 90.7 Muscle - D51(NTK) loop System 402 - 405 33 [17] 2008 Square Skin Tissue Glucose Monitoring 22.5 x 22.5 x 2.5 1265.6 Roger RO3210 Serpentine YES - 2400 - 2480 5.5 [55] 2008 Circular Skin Tissue Biotelemetry π x 52 x 1.9 149.22 402 - 405 Roger 3210 Hook-Shaped Slot YES YES 148.8 [53] 2008 Square Skin Tissue Biotelemetry 10 x 10 x 1.9 190 402 - 405 Roger 3210 Rectangular Spiral YES YES 74.4 [56] 2009 Square Skin Tissue Biotelemetry 8 x 8 x 1.9 121.6 402 - 405 Roger 3210 Hook-Shaped Slot YES YES 93 - Ceramic ANMg [49] 2010 Rectangular Skin Tissue Biotelemetry 18 x 16 x 2 288 402 - 405 L-type - YES 31 - MgTa1.5Nb0.5O6 [35] 2011 Circular Human Head Biotelemetry π × 42 × 0.65 32.7 402 - 405 Αlumina Circular Meandered YES YES 186 402 74.4 Μulti-band 10 x 10 x 2.54 [59] 2011 Square Skin Tissue 254 433 Roger 3210 π-shaped Spiral YES YES 69.2 Biotelemetry 2450 12.2 - SiO2 [4] 2011 Square Skin Tissue In Vivo Dosimeter 1 x 1 x 1 1 402 ANTs - Spiral - - 744 - N-doped Silicon Retinal Prosthesis [58] 2011 Square Human Eyeball 12 x 12 x 1.9 273.6 402 - 405 Rogers RO3210 YES YES 62 System Meandered - Open [29] 2012 Rectangular Skin Tissue biotelemetry 10 x 16 x 1.27 203.2 402 Rogers RO3210 YES - 39.5 Slots Wireless Telemetry 401 - 406 47.6 [31] 2012 Rectangular Skin Tissue 10 x 12 x 1.5 180 FR4 Epoxy Slot Line - - Systems 433 - 434.8 44.3 16.5 x 16.5 x 402 - 405 45 [34] 2012 Square Skin Tissue Biotelemetry 691.5 Rogers 3010 Spiral YES - 2.54 2400 - 2480 7.57 2/3 Human Meander Inverted F [57] 2012 Rectangular Biotelemetry 13 x 16 x 2 416 402 - 405 LTCC YES YES 36 Muscle Structure Medical Wireless 401 - 406 Symmetrical [32] 2013 Rectangular Skin Tissue 18 x 16 x 1.27 366 Rogers RO3210 - - 30 Telemetry 2400 - 2480 Spiraling Cond.

35 Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

2.6 CST Microwave Studio - 3D EM Simulation Software [72]

CST MICROWAVE STUDIO is a fully featured software package for electromagnetic analysis and design in the high frequency range. It simplifies the process of creating the structure by providing a powerful graphical solid modeling front end which is based on the ACIS modeling kernel. After the model has been constructed, a fully automatic meshing procedure is applied before a simulation engine is started. CST MICROWAVE STUDIO uses a Method on Demand approach which gives the choice of simulator or mesh type that is best suited to a particular problem.

Since no one method works equally well for all applications, the software contains several different simulation techniques (transient solver, frequency domain solver, integral equation solver, multilayer solver, asymptotic solver, and eigen mode solver) to best suit various applications. Each method in turn supports meshing types best suited for its simulation technique. Applying these highly advanced techniques usually increases the accuracy of the simulation significantly.

The most flexible tool is the transient solver using a hexahedral grid, which can obtain the entire broadband frequency behavior of the simulated device from only one calculation run. This solver is remarkably efficient for most high frequency applications such as connectors, transmission lines, filters, antennas, amongst others.

The transient solver is less efficient for structures that are electrically much smaller than the shortest wavelength. In such cases it is advantageous to solve the problem by using the frequency domain solver. The frequency domain solver may also be the method of choice for narrow band problems such as filters or when the use of tetrahedral grids is advantageous.

Antenna Calculations

The following summarizes the input necessary for antenna calculations:

1. Select an antenna project template 2. Set units 3. Set background material 4. Define structure 5. Set frequency range 6. Set (open) boundary conditions 7. Define excitation ports.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

8. Set (farfield) monitors and/or probes. 9. Specify farfield result processing templates 10. Start transient or general purpose frequency domain solver. 11. Analyze results (input impedance, farfields, etc.).

For the calculation of the antenna farfield gain or directivity patterns (farfield distribution in spherical or Ludwig coordinate systems, left and right hand polarization, axial ratio), “farfield monitors” need to be defined before the simulation starts. An arbitrary number of these monitors can be defined for various frequencies. This means that the antenna farfield for multiple frequency points can be computed from a single transient analysis.

Meshing (Discretization)

Initially, to prepare a model for simulation the work-space it occupies is divided, into a large number of spatial elements or cells. These cells are small rectangular blocks arranged on a Cartesian mesh. Each cell is required to be homogeneous by CST, which means that the cell must be entirely empty or entirely filled with the same metal or dielectric material. This condition is met by means of the Auto-mesh facility which aligns the cell-faces with the surfaces of the model. Failure of aligning the cell-faces with the model, results in the surfaces of the model effectively jumping to the nearest cell face.

Secondly, the time-step of the simulation depends on the size of the smallest cell. If even one cell is made smaller than the others by a small factor (5) in linear dimensions, then the time-step of the simulation will be reduced by the same factor. And it will then take 5 times as many time-steps for the simulator to cover a given period of time, and the computer run-time will be increased proportionately.

For the purpose of determining the time-step, the exact definition of smallest cell is a little complicated. To a fair approximation however, it will take at least two time-steps for light to cross any cell in any direction so that the time-step depends critically on the smallest dimension of any cell.

In summary it can be said that the largest dimension of any cell limits the frequency range and the smallest dimension of any cell limits the time-step.

Simulation

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

After defining all necessary parameters, the simulation process can start where the most important stages are listed below:

1. Analyzing port domains: During this first step, the port regions are analyzed for the port mesh adaptation to follow.

2. Port mode calculation: Here, the port modes are calculated during the port mesh adaptation. This step is performed several times for each port until a defined accuracy value or a maximum number of passes has been reached.

3. Calculating matrices, preparing and checking model: During this step, the input model is checked for errors such as invalid overlapping materials.

4. Calculating matrices, normal matrix and dual matrix: During these steps, the system of equations which will subsequently be solved is set up.

5. Transient analysis, calculating port modes: In this step, the solver calculates the port mode field distributions and propagation characteristics as well as the port impedances if they have not been previously calculated.

6. Transient analysis, processing excitation: During this stage, an input signal is fed into the stimulation port. The solver then calculates the resulting field distribution inside the structure as well as the mode amplitudes at all other ports. From this information, the frequency dependent S-parameters are calculated in a second step using a Fourier Transformation.

7. Transient analysis, transient field analysis: After the excitation pulse has vanished, there is still electromagnetic field energy inside the structure. The solver then continues to calculate the field distribution and the S-parameters until the energy inside the structure has decayed below a certain limit.

