Implanted Antennas in Biomedical Telemetry · 2017-08-25 · Implanted Antennas in Biomedical...

Implanted Antennas in Biomedical Telemetry

Asimina Kiourtia* and Konstantina S. NikitabaElectroScience Laboratory, Department of Electrical and Computer Engineering, The Ohio State University,Columbus, OH, USAbSchool of Electrical and Computer Engineering, National Technical University of Athens, Zografos, Athens, Greece

Abstract

Biomedical telemetry permits the measurement of physiological signals at a distance, through either wiredor wireless communication technologies. One of the latest developments in wireless biomedical telemetryis in the field of implantable medical devices (IMDs). Such devices are implanted inside the patient’s bodyby means of a surgical operation and can be used for a number of diagnostic, monitoring, and therapeuticapplications. Implantable antennas, i.e., antennas which are integrated into RF-enabled IMDs, exhibitnumerous challenges in terms of design, fabrication, and testing and are, therefore, currently attractingsignificant research attention. Contributions from researchers of various disciplines build a rich pool ofbackground information, while highlighting future prospects.

Keywords

Biocompatibility; Biomedical telemetry; Industrial scientific and medical (ISM) applications band;Medical implant communication service (MICS) band; Miniaturization; Implantable antennas; In vitro;In vivo; Specific absorption rate

Introduction

Biomedical telemetry permits the measurement of physiological signals at a distance, through eitherwired or wireless communication technologies. Physiological signals are obtained by means of appro-priate transducers, post-processed, and eventually transmitted to exterior monitoring/control equipment.One of the latest developments in wireless biomedical telemetry is in the field of implantable medicaldevices (IMDs). Such devices are implanted inside the patient’s body bymeans of a surgical operation andcan be used for a number of diagnostic, monitoring, and therapeutic applications (Greatbatch and Homes1991; Chow et al. 2013; Kiourti et al. 2014a).

Nowadays, millions of people worldwide depend upon IMDs to support and improve the quality oftheir lives. Advances in biological, chemical, electrical, and mechanical sensor technologies as well as inmicroelectromechanical systems (MEMS) have led to a wide range of IMDs. Specifically, wireless IMDsare already in use for a wide variety of applications, including temperature monitors (Scanlon et al. 1997);pacemakers and cardioverter defibrillators (Wessels 2002); functional electrical stimulators (FES)(Guillory and Normann 1999); blood glucose sensors (Shults et al. 1994); cochlear (Bucheggeret al. 2005), gastric, and bladder controllers (Sani et al. 2009); and retinal (Gosalia et al. 2004) implants.

*Email: [email protected]

*Email: [email protected]

Handbook of Antenna TechnologiesDOI 10.1007/978-981-4560-75-7_94-1# Springer Science+Business Media Singapore 2015

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As technology continues to evolve, new IMDs are being developed, and their use is expected to rapidlyincrease from this already large base.

Traditionally, low-frequency (tens to hundreds kHz) inductive links have been employed for thewireless telemetry of IMDs (Tang et al. 1995; Valdastri et al. 2004). However, inductive links sufferfrom: (a) low data rates (1–30 kbps), (b) restricted communication range (<10 cm), and (c) increasedsensitivity to inter-coil positioning. To overcome these limitations, research is recently oriented towardradio-frequency (RF)-linked IMDs. Design of such RF telemetry systems is facilitated by the rapidadvances in wireless communications and electronics. As shown in Fig. 1, a key and critical componentof RF-linked IMDs is the integrated implantable antenna. In principle, picked-up physiological signals areamplified, digitized, and fed to a transceiver, which will code and modulate the data, and finally lead themto the antenna. The latter enables the IMD’s bidirectional communication with the exterior monitoring/control equipment. In a realistic scenario, these implantable antennas are mounted on the existinghardware of the IMD.

Implantable antennas, i.e., antennas which are integrated into RF-enabled IMDs, exhibit numerouschallenges in terms of design, fabrication, and testing and are, therefore, currently attracting significantresearch attention (Kiourti and Nikita 2012a; Chow et al. 2013). Specifically, numerical design ofimplantable antennas needs to be performed fast and in a way which optimally addresses issues relatedto operation frequency selection, miniaturization, biocompatibility, patient safety, high-quality commu-nication with exterior equipment, and intersubject variability. Furthermore, prototype fabrication of suchminiature antenna structures is highly challenging given their critical tolerance to potential experimentalversus numerical inconsistencies. Finally, in vitro and in vivo testing of implantable antennas ishighly intriguing given the requirements for (a) phantom formulation that matches the theoreticalelectrical properties and (b) implantation inside living model animals, respectively. Contributionsfrom researchers of various disciplines build a rich pool of background information, while highlightingfuture prospects.

Fig. 1 Schematic of a typical wireless biomedical telemetry system with example IMD applications


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Design Considerations for Implantable Antennas

Considerations related to the design and performance of implantable patch antennas include (a) selectionof the frequency band of operation, (b) miniaturization of the antenna’s occupied volume/size,(c) biocompatibility of the antenna structure, (d) conformance of the implantable antenna to internationalguidelines which preserve patient safety against radiated electromagnetic (EM) fields (IEEE 1999, 2005),(e) high-quality communication of the implantable antenna with exterior monitoring/control equipment,and (f) intersubject variability. These considerations are discussed in detail in the following.

Frequency Bands of OperationSelection of the operation frequency of implantable antennas, or equivalently IMDs, is receivingconsiderable attention from the scientific community as attributed to a number of competing factors.

Until recently, no globally accepted frequency band had been dedicated to the biomedical telemetry ofIMDs. The situation changed with the ITU-R Recommendation SA.1346 (ITU-R 1998) which outlinedthe use of the 402.0–405.0 MHz frequency band for medical implant communication services (MICS).The most recent contribution in the field is the Institute of Electrical and Electronics Engineers (IEEE)802.15.6 standard (IEEE 2012) which deals with short-range, wireless communications in the vicinity of,or inside, the human body. The standard refers to existing industrial, scientific, and medical (ISM) bandsas well as frequency bands approved by national medical and/or regulatory authorities. According to thislatest standard, an IMD shall be able to support transmission and reception in at least one of the followingfrequency bands: 402.0–405.0, 420.0–450.0, 863.0–870.0, 902.0–928.0, 950.0–958.0, 2,360.0–2,400.0,and 2,400.0–2,483.5 MHz. Ultrawide band (UWB) IMDs which implement low band(3,494.4–4,492.8 MHz) or high band (6,489.6–9,984.0 MHz) channels are also supported. For example,Table 1 summarizes recent research studies on implantable antennas operating within various frequencybands.

The ISM band of 2,400.0–2,500.0 MHz (ITU-R 2008) is appearing as one of the most promisingsolutions. This is because it is already well developed in terms of technology (Bluetooth, Wi-Fi, andWLAN), antennas, integrated circuits, and embedded systems. Furthermore, higher operation frequenciesallow the use of smaller-sized antennas and components. It is for this purpose that implantable antennasoperating at much higher frequencies (e.g., 5.85 and 31.5 GHz) have also been reported in the literature(see Table 1). However, a high number of operating services are colocated in the aforementioned bands.Therefore, interference issues constitute a limiting factor. Interference may cause harmful effects in termsof false IMD activation, link unavailability, and data corruption.

Table 1 Operation frequencies of implantable antennas reported in the literature

Frequency References for implantable MDs

402 MHz Kim and Rahmat-Samii 2004; Soontornpipit et al. 2004, 2005; Abadia et al. 2009; Chen et al.2009; Saniet al. 2009; Karacolak et al. 2009; Sánchez-Fernández et al. 2010; Gemio et al. 2010; Huang et al. 2011; Vidalet al. 2012; Kiourti and Nikita 2012b

433 MHz Weiss et al. 2009; Gemio et al. 2010; Huang et al. 2011; Kiourti and Nikita 2012b

868 MHz Sani et al. 2009; Sani et al. 2010; Kiourti and Nikita 2012b

915 MHz Scanlon et al. 2000; Gemio et al. 2010; Kiourti and Nikita 2012b

1,575 MHz Azad and Ali 2009

2,400 MHz Kawoos et al. 2008; Karacolak et al. 2009; Xia et al. 2009; Sánchez-Fernández et al. 2010; Gemio et al. 2010;Huang et al. 2011; Scarpello et al. 2011

31.5 GHz Ahmed et al. 2008


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To deal with interference issues, focus for IMDs is mainly on the 402.0–405.0 MHz band, which hasbeen exclusively allocated for medical implant communication services (MICS). The MICS band iscurrently regulated by the United States Federal Communications Commission (FCC) and the EuropeanRadiocommunications Committee (ERC). Its spectrum of 3 MHz allows for 10 channels (bandwidth of300 KHz each) to operate simultaneously (i.e., multiple IMDs in the same area). It also limits potentialinterferences from the colocated Meteorological Aids Service band (401–406 MHz). Additional ways ofenhancing interference tolerance include (a) automatic repeat request (ARQ) and forward error correction(FEC) techniques to mitigate the effects of impulsive noise (noise which is very short in duration and oftenof greater amplitude than the IMD signal levels) and (b) the use of frequency agility and channelization asa means of avoiding narrowband interferers (sources with bandwidths comparable to the IMD signalwaveform). TheMICS band is internationally available and feasible with low power circuits, falls within arelatively low noise portion of the spectrum, and allows for acceptable propagation through human tissue.A review on the regulatory standards for IMDs and the characteristics of MICS transceivers is performedin Savci et al. (2005).

