Wirelessly-Powered Cage Designs for Supporting Long-Term Experiments on Small Freely Behaving Animals in a Large Experimental Arena

electronics

Review Wirelessly-Powered Cage Designs for Supporting Long-Term Experiments on Small Freely Behaving Animals in a Large Experimental Arena

Byunghun Lee 1 and Yaoyao Jia 2,*

1 Department of Electrical Engineering, Incheon National University, Incheon 22012, Korea; [email protected] 2 Department of Electrical and Computer Engineering, North Carolina State University, 890 Oval Dr, Raleigh, NC 27606, USA * Correspondence: [email protected]; Tel.: +1-919-515-7350

Received: 22 October 2020; Accepted: 18 November 2020; Published: 25 November 2020

Abstract: In modern implantable medical devices (IMDs), wireless power transmission (WPT) between inside and outside of the animal body is essential to power the IMD. Unlike conventional WPT, which transmits the wireless power only between fixed Tx and Rx coils, the wirelessly-powered cage system can wirelessly power the IMD implanted in a small animal subject while the animal freely moves inside the cage during the experiment. A few wirelessly-powered cage systems have been developed to either directly power the IMD or recharge batteries during the experiment. Since these systems adapted different power carrier frequencies, coil configurations, subject tracking techniques, and wireless powered area, it is important for designers to select suitable wirelessly-powered cage designs, considering the practical limitations in wirelessly powering the IMD, such as power transfer efficiency (PTE), power delivered to load (PDL), closed-loop power control (CLPC), scalability, spatial/angular misalignment, near-field data telemetry, and safety issues against various perturbations during the longitudinal animal experiment. In this article, we review the trend of state-of-the-art wirelessly-powered cage designs and practical considerations of relevant technologies for various IMD applications.

Keywords: wirelessly-powered cage; inductive power transmission; implantable medical device; animal experiment

1. Introduction Implantable medical devices (IMDs) have been developed for behavioral neurosciences which research on small freely moving animal subjects, such as rodents [1–5]. Conventional hardwired IMD in Figure1a, which is restricting experiments for freely behaving animal subjects [ 1,2], have been replaced with battery-powered IMDs. However, the battery needs be replaced after 2–4 h animal experiments [3,4], resulting in the interruption for continuous and smooth ﬂow of the experiments, as shown in Figure1b. When the IMD is fully implanted inside the animal body, the risk for replacing the battery dramatically increases due to the potential infection in the animal body during the surgery.

Electronics 2020, 9, 1999; doi:10.3390/electronics9121999 www.mdpi.com/journal/electronics Electronics 2020, 9, x FOR PEER REVIEW 2 of 28

weeks, or months [5]. Therefore, several concepts of the wirelessly-powered cage have been proposed to extend the wireless coverage and also provide the homogenous power transfer efficiency (PTE) while the small animal is freely behaving inside the cage, as shown in Figure 1d. The key techniques for the wirelessly-powered cage include: (1) the optimized transmitter (Tx) coil design for the extended area; (2) the closed-loop power control (CLPC) for safe wireless power transmission (WPT) [6]; (3) the compensation technique for spatial/angular misalignments of receiver (Rx) coil; (4) the animal tracking technique for the scalability of wireless coverage; and (5) near-field data transmission, while other technologies can also be considered depending on the intended medical applications. Despite the exciting current art, all the requirements of an application involving high- performance or mm-scaled IMDs cannot be addressed by a current existing wirelessly-power cage design. It is important for designers to select suitable wirelessly-powered platforms considering their practical limitations with respect to the IMD design. This review article focuses on the overview of related technologies in recent wirelessly-powered cage platforms including the fundamental principles and practical considerations and provides the guidelines for designers to customize the appropriate wirelessly-powered cages with respect to their IMD applications. The optimized wirelessly-powered cage for the target application based on this article will enable the automated, high throughput, and long-term experiments in a large number of parallel standard cages or in a cage with specific shape for single or multiple animal subjects. In this article, these key technologies for wirelessly-powered cages are categorized and discussed with the practical design considerations that include the CLPC, coil design/optimization, scalability for wireless coverage, special/angular misalignment, near-field data telemetry, and safety issues. In Section 2, the main blocks for wirelessly-powered cage systems are introduced with the practical considerations related to the key technologies. Section 3 provides the different designs of Electronicswirelessly-powered2020, 9, 1999 cages with the performance comparison. Section 4 introduces the state-of-the-art2 of 28 wirelessly-powered cages for mm-sized IMDs, followed by a conclusion.

(a) (b)

FigureFigure 1. 1.Neural Neural interfacesinterfaces categorized by by power power sources: sources: (a) ( ahard-wired,) hard-wired, (b) (bbattery-powered,) battery-powered, (c) (c)wirelessly-powered wirelessly-powered at ata fixed a ﬁxed distance, distance, and and(d) wirelessly-powered (d) wirelessly-powered neural neuralinterfaces interfaces for freely- for freely-movingmoving animal. animal.

2. MainIn an Blocks attempt for to Wirelessly-Powered overcome the limitations Cage System imposed by the replacement of battery in IMDs, the wirelessly-poweredFigure 2 shows a cagesimplified systems block have diagram been developed of the wirelessly-powered to recharge the batteries cage system without and detaching the IMD theattached IMD from to or the implanted animal duringin the freely-moving the experiment. anim However,al subject. sinceThe wirelessly-powered these systems are designedcage system to providetypically the includes wireless a charging main controller, in the fixed a power distance, amplifier as shown (PA),in a pair Figure of 1datac, similar Tx/Rx, to a theTx mobilecoil array, phone and chargers, they are still not suitable for longitudinal animal studies over the span of several days, weeks, or months [5]. Therefore, several concepts of the wirelessly-powered cage have been proposed to extend the wireless coverage and also provide the homogenous power transfer efficiency (PTE) while the small animal is freely behaving inside the cage, as shown in Figure1d. The key techniques for the wirelessly-powered cage include: (1) the optimized transmitter (Tx) coil design for the extended area; (2) the closed-loop power control (CLPC) for safe wireless power transmission (WPT) [6]; (3) the compensation technique for spatial/angular misalignments of receiver (Rx) coil; (4) the animal tracking technique for the scalability of wireless coverage; and (5) near-field data transmission, while other technologies can also be considered depending on the intended medical applications. Despite the exciting current art, all the requirements of an application involving high-performance or mm-scaled IMDs cannot be addressed by a current existing wirelessly-power cage design. It is important for designers to select suitable wirelessly-powered platforms considering their practical limitations with respect to the IMD design. This review article focuses on the overview of related technologies in recent wirelessly-powered cage platforms including the fundamental principles and practical considerations and provides the guidelines for designers to customize the appropriate wirelessly-powered cages with respect to their IMD applications. The optimized wirelessly-powered cage for the target application based on this article will enable the automated, high throughput, and long-term experiments in a large number of parallel standard cages or in a cage with specific shape for single or multiple animal subjects. In this article, these key technologies for wirelessly-powered cages are categorized and discussed with the practical design considerations that include the CLPC, coil design/optimization, scalability for wireless coverage, special/angular misalignment, near-field data telemetry, and safety issues. In Section2, the main blocks for wirelessly-powered cage systems are introduced with the practical considerations related to the key technologies. Section3 provides the di fferent designs of wirelessly-powered cages with the performance comparison. Section4 introduces the state-of-the-art wirelessly-powered cages for mm-sized IMDs, followed by a conclusion. Electronics 2020, 9, 1999 3 of 28

2. Main Blocks for Wirelessly-Powered Cage System Figure2 shows a simplified block diagram of the wirelessly-powered cage system and the IMD attachedElectronics to2020 or, 9 implanted, x FOR PEER inREVIEW the freely-moving animal subject. The wirelessly-powered cage system3 of 28 typically includes a main controller, a power amplifier (PA), a pair of data Tx/Rx, a Tx coil array, and a positiona position sensor sensor to localize to localize the Rx the coil Rx in coil the IMD. in the The IMD. wireless The link wireless for the link power for theand power data transmission and data istransmission established betweenis established the Tx between coil array the and Tx the coil Rx array coil throughand the Rx the coil skin through/air while the the skin/air PA driving while the the Tx coilPA arraydriving tuned the Tx at thecoil power array tuned carrier at frequency, the powerf pcarrier. The amount frequency, of wireless fp. The amount power drivenof wireless from power the PA isdriven typically from controlled the PA is by typically the main controlled controller, by namely the main PC controller, or microcontroller namely PC unit or (MCU),microcontroller depending unit on the(MCU), amount depending of received on the power amount by of the received Rx. Unlike power the by WPT the Rx. system Unlike including the WPT Txsystem and including Rx coils with Tx fixedand Rx distance, coils with the fixed coupling distance, between the coupling the Tx and between Rx coils the inTx the and wirelessly-powered Rx coils in the wirelessly cage- ispowered varying duecage to is thevarying movements due to the of movements the animal subject,of the animal resulted subject, in the resulted received in the power received variations. power variations. The CLPC, composedThe CLPC, of composed the received of the data received from the data IMD, from the the main IMD, controller, the main and controller, the PA, increasesand the PA, the increases Tx power inthe the Tx PA power until in enough the PA power until enough is delivered power to is the delivered Rx while to it the reduces Rx while the Txit reduces power whenthe Tx the power Rx receives when enoughthe Rx receives power. enough power. r

Wirelessly-powered i A cage system Position /

Position n i

Rx position data Sensor k Implantable S Recording device /Control data Data Data Telemetry Data

e Data Rx/Tx l Tx/Rx c a

Telemetry a r f u Power r e e

Power t Main N n Control Power Tx Coil Rectifier I Controller Rx Coil Amplifier Array AC / Charger DC (MCU/PC) power Data Coil Selection

FigureFigure 2.2.Simpliﬁed Simplified block block diagram diagram of of the the wirelessly-powered wirelessly-powered cage cage system system as as a stationarya stationary unit unit and and the IMDthe IMD as a mobileas a mobile unit unit attached attached to or to implanted or implanted in the in the freely-moving freely-moving animal animal subject. subject.

TheThe TxTx coilcoil arrayarray isis oneone ofof thethe importantimportant design considerations in in the the wirelessly wirelessly-powered-powered cage cage systemsystem to to provideprovide thethe highhigh andand homogeneoushomogeneous PTE and power power delivered delivered to to the the load load (PDL) (PDL) within within the the cage.cage. TheThe TxTx coilcoil arrayarray can be optimized for a a designated arena arena while while it it also also can can be be extended extended for for a a largerlarger areaarea withwith thethe modularmodular typetype design.design. Although the the achievable achievable PTE PTE and and PDL PDL by by the the inductive inductive linklink areare importantimportant forfor thethe TxTx coilcoil array,array, thethe homogeneityhomogeneity of of the the PTE PTE across across the the cage cage should should also also be be consideredconsidered duedue toto thethe animalanimal subject’ssubject’s freelyfreely movements.movements. In In most most of of the the wirelessly wirelessly-powered-powered cages cages withwith thethe TxTx coil array, the positi positionon sensor sensor selects selects the the nearest nearest Tx Tx coil coil among among the theTx coil Tx coilarray array to the to Rx the Rxcoil. coil. Since Since only onlythe selected the selected Tx coil Tx will coil be willactivated, be activated, this position this sensing position mechanism sensing mechanism helps to reduce helps topower reduce loss power significantly. loss significantly. Furthermore, Furthermore, each single eachTx coil single in the Tx modular coil in thedesig modularn of Tx designcoil array of is Tx coiloptimized array is with optimized the Rx coil with to theimprove Rx coil the to PTE improve within thethe entire PTE within wirelessly the- entirepowered wirelessly-powered arena. The near- arena.field data The telemetry near-field between data telemetry the Tx and between Rx coilsthe canTx be andusedRx to control coils can the be IMD, used monitor to control the received the IMD, monitorpower, and/or the received acquire power, the biomedi and/orcal acquire data from the biomedical the IMD. Even data though from the far IMD.-field Even communication though far-field has communicationthe advantage of has longer the advantage data transmission of longer distance, data transmission near-field communication distance, near-field is regarded communication as a more is regardedsuitable method as a more in suitableterms of methodpower saving in terms in ofIMDs. power Given saving that in the IMDs. IMD Givenis always that located the IMD within is always the locatedwirelessly within-powered the wirelessly-powered cage or arena, the coupling cage or arena, between the Tx coupling and Rx between coils is always Tx and enough Rx coils to is deliver always enoughboth power to deliver and data both [7]. power Here and are data some [7]. practical Here are considerations some practical for considerations different types for of di ffwirelesslyerent types- ofpowered wirelessly-powered cage used in cagethe freely used-moving in the freely-moving animal experimen animalts. experiments.

2.1. Coil Design and Optimization

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2.1. Coil Design and Optimization

2.1.1. Coil Optimization for Conventional Two-/Three-/Four-Coil Inductive Links IMDs typically have a tight limitation in their size depending on the applications, resulting in the diameter limitation of Rx coil in the body. In contrast, the Tx coil in the wirelessly-powered cage has more size relaxation in its design. In the inductive link, the coil geometries should be carefully designed to achieve the efficient inductive coupling considering the load and coil separation. Figure3 shows the physical and electrical configurations of a two-coil inductive link with geometrical parameters used for Electronics 2020, 9, x FOR PEER REVIEW 4 of 28 primary (L1) and secondary (L2) coils, where din is inner diameter of each coil, dout is outer diameter of coils, N is number of turns, and z is the coil separation between L and L coils. The mutual inductance is outer diameter of coils, N is number of turns, and z is the coil1 separation2 between L1 and L2 coils. between Tx and Rx coils (M12) is defined by M12 = k √L1L2, where k is the coupling coefficient of the The mutual inductance between Tx and Rx coils (M12) is defined by 𝑀 =𝑘𝐿𝐿, where k is the two-coil link. M shows the ratio of magnetic flux common to both L and L . R and R are the series coupling coefficient12 of the two-coil link. M12 shows the ratio of magnetic1 flux2 common1 to2 both L1 and resistance of the Tx and Rx coils, respectively. The quality factor (Q) of each coil is defined by Q = ωL/R, L2. R1 and R2 are the series resistance of the Tx and Rx coils, respectively. The quality factor (Q) of where ω = 2πf and f is the power carrier frequency. each coil is defined0 by0 Q = ωL/R, where ω = 2πf0 and f0 is the power carrier frequency.

FigureFigure 3.3. PhysicalPhysical andand electricalelectrical conﬁgurationsconfigurations in a two-coil inductive inductive link. link.