Adaptive Meshing

As mentioned above, the mesh resolution influences the results. The expert system-based mesh generator analyzes the geometry and tries to identify the parts that are critical to the electromagnetic behavior of the device. The mesh will then automatically be refined in these regions. The automatic mesh generation always tries to choose a mesh that provides a good tradeoff between accuracy and simulation speed. The solver performs several mesh refinement passes until the S-parameters no longer change significantly between two subsequent passes.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Chapter 3 Antenna Designs - Simulations - Parametric Studies - Effects

The objectives of this chapter are the analytical parametric studies of two small implantable Planar Inverted F Antennas based on the design in [42]. Our goal is to miniaturise as much as possible, smaller than 10 x 10 mm, both types of these low-profile antennas.

3.1 Initial Antenna Designs

Searching from literature a few potential designs were found that could be good candidates for our initial antenna designs to start with. Based on this research, the best designs for simplicity and performance, to be selected, were the Meander PIFA and the spiral PIFA. In [42] two compact planar antennas were designed, constructed and measured using ﬁnite difference time domain (FDTD) simulations and measurement setup for active implantable medical devices at the medical implant communications service (MICS) frequency band, 402 - 405 MHz. A planar inverted F antenna (PIFA) structure was applied to design two small low- proﬁle antennas: meandered-type and spiral-type PIFA.

Both of the antennas were redesigned, according to the configurations given, and simulated in CST Microwave Studio (3D EM Simulation Software) in order to verify and compare their return-loss characteristics as well as their performance to the ones shown in [42].

For the ease of designing implanted antennas, planar antennas are located inside a simpliﬁed body model instead of an anatomic complete body model. Because implantable medical devices are positioned under a skin tissue, electrical effects of the skin tissue on implanted antennas are very strong, a simpliﬁed body model consists of only one skin tissue (dielectric constant (εr) = 46.7, conductivity (σ) = 0.69 S/m at 402 MHz, mass density (ρ) = 1.01 g/cm3), as shown in Fig. 3.1. The dimension of the hexahedral body model is 10 cm ×10 cm×5 cm, and the two planar antennas were positioned at the center of the body model while the location of the antenna from the bottom of the body model is 1 cm.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Figure 3.1 Simpliﬁed body model for the design of planar antennas implanted in a human body [42]

Meandered PIFA

According to [42] a meandered antenna was designed for implantable medical device inside a human body at a biomedical frequency range of 402 - 405 MHz. Because the designed antenna uses a grounding pin at the end of the radiator, the operation mechanism is the same as a planar inverted F antenna (PIFA). The printed radiator is located between substrate and superstrate dielectric layers whose dielectric constant is 10.2 and thickness is 1.25 mm. The origin of the coordinate system is located at the center of the ground plane which is 24 mm in width and 20 mm in length.

To understand the construction method of the meandered PIFA in Fig. 3.2, it is considered that the meandered radiator consists of four rectangular strips (15 mm × 3.8 mm) which are electrically connected to each other with three connection strips (1.2 mm × 1.2 mm). The spacing among the rectangular strips is the same as the distance (1.2 mm) between the radiator and the ground plane in order to reduce coupling effects between the rectangular strips and achieve a small antenna. The location of a coaxial feeding was determined to make the antenna match well to 50 Ω systems at the desired frequency (402 - 405 MHz).

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Figure 3.2 Meandered PIFA configuration [42]

Figure 3.3 3D illustration of the Meandered PIFA in CST Microwave Studio environment

Graph 3.1 Simulated and measured return-loss characteristics of meandered PIFA [42]

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Graph 3.2 Simulated return-loss characteristics of the meandered PIFA in CST Microwave Studio environment

From the above graph it is clear that our results have good agreement with the results from [42], as the meandered PIFA is matched almost at MICS band.

Spiral PIFA

The spiral PIFA antenna as described in [42] is shown in Fig. . The uniform width radiator is sandwiched between substrate and superstrate dielectric layers whose thickness is 1.25 mm each using Rogers RO3210 as a material with dielectric constant 10.2. The origin of the coordinate system is located at the center of the ground plane (24 mm × 20 mm). Similar to the meandered PIFA, the spacing among the metallic strips (3.8 mm in width) is 1.2 mm. In contrast to meandered PIFA, the operating frequency of the spiral antenna was tuned by changing the length of the innermost metallic strip.

Figure 3.4 Spiral PIFA configuration [42]

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Figure 3.5 3D illustration of the Spiral PIFA in CST Microwave Studio environment

Graph 3.3 Simulated and measured return-loss characteristics of spiral PIFA [42]

Graph 3.4 Simulated return-loss characteristics of spiral PIFA in CST Microwave Studio environment

From the above graph it is clear that our results have good agreement with the results from [42], as the spiral PIFA is matched almost at MICS band.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

3.2 Parametric Studies of Meandered PIFA – Effects

In order to understand the performance of those two microstrip antennas when implanted in a lossy material (phantom) and to achieve a thorough miniaturization of their size based in these designs, a complete study of the effect of each parameter was undertaken.

Effect of the Electrical Length

To determine the effect of antenna length, the number of the meandered strip arms were changed. More strips increase the length of the current path on the radiating patches, or, equivalently, the effective size of the PIFA and achieve a more-compact size for the implantable antenna. The current is directed from the shorting pin to the upper end of the antenna and as expected, and as shown in Graph 3.5, longer antennas have lower resonant frequencies.

Table 3.1 Number of Meandered strips with the corresponding resonant frequency

No of Strips fres (MHz) 4 403 8 348 16 286 32 237 64 209

Frequency (MHz) Number of Strips vs fres 500 450 400 350 300 250 200 150 100 50 0 0 5 10 15 20 25 30 35

Graph 3.5 Number of Meandered strips with the corresponding resonant frequency

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Figure 3.6 Top view of the Meandered PIFA with (a) 4 strips and (b) 8 strips

It is well known that the higher the number of strips: - the lower the bandwidth of the antenna, as well as - the smaller the width of the strips having as a result to potentially increase the manufacturing cost.

For our meandered PIFA design, 8 strips were chosen because it is a good compromise. All the following simulations are considered to be with an 8 strip design for the meandered antenna. In addition, as shown Graph 3.6, in this simulation the antenna matching changes, but as we will see in the following sections, this can be tuned by adjusting other antenna parameters.

Figure 3.7 Front view and side view of Meandered PIFA configuration parameters

Table 3.2 Parameters of Meandered PIFA configuration with 8 strips

Dimension Description Value (mm)

Lsub Substrate Length 24.4 Wsub Substrate Width 20 Lp Patch Length 19.4 Wp Patch Width 15 Wstrp Stripline Width 1.9 Gstrp Stripline Gap 0.6 Wds π-turn Width 4.4 Ha Antenna Height 2.75 Tsub Substrate Thickness 1.25

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Graph 3.6 Reflection Coefficient of Meandered PIFA configuration with 8 strips

Effect of the Dielectric Constant (Substrate - Superstrate)

The choice of substrate and superstrate materials is critical in the design of long-term biocompatible antennas. The effect of varying the electrical permittivity of the superstrate and substrate is shown in Graph 3.7. As expected, higher permittivity results in lower resonant frequency because the effective wavelength is shorter. Higher dielectric constant-materials shorten the PIFA’s wavelength, increase its electrical length and are, thus, found to decrease its resonance frequency.