The effect of operation frequency upon the performance of IMDs has been addressed in the literature.Single-cell excitation used to simulate vaginal (Scanlon et al. 2000) and gastric/bladder/cardiac (Saniet al. 2009) implants has been shown to exhibit increased power absorption, higher net body losses, andreduced penetration depths with increasing frequency. Recently, implantable antennas at higher frequen-cies were found to achieve enhanced gains (a 10.7 % increase in the maximum far-field gain at 915 MHz,compared to the gain at 402 MHz), increased maximum allowable net input power levels (10.1 % and1.3 % increases imposed by the IEEE C95.1-1999 (IEEE 1999) and IEEE C95.1-2005 (IEEE 2005) safetystandards, respectively), and more expanded specific absorption rate (SAR) distributions (Kiourti andNikita 2012b). Results were attributed to the authors’ choice of keeping the antennas’ physical dimen-sions identical and modifying their effective size. Furthermore, the selection of the operation frequency isdirectly related to the bit rate: higher operation frequencies allow for an increase in bandwidth and enablehigher bit rates, which are favorable for high data rate applications. For example, the maximum channelcapacity for a band-limited additive white Gaussian noise (AWGN) channel is given by

C ¼ BW log2 1þ S

N

� �(1)

where BW is the channel bandwidth, S is the mean signal power, and N is the mean noise power.Therefore, the MICS band (BW 300 kHz) enables low bit rates, and the WMTS (BW 8.5 kHz to 6 MHz)and ISM (selectable BW) bands enable medium bit rates, whereas the UWB (BW > 500 MHz) isappearing as the most promising solution for high data rates. It is remarked that according to the recentIEEE 802.15.6 standard (IEEE 2012), data rates of typically up to 10 Mbps are required to satisfyevolutionary health-care services.

MiniaturizationRecent advances in the technology of IMD electronics lead to ultrasmall designs for IMDs. For instance,implantable retinal prostheses should be small enough to be inserted inside the eyeball (radius of~12.5 mm), while intracranial pressure monitors should be small enough to fit in standard 12 mm burrholes in the skull (Warty et al. 2008). However, dimensions of the traditional half-wavelength (l/2) orquarter-wavelength (l/4) antennas at the frequency bands allocated for medical implants and especially atthe low-frequency MICS band make them useless for implantable applications. As an example, the free-space wavelength at 402, 433, 868, and 915 MHz can be computed as 74.6, 69.3, 34.6, and 32.8 cm,


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respectively. Therefore, miniaturization becomes one of the greatest challenges in implantable antennadesign.

Fortunately, human tissues in which implantable antennas are intended to operate exhibit highpermittivity (e.g., the permittivity of skin at 402 MHz is 46.7 (Gabriel et al. 1996a, b, c)) or, equivalently,reduced wave propagation velocity. This, in turn, increases the effective dielectric constant of the antennaand works to advantageously miniaturize its physical size. As would be expected, additional antennaminiaturization techniques can be concurrently employed. Specifically, miniaturization techniques thathave been proposed in the literature for implantable antennas include:

• Use of high-permittivity dielectric materials. High-permittivity dielectrics are selected for implant-able antennas (e.g., ceramic alumina (er = 9.4) (Kiourti et al. 2011a) or Rogers RO3210 (er = 10.2)(Kiourti and Nikita 2012b)) because they shorten the effective wavelength and result in lowerresonance frequencies, thus assisting in antenna miniaturization.

• Lengthening of the current flow path on the antenna surface. Longer effective current flow pathsexcited on the antenna can reduce the resonance frequency and achieve a more compact size for theimplantable antenna. For this purpose, meandered (Kiourti and Nikita 2011), spiral (Kiourti and Nikita2011), waffle-type (Soontornpipit et al. 2005), and hook-slotted (Liu et al. 2008a) shaped implantableantennas have been suggested, as shown in Fig. 2.

• Addition of shorting pins. In the case of implantable patch antennas, inserting a shorting pin betweenthe ground and patch planes increases the effective size of the antenna. This, in turn, reduces therequired physical dimensions, given a specific operation frequency scenario. The technique works inmuch the same way a ground plane doubles the height of a monopole antenna, i.e., it typically produces

a b

c d

Y

1

2

4

5

X

Y

X

Fig. 2 Lengthening of the current flow path for miniature implantable antennas: (a) meandered (Kiourti and Nikita 2011),(b) spiral (Kiourti and Nikita 2011), (c) waffle-type (Soontornpipit et al. 2005), and (d) hook-slotted (Liu et al. 2008a) shapedpatches


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a planar inverted-F antenna (PIFA) with the same resonance performance as a double-sized antennawithout the shorting pin (Soontornpipit et al. 2004).

• Patch stacking. In the case of implantable patch antennas, vertically stacking two radiating patchesreduces antenna size by increasing (nearly doubling) the length of the current flow path (Kiourtiet al. 2011a; Kiourti and Nikita 2012b).

Implantable antennas reported in the literature combine some (or all) of these miniaturization tech-niques in order to reduce size. Of course, all antenna design parameters, including the location of thecoaxial feed, have to be appropriately selected (optimized) for a good 50Omatch at the desired operationfrequency. For example, the skin-implantable antenna of Fig. 3 adapts a stacked PIFA structure ofmeandered patches built on Rogers RO3210 (er = 10.2) substrate to achieve a miniaturized structure(volume of 214.9 mm3) resonating in the MICS band (Kiourti and Nikita 2012b).

Nevertheless, it is important to emphasize that implantable antenna miniaturization comes in expenseof its radiation and patient safety performance. Specifically, generic results for a skin-implantable antennaplaced inside a tissue-simulating cube have indicated degraded gain and SAR performance with areduction in size. Antenna miniaturization by 32 % and 65 % has been found to reduce the maximumfar-field gain values by 5% and 19%, respectively, and the maximum allowable input powers imposed bythe IEEE C95.1-1999 safety standard (IEEE 1999) by 21 % and 44 %, respectively. The significance ofapplication-specific rather than miniaturization-oriented implantable antenna design is, thus, highlighted.

BiocompatibilityImplantable antennas must be biocompatible in order to preserve patient safety and prevent rejection ofthe implant. Another consideration is that human tissues are conductive and would short-circuit theimplantable antenna if they were allowed to be in direct contact with its metallization. Biocompatibilityand prevention of undesirable short circuits are especially crucial in the case of antennas which areintended for long-term implantation. In the literature, there have been reported two approaches forpreserving the biocompatibility of implantable antennas and separating their metallic parts from thesurrounding biological tissues: (a) covering the antenna structure with a biocompatible superstratedielectric layer and (b) insulating the antenna with a thin layer of low-loss biocompatible coating.

Specifically, the most widely used approach for preserving the biocompatibility of an implantableantenna, while at the same time separating its metal radiator from the human tissues, is to cover thestructure by a superstrate dielectric layer (e.g., Fig. 4a (Karacolak et al. 2008)). Commonly usedbiocompatible materials include Teflon (permittivity, er = 2.1; dielectric loss tangent, tand = 0.001),

Fig. 3 Geometry of a stacked skin-implantable PIFAwith meandered patches (Kiourti and Nikita 2012b)


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MACOR® (er = 6.1, tand = 0.005), and ceramic alumina (er = 9.4, tand = 0.006) (Soontornpipitet al. 2004). It is important to highlight, however, that ceramic substrates do not lend themselves easilyto drilling and round cuts (Warty et al. 2008).

Insulating the implantable antenna with a thin layer of low-loss biocompatible coating is anotherreported approach (e.g., Fig. 4b; Karacolak et al. 2010). Materials proposed for biocompatible encapsu-lation include zirconia (er = 29, tand � 0) (Skrivervik and Merli 2011), polyetheretherketone (PEEK)(er = 3.2, tand = 0.01) (Abadia et al. 2009), polydimethylsiloxane (PDMS) (er = 3, tand = 0.005),Silastic MDX4-4210 Biomedical Grade Base Elastomer (er = 3.3, tand � 0), and Parylene (er = 2.95,tand = 0.005) (Karacolak et al. 2010). Because of its electrical properties, zirconia is a better candidatematerial for biocompatible insulation from an electromagnetic point of view. High permittivity andlow-loss tangent values allow the near fields of the antenna to concentrate inside the low-loss encapsu-lation layer, thus mitigating power loss. However, easiness of preparation and handling must also be takeninto account. For example, PEEK and Silastic MDX4-4210 Biomedical Grade Base Elastomer are mucheasier to prepare and handle. Furthermore, thickness of the biocompatible insulation layer is an importantfactor in antenna design. Computation of its optimum thickness is, thus, considered to be highlysignificant for lowering power loss without aimlessly increasing antenna size.