AlthoughAlthough the the two-coiltwo-coil inductiveinductive linklink hashas its optimized solution for for highest highest PTE PTE depending depending on on a a givengiven set set of ofQ 1Q, 1Q, Q2,2 and, andk12 k12based based on on [8 [8],], the the optimized optimized load load resistance, resistance,RL,PTE RL,PTE, is, is sometimes sometimes far far from from the nominalthe nominal target target load resistance, load resistance,RL, which RL, iswhich predeﬁned is predefined in the IMD in application.the IMD application.Therefore, Therefore, the multi-coil the solutionmulti-coil such solution as three- such or as four-coil three- or inductive four-coil inductive links has beenlinks widelyhas been studied widely to studied provide to theprovide designer the withdesigner more with degrees more of degrees freedom of tofreedom convert to RconvertL to RL,PTE RL toand RL,PTE thereby and thereby maximize maximize PTE, PTE, while while they they have ahave potential a potential negative negative impact impact on the on the size-constrained size-constrained applications. applications. Figure Figure4 4shows shows the the two-,two-, three-, three-, andand four-coil four-coilinductive inductive linkslinks withwith thethe lumpedlumped circuitcircuit model. The The PTE PTE of of two-, two-, three-, three-, and and four-coil four-coil inductiveinductive links links can can be be calculated calculated basedbased onon thethe basicbasic circuitcircuit theorytheory found in [9]. [9]. 𝑘 𝑄 𝑄 𝑄 2 𝜂 =𝜂 = k Q2Q3L .Q , (1) 1+𝑘23 𝑄 𝑄 𝑄3L η2 coil = η23 = , (1) − + 2 · Q 1 k23Q2Q3L L (𝑘𝑄𝑄)(𝑘𝑄𝑄) 𝑄 𝜂 =𝜂 𝜂 = . , (2) [(1 + 𝑘 𝑄 k𝑄2 Q+𝑘Q 𝑄k2𝑄Q)(1Q + 𝑘 𝑄 𝑄 )] 𝑄 23 2 3 343 4L Q4L η3 coil = η23η34 = h i , (2) − 2 2 2 Q 1 + k Q2Q3 + k Q3Q4L 1 + k Q3Q4L · L 𝜂 =𝜂.𝜂.𝜂 23 34 34 (𝑘𝑄𝑄)(𝑘𝑄𝑄)(𝑘𝑄𝑄) 𝑄 (3) = η4 coil = η12 η23 η34 ⋅ − · · [(1 + 𝑘𝑄𝑄). (1 + 𝑘(k2 𝑄QQ𝑄)()+𝑘k2 Q Q 𝑄)(k𝑄2Q]. [1Q +) 𝑘𝑄𝑄 +𝑘𝑄𝑄] 𝑄 (3) = 12 1 2 23 2 3 34 3 4L Q4L [(1+k2 Q Q ) (1+k2 Q Q )+k2 Q Q ] [1+k2 Q Q +k2 Q Q ] QL where Q3L and Q4L are the loaded12 1 2 · quality34 3 factor,4L 23 Q23L 3=· Q3Q23L/(2Q3 + Q34 L)3 and4L ·Q4L = Q4QL/(Q4 + QL), in wherewhichQ the3L andloadQ quality4L are thefactor, loaded QL = quality RL/ωL. factor,Note thatQ3L the= Q source3QL/(Q output3 + QL )resistance, and Q4L = RQS, 4isQ includedL/(Q4 + Q inL), inthe which driver the coil load resistance. quality factor, The EquationQL = RL (2)/ωL. impliesNote thatthat thethe sourcethree-coil output link resistance,gives the designersRS, is included with an in theadditional driver coil degree resistance. of freedom The Equation(k23) to adjust (2) implies the reflected that the load three-coil onto L2 link to be gives the theoptimal designers value, with RL,PTE an, compared to the two-coil link. The PTE of the three-coil inductive link is related with k23, k34, Q2, Q3, and Q4, for a given load condition. The four-coil link can provide an additional degree of freedom (k12) from the three-coil link for the impedance matching on the source side based on (3). Since the two-, three-, and four-coil inductive links have different strengths and weaknesses in the coupling coefficient (k), PDL, and coupling variations as summarized in Table 1, the designers can select the appropriate inductive link configuration depending on the specifications of the WPT system [9].

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additional degree of freedom (k23) to adjust the reflected load onto L2 to be the optimal value, RL,PTE, compared to the two-coil link. The PTE of the three-coil inductive link is related with k23, k34, Q2, Q3, and Q4, for a given load condition. The four-coil link can provide an additional degree of freedom (k12) from the three-coil link for the impedance matching on the source side based on (3). Since the two-, three-, and four-coil inductive links have different strengths and weaknesses in the coupling coefficient (k), PDL, and coupling variations as summarized in Table1, the designers can select the appropriate inductiveElectronics 2020 link, 9 configuration, x FOR PEER REVIEW depending on the specifications of the WPT system [9]. 5 of 28

(a) (b)

(c)

FigureFigure 4. 4.The The lumpedlumped circuitcircuit modelmodel forfor ((aa)) two-coil,two-coil, (b) three-coil, and ( (cc)) four-coil four-coil inductive inductive links. links.

TableTable 1. 1.Comparison Comparison betweenbetween two-,two-, three-,three-, and four-coil inductive links.

InductiveInductive Link ConﬁgurationsLink Configurations 2-Coil2-Coil 3-Coil3-Coil 4-Coil 4-Coil SizeSize constrain constrain √ √ × × × × StrongStrong coupling coupling (k)) √√ √ √ × × Weak coupling (k) √ √ Weak coupling (k) × × √ √ Application Conditions Large PDL (small Rs) √ √ Application Conditions Large PDL (small Rs) √ √ × × Small PDL (large Rs) √ Small PDL (large Rs) ×× × × √ k variation w/small Rs √ √ × k variationk variation w/small w/large RRss √ √ √ × × × k variation w/large Rs × × √

The optimization procedures of two-, three-, and four-coil inductive links start with the design The optimization procedures of two-, three-, and four-coil inductive links start with the design constrains imposed by the application and coil fabrication technology. The design constrains in the Rx constrains imposed by the application and coil fabrication technology. The design constrains in the definesRx defines the maximum the maximum outer outer diameter diameter of coils of in coils the IMD,in the and IMD, the and coil fabricationthe coil fabrication technology technology indicates theindicates minimum the minimum line width line and width line spacing. and line Dependingspacing. Depending on the application, on the application, the coil separation the coil separation between L and L (z ), the nominal load resistance (R ), and the source resistance (R ) are also determined. between2 3 L223 and L3 (z23), the nominal load resistanceL (RL), and the source resistances (Rs) are also In the two-coil optimization procedure, k Q Q should be maximized to achieve the maximum η determined. In the two-coil optimization23 procedure,2 3 k23Q2Q3 should be maximized to achieve 2-coilthe k Q Q basedmaximum on (1). η The2-coil based optimum on (1).23, The2, andoptimum3 can k be23, Q derived2, and Q by3 thecan properbe derived outer by and the inner proper diameter outer ofand Tx coil,inner inner diameter diameter of Tx of coil, Rx coil, inner and diameter number of of Rx turns coil, forand Tx number and Rx of coils turns at for given Tx designand Rx constraintscoils at given [8]. Thedesign three-coil constraints link optimization [8]. The three-coil procedure link maximizesoptimizationη23 procedureand Q4 in maximizes (2), and additionally η23 and Q4 adjustin (2),k and34 in theadditionally Rx to provide adjust the k34 maximum in the Rx to PTE, provideη3-coil .the A moremaximum detailed PTE, flow η3-coil chart. A more is discussed detailed inflow [9]. chart In this is optimizationdiscussed in procedure,[9]. In this optimization the additional procedure,L3 coil in the the additional Rx plays L the3 coil role in ofthe an Rx impedance-matching plays the role of an circuit,impedance-matching which can convert circuit, an which arbitrary can RconvertL to RL,PTE an arbitraryfor optimal RL to PTE RL,PTE compared for optimal to PTE the conventionalcompared to two-coilthe conventional link. In other two-coil words, link. the In reflected other words, load on the the reflected Tx can beload adjustable on the Tx for can maximizing be adjustable the PTEfor ifmaximizing the designer the can PTE choose if the designer the suitable can choosek23 and thek34 suitablein the k design23 and k of34 in a the three-coil design link.of a three-coil As shown link. in FigureAs shown5 which in Figure shows 5 the which exemplar shows designs the exemplar of two- de andsigns three-coil of two- inductiveand three-coil links, inductive the three-coil links, linkthe canthree-coil maintain link the can maximum maintain PTE the bymaximum adjusting PTEk34 bywhile adjusting the two-coil k34 while link the only two-coil reaches link the only optimal reaches PTE forthe a optimal specific PTERL = for200 a Ωspecific[9]. RL = 200 Ω [9].

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(a) (b) (a) (b) FigureFigure 5. 5. a(a) Optimized three-coil inductive link, and (b) PTE variation in the two-coil and three-coil Figure 5. ((a)) OptimizedOptimized three-coilthree-coil inductive link, link, and and ( (b)) PTE PTE variation variation in in the the two-coil two-coil and and three-coil three-coil inductiveinductive links links vs. vs.R RL [[9].9]. inductive links vs. RLL [9]. TheThe four-coil four-coil link link is is sometimes sometimes veryvery usefuluseful especiallyespecially for large coil distance between between the the Tx Tx and and The four-coil link is sometimes very useful especially for large coil distance between the Tx and RxRx and and the the large large source source impedance, impedance,R Rss,, sincesince itit providesprovides the additional degree degree of of freedom freedom on on the the Tx Tx Rx and the large source impedance, Rs, since it provides the additional degree of freedom on the Tx side.side. Therefore, Therefore, thethe four-coilfour-coil linklink isis widelywidely implementedimplemented in the high carrier frequency applications side. Therefore, the four-coil link is widely implemented in the high carrier frequency applications becausebecauseR Rs sin in the the PA PA is is typically typically increasedincreased inin thethe higherhigher frequency. The four-coil link link can can tolerate tolerate the the because Rs in the PA is typically increased in the higher frequency. The four-coil link can tolerate the variations in k23 caused by the coil separation varying and maintain the high PTE by keeping k12 large. variations in k23 caused by the coil separation varying and maintain the high PTE by keeping k12 large. variations in k23 caused by the coil separation varying and maintain the high PTE by keeping k12 large. TheThe four-coil four-coil link link optimization optimization maximizes maximizes the the individual individual parameters parameters of ofk k23QQQ2Q3,, QQ1,Q, Q4,, kk12 asas similar similar The four-coil link optimization maximizes the individual parameters of 23k23Q22Q33, Q1, Q44, k1212 as similar inin the the two- two- and and three-coil three-coil link link optimization. optimization. Then,Then, thethe optimaloptimal kk34 isis chosen chosen to to provide provide the the maximum maximum in the two- and three-coil link optimization. Then, the optimal k3434 is chosen to provide the maximum PTE as discussed in [9]. The optimization geometries of two-, three-, or four-coil links should satisfy PTEPTE asas discusseddiscussed inin [[9].9]. The optimizationoptimization geometries of of two-, two-, three-, three-, or or four-coil four-coil links links should should satisfy satisfy the specific absorption rate (SAR) limit which can be verified by a field solver. If the resulted design thethe specific specific absorptionabsorption raterate (SAR)(SAR) limit which can can be be verified verified by by a a field field solver. solver. If If the the resulted resulted design design cannot satisfy the SAR limit, the designer needs to modify the design constraints and perform the cannotcannot satisfysatisfy thethe SARSAR limit,limit, thethe designerdesigner needs to modify the the design design constraints constraints and and perform perform the the optimization procedure again. The segmented coil design in [10] helps to reduce the average SAR optimizationoptimization procedureprocedure again.again. The segmented segmented coil coil design design in in [10] [10 ]helps helps to to reduce reduce the the average average SAR SAR while the loss of the overall link is decreased by using a segmented Tx coil. In Figure 6, the segmented whilewhile the the loss loss ofof thethe overalloverall linklink is decreased by us usinging a segmented segmented Tx Tx coil. coil. In In Figure Figure 6,6, the the segmented segmented coil shows a more uniform E-field distribution compared to a normal coil with the same geometry, coilcoil showsshows aa moremore uniformuniform E-fieldE-field distribution compared compared to to a a normal normal coil coil with with the the same same geometry, geometry, resulting in the reduced peak E-field. Therefore, more Tx power is allowable under the same tissue resultingresulting inin thethe reducedreduced peakpeak E-field.E-field. Therefore, more Tx Tx power power is is allowable allowable under under the the same same tissue tissue environment and SAR limit. environmentenvironment andand SARSAR limit.limit.

(a) (b) (a) (b) Figure 6. E-field distribution in V/m at skin surface for (a) a conventional coil and (b) a segmented Figure 6. E-field distribution in V/m at skin surface for (a) a conventional coil and (b) a segmented Figurecoil [10]. 6. E-field distribution in V/m at skin surface for (a) a conventional coil and (b) a segmented coilcoil [[10].10]. Although previous studies provide the optimization of two-, three-, or four-coil inductive link AlthoughAlthough previous studies studies provide provide the the optimizati optimizationon of two-, of two-, three-, three-, or four-coil or four-coil inductive inductive link [8,9], they only focus on the optimization procedure for fixed Tx and Rx coils that is not simply link[8,9], [8 ,they9], they only only focus focus on onthe the optimization optimization procedur proceduree for forfixed fixed Tx Txand and Rx Rx coils coils that that is isnot not simply simply applicable for powering large arena in the cage. If the designer uses the large Tx coil around the cage, applicableapplicable forfor poweringpowering largelarge arenaarena in the cage. If If the the designer designer uses uses the the large large Tx Tx coil coil around around the the cage, cage, the overall PTE will significantly drop and show large variations depending on the location of the the overall PTE will significantly drop and show large variations depending on the location of the theIMD. overall Several PTE willapproaches significantly have drop been and studying show large to improve variations the depending PTE from on theTx locationto Rx coils of the while IMD. IMD. Several approaches have been studying to improve the PTE from Tx to Rx coils while maintaining the homogeneity of wireless power distribution. These approaches can be mainly maintaining the homogeneity of wireless power distribution. These approaches can be mainly

Electronics 2020, 9, x FOR PEER REVIEW 7 of 28

classified into two categories: modular design of coil array, as shown in Figure 7, and resonance- based multi-coil inductive link, as shown in Figure 8 in the following section.

2.1.2. Coil Optimization for Inductive Links Implemented in Wirelessly-Powered Cages For the abovementioned two categories of inductive link configurations, the optimization method for each category is different. First, we talk about the optimization method for the inductive link incorporating Tx coil array [11,12]. Instead of having a single Tx coil, identical Tx coils are repeated and tiled at the bottom of the cage so that wireless power transmission covers the entire cage arena. One of the Tx coils, which is closest to the Rx, is activated, and together with the Rx coil forms a two-coil inductive link. Therefore, the optimization of the Tx coil and the Rx coil can refer to the optimization procedure of the conventional two-coil inductive link. Furthermore, other strategies in terms of the Tx coil array design are implemented to improve the homogeneous distribution of the electromagnetic (EM) field within the entire cage arena. As shown in Figure 7a, the effective area of a single Tx coil, where most of transmitted power are focalized, is located at the center of the Tx coil and has same distances from the edges of the Tx coil. The PTE drops at the boundaries of the adjacent Tx coils. Multi-layer Tx coil array is typically utilized to provide uniform power transmission over large areas. Instead of fully overlapping, one layer is shifted from the other layer, so that the point of intersection of every three adjacent coils is on the center of the coil in the previous layer, as shown in Figure 7a. With the configuration of multi-layer Tx coil array, the effective areas of the Tx coils cover the entire cage arena. Moreover, the Tx coil array should cover larger area than the cage arena so that the edges of the cage arena are still covered by the multi-layer Tx coil array for homogeneous wireless power transmission. As shown in Figure 7b, the overall PTE distribution across the multi-layer Tx coil array has variations within ±24% of the average PTE. The higher PTE peaks are resulted from the Tx coils on layer 1, while the lower peaks are associated with the Tx coils in layers 2 and 3. On the one hand, the layer 1 is slightly closer to the Rx coil. On the other hand, the Tx coils in layers 2 and 3 are more overlapped and surrounded by other Tx coils. This condition leads to larger parasitic capacitance and Electronics 2020, 9, 1999 7 of 28 resistance, resulting in the lower Q and PTE. Figure 7c shows the PDL distribution when the Rx is swept within the cage at the height of 70 mm. Thanks to the proposed configuration of multi-layer SeveralTx coil approachesarray together have with been the studying CLPC tomechanism, improve thethe PTE PDL from can Txbe tomaintained Rx coils while at 20 maintaining mW with thefluctuations homogeneity of less of wirelessthan 2 mW. power It distribution.should be no Theseticed approachesthat one of canthe bemain mainly disadvantages classiﬁed intoin this two categories:modular system modular is designthat one of coilPA is array, required as shown for each in Figure driving7, and coil resonance-based in the coil array, multi-coil resulting inductive in the link,increased as shown complexity in Figure and8 in cost the of following the Tx design. section.

