2170 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 61, NO. 5, MAY 2013

Analysis of Electromagnetic Fields Inducedin Operation of a Wireless Fully PassiveBackscattering Neurorecording Microsystem

in Emulated Human Head TissueHelen N. Schwerdt, Félix A. Miranda, Senior Member, IEEE, and Junseok Chae, Senior Member, IEEE

Abstract�—This paper reports on a fully passive microsystemthat wirelessly records and transmits neuropotentials exclusivelyby means of electromagnetic backscattering techniques, affordingsubstantially simpler circuitry and potentially safer and morereliable approach for implantable wireless neurorecording. Afundamental practical barrier for wireless brain-implantablemicrosystems includes heat dissipation by on-chip circuitry, whichmay cause permanent brain damage. Hence, measurement ofthermal proles of surrounding tissue induced by operation ofwireless implants is imperative in assessing the safety of thesedevices. Evaluation of specic absorption rate (SAR) is espe-cially relevant for wireless electromagnetic transmission schemesoperating at microwave frequencies and directly relates to theheat generated within biological tissue media. In this study,computational and empirical methods are used to measure SARwithin a human-head-equivalent phantom during operation of theembedded fully passive wireless neurorecording microsystem. Themaximum average SAR, coinciding with the worst case scenario,measured within 1 g of brain tissue is 0.45 0.11 W/kg,complying with the U.S. FCC threshold (1.6 W/kg).

Index Terms�—Backscattering, brain�–machine interfaces, im-plantable electronics, microelectromechanical systems (MEMS),passive devices, specic absorption rate (SAR).

I. INTRODUCTION

W IRELESS cortical neurorecording implant devicesestablish a pivotal interface to intricate brain processes.

Cortical neurorecording involves embedding electrodes andintegrated electronics on the surface of the brain to wire-lessly extract electrophysiological signals, neuropotentials,that provide the key data necessary to control prostheticand/or brain�–machine interfaces as well as diagnose and treatnumerous neurological disorders. The practical utility of im-plantable neurorecording systems in the clinical and scientic

Manuscript received October 15, 2012; revised February 27, 2013; acceptedMarch 01, 2013. Date of publication April 03, 2013; date of current versionMay02, 2013. This work was supported in part by the National Science Founda-tion under Grant ECCS-0702227, the National Institutes of Health under Grant5R21NS059815-02, and the NASA Graduate Student Research Program underGrant NNX09AK93H.H. N. Schwerdt and J. Chae are with the Department of Electrical

Engineering, Arizona State University, Tempe, AZ 85287 USA (e-mail:hschwerd@asu.edu).F. A. Miranda is with the NASA Glenn Research Center, Cleveland, OH

44135 USA.Color versions of one or more of the gures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identier 10.1109/TMTT.2013.2252916

elds can only be realized by ascertaining their safety andlong-term viability and durability [1]�–[5]. In order to conformto the stringent safety requirements, especially for chronicbrain implantation, considerable research has been investedin enhancing such device aspects as biocompatibility [5]�–[7],miniaturization [2], [5], [8]�–[12], elastic material platforms[3], [5], [13], and power and heat management [11], [14]�–[16].A biocompatible encapsulation is required to avoid evokingimmune reaction or harmful response from the human body [6],[7]. Implant devices are also miniaturized and/or translated tomore compliant materials to minimize their physical obtrusionand contortion of the brain [2]�–[5]. In addition to mechanicallyinstilled perturbation, a major safety concern associated withoperation of the implanted device relates to heat dissipatedfrom the embedded electronics or permeating wireless elec-tromagnetic (EM) elds into surrounding tissue [17]. It isimperative to limit the heat generated during operation of thecortically implanted device, as temperature rises greater than1 C have been shown to induce irreversible damage on thebrain [18]. Thermal effects caused by the implanted systemmay be attributed to the heat dissipated by internal circuitryand/or propagating EM waves. In the former instance, heatdissipation is directly related to the power consumed by thecircuits and power supply/regulatory components. In the lattercase, heat is generated due to the radiated EM elds that areabsorbed into the dissipative biological media. The fully pas-sive neurorecording microsystem utilized in this work operatesexclusively by means of wireless EM backscattering in theabsence of onboard active electronics or internal poweringelements [Fig. 1(a)] [11], [15]. Hence, the present study focuseson characterizing induced EM elds associated with operationof the microsystem that may be correlated to potentially adversethermal consequences.The fully passive microsystem acquires and mixes internal

