Biomed Eng Lett (2015) 5:203-212 DOI 10.1007/s13534-015-0193-z

REVIEW ARTICLE

Antennas for Biomedical Applications Gurveer Kaur, Amandeep Kaur, Gurpreet Kaur Toor, Balwinder S. Dhaliwal and Shyam Sundar Pattnaik

Received: 19 April 2015 / Accepted: 4 August 2015 © The Korean Society of Medical & Biological Engineering and Springer 2015

Abstract Biomedical engineering today holds a prominent place as a means of improving medical diagnosis and treatment, and as an academic discipline. Today glucose monitoring, insulin pumps, deep brain simulations and endoscopy are a few examples of the medical applications that can take advantage of remote monitoring system and body implantable unit. Body implantable devices are widely researched for humans, in the applications such as monitoring blood pressure and temperature, tracking dependent people or lost pets, wirelessly transferring diagnostic information from an electronic device implanted in the human body for human care and safety, such as a pacemaker, to an external RF receiver. Antennas can be implanted into human bodies or can just be mounted over the torso (skin-fat-muscle) to form a bio-communication system between medical devices and exterior instruments for short range biotelemetry applications. In addition to the clear benefits to the healthcare system provided by body implanted devices, economical aspects are also relevant. Remote monitoring systems facilitate the diagnosis of diseases and favor the hospital at home by reducing the hospitalization period. Keywords WBAN (Wireless Body Area Network), WCE (Wireless Capsule Endoscopy), MICS (Medical Implant Communication Services)

Gurveer Kaur ( ), Amandeep Kaur, Gurpreet Kaur Toor, Balwinder S. Dhaliwal Guru Nanak Dev Engineering College, Ludhiana, India, 141006 Tel: +91-9779408165 / Fax : +91-9779408165 E-mail: [email protected] Shyam Sundar Pattnaik ( ) National Institute of Technical Teachers Training and Research, Chandigarh Tel: +91-9872879362 / Fax : +91-9872879362 E-mail: [email protected]

INTRODUCTION One of the fastest growing fields of technology−a field of recent achievements and even more ambitious hopes−is biomedical engineering. Laboratory instrumentation, medical imaging, cardiac pacemakers [1], artificial limbs, and computer analysis of the human genome are some of its familiar products. The design of a specific radiator is the key aspect of a WBAN of a few meter ranges. The characteristics such as radiation efficiency, bandwidth, the coupling with the lossy biological tissues and the use of available volume are essential for the data communication [2]. In recent years, various types of medical applications of antennas have widely been investigated and reported, which includes diagnosis as well as treatment of various chronic diseases.

MEDICAL APPLICATIONS OF ANTENNAS Diagnosis - MRI (magnetic resonance imaging)/FMRI (functional MRI) - Biomedical Telemetry - Wireless capsule endoscopy Treatment (often referred to as “thermal therapies”) - Microwave hyperthermia - Microwave coagulation therapy Diagnosis Role of antennas in diagnosis of medical ailments include various medical applications such as MRI (magnetic resonance imaging)/FMRI (functional MRI), biomedical telemetry and WCE (Wireless Capsule Endoscopy). MRI (magnetic resonance imaging)/FMRI (functional MRI) During MRI, antennas radiate electromagnetic pulses to the human body and in response receive the NMR nuclear magnetic resonance signals emitted from the nuclei, which

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The main goal of a healthcare monitoring system is to provide reliable information from inside of the human body to an external Base Station (BS). Such aspects, identified from the antenna engineer point of view, are illustrated in Fig. 3. Fig. 1. Various types of Antennas for MRI systems: (a) Head Coil (b) Surface Coil (c) Knee Coil.

constitutes the human body. Fundamental frequency of the EM pulses and frequency of the NMR signals are placed in VHF band [3]. Therefore, these antennas are electrically small antennas as shown in Fig. 1. Biomedical telemetry The introduction of pacemakers and the first swallowable pills with sensing capabilities showed the great importance of implantable devices enabling the monitoring and the treatment within the human body. Such devices called for the use of wireless communication system in order to control the functioning of the system and report the patient’s status. However, to be truly beneficial, the telemetry device must be able to communicate with an external receiver unit. To do this, the device must have an integrated antenna [4]. An example of a home healthcare monitoring system is illustrated in Fig. 2.