Graph 3.7 Reflection coefficient responses for different dielectric constant materials

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Table 3.3 Effect of dielectric material on the resonance frequency, and reflection coefficient at 350 MHz

Dielectric Material εr fres (GHz) |S11|@350 MHz

Teflon 2.1 0.754 - 0.13 Macor 6.1 0.443 - 1.36 Alumina (96%) 9.4 0.361 - 7.65 Rogers 3210 10.2 0.350 - 8.86

It can be easily find out that with the increasing dielectric constant, the resonant frequency is shifting to the left, in other words, the resonant frequency decreases. At the same time, the magnitude of the return loss becomes larger in dB. It indicates that with much larger dielectric constant, the input return loss is higher. The choice of substrate and superstrate materials is thus proved to be highly critical in the design and performance of miniature implantable PIFAs. For our designs the material used for substrate and superstrate was Rogers

3210 (εr = 10.2).

Generally, conventional PIFA is a quarter-wavelength structure. It has a resonant length almost proportional to 1/√εr. The resonant frequency can be calculated approximately using the equation below.

Where C is the speed of light in air. In addition, LT is the total surface current length. Finally, the εr is the relative permittivity of the microwave substrate and superstrate. Thus, the simulation results are following the theoretic rule. As the dielectric constant increases, the resonant frequency decreases.

 When the thickness of the substrate is increased, the effective dielectric constant is also increased, and the antenna will appear electrically longer and, hence, have a slightly lower resonant frequency.  When the superstrate thickness is increased, the resonant frequency is decreased, because the thicker superstrate actually reduces the effective permittivity by insulating the antenna from the higher dielectric body material.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Effect of background material and size (Phantoms)

This section analyses the impact that the size of the cubic model may have that simulates the properties of human tissue in the electromagnetic properties of the final layout which includes the system tissue and the implantable antenna.

 Fat - Phantom Size

The dependence of the behaviour of the size of the fat tissue, with relative permittivity εr = 4.6023, on the resonance frequency of the antenna is shown. First the dependence of the cube length and width behaviour of the antenna was studied and then the height. This was done to give a clearer picture of the effect of the dimensions of the tissue surrounding the antenna. It was observed that if the height was changed to a value less than 10 cm the function of the antenna would be altered dramatically affecting both the resonance frequency and the reflection coefficient.

Graph 3.8 Reflection coefficient responses for different fat phantom sizes

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Graph 3.9 Zoomed in view of reflection coefficient responses for different dielectric constant materials

Specifically, the change in the dimensions of the tissue model does not significantly affect the behaviour of the antenna, i.e., the resonant frequency and the reflection coefficient. The length and width of the tissue surrounding the antenna generally affects much less the characteristics of the antenna in relation to the height of the fat tissue.

In summary, although there are changes in the characteristics of the antenna when changing the size of the tissue that surrounds them, the impact is insignificant. The fact that the size of the box of tissue does not affect the characteristics of the antenna is very important. The as much as possible less dependent characteristics of implantable antenna the size of the tissue that surrounds it can be crucial to use the antenna in a real environment.

 Free Space

The implantation of the antenna in human tissue, as already mentioned, increases the dielectric constant of the active antenna. The reason this happens is because the human tissue has a much higher dielectric constant than air.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Graph 3.10 Reflection coefficient response for the Meandered PIFA with 8 strips in free space

As expected, the tuning of the antenna has changed and the antenna does not resonate in the same frequency or at least close to it as in the previous case, the main resonant frequency in this case is around 3 GHz. The antenna is not tuned anymore at 350 MHz. The behavior of the antenna has changed significantly. The introduction of material with different dielectric constant of vacuum changes the dielectric constant of the active antenna thereby displaying tuning in a different frequency from that experienced by the antenna in free space. The presence of biological tissue in the previous simulation affects the active dielectric constant of the device, leading to a reduction of the wavelength and increasing the active electrical length of the antenna. As a result of this whole process, the resonant frequency is significantly affected.

 Skin - Phantom Size

At this point, the same antenna is placed within a skin tissue phantom in order to see the effect of different background material with different dielectric constant (εr = 46.741) and its size. The different background material would contribute to the performance of antenna which is immersed inside it. The skin tissue is modeled by a brick model with the following three dimensions as seen on Graph 3.11. The length and width of the phantom are kept stable and only the effect of changing the height (z-plane) is observed. The relative dielectric constant of the cubic model skin used was stable for these simulations and equal to 46.741, a value that represents a dielectric constant of the human tissue at 402 MHz. Correspondingly, the conductivity value was constant and equal to σ = 0.6889925 S/m.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Graph 3.11 Reflection coefficient responses for different Skin phantom sizes

By changing the height of phantom not only the resonant frequency is changed but also the magnitude of the reflection coefficient, in more particular the smaller the height the bigger the value of the resonant frequency and on the contrary the smaller the absolute value of reflection coefficient in dBs. The frequency shift indicates that the lossy and dielectric effects of biological tissue on the PIFAs diminish as the skin tissue becomes thinner.

Effect of Rectangular Strip Width

Another parameter of interest is the dependence of the rectangular strip width of the antenna on the resonant frequency. In the following figures we can see five different strip widths starting from P1 - P2 = 0.52 mm (i), 0.77mm (ii), 1.02mm (iii), 1.27mm (iv) and end with 1.52 mm (v): (i) (ii) (iii) (iv) (v)

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Graph 3.12 Reflection coefficient responses for different strip widths

Both the resonant frequency and the magnitude of the reflection coefficient are changed when the strip width is altered. The smaller the width is, the lower the resonant frequency becomes. Only in the case when moving from 0.77 mm to 0.52 mm we can see that there is no effect on the resonant frequency. It can be noted as well that the matching is getting poorer as the strip width is getting smaller. The optimal strip width for our meandered PIFA design is 1.52 mm for better matching and closer to the MICS band.

Effect of Feeding Point Position

The antenna is fed with a standard discrete port, and the location of the feed would be expected to affect the tuning of the antenna. The dependence on the location of feeding point in the resonant frequency is also an interesting parameter. In this case, the feeding point is placed in different positions along one strip arm (x- axis) and between arms (y-axis).

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

 One strip arm (x-axis)

Graph 3.13 Reflection coefficient responses for different feeding point positions in the same strip

Graph 3.14 Zoomed in view of reflection coefficient responses for different feeding point positions in the same strip

As shown in Graphs 3.13 and 3.14, the locations of the feed point impact the antenna matching, but have little effect on the resonant frequency. There is no evident difference in the resonant frequency, but there is a change in the propagation. The magnitude of the return loss has become smaller, which indicates better impedance matching when the distance is being moved from the initial port position by -2 mm.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

 Different strip arms (y-axis)

Figure 3.8 Position of feeding point along different strip arms

Graph 3.15 Reflection coefficient responses for different feeding point positions in different strip arms

The changes in the resonant frequency are not significant. The graph shows that when the feeding point is set in the third arm, the return loss is smallest. Therefore, in order to have a better return loss, the location of the coaxial feeding should be set in this strip arm. However, at this point, our antenna is resonating at 348 MHz with magnitude of - 12.6 dB. Better matching could be achieved by changing the port impedance as shown in the next step.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Effect of Port Impedance

At this point of the simulations, the designs where the feeding point is placed -2 mm on the x-axis, comparing to its initial position, and on the third strip arm, were chosen in order to observe the effect of changing the port impedance. It has to be also noted that the dimensions of the meandered antenna for this simulation were 19.52 x 16 x 2.75 mm.