It is remarked that addition of an insulation coating or superstrate layer significantly affects theperformance of the antenna and, thus, needs to be taken into account within the design. Furthermore, abiocompatible hermetic package is necessary for housing the electronics, which may be made oflow-temperature co-fired ceramic (LTCC), Parylene, liquid crystal polymer (LCP), silicon, or alumina(Chow et al. 2010).

Patient Safety and Specific Absorption Rate (SAR)It was not until recently that research on the biological effects of IMDs started being carried out.Specifically, issues related to patient safety limit the maximum allowable power incident to the implant-able antenna. The specific absorption rate (SAR) (rate of energy deposited per unit mass of tissue) isgenerally accepted as the most appropriate dosimetric measure, and compliance with internationalguidelines is assessed. For example, the IEEE C95.1-1999 standard restricts the SAR averaged overany 1 g of tissue in the shape of a cube to less than 1.6 W/kg (SAR1g,max � 1.6 W/kg) (IEEE 1999). TheICNIRP’s basic restrictions limit the SAR averaged over 10 g of contiguous tissue to less than 2 W/kg(ICNIRP 1998). To harmonize with the ICNIRP guidelines, the IEEE C95.1-2005 standard restricts theSAR averaged over any 10 g of tissue in the shape of a cube to less than 2 W/kg (SAR10g,max � 2 W/kg)(IEEE 2005).

Fig. 4 Biocompatibility issues for implantable antennas: (a) addition of a superstrate (Karacolak et al. 2008) and (b) thin-layerencapsulation (Karacolak et al. 2010)


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Specifically, the power absorbed by the human body in the presence of an incident electromagnetic fieldis given by

Pabs ¼ 1

2

ðsjEj2dV ; (2)

where s is the conductivity of the human tissues and jEj is the intensity of the electric field inside the body(Kim and Rahmat-Samii 2004). Equation 2 indicates that the absorbed power is related to the electric field,so that maximum SAR values are recorded in the areas where maximum electric field intensities occur.

Based on the deduction that peak averaged SAR values are generated from high near fields, novelimplantable patch antennas can be designed, which aim at lower electric field intensities. For example, inKim and Rahmat-Samii (2006), the radiation mechanism of an implantable antenna was discussed, in anattempt to modify its design for reducing the spatial-averaged SAR in human tissue. Replacing theuniform-width spiral radiator of an implantable MICS PIFAwith a nonuniform-width radiator was foundto decrease the electric field intensity and, in turn, SAR1g,max. The simulated near electric field distributionshowed that the high electric field area of the PIFA employing the nonuniform-width radiator was muchsmaller than that of the original PIFA. The value of SAR1g,max was, thus, limited from 310 to 210 W/kg,considering a net input power of 1 W.

Several safety evaluation studies have been performed for implantable antennas. Of importance is thatthe IEEE C95.1-1999 standard has been found to be much stricter than the recent IEEE C95.1-2005standard (Kiourti and Nikita 2012b). The latter has shown to be almost insensitive to changes in the tissuemodel properties (anatomical features and dielectric parameters) (Kiourti and Nikita 2013a).

Radiation PerformanceBiomedical telemetry systems for IMDs are comprised of the IMD and an exterior monitoring/controldevice, which is placed at some distance (typically 2 m) away from the body. Biotelemetry links may beused for device parameter adjustment, transmission of stored information, as well as real-time transmis-sion of vital monitoring information. Therefore, the implantable antenna should provide a signal that isstrong enough to be picked up by the exterior device, regardless of any power limitations. It is important tohighlight that apart from patient safety, interference issues also limit the maximum allowable powerincident to the implantable antenna. For example, a strict limit of �16 dBm (25 mW) has been set on theeffective radiated power (ERP) of IMDs operating in theMICS band in order to prevent interference to thecolocated Meteorological Aids Service band (ITU-R 1998).

Given the SAR and ERP power limitations, far-field gain of the implantable antenna indicates thedesired receiver sensitivity for achieving reliable biotelemetry communication. In order to increase therange of biotelemetry communication, implantable antennas with enhanced gain are solicited. However,reduced-size antennas exhibit degraded electromagnetic performance: miniaturization degrades gain,while high-gain antennas exhibit relatively increased size. Low values of gain imply poor radiationefficiencies; however, compromises on the system performance are inevitable given the miniaturizedantenna dimensions.

Of importance is that (a) the symmetry of the implantation tissue model affects the symmetry of theantenna’s far-field radiation pattern, accordingly. Omnidirectional, monopole-like radiation is observedinside symmetrical tissue models (see Fig. 5a; Kiourti et al. 2011a; Kiourti and Nikita 2012b), whereasasymmetrical radiation is recorded within anatomical tissue models which are irregular and inhomoge-neous (see Fig. 5b; Kiourti et al. 2011a; Kiourti and Nikita 2012b).


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Powering ConsiderationsAll the components of an IMD, including the implantable antenna, require power. Integrated powersupplies, such as batteries, are only suitable for applications where the IMD lifespan is short (e.g., IMDswhich are intended to be implanted for months or a few years, depending on the duty cycle). To elongatethe battery life of IMDs, external power transmission has been proposed for recharging purposes (Kendiret al. 2005). The technique is based on electromagnetic (EM) induction between an exterior-transmittingand an implantable receiving coil, which is placed in close distance and is often wound around a dielectricor ferrite core to improve the efficiency. Furthermore, power scavenging sources including motion,vibration, air flow, temperature difference, light, and infrared radiation have been suggested. For example,a vibration-based generator for implantable IMDs, capable of delivering 2 mJ/cycle, has been designed,while ambient EM energy harnessing has recently been investigated (Mitcheson et al. 2004). Suchsolutions are solicited for IMDs which are intended for an implantation period of several years or evena lifetime (cochlear implants for the deaf or retina implants for the blind).

To meet potential longevity requirements of the IMDs and guarantee their on demand availability,power conservation techniques can additionally (or instead) be applied. Suggested ideas includepre-configured on-off function of the IMD (Furse 2009) and transmission/detection of a “wake-up”alarm signal (Karacolak et al. 2008). In the first case, devices spend most of their time in an ultraefficientsleep mode followed by short bursts of data transmission. Data mining or compression techniques may beused to reduce the actual bits of data to be transmitted. In the second case, the system uses two frequencybands, one for “wake up” and one for transmission. The transceiver stays in “sleep mode”with low powerconsumption (1 mW) until a “wake-up” signal is sensed in the 2,450 MHz ISM band. In the normal mode,the IMD is fully powered and exchanges data in the MICS band. Following the data transfer, the IMDtransceiver returns back to the “sleep mode.” To do so, a transceiver with dual-band operation may beused, such as the commercially available Zarlink ZL70101 Transceiver (Zarlink 2006). The system usestwo frequency bands, one for “wake up” and one for transmission. The exterior device may beprogrammed to wake up the implanted device according to a physician-defined schedule or only whena patient event is detected (Savci et al. 2005).

It is remarked that the employment of “wake-up” techniques requires the design of multiband antennas,i.e., a separate band for data biotelemetry and a separate band for transmission/detection of the “wake-up”alarm signal. For example, a dual-band (MICS and ISM) implantable antenna has been proposed in theliterature for continuous glucose monitoring (Karacolak et al. 2008). A meandered antenna configurationwas considered for optimizing the antenna surface area, and particle swarm optimization was applied to

Fig. 5 Far-field gain radiation pattern of the skin-implantable antenna proposed in Kiourti et al. (2011a) inside (a) a 100 mm-edge skin cube and (b) the skin tissue of an anatomical human head model


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achieve the desired resonance characteristics. The simulated and measured bandwidths were found to be82 and 142 MHz in the MICS band and 103 and 174 MHz in the ISM band, respectively. An innovativedual-band (MICS and ISM) patch antenna with a multilayer configuration and electromagnetic coupling-based feeding has further been proposed for implantation inside the left subpectoral region (Sánchez-Fernández et al. 2010). Recently, a novel antenna design was suggested using a p-shaped radiator withstacked and spiral structure, to support triple-band operation with data telemetry (402 MHz), wirelesspower transmission (433 MHz), and wake-up controller (2,450 MHz) (Huang et al. 2011). The simulatedand measured bandwidths were 86 and 114 MHz in the MICS band and 60 and 70 MHz in the ISM band,respectively.

Intersubject VariabilitySince implantable antennas are intended to operate inside human tissue, their performance stronglydepends on the surrounding tissue environment. The latter includes (a) the anatomical features of theindividual and (b) the dielectric parameters (permittivity, er, and conductivity, s) of the biological tissues.

For example, smaller-sized (female and low body mass index male) anatomical models have beenfound to exhibit higher radiated power levels and far-field gain values (Sani et al. 2009). Simplified,single-cell excitation was considered in this study to mimic gastric, bladder, and cardiac implants at402 and 868 MHz. Anatomical differences considering a realistic model of an implantable antenna havealso been assessed (Vidal et al. 2012). In this study, two implantable antennas were designed to operate at403 MHz within mean head and mean body tissues, respectively, and the exhibited resonance perfor-mance was studied. Simulations for head, arm, and abdomen implantation were carried out within fouranatomical models (two adults and two children), indicating a maximum detuning of 14 MHz from thereference frequency of 403 MHz.