Fig. 2. Sketch of a generic home healthcare system with a wireless implantable device working in a Wireless Body Area Network (WBAN).

1-BASE STATION A general Base Station comprises several sub-systems: • a control module to drive the entire system and to store the measurements. • a receiver module including antennas. • an internet modem (or any other device to connect to the data collecting system). 2-CHANNEL PROPAGATION The analysis of the EM propagation from the implant to the Base Station is an important aspect of the channel propagation and is taken care of for data telemetry. 3-HUMAN BODY The human body is of primary relevance for an implantable device. Its complex, dispersive and highly lossy characteristics unavoidably affect the analysis, design, realization and characterization of body implantable antennas, thus the wireless performances of the entire system. 4-INSULATIONS The presence of a biocompatible insulation is mandatory for any implantable device so as to avoid any adverse reaction of the living tissues. Such insulation is of paramount interest from the antenna point of view, as the human body is a “hostile” environment for the Radio Frequency (RF) radiation. In fact, insulating layers either placed around the antenna or on the surface of the human skin, enhance the EM transmission from an implantable radiator to the Base Station. 5-IMPLANTABLE ANTENNAS The key and critical component of RF linked medical devices is the integrated implantable antenna, which enables bidirectional communication with the exterior monitoring/ control equipment. Patch designs are currently receiving considerable attention for implantable antennas because they are highly flexible in design, shape and conformability [5-7], thus allowing for easy miniaturization and integration into the shape of implantable medical device.

Fig. 3. Wireless implantable systems for data telemetry.

6-ELECTRONICS AND POWER SUPPLY The electronic components of an implantable device allow the functioning of the system, provide data communication and signal processing capabilities and delineate the working power requirements. It thus defines the overall capabilities of

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the device itself. The power supply, often having the largest volume occupation, sets the life time of the apparatus. Several solutions such as energy harvesting, internal power supplies, or wireless power transfer are possible [8, 9]. 7-BIO-SENSORS AND BIO-ACTUATORS The bio-sensors and/or bio-actuators determine the application of an implantable device and its placement in the human body. Monitoring devices (for instance measuring temperature, pH, glucose, etc.) or active systems (drug delivery apparatus) are nowadays being investigated for implantable applications. 8-CHARECTRIZATION AND EXPERIMENTS Each component constituting the healthcare monitoring system must be characterized to validate its proper functioning and the conformity with safety and regulatory requirements. For these purposes, in vitro and in vivo (in animal) experiments are necessary steps before the application in humans [2]. Wireless capsule endoscopy The organization of American Cancer Society reported that the total number of cancer related to GI (Gastro-Intestinal) track is about 149,530 in the United State only for 2010. Wireless Ingestible Capsule Endoscopy (WCE) enables the visualisation of the whole GI track cable freely and is used for examining OGIB (Obscure Gastrointestinal Bleeding), tumours, cancers, Crohn’s Disease, Celiac Disease [10]. The swallowable pill endoscope is a miniature telecamera that acquires images as it travels through the gastrointestinal tract. The imaging system consists of a swallowable capsule,

