Modeling of a compact, implantable, dual-band antenna for biomedical applications

Ci a ion: Bah ouni, M.; Houze , G.; Vuong, T.P.; Mendes, P.M.; Dinis, H.; Sil a, R.; T abelsi, H. Modeling o a Compac , Implan able, Dual‑Band An enna o Biomedical Applica ions. Elec onics 2023,12, 1475. h ps:// doi.o g/10.3390/elec onics12061475 Academic Edi o : Gio anni Leone Recei ed: 8 Janua y 2023 Re ised: 20 Feb ua y 2023 Accep ed: 20 Feb ua y 2023 Published: 21 Ma ch 2023 Copy igh : © 2023 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). elec onics A icle Modeling o a Compac , Implan able, Dual‑Band An enna o Biomedical Applica ions Majdi Bah ouni 1,2,3,*, G ego y Houze 2,3, Tan Phu Vuong 2, Paulo M. Mendes 4,5 , Hugo Dinis 4,5, Rui Sil a 4,5 and Hichem T abelsi 6 1Mic owa e Elec onics Resea ch Labo a o y, Depa men o Physics, Facul y o Sciences o Tunis, Uni e si y o Tunis El Mana , Tunis 2092, Tunisia 2Ins i u e o Mic oelec onics Elec omagne ism and Pho onics‑Mic owa e Labo a o y and Cha ac e iza ion, G enoble INP, G enoble Alps Uni e si y, 38000 G enoble, F ance 3Ins i u e o Mic oelec onics Elec omagne ism and Pho onics‑Mic owa e Labo a o y and Cha ac e iza ion, Uni e si éSa oie Mon Blanc, 73000 Le Bou ge du Lac, F ance 4Cen e o Mic oElec omechanical Sys ems (CMEMS‑UMinho), Uni e si y o Minho, 4800‑058 Guima ães, Po ugal 5LABBELS—Associa e Labo a o y, 4710‑057 B aga, Po ugal 6Na ional School o Enginee s o Ca hage, Uni e si y o Ca hage, Cha guia II 2035, Tunisia *Co espondence: [email p o ec ed] Abs ac : Di e en implan able an enna designs exis o es ablish communica ion wi h implan able de ices depending on he domain o use and he implan a ion space. Owing o hei na u e and pu poses, hese an ennas ha e many imposed c i e ia on a ious cha ac e is ics, such as bandwid h, mul iband beha io , adia ion pa e n, gain, and speci ic abso p ion a e (SAR). This p esen s a chal‑ lenge when i comes o achie ing sa is ying esul s wi hou a majo comp omise in any o hese c ucial pa ame e s. Addi ionally, many o he exis ing designs do no ollow a speci ic app oach o ob ain esul s. Measu ing di e en pa ame e s o such ab ica ed s uc u es equi es special condi‑ ions and special en i onmen s mimicking he issues whe e hey a e supposed o be placed. Fo such issues, he use o biological o syn he ic phan oms is widely employed o alida e wha is ob‑ ained in simula ion, and a mul i ude o o mulas exis o he c ea ion o such phan oms, each wi h i s ad an ages and d awbacks. In his pape , a minia u e dual‑band s uc u e de i ed om he i s i e a ion o he Koch ac al s uc u e is designed o ope a e 2 mm below he skin in he a m o he hu‑ man body, wi h he MICS (Medical Implan Communica ion Sys em) and ISM (Indus ial, Scien i ic, Medical) 2.4 GHz bands. The pu poses o he design a e o de i e s uc u es om commonly used shapes wi h ce ain beha io while main aining minia u iza ion, and o easily design dual‑band im‑ plan able an ennas. Mo e han one band is used o di e si y uses, since bands such as he MICS band a e mainly dedica ed o eleme y. The s uc u e is cha ac e ized no only by i s low p o ile compa ed o a ious s uc u es ound in he li e a u e wi h dimensions o 17.2 ×14.8 ×0.254 mm3, bu also i s ease o design, independen shi ing o esonan equencies, and he absence o he need o a ma ching ci cui and a sho ing pin ( ia) o minia u iza ion. I exhibi s sa is ying pe o mance: bandwid hs o 23 MHz in he MICS band and 190 and 70 MHz in he icini y o he ISM 2.4 GHz band, and measu ed gain in he la e band o −18.66 and −17 dBi in he azimu h and ele a ion adia ion pa e ns, espec i ely. To alida e he an enna’s p ope ies in a skin‑mimicking en i onmen , wo simple phan om o mulas ound in he li e a u e we e explo ed and compa ed in o de o iden i y he bes op ion in e ms o accu acy and ease o ab ica ion. Keywo ds: an enna; implan able; minia u e; ac al; phan om cha ac e iza ion; biocompa ibili y 1. In oduc ion Wi h he e olu ion o a ious exis ing echnologies, implan able medical de ices (IMDs) con inue o e ol e no only o sol e he al eady exis ing challenges, bu also o ace newe Elec onics 2023,12, 1475. h ps://doi.o g/10.3390/elec onics12061475 h ps://www.mdpi.com/jou nal/elec onics Elec onics 2023,12, 1475 2 o 15 ones. This all e ol es a ound he objec i e o simpli ying he li es o pa ien s by imp o ‑ ing pe o mance and es ablishing new app oaches, pu