Tissue dielectric parameters (permittivity and conductivity) also affect the design and performance ofimplantable antennas. Therefore, variations due to uncertainties in dielectric properties and intersubjectvariability have to be taken into account (Virtanen et al. 2006). It is remarked that maximum standarddeviations of 16 % have been reported in the dielectric parameter values of rat brain tissue (Baoet al. 1997), while age dependency has repeatedly been emphasized (Gabriel 2005; Conil et al. 2008).Decrease in permittivity and conductivity values by 4 % and 10 % has been recorded, respectively, for pigtissue within 4 h after death (Schmid et al. 2003).

Recently, the performance of aMICS scalp-implantable antenna was assessed with respect to variationsin head properties, i.e., anatomy and dielectric parameters (Kiourti and Nikita 2013a, b). Five head models(3- and 5-layer spherical and 6-, 10-, and 13-tissue anatomical; see Fig. 6) and seven dielectric valuescenarios (variations by �20 % in the reference permittivity and conductivity values) were considered.Compared with the reference dielectric parameter scenario within the 3-layer spherical head model,maximum variations of�19.9 %, +3.7 %,�55.1 %, and�39.2 % were overall recorded in IEEE C95.1-1999 (IEEE 1999), IEEE C95.1-2005 (IEEE 2005), RL, and Gmax at 403.5 MHz. More specifically, theperformance of implantable antennas was found to considerably depend on the anatomy of the tissuemodel in the area that is immediately surrounding the implant. In particular, the 6-tissue anatomical headmodel, which exhibited a bump of skin tissue at the top side of the head (implantation site), demonstratedthe highest deviations. As long as anatomical features around the implant are relatively similar, overallanatomy and tissue composition/distribution of the numerical model were found to insignificantlyinfluence the antenna performance. On the other hand, tissue dielectric parameters were found toinsignificantly affect the implantable antenna performance. As compared to the reference dielectricvalue scenarios, variations in IEEE C95.1-1999 (P1999) and IEEE C95.1-2005 (P2005) ranged, on average,between�0.4 % and +0.1 % and between�0.4 % and +0.8 %, respectively. RL varied between�33.1 %and +17.6 % and Gmax between �0.6 % and +3.1 %.


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In summary, taking tissue anatomy and dielectric parameter uncertainties into account has been shownto be important for implantable antenna design and performance evaluation. As far as compliance withsafety guidelines is concerned, uncertainties inherent to variations in tissue anatomy and dielectricparameters must be taken into account. Nevertheless, it is important to highlight that in contrast to theIEEE C95.1-1999 guidelines, compliance with the recent IEEE C95.1-2005 guidelines occurs to bealmost insensitive to tissue properties. Furthermore, results indicate the need for designing implantableantennas with enhanced bandwidth in order to compensate for detuning and impedance mismatch

Fig. 6 Antenna implanted inside five numerical human head models: (a) 3-layer spherical, (b) 5-layer spherical, (c) 6-tissueanatomical, (d) 10-tissue anatomical, and (e) 13-tissue anatomical (Kiourti and Nikita 2013a)


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inherent to intersubject variability. A conservative evaluation of the link budget between the implantableantenna and exterior equipment is also required in order to account for potential degraded gain values anddeteriorated symmetry in radiation.

Numerical Design and Performance Assessment of Implantable Antennas

Tissue Models and Design Strategies for Implantable AntennasThe fact that implantable antennas are intended to operate inside biological tissue rather than in free spaceaffects their design and performance in a number of ways. Specifically, given the presence of the humanbody, design of implantable antennas should be performed either (a) inside free space and further refinedfor tissue implantation or (b) directly inside an environment surrounded by human tissue. It is noted that innumerical simulations, biological tissues are analyzed as inhomogeneous lossy mediums that have theirown permittivity (er), conductivity (s), and mass density values. These can be either approximated asconstant within a narrow frequency range (Kiourti et al. 2011a; Kiourti and Nikita 2012b) or described bymeans of a Cole–Cole formulation for the complex relative permittivity, according to

ec oð Þ ¼ e1 þXn

Den1þ jotnð Þ 1�anð Þ þ

sijoe0

(3)

where o is the angular frequency, n is the order of the Cole–Cole model, e1 is the high-frequencypermittivity, tn is the relaxation time, Den is the pole amplitude, an is the parameter that allows for thebroadening of the dispersion, and si is the static ionic conductivity (Karacolak et al. 2009; Noroozi andHojjat-Kashani 2012). Canonical tissue models are often used to speed up simulations and ease the designof implantable antennas. These may be either single layer (e.g., Fig. 7a; Kiourti and Nikita 2012b) ormultilayer (e.g., Fig. 7b; Karacolak et al. 2008). To obtain more realistic results, anatomical tissue models(e.g., Fig. 7c; Kiourti and Nikita 2012b) produced by the combination of magnetic resonance imaging(MRI) or computed tomography (CT) data can also be applied.

Suggested design strategies for implantable antenna design include:

• Strategy #1. Antenna design in free space and further refinement inside an anatomical model of theintended implantation site. For example, in Rucker et al. (2007), aMICS patch antenna was designed ina free-space environment and further implanted inside the skin tissue of an anatomical head model.Resonance frequency detuning was observed, as attributed to the capacitive loading effect of thesurrounding tissues. To refine the resonance, a varactor diode with tuning capabilities was subsequentlyintegrated.

• Strategy #2.Antenna design in free-space targeting at high-gain values and further refinement inside asingle-layer tissue model. For example, in Abadia et al. (2009), a MICS antenna was designed in freespace aiming at high gain (>�20 dB) in order to account for subsequent body absorption losses. Theantenna was optimized in free space to minimize size and further covered by a biocompatible layer andplaced inside tissue material. Design modifications were performed to account for the frequency shiftinduced by the presence of encapsulation and human tissue.

• Strategy #3.Antenna design directly inside a canonical single-layer tissue model (cubical, rectangularparallelepiped, or cylindrical) of the intended implantation tissue. As would be expected, use of asingle-layer tissue model is the simplest and fastest option when designing implantable antennasdirectly inside tissue material. Following this design strategy, antennas are designed for a “generic”tissue-implantation scenario. Simplified tissue models in the shape of a cube (Kiourti et al. 2011a, b), a


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rectangular parallelepiped (Liu et al. 2008a; Kim and Rahmat-Samii 2004, 2006), and a cylinder (Liuet al. 2008b, 2009) have been used for this purpose. Design is performed by selecting the dielectricmaterial and subsequently optimizing all antenna design parameters to refine tuning at the desiredoperation frequency.

• Strategy #4. Antenna design directly inside a canonical multilayer tissue model (cubical, rectangularparallelepiped, or cylindrical) of the intended implantation tissue. This strategy intends to design theantenna for a specific implantation site by taking into account a specific region of the body.A multilayer tissue model, with either finite or infinite dimensions, is selected. For example, implant-able antennas intended for trunk (Karacolak et al. 2008) and chest (Kim and Rahmat-Samii 2004)implantation have been directly designed inside three-layer planar tissue models consisting of skin, fat,and muscle tissues.

• Strategy #5. Approximate antenna design inside a cube filled with the intended tissue material andfurther quasi-Newton optimization inside a canonical model of the intended implantation site (Kiourtiand Nikita 2012b). This strategy emphasizes on design speedup and optimized resonance performance

Fig. 7 Tissue models: (a) single-layer canonical (skin cube) (Kiourti and Nikita 2012b), (b) three-layer (skin/fat/muscle)canonical (Karacolak et al. 2008), and (c) anatomical human head (Kiourti and Nikita 2012b)


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of the implantable antenna inside a specific implantation site. It is remarked that, as far as antennadesign is concerned, multilayer canonical models have been shown to provide an acceptable model forthe human body. Specifically, highly similar return loss characteristics have been found for implantablepatch antennas inside a three-layer planar geometry and a realistic model of the human chest (Kim andRahmat-Samii 2004), as well as inside a three-layer spherical and an anatomical model of the humanhead (Kiourti and Nikita 2012b).

• Strategy #6. Antenna design inside a small-sized single-layer or multilayer tissue box (Kiourti andNikita 2012c). Numerical results inside small-sized tissue boxes have been found to be almost identicalto those inside canonical models of the intended implantation site, thus, rendering design directly intothe latter unnecessary and inadequately slow. The aim is to minimize the use of required computationalresources or, equivalently, the required simulation time toward designing implantable antennasoptimized for specific medical implantation scenarios. Incorporation of the dielectric loading of thesurrounding tissues and exterior air in the design and use of canonical, small-sized tissue models havebeen found to form the optimum solution for design purposes.