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a data recorder, and a workstation. The patient swallows the capsule, which contains a camera, lights, transmitter, and battery. The capsule acquires images and transmits video signals to receiver. These capsules move through the digestive tract by the natural motility of the tract itself. WCE takes images in its 8-hour journey through the digestive tract, after it is swallowed by the patient. The capsule is then excreted naturally in the patient’s bowel movement, and the data it contains is retrieved and interpreted. Fig. 4 shows the human GI tract consisting of four main parts, that is, the oesophagus, stomach, small intestine/bowel, and large intestine or colon. Antennas for diagnosis Antennas employed for Diagnosis are mainly categorized as Body Centric Antennas, Implantable Antennas and Ingestible Antennas as shown below in Fig. 5. Body centric antennas These are the antennas mounted on the human torso covered with three layer tissue (skin-fat-muscle) mimicking gels. These operate in the following bands: • ISM band: 2.40 GHz-2.48 GHz • UWB band: 3 GHz-6 GHz • RFID frequencies (MHz) The body-centric wireless communications were first devised and demonstrated by Zimmerman [11]. His prototype operated at the very low frequency of 330 kHz. Since then, several researchers followed his lead and some of them used slightly higher frequencies such as 10 MHz [12-14]. Meanwhile, ultra high frequency (UHF) bands such as 403 MHz, 868 MHz, and 2.45 GHz have become popular for the body-centric wireless communications [15, 16]. The Fig. 6 below highlights various applications in which the Body Centric Antennas can be used. Design requirements for body-centric antennas • Antennas are required to be miniaturized. • Antennas should have sufficient gain to transmit data to

Fig. 4. Complete digestive tract that the bio-telemetric capsule would be able to map and typical components found within an imaging capsule system.

Fig. 5. Antennas for diagnosis.

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Fig. 8. Measurement set-up. Fig. 6. Applications of body centric antennas.

– ε∞- + ----------σs εr ( ω ) = ε∞ + εs ---------------1 + jωτ jωε0

Fig. 7. Structure of body-centric antenna.

required distance. • The mutual coupling between body-centric antennas should be minimized. • The mutual coupling between central unit and body-centric antennas should be maximized for accurate data transmission. Typical structure of body-centric antenna is shown in Fig. 7. There is a 4-mm separation between the antennas and the body surface. As we all know, dielectric properties of human tissue are dependent on frequency. In order to solve the problem in the FDTD method, the complex relative permittivity of the human body tissue should be approximated by the Debye equation:

where ω is the angular frequency, ε0 is the permittivity of free space, εs is the static permittivity at zero frequency, ε∞ is the optical permittivity at infinite frequency, τ is the relaxation time, and σs is the static conductivity. As the human body is composed of muscle equivalent tissue. However, the Debye-type dispersion cannot approximate the actual muscle over the whole range because the actual muscle has gentler dispersion [17]. For this reason, the FDTD calculation was separated into those for three ranges of 3-30 MHz, 30-300 MHz, and 300 MHz-3 GHz, and the parameters were determined for each range so as to be continuous at boundaries between neighbouring ranges, as shown in Table 1 below. Design Methodology of Body Centric Antennas A mannequin torso is covered with three layered tissue mimicking gel and the antenna is mounted on the female mannequin’s torso as shown in Fig. 8. Then the mutual coupling measurements are taken for different configurations. The measurements can be performed with Agilent’s 85070E dielectric probe kit and E8362B PNA network analyzer. Karacolak et al. in their paper [11] presented a small-size dual medical implant communications service (MICS) (402405 MHz) and industrial, scientific, and medical (ISM) (2.42.48 GHz) band antenna for continuous glucose-monitoring

Table 1. Computation Parameters for human muscle tissues at different frequencies. Parameter Static Permittivity (εs) Optical Permittivity (ε∞) Relaxation Time (τ) Static Conductivity (σs)

3-30 MHz 706 81 3.98 × 10−8 s 0.52 S/m

30-300 MHz 105 57 3.18 × 10−9 s 0.63 S/m

300 MHz-3 GHz 58 40 3.54 × 10−10 s 0.75 S/m

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applications. The antenna is optimized for dual-band operation by combining an in-house finite-element boundary integral electromagnetic simulation code and particle swarm optimization algorithm. In order to test the designed body centric antenna in vitro, gels mimicking the electrical properties of human skin are also developed. The optimized antenna is fabricated and measured in the gel. Implantable antennas These are the antennas mounted inside the human body, for continuous health monitoring. For example: Monitoring cardiac activity, brain activity etc [18, 19]. Implantable Antennas must be biocompatible in order to preserve patient safety and prevent rejection of the implant. Furthermore, human tissues are conductive, and would short circuit the implantable antenna if they were allowed to be in direct contact with its metallization. The most widely used approach for preserving the biocompatibility of the antenna – while at the same time separating the metal radiator from human tissue – is to cover the structure with a superstrate dielectric layer such as Teflon, Alumina Ceramic, Macor etc. These operate in the following bands: • Dual band MICS (Medical Implant Communication Services): 402 MHz-405MHz • ISM band: 2.40 GHz-2.48 GHz The Fig. 9 highlights various applications in which the implantable antennas can be used. Design requirements for implantable antennas • Characterization of the antenna implanted. • Design of low-profile antennas matched to the environment of the human body. • Proposal of simplified geometries for low-profile antennas