neuropotentials ( at 5�–1000 Hz) [Fig. 1(b)] with anexternally supplied carrier ( 2.25�–2.45 GHz) to producethird-order intermodulation products (4.5�–4.9 GHz) through onboard nonlinear varactors that are thenwirelessly backscattered to the external interrogator, wherethe original can be extracted [Fig. 1(c)]. The fully passiveneurorecording circuitry is inherently simple as it is composedof only two off-chip varactors, three integrated metal�–insu-lator�–metal (MIM) capacitors for antenna loading andneuropotentials electrode port isolation , and a planar

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SCHWERDT et al.: EM FIELDS INDUCED IN OPERATION OF A PASSIVE NEURORECORDING MICROSYSTEM IN EMULATED HUMAN HEAD TISSUE 2171

Fig. 1. Overview of wireless fully passive backscattering neurorecording mi-crosystem. (a) Themicrosystem is subcranially implanted to record cortical neu-ropotentials and wirelessly transmit these signals to the external interrogator.(b) Illustration of neuronal structures surrounding the microsystem, from whichpropagating neuropotentials are acquired by the microsystem. (c) Sim-plied schematic of wireless fully passive backscattering neurorecording oper-ation. At the external interrogator (right), a signal generator supplies carrierthat is low-pass ltered (LPF) and wirelessly transmitted via dual band antennato the microsystem (left) antenna , which concurrently backscattersthird-order mixed harmonic signals . The sig-nals are high-pass ltered (HPF) after retrieval by external antenna and are de-modulated to retrieve . The microsystem (left) mixes incident carrier withinternal neuropotentials through varactors to produce . A by-pass capacitor isolates the neuropotential port from RF. (d) Fabricatedmicrosystem 4 12 0.5 mm above a U.S. dime. (e) Close-up of a fabri-cated microsystem showing electrodes that connect input neuropotentials ,MIM bypass and slot antenna loading capacitors, and placement ofvaractors .

slot antenna [Fig. 1(d) and (e)]. Conversely, state-of-the-artneurorecording systems usually retain a complex multifunc-tional platform possessing several components; microelectrodearrays (MEAs) to probe individual neurons, complex circuitryto process the signals, and a wireless link (antenna or inductivecoil) [2], [5], [19]. In order to reduce heat diffusion into thecortex, the electronics platform is usually dissociated fromthe subdural MEA and embedded immediately above theskull. However, the intracranial wiring between electrodes andextracranial circuits may introduce separate supplementarytraumatic effects as well as complicate and lengthen surgicalinstallation [2], [3], [5]. Alternatively, the fully passive tech-nique may substantially simplify the neurorecording methodand potentially lessen introduced trauma. Detailed design, op-eration, fabrication, and preliminary testing of the microsystemhave been previously reported [11], [15].

In order to assure minimal risk in operating wireless im-plants, it is critical to assess the impact of heat emanated byintegrated circuit as well as impinging EM waves in the humanbody. Thermal analyses for multichannel MEA have beenperformed using nite element analysis (FEA) and infrared(IR) thermography in brain phantom and in vivo [19]. Thisprior study estimated heat dissipation of the microsystem IC(integrated circuit) by mounting Ti/Pt micro heating elementsonboard the MEA to emulate IC power dissipation. A temper-ature prole of 0.029 C/mW due to power dissipated by theIC or an equivalent maximum temperature rise of 0.38 C (at13 mW) was demonstrated. Unlike prevalent low-frequencyinductive coupling schemes, the microsystem of this workoperates via EM backscattering at microwave frequencies( 2.25�–4.9 GHz) wherein most of the irradiated EM signalsexperience signicant scattering and absorption in biologicalmedia. As such, it is relevant to measure the specic absorptionrate (SAR) or localized power density of the EM elds absorbedby the biological tissue. The SAR value is commonly appliedto quantify the safety of mobile communication devices andis regulated by the U.S. FCC (1 g average SAR 1.6 W/kg)[20]. SAR thresholds are maintained to account for knownthermal consequences as related to specic SAR doses [21].SAR may be measured through thermographic procedures,of which the operation and accuracy have been extensivelyexplored [22]�–[24].The fully passive backscattering microsystem may present a