Regarding the numerical performance analysis of implantable antennas, it is noted that analyticalmethods can only be applied for simplified implantable antennas placed inside canonical tissue models.For example, a spherical dyadic Green’s function (DGF) code has been implemented in the literature tocharacterize a MICS dipole antenna implanted inside a multilayer spherical human head model (Kim andRahmat-Samii 2004). As a result, emphasis is mainly on numerical methods implemented on commercialelectromagnetic simulation platforms. The electromagnetic solvers which are most commonly used in theliterature for implantable antenna design are based on the finite element (FE) method (e.g., Ansoft HFSS)(Kiourti et al. 2011b; Liu et al. 2008a, b; Huang and Kishk 2011). The finite difference time domain(FDTD) method is also applied in some studies, because it exhibits simplicity in the implementation ofinhomogeneous media and assessment of bioelectromagnetic interactions, while enabling efficientmodeling of detailed anatomical human body parts (e.g., CST Microwave Studio, Remcom XFdtd)(Soontornpipit et al. 2004; Kiourti and Nikita 2012b; Kim and Rahmat-Samii 2004, 2006). In all cases,the computational cost heavily depends on the complexity of the tissue and antenna models. Absorbingboundaries (e.g., Mur (Soontornpipit et al. 2004)) or perfectly matched layer boundaries (Soontornpipitet al. 2005) are placed at some distance away from the setups to truncate the simulation domain whileextending radiation infinitely far.

A Step-by-Step Example Tutorial to Implantable Antenna DesignIn this section, a simple, yet analytical and complete, step-by-step example tutorial is provided onimplantable antenna design (Kiourti and Nikita 2014). Simulations are carried out within the frameworkof a MICS implantable patch antenna for intracranial pressure (ICP) monitoring applications. Neverthe-less, the same steps may be applied for any implantable antenna design that the designer may have onhand. It is remarked that the design strategy followed hereafter is the strategy #6 that was discussed in theprevious section. As compared to the other strategies, strategy #6 has been shown to result in the fastestdesign of implantable antennas with optimized resonance characteristics within the medical applicationon hand. The reason is that it incorporates dielectric loading of both the surrounding tissues and exteriorair on the antenna, while employing a canonical (parallelepiped) miniature tissue model, which can bemeshed and solved in a relatively easy and fast way (Kiourti and Nikita 2012c). Equivalently, the tissue-simulating box considered in this methodology has been found to be the simplest and smallest tissuemodel in which the implantable antenna exhibits almost identical reflection coefficient frequencyresponse as it would exhibit inside a canonical or anatomical tissue model of the intended implantationsite (Kiourti and Nikita 2012c).


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The parametric implantable antenna model of Fig. 8 is employed to serve the goals of this tutorial. Themodel consists of a ground plane (radius of R) and two vertically stacked patches (radius of R = 0.1 mmeach), printed on dielectric substrates (permittivity of erd and thicknesses of h1 and h2, respectively).Origin of the coordinate system is considered to be located at the center of the antenna ground plane.A dielectric superstrate (permittivity of erd and thickness of h3) covers the structure for biocompatibilitypurposes. Meanders of variable lengths (Li, i = 1–5, 1–60) and identical widths (0.4 mm) are inserted intothe patches to assist in miniaturization. A shorting pin (S: (sx, sy)) connects the ground plane to the lowerpatch, while a 50 Ω coaxial cable of variable type and length (L) excites both patches (F: (fx, fy)). Coppersheets (thickness of hm) are considered for the ground plane and patches, while glue layers (permittivity oferg and thickness of hg) bond the dielectric layers together. It is remarked that, in fabrication, glue layersare to be inserted between multiple substrate layers and/or between substrate and superstrate layers of theantenna for bonding purposes (Kiourti and Nikita 2012a; Abadia et al. 2009). Gluing has been found to bea very critical factor for implantable antenna design and has to be accounted for the following:low-permittivity glue layers isolate the high-permittivity substrate layers, thus decreasing the effectivepermittivity and electrical length of the antenna, while increasing its resonance frequency. Importantly,due to the miniature size of implantable antennas, inconsistencies between the numerical antenna modeland the fabricated prototype might result in a nonfunctional prototype. Therefore, gluing considerationshave to be taken into account within simulations.

Once the parametric implantable antenna model has been selected, the next step is to appropriately tuneits design parameters using an electromagnetic (EM) modeling and simulation program. The goal is toquickly calculate those parameter values which will optimize the antenna design in terms of impedancematching as well as exhibited radiation and patient safety performance at the desired operation frequency.The flowchart of the design strategy #6 is shown in Fig. 9 (Kiourti and Nikita 2012c). It is hereafterapplied within the framework of tuning the parametric implantable antenna model of Fig. 8 for ICPmonitoring (Warty et al. 2008) at 402 MHz (MICS band) (Kiourti and Nikita 2012b; Kiourti et al. 2012).

• “Fabrication-related” parameters: Initially, the “fabrication-related” parameters of the antenna are setto the values dictated by the intended fabrication procedure (Table 2). As part of this tutorial, Rogers

Fig. 8 Parametric implantable antenna model: (a) ground plane, (b) lower patch, (c) upper patch, and (d) side view (Kiourtiand Nikita 2014)


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RO3210 (erd = 10.2) dielectric sheets with a thickness of 0.635 mm (h1 = h2 = h3 = 0.635 mm) areconsidered. The aforementioned dielectric sheets come premetallized with a 0.017 mm-thickelectrodeposited copper foil (hm = 0.017 mm). Sprayable glue 3 M 77 is used to bond the layers(erg = 2.0), which has been found to exhibit an average thickness of 0.3 mm (hg = 0.3 mm) for thefabrication process to be followed. The antenna is to be fed by means of a 50 mm-long (L = 50 mm)EZ-47 (center conductor diameter of 0.29 mm, PTFE dielectric with a diameter of 0.93 mm, outerconductor diameter of 1.19 mm) semirigid coaxial cable.

• “Size-related” parameters: Next, the “size-related” parameters of the antenna have to be selected, i.e.,the parameters which determine the outer dimensions (physical size) of the antenna. In the parametricimplantable antenna model considered in this tutorial, these are dictated by the antenna radius, R.Selection of the outer dimensions relies on the expertise and knowledge of the designer and must beperformed based on the following two considerations. Firstly, size of the implantable antenna needs totake into account the desired implantation site and medical application scenario, as well as the size ofthe IMD in which it will be integrated. Secondly, miniaturization should not be set as the sole goal ofthe design. Previous studies have demonstrated degraded radiation and patient safety performance withsize reduction for implantable antennas and have quantified this degradation as a function of size(Kiourti and Nikita 2012d). Given these considerations, a radius of R = 6 mm is selected for the ICPmonitoring antenna under study (see Table 2).

• “To-be-optimized” parameters: The rest of the design parameters are considered as dimensions in thesolution space and have to be tuned for an optimized 50 Ω impedance match at the desired operatingfrequency (“to-be-optimized”). Design is performed by: (a) setting the “fabrication-related” and “size-related” parameters to the values selected in the previous steps, (b) initializing the “to-be-optimized”parameters to random values, and (c) placing the antenna at a distance d under the outer surface of thetissue-simulating box shown in the inset of Fig. 3 (Kiourti and Nikita 2012c). The distance d corre-sponds to the actual air-to-antenna separation distance for the desired medical application scenario(implantation depth). The tissue-simulating box extends by R + 4 mm in the x- and y-directions (R is

Fig. 9 Flowchart of the methodology for numerical design of implantable antennas


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the maximum dimension of the antenna in the positive y-axis) and simulates the electrical properties ofthe intended implantation tissue.

In the “refinement” step of the design strategy, an approximate design is performed for the antenna.The “to-be-optimized” parameters are manually updated in an iterative way, until the magnitude of thereflection coefficient (|S11|) at the desired operating frequency (fres) satisfies:

S11j j@fres< �15 dB (4)

Manual update relies on the skills and expertise of the designer, who is considered to be aware ofthe theoretical background related to antenna miniaturization (e.g., longer meanders are expected toincrease the length of the current flow and result in lower resonance frequencies (Dey and Mittra 1996).

In the “optimization” step of the design strategy, antenna design is optimized. The “to-be-optimized”parameters are initialized to the values of the refinement step and are optimized based on a software-integrated optimization algorithm. The optimization process terminates when:

S11j j@fres¼ min (5)

Table 2 Parameter values selected for optimally tuning the implantable antenna model of Fig. 8 at 402 MHz (MICS band)

Parameters Values

“Fabrication-related” erd 10.2

erg 2.0

h1 0.635 mm

h2 0.635 mm

h3 0.635 mm

hm 0.017 mm

hg 0.3 mm

L 50 mm

Coaxial type EZ-47

“Size-related” R 6 mm

“To-be-optimized” L1 7.597 mm

L2 10.146 mm

L3 10.146 mm

L4 3.019 mm

L5 3.019 mm

L1’ 11.397 mm

L2’ 11.146 mm

L3’ 11.146 mm

L4’ 10.519 mm

L5’ 10.519 mm

L6’ 8.993 mm

sx 1 mm

sy �4 mm

fx 0 mm

fy 4 mm


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or when the number of iteration exceeds a pre-defined maximum number.As part of this tutorial, simulations are performed at a distance of d = 5mm under the outer surface of a

small (R = 6 mm) tissue-simulating box, which corresponds to the actual average implantation depth ofan ICP monitor inside the human scalp. The tissue-simulating box represents skin tissue (scalp) electricalproperties at fres = 402 MHz (er = 46.7, s = 0.69 S/m), which are approximated as constant inside the300 to 500MHz range. Using this approximation, the maximum errors of er and s are given by 6.59% and8.89 %, respectively. The “to-be-optimized” parameters of the antenna are optimized based on quasi-Newton optimization (the maximum number of iterations is set to 300), due to its speed and accuracy incases of insignificant numerical noise (Sun and Yuan 2006). Optimal parameter values are given inTable 2, whereas the reflection coefficient frequency response of the designed antenna is shown in Fig. 10(“numerical model”). The antenna resonates at 402 MHz with a reflection coefficient of �27.9 dB and awide 10 dB bandwidth of 44 MHz, which covers the MICS band.