Electronics 2020, 9, x FOR PEER REVIEW 8 of 28 (a)

(b) (c)

ElectronicsFigureFigure 2020 7. , (9a, )(x aThe FOR) The configuration PEER conﬁguration REVIEW of the of scalable the scalable multi-layer multi-layer Tx coil array Tx implemented coil array implemented in the wirelessly- in the9 of 28 wirelessly-poweredpowered cage [11,12]. cage (b) [Measured11,12]. (b )PTE Measured distribution PTE within distribution the cage within arena the when cage the arena Rx is when at the the powerRxheight isTx. at of theTherefore, 70 height mm from of 70the the mm designer bottom from theof needsthe bottom cage to of[11,12]. ch theoose cage (c ) the [Measured11 ,12coil]. (cdesign) PDL Measured distribution and PDL optimization distribution within the procedurecage within whetherthearena cage to when adopt arena the whena Rx modular is theswept Rx coil at is sweptthe design height at theor of a height70 specific mm ofwith 70 coil mmCLPC design with set CLPCatdedicated 20 mW set at[11,12]. to 20 a mW designated [11,12]. area.

The other type of coil optimization is relevant to the inductive link incorporating Tx and/or Rx resonator, for instance the resonance-based four-coil inductive link implemented in the EnerCage- HC system families [13,14]. The key factor in determining the Tx resonator geometries in EnerCage- HC system is the compatibility with dimensions of the standard-sized rodent homecage and the maximum overlap with the Tx coil. Instead of having an array of identical Tx coils tiled at the bottom of the cage, in the EnerCage-HC system, multiple Tx resonators wrap around the cage to provide wireless power coverage of the entire cage (see Figure 8). In this case, optimizing the four-coil inductive link means increasing the minimum PTE within the homecage to ensure PDL is enough to keep the headstage on when the CLPC adjusts the Tx power, as opposed to maximizing PTE in the perfectly aligned regions in traditional coil optimization. The coupling between loosely coupled Tx and Rx coils is the dominant factor in determining the PTE of the four-coil inductive(a )link. Since the size of the headstage is considerably(b) smaller than the homecage, the effective area of the Rx resonators should be maximized so that more Tx magnetic flux Figure 8. Cont. can pass through the Rx resonator, thereby improving the coupling between the Tx and Rx resonators. In [15], the Rx resonator wrap around the headstage, maximizing the area encompassed by the Rx resonator without enlarging the size of the headstage, as shown in Figure 8a. In [14], the largest possible area of the headstage is the diagonal planes of the headstage cube, therefore, the Rx resonators are tilting an angle of 25° compared to the horizontal plane in each four directions of the cubical headstage, as shown in Figure 8b. Due to the large separation and size difference between the Tx and Rx structures, the Tx and Rx resonators are loosely coupled. Hence, a single target resonance frequency for this system can be set, regardless of the Rx location in the homecage. Additionally, because of the strong coupling among the Tx resonators, only one Tx resonator needs to be finely tuned to match the resonance frequency of the entire Tx structure with the target carrier(c) frequency. Such practical and convenient characteristics are also applicable on the Rx resonators, which are also strongly coupled with each Figure 8. The configuration of the resonance-based four-coil inductive link implemented in (a) the other. EnerCage-HC system [13] and (b) the EnerCage-HC2 system [14]. (c) Measured PTE when the Figure 8c shows the PTE distribution within the 3D volume of the cage. As we can see that while headstage is swept inside the homecage at the heights of 4 cm, 8 cm, 12 cm, 16 cm, and 20 cm. the center area has a weaker magnetic flux density, mutual coupling, and thereby lower PTE, the PTE measured2.2. Closed-Loop at each Power height Control is more (CLPC) uniform, with smaller variations of less than 7%. Although the PTE reduces as the height increases, the deduction of the PTE is slowed, which is mainly credited to the enhancementCompared of toEM the field wireless by the power Tx resonator transmission at the system top of betweenthe cage. the Although fixed Tx the and optimization Rx coils, as proceduresshown in Figure in [13,14] 1c, the are Rx only coil dedicated attached to to the the anim specifalic body geometry continuously of the cage, moves which inside is difficultthe wirelessly- to be extendedpowered forcage a largeresulting arena, in thesethe coupling techniques variation show highbetween and Txhomogeneous and Rx coils. PT InE insideaddition, the thestandard power geometryconsumption of cage. in the Besides, mobile only device one isPA typically is needed, not which constant can significantlyfor recording simplify or stimulation the design operation. of the Therefore, CLPC, which can dynamically compensate for coupling distance and load variations due to animal movements and implant functions, is required to provide enough power for the mobile

device [16]. When the mobile device receives more than enough power, the CLPC reduces the Tx power automatically to minimize the power dissipation on the Tx and the EM exposure on the animal subject, resulting in the improvement of wireless link efficiency and ensuring safety. The CLPC is typically composed of the data communication channel from the Rx to the Tx, the control unit, DC-DC converter, and the PA as shown in the exemplar design of Figure 9a. The rectifier voltage in the Rx, Vrec, is monitored and sent to the Tx through the data communication channel which can be either near-field or far-field data communication. The control unit, such as a microcontroller, in the Tx collects the rectifier information through data demodulator block and determines whether the Rx receives enough power or not. If the Rx is not receiving sufficient power, the microcontroller controls the digital potentiometer to reduce the feedback voltage of DC-DC converter. Then, the DC- DC converter increases the PA supply voltage, VDD_Tx, to increase the amount of transmitted power. Otherwise, the microcontroller adjusts the digital potentiometer to decrease VDD_Tx if the Rx is receiving surplus power. Figure 9b shows the exemplar operation of CLPC in [16]. The CLPC starts to increase the transmitted power by increasing VDD_Tx when the rat moved to low PTE areas or stood

Electronics 2020, 9, x FOR PEER REVIEW 9 of 28

power Tx. Therefore, the designer needs to choose the coil design and optimization procedure whether to adopt a modular coil design or a specific coil design dedicated to a designated area.

Electronics 2020, 9, 1999 8 of 28 (a) (b)

(c)

FigureFigure 8.8. TheThe conﬁgurationconfiguration of the resonance-based four-coil four-coil inductive inductive link link implemented implemented in in (a ()a )the the EnerCage-HCEnerCage-HC system system [13 [13]] and and (b) the(b) EnerCage-HC2the EnerCage-HC2 system system [14]. (c[14].) Measured (c) Measured PTE when PTE the when headstage the isheadstage swept inside is swept the homecageinside the athomecage the heights at th ofe 4heights cm, 8 cm,of 4 12cm, cm, 8 cm, 16 cm,12 cm, and 16 20 cm, cm. and 20 cm.

2.1.2.2.2. Closed-Loop Coil Optimization Power Control for Inductive (CLPC) Links Implemented in Wirelessly-Powered Cages ForCompared the abovementioned to the wireless two power categories transmission of inductive system link configurations,between the fixed the optimizationTx and Rx coils, method as forshown each in category Figure 1c, is the diff Rxerent. coil attached First, we to talk the anim aboutal the body optimization continuously method moves forinside the the inductive wirelessly- link incorporatingpowered cage Tx resulting coil array in the [11, 12coupling]. Instead variation of having between a single Tx Txand coil, Rx identicalcoils. In addition, Tx coils are the repeated power andconsumption tiled at the in bottom the mobile of the device cage sois thattypically wireless not constant power transmission for recording covers or stimulation the entire operation. cage arena. OneTherefore, of the CLPC, Tx coils, which which can is dynamically closest to the compensate Rx, is activated, for coupling and together distance withand load the Rxvariations coil forms due a two-coilto animal inductive movements link. and Therefore, implant functions, the optimization is required of the to Txprovide coil and enough the Rxpower coil canfor the refer mobile to the optimizationdevice [16]. When procedure the mobile of the conventionaldevice receives two-coil more than inductive enough link. power, Furthermore, the CLPC other reduces strategies the Tx in termspower of automatically the Tx coil array to minimize design arethe implementedpower dissipatio ton improve on the Tx the and homogeneous the EM exposure distribution on the animal of the electromagneticsubject, resulting (EM) in the field improvement within the entireof wire cageless arena.link efficiency As shown and in ensuring Figure7a, safety. the e ffective area of a singleThe Tx coil,CLPC where is typically most of composed transmitted of powerthe data are commu focalized,nication is located channel at thefrom center the Rx of theto the Tx Tx, coil the and hascontrol same unit, distances DC-DC from converter, the edges and the of thePA Txas shown coil. The in the PTE exemplar drops at design the boundaries of Figure 9a. of The the rectifier adjacent Txvoltage coils. in Multi-layer the Rx, Vrec, Tx is monitored coil array isand typically sent to the utilized Tx through to provide the data uniform communication power transmission channel which over largecan be areas. either Instead near-field of fully or far-field overlapping, data communication. one layer is shifted The fromcontrol the unit, other such layer, as soa microcontroller, that the point of intersectionin the Tx collects of every the threerectifier adjacent information coils is thro on theugh center data demodulator of the coil in block the previous and determines layer, as whether shown in Figurethe Rx7 a.receives With theenough configuration power or ofnot. multi-layer If the Rx is Tx not coil receiving array, the sufficient e ffective power, areas the of the microcontroller Tx coils cover thecontrols entire the cage digital arena. potentiometer Moreover, the to Txreduce coil arraythe fe shouldedback covervoltage larger of DC-DC area than converter. the cage Then, arena the so DC- that theDC edges converter of the increases cage arena the are PA still supply covered voltage, by the VDD_Tx multi-layer, to increase Tx coil the arrayamount for of homogeneous transmitted power. wireless powerOtherwise, transmission. the microcontroller adjusts the digital potentiometer to decrease VDD_Tx if the Rx is receivingAs shown surplus in power. Figure7 Figureb, the 9b overall shows PTE the distributionexemplar operation across theof CLPC multi-layer in [16]. Tx The coil CLPC array starts has variationsto increase within the transmitted24% of thepower average by increasing PTE. The V higherDD_Tx when PTE the peaks rat moved are resulted to low from PTE areas the Tx or coils stood on ± layer 1, while the lower peaks are associated with the Tx coils in layers 2 and 3. On the one hand, the layer 1 is slightly closer to the Rx coil. On the other hand, the Tx coils in layers 2 and 3 are more overlapped and surrounded by other Tx coils. This condition leads to larger parasitic capacitance and resistance, resulting in the lower Q and PTE. Figure7c shows the PDL distribution when the Rx is swept within the cage at the height of 70 mm. Thanks to the proposed configuration of multi-layer Tx coil array together with the CLPC mechanism, the PDL can be maintained at 20 mW with fluctuations of less than 2 mW. It should be noticed that one of the main disadvantages in this modular system is that one PA is required for each driving coil in the coil array, resulting in the increased complexity and cost of the Tx design. The other type of coil optimization is relevant to the inductive link incorporating Tx and/or Rx resonator, for instance the resonance-based four-coil inductive link implemented in the EnerCage-HC system families [13,14]. The key factor in determining the Tx resonator geometries in EnerCage-HC system is the compatibility with dimensions of the standard-sized rodent homecage and the maximum overlap with the Tx coil. Instead of having an array of identical Tx coils tiled at the bottom of the cage, Electronics 2020, 9, 1999 9 of 28 in the EnerCage-HC system, multiple Tx resonators wrap around the cage to provide wireless power coverage of the entire cage (see Figure8). In this case, optimizing the four-coil inductive link means increasing the minimum PTE within the homecage to ensure PDL is enough to keep the headstage on when the CLPC adjusts the Tx power, as opposed to maximizing PTE in the perfectly aligned regions in traditional coil optimization. The coupling between loosely coupled Tx and Rx coils is the dominant factor in determining the PTE of the four-coil inductive link. Since the size of the headstage is considerably smaller than the homecage, the effective area of the Rx resonators should be maximized so that more Tx magnetic flux can pass through the Rx resonator, thereby improving the coupling between the Tx and Rx resonators. In [15], the Rx resonator wrap around the headstage, maximizing the area encompassed by the Rx resonator without enlarging the size of the headstage, as shown in Figure8a. In [ 14], the largest possible area of the headstage is the diagonal planes of the headstage cube, therefore, the Rx resonators are tilting an angle of 25◦ compared to the horizontal plane in each four directions of the cubical headstage, as shown in Figure8b. Due to the large separation and size difference between the Tx and Rx structures, the Tx and Rx resonators are loosely coupled. Hence, a single target resonance frequency for this system can be set, regardless of the Rx location in the homecage. Additionally, because of the strong coupling among the Tx resonators, only one Tx resonator needs to be finely tuned to match the resonance frequency of the entire Tx structure with the target carrier frequency. Such practical and convenient characteristics are also applicable on the Rx resonators, which are also strongly coupled with each other. Figure8c shows the PTE distribution within the 3D volume of the cage. As we can see that while the center area has a weaker magnetic flux density, mutual coupling, and thereby lower PTE, the PTE measured at each height is more uniform, with smaller variations of less than 7%. Although the PTE reduces as the height increases, the deduction of the PTE is slowed, which is mainly credited to the enhancement of EM field by the Tx resonator at the top of the cage. Although the optimization procedures in [13,14] are only dedicated to the specific geometry of the cage, which is difficult to be extended for a large arena, these techniques show high and homogeneous PTE inside the standard geometry of cage. Besides, only one PA is needed, which can significantly simplify the design of the power Tx. Therefore, the designer needs to choose the coil design and optimization procedure whether to adopt a modular coil design or a specific coil design dedicated to a designated area.

2.2. Closed-Loop Power Control (CLPC) Compared to the wireless power transmission system between the fixed Tx and Rx coils, as shown in Figure1c, the Rx coil attached to the animal body continuously moves inside the wirelessly-powered cage resulting in the coupling variation between Tx and Rx coils. In addition, the power consumption in the mobile device is typically not constant for recording or stimulation operation. Therefore, CLPC, which can dynamically compensate for coupling distance and load variations due to animal movements and implant functions, is required to provide enough power for the mobile device [16]. When the mobile device receives more than enough power, the CLPC reduces the Tx power automatically to minimize the power dissipation on the Tx and the EM exposure on the animal subject, resulting in the improvement of wireless link efficiency and ensuring safety. The CLPC is typically composed of the data communication channel from the Rx to the Tx, the control unit, DC-DC converter, and the PA as shown in the exemplar design of Figure9a. The rectifier voltage in the Rx, Vrec, is monitored and sent to the Tx through the data communication channel which can be either near-field or far-field data communication. The control unit, such as a microcontroller, in the Tx collects the rectifier information through data demodulator block and determines whether the Rx receives enough power or not. If the Rx is not receiving sufficient power, the microcontroller controls the digital potentiometer to reduce the feedback voltage of DC-DC converter. Then, the DC-DC converter increases the PA supply voltage, VDD_Tx, to increase the amount of transmitted power. Otherwise, the microcontroller adjusts the digital potentiometer to decrease Electronics 2020, 9, 1999 10 of 28

VDD_Tx if the Rx is receiving surplus power. Figure9b shows the exemplar operation of CLPC in [ 16]. TheElectronics CLPC 2020 starts, 9, x to FOR increase PEER REVIEW the transmitted power by increasing VDD_Tx when the rat moved10 of to 28 low PTE areas or stood up resulting in the weak coupling from the Tx to Rx coils as shown in the inset t =up14,817 resulting s. When in the the weak Rx coilcoupling was closefrom tothe the Tx Txto coilRx coils located as shown the bottom in the ofinset the t homecage= 14,817 s. When or high the PTE Rx coil was close to the Tx coil located the bottom of the homecage or high PTE areas as shown in the areas as shown in the inset t > 14,817 s, the Rx receives more power than necessary resulting in the inset t > 14,817 s, the Rx receives more power than necessary resulting in the increase of rectifier increase of rectiﬁer voltage, Vrec. Then, the CLPC immediately decreases the VDD_Tx to reduce the voltage, Vrec. Then, the CLPC immediately decreases the VDD_Tx to reduce the transmitted power for transmitted power for the regulation of received Rx power. In the result, the Rx can always receive the the regulation of received Rx power. In the result, the Rx can always receive the constant power from constant power from the Tx regardless of any environmental variations during the experiment. the Tx regardless of any environmental variations during the experiment.