safer neurorecording technique and potentially deect problem-atic power and heat challenges characteristic of conventionalactive or semipassive implantable neurorecording systems [2],[5], [14], [19], as may be elucidated through the SAR analyseswork of this paper.

II. EXPERIMENTAL AND NUMERICAL TECHNIQUES

A. Wireless Backscattering Neurorecording OperationFully passive wireless neurorecording operations are real-

ized by EM coupling between the internal microsystem and theexternal interrogator. In order to activate nonlinear mixing andbackscattering operations of microsystem, a dual-band externalinterrogator wirelessly supplies the fundamental carrierto the microsystem and receives backscattered harmonics fromthe microsystem . The microsystem possesses twooff-chip mounted varactors to mix incident carrier at2.25�–2.45 GHz with internally retrieved neuropotentialsat 5�–1000 Hz and produce third-order intermodulationproducts at 4.5�–4.9 GHz. The back-to-backvaractor conguration optimizes generation of odd-orderharmonics. The integrated subresonant slot antenna operatesat fundamental and backscatter harmonic frequencies

. As its maximum dimensions are restricted tofor practical and safety purposes as a subcranial

implant, the antenna takes the form of an electrically small an-tenna (ESA), where its directivity is fundamentally constrainedto 1.76 dBi [25]. The antenna�’s ability to backscatter powermay be evaluated by its radar cross section [26]

(1)

2172 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 61, NO. 5, MAY 2013

where the subscripts and correspond to receive and transmitmodes of the microsystem antenna, respectively, and arethe backscatter (transmit) and incident (receive) power ux den-sities, respectively, is the conversion gain of the nonlinearmixer from to , is antenna directivity, and isantenna efciency. As the microsystem is intended for implan-tation in brain tissue, performance of EM backscattering opera-tions largely depends on traversing media. EM signal losses dueto wave propagation through the dissipative biological mediamay be estimated by skin depth

(2)

where is angular frequency, is permeability, is permit-tivity, and is conductivity. Evidently, higher frequencies allowreduction in antenna size but augment signal loss.

B. Thermal Analyses

SAR is a measure of the rate at which energy is absorbed bythe body and is dened as the power absorbed per mass of tissue.SAR is generally measured in reference to the electric eld inthe sample by

(3)

where kg/m is sample density and (V/m) is root meansquared (rms) electrical eld strength, respectively. The U.S.FCC designates measurement of the SAR as averaged within1 g of tissue , usually enclosed in the brain [20], [21].Due to strong heterogeneity of the EM elds induced in

tissue, as well as the close orientation between source inter-rogator and implant to realize optimal backscattering, SARanalysis must consider the effects of localized pulse energy ab-sorption in the head. In the present study, the external antenna,implanted microsystem, and head-mimicking phantom aresituated in a manner that would produce the highest magnitudethird order backscattering products , aswell as elicit maximal SAR, the worst-case scenario, in thebrain tissue. A linearly tapered slot antenna (LTSA) is used asthe external interrogator antenna as it provides the wideband(1�–5 GHz) characteristics to operate at and . The LTSAalso integrates a diplexer through onboard lters coupled totransmit and receive ports. The microsystem ispositioned parallel to the plane of the LTSA substrate and to-wards the taper origin, where near-eld forms of EM couplingare maximized. The arrangement maintained throughout SARtesting is shown in Fig. 2. The dielectric properties of individualtissue layers used in measurements are listed in Table I [22],[24], [27].