Table 3.4 Table Parameters of Meandered PIFA configuration

Dimension Description Value (mm)

Lsub Substrate Length 19.52 Wsub Substrate Width 16 Lp Patch Length 15.52 Wp Patch Width 12 Wstrp Stripline Width 1.52 Gstrp Stripline Gap 0.48 Wds π-shaped turn Width 3.52 Ha Antenna Height 2.75 Tsub Substrate Thickness 1.25

Matching for maximum power transfer indicates that the resonant frequency of the antenna should be closer to the axis 0 and 1, position z = 1 and z = 0 on the smith chart, where 1 represents 50 Ohms impedance. This is a typical curve of the highly resonant antenna. What we are trying to do is to get the resonant frequency to the 50 Ohms line. In our case, as it can be seen at Graph 3.16, the resonant frequency from the previous simulation was 348 MHz which has 92.19 Ohms impedance and is not located next position z = 1 in order to obtain impedance matching. After further investigation and a number of simulations the impedance port was moved up to 100 Ohms and as we can see from Graph 3.17, the resonant frequency now has 107.67 Ohms impedance and it is located next to z = 1.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Graph 3.16 S-parameter impedance view (smith chart) of fres 348 MHz at 50 Ohms

Graph 3.17 S-parameter impedance (smith chart) view of fres 341 MHz at 100 Ohms

Graph 3.18 Reflection coefficient responses for different feeding point impedances

It is clearly observed that when the feeding point impedance is changed from 50 Ohms to 100 Ohms better matching is achieved. There is also a slight reduction to the resonant frequency from 348 MHz to 341 MHz.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

The result of the graph indicates that with some circuit design we can artificially improve the input impedance matching of the antenna.

Effect of Antenna Size

In order for the antenna to be tuned at the MICS band, the antenna size was minimized (in x-plane and y- plane only). Five different antenna sizes were investigated and simulated as follows.

Graph 3.19 Reflection coefficient responses for different antenna sizes

The resonant frequency was being shifted to the right and was getting bigger as the antenna size was getting smaller. The optimal antenna size for tuning at 403 MHz was found to be 17.21 x 14.11 x 2.75 mm. However, the s-parameter magnitude has dropped significantly from - 28.633 dB to -19.3 dB. The following table summarizes the results from the above graph and the bandwidth for each antenna size is also calculated.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Table 3.5 Effect of antenna size on the resonance frequency, reflection coefficient at 403 MHz and antenna bandwidth

Antenna Size (mm) fres (GHz) |S11|@403 MHz (dB) Bandwidth (%)

19.52 x 16 x 2.75 0.341 -2 9.07

18.54 x 15.2 x 2.75 0.366 -4.14 8.74

17.56 x 14.4 x 2.75 0.393 -13.45 8.39

17.39 x 14.25 x 2.75 0.398 -17.07 8.54

17.21 x 14.11 x 2.75 0.403 -19.3 8.18

It can be derived from the table that the bandwidth of the different antenna sizes has approximately the same value from 8 - 9 %.

3.3 Proposed Meandered PIFA design

After several parametric studies the proposed meandered PIFA design has the following configuration parameters. As shown from Table 3.7, size of the proposed meandered PIFA design is 17.21 x 14.11 mm, thus a 50.3 % reduction in size of the initial antenna with dimensions 24.4 x 20 mm is achieved.

Table 3.6 Table Parameters of proposed Meandered PIFA configuration

Dimension Description Value (mm)

Lsub Substrate Length 17.21 Wsub Substrate Width 14.11 Lp Patch Length 13.69 Wp Patch Width 10.58 Wstrp Stripline Width 1.34 Gstrp Stripline Gap 0.42 Wds Turn Width 3.10 Ha Antenna Height 2.75 Tsub Substrate Thickness 1.25

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Graph 3.20 Reflection coefficient of the proposed meandered antenna resonating at 403 MHz

From the above graph we can see that the proposed meandered antenna is well matched at 403 MHz with -

19.3 dB reflection coefficient. The bandwidth of the antenna was calculated earlier to be 8.18 %.

Figure 3.9 3D farfield radiation pattern of the proposed meandered PIFA at 403 MHz

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Graph 3.21 H-plane radiation pattern of the proposed meandered PIFA (θ = 0°) at 403MHz

Graph 3.22 E-plane radiation pattern of the proposed meandered PIFA (θ = 90°) at 403 MHz The antenna has a fully omnidirectional pattern over the H-plane plane with the main lobe magnitude at -

6.3 dBi and it has E-plane directivity of 3.2 dBi.

Table 3.7 Table of Performance Parameters of proposed Meandered PIFA

Performance Parameter Value Dimensions 17.21 x 14.11 x 2.75 mm fres 403 MHz |S11|@403 MHz - 19.29 dB Bandwidth 8.18 % Radiated Power - 6.5 dBmW Radiation Efficiency - 33.48 dB Total Efficiency - 33.53 dB Directivity 3.228 dBi

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

3.4 Parametric Studies of Spiral PIFA - Effects

In this section, the effect of changing different parameters at the Spiral PIFA antenna is investigated. As already mentioned the initial design of the spiral PIFA has been described in section 3.1 of this chapter. The top view and side view of this spiral PIFA antenna are also shown in Figure 3.4.

Effect of Electrical Length

In order to have a much better understanding of the dependence of electrical length on the resonant frequency of the spiral PIFA antenna, the number of turns have to be increased. Therefore, the simulations of different number of multi-turn Spiral PIFA antennas are processed in this section.

The results of resonant frequency for different number of turns spiral PIFA antennas are listed in Table 3.8. In addition, the resonant frequency versus number of turns is drawn in Graph 3.23.

Table 3.8 Number of spiral turns with the corresponding resonant frequency

No of turns fres (MHz) 1 403 2 212 3 164 4 123

Frequency (MHz) fres vs No of turns 500

400

300

200

100

0 0 1 2 3 4 5

Graph 3.23 Resonant frequency versus number of spiral turns

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Graph 3.24 Return loss for 3 and 4 turns Spiral PIFA

It can be concluded from Graph 3.23 and Graph 3.24, that by increasing the number of turns, the resonant frequency is shifting towards left, which means the resonant frequency is becoming smaller and smaller. However, with smaller resonant frequency, the magnitude of the return loss increases. The reason for this consequence may be the reduction of strip width when more spiral turns are needed to be placed in the same area of substrate. The next figures depicts four different multi-turn spiral PIFA designs staring from 1-turn (i), 3-turn (ii), 9-turn (iii) and 10-turn (iv) spirals accordingly. (i) (ii) (iii) (iv)

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Chapter 4

Optimised Square Spiral Antenna Design

In the previous chapter, in section 3.2, we had a meandered PIFA that was matched at 403 MHz having the optimal size of 17.21 x 14.11 x 2.75 mm. In this chapter we will try to further miniaturise an antenna design and see if we can optimise the antenna performance at 402 - 405 MHz. In order to maximise the electrical length and achieve thorough miniaturisation, the spiral is a better candidate because in smaller area we can design longer strip lines.