(a) (b)

FigureFigure 9. 9.(a ()a Block) Block diagram diagram of of CLPCCLPC inin thethe two-coiltwo-coil inductive inductive link, link, and and (b (b) )the the experimental experimental CLPC CLPC waveformswaveforms in in the the wirelessly-powered wirelessly-powered cage cage to to compensate compensate for for the the distance distance variation variation betweenbetween TxTx andand Rx inRx transient in transient caused caused by the by animalthe animal movement movement [16 ].[16].

2.3.2.3. Scalability Scalability for for Wireless Wireless Coverage Coverage TheThe animal animal experimental experimental arenaarena cancan be either a a sq squareuare standard standard cage cage or or a aspecific specific shape shape with with a a largerlarger area area depending depending on on the the experimental experimental purposes.purposes. Some Some wirelessly-powered wirelessly-powered cages cages are are dedicated dedicated toto the the standard standard cage cage [ 13[13,16],,16], which which havehave anan advantageadvantage in in terms terms of of compatibility compatibility with with the the standard standard racksracks of of rodent rodent cages cages in in the the animalanimal facilitiesfacilities comp comparedared to to the the modular modular design design for for a specific a specific shape. shape. ThisThis compatibility compatibility is beneficialis beneficial for for longitudinal longitudinal studies studies on on multiple multiple animals animals in separatein separate standard standard cages resultingcages resulting in the simultaneous in the simultaneous and massive and massive data collection data collection from many from animal many animal subjects subjects [15]. The [15]. modular The designsmodular can designs be easily can extended be easily forextended large areafor large or specific area or shapespecific [11 shape,17,18 [11,17,18]] while the while optimized the optimized wireless cageswireless dedicated cages todedicated the standard to the cagestandard are hard cage toare modify hard to the modify wireless the coverage.wireless coverage. One of the One important of the considerationsimportant considerations for the scalability for theusing scalability the modular using the coil modular design coil is todesign choose is theto choose suitable the method suitablefor method for tracking the Rx coil position because the modular coil design needs to select the nearest tracking the Rx coil position because the modular coil design needs to select the nearest Tx coil to the Tx coil to the Rx coil. If the modular system does not equip the tracking method, all the Tx coils in Rx coil. If the modular system does not equip the tracking method, all the Tx coils in the array will be the array will be driven simultaneously, resulting in significant power loss. In [11], a small permanent driven simultaneously, resulting in significant power loss. In [11], a small permanent magnet is attached magnet is attached to the mobile device, and the wirelessly-powered cage detects the mobile device to the mobile device, and the wirelessly-powered cage detects the mobile device using three-axis using three-axis magnetic sensors to select the nearest Tx coil to the freely moving animal subject. magneticThe permanent sensors magnet to select is thealso nearest utilized Txin coil[18] for to thethe freelyRx coil moving tracking, animal where subject.a single Tx The coil permanent moves magnetmechanically is also utilizedon XY-rails in [18 located] for the at Rx the coil bottom tracking, of the where cage. a singleHowever, Tx coil in movesthe WPT mechanically system, the on XY-railsperformance located of at magnetic the bottom sensors of the might cage. be However, degraded indu thee to WPTthe strong system, magnetic the performance fields inside of the magnetic cage, sensorsresulting might in not be degradedsufficient tracking due to the resolution strong magnetic and quality. fields inside the cage, resulting in not sufficient trackingThe resolution optical animal and quality. tracking techniques are studied in [16,19] using an infrared range camera or a MicrosoftThe optical Kinect animal®. The tracking Microsoft techniques Kinect includes are studied infrared in [16 depth,19] using(IR-3D) an and infrared red-green-blue range camera (RGB- or a Microsoft2D) cameras Kinect allowing®. The Microsoftanimal tracking Kinect in includes both bright infrared and dark depth conditions. (IR-3D) and Since red-green-blue the optical tracking (RGB-2D) camerasmethod allowing can obtain animal the information tracking in bothabout bright both andthe Rx dark coil conditions. position and Since the the animal optical subject tracking behavior method canat obtaina time, theit is information more beneficial about than both the thepermanent Rx coil positionmagnet sensing and the in animal terms of subject the additional behavior analysis at a time, it isof moreanimal beneficial locomotion than and the behavior. permanent However, magnet the sensing optical in cameras terms of should the additional be installed analysis on the of top animal of locomotionthe cage with and a behavior. few tens However,of centimeters. the optical As such, cameras the lid should of the cage be installed should onnot thebe closed top of theduring cage the with a fewexperiment. tens of centimeters. As an alternative, As such, the the resonator-based lid of the cage cage should design, not bewhich closed allows during for theautomatic experiment. magnetic As an alternative,field localization, the resonator-based obviating the cageneed design,for a tracking which system allows or for switching automatic the magnetic coils, are field introduced localization, in obviating[13–15,17]. the In need these for systems, a tracking the multi-resonator system or switching coil arrangement the coils, are around introduced the cage, in [simply13–15, 17driven]. In theseby a single LC-tank located at the bottom of the cage, can dynamically focus the magnetic field at the position of Rx coil. However, the parasitic resistance in multi-resonator coils still dissipates power all

Electronics 2020, 9, x FOR PEER REVIEW 11 of 28 the time regardless of the Rx coil position, resulting in the additional power loss compared to the switchable modular Tx coil design.

2.4. Spatial and Angular Misalignment between Tx and Rx Coils Electronics 2020, 9, 1999 11 of 28 Spatial and angular misalignments and distance variation between Tx and Rx coils inside the wirelessly-powered cage happen quite frequently in practice as freely behaving animal subjects walk, sniffsystems, around, the rear, multi-resonator and climb the coil walls arrangement of the cage. around This will the cage,result simplyin a significant driven by reduction a single in LC-tank the PTElocated and atPDL, the bottomwhich might of the cause cage, malfunctions can dynamically in the focus mobile the magneticdevice which field does at the not position equip ofenergy Rx coil. storage.However, The the homogeneity parasitic resistance of wireless in multi-resonator coverage achieved coils by still the dissipates Tx coil array power alleviates all the time the regardlessspatial misalignmentof the Rx coil position,as shown resulting in Section in the2.1.2, additional and the powerCLPC in loss Section compared 2.2 [11,13,16] to the switchable compensates modular the Tx reducedcoil design. power in the Rx against some angular misalignments or distance variations. However, the Rx coil rarely receives the wireless power from the Tx coil with maximum angular misalignment of 90°.2.4. Therefore, Spatial and the Angular designer Misalignment should consider between the Tx worst-case and Rx Coils condition in the wirelessly-powered cage. The most common technique to address the angular misalignment with omnidirectional powering is Spatial and angular misalignments and distance variation between Tx and Rx coils inside the to implement 3D Rx coils in x-, y-, and z-axis, as shown in Figure 10a [20]. The 3D Rx coil is composed wirelessly-powered cage happen quite frequently in practice as freely behaving animal subjects walk, of three individual Rx coils followed by each rectifier as shown in Figure 10b. For the nominal sniff around, rear, and climb the walls of the cage. This will result in a significant reduction in the PTE condition which angular misalignment is 0°, the Rxz coil mainly receives the power carrier from the and PDL, which might cause malfunctions in the mobile device which does not equip energy storage. Tx coils while Rxx and Rxy coils rarely receive the power. When the 3D Rx coil has large angular The homogeneity of wireless coverage achieved by the Tx coil array alleviates the spatial misalignment misalignments from the Tx coil, the received power in the Rxx or Rxy coil is increased depending on as shown in Section 2.1.2, and the CLPC in Section 2.2[ 11,13,16] compensates the reduced power in the the misalignment axis to compensate the reduced power in the Rxz coil. The 3D Rx coil can also be Rx against some angular misalignments or distance variations. However, the Rx coil rarely receives the implemented by using the multiple resonator coils, L3, instead of multiple load coils, L4, as introduced wireless power from the Tx coil with maximum angular misalignment of 90◦. Therefore, the designer in Figure 11 [14]. As shown in Figure 11b, the multiple resonators are strongly coupled with L4, while should consider the worst-case condition in the wirelessly-powered cage. The most common technique the multiple resonators can receive the power against angular misalignment inside the cage. This to address the angular misalignment with omnidirectional powering is to implement 3D Rx coils in technique only utilizes one rectifier resulting in the reduced circuit complexity compared to Figure x-, y-, and z-axis, as shown in Figure 10a [20]. The 3D Rx coil is composed of three individual Rx 11b. Since these 3D Rx coils receive the inductive power from each direction, it provides better homogeneouscoils followed PTE by eachand PDL rectifier against as shownthe angular in Figure misa lignments10b. For theof the nominal mobile condition device. However, which angular the 3Dmisalignment Rx coil design is in 0◦ the, the mobile Rxz coil unit mainly increases receives the volume the power compared carrier to fromthe conventional the Tx coils single while planar Rxx and RxRx coil,y coils resulting rarely receivein the limited the power. applications When thefor tiny 3D Rximplants coil has inside large the angular animal misalignments body. from the Tx coil,Instead the receivedof the 3D powerRx coil, in the the new Rx xTxor coil Rx yconfigurationscoil is increased are dependingstudied to provide on the misalignment the homogeneous axis to magneticcompensate field the along reduced with power x-, y-, inand the z Rx-axisz coil. [21,22]. The 3DThe Rx proposed coil can alsoarch beitecture implemented in Figure by 12a using [21] the includesmultiple three resonator layers coils, of hexagonalL3, instead planar of multiple spiral loadcoil (hex-PSCs) coils, L4, as array introduced for homogeneous in Figure 11[ distribution14]. As shown ofin the Figure PTE against11b, the angular multiple misalignments. resonators are The strongly individual coupled Tx coil with arrayL4 generates, while the power multiple carriers resonators with threecan receivephases (0°, the 120°, power 240°) against as shown angular in Figure misalignment 12b, and insideprovides the both cage. vertical This techniqueand lateral only magnetic utilizes fluxesone rectifier at the same resulting time in over the reducedthe entire circuit powered complexity 3D volume compared inside tothe Figure cage. 11Whenb. Since the Rx these coil 3D is Rx alignedcoils receive with the the Tx inductive coil, the powerRx coil from can receive each direction, the power it providesfrom the vertical better homogeneous magnetic flux. PTE When and the PDL Rxagainst coil has the the angular angular misalignments misalignment of against the mobile the Tx device. coil by However, 90°, the lateral the 3D magnetic Rx coil design flux between in the mobile the Txunit coils increases mainly the provides volume the compared power to to the the Rx, conventional resulting in single ~4% PTE planar improvement Rx coil, resulting compared in the to limited the conventionalapplications Tx for coil tiny array implants as shown inside in the Figure animal 12b. body.

(a) (b)

Figure 10. (a) The design of the 3D Rx coil structure and (b) circuit diagrams in [20] to compensate for the angular misalignment of Rx coil. Electronics 2020, 9, x FOR PEER REVIEW 12 of 28

ElectronicsFigure2020 10., 9(,a 1999) The design of the 3D Rx coil structure and (b) circuit diagrams in [20] to compensate for 12 of 28 the angular misalignment of Rx coil.

Electronics 2020, 9, x FOR PEER REVIEW 12 of 28

Figure 10. (a) The design of the 3D Rx coil structure and (b) circuit diagrams in [20] to compensate for the angular misalignment of Rx coil. (a) (b)

FigureFigure 11. 11. (a()a The) The design design of of the the 3D 3D Rx Rx coil coil using using multiple multiple resonators resonators in inthe the mobile mobile device, device, and and (b) (theb) the schematicschematic diagram diagram of of key key circuits circuits [14]. [14 ].

Instead of the 3D Rx coil, the new Tx coil configurations are studied to provide the homogeneous magnetic field along with x-, y-, and z-axis [21,22]. The proposed architecture in Figure 12a [21] includes three layers of hexagonal planar spiral coil (hex-PSCs) array for homogeneous distribution of the PTE against angular misalignments. The individual Tx coil array generates power carriers with three phases (0◦, 120◦, 240◦) as shown in Figure 12b, and provides both vertical and lateral magnetic fluxes at the same time over the entire powered 3D volume inside the cage. When the Rx coil is aligned with the Tx coil, the Rx coil can receive the power from the vertical magnetic flux. When the Rx coil has the (a) (b) angular misalignment against the Tx coil by 90◦, the lateral magnetic flux between the Tx coils mainly providesFigure the 11. power (a) The todesign the Rx,of the resulting 3D Rx coil in using ~4% multiple PTE improvement resonators in comparedthe mobile device, to the and conventional (b) the Tx coil arrayschematic as shown diagram in of Figure key circuits 12b. [14].

(a) (b)

Figure 12. (a) The design of the 3D Tx coil using multiple resonators in the wirelessly-powered cage, and (b) the three-phase magnetic field excitation scheme [21].

In [22], the omnidirectional powering is achieved by two dipole coils using DQ rotating magnetic field. This technique utilizes the phase differences in two Tx coils, Tx-D, and Tx-Q, and then, generates the rotating magnetic field in the 3D volume as shown in Figure 13. The AC currents in the Tx-D and Tx-Q have same magnitude with 90° phase difference from each other. These two currents generate the transient magnetic field along with x-, y-, and z-directions by rotating the angular frequency from 0 to 2π as partly shown in Figure 13b–d. Then, the crossed dipole Rx coil can still receive the wireless power from a portion of rotating magnetic fields even though it has the maximum 90° angular misalignment from(a) the Tx coils. Although the Rx coil in this system(b) should be designed in theFigureFigure crossed 12. 12. dipole((aa)) The The structuredesign design of of thewith the 3D 3D the Tx Tx coilferrite coil using using material multiple multiple resulting resonators resonators in in the inthe thelimited wirelessly-powered wirelessly-powered applications cage, for cage, tiny biomedicalandand ( (bb )implants,) the the three-phase three-phase the rotating magnetic magnetic magnetic field field excitation excitation field te scheme schemechnique [21]. [21 can]. be potentially applicable for external portable biomedical devices attached to the animal body. InIn [22], [22], the the omnidirectional omnidirectional powering powering is is achieved achieved by by two two dipole dipole coils coils using using DQ DQ rotating rotating magnetic magnetic field.field. This techniquetechniqueutilizes utilizes the the phase phase di ffdifferenceserences in twoin two Tx coils, Tx coils, Tx-D, Tx-D, and Tx-Q, and andTx-Q, then, and generates then, generatesthe rotating the magnetic rotating magnetic field in the field 3D in volume the 3D as volu shownme as in shown Figure in 13 Figure. The AC13. The currents AC currents in the Tx-D in the and Tx-DTx-Q and have Tx-Q same have magnitude same magnitude with 90◦ phase with 90° diff erencephase difference from each from other. each These other. two These currents two generate currents the generatetransient the magnetic transient field magnetic along with field x-, y-,along and with z-directions x-, y-, and by rotatingz-directions the angular by rotating frequency the angular from 0 to frequency2π as partly from shown 0 to in 2 Figureπ as partly 13b–d. shown Then, in the Figure crossed 13b–d. dipole Then, Rx coil the can crossed still receive dipole theRx wirelesscoil can powerstill receivefrom a portionthe wireless of rotating power magneticfrom a portion fields of even rotating though magnetic it has thefields maximum even though 90◦ angular it has the misalignment maximum 90°from angular the Tx misalignment coils. Although from the the RxTx coilcoils. in Although this system the Rx should coil in be this designed system inshould the crossed be designed dipole instructure the crossed with dipole the ferrite structure material with resulting the ferrite in material the limited resulting applications in the limited for tiny applications biomedical for implants, tiny biomedical implants, the rotating magnetic field technique can be potentially applicable for external portable biomedical devices attached to the animal body.