C. Computational SAR

FEA is performed using Ansoft�’s High Frequency StructureSimulator (HFSS) to compute SAR generated during opera-tion of the microsystem. The implant-phantom assembly ispositioned parallel to the plane of the LTSA substrate with anair gap of 0.5 mm, as shown in Fig. 3. SAR computations are

Fig. 2. Arrangement of phantom tissue layers, implanted microsystem, and ex-ternal antenna, with indicated thicknesses of individual phantom layers.

TABLE IDIELECTRIC PROPERTIES OF HUMAN HEAD TISSUE LAYERS

Fig. 3. Simulation setup for computing SAR. Averaging 1-g and 1-mg boxesare seen along the normal axis of the microsystem. The transverse cross sectionon which spatial SAR is computed is also indicated.

divided by the two transmit and receive channels of propa-gating EM waves: primary downlink mode corresponding totransmission from the external antenna at to the microsystemand secondary uplink mode corresponding to backscatteringfrom the microsystem to the external antenna at . Thesignal loss (including mixer conversion loss) between suppliedtransmit carrier from external antenna and backscatteredharmonic products is 90 dB [15]. This substantial losssuggests that the SAR contributions of the uplink mode causedby backscattering of the microsystem are negligibly smallrelative to the primary downlink channel. The local maximumSAR is computed for both modes to realize the isolated effectsof both transmit and receive channels. Spatial SAR distributionacross the transverse plane of tissue layers aligned with mul-tiple axes along the microsystem plane is measured to visualizethe distribution along the main propagation path. is

SCHWERDT et al.: EM FIELDS INDUCED IN OPERATION OF A PASSIVE NEURORECORDING MICROSYSTEM IN EMULATED HUMAN HEAD TISSUE 2173

Fig. 4. Temperature rise measured in phantom as a function of radiation ex-posure duration (at 20 W) with tted trend-line having coefcient ofdetermination of 0.9988. These measurements indicate a nearly linear re-lationship between and , validating SAR derivations based on (4).

obtained by averaging SAR within a 1-g cube underlying themicrosystem and incorporating both dura/gray and white matterlayers 1030 kg/m . along the surface of thetissue mass is obtained by averaging SAR within distal skinand bone strata ( 1100 kg/m for skin and 1800 kg/mfor bone). Average 1-mg SAR is also computed atvarious depths of the phantom.

D. Empirical Derivation of SAR

Thermographic measurements are performed to empiricallyestimate SAR and involve recording the temperature rise afl-iated with the induced radiating wireless elds through the useof an infrared (IR) camera [22]�–[24]. The temperature elevationmay be directly correlated to SAR by

(4)

where [J/(kg K)] is specic heat of the sample and (seconds)is duration of exposure to radiation. is 1300 J/(kg Kfor bone and 3750 J kg K for brain phantoms [23], [24].Equation (4) is valid when thermal diffusion is negligible and

is linearly related to . Deviations caused by thermal dif-fusion may be reduced by generating sufcient (increasing) and reducing lag time between procuring IR images and ter-

mination of . A nearly constant may be achieved bylimiting . is set at 20 W throughout testing to achieve suf-cient and was measured as a function of demon-strating near linear for seconds, as shown in Fig. 4.Throughout testing, backscattering functionality was veried atthis higher operating power (note�—maximum power suppliedby interrogator for normal operation is 50 mW) by conrmingacquisition of through a spectrum analyzer.Unlike existing procedures for thermography-based SAR

measurement [23], measurements of the present work involve aradiating implant. Phantom layers emulating the dielectric andthermal properties of individual human head tissue layers are

Fig. 5. Setup for IR-based measurement of SAR induced by operation of mi-crosystem implanted inside human-head mimicking phantom. The microsystemis implanted in the gray/dura layer towards the exposed crosssectional surface.Phantom cross-sectional surface is spray painted black to normalize apparentemissivity for IR measurements. The external antenna is separated from thephantom by thin plastic wrap ( 0.5 mm) to estimate worst case SAR. The IRcamera (lower right) captures thermographic images for subsequent determina-tion of SAR.