Example Implantable AntennasRelative position and orientation between an IMD and its exterior monitoring/control equipment coun-terpart is known a priori. As a result, patch designs are most commonly chosen for implantable antennasbecause they exhibit directive radiation patterns, lend themselves easily to a number of miniaturizationtechniques, and are highly flexible in design, conformability, and shape. In a realistic scenario, theimplantable patch antenna will be mounted on the existing hardware of the IMD, which will also serveas its ground plane. Table 3 compares the volume occupied byMICS implantable antennas reported in theliterature with respect to the applied miniaturization techniques. The bands of operation covered are alsoincluded in Table 3. When the number of bands of operation is increased, the size of the antenna istypically increased to cover them. Circular shape is generally preferred in order to avoid sharp edges thatcould cause injury. The performance of these antennas is further compared in Table 4 in terms of their10 dB bandwidth (BW), maximum allowable input power levels imposed by the IEEE C95.1-1999 (1 g-avg SAR � 1.6 W/kg (IEEE 1999)) (P1999) and IEEE C95.1-2005 (10-g-avg SAR � 2 W/kg (IEEE2005)) (P2005) safety guidelines, and maximum far-field gain (Gmax). In general, increased-size implant-able antennas exhibit more uniform distributions of the electric field and current density across anincreased patch surface area, so that lower SAR values are obtained.

Nevertheless, it is remarked that dipole (Kim and Rahmat-Samii 2004), loop (Chen et al. 2009),monopole (Weiss et al. 2009), modified dipole (Scarpello et al. 2011), and 3D spiral (Abadiaet al. 2009) antennas have also been reported for implantation purposes.

Fig. 10 Numerical and in vitro measured reflection coefficient frequency response of the proposed implantable antenna forICP monitoring


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Fabrication and Testing of Implantable Antennas

Experimental investigations are required in order to confirm the validity of numerical simulations forimplantable antennas. Of course, it is not possible to carry out measurements inside real operatingscenarios (i.e., inside the human body). Therefore, experimental testing is performed by measuring thefabricated antenna prototypes inside either tissue-equivalent mediums (phantoms) or animal tissue.

Fabrication of Implantable AntennasFabrication of implantable antenna prototypes needs to deal with all challenges related to the fabricationof miniature antenna structures. For example, glue layers used to affix all components together stronglyaffect the antenna performance by shifting its resonance frequency and degrading its impedancematching. Furthermore, the coaxial cable feed used to connect the antenna with the network analyzermay give rise to radiating currents on the outer part of the cable, which, in turn, may deteriorate the

Table 3 A size comparison of implantable patch antennas reported in the literature

ReferencesBands[MHz]

Miniaturization technique

Volume [mm3]Dielectricpermittivity Patch shape

Shortingpin

Patchstacking

Kim and Rahmat-Samii2004

402–405 10.2 Spiral No No 10,240.0

Soontornpipit et al. 2005 402–405 2.94 Waffle Yes No 6,480.0


402–405 10.2 Spiral Yes No 6,144.0

Soontornpipit et al. 2004 402–405 6.1 Spiral Yes No 3,457.4

Sánchez-Fernándezet al. 2010

402–4052,400–2,800

6.1 SRR coupled tospiral

Yes No 1,375.4

Karacolak et al. 2008 402–4052,400–2,800

10.2 Meandered Yes No 1,265.6


402–405 10.2 Spiral Yes No 1,200.0


402–405 10.2 Meandered Yes No 1,200.0

Huang and Kishk 2011 402–405 9.4 Spiral Yes No 823.0

Lee et al. 2009 402–405 10.2 p-shaped Yes No 790.9

Vidal et al. 2013 402–405 6.7 Folded square Yes Yes 448.0

Lee et al. 2006 402–405 10.2 Hook-slotted Yes Yes 335.8

Permana et al. 2011 402–405 10.2 Spiral Yes Yes 273.6

Huang et al. 2011 402–405433–4352,400–2,480

10.2 Comb- andp-shaped

Yes Yes 254.0

Permana et al. 2013 402–405 10.2 Spiral Yes Yes 254.0

Kiourti and Nikita 2012b 402–405 10.2 Meandered Yes Yes 203.6

Liu et al. 2008b 402–405 10.2 Spiral Yes Yes 190.0

Liu et al. 2008a 402–405 10.2 Hook-slotted Yes Yes 149.2

Liu et al. 2009 402–405 10.2 Hook-slotted Yes Yes 121.6

Kiourti et al. 2011b 402–405 10.2 Meandered Yes Yes 110.4

Kiourti et al. 2011a 402–405 9.4 Meandered Yes Yes 32.7


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measurements. Therefore, fabrication of implantable antennas is highly intriguing. In the following, thefabrication of implantable antennas will be discussed as part of fabricating the prototype of the designshown in Fig. 8. The latter incorporates several fabrication challenges that any prototype fabrication mighthave to deal with (e.g., multiple layers, inclusion of a shorting pin, etc.).

Specifically, three key aspects of the fabrication are considered to mainly influence the final antennabehavior: (a) substrate cutting, (b) substrate gluing, and (c) layer alignment. One of the problems is thatthese three steps are not necessarily independent. In fact, because the substrate material is relatively stiff, itcannot (or is hard to) be cut after the antenna has been assembled: micro-soldering of the coaxial cable andshorting pin are very fragile and cannot withstand the vertical pressure and torsion of the cutting tool.Furthermore, external alignment points are proved to be required for the assembling, to be removed afterfabrication. Therefore, a mounting base (Fig. 11a) is suggested to be fabricated in order to help with theantenna’s assembly. This base ensures the correct alignment between the three layers, while serving as theantenna support for the different soldering procedures. Based on the above, a typical fabricationmethodology includes the following steps (Kiourti and Nikita 2014):

• Photolithography masks. Photolithography masks are prepared and printed, as shown in Fig. 12. Themasks include: (a) a circular circumference which is used to guide the antenna cutting, (b) four circularmarks which indicate the position of the holes that match the four pins of the mounting base (Fig. 11a)during the assembly procedure, (c) a square frame which matches the dimensions of the mounting base(Fig. 11a), and (c) complementary alignment marks to help in the alignment of the two sides of thebottom substrate layer (i.e., ground plane and lower patch).

Table 4 A performance comparison of implantable patch antennas reported in the literature with respect to their occupiedvolume: 10 dB bandwidth (BW), maximum allowable input power levels imposed by the IEEE C95.1-1999 (P1999) and IEEEC95.1-2005 (P2005) standards, and maximum far-field gain (Gmax) (N/A denotes that this information is not available)

References Volume [mm3] BW [MHz] P1999 [mW] P2005 [mW] Gmax [dBi]

Kim and Rahmat-Samii 2004 10,240.0 20 8.791 N/A N/A

Soontornpipit et al. 2005 6,480.0 16 N/A N/A N/A


Soontornpipit et al. 2004 3,457.4 28 N/A N/A N/A

Sánchez-Fernández et al. 2010 1,375.4 12 N/A N/A �6

Karacolak et al. 2008 1,265.6 142 N/A N/A �25



Huang and Kishk 2011 823.0 25 5.820 N/A N/A

Lee et al. 2009 790.9 120 5.714 N/A �27

Vidal et al. 2013 448.0 110 3.7 N/A N/A

Lee et al. 2006 335.8 50 4.798 N/A �26

Permana et al. 2011 273.6 39 N/A N/A �24

Huang et al. 2011 254.0 113 4.692 N/A �7

Permana et al. 2013 254.0 5 N/A 60.6 �40

Kiourti and Nikita 2012b 203.6 27 4.928 30.030 �37

Liu et al. 2008b 190.0 50 4.762 N/A �26

Liu et al. 2008a 149.2 84 2.235 N/A N/A

Liu et al. 2009 121.6 122 1.778 N/A �38

Kiourti et al. 2011b 110.4 50 1.932 20.704 �46

Kiourti et al. 2011a 32.7 40 2.354 24.390 �45


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• Photolithography. The acquired Rogers RO3210 dielectric layers (erd = 10.2, h1 = h2 = h3 =0.635 mm, hm = 0.017 mm) are etched by means of a photolithographic process which makes useof the photolithography masks of the previous step. The lower substrate layer contains the ground planeand the lower patch, the upper substrate contains the upper patch, while the superstrate has nometallization. It is highlighted that due to the unavailability of biocompatible materials in somelaboratories, other dielectrics with similar electrical properties may be selected for prototype fabrica-tion. For instance, Rogers RO3210 is often used because it has similar properties as the biocompatibleceramic alumina (er = 9.4, tand = 0.006) (Karacolak et al. 2008; Kiourti and Nikita 2012b;Liu et al. 2008a).