After analysing the different parametric studies on the previous chapter, we designed a 10 x 10 mm 9-turn square spiral PIFA with the following characteristics and the aim was to develop this design into an antenna smaller than 5 x 5 mm in size which operates at 402 - 405 MHz MICS band.

Figure 4.1 Front view and side view of 9-turn square spiral PIFA (10 x 10 mm in size) configuration parameters

Table 4.1 Parameters of 9-turn square spiral PIFA (10 x 10 mm in size) configuration Dimension Description Value (mm)

Ntur Number of Turns 9 Lsub Substrate Length 10 Wstrp Stripline Width 0.42 Gstrp Stripline Gap 0.2 Gsub Substrate Gap 0.25 Ha Antenna Height 0.525 Tsub Substrate Thickness 0.25

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

4.1 Effect of Antenna Size

Graph 4.1 Reflection coefficient responses for different antenna sizes

As it can be observed from the above graph, our initial 10 x 10 x 0.525 mm antenna resonates at two different frequencies, the first resonant frequency is at 135 MHz and the second one is closer to the MICS band in which we are interested. This indicates that there is space for improvement for this antenna such as further miniaturization. The result would be for the first resonant frequency to be shifted to the MICS band in order to achieve matching. The next antenna that was designed had the dimensions of 8 x 8 x 0.525 mm and the resonant frequency was moved to 175 MHz, with further miniaturization we run a 6 x 6 x 0.525 mm antenna design which resonated at 250 MHz. Even the fact that the matching is getting poorer, there is still resonance which was actually achieved with the 4.2 x 4.2 x 0.525 design at MICS band and in particular at 403 MHz.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Table 4.2 Effect of antenna size on the resonance frequency and reflection coefficient at 403 MHz

Antenna Size (mm) fres (GHz) |S11|@403 MHz (dB)

10 x 10 x 0.525 0.135 -13.79

8 x 8 x 0.525 0.175 -6.02

6 x 6 x 0.525 0.250 -4.13

4.2 x 4.2 x 0.525 0.403 -6.44

The last design proved to be the optimal for getting the resonant frequency we were looking after. However, matching is not succeeded as the reflection coefficient is - 6.44 dB making the antenna act like a broadband one. In order to achieve matching, we carried on to changing the feeding port impedance as we have already reached the desired resonant frequency.

4.2 Effect of Port Impedance

As in the previous chapter, in section 3.2.6 in particular, what we are trying to do is to get the resonant frequency to the 50 Ohms line. In our case, as it can be seen at Graph 4.2, the resonant frequency from the previous simulation was 403 MHz which has 65.42 Ohms impedance and is not located next position z = 1 in order to obtain impedance matching. After further investigation and a number of simulations the impedance port was moved up to 130.8 Ohms and as we can see from Graph 4.3, the resonant frequency now has 151.78 Ohms impedance and it is located next to z = 1.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Graph 4.2 S-parameter impedance view (smith chart) of fres 403 MHz at 50 Ohms

Graph 4.3 S-parameter impedance (smith chart) view of fres 347 MHz at 130.8 Ohms

Graph 4.4 Reflection coefficient responses for different feeding point impedances

It is clearly observed that when the feeding point impedance is changed from 50 Ohms to 130.8 Ohms better matching is achieved. There is also a reduction to the resonant frequency from 403 MHz to 347 MHz. The

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications result of the graph indicates once again that with some circuit design we can artificially improve the input impedance matching of the antenna.

In order for the antenna to be tuned at the MICS band, the antenna size was minimized (in x-plane and y- plane only). Five different antenna sizes were investigated and simulated as follows.

Graph 4.5 Reflection coefficient responses for different antenna sizes

The resonant frequency was being shifted to the right and was getting bigger as the antenna size was getting smaller. The optimal antenna size for tuning at 402 MHz was found to be 3.696 x 3.696 x 0.525 mm. However, the s-parameter magnitude has dropped slightly from - 22.251 dB to - 17.121 dB. The following table summarizes the results from the above graph and the bandwidth for each antenna size is also calculated.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Table 4.3 Effect of antenna size on the resonance frequency, reflection coefficient at 402 MHz and antenna bandwidth

Antenna Size (mm) fres (GHz) |S11|@402 MHz (dB) Bandwidth (%)

4.2 x 4.2 x 0.525 0.347 -7.31 20.9

3.99 x 3.99 x 0.525 0.368 -10.93 20.3

3.78 x 3.78 x 0.525 0.392 -16.46 19.57

3.738 x 3.738 x 0.525 0.397 -17.13 19.55

3.696 x 3.696 x 0.525 0.402 -17.12 19.53

It can be derived from the table that the bandwidth of the different antenna sizes has approximately the same value from 19 - 20 %.

4.3 Effect of Superstrate

Graph 4.6 Reflection coefficient responses for the antenna with and without superstrate

As expected without the superstrate the bandwidth is increased, in particular from 19.53 % to 26 %. However, the necessary presence of superstrate protects the circuits and the antenna from the dielectric properties of the skin, it actually acts as a physical insulator and it is used for biocompatibility. The dielectric constant of the skin is higher than the one from the substrate, and the effective εr is reduced when having the superstrate. The

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications superstrate material (which also should be low loss) also reduces the power deposited in the body very near the antenna. In effect, the superstrate is preventing the body from shorting out the antenna.

4.4 Effect of Antenna Implanted Depth in Fat Phantom

Figure 4.2 Schematic of different antenna implanted depth in Fat Phantom (50 x 50 x 30 mm,εr=4.6023)

Graph 4.7 Reflection coefficient responses for different antenna implanted depth

As we can see in the graph by changing the depth of our implanted antenna, in the phantom, there is no significant alteration in the performance of the antenna. Only the absolute value of the reflection coefficient is getting slightly smaller in the extreme scenario where the antenna is placed just 0.5 mm from the skin. The most important aspect of this graph is that the antenna does not detune even in 0.5 mm depth and the resonant

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications frequency remains almost stable at MICS band. It is very important for an implantable antenna system not to detune without concerning where it is placed inside the body.

4.5 Proposed Spiral Antenna

After several parametric studies the proposed meandered PIFA design has the following configuration parameters. As shown from Table 4.5, the size of the proposed spiral PIFA design is 3.7 x 3.7 mm, thus a 97.2 % reduction in size of the initial antenna with dimensions 24.4 x 20 mm is achieved.