constructed using pre-established techniques [22]�–[24]. Theirdielectric properties are measured using a dielectric coaxialprobe (Agilent 85070D) and closely correspond to equivalentreal tissue layers (Table I). The dura matter and gray matterlayers are merged for ease of experimental testing as the duralayer is inherently thin ( 0.5 mm) and has little inuence onradiating EM elds as may be corroborated by their similardielectric properties [27]. The phantom is sliced across the im-plant axis to measure surface SAR distribution along the mainradiating axis of the implant and across the various phantomstrata. An IR camera (FLIR, SC620) possessing a resolution of640 480 or equivalent 0.09 0.12 mm spatial resolution,is used for thermographic measurements. A thin layer of blackpaint is sprayed onto exposed phantom halves to normalizeemissivity ( ) to 0.95 across the various layers and reduceerrors caused by apparentmeasured by the IR camera. Plastic wrap is used to separatethe phantom surface from the underlying external antenna andcreate an effective separation of 0.3�–0.5 mm that closelymatches the air gap used in computational measurements.The procedure for measuring thermal rise during wirelessoperation involves: 1) capturing an IR image of the surfaceof the microsystem enclosed phantom half to measure initialtemperature, , distribution; 2) merging two phantom halves,turning on function generator to supply (50 mV ) tomicrosystem, and turning on signal generator and microwaveamplier to supply (20 W) to the external antenna (LTSA);3) maintaining wireless operation and conrm backscatteringoperation by concurrent monitoring of wirelessly retrieved

in spectrum analyzer; and 4) after designated (45 to60 s), detaching secondary phantom half and capturing an IRimage of the radiation-exposed primary phantom half surface tomeasure nal temperature and obtain targeted temperaturerise . Convective cooling caused by phantomsplitting in step (iv) is expected to be minimal (i.e., of the orderof 0.1 C) and within the IR camera�’s resolving temperaturerange. The experimental setup is illustrated in Fig. 5.

2174 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 61, NO. 5, MAY 2013

Fig. 6. (a) Computed SAR distribution on cross section of phantom (50 mm40 mm) for primary transmit (uplink) mode at 2.45 GHz for

50 mW. (b) Computed SAR for secondary backscattering (downlink) mode at4.9 GHz using a specied backscatter power of 0.158 mW irradiated

from the microsystem as derived by factoring computed (idealized) wirelesspropagation and mixer conversion loss ( 25 dB).

III. RESULTS

A. FEA Computed SAR

SAR data are obtained for isolated primary downlink andsecondary uplink modes of the wireless backscattering neu-rorecording operation. All simulations are performed with a setsource power of 50 mW, which represents the highestpower employed in practical operation of the microsystem [11],[15]. The simulation of the uplink backscattering channel ismade under the consideration that the onboard nonlinear mixerpossesses an idealized conversion gain of 5 dB be-tween translation of incident carrier to third-order harmonicoutput products at . Concomitantly, given a computedoptimal downlink wireless path loss of 20 dB at fac-tored in with the aforementioned mixer conversion loss, thepower backscattered by the microsystem antenna at isspecied as 0.158 mW. The peak local SAR observed in thesurrounding dura/gray mass of the microsystem is 4.648 W/kg

0.216 W/kg for the primary downlink mode at2.45 GHz and 0.6 W/kg 0.0318 W/kg for

the downlink mode at 4.9 GHz. The spatial SAR dis-tribution for the fundamental transmit mode and for secondarybackscatter mode is shown in Fig. 6(a) and (b), respectively.

Fig. 7. Spatial SAR distribution measured using thermographic approach. Im-planted microsystem (not visible) is located inside dura/gray layer. Higher SARdistribution is observed to concentrate towards the external skin layers of thephantom and decay rapidly towards more critical brain tissue mass.