Electronics 2020, 9, 1999 13 of 28 the rotating magnetic ﬁeld technique can be potentially applicable for external portable biomedical devicesElectronics attached 2020, 9, x FOR to the PEER animal REVIEW body. 13 of 28

(a) (b) (c) (d)

FigureFigure 13.13. ((aa)) TheThe design of two crossed-dipole coils coils for for Tx-D Tx-D (Tx1, (Tx1, xx-axis)-axis) and and Tx-Q Tx-Q (Tx2, (Tx2, yy-axis)-axis) coils coils forfor generating generating omnidirectionalomnidirectional magneticmagnetic fields, ﬁelds, and the the simulated simulated rota rotatingting magnetic magnetic flux ﬂux at at relative relative angularangular frequencyfrequency ofof ((bb)) 00 ((cc)) ππ//4,4, andand ((dd)) ππ//22 [[22].22].

2.5.2.5. Near-FieldNear-Field DataData TelemetryTelemetry SinceSince thethe IMDIMD inin thethe wirelessly-poweredwirelessly-powered cage ty typicallypically monitors monitors different different types types of of biomedical biomedical signalssignals recordedrecorded fromfrom thethe electrodeselectrodes andand performs st stimulationimulation to to tissue tissue for for the the treatment, treatment, the the data data transmissiontransmission between between the the mobile mobile device device and and the the wirelessly-powered wirelessly-powered cage cage is requiredis required to collectto collect the datathe anddata to and control to control the device. the device. In terms In of terms the average of the poweraverage dissipation power dissipation in the recent in the IMDs, recent composed IMDs, ofcomposed amplifiers, of signalamplifiers, conditioning, signal conditioning, digitization, digitization, processing, andprocessing, radio frequency and radio (RF) frequency transmission (RF) blockstransmission for neural blocks recording for neural application, recording the application, traditional far-fieldthe traditional data communication far-field data blockscommunication dedicate a relativelyblocks dedicate large portion a relatively of the large IMD portion power budget of the IM comparedD power to budget the other compared functional to the blocks other [7]. functional Therefore, theblocks near-field [7]. Therefore, communication the near-field can be regardedcommunication as a more can suitablebe regarded method as a in more terms suitable of power method saving in in IMDs,terms consideringof power saving that thein IMDs, animal considering subject moves that within the animal the wirelessly-powered subject moves within cage, the and wirelessly- the Tx/Rx coilspowered are already cage, and implemented the Tx/Rx forcoils the are wireless already powering. implemented While for there the wireless have been powering. many data While telemetry there techniqueshave been aremany introduced, data telemetry it is important techniques for are the introd designersuced, toit is select important suitable for data the designers telemetry to methods, select consideringsuitable data the telemetry practical methods, limitations considering of IMD designthe practical related limitations to key aspects, of IMD such design as powerrelated budget,to key siliconaspects, area, such IMD as power dimensions, budget, temperature silicon area, elevation,IMD dimensions, data bandwidth, temperature sensitivity, elevation, reliability data bandwidth, (Bit-Error Rate,sensitivity, BER), andreliability robustness (Bit-Error against Rate, various BER), perturbationsand robustness [7 against]. various perturbations [7].

2.6.2.6. SpecificSpecific AbsorptionAbsorption RatioRatio (SAR)(SAR) and Safety Issues TheThe transmittedtransmitted powerpower from the Tx coil should be less than allowable allowable specific specific absorption absorption rate rate (SAR)(SAR) limit,limit, determined by by the the induced induced electric electric field field intensity, intensity, E, in E, tissue in tissue when when exposed exposed to a RF to aEM RF EMfield. field. There There are mainly are mainly two official two offi internationalcial international safety safetystandard standard establishment establishment organizations organizations which whichhave deeply have deeply impacted impacted each country each country for their for safety their standards. safety standards. The International The International Commission Commission on Non- on Non-IonizingIonizing Radiation Radiation Protection Protection (ICNIR (ICNIRP)P) has the has series the exposure series exposure guideline guideline for each forE-field each and E-field H-field. and H-field.The recent The recentICNIRP ICNIRP (2010) (2010)used usedthe induced the induced E-Field E-Field instead instead of induced of induced current current density density metric. metric. Meanwhile,Meanwhile, the the guideline guideline also also provided provided the the exposure exposure limits limits in in central central nervous nervous system system (CNS) (CNS) tissues tissues of theof the head head and and other other tissues tissues of of the the body. body. Institute Institute of of Electrical Electrical andand Electronics Engineers (IEEE) (IEEE) also also providedprovided [ 23[23]] for for exposure exposure standard. standard. Compared Compared with with ICNIRP ICNIRP guideline, guideline, IEEE IEEE standard standard is widely is widely used inused US, in Canada, US, Canada, Japan, Japan, Korea, Korea, etc. It alsoetc. It has also basic has restriction basic restriction and reference and reference restriction. restriction. When When setting protectivesetting protective limits for limits localized for localized tissue heating tissue dueheatin to theg due diffi toculty the difficulty of SAR measurement, of SAR measurement, the point SARthe ispoint mass SAR averaged, is mass in recognitionaveraged, in of recognition the thermal of di thffusione thermal properties diffusion of tissue. properties The averagingof tissue. massThe inaveraging ICNIRP ismass any in 10 ICNIRP g of contiguous is any 10 tissue. g of contiguo IEEE standardus tissue. specifies IEEE standard a smaller specifies 1 g averaging a smaller mass 1 g in theaveraging shape of mass a cube. in the The shape IEEE of limitsa cube. di Theffer IEEE from limits the ICNIRP differ from guidelines the ICNIRP in both guidelines the maximum in both levelthe 2 2 specifiedmaximum which level arespecified 100 W which/mm2 forare IEEE100 W/mm and 50 for W /IEEEm2 for and ICNIRP, 50 W/m and for the ICNIRP, frequency and the at which frequency these peakat which values these reach peak are values 2000 MHzreach for are ICNIRP 2000 MHz and for 3000 ICNIRP MHz and for IEEE3000 [MHz24]. for IEEE [24]. SAR can be calculated by σ|E|2/ρ, where σ and ρ are tissue conductivity and density, respectively. Since the SAR limit allows the less amount of RF exposure for the higher carrier frequency, the designer also needs to select a suitable power carrier frequency for the wirelessly- powered cage system based on the expected maximum power used in the Tx and SAR limit for safety, prior to the PTE or PDL optimization. RF exposure can be evaluated by the simulation using a

ElectronicsElectronics2020 2020, 9, ,9 1999, x FOR PEER REVIEW 1414 of of 28 28

“phantom”, which emulates the electrical characteristics of the human head, body, or tissue. In [25], the SAREM simulator, can be calculated namely HFSS, by σ|E is|2 /utilizedρ, where toσ estimaand ρteare the tissue peak conductivityof average SAR and values density, of tissue respectively. layers Sincewhen the a mm-size SAR limit Rx allows coil implanted the less amount in the oftissue RF exposure receives for1.3 themW higher power. carrier Under frequency, different the settings designer of alsotissue needs layers, to select the SAR a suitable values with power the carrier WPT configuratio frequency forn of the a two-coil wirelessly-powered link, a three-coil cage link system with based loop oncoils, the and expected a three-coil maximum link powerwith segmented used in the coils Tx are and compared, SAR limit as for shown safety, in prior Figure to the14a. PTEFigure or PDL14b optimization.shows the SAR RF simulation exposure can results be evaluated with Poynting by the vectors simulation (the using real component a “phantom”, of the which power emulates density) the electricalgenerated characteristics by the EnerCage-HC2 of the human when head, the Tx body, power or is tissue. 1 W for In the [25 rat], the model. EM simulator,In this simulation namely setup, HFSS, isthe utilized rat model to estimate is made the entirely peak of out average of brain SAR tissue values with of σ tissue = 105 S/m layers and when ρ = 1040 a mm-size kg/m3, Rx which coil implantedgenerates inthe the worst tissue case receives for SAR 1.3 simulation mW power. [14]. Under diﬀerent settings of tissue layers, the SAR values with the WPTTo ensure conﬁguration the safety of aof two-coil EM exposure, link, a SAR three-coil can be link simulated with loop by various coils, and EM a simulators, three-coil link such with as segmentedHFSS, in the coils design are compared, stage as shown as shown in Figure in Figure 14. Additionally,14a. Figure 14 otherb shows measures the SAR include simulation monitoring results withthe Poyntingtissue heating vectors using (the realtemperature component sensor of the in powerthe IMD density) [26,27]. generated Based on by the EnerCage-HC2tissue heating wheninformation the Tx powersent from is 1 Wthe for IMD, the ratthe model.wirelessly-pow In this simulationered cage can setup, limit the the rat amount model is of made transmitted entirely outpower of brain to conform tissue with to theσ = SAR105 Slimitation/m and ρ =and1040 prev kg/entm3 ,tissue which damage generates from the the worst abnormal case for heat SAR simulationgeneration [ 14of]. the IMD.

(a) (b)

FigureFigure 14. 14.Examples Examples of of EMEM simulationsimulation ofof SARSAR limitslimits forfor (a)) lamb’s head layers [25] [25] and and ( (bb)) a a rat rat model model mademade entirely entirely out out of of brain brain tissue tissue [ 14[14].].

3. DesignsTo ensure of Wirelessly-Powered the safety of EM exposure, Cage Systems SAR can be simulated by various EM simulators, such as HFSS, in the design stage as shown in Figure 14. Additionally, other measures include monitoring the In this section, we will introduce different designs of wirelessly-powered cage systems, which tissue heating using temperature sensor in the IMD [26,27]. Based on the tissue heating information can be classified into four categories. Based on the design considerations discussed in Section 2, we sentwill from discuss the the IMD, main the features wirelessly-powered of each wirelessly-powered cage can limit the cage amount design. of transmitted power to conform to the SAR limitation and prevent tissue damage from the abnormal heat generation of the IMD. 3.1. Wirelessly-Powered Cage Systems with Single Tx Coil Configuration 3. Designs of Wirelessly-Powered Cage Systems The single Tx coil configuration can simplify the design of the driver control in the wirelessly- In this section, we will introduce different designs of wirelessly-powered cage systems, which can powered cage systems, significantly improving the system robustness. However, the following be classified into four categories. Based on the design considerations discussed in Section2, we will examples of the wirelessly-powered cage systems with such configuration also suffer from other discuss the main features of each wirelessly-powered cage design. limitations, which we will discuss in detail [18,28–30]. 3.1. Wirelessly-PoweredFigure 15a,b show Cage two Systems wirelessly-powered with Single Tx cage Coil Configurationdesigns, each of which a large Tx coil wraps around the cage to enhance the magnetic field within the volume covered by the Tx coil [28,29]. In FigureThe 15a, single the Tx Tx coil coil configuration comprises two can simplifyseries-conne the designcted coaxial of the driver rectangular control windings, in the wirelessly-powered generating an cageeffective systems, operating significantly zone of improving 40 × 24 × 4 the (height) system cm robustness. [28]. The centerHowever, of such the an following operationexamples zone is of7 cm the wirelessly-poweredheight from the bottom cage systemsof the cage with to such cover configuration the vertical alsomovement suffer from of the other mobile limitations, device while which the we willanimal discuss is walking in detail and [18,28 standing.–30]. A π-capacitor matching circuit, which matches the Rx coil to the load,Figure is also 15 a,bdesigned show twoto ensure wirelessly-powered the wireless power cage designs,delivery eachmaintaining of which at a the large maximum Tx coil wraps PDL aroundcondition. the In cage Figure to enhance 15b, the Tx the coil magnetic completely field encompasses within the volumethe area coveredin which bythe theanimals Tx coil can [ 28roam,29]. Infreely, Figure resulting 15a, the in Tx an coil effective comprises space two of 40 series-connected × 40 × 20 (height) coaxial cm3 [29]. rectangular These two windings, designs can generating improve anthe eff WPTective resilience operating to zonelateral of and 40 vertical24 4 misalign (height)ments cm [28 between]. The center the Tx of and such Rx an coils. operation However, zone the is × × 7angular cm height misalignments from the bottom still ofcan the negatively cage to cover impact the verticalthe PTE movementof the inductive of the mobilelink. Moreover, device while due theto animalhuge size is walking difference, and the standing. pair of ATxπ -capacitorand Rx coils matching is away circuit, from their which optimal matches size the matching, Rx coil to resulting the load, isin also the designed low PTE toof ensurethese two the designs. wireless For power a larger delivery experimental maintaining arena, at thethe maximumsize of the PDLTx coil condition. will be

Electronics 2020, 9, x FOR PEER REVIEW 15 of 28 enlarged accordingly, which will aggravate the coil size mismatch and then the PTE reduction. For the designs in Figure 15a,b, the Tx coil covers the 3D volume of the experimental arena, which can ensure the strength of the magnetic field within the covered volume. For the wirelessly-powered cage design shown in Figure 15c, the Tx coil is located underneath the circular mouse cage with a diameter of 8 inches, and the power Rx is a 15 mm-long, 2 mm- diameter solenoid coil wrapping around a copper ferrite core [30]. The coil size mismatch is eased in this design, at the cost of smaller wirelessly-powered arena, compared to the designs in Figure 15a,b. The Rx coil continuously receivers a large amount of power of 2 W over distances of several centimeters within the cage. Given that the inductive link is non-optimal, it is challenging to ensure the wireless power transmission safety by not overcoming the SAR limitation in the case of strong power delivery. In this wirelessly-powered cage system, the Tx coil also needs to cover the bottom area of the circular mouse cage. Such a configuration is also not conductive for the system scalability. Compared to the abovementioned wirelessly-powered cage designs which suffer from the size mismatch between the Tx and Rx coils, the wirelessly-powered cage in Figure 15d does not have this issue [18]. The coils in the inductive link are geomecally optimized. Besides, the optimum alignment between the Tx and Rx coils is also achieved. In Figure 15d, the IRPower system uses a servo- controlledElectronics 2020 Tx, 9coil, 1999 moving under the cage on the X and Y rails. A permanent magnet in the mobile15 of 28 device and an array of magnetic sensors placed around the Tx coil form the animal tracking system. GivenIn Figure the real-time15b, the optimized Tx coil completely size and optimized encompasses alignment the area of inthe which Tx and the Rx animals coils, the can PTE roam of this freely, inductiveresulting inlink an design effective is space high. of The 40 IRPower40 20 (height) system cm in3 Figure[29]. These 15d twoalso designs features can a improveCLPC, which the WPT × × compensatesresilience to the lateral PDL and variations vertical misalignmentsby adjusting the between amount the of Txpower and transmitted Rx coils. However, into thethe Tx angularcoil. However,misalignments the mechanical still can negatively setup of and impact X and the Y rails PTE enlarges of the inductive the size of link. the Moreover,IRPower system, due to making huge size itdi incompatiblefference, the pairwithof the Tx standard and Rx coils racks is of away rodent from cages their in optimal animal size facilities. matching, For resultinghigh throughput in the low experiments, a large number of cages can be located in standard racks in animal research facilities PTE of these two designs. For a larger experimental arena, the size of the Tx coil will be enlarged without occupying precious laboratory space. In addition, with the Tx coil underneath the cage, the accordingly, which will aggravate the coil size mismatch and then the PTE reduction. For the designs systems in Figure 15c,d will have the dramatic reduction in the magnetic field strength and then the in Figure 15a,b, the Tx coil covers the 3D volume of the experimental arena, which can ensure the PTE, as the distance between the Tx and Rx coil increases. Table 2 compares the features of the strength of the magnetic field within the covered volume. abovementioned systems.

(a) (b)

Figure 15. Examples of wirelessly-powered cage designs with a single Tx coil conﬁguration presented

in (a)[28], (b)[29], (c)[30], and (d)[18].