Fig. 9. Applications of Implantable Antennas.

implanted in the human chest. • Evaluation of the characteristics of low-profile antennas in terms of return loss and radiation efficiency. • Estimation of the performance of communication links utilizing the implanted antennas with consideration given to the maximum ERP and SAR limitations. • Table 2 compares the volume occupied by the implantable antennas presented in the literature. Design methodology of implantable antennas First, numerical simulation tools are used to develop a dual band MICS (402 MHz-405 MHz) and ISM (2.4 GHz-2.48 GHz) antennas. Then antennas are fabricated and their performance is measured using gels that can mimic the electrical properties of the tissue as shown in Fig. 10. Characterizations of tissue-mimicking materials such as

Table 2. Various implantable antennas reported in the literature. Ref

Substrate

[20] [21] [20] [22]

Rogers 3210 RT Duroid 6002 Rogers 3210 ARLON 1000

Substrate Shape Rectangular Rectangular Rectangular Square

[23] [24] [25] [26] [27] [28] [29] [30]

Rogers 3210 Rogers 3210 Rogers 3210 Rogers 3210 Rogers 3210 Rogers 3210 Rogers 3210 Alumina

Rectangular Rectangular Circular Square Square Circular Circular Circular

Frequency Implantation Band [MHz] Tissue Spiral 402-405 Skin Waffle 402-405 2/3 Muscle Spiral 402-405 Skin SRR coupled 402-405 Skin spiral 2400-2480 meandered 402-405 Skin -shaped 402-405 Muscle Hook-slotted 402-405 Skin Spiral 402-405 Viterous humor Spiral 402-405 Skin Hook-slotted 402-405 Skin meandered 402-405 Skin meandered 402-405 Skin Patch Shape

Shorting Pin Shorting Pin Shorting Pin

Vol. [mm3] 10240 6480 6144 1375.4

Shorting Pin Shorting Pin Shorting Pin and Patch stacking Shorting Pin and Patch stacking Shorting Pin and Patch stacking Shorting Pin and Patch stacking Shorting Pin and Patch stacking Shorting Pin and Patch stacking

1200 790.9 335.8 273.6 190 149.2 110.4 32.7

Miniaturization Technique

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Fig. 10. Fabricated Dual Band Antenna and Skin/fat/muscle mimicking gels.

SAR measurements are important for in vitro testing of implantable systems. Since the implanted antenna must operate through the skin, in vitro measurements require the characterization of skin-mimicking materials. These materials are developed by mixing de-ionized water, diethylene glycol butyl ether (DGBE), polyethylene glycol mono phenyl ether (Triton X-114), and salt. Finally, the antennas are implanted in rats to perform vitro measurements. Kumar and Shanmuganantham in their paper [31] proposed a Coplanar Waveguide fed dual V-shaped Implantable monopole antenna for biomedical applications. The antenna has a simple structure with low profile and is placed on human tissues like Muscle, Fat and Skin. The designed antenna is made compatible for implantation by embedding it in a FR4 substrate. The proposed antenna is simulated using the method of moment’s software IE3D by assuming the predetermined dielectric constant for the human muscle tissue, fat and skin. The antenna works in the Industrial, Scientific and Medical Band (900-915 MHz and 2.4-2.48 GHz). Duan et al. in their paper [32] proposed a novel differentially fed dual-band implantable antenna for the first time for a fully implantable neuro-microsystem. The antenna operates at two center frequencies of 433.9 MHz and 542.4 MHz, which are close to the 402-405 MHz medical implant communication services (MICS) band, to support sub-GHz wideband communication for high-data rate implantable neural recording application. The size of the antenna is 480.06 mm (27 mm 14 mm 1.27 mm). The specific absorption rate (SAR) distribution induced by the implantable antenna inside a tissue-mimicking solution is evaluated. The performance of the communication link between the implanted antenna and external half-wavelength dual-band dipole is also examined. Liu et al. in their paper [33] presented a miniaturized dualband implantable antenna for medical communications service (MICS) (402-405 MHz) and industrial, scientific, and medical (ISM) (2.4-2.48 GHz) applications. Compared to traditional dual-band antennas, the proposed antenna is small in size and also covers the suitable wide bandwidth at