B. Empirical SAR

Thermal data taken from the IR camera are exported toMATLAB where measured over specied are convertedto correlated SAR based on (4). The evaluated SAR data arere-normalized to a source power of 50 mW, the max-imum operating power of the external interrogator to activatemicrosystem. The plotted local SAR distribution for operationof the microsystem in the constructed phantom is shown inFig. 7. In order to predict average volumetric SAR values cor-responding to the computed average and , themeasured surface SAR distribution is approximated to have anequal distribution along its orthogonal plane. This coarse inter-pretation may constitute the maximum possible average SARas the assimilated SAR elds rapidly decay away from the axisof the microsystem, where thermal measurements are taken.As follows, the areal SAR distribution (1 mm and 1 cm ) isevaluated alongside average volumetric SAR ( 1 mm and1 cm ). Based on these assessments, the maximumis 0.45 0.11 W/kg, which is, as expected, higher thanthe computed value (0.216 and 0.102 W/kg for primary andsecondary modes, respectively). is plotted relativeto depth into the phantom mass in comparison to computedvalues, as shown in Fig. 8. computed for two dif-ferent vertical axes aligned with the microsystem are includedin the plot to adjust for variation in SAR. The experimentallyestimated average and maximum local SAR data are listedalongside computed data in Table II.

IV. DISCUSSION

The computed and measured associated with opera-tion of the fully passive wireless neurorecording microsystem(0.216 W/kg computed and 0.45 0.11 W/kg measured at

50 mW) in the human head equivalent phantom com-plies with formal U.S. FCC standards and informal clinicalguidelines [20], [21] and is substantially lower in comparisonto available applicable studies of SAR for implantable wirelesssystems operating at or near microwave frequencies [28], [29].Unlike EM propagating modes, inductive coupling schemes,

SCHWERDT et al.: EM FIELDS INDUCED IN OPERATION OF A PASSIVE NEURORECORDING MICROSYSTEM IN EMULATED HUMAN HEAD TISSUE 2175

Fig. 8. Computed and measured maximum average as a function ofpenetration depth into multilayered tissue volume. Computed for twodifferent vertical axes aligned with implanted microsystem are included.

TABLE IISUMMARY OF SAR MEASUREMENTS

The primary and secondary columns refer to values computed forthe separated primary uplink transmit and secondary downlinkbackscattering modes of the wireless neurorecording operation. Brain(1 g) SAR value corresponds to average between dura/gray and white matterlayers associated with tissue mass immediately surrounding implant. Skin (1g) SAR value corresponds to average between skin and bone volumes.

more commonly employed in implant systems, primarily gen-erate magnetic ux, whereby induced wireless power may bevirtually negated. As such, it is inconsequential to compare theSAR measurements of the current work to those of inductivecoupling systems. Existing implant studies employing EM cou-pling antennas, however, suggest signicantly higherof 14.5 and 19.1 W/kg (289 and 382 W/kg at 1 W) atlower microwave frequencies (0.9�–2.45 GHz) when adjustedfor an equivalent power of 50 mW [28], [29]. The higher

is largely owing to the fact that these studies referenceto power supplied at the implant, while the wireless fully-pas-sive microsystem of this work excludes powering componentsas it is exclusively activated by the external interrogator. Inorder to conform to FCC regulated 1.6 W/kg,the existing studies suggest limiting input implant power to1�–4.2 mW, but despite these restrictions, the potentially

higher SAR associated with the induced EM elds generated

by an external interrogator to activate implant functionalityremains nonetheless unaccounted for.The higher SAR measured within the bone layer may be due

to a combination of thermal diffusion, moisture loss across thephantom surface throughout the course of testing, diffusion ofmoisture from neighboring skin and dura/gray layers increasingthermal conductivity at the interface, and unadjusted specicheat capacity, , for the bone mass. is xed at 3750 J kg Kfor all experimental SAR estimations (as taken from reportedthermal properties of skin, gray matter, and white matter phan-toms) despite the considerably different 1300 J kg Kfor bone medium [22], [27]. Due to these uncertainties, the ap-parent discontinuity observed in computed SAR distribution be-tween skin and bone strata does not manifest itself in the empir-ical distribution plot. Moreover, the higher empiricaland peak local SAR values observed at deeper strata of thephantom are most likely caused by nite thermal diffusion ofthe higher SAR established in the skin layer. These discrepan-cies may be subdued by using a higher output power and lower.As the microsystem involves operation in the near-eld