• Cutting of the layers. A circular cutting tool is then used to cut the antenna layers, as shown inFig. 11b. The cutting tool exhibits a nominal diameter of 12 mm, which corresponds to the diameter ofthe intended antenna prototype (R = 6 mm). The adopted strategy is to precut the substrate down to acritical depth, just enough to keep the alignment points solidary with the patch, but weak enough toallow easy detaching without much mechanical stress to the antenna.

• Antenna assembly. The antenna is further assembled by making use of the mounting base of Fig. 11a.Layers are aligned and glued (3 M 77 glue: erg = 2.0, hg = 0.3 mm), while the shorting pin is set toconnect the ground plane to the lower patch through a via. The outer conductor of the coaxial cable getsconnected to the antenna ground plane, while the inner conductor gets simultaneously soldered to thelower and upper patches through vias. Nevertheless, detaching the antenna from the excess alignmentmaterial has been shown to be relatively hard, thus resulting in some stress to the fragile antenna.

Fig. 11 Fabrication of implantable antennas: (a) mounting base, (b) circular cutting tool, and (c) fabricated antenna prototype


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The assembled antenna is shown in Fig. 11c. It is remarked that the most critical aspect regarding thefabrication of such miniature implantable antennas occurs to be the control of the glue layer thickness.This is impaired not only by the glue itself but also by the slight bump of the micro-solder near the coaxialcable and the shorting pin that prevents perfect contact between the layers.

Fig. 12 Photolithography masks for printing the implantable antenna shown in Fig. 8: (a) ground plane, (b) lower patch, (c)upper patch, and (d) superstrate

Fig. 13 Canonical phantoms used for testing of implantable patch antennas: (a) liquid (Kiourti and Nikita 2012b) and (b)multilayer gel (Sani et al. 2010)


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In Vitro Testing of Implantable AntennasTesting inside phantoms is relatively easy and practical to implement. The fabricated prototype isimmersed inside a tissue phantom (i.e., a container filled with a liquid or gel material which mimics theelectrical properties of biological tissue) and tested.

• Phantom formulation. Canonically shaped phantoms have, most commonly, been used for implant-able antenna testing (see Fig. 13). In this case, the main challenge lies in the formulation (and,subsequently, characterization) of the tissue-emulating materials. In the literature, there have beenseveral recipes presented for emulation of biological tissues at various frequencies (see Table 5). Gelsrather than liquids are to be preferred in cases where multilayer phantoms and, thus, increased realismin experimental modeling are solicited. Deionized or distilled water usually acts as the base ingredientof the phantoms. Addition of sugar or glycerol reduces the permittivity, without, almost, affecting theconductivity. Salt increases the conductivity and, slightly, increases the permittivity (Karacolaket al. 2008). Solidification is usually made possible with agar. Other ingredients may also be used inorder to vary the viscosity, preserve the mixture, and further control the permittivity and conductivity(Kiourti et al. 2014b; Ito et al. 2001). To prevent the formation of air bubbles and/or gaps, the mixturemust be carefully heated and stirred and slowly poured inside the container of the phantom. Since it isnot possible to produce a valid approximation to human tissue for a broad frequency spectrum using asingle formula, separate recipes are given for different frequency bands (Karacolak et al. 2008).

• Measurement of the phantom electrical properties. In vitro testing of implantable antennas insidephantoms requires experimental measurement of the exhibited electrical properties in order to ensure

Table 5 Phantoms used in the literature for testing of implantable antennas

PhantomMeasured electricalproperties

ReferencesTissue(s) State Ingredientsf[MHz] er s [S/m]

Skin Liquid Deionized water, sugar, salt 402 46.7 0.69 Kiourti and Nikita2012b

Liquid Deionized water/sugar/salt/cellulose 402 49.6 0.51 Kim and Rahmat-Samii2004

Liquid Deionized water, fruit sugar, salt, cellulose 402 46.7 0.69 Liu et al. 2008a

Liquid – 402 49.6 0.51 Kim and Rahmat-Samii2004

Gel Deionized water, sugar, salt, agarose 402 46.7 0.69 Karacolak et al. 2008

Gel Deionized water, sugar, agarose 2,450 38.1 2.27 Karacolak et al. 2008

2/3 muscle Liquid Water, sugar, salt, TX-151 powder 402 48.9 0.71 Soontornpipitet al. 2004

Liquid Water, sugar, salt, cellulose cetylpyridiniumchloride

403 41.3 – Huang and Kishk 2011

Scalp Gel Water, salt, acrylamide, TMEDA, ammoniumpersulfate

2,450 50 2.2 Warty et al. 2008

Rat tissue Gel Deionized water, salt, DGBE 402 0.78 1.3 Karacolak et al. 2009

Gel Deionized water, DGBE, Triton X-100 2,450 0.73 1.27 Karacolak et al. 2009

(1) Skin(2) Fat(3) Muscle

(Multilayer)Gel

(1) Deionized water, sugar(2) Deionized water, salt, vegetable oil, flour(3) Deionized water, sugar, salt

868 (1) 38.7(2) 4.9(3) 53.0

(1) 0.77(2) 0.04(3) 0.92

Sani et al. 2010


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conformance with the corresponding theoretical values. Recently, an in-depth analysis has beenprovided for the measurement of the electrical properties of biological media (Hofmann et al. 2013).In the market, there exist some commercial complex permittivity measurement systems, such as theAgilent Technologies 85071E (Agilent Technologies, Santa Clara, California, USA) or the SPEAGDielectric Assessment Kit (SPEAG, Switzerland). However, alternative approaches are furthersolicited for laboratories that are not equipped with such commercial systems. For example, alow-cost and reliable complex permittivity measurement technique has recently been proposed(Kiourti et al. 2012). The measurement setup consists of a parallelepiped container intercepted bythe inner conductor of a coaxial cable, as shown in Fig. 14a (exterior container size of 52mm � 32mm � 32.2 mm, interior cavity size of 40 mm � 20 mm � 20 mm). The coaxial container is filledwith the liquid or gel dielectric material under investigation, and once the lid is closed, it represents atransition between coaxial guides with a step characteristic impedance discontinuity. The transferfunction between the two coaxial connectors outside the container depends upon the complex permit-tivity of the container’s filling material. This can be de-embedded by comparing the measuredscattering matrix (S-matrix) with simulation results for the same structure (Fig. 14b).

• In vitro measurement of the implantable antenna. Most studies for implantable antennas are limitedto reflection coefficient (S11) measurements (e.g., Kiourti and Nikita 2012b; Soontornpipit et al. 2005;Liu et al. 2008a; Sánchez-Fernández et al. 2010). As part of the experimental setup, prototype antennasare connected to a network analyzer through a coaxial cable, immersed inside the tissue-emulatingphantom, and measured. Transmission coefficient (S21) measurements have also been reported. Forexample, in Warty et al. (2008), a 2,450 MHz PIFA was implanted inside a gel scalp phantom forintracranial pressure monitoring, and a linearly polarized 2,450 MHz chip antenna was used as thereceiving (probing) antenna. Recently, in Kiourti et al. (2014b), a miniature broadband implantableantenna and a dual-band on-body antenna were presented along with the transmission performancebetween the two. The antennas were intended for integration into IMDs and on-body repeaters,respectively.

In Vivo Testing of Implantable AntennasIn vitro verification of an implantable antenna does not guarantee its proper functioning when implantedinside actual biological tissues (Karacolak et al. 2010). Therefore, once functionality of an implantableantenna prototype has been verified in vitro, in vivo testing is recommended to be further performed.Specifically, in vivo testing can be performed either (a) by embedding the implantable antenna insidetissue samples from donor animals (b) or by surgically implanting the antenna inside live model animals.

Use of animal tissue samples provides an easy approach to mimicking the frequency-dependencycharacteristic of the electrical properties of tissues. This can prove highly advantageous when carrying outmeasurements for multiband implantable antennas. In this case, the electrical properties of the test tissue

Fig. 14 Complex permittivity measurement of liquid and semisolid phantoms: (a) coaxial container and (b) numerical model(Kiourti et al. 2012)


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can be measured using a dielectric probe kit and a network analyzer. In the literature, an implantable patchantenna with dual resonances at 380 and 440 MHz has been tested inside test tissue obtained by grindingthe front leg of pork (Lee et al. 2009). The electrical properties of the adopted pork were found to bebetween those of human skin and muscle in the MICS band. A dual-band skin-implantable patch antennaoperating in the MICS and 2,450 MHz ISM bands has also been tested in real animal skin (Karacolaket al. 2009). Skin samples with dimensions of 50 � 50 � 5 mm3 were extracted from the dorsal area ofthree donor rats to cover the designed antenna, and measurements were performed within 30 min ofeuthanization (Fig. 15a). Finally, a triple-band implantable patch antenna has been tested inside a mincedfront leg of pork (Huang et al. 2011). Electrical properties of the minced pork were measured and found tocorrespond to those of human skin and muscle between 100 MHz and 3 GHz.