Table 4.4 Table Parameters of proposed Meandered PIFA configuration Dimension Description Value (mm)

Ntur Number of Turns 9 Lsub Substrate Length 3.7 Wstrp Stripline Width 0.11 Gstrp Stripline Gap 0.07 Gsub Substrate Gap 0.13 Ha Antenna Height 0.525 Tsub Substrate Thickness 0.25

Graph 4.8 Reflection coefficient of the proposed spiral antenna resonating at 402 MHz

From the above graph we can see that the proposed spiral antenna is well matched at 402 MHz with -17.12 dB reflection coefficient. The bandwidth of the antenna was calculated earlier to be 19.53 %.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Figure 4.3 3D farfield radiation pattern of the proposed spiral PIFA at 402 MHz

Graph 4.9 H-plane radiation pattern of the proposed spiral PIFA (θ = 0°) at 403MHz

Graph 4.10 E-plane radiation pattern of the proposed spiral PIFA (θ = 90°) at 403 MHz

The antenna has a fully omnidirectional pattern over the H-plane plane with the main lobe magnitude at -

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

16.1 dBi and it has E-plane directivity of 2.3 dBi.

Table 4.5 Table of Performance Parameters of proposed Spiral PIFA Parameter Value Dimensions 3.696 x 3.696 x 0.525 mm fres 402 MHz |S11|@402 MHz - 17.12 dB Bandwidth 19.53 % Radiated Power - 36.36 dBmW Radiation Efficiency - 63.26 dB Total Efficiency - 63.35 dB Directivity 2.284 dBi

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Chapter 5

Future Work

5.1 Antenna Design (1 x 1 mm)

In paper [64], the radiation characteristics of a miniature (1 x l mm) printed square spiral chip antenna for contact-less powering and RF telemetry from implantable bio-MEMS sensors are presented. The antenna is fabricated on a high resistivity silicon wafer and has a serrated ground plane on the opposite side to mitigate image current effects. This miniaturised antenna is part of a space program at NASA (Robotic and Human exploration) where implantable wireless sensors are developed the physiological parameters of humans during flights [73] and is a perfect candidate for this project.

Figure 5.1(a) shows a schematic of the miniature implantable square spiral chip antenna. The outer dimensions of the antenna are about 1 x 1 mm, and the circuit is fabricated on a 400 μm thick HR-Si wafer to reduce the attenuation of the signals. Figure 5.1(b) shows the serrated ting ground plane on the opposite side of the wafer to mitigate image current effects.

Figure 5.1 Miniature implantable square spiral chip antenna on high resistivity silicon wafer [64]. (a) Top view: Square spiral conductors. (b) Bottom view: serrated ring ground plane

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Figure 5.2 3D illustration of spiral chip antenna in CST Microwave Studio environment

According to [64] the measured Q and L as a function of frequency are presented in Graph 5.1. The peak Q and the corresponding L are about 8.2 and 130 nH, respectively. By comparing the two sets of results it is evident that there is a small degradation in the peak Q and corresponding L values. Nevertheless, the serrated ground plane clearly demonstrates its efficacy to mitigate image currents effects.

Graph 5.1 Measured inductance and quality factor for inductor on SOG/HR-Si [64]

The proposed antenna was redesigned, according to the configurations given, and simulated in CST Microwave Studio (3D EM Simulation Software) in order to verify and compare their return-loss characteristics as well as their performance to the one shown in [64]. The return loss of our simulated version of the square spiral chip antenna is presented in Graph 5.2. The antenna for this simulation was placed in a fat phantom (50 x 50 x 30 mm) with εr = 4.6023. At this stage of this work, the resonant frequency is 4.35 GHz with - 8.76 dB return loss.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Graph 5.2 Simulated return-loss characteristics of the square spiral chip antenna in CST Microwave Studio environment

However, it was not possible to fulfil the requirements of this challenging work. Since, the simulation running time was extremely high (at least 3 days needed for one simulation to run in a super computer owned by Mediwise Ltd.), due to the enormous number of mesh cells (48,000,000), further work is required, and this could be considered as a future research project.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Chapter 6

Conclusions

In the context of this thesis the following were presented:  the potential of biomedical technology and the significant boost that can deliver in the field of medicine,  several implantable devices used today, but also  the importance of designing and manufacturing miniature antennas offering interactivity to these provisions with the outside world,  the properties of the biological environment, in which such an implantable antenna will be required to operate,  summary of the microstrip antenna theory as well as their miniaturization techniques.

Two types of implantable antenna structures, that are suitable for miniaturisation, were designed, a meandered PIFA and Spiral PIFA. In order to understand the performance of those two microstrip antennas, when implanted in a lossy material (phantom) and to achieve a thorough miniaturization of their size based in these designs, a complete study of the effect of each parameter was undertaken.

An implantable multi-turn square spiral antenna design, that is smaller than 5 x 5 mm in size, operating at 402 - 405 MHz MICS band, was developed. The size of the proposed spiral PIFA design is 3.7 x 3.7 mm, thus a 97.2 % reduction in size of the initial antenna with dimensions 24.4 x 20 mm is achieved. The proposed spiral antenna is well matched at 402 MHz with -17.12 dB reflection coefficient. The bandwidth of the antenna was calculated to be 19.53 %. The Radiated Power is enough for communication outside of the body, however, further work could be fulfilled to improve the Radiation Efficiency.

Concerning the 1 x 1 mm design of the miniaturized antenna, this can be considered as future work, since it was not possible to fulfill the requirements of this challenging work.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