For the wirelessly-powered cage design shown in Figure 15c, the Tx coil is located underneath the circular mouse cage with a diameter of 8 inches, and the power Rx is a 15 mm-long, 2 mm-diameter solenoid coil wrapping around a copper ferrite core [30]. The coil size mismatch is eased in this design, at the cost of smaller wirelessly-powered arena, compared to the designs in Figure 15a,b. The Rx coil continuously receivers a large amount of power of 2 W over distances of several centimeters within the cage. Given that the inductive link is non-optimal, it is challenging to ensure the wireless power transmission safety by not overcoming the SAR limitation in the case of strong power delivery. In this wirelessly-powered cage system, the Tx coil also needs to cover the bottom area of the circular mouse cage. Such a configuration is also not conductive for the system scalability. Compared to the abovementioned wirelessly-powered cage designs which suffer from the size mismatch between the Tx and Rx coils, the wirelessly-powered cage in Figure 15d does not have this issue [18]. The coils in the inductive link are geomecally optimized. Besides, the optimum alignment between the Tx and Rx coils is also achieved. In Figure 15d, the IRPower system uses a servo-controlled Tx coil moving under the cage on the X and Y rails. A permanent magnet in the mobile device and Electronics 2020, 9, 1999 16 of 28 an array of magnetic sensors placed around the Tx coil form the animal tracking system. Given the real-time optimized size and optimized alignment of the Tx and Rx coils, the PTE of this inductive link design is high. The IRPower system in Figure 15d also features a CLPC, which compensates the PDL variations by adjusting the amount of power transmitted into the Tx coil. However, the mechanical setup of and X and Y rails enlarges the size of the IRPower system, making it incompatible with the standard racks of rodent cages in animal facilities. For high throughput experiments, a large number of cages can be located in standard racks in animal research facilities without occupying precious laboratory space. In addition, with the Tx coil underneath the cage, the systems in Figure 15c,d will have the dramatic reduction in the magnetic field strength and then the PTE, as the distance between the Tx and Rx coil increases. Table2 compares the features of the abovementioned systems.

Table 2. Benchmarking of the wirelessly-powered cage systems with a single Tx coil conﬁguration.

Publications [28][29][30][18] Inductive link 2-coil 2-coil 2-coil 2-coil Frequency 6.78 MHz 13.56 MHz 120 kHz 13.56 MHz WPT coverage, 40 24 4 cm3 40 40 20 cm3 diameter of 8 inch 34 18 cm2 length width height × × × × × Mobile device× size,× length × 24.5 13 16 mm3 10 10 10 mm3 <1 cm3 12 12 mm2 width height × × × × × PTE× 3.8–7.5% >0.56% N/A 17% PDL 100 mW 8.5 mW 2 W 1.7 mW CLPC √ × × × Scalability √ × × × Compatible to racks √ √ √ × Coil size matching √ × × × Vertical misalignment Resilience √ √ × ×

3.2. Wirelessly-Powered Cage Systems with a Scalable Tx Coil Array With the configuration of the scalable Tx coil array, the wirelessly-powered cage systems introduced in this section have scalable designs [11,17,31]. Each Tx coil in the array has its optimum diameter matched with the Rx coil at a given Rx-Tx separation distance. Within the array, the Tx coil closest to the animal, which is often in the best position to power the mobile unit, is activated. Several different methods have been used to dynamically and automatically activate the Tx coil that is in the optimum alignment with the Rx coil. The inductive link configuration in Figure 16a includes a single Rx coil embedded in the mobile device and two overlapping arrays of planar Tx coils placed at the bottom of the cage [31]. Every four coils are connected to a single driver module. The 50% overlap in both X and Y directions eliminates dead spots on the power transmission side. Rx tracking is performed by the impedance tracking technique. Particularly, the proximity of the Rx coil will induce a reflected impedance adding onto the Tx coil, causing a reduction in the current drawn from the driver. The Tx power controller detects the impedance variation of each Tx coil by sensing its supply current variation so that the Tx coil closest to the Rx can be identified as the one with supply current reduction. In Figure 16b, the wirelessly-power cage system includes a resonance-based inductive link with its configuration adjustable [11]. The stationary unit includes a three-layer array of overlapping hex-PSCs that tile the floor of the experimental arena, Tx coils on the third layer driven by PAs and Tx resonators on the first and second layers. The Tx coil can directly link to the mobile unit by forming a three-coil inductive link if the nearest to the Rx is a Tx coil. Otherwise, a four-coil inductive link will be formed if a Tx resonator is closest to the Rx. The animal location tracking is performed via an array of magnetic sensors under the stationary unit and a magnetic tracer in the mobile device. This system is equipped with CLPC, which can enhance the robustness of the wireless power delivery against the Tx-Rx distance variation. Electronics 2020, 9, x FOR PEER REVIEW 17 of 28 angular misalignments, as the Tx resonators at two surfaces will contribute to directing the EM filed towards the mobile device. The Tx resonator array configuration features the natural power localization mechanism that the Tx resonators located directly below and above the mobile device will be automatically activated. Without the requirements of Rx location detection and Tx coil switching, the system implementation can be significantly simplified. However, placing a resonator array on the top surface of the cage limits access to the animals and blocks the field of view of video recording for analyzing animal behavior. In sum, the abovementioned wirelessly-powered cage systems feature the modular and scalable architecture, which allows for experimental arenas with arbitrary shapes and dimensions. While the

Tx includesElectronics multiple2020, 9, 1999 coils, these systems can still minimize the power interferences as only17 of one 28 Tx coil is activated at a time. Table 3 compares the features of the abovementioned systems.

(a) (b)

(c)

FigureFigure 16. Examples 16. Examples of wirelessly-powered of wirelessly-powered cage cage system system designsdesigns with with the the Tx Tx coil coil array array conﬁguration configuration presentedpresented in (a in) [31], (a)[31 (b],) ( b[11],)[11 and], and (c ()c )[[17].17].

Figure 16c shows another wirelessly-powered cage design based on a 13.56 MHz resonance-based Table 3. Benchmarking of the wirelessly-powered cage systems with scalable Tx coil array four-coil inductive link [17]. The power Tx includes a Tx coil powered by a PA and two arrays configuration. of Tx resonators connected in parallel and tiled at the top and bottom surfaces of the chamber to form uniformPublications wireless power distribution in 3D. The[31] top and bottom resonator[11] arrays have more[17] or less contributionInductive to the link power transmission, depending 2-coil on the location3/4-coil of the mobile device4-coil in the z-direction. TheFrequency distance between the surfaces is 1.5 set MHz for ensuring almost 13.56 constantMHz power 13.56 delivery MHz to the mobileWPT device coverage, in the z-direction. Such an arrangement also can improve the system tolerance 42 × 18 × (8–11) cm3 3538 × 12 cm3 27 × 27 × 16 cm3 to angularlength misalignments, × width × height as the Tx resonators at two surfaces will contribute to directing the EM Mobilefiled device towards size, the length mobile × width device. × height The Tx resonator 40 × 40 array mm configuration2 π × 202 features × 20 mm the3 natural 42 mm powerdiameter localization mechanismPTE that the Tx resonators located13–39% directly below and5.6–12.6% above the mobile device59% will be automaticallyPDL activated. Without the requirements 21–225 of mW Rx location detection 20 mW and Tx coil switching, 100 mW the system implementationCLPC can be significantly simplified.× However, placing√ a resonator array on× the top surfaceTx control of the cage simplicity limits access to the animals and × blocks the field of view × of video recording√ for analyzingCompatible animal behavior. to racks √ × √ AngularIn sum, misalignment the abovementioned resilience wirelessly-powered × cage systems feature × the modular and scalable√ architecture,No whichview-blocking allows for experimental arenas with√ arbitrary shapes and√ dimensions. While× the Tx includes multiple coils, these systems can still minimize the power interferences as only one Tx coil is activated at a time. Table3 compares the features of the abovementioned systems.

Electronics 2020, 9, x FOR PEER REVIEW 18 of 28

3.3. Wirelessly-Powered Cage Systems with Slanted Tx Resonators The wirelessly-powered cage systems introduced in this section have multiple slanted Tx resonators wrapping around the cage [13,16,32,33]. The slanted Tx resonators help bend and direct the magnetic flux towards the Rx coil. Such a configuration can improve the system tolerance to angular misalignments. Additionally, the geometry of the Tx and Rx coils are optimized and matched, which is beneficial to improve PTE. ElectronicsThe2020 wirelessly-powered, 9, 1999 cage system, shown in Figure 17a, is built based on a resonance-based18 of 28 inductive link whose configuration is adjustable [16]. The stationary unit includes one square-shaped central Tx coil at the bottom of the cage and four triangular-shaped slanted Tx resonators on the Table3. Benchmarking of the wirelessly-powered cage systems with scalable Tx coil array configuration. corners of the cage. The Tx coil can directly form a three-coil inductive link with the coils in the mobile device when thePublications animal moves to the center of the [31][ cage, or form a four-coil11][ inductive link17] with one Tx resonator whenInductive the linkmobile device is at a corner 2-coil of the cage. The 3/ 4-coilposition of the mobile 4-coil device is tracked in real-timeFrequency by a Microsoft Kinect inst 1.5alled MHz above the 13.56cage. MHz Depending on 13.56 the MHz recorded WPT coverage, position information, the stationary unit 42will18 either(8–11) deactivate cm3 3538all Tx12 resonators cm3 or27 activate27 16 cm the3 Tx length width height × × × × × × × resonatorMobile device closest size, to length the mobilewidth deviceheight by shorting40 40 mm this2 Tx resonatorπ 202 with20 mm a3 resonant42 mm capacitor diameter for × × × × × forming an LC-tankPTE with a high quality-factor. 13–39%However, installing 5.6–12.6% the Microsoft Kinect 59% will enlarge PDL 21–225 mW 20 mW 100 mW the experimental volumeCLPC needed for the entire system setup, making√ the system not compatible to × × Tx control simplicity √ the racks of standard-sized rodent cages. × × Compatible to racks √ √ In Figure 8a, the wirelessly-powered cage system, named EnerCage-HC,× includes four-coil Angular misalignment resilience √ × × inductive linkNo built view-blocking around a standard-sized rodent√ cage [13]. The inductive √ link has a square-shaped × Tx coil attached at the center and bottom of the cage and four Tx resonators wrapping around the 3.3.cage Wirelessly-Powered at different heights/orientations, Cage Systems with and Slanted planar Tx Rx Resonators resonator and Rx coil in the mobile device. The slanted Tx resonators, which direct the magnetic field to the mobile device, together with the CLPC can supportThe wirelessly-powered sufficient and constant cage systems PDL up introduced to 80-deg inree this rotation. section In have Figure multiple 17b,c, slanted two similar Tx resonators wireless wrappingcage designs around are presented the cage [13 [32,33].,16,32,33 In]. the The wirele slantedssly-powered Tx resonators cage help system, bend and shown direct in the Figure magnetic 17b, fluxtwo towardsmore Tx theresonators, Rx coil. Suchwrapping a configuration around the can bottom improve and the top system rim of tolerancethe cage, toare angular added misalignments.to enhance the Additionally,homogenous thedistribution geometry of of the the transmitted Tx and Rx coils power are in optimized the z-direction and matched, [32]. The which Tx coil is beneficial consists toof improvemultiple PTE.identical spiral coils connected in series. However, the Tx coil located at the top of the cage may Theblock wirelessly-powered the line of view of the cage camera. system, The shown wireless in Figure cage design, 17a, is builtshown based in Figure on a resonance-based17c, has a similar inductiveTx resonator link configuration whose configuration [33]. The is adjustableTx coil is fo [16rmed]. The by stationary three identical unit includes coils connected one square-shaped in parallel centraland tiled Tx at coil the atbottom the bottom of the cage of the to cagemake and the fourtransmitted triangular-shaped power more slanted evenly Txdistributed resonators in on3D.the cornersThanks of the to cage. the Theslanted Tx coil Tx can resonators, directly form the aab three-coilovementioned inductive wirelessly-powered link with the coils cage in the systems mobile deviceshow improved when the animalrobustness moves to tothe the angular center misalignment. of the cage, or formAdditionally, a four-coil the inductive Tx coil linkconfiguration with one Txmakes resonator the magnetic when the field mobile more device evenly is distributed at a corner in of the the 3D cage. volume The positionof the cage, of the and mobile the required device isnumber tracked of inPAs real-time is reduced by ato Microsoft only one. KinectFor the installed abovementioned above the four cage. designs, Depending there is on no the need recorded for Tx positionresonator information, switching, significantly the stationary reducing unit will the eitherdesign deactivate complexity, all and Tx they resonators feature or the activate compatibility the Tx resonatorto the standard closest racks. to the However, mobile device the byabovementioned shorting this Txwirelessly-powered resonator with a resonantcage designs capacitor are not for formingscalable. anThe LC-tank size of withthe Tx a high resonators quality-factor. increases However, as the cage installing side increases, the Microsoft which Kinect will willweaken enlarge the theadvantage experimental of promoting volume needed EM foruniform the entire distributi systemon. setup, Table making 4 compares the system the not compatiblefeatures of to the racksabovementioned of standard-sized systems. rodent cages.

(a)

Figure 17. Cont. Electronics 2020, 9, 1999 19 of 28 Electronics 2020, 9, x FOR PEER REVIEW 19 of 28

(b) (c)

Figure 17. ExamplesExamples of of wirelessly-powered wirelessly-powered cage cage system system designs designs with with the slanted Tx resonator conﬁgurationconfiguration presented inin ((a)[) [16],16], ((b)[) [32],32], andand ((cc)[) [33].33].

InTable Figure 4. 8Benchmarkinga, the wirelessly-powered of the wirelessly-powered cage system, cage named systems EnerCage-HC, with slanted includes Tx resonator four-coil inductiveconfiguration. link built around a standard-sized rodent cage [13]. The inductive link has a square-shaped Tx coil attached at the center and bottom of the cage and four Tx resonators wrapping around the cage Publications [16] [13] [32] [33] at different heights/orientations, and planar Rx resonator and Rx coil in the mobile device. The slanted Inductive link 3/4-coil 4-coil 4-coil 42-coil Tx resonators, which direct the magnetic field to the mobile device, together with the CLPC can Frequency 13.56 MHz 13.56 MHz 13.56 MHz 13.56 MHz support sufficient and constant PDL up to 80-degree rotation. In Figure 17b,c, two similar wireless WPT coverage, 30 × 30 × 17 46 × 24 × 20 28.5 × 18 × 13 46 × 24 × 20 cage designs are presented [32,33]. In the wirelessly-powered cage system, shown in Figure 17b, length × width × height cm3 cm3 cm3 cm3 two more Tx resonators, wrapping around the bottom and top rim of the cage, are added to enhance Mobile device size, length × width 40 × 40 × 20 20 × 22 × 5 26 × 26 × 35 15 × 15 × 10 the homogenous distribution of the transmitted power in the z-direction [32]. The Tx coil consists of × height. mm3 mm3 mm3 mm2 multiple identical spiral coils connected in series. However, the Tx coil located at the top of the cage PTE 16.1–36.3% 14% 34–42% 17% may block the line of view of the camera. The wireless cage design, shown in Figure 17c, has a similar PDL 24 mW 42 mW 13 mW 62 mW Tx resonator configuration [33]. The Tx coil is formed by three identical coils connected in parallel and CLPC √ √ × √ tiled at the bottom of the cage to make the transmitted power more evenly distributed in 3D. Tx control simplicity × √ √ √ Thanks to the slanted Tx resonators, the abovementioned wirelessly-powered cage systems show Compatible to racks × √ √ √ improved robustness to the angular misalignment. Additionally, the Tx coil configuration makes the No view blocking √ √ × √ magnetic field more evenly distributed in the 3D volume of the cage, and the required number of PAs is reduced to only one. For the abovementioned four designs, there is no need for Tx resonator switching, 3.4. Wirelessly-Powered Cage Systems for Omnidirectional Power Transmission significantly reducing the design complexity, and they feature the compatibility to the standard racks. However,In the the experiments abovementioned involving wirelessly-powered small freely-movin cageg designsanimals, are such not as scalable. rodents, The the size horizontal, of the Tx resonatorsdistance, and/or increases angular as the misalignments cage side increases, between which the power will weaken Tx and the Rx advantagecomponents of may promoting frequently EM uniformhappen due distribution. to the animal’s Table 4free compares movements. the features The misalignments of the abovementioned will result systems.in a significant reduction in the PTE and the PDL. In this section, we will introduce several omnidirectional power transmission solutionsTable that 4. Benchmarking can address of robust the wirelessly-powered wireless power cage de systemslivery within the slanted presence Tx resonator of any configuration. position and/or orientation variations of the Rx device. Publications [16][13][32][33] One omnidirectional power transmission solution is a cavity resonator-based wireless cage Inductive link 3/4-coil 4-coil 4-coil 42-coil design, as shownFrequency in Figure 18a [34]. 13.56 On MHzthe power Tx 13.56side, MHz the H-field 13.56produced MHz is rotational 13.56 MHz around the centerWPT of coverage,the cavity. The amplitude of 3the H-field increases3 radially28.5 18 from13 the center to 3the 30 30 17 cm 46 24 20 cm × 3 × 46 24 20 cm peripherylength andwidth is notheight a function of ×height.× On the Rx× side,× the implantablecm device has× two× Rx coils Mobile device× size,× length × 40 40 20 mm3 20 22 5 mm3 26 26 35 mm3 15 15 10 mm2 oriented widthperpendicularheight. to each other.× × This placement× of× the two Rx× coils× can address× the× device × orientation insensitivityPTE within two 16.1–36.3% planes. To achieve 14%truly omnidirectional 34–42% powering, 17%a third Rx PDL 24 mW 42 mW 13 mW 62 mW coil orientedCLPC perpendicular to the first√ two Rx coils should √ be added. Even then, there are√ still five × WPT blindTx control spots, simplicity namely the center and four corners of the√ cavity, where √the H-field is very √ weak. × Compatible to racks √ √ √ For the wirelessly-power cage system,× shown in Figure 11, we call it EnerCage-HC2 system [14]. No view blocking √ √ √ Based on the EnerCage-HC system shown in Figure 8a, the fifth Tx resonator× is added at the top rim of the homecage to enhance the magnetic flux density at top of the cage. More importantly, the Rx resonators in the EnerCage-HC2 system encompass four diagonal planes of the mobile device. Such a novel Rx coil arrangement directs the transmitted EM field, which is already homogenized by the Tx resonators, towards the Rx coil at the bottom of the mobile device at any arbitrary orientations.