Investigations inside living animals are also vital in order to investigate the effects of live tissue on theperformance of implantable antennas, while providing valuable feedback for antenna design and analysis.The goal is to assess the effects of the following factors which are not accounted for in in vitroexperimentation: (a) air gaps between the implanted antenna and the surrounding tissues, (b) presenceof multiple types of tissues around the antenna, (c) dependence of tissue electrical properties uponfrequency, (d) intersubject variability (anatomy and dependence of tissue electrical properties uponeach rat’s age, size, sex, internal body temperature, etc.), and (e) variations in the surgical proceduresfollowed (implantation depth, implantation site, length of the wound, closure of the wound with sutures,etc.). In such measurements, the first step is the development of an experimental measurement protocol.The protocol has to be developed in cooperation with an experimental surgery unit, take into account legalrequirements regarding the care and use of laboratory animals and address issues related to the type andnumber of model animals, implantation site of the antenna, anesthesia, surgical procedure, measurements,and post-surgery treatment. As an example, an in vivo experimentation protocol was recently developedfor implantable antenna testing (Kiourti et al. 2013). This protocol takes into account the legal require-ments regarding the care and use of laboratory animals and can be summarized as follows:

• Type and number of model animals. Implantation and measurements are carried out inside rats,which have long been used in the literature as model animals (Karacolak et al. 2010). Wistar outbred

Fig. 15 In vivo testing of implantable antennas: (a) Karacolak et al. (2009), (b) (Karacolak et al. (2010), and (Kiourtiet al. (2013)


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rats (HsdOla:WI) are employed, which exhibit a mean and standard deviation (SD) body weight of331.3 � 9.2 g. In order to assess intersubject and surgical procedure variability, each antenna isimplanted and measured inside three different rats.

• Implantation site of the antenna. Since the antennas under study have been designed for operationinside soft tissues, implantation is carried out within the subcutaneous tissue of the rats’ abdomen.

• Anesthesia. Each rat is first anesthetized with an intraperitoneal (i.p.) injection of 70 mg/kg ketamine(Ketaset, Fort Dodge, Iowa, USA) and 5 mg/kg xylazine (Rompun, Bayer, Leverkusen, Germany).

• Surgical procedure. Awound is further made in the rat’s abdomen area, and the antenna is implantedwithin the abdominal subcutaneous tissue. Following implantation, the wound is closed with 0/4 silksutures (Silkam, Braun, Aesculap, Tuttlingen, Germany), leaving a 2 mm opening for the feedingcoaxial cable to exit the skin.

• Measurements. Right after surgery, the feeding coaxial cable is connected to a network analyzer whichmeasured the exhibited reflection coefficient frequency response.

• Post-surgery treatment. Once measurement is completed, the implanted antenna is removed, and therat is euthanized in a CO2 chamber. Time lapse from the start of the surgical procedure to euthanasia ofeach rat does not exceed 8 min.

In vivo studies reported in the literature are very limited. The return loss frequency response of a skin-implantable antenna has been measured using rats as model animals (Karacolak et al. 2010; Fig. 15b). Inthis study, the antenna was implanted by means of a surgical operation inside the dorsal midline of threerats (for validation purposes), and euthanasia was applied after the measurements (approximately13–15 min after the surgery). Canine studies for trans-scalp evaluation of a scalp-implantable antennaat 2,450 MHz have also been presented (Kawoos et al. 2008). Canine models were selected to ensure alarge head size, and an intracranial pressure monitoring device with an integrated antenna was fixed to theskull. The monitor was tested while the dog was still under anesthesia. After the measurements, the animalwas allowed to emerge from anesthesia and taken to the recovery area. Recently, implantable antennaswere tested inside three different rats to assess intersubject variability considerations (Fig. 15c; Kiourtiet al. 2013). In this case, numerical and experimental results were found to exhibit quite good agreement.Compared to numerical simulations, percentage changes in the exhibited resonance frequency (fres),reflection coefficient at this frequency (|S11|@fres), and 10 dB bandwidth (BW) are found to equal +6.9 %,+51.9 %, and +30.2 %, respectively. Maximum deviations in fres, |S11|@fres, and BW recorded among thethree measurements in different rats were found to equal 43 MHz, 12.6 dB, and 23 MHz, respectively.

Full Solutions of Commercial IMDs

A number of commercial IMDs have already been reported, as shown in Table 6. Defibrillators andpacemakers are the most common examples of IMDs. Biotronik has recently proposed a small battery-powered electrical impulse generator to be implanted in patients who are at risk of sudden cardiac deathdue to ventricular fibrillation and ventricular tachycardia (Lumax 540 DR-T) (Biotronik 2012). TheMedtronic Adapta with MVP pacing system offers managed ventricular pacing, atrial therapy, ventricularcapture, and remote cardiac telemetry (Medtronic 2010a), whereas the Medtronic Revo MRI SureScanpacing system is magnetic resonance (MR) conditional designed to allow patients to undergo magneticresonance imaging (MRI) under the specified conditions of use (Medtronic 2011a). VeriChip (renamed toPositiveID in 2010) was the only human-implantable microchip to receive approval by the Food and DrugAdministration (FDA) in 2004 (PositiveID 2004). The device was typically implanted between theshoulder and elbow of the individual, and, once scanned, it replied with a unique 16-digit number,


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which could be used to retrieve personalized information, for example, identity verification, medicalrecords, etc. However, privacy concerns generated controversy and debate, and marketing wasdiscontinued in 2010. The Nucleus Freedom cochlear implant includes a sound processor which isworn behind the ear and a cochlear implant which is placed under the skin, behind the ear (NucleusFreedom 2010). The sound processor captures sounds, digitizes them, and sends the digital code to theimplant. The implant converts the digitally coded sound to electrical impulses and sends them along anelectrode array to further stimulate the cochlea’s hearing nerve. The Medtronic SynchroMed Pump is adrug infusion system which provides precise drug delivery for chronic therapy of severe spasticity(Medtronic 2012). In addition to the implanted pump, the system uses a catheter to deliver programmedamounts of intrathecal baclofen to the intrathecal space and cerebrospinal fluid. The Argus II retinalimplant was approved by FDA’s Ophthalmic Devices Advisory Panel in 2012 (Second Sight 2012). Itincludes a video camera, a transmitter mounted on a pair of eyeglasses, a video processing unit, and a60-electrode implanted retinal prosthesis that replaces the function of degenerated cells in the retina.Although it does not fully restore vision, this setup can improve a patient’s ability to perceive images andmovement. Finally, implantable glucose monitoring systems appear as a promising treatment for diabeteson a continuous basis (e.g., Medtronic Guardian REAL-Time (Medtronic 2010b), Medtronic MiniMedParadigm Veo (Medtronic 2011b), Dexcom SEVEN PLUS (Dexcom 2008), Abbott FreeStyle Navigator(Abbott 2011)). A tiny sensor is inserted under the skin to measure glucose levels and further transmitsthis information to an exterior monitor via radio waves.

Conclusion

IMDs with wireless telemetry functionalities appear as a highly promising option toward improving thepatients’ quality of life and providing medical systems with constant availability, context awareness,reconfigurability, and unobtrusiveness. As such, a key and critical component of RF-linked IMDs is theintegrated implantable antenna.

In this chapter, issues related to the design, simulations, and experimental investigations of implantableantennas for biomedical telemetry were discussed. Specifically, design mainly emphasizes on miniatur-ization issues and biocompatibility. However, electrically small antennas present poor radiation perfor-mance and relatively narrow bandwidth. Even though gain enhancement is considered crucial,compromises on the system performance are generally inevitable. Conserving energy to extend thelifetime of the IMD is also significant. Multiband antennas are being designed for this purpose, which

Table 6 Example full solutions of commercial IMDs

Commercial IMD Function of the IMD

Biotronik Lumax Defibrillator

Medtronic Adapta with MVP Pacing system

Medtronic Revo MRI SureScan Pacing system for magnetic resonance imaging

VeriChip/PositiveID Identity verification

Nucleus Freedom cochlear Cochlear implant

Medtronic SynchroMed Pump Drug infusion system

Second Sight Argus II Retinal prosthesis

Medtronic Guardian REAL-Time Glucose monitor

Abbott FreeStyle Navigator Glucose monitor

Dexcom SEVEN PLUS Glucose monitor

Medtronic MiniMed Paradigm Veo Glucose monitor


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“wake up” the IMD only when there is a need for information exchange. Several methodologies have beenproposed for implantable antenna design, all of which need to take into account the host body. Simplifiedtissue models are proved to be able to substitute for complex anatomical tissue models, thus speeding upsimulations. Although a homogenous model is sufficient for basic antenna design, a more realistic modelis needed to refine the final antenna design and provide accurate results. Using efficient and accuratesimulation tools and tissue models is a key issue for both design and performance analysis. Regardingexperimental investigations, implantable antennas exhibit tight fabrication tolerances as attributed to theirminiature size. Testing inside tissue-emulating phantoms mainly needs to deal with the formulation andcharacterization of the tissue-mimicking liquid or gel. To benefit from frequency-dependent tissueelectrical properties, testing in animal tissue samples can additionally be performed. The highest chal-lenge, however, lies in measurements within living animals, in which careful consideration is required fordeveloping the optimal testing protocol.

IMDs are a growing technology with a high potential for improving the patients’ life and the quality ofhealthcare. RF technology for IMDs promises many benefits for both patients and caregivers. With risinghealth-care costs, an aging population, a growing acceptance of home-based medical monitoring, andadvances in supporting technology, IMDs are gaining a continually increasing interest in both academiaand industry.

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