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both bands. Kim et al. in [34] applied the spherical dyadic Green’s function (DGF) expansions and finite-difference time-domain (FDTD) to analyze the electromagnetic characteristics of dipole antennas and low-profile patch antennas implanted in the human head and body. All studies to characterize and design the implanted antennas are performed at the biomedical frequency band of 402-405 MHz. By comparing the results from two numerical methodologies, the accuracy of the spherical DGF application for a dipole antenna at the center of the head is evaluated. The impact of a shoulder on the performance of the dipole inside the head is studied using FDTD. For the ease of the design of implanted low-profile antennas, i.e., a spiral microstrip antenna and a planar invertedF antenna, with superstrate dielectric layers simplified planar geometries based on a real human body are proposed. The radiation performances of the designed low-profile antennas are estimated in terms of radiation patterns, radiation efficiency, and specific absorption rate. Ingestible antennas These are the antennas inside the capsule that travels through the digestive track and other parts of the internal organs and diagnoses GI related cancers. However, the particularity of the alimentary track restricts the utilization of the current available examine techniques. The upper gastrointestinal tract can be examined by Gastroscopy. The bottom 2 meters makes up the colon and rectum, and can be examined by Colonoscopy. In between, lays the rest of the digestive tract, which is the small intestine characterised by being very long (average 7 meters) and very convoluted. However, this part of the digestive tract lies beyond the reach of the two previously indicated techniques. Therefore, the non-invasive technique, ‘Wireless Ingestible Capsule Endoscopy (WCE)’ enables the visualisation of the whole GI track cable freely. The WCE contains a colour video camera, LEDs, wireless radio frequency transmitter, antenna and battery. These operate in the following bands: • Dual band MICS (Medical Implant Communication Services): 402 MHz-405 MHz • ISM band: 2.40 GHz-2.48 GHz • WMTS bands (608 MHz-614 MHz, 1395 MHz-1400 MHz, 1427 MHz-1432 MHz) • Or even lower frequencies: 40 MHz The Fig. 11 below highlights how the endoscopic capsule moves through the bowels and scans the entire digestive tract from mouth to anus: Design requirements for ingestible antennas • The antenna should have enough bandwidth to transmit

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Fig. 12. Human body model (torso) with the capsule located 5 cm within the body from xy-plane.

Fig. 11. Ingestible Capsule travelling through the digestive tract of the human body.

high resolution images and large amount of data such as temperature, pressure, pH and oxygen concentration of GI tract. • The enhancement of antenna efficiency should facilitate lower power consumption and high data rate transmission. • Since the endoscopic capsule operates inside the human body, which is a dissipative multi-layer medium having electric properties different to these of free space: it has conductivity, which is different to zero and a high dielectric constant. So the electric properties of the human body are going to modify the antenna performances (radiation pattern, resonant frequency, radiation efficiency) and must be considered while designing the antenna. Table 3 below presents the points of difference between probe endoscopy and wireless capsule endoscopy. Design methodology of ingestible antennas Human body model is treated as a uniform model for initial characterization purposes with properties of human tissue (muscle). The variation of dielectric constant and conduc-