Chapter 7

References

[1] P. D. Bradley, “Wireless Medical Implant Technology - Recent Advances and Future Developments,” Esscirc 2011 Proc., pp. 37 – 41, 2011. [2] J. Kim and Y. Rahmat-Samii, “Implanted Antennas Inside a Human Body: Simulations, Designs and Characterizations,” Ieee Trans. Microw. Theory Tech., vol. 52, no. 8, pp. 1934–1943, Aug. 2004. [3] W. G. Scanlon, Z. M. McCreesh, and N. E. Evans, “RF Performance of a 418 MHz Radio Telemeter Packaged for Human Vaginal Placement,” Ieee Trans. Biomed. Eng., vol. 44, no. 5, pp. 427–430, May 1997. [4] J. Riistama, J. Vaisanen, S. Heinisuo, J. Lekkala, and J. Kaihilahti, “Evaluation of an implantable ECG monitoring device in vitro and in vivo,” Eng. Med. Biol. Soc., pp. 5703 – 5706, 2007. [5] M. C. Shults, R. K. Rhodes, S. J. Updike, B. J. Gilligan, and W. N. Reining, “A Telemetry- Instrumentation System for Monitoring Multiple Subcutaneously Implanted Glucose Sensors,” Ieee Trans. Biomed. Eng., vol. 41, no. 10, pp. 937–942, Oct. 1994. [6] D. Wessels, “Implantable Pacemakers and Defibrillators: Device Overview and EMI Considerations,” Proc. Ieee Int. Symp. Electromagn. Compat. Emc 2002, Aug. 2002. [7] K. Guillory and R. A. Normann, “A 100-Channel System for Real Time Detection and Storage of Extracellular Spike Waveforms,” J. Neurosci. Methods, vol. 91, pp. 21–29, 1999. [8] T. Buchegger, G. Obberger, A. Reisenzahn, E. Hochmair, A. Stelzer, and A. Springer, “Ultra-Wideband Transceivers for Cochlear Implants,” Eurasip J. Appl. Signal Process., vol. 18, pp. 3069–3075, 2005. [9] K. Gosalia, G. Lazzi, and M. Humayun, “Investigation of Microwave Data Telemetry Link for a Retinal Prosthesis,” Ieee Trans. Microw. Theory Tech., vol. 52, no. 8, pp. 1925–1932, Aug. 2004. [10] “http://www.nhlbi.nih.gov/health/health-topics/topics/pace/howdoes.html.” . [11] “http://www.evmsent.org/cochlear_implant.asp.” . [12] “http://vectoracademy.net/bionic-eye-received-fda-approval-2/.” . [13] “http://news.cnet.com/8301-27083_3-10390780-247.html.” . [14] A. Kiourti and K. S. Nikita, “A Review of Implantable Patch Antennas for Biomedical Telemetry: Challenges and Solutions,” Antennas Propag. Mag. Ieee, vol. 54, no. 3, pp. 210 – 228, Jun. 2012. [15] F. Merli, “Implantable Antennas for Biomedical Applications,” Ecole Polytechnique Federale De Lausanne, 2011. [16] A. K. Skriverviky and F. Merli, “Design strategies for implantable antennas,” Antennas Propag. Conf. Lapc 2011 Loughb., pp. 1 – 5, Nov. 2011. [17] T. Karacolak, A. Z. Hood, and E. Topsakal, “Design of a dual-band implantable antenna and development of skin mimicking gels for continuous glucose monitoring,” Ieee Trans Microw Theory Tech, vol. 56, no. 4, pp. 1001–1008, Apr. 2008. [18] A. K. Skriverviky and F. Merli, “Design and characterization of bio-implantable antennas,” Icecom 2010 Conf. Proc., pp. 1 – 5, Sep. 2010. [19] “http://www.mediwise.co.uk/.” . [20] Konstantinos Psathas, “DESIGN OF IMPLANTABLE DUAL-BAND ANTENNA FOR BIOTELEMTRY,” National Technical University of Athens, Athens - Greece, 2012. [21] G. Rizzi, A. Scrivani, M. Fini, and R. Giardino, “Biomedical coatings to improve the tissue- biomaterial interface,” Int J Artif Organs, pp. 49–57, Aug. 2004. [22] G. Llanos, P. Narayanan, M. Roller, and A. Scopelianos, “Coatings for Medical Devices,” US 6,746,773 B2. [23] J. Burgahard, C. Campbell, T. Younkin, M. Kuhn, D. Shykind, and J. Maiz, “Biocompatible coatings for Medical Devices,” US 2009/0169714 A1.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

[24] M. Meyers, P. Chen, A. Lin, and Y. Seki, “Biological materials: Structure and mechanical properties,” Prog. Mater. Sci., no. 53, pp. 201–206, 2008. [25] S. Stupp and P. Braun, “Molecular Manipulation of Microstructures: Biomaterials, Ceramics, and Semiconductors,” Science, no. 277, pp. 1242–1248, 1997. [26] A. Kumar, A. Srivastava, I. Galaev, and B. Mattiasson, “Smart polymers: Physical forms and bioengineering applications,” Prog. Polym. Sci., vol. 32, no. 10, pp. 1205–1237, 2007. [27] IEEE, “Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3kHz to 300GHz,” 1999. [28] IEEE, “Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3kHz to 300GHz,” 2005. [29] C. A. Balanis, Antenna Theory, Analysis and Design, 2nd ed. John Wiley & Sons, INC, 1997. [30] “http://www.rfglobalnet.com/content/news/article.asp?docid=%7B75e3bff5-2b87-479f-8d32- 964f1fd04715%7D.” . [31] “http://www.antenna-theory.com/antennas/patches/pifa.php.” . [32] P. S. Neelakantra and R. Chatterjee, Antennas for Information Super Skyways: An Exposition on Outdoor and Indoor Wireless Antennas. Research Studies Press Ltd., 2002. [33] A. Kiourti, M. Tsakalakis, and K. S. Nikita, “Parametric Study and Design of Implantable PIFAs for Wireless Biotelemetry,” Proc. 2nd Icst Int. Conf. Wirel. Mob. Commun. Healthc. Mobihealth 2012 Kos Isl. Greece, Oct. 2011. [34] P. Soontornpipit, C. M. Furse, and Y. C. Chung, “Design of Implantable Microstrip Antenna for Communication with Medical Implants,” Ieee Trans. Microw. Theory Tech., vol. 52, no. 8, pp. 1944– 1951, Aug. 2004. [35] A. Kiourti, K. S. Nikita, and M. Christopoulou, “Performance of a Novel Miniature Antenna Implanted in the Human Head for Wireless Biotelemetry,” Ieee Int. Symp. Antennas Propag. Spokane Wash., Jul. 2011. [36] A. Kiourti and K. S. Nikita, “Miniature Scalp-Implantable Antennas for Telemetry in the MICS and ISM Bands: Design,Safety Considerations and Link Budget Analysis,” Ieee Trans. Antennas Propag. [37] I. J. Bahl, S. S. Stuchly, and M. A. Stuchly, “A new microstrip radiator for medical applications,” Ieee Trans. Microw. Theory Tech., vol. MTT-28, pp. 1464–1469, Dec. 1980. [38] C. M. Furse, “Design an antenna for pacemaker communication,” Microw. Rf J., pp. 73–76, Mar. 2000. [39] A. Kiourti, J. R. Costa, C. A. Fernandes, A. G. Santiago, and K. S. Nikita, “Miniature Implantable Antennas for Biomedical Telemetry: From Simulation to Realization,” Ieee Trans. Biomed. Eng., vol. 59, no. 11, Nov. 2012. [40] K. Y. Yazdandoost and R. Kohno, “An Antenna for Medical Implant Communications System,” Proc. 37th Eur. Microw. Conf., pp. 968–971, Oct. 2007. [41] M. L. Scarpello, D. Kurup, and H. Rogier, “Design of an Implantable Slot Dipole Conformal Flexible Antenna for Biomedical Applications,” Ieee Trans. Antennas Propag., vol. 59, no. 10, pp. 3556–3554, Oct. 2011. [42] Y. Rahmat-Samii and J. Kim, Implanted Antennas in Medical Wireless Communications, 1st ed. Morgan & Claypool Publishers, 2006. [43] J. Kim and Y. Rahmat-Samii, “Planar inverted-f antennas on implantable medical devices: Meandered type versus spiral type,” Microw Opt Technol Lett, vol. 48, no. 3, pp. 567–572, 2006. [44] P. Soontornpipit, C. M. Furse, and Y. C. Chung, “Miniaturized biocompatible microstrip antenna using genetic algorithm,” Ieee Trans Antennas Propag, vol. 53, no. 6, pp. 1939–1945, Jun. 2005. [45] C. Miry, R. Gillard, and R. Loison, “An application of the multi-level DG-FDTD to the analysis of the transmission between a dipole in free-space and an implanted antenna in a simplified body model with various positions,” Proc 3rd Eur. Conf. Antennas Propag. Eucap 2009, pp. 67–70, Mar. 2009. [46] C. Miry, R. Gillard, and R. Loison, “Multi-level dual-grid finite-difference time-domain approach for the analysis of body implanted antennas,” Microwaves Antennas Propag. Iet, vol. 4, no. 6, Jun. 2010.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