regime, variations in individual phantom layer thicknesseswould alter SAR considerably [17]. When the propagation dis-tance is on the order of the wavelength, interference phenomenaof the impinging EM elds become increasingly prominent.Nevertheless, the present study targets a realistic and worst caseinterpretation of SAR distribution in the ultimately intendedsubcranial implant setting and does not account for the highlyvariable thicknesses that may be more inuential for applicationin other parts of the human body.

V. CONCLUSIONCharacterization of the highest possible SAR generated

in operation of the fully passive wireless neurorecordingmicrosystem demonstrates compliance with current safetythresholds [20], [21] and suggests signicantly reduced SARin the vicinity of critical underlying brain tissue layers incomparison to existing SAR studies [28], [29]. Furthermore,as may be visualized in Fig. 6�–8, it has been shown that SARcontributions are primarily dominated by the radiating source(external interrogator) rather than the implanted backscatteringmicrosystem, as expected. In consequence, SAR distributionmeasurements reveal that induced EM elds are concentratedtowards the skin surface, and rapidly decay towards morecritical brain tissue mass. This externalization of SAR maycontrast with inductively coupled or other wireless schemescontaining active on-chip electronics, where dissipated heatmay be localized towards implant surrounding due to internalpowering components [19]. Ongoing and future work involvesenhancing sensitivity of the fully passive neurorecordingmicrosystem to acquire low-level cortical neuropotentials

10�–100 V as well as produce higher amplitudebackscattering products to potentially extend the wirelesspropagation distance and further reduce its thermal footprint.

ACKNOWLEDGMENTThe authors would like to thank Dr. F. W. Van Keuls,

NASA Glenn Research Center (GRC), Cleveland, OH,

2176 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 61, NO. 5, MAY 2013

USA, for assistance in experimental testing, Dr. D. Zhuand Dr. R. E. Martin, NASA GRC, for assistance with ther-mographic measurements, and Dr. B. D. Bauman, InhanceFluoro-Seal, Ltd., for supplying samples of polyethylenepowder used for phantom preparation.

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Helen N. Schwerdt received the B.S. and M.S.E. degrees from Johns Hop-kins University, Baltimore, MD, USA, in 2008 and 2009, respectively. She iscurrently working toward the Ph.D. degree in electrical engineering at ArizonaState University, Tempe, AZ, USA.

Félix A. Miranda received the Ph.D. degree in physics from Case Western Re-serve University, Cleveland, OH, USA, in 1991.He is currently the Chief of the Antenna and Optical Systems Branch, NASA

Glenn Research Center, Cleveland, OH, USA. His research areas of interest areantenna technology, high-transition-temperature superconductors, ferroelectrictunable microwave components, and microwave integrated circuits and devicesfor space and ground-based communications. He has authored or coauthoredmore than 160 technical publications and several book chapters and is the coin-ventor of six U.S. patents, with three patents pending.Dr. Miranda was the recipient of the 2007 NASA Exceptional Service Medal

for outstanding technical and managerial leadership in antenna and microwavetechnologies for space communications.

Junseok Chae received the Ph.D. degree in electrical engineering and computerscience from the University of Michigan, Ann Arbor, MI, USA, in 2003.He joined Arizona State University, Tempe, AZ, USA, as an Assistant Pro-

fessor in electrical engineering in 2005, where he is currently an Associate Pro-fessor. His research areas of interest are MEMS for biomedical applications. Hehas authored or coauthored over 100 journal and conference articles, one book,and four book chapters and holds two U.S. patents.Prof. Chae was the recipient of the National Science Foundation CAREER

Award.