tivity with frequency is incorporated into the human-body model. The dielectric constant varies from 54.81 to 53.55, and the conductivity varied from 0.98 to 1.34 S/m for the frequency range from 1 to 1.8 GHz [35]. As can be seen in Fig. 12 the capsule is located, inside from the surface (xyplane) of the human body model, with its axis along the zaxis. A linearly polarized receiver antenna is placed some distance away (nearly 20 cm) from the human-body model for link budget characterization. A practical procedure for antenna performance evaluation in these ingestible systems is the utilization of body-tissuesimulating liquids (made from deionised water, sugar, salt, cellulose, etc.) in a container with an anatomical shape. The antenna system is immersed into the liquid, and measurements are carried out. The main advantage of body-tissuesimulating liquids method is that it is practical to implement for initial prototype measurements. Single-layer homogeneous human-body models are used in simulations here because they are simple and have faster computational times and, even though less accurate, they can agree with the phantom measurements. Basar et al. in their paper [10] reported the status of several activities related to WCE, including improvement of capsule technology, research progress, technical challenges, and key indicators concerning the next-generation, active, medical robot. Rajagopalan and Samii in their letter [35] presented a comprehensive and systematic antenna performance evaluation and wireless medical telemetry characterization between two

Table 3. Comparison of WCE and probe endoscopy. WCE Effective visualization of GI tract It is not painful and does not create discomfort Good insertion capability into GI tract, scans the entire convoluted bowels Helps in detection and diagnosis of GI tract cancer

Probe Endoscopy Incapable of reaching some hidden locations in the GI tract Painful and creates discomfort Limited insertion capability into GI tract, reaches as far as upper portion of the small bowel. Incapable in detection and diagnosis of GI tract cancer

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different ingestible antenna designs, an inverted conical helical antenna and a reference conformal meandered offset dipole antenna. The antenna performance comparison matrix includes matching, radiation patterns, and polarization. Both these antennas are independently characterized inside a human body environment (box-model human). Specific absorption rate (SAR) evaluations are described, and finally a link budget is developed for both the antenna systems based on certain parameters (frequency, power limitations) with the goal of establishing a reliable communication link for indoor communication for biotelemetry applications. Hatmi et al. in their paper [36] proposed that the magnetic antennas are supposed to be less perturbed than electrical antennas in the presence of human body. In this study, initially transmitter spiral coil ingestible capsule (in-body) antenna and receiver magnetic square coil (on-body) antenna have been designed separately at 40 MHz with a matching system. Then, a near field magnetic induction link budget has been established in the presence of a human body model. The efficiency (received power/transmitted power) of the magnetic induction link is found to be better than that of an electromagnetic link when both TX and RX antennas are opposite to each other. This phenomenon facilitates to minimize the power consumption and hence to increase the battery life of the wireless capsule. Lee et al. in their paper [37], presented the design of a wideband spiral antenna for ingestible capsule endoscope systems and a comparison between the experimental results in a human phantom and a pig under general anesthesia. As wireless capsule endoscope systems transmit real-time internal biological image data at a high resolution to external receivers and because they operate in the human body, a small wideband antenna is required. To incorporate these properties, a thick-arm spiral structure is applied to the designed antenna. To make practical and efficient use of antennas inside the human body, which is composed of a high dielectric and lossy material, the resonance characteristics and radiation patterns were evaluated through a measurement setup using a liquid human phantom. Psathas et al. in their paper [38] proposed a miniature conformal antenna is proposed for ingestible capsule endoscopy in the Medical Device Radio communication Services (MedRadio) band (401-406 MHz). Finite Element (FE) numerical simulations were performed assuming the capsule to be surrounded by muscle tissue. Treatment Nowadays, antennas have started to play an important role in the treatment of deadly diseases like that of cancers in the form of therapies like hyperthermia and hence increase the effectiveness of Chemotherapy and Radiotherapy.

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Fig. 13. Patient during the Treatment.