Electronics 2020, 9, 1999 20 of 28

3.4. Wirelessly-Powered Cage Systems for Omnidirectional Power Transmission In the experiments involving small freely-moving animals, such as rodents, the horizontal, distance, and/or angular misalignments between the power Tx and Rx components may frequently happen due to the animal’s free movements. The misalignments will result in a significant reduction in the PTE and the PDL. In this section, we will introduce several omnidirectional power transmission solutions that can address robust wireless power delivery in the presence of any position and/or orientation variations of the Rx device. One omnidirectional power transmission solution is a cavity resonator-based wireless cage design, as shown in Figure 18a [34]. On the power Tx side, the H-field produced is rotational around the center of the cavity. The amplitude of the H-field increases radially from the center to the periphery and is not a function of height. On the Rx side, the implantable device has two Rx coils oriented perpendicular to each other. This placement of the two Rx coils can address the device orientation insensitivity within two planes. To achieve truly omnidirectional powering, a third Rx coil oriented perpendicular to the first two Rx coils should be added. Even then, there are still five WPT blind spots, namely the center and four corners of the cavity, where the H-field is very weak. Electronics 2020, 9, x FOR PEER REVIEW 21 of 28

(a) (b)

FigureFigure 18. Examples18. Examples of wirelessly-powered of wirelessly-powered cage systems cage systems oﬀering omnidirectionaloffering omnidirectional power transmission, power presentedtransmission, in (a)[34 presented], (b)[35 in], ( ac))[ [34],36], ( andb) [35], (d )[(c22) [36],]. and (d) [22].

For theTable wirelessly-power 5. Benchmarking of cage the system,wirelessly-powered shown in cage Figure systems 11, we offering call itomnidirectional EnerCage-HC2 power system [14]. transmission. Based on the EnerCage-HC system shown in Figure8a, the fifth Tx resonator is added at the top rim of the homecagePublications to enhance the magnetic[34] flux density[14] at top of the[35] cage. More[36] importantly,[22] the Rx resonators inInductive the EnerCage-HC2 link system Cavity encompass 4-coil four diagonal 2-coil planes ofthe 2-coil mobile 2-coil device. Such 530 a novel Rx coilFrequency arrangement directs 346.6 the transmittedMHz 13.56 EM MHz field, which 6.78 MHz is already homogenized280 kHz by the Tx resonators, towards the Rx coil at the bottom of the mobile device at anykHz arbitrary orientations. WPT coverage, 61 × 61 × 30 46 × 24 × 20 30 × 30 × 30 Together with CLPC, the inductive link achieves omnidirectional power deliveryN/A to the mobile1 m3 device, length × width × height cm3 cm3 cm3 irrespective of the subject’s location or head orientation. When the mobile device is 90 rotated while Mobile device size, length × π × (7)2 × 25 20 × 22 × 11 ~64 × 64 × 64 ◦20 × 20 × locating along the center lines of the homecage, the magnetic fields passingN/A through the two Rx width × height mm3 mm3 mm3 0.5 cm3 resonators are bentPTE in the opposite direction14.32% and23.6–33.3% cancel with each7.9% other. In thisN/A case, little33.6% magnetic PDL 6.1–13 mW 42 mW 1.4 W N/A 10 W CLPC × √ × × × Scalability × × × × × Tx control simplicity √ √ √ × × Compatible to racks √ √ √ √ × Center + 4 Center Blind spots 2 corners None None corners lines No view-blocking × √ √ × √ No Rx access blocking √ √ × √ √

4. Designs of Wirelessly-Powered Cage for mm-Sized Implants Instead of a single, large, and centralized mobile device [37,38], distributed neural interfaces made of a large number of tiny implants are poised to play a key role in future brain-computer interfaces (BCI) because of their less damage to the surrounding tissue. However, it is challenging to

Electronics 2020, 9, 1999 21 of 28

field can pass through the Rx coil, resulting in PTE drop. It should be noted that the probability of this case happening is quite low in practice. Therefore, we add a small supercapacitor following the rectifier in the mobile device to prevent any sudden drop in the supply voltage. In addition to the EnerCage-HC2 system, there are three other inductive link configurations that have been proposed to address omnidirectional wireless powering [22,35,36]. One possible coil configuration is a set of triple orthogonal Rx coils together with a single Tx coil. For achieving higher uniformity of magnetic flux, a pair of Tx coils facing each other can be used instead of a single Tx coil. On the Rx side, each Rx coil is followed by an AC-DC converter, such as a rectifier. The three rectifiers can be connected in series or in parallel to drive their common load. In the series connection scenario, the three DC output voltages of the rectifiers are stacked for providing supply voltage for the load. With the parallel connection scheme, the highest Rx coil voltage is automatically delivered to the load. In Figure 18b, two pairs of Tx coils facing each other are attached to the vertical sides of the cage and driven by their own class-D PA simultaneously [35]. On the Rx side, three coils are placed at 90◦ from each other, which can make the system insensitive to angular misalignments. However, the Tx coil configuration causes two “cold corners” formed by opposite magnetic fields and two “hot corners” formed by constructive magnetic fields in the cage. In the “cold corners”, the triple orthogonal Rx coils still cannot ensure omnidirectional powering. Besides, the Rx coils block the access to the electronics inside the mobile device, making it difficult to repair or to have electrode feed-through. The triple orthogonal Tx coils together with a single Rx coil is another coil configuration offering the omnidirectional powering. If the three Tx coils are driven by the same current, a constant magnetic field in a fixed direction will be generated. For omnidirectional powering, the three Tx coil currents should be driven appropriately different from each other in order to generate a rotating magnetic field. To achieve this objective, there are three possible modulation methods in general: (1) phase-domain modulation (PDM); (2) amplitude-domain modulation (ADM); and (3) frequency-domain modulation (FDM). Given the simplicity of implementing system control, the PDM method is widely used. In Figure 18c, the PDM method is used. More specifically, the three Tx coils excited with three-phase currents with a peak magnitude of 250 mA and 120◦ displacement are wound on the outer surface of a glass bowl [36]. The magnetic field vectors rotate periodically in all directions. Three loads hung inside the glass bowl with the Rx coil facing in different directions. Each load consists of a planar Rx coil, a rectifier, and four LEDs. It can be seen that the LEDs in all three loads are on, confirming the omnidirectional nature of the wireless power transmission. The third viable coil configuration offering omnidirectional powering is the double orthogonal Tx coils together with double orthogonal Rx coils. The two orthogonal Tx coils should be driven by two nonidentical currents to form a rotating magnetic field. Figure 18d shows an example of such a coil configuration [22]. Instead of using loop coils, both the Tx and Rx coil, operating at 280 kHz, have crossed-dipole coil structure. The crossed-dipole Tx and Rx coils are composed of a dual set of two orthogonal coils, wire-wounded around ferromagnetic cores. The currents of the two Tx coils have the same magnitude with a 90◦ phase difference from each other in order to generate a rotating magnetic field. The magnetic field produced can always pass through the crossed-dipole Rx coils at any orientations, to allow for the omnidirectional power receiving. Additionally, the physical dimensions of Tx and Rx are reduced from volume to plane, which is crucial for practical applications, where volumetric coil structure is highly prohibited. Table5 compares the features of the abovementioned systems for omnidirectional power transmission.

Table 5. Benchmarking of the wirelessly-powered cage systems oﬀering omnidirectional power transmission.

Publications [34][14][35][36][22] Inductive link Cavity 4-coil 2-coil 2-coil 2-coil Frequency 346.6 MHz 13.56 MHz 6.78 MHz 530 kHz 280 kHz WPT coverage, 61 61 30 cm3 46 24 20 cm3 30 30 30 cm3 N/A 1 m3 length width height × × × × × × × × Electronics 2020, 9, 1999 22 of 28

Table 5. Cont.

Publications [34][14][35][36][22] Mobile device size, length × π (7)2 25 mm3 20 22 11 mm3 ~64 64 64 mm3 N/A 20 20 0.5 cm3 width height × × × × × × × × PTE× 14.32% 23.6–33.3% 7.9% N/A 33.6% PDL 6.1–13 mW 42 mW 1.4 W N/A 10 W CLPC √ × × × × Scalability × × × × × Tx control simplicity √ √ √ Electronics 2020, 9, x FOR PEER REVIEW × ×22 of 28 Compatible to racks √ √ √ √ × Blind spots Center + 4 corners Center lines 2 corners None None wirelesslyNo view-blocking operate the tiny implants in a cage. As√ a result of the √ significant size difference,√ the × × No Rx access blocking √ √ √ √ coupling between the Tx and Rx coils is quite weak. The optimal operating× frequency for a mm-sized coil to maximize its quality factor is within several tens to hundreds of megahertz range. At such high 4.operating Designs offrequencies, Wirelessly-Powered it also increases Cage the for risk mm-Sized of unsafe Implants exposure to EM radiation and interference, which,Instead in ofturn, a single, imposes large, new and challenges centralized on mobile SAR compliance. device [37,38 The], distributed misalignments neural caused interfaces by free made movements of the animal subject and/or the arbitrary placement of the tiny implants, which combine of a large number of tiny implants are poised to play a key role in future brain-computer interfaces (BCI) with the other challenges, make it challenging to maintain sufficient power delivery to the tiny because of their less damage to the surrounding tissue. However, it is challenging to wirelessly operate implants within a cage. Thus, it is important to develop WPT systems that can overcome these the tiny implants in a cage. As a result of the significant size difference, the coupling between the Tx challenges. and Rx coils is quite weak. The optimal operating frequency for a mm-sized coil to maximize its quality A wireless cage based on the resonant cavity was introduced in [39] to power and control tiny factorimplanted is within devices. several An tens aluminum to hundreds resonant of megahertz cavity, wh range.ich has At 21 such cm diameter high operating and 15 frequencies, cm height with it also increasesa surface the lattice risk of hexagons unsafe exposure (2.5 cm diameter), to EM radiation is utilized and to interference, couple EM energywhich, with in turn, 1.5 GHzimposes carrier new challengesfrequency on toSAR the tissue compliance. of a mouse The as misalignments shown in Figure caused 19a. Compared by free movements to the conventional of the animal inductive subject andpower/or the transfer arbitrary system placement through of thedirect tiny coupling implants, be whichtween combine two coils, with the the energy other in challenges, this system make is it challengingconfined to to the maintain mouse placed sufficient on the power grid deliverydue to the to resonance the tiny implantsexcitation within of the limited a cage. EM Thus, field it is importantpattern. This to develop system WPTcan provide systems self-tracking that can overcome wireless thesepower challenges. to the tiny implant within the mouse resultingA wireless in no cageneeds based of tracking on the algorithms resonant over cavity the was experimental introduced area. in [Figure39] to 19b power shows and a schematic control tiny implantedof wireless devices. implant An for aluminum optogenetic resonant stimulation, cavity, designed which in has the 21 size cm of diameter 10~25 mm and3. This 15 cm light-emitting height with a surfaceimplants lattice can of receive hexagons 5.6~15.7 (2.5 cmmW diameter), with 3.2 W is utilizedof averaged to couple input EMpower. energy Although with 1.5 the GHz wireless carrier frequencyoperation to in the RF tissue bands of are a mousetypically as susceptible shown in Figure to signal 19 a.reflection, Compared interference, to the conventional and absorption inductive by powerobstructions, transfer systemthe proposed through system direct in coupling [39] has betweenbeen successfully two coils, demo the energynstrated in by this behavioral system is testing confined toenvironments the mouse placed and has on shown the grid the duepossibility to the of resonance using RF bands excitation for wireless of the cage limited applications. EM field pattern. This systemEM WPT can provideat high self-trackingfrequency in wirelessthe GHz power range tofacilitate the tiny the implant Rx size within reduction the mouse due to resulting shorter in nowavelength, needs of tracking but they algorithms suffer from over difficulties the experimental in creating area. homogeneous Figure 19b WPT shows in aa schematiclarge experimental of wireless arena [40]. The SAR of EM field in the tissue, which mainly consists of water, at high frequencies is implant for optogenetic stimulation, designed in the size of 10~25 mm3. This light-emitting implants rather high [23]. At high frequency, it also increases the risk of unsafe personnel exposure to EM can receive 5.6~15.7 mW with 3.2 W of averaged input power. Although the wireless operation in radiation and interference with nearby instruments [40]. The microwave chamber delivers power at RF bands are typically susceptible to signal reflection, interference, and absorption by obstructions, low efficiency through the animal body. Its extension to larger animals and eventually humans the proposed system in [39] has been successfully demonstrated by behavioral testing environments without surpassing SAR limits at this frequency does not seem to be feasible, even though it might andbe has a reasonable shown the solution possibility for ultra-low-power of using RF bands applications. for wireless cage applications.

(a) (b)

FigureFigure 19. 19.(a ()a A) A diagram diagram of of wirelessly-powered wirelessly-powered cage system, system, and and (b (b) )a aschematic schematic of of wireless wireless implant implant customizedcustomized for for the the brain brain in in [ 39[39].].