[47] Α. Sani, Μ. Rajab, R. Foster, and Y. Hao, “Antennas and propagation of implanted RFIDs for pervasive healthcare applications,” Proc Ieee, vol. 98, no. 9, pp. 1648–1655, 2010. [48] C.-M. Lee, T.-C. Yo, F. J. Huang, and C.-H. Luo,, “Dual-resonant π-shape with double L-strips PIFA for implantable biotelemetry,” Electron. Lett., vol. 44, no. 14, pp. 837–838, Jul. 2008. [49] T.-F. Chien, C.-M. Cheng, H.-C. Yang, J.-W. Jiang, and C.-H. Luo, “Development of nonsuperstrate implantable low-proﬁle CPW-fed ceramic antennas,” Ieee Antennas Wirel. Propag Lett, vol. 9, pp. 599– 602, 2010. [50] F. Merli, L. Bolomey, E. Meurville, and A. K. Skrivervik, “Implantable antenna for biomedical applications,” Proc Ieee Antennas Propag Soc Int Symp San Diego Ca, pp. 584–587, 2008. [51] C.-M. Lee, T.-C. Yo, F. J. Huang, and C. H. Luo, “Bandwidth enhancement of planar inverted-F antenna for implantable biotelemetry,” Mi- Crowave Opt Tech Lett, vol. 51, no. 3, pp. 749–751, Mar. 2009. [52] A. Kiourti, M. Christopoulou, S. Koulouridis, and K. S. Nikita, “Design of a novel miniaturized implantable PIFA for biomedical telemetry,” Proc Int. Icst Con- Ference Wirel. Mob. Commun. Healthc. Mobihealth 2010, pp. 18–20, Oct. 2010. [53] W.-C. Liu, F.-M. Yeh, and M. Ghavami, “Miniaturized implantable broadband antenna for biotelemetry communication,” Microw Opt Technol Lett, vol. 50, pp. 2407–2409, Sep. 2008. [54] C.-M. Lee, T.-C. Yo, and C.-H. Luo,, “Compact broadband stacked implantable antenna for biotelemetry with medical devices,” Proc Ieee Annu Wirel. Microw Technol Conf, pp. 1–4, Dec. 2006. [55] W.-C. Liu, S.-H. Chen, and C.-M. Wu, “IMPLANTABLE BROADBAND CIRCULAR STACKED PIFA ANTENNA FOR BIOTELEMETRY COMMUNICATION,” J Electromagn Waves Appl, vol. 22, pp. 1791–1800, 2008. [56] W.-C. Liu, S.-H. Chen, and C.-M. Wu, “Bandwidth enhancement and size reduction of an implantable PIFA antenna for biotelemetry devices,” Microw Opt Technol Lett, vol. 51, pp. 755–757, Mar. 2009. [57] Vidal, N., Villegas, J.M.L., Curto, S., and Sieiro, J., “Design of a LTCC compact implantable broadband antenna for wireless biotelemetry,” Antennas Propag. Soc. Int. Symp. Apsursi 2012 Ieee, pp. 1–2, Jul. 2012. [58] Permana, H., Qiang Fang, and Cosic, I., “3-layer implantable microstrip antenna optimised for retinal prosthesis system in MICS band,” Bioelectron. Bioinforma. Isbb 2011 Int. Symp., pp. 65 – 68, Nov. 2011. [59] Fu-Jhuan Huang, Chien-Ming Lee, Chia-Lin Chang, and Liang-Kai Chen, “Rectenna Application of Miniaturized Implantable Antenna Design for Triple-Band Biotelemetry Communication,” Ieee Trans. Antennas Propag., vol. 59, no. 7, p. JULY 2011. [60] A. Kiourti and K. S. Nikita, “Meandered versus spiral novel miniature PIFAs implanted in the human head: Tuning and performance,” Proc 2nd Icst Int Conf Wirel. Mob. Commun Healthc., pp. 80– 87, Oct. 2011. [61] C. L. Chen and R. W. P. King, “The small bare loop antenna immersed in a dissipative medium,” Ieee Trans Antennas Propag., pp. 266–269, May 1963. [62] G. S. Smith, “On the electrically small bare loop antenna in a dissipative medium,” Ieee Trans Antennas Propag., vol. 24, no. 4, pp. 533–537, Jul. 1976. [63] G. S. Smith, “A theoretical and experimental study of the insulated loop antenna in a dissipative medium,” Radio Sci., vol. 8, pp. 711– 725, Jul. 1973. [64] Simons, Rainee N. and Miranda, F.A., “Radiation characteristics of miniature silicon square spiral chip antenna for implantable bio-MEMS sensors,” Antennas Propag. Soc. Int. Symp. 2005 Ieee, vol. 1B, pp. 836 – 839, 2005. [65] R.N. Simons, D.G. Hill, and F.A. Miranda, “Spiral Chip Implantable Radiator and Printed Loop Extemal Receptor for RF Telemetry in Bio-Sensor Systems,” Proc 2004 Ieee Radio Wirel. Conf. Rawcon2004atlanta Ga, pp. 203–206, Sep. 2004.

Panagiotis Blanos - Miniaturization Of Implantable Antennas For Medical Applications

[66] F.A. Miranda, R.N. Simons, and D.G. Hall, “Validation of Radio Frequency Telemetry Concept in the Presence of Biological Tissue-Like Stratified Media,” Ieee Antennas Propag. Soc. Inter Symp Dig. Monterey Ca, vol. 2, pp. 1335–1338, Jun. 2004. [67] M. Ahmadian, et al., “Miniature Transmitter for Implantable Micro Systems,” Proc 25th Annu. Inter Conf Ofthe Ieee Eng. Med. Biol. Soc. Cancun Mex., vol. 4, pp. 3028–3031, Sep. 2003. [68] S. Ullerich, et al., “Micro Coils for an Advanced System forMeasuring Intraocular Pressure,” Proc Ofthe 1st Annu. Inter Ieee-Embs Spec. Top. Conf Microtechnologies Med. Biol. Lyonfrance, pp. 470– 474, Oct. 2000. [69] W. Mokwa, “Ophthalmic Implants,” Proc Ofthe 2nd Ieee Inter Conf Sensors Tor. Can., vol. 2, pp. 980–986, Oct. 2003. [70] K. Van Schuylenbergh, et al., “Monitoring Orthopedic Implants Using Active Telemetry,” Proc 14th Annu. Inter Conf Ofthe Ieee Eng. Med. Biol. Soc. Embs-92 Paris Fr., vol. 6, pp. 2672–2673, Nov. 1992. [71] T. Akin, et al., “A Wireless Implantable Multichannel Digital Neural Recording System for a Micromachined Sieve Electrode,” Ieee Jour Solid-State Circuits, vol. 33, no. 1, pp. 109–118, Jan. 1998. [72] CST STUDIO SUITETM 2010, CST Microwave Studio - Workﬂow & Solver Overview. CST – Computer Simulation Technology AG, 2010. [73] “http://www.nasa.gov/missions/solarsystem/explore_main.html.” .