Hyperthermia therapy Several applicators containing antennas have been developed for interstitial as well as intracavitary microwave hyperthermia. Hyperthermia is one of the modalities for cancer treatment, utilizing the difference of thermal sensitivity between tumor and normal tissue. In this treatment, the tumor is heated up to the therapeutic temperature between 42 and 45°C without overheating the surrounding normal tissues. We can enhance the treatment effect of other cancer treatments such as radiotherapy and chemotherapy with hyperthermia [39]. Microwave coagulation therapy This is used for the treatment of Hepatocellular carcinoma. It involves the heating of hepatocells in the blood clots. Antennas for treatment For heating cancer cells inside the human body coaxial-slot antenna has been used, which is one of the thin microwave antennas for hyperthermia. Fig. 13 highlights the coaxial slot antenna used for the treatment. Design methodology of coaxial slot antennas for the treatment of patient with brain tumor • The coaxial-slot antenna is placed almost at the center of the tumor and a region close to the antenna is picked up for the calculations. • First, the EM calculation around the antenna is performed by the FDTD (finite difference time domain) computation. • Then, the SAR distribution around the antenna is calculated. • Finally, we calculate the temperature distribution around the antenna. In order to obtain the temperature distribution in the tissue, we numerically analyze with the bio-heat transfer equation using the FDM (Finite Difference Method) [3]. The temperature distribution model of tumor is shown in Fig. 14. Ito et al. in their paper [3] have developed a coaxial-slot

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of batteries that limits the lifetime of such systems. • The antennas employed in capsules must be miniaturized.

CONCLUSION

Fig. 14. Calculated temperature distribution model.

antenna, which is one of thin microwave antennas to be employed for interstitial as well as intracavitary microwave hyperthermia. Two different types of coaxial-slot antennas are then introduced for the treatment of brain tumor and bile duct carcinoma. Ito in his paper [39] studied coaxial-slot antennas for thermal therapies and presented several cases of actual interstitial hyperthermia treatments for neck tumors by use of coaxial-slot antennas and have confirmed their effectiveness with clinical trials [40]. Saito et al. in their paper [40], studied the coaxial-slot antenna, which is one of the thin microwave antennas, for the minimally invasive microwave thermal therapies, such as interstitial microwave hyperthermia. From the results of the previous studies, it was clear that the coaxial-slot antenna with two slots generates a localized heating region only around the tip of the antenna. In this paper, the authors confirm the heating characteristics of the coaxial-slot antennas with two slots from a viewpoint of clinical use, and introduce the results of two clinical trials.

FUTURE TRENDS • The antenna should have enough bandwidth to transmit high resolution images and large amount of data such as temperature, pressure, pH and oxygen concentration of GI tract. • The enhancement of antenna efficiency should facilitate lower power consumption and high data rate transmission. • Since the endoscopic capsule operates inside the human body, which is a dissipative multi-layer medium having electric properties different to these of free space: it has a conductivity, which is different to zero and a high dielectric constant. So the electric properties of the human body are going to modify the antenna performances (radiation pattern, resonant frequency, radiation efficiency…) and must be considered while designing the antenna. • To wirelessly transmit power to ingestible antennas using principles of magnetic induction in order to avoid the use

The usage of antennas in Bio-medical applications has made the detection and diagnosis of various diseases easier, faster and comfortable. Recently much of the research work is in progress in the designing of miniaturized Capsule antennas, the multifunctional, medical robots that can overcome the limitations of the current capsule endoscopes. Also the antennas have started to play an important role in the treatment of deadly diseases like that of cancers in the form of Hyperthermia and hence increase the effectiveness of Chemotherapy and Radiotherapy.

ACKNOWLEDGEMENTS The authors would like to express sincere appreciation and gratitude to Dr. M.P Poonia and Dr. S.S Pattnaik for their enthusiastic guidance, valuable support and technical assistance at NITTTR, Chandigarh.

CONFLICT OF INTEREST STATEMENTS Kaur G declares that she has no conflict of interest in relation to the work in this article. Kaur A declares that she has no conflict of interest in relation to the work in this article. Toor GK declares that she has no conflict of interest in relation to the work in this article. Dhaliwal BS declares that he has no conflict of interest in relation to the work in this article. Pattnaik SS declares that he has no conflict of interest in relation to the work in this article.

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