To address this issue, Figure 20 shows the wireless cage for powering tiny implants using inductive coupling at 13.56 MHz frequency with near-field communication hardware [41]. The wireless cage equips a double-loop coil with turns at heights of 4 and 11 cm from the bottom of the animal enclosure as shown in Figure 20a. This technique is similar with the design in Figure 15a, which improves the inductive coupling at further distance from the bottom of the cage resulting in the extension of wireless coverage to the higher side. The wireless cage covers 30 × 30 × 11 cm and the power is relatively uniform across the region of interest as shown in Figure 20b although the

Electronics 2020, 9, 1999 23 of 28

EM WPT at high frequency in the GHz range facilitate the Rx size reduction due to shorter wavelength, but they suffer from difficulties in creating homogeneous WPT in a large experimental arena [40]. The SAR of EM field in the tissue, which mainly consists of water, at high frequencies is rather high [23]. At high frequency, it also increases the risk of unsafe personnel exposure to EM Electronicsradiation 2020, 9 and, x FOR interference PEER REVIEW with nearby instruments [40]. The microwave chamber delivers power23 of 28 at low efficiency through the animal body. Its extension to larger animals and eventually humans normalizedwithout surpassingreceived power SAR limits in atthe this implant frequency decreases does not seemto 0~0.9 to be for feasible, 80° angular even though misalignment. it might be a The heightreasonable for wireless solution coverage for ultra-low-power and its uniformity applications. in coverage can be optimized for a given experimental area basedTo on address the separati this issue,on Figure of double-loop 20 shows the coil. wireless cage for powering tiny implants using inductive couplingWhile the at wireless 13.56 MHz cage frequency with high with frequency near-field communicationis typically suffer hardware from the [41]. SAR The wirelessin the tissue, cage the wirelessequips cage a double-loop in the near-field coil with regime turns athas heights a difficulty of 4 and in 11 the cm fromreduction the bottom of Rx of size the animaldue to enclosurethe poor EM field asfocusing. shown in In Figure Figure 20 a.20b, This the technique two-coil is inductive similar with link the configuration design in Figure results 15a, which in the improves Rx coil the with a diameterinductive of 9.8 coupling mm, atwhich further becomes distance fromthe themain bottom limiting of the cagefactor resulting in the in device the extension miniaturization. of wireless In coverage to the higher side. The wireless cage covers 30 30 11 cm and the power is relatively addition, the single Tx coil configuration, which can simplify× × the design of the power control uniform across the region of interest as shown in Figure 20b although the normalized received power circuitry, suffers from low PTE at the center of the cage. Therefore, they transmitted a large amount in the implant decreases to 0~0.9 for 80◦ angular misalignment. The height for wireless coverage and of powerits uniformity to energize in coverage the entire can becage. optimized This will for a re givensult experimentalin excessive area heat based dissipation on the separation and large of EM interferencedouble-loop between coil. the cage and other lab instruments.

(a) (b)

FigureFigure 20. 20.(a) Simulated(a) Simulated normalized normalized output output power power inside thethe wirelesswireless cage cage without without angular angular misalignment,misalignment, and and (b) (b wireless) wireless operation operation ofof 13 13 implants, implants, placed plac ated height at height of 3 cm of from 3 cm the from bottom the [ 41bottom]. [41]. While the wireless cage with high frequency is typically suffer from the SAR in the tissue, the wireless cage in the near-field regime has a difficulty in the reduction of Rx size due to the poor In Figure 21a, the dual-band EnerCage-HC system, built upon the EnerCage-HC2 system (Figure EM field focusing. In Figure 20b, the two-coil inductive link configuration results in the Rx coil 11), uses a two-stage inductive power transmission technique to wirelessly deliver sufficient PDL to with a diameter of 9.8 mm, which becomes the main limiting factor in the device miniaturization. the tinyIn addition, implant the within single the Tx coilcage configuration, [42]. Given whichthe mismatch can simplify between the design the ofoptimal the power operating control circuitry,frequencies of thesu Txffers and from Rx low coils PTE due at theto their center size of thedifferen cage.ce, Therefore, the dual-band they transmitted EnerCage-HC a large system amount includes of power two inductiveto energize links theoperating entire cage. simultaneously This will result at in their excessive optimal heat frequencies. dissipation and To largedo so, EM this interference system also includesbetween an theintermediate cage and other unit lab (a instruments. headstage device) in addition to the power Tx and Rx. This intermediateIn Figure unit 21 actsa, the as dual-band a power EnerCage-HC converter, system,which builtreceives upon thepower EnerCage-HC2 from the systemcage via (Figure a four-coil 11), inductiveuses a link two-stage at 13.56 inductive MHz powerand delivers transmission it to technique the tiny toimplant wirelessly via deliver a three-coil sufficient inductive PDL to the link tiny at 60 MHz.implant This intermediate within the cage unit [42 ].is Givenalso used the mismatch as a powe betweenr buffer the that optimal uses operatingits stored frequencies energy to ofsupport the Tx the continuousand Rx coilsoperation due to theirof the size tiny difference, implants the in dual-band the presence EnerCage-HC of misalignments system includes in the two four-coil inductive inductive links operating simultaneously at their optimal frequencies. To do so, this system also includes an intermediate link. A dual-loop sequential CLPC mechanism, which includes two loops, further stabilizes the unit (a headstage device) in addition to the power Tx and Rx. This intermediate unit acts as a power amount of power delivered to the tiny implant despite any misalignments by adjusting PDLs of the converter, which receives power from the cage via a four-coil inductive link at 13.56 MHz and delivers four-coilit to theinductive tiny implant link and via athe three-coil three-coil inductive inductive link atlink 60 at MHz. designated This intermediate levels, respectively. unit is also used Figure as 21b shows the final in vitro measurement setup for enabling optical stimulation in the dual-band EnerCage-HC system. The experiment was performed on 6 × 6 × 7 cm cube from a fresh sheep head, including brain, skull, fat, and skin, and placed the cube in the center of the cage. The headstage was mounted on top of the tissue cube to receive 13.56 MHz carrier from the EnerCage-HC coils while the mm-sized implant was placed ~2 mm under the skull. It shows both the orange LED that is an indicator of the headstage receiving sufficient power at 13.56 MHz, and the selected μLED (blue color) for optogenetic stimulation that is powered at 60 MHz.

Electronics 2020, 9, 1999 24 of 28 a power buffer that uses its stored energy to support the continuous operation of the tiny implants in the presence of misalignments in the four-coil inductive link. A dual-loop sequential CLPC mechanism, which includes two loops, further stabilizes the amount of power delivered to the tiny implant despite any misalignments by adjusting PDLs of the four-coil inductive link and the three-coil inductive link at designated levels, respectively. Figure 21b shows the final in vitro measurement setup for enabling optical stimulation in the dual-band EnerCage-HC system. The experiment was performed on 6 6 7 cm cube × × from a fresh sheep head, including brain, skull, fat, and skin, and placed the cube in the center of the cage. The headstage was mounted on top of the tissue cube to receive 13.56 MHz carrier from the EnerCage-HC coils while the mm-sized implant was placed ~2 mm under the skull. It shows both the orange LED that is an indicator of the headstage receiving sufficient power at 13.56 MHz, and the selected µLED (blue color) for optogenetic stimulation that is powered at 60 MHz. Electronics 2020, 9, x FOR PEER REVIEW 24 of 28

(a) (b)

FigureFigure 21. 21.A A prototype prototype of inductive power power transfer transfer systems systems for fortiny tiny implants, implants, proposed proposed in [42] in for [42 (a]) for (a)conceptual conceptual system system overview overview and and (b) ( bin) vitro in vitro measurement measurement setup. setup.

FigureFigure 22 22aa presents presents a a newnew wirelesswireless powerpower transf transferer system system that that can can wirelessly wirelessly power power a large a large numbernumber of of tiny tiny implants implants arbitrarily arbitrarily distributeddistributed over a a large large tissue tissue area. area. This This is isachieved achieved byby a power a power TxTx platform platform which which can can generate generate aa verticalvertical andand lateral magnetic magnetic field field alternately alternately at atany any places places within within thethe experimental experimental arena arena [ 21[21].]. This This powerpower TxTx platformplatform includes includes three three overlapping overlapping layers layers of ofhex-PSCs hex-PSCs tiled at the bottom of the cage. The three layers are horizontally shifted in a way that the centers of tiled at the bottom of the cage. The three layers are horizontally shifted in a way that the centers of every three adjacent hex-PSCs on three different layers are on the corners of an equilateral triangle. every three adjacent hex-PSCs on three different layers are on the corners of an equilateral triangle. In each layer, every three adjacent hex-PSCs are driven at carrier signals with the same magnitude In each layer, every three adjacent hex-PSCs are driven at carrier signals with the same magnitude and and phase of 0°, 120°, and 240°, respectively, as shown in Figure 22b. To avoid any counteracts among phase of 0 , 120 , and 240 , respectively, as shown in Figure 22b. To avoid any counteracts among the the three◦ layers,◦ they are◦ controlled by time-division multiplexing (TDM). The appropriate TDM threeperiod, layers, T, is they decided are controlled by the carrier by time-divisionfrequency, time multiplexing constant of the (TDM). capacitance The appropriate following TDMthe Rx period, coil T, isand decided rectifier, by Rx the loading, carrier frequency, and the acceptable time constant level of of the ripple capacitance on the regulated following supply the Rx voltage coil and of rectifier, the Rxelectronics loading, and that theare acceptablepowered by level the Rx of coil ripple as shown on the in regulated Figure 22c. supply To this voltage end, this of power the electronics Tx platform that arein powered Figure 22d, by thewhich Rx coiltakes as advantage shown in of Figure the overla 22c. Topping this placements end, this power of the Tx three platform layers, in the Figure three- 22d, whichphase takes power advantage carrier signals, of the overlapping and the TDM placements control mechanism, of the three eventually layers, the achieves three-phase omnidirectional power carrier signals,WPT towards and the a TDM large control number mechanism, of tiny implants eventually with arbitrary achieves angular omnidirectional and/or spatial WPT misalignments. towards a large numberIn the ofmeasured tiny implants result, withthe proposed arbitrary method angular in and [21]/or achieves spatial misalignments.5% of PTE distributions In the measured and 5 mW result, of thePDL proposed within methodthe area in for [21 90°] achieves angular 5% misalignment of PTE distributions of Rx coil and compared 5 mW ofto PDL 0.8% within of PTE the for area the for 90◦conventionalangular misalignment WPT system. of Rx coil compared to 0.8% of PTE for the conventional WPT system.

(a) (b)

Electronics 2020, 9, x FOR PEER REVIEW 24 of 28

(a) (b)

Figure 21. A prototype of inductive power transfer systems for tiny implants, proposed in [42] for (a) conceptual system overview and (b) in vitro measurement setup.

Figure 22a presents a new wireless power transfer system that can wirelessly power a large number of tiny implants arbitrarily distributed over a large tissue area. This is achieved by a power Tx platform which can generate a vertical and lateral magnetic field alternately at any places within the experimental arena [21]. This power Tx platform includes three overlapping layers of hex-PSCs tiled at the bottom of the cage. The three layers are horizontally shifted in a way that the centers of every three adjacent hex-PSCs on three different layers are on the corners of an equilateral triangle. In each layer, every three adjacent hex-PSCs are driven at carrier signals with the same magnitude and phase of 0°, 120°, and 240°, respectively, as shown in Figure 22b. To avoid any counteracts among the three layers, they are controlled by time-division multiplexing (TDM). The appropriate TDM period, T, is decided by the carrier frequency, time constant of the capacitance following the Rx coil and rectifier, Rx loading, and the acceptable level of ripple on the regulated supply voltage of the electronics that are powered by the Rx coil as shown in Figure 22c. To this end, this power Tx platform in Figure 22d, which takes advantage of the overlapping placements of the three layers, the three- phase power carrier signals, and the TDM control mechanism, eventually achieves omnidirectional WPT towards a large number of tiny implants with arbitrary angular and/or spatial misalignments. In the measured result, the proposed method in [21] achieves 5% of PTE distributions and 5 mW of PDL within the area for 90° angular misalignment of Rx coil compared to 0.8% of PTE for the Electronics 2020, 9, 1999 25 of 28 conventional WPT system.

Electronics 2020, 9, x FOR PEER REVIEW 25 of 28 (a) (b)

FigureFigure 22. 22. (a)( aRendered) Rendered view view of ofdistributed distributed BCI BCI concept, concept, (b ()b phase) phase distribution distribution among among the the hex-PSCs hex-PSCs in inone oneconductive conductive layer, layer, (c ()c) TDM TDM driving driving techniques techniques to to avoid avoid the the magnetic magnetic interference interference between between three three layers, layers,and ( dand) a prototype(d) a prototype measurement measurement setup forsetup the for three-phase the three-phase TDM overlapping TDM overlapping hex-PSC hex-PSC array in array [21]. in [21]. 5. Conclusions 5. ConclusionsWirelessly-powered cages for IMD applications are categorized and discussed with their key techniques,Wirelessly-powered such as closed-loop cages for IMD power applications control, coil are designcategorized/optimization, and discussed scalability with their for wireless key techniques,coverage, such spatial as/angular closed-loop misalignments, power control, near-field coil design/optimization, data telemetry, and scalability safety issues. for wireless Since the coverage,high-performance spatial/angular IMDs misalignments, mainly focus near-field on long-term data telemetry, recording and safety stimulation issues. functionalitiesSince the high- for performancecentral and IMDs peripheral mainly nervous focus on systems long-term on freely recording behaving and animalstimulation subjects functionalities [37,38], it is for important central to anddesign peripheral a suitable nervous wireless systems power on platformfreely behaving covering an aimal large subjects experimental [37,38], arena.it is important Different totechniques design a forsuitable wirelessly-powered wireless power cagesplatform that covering are benchmarked a large experimental in Tables2–5 arena. consider Different various techniques practical issuesfor wirelessly-powereddescribed in this article. cages Althoughthat are benchmarked most wirelessly-powered in Tables 2–5 cages consider provide various PTE and practical PDL for issues specific describedexperimental in this conditions, article. Although the physical most wirele constraintsssly-powered of Rx coils,cages coilprovide separations, PTE and andPDL magneticfor specific field experimentalhomogeneity conditions, over the experimental the physical areaconstraints should beof comparedRx coils, coil for aseparations, more suitable and choice. magnetic As thefield most homogeneityrecent class ofover IMDs the areexperimental millimeter-sized area should and distributed be compared over for a a large more area suitable in the choice. brain or As the the rest most of the recentbody class [43– 46of], IMDs the importance are millimeter-sized of wirelessly-powered and distributed cages over fora large tiny area multiple in the implants brain or isthe elevated rest of as theintroduced body [43–45], in Section the importance4. Hence, it of is wirelessly-powered important for designers cages to for select tiny suitable multiple wirelessly-powered implants is elevated cages, asconsidering introduced thein Section practical 4. limitationsHence, it is of importan IMD designt for related designers to key to select aspects, suitable such as wirelessly-powered PTE, PDL, closed-loop cages,power considering control, scalability, the practical spatial limi/angulartations of misalignment, IMD design related near-field to key data aspects, telemetry, such andas PTE, safety PDL, issues closed-loopagainst various power perturbations control, scalability, during spatial/angular the longitudinal misalignment, animal experiment. near-field With data respecttelemetry, to variousand safetyIMD issues applications against fromvarious conventional perturbations implantable during the devices longitudinal to recent animal mm-sized experiment. implants, With thisrespect article tocontributes various IMD the applications design strategy from conventional of the appropriate implantable wirelessly-powered devices to recent cage mm-sized designs, implants, which have this the articleability contributes to create anthe automated design strategy enriched of the environment appropriate inside wirelessly-powered the experimental cage arena designs, for long-termwhich haveelectrophysiology the ability to create experiments. an automated enriched environment inside the experimental arena for long- term electrophysiology experiments.

Author Contributions: Conceptualization: B.L. and Y.J.; methodology: B.L. and Y.J.; validation: B.L. and Y.J.; formal analysis: B.L. and Y.J.; investigation: B.L. and Y.J.; data curation: B.L. and Y.J.; writing—original-draft preparation: B.L. and Y.J.; writing—review and editing: B.L. and Y.J.; visualization: B.L. and Y.J.; supervision, project administration, and funding acquisition: B.L. and Y.J. All authors have read and agreed to the published version of the manuscript.

Funding: This work was supported by the Incheon National University (#2019-0299) research grant in 2019.

Conflicts of Interest: The authors declare no conflict of interest.

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