poses, and ways o ope a ion, while main aining less in asi eness and lowe cos s. This is pe o med wi hou comp omising he heal h o pa ien s, mainly in e ms o biocompa ibili y and unwan ed issue s imula ion, o en e alua ed h ough he speci ic abso p ion a e (SAR). Implan able an ennas, pa o an IMD ha needs o connec o he ou side wo ld, a e he e o e also changing. Minia u iza ion o hese adia ing s uc u es is c ucial o main‑ ain sizes compa ible wi h hose o downsized implan able de ices, making hei design p ocesses challenging. A la ge a ie y o an enna design concep s and app oaches exis o main ain his pe o mance/minia u iza ion ule. The design p esen ed in [1] has a compac s uc u e o only 8 ×8×0.2 mm3. I was simula ed in a skin‑mimicking homogeneous phan om. I has a bandwid h o 290 MHz co e ing he ISM 2.4 GHz band and a simula ed gain alue o −14 dBi a i s esonan equency. Despi e i s cha ac e is ics, he s uc u e includes slo s on bo h sides, along wi h a ia o minia u iza ion, making he manu ac‑ u ing p ocess complica ed. All his, in addi ion o i s monoband na u e, makes i s uses limi ed o he men ioned band. A simila s uc u e is p esen ed in [2]. Al hough smalle , a jus 7 ×7×0.2 mm3, he an enna has a simula ed gain o −15 dBi and a o al bandwid h o 420 MHz, including he ISM 2.4 GHz band. The simula ion was pe o med in skin, s omach, and b onchi‑mimicking en i onmen s. The an enna con ains ela i ely complex shapes on bo h sides and a ia, which makes i e en mo e complex in e ms o manu ac u ‑ ing, aside om he ac ha , simila ly o he p e iously p esen ed s uc u e, i emains a monoband an enna. In [3], a dual‑band meande ‑line/ ing shape is p esen ed, wi h dimen‑ sions o 6 ×6×2.54 mm3. The an enna ope a es in he ISM 915 MHz and 2.4 GHz bands, in a homogeneous skin box, and has measu ed gain alues o −13.14 and −28 dBi in he men ioned bands, espec i ely. Al hough simple in design and ha ing no ias, he an enna has a ela i ely la ge olume o 91.44 mm3and low gain in he ISM 2.4 GHz band. In [4], an e en simple s uc u e ope a ing in he MICS and ISM 2.4 GHz bands is p esen ed, wi h dimensions o 22.5 ×22.5 ×2.5 mm3. I was simula ed in a h ee‑laye muscle– a –skin issue model, p o iding measu ed bandwid hs o 141.2 and 170.4 MHz in he MICS and ISM 2.4 GHz bands, espec i ely. The s uc u e is no e y e icien in e ms o minia u iza‑ ion, as i has la ge dimensions and a olume o 1265.63 mm3and equi es a sho ing pin, making he ab ica ion p ocess complica ed. A dual‑band ISM 915 MHz/2.4 GHz an enna was p oposed in [5]. I has simula ed bandwid hs o 200 and 450 MHz, and le els o gain eaching −26.71 dBi and −17.5 dBi in he men ioned bands, in a se en‑laye human head model. I s o e all dimensions a e 11 ×19 ×1.25 mm3, and i includes a ia. In his pape , an app oach based on ac al s uc u es is adop ed, as hey a e known o exhibi mul iband beha io depending on he chosen shape. The pu pose o he app oach is o design dual‑band s uc u es wi h known shapes ha allow ob aining esonan e‑ quencies in he desi ed bands, while main aining a educed size. The eason behind he adop ion o he dual‑band cha ac e is ic is o di e si y he use o he an enna. In he case o he p esen ed wo k, he MICS band is dedica ed o eleme y, whe eas he ISM 2.4 GHz band can ha e o he uses. One applica ion o in e es is wi eless powe ans e (WPT), as i has a mul i ude o ad an ages in he ield o implan able de ices. These include a educ ion in he dependency on ba e ies, allowing smalle ba e y sizes o dependen implan able de ices, and di ec ly powe ing up implan able de ices when he ba e y is no needed. The p oposed design is based on he i s i e a ion o Koch’s ac al s uc u e be o e i is modi ied o minia u iza ion pu poses, wi h inal dimensions o 17.2 ×14.8 × 0.254 mm3. The esonan equencies, which a e independen ly modi iable, a e ob ained by uning he speci ic a eas whe e he an enna esona es. The model is designed o be placed 2 mm below he skin o a human a m, wi h no ma ching ci cui , and does no e‑ qui e any sho ing pin o minia u iza ion. The bands o in e es a e he 400 MHz MICS and he ISM 2.4 GHz. To alida e simula ion esul s h ough measu emen s, a phan om imi a ing a biological issue is equi ed. Va ious me hods and mix u es exis o c ea e such phan oms, bu wo mix u es ound in he li e a u e we e a ma e o in e es . One is based Elec onics 2023,12, 1475 3 o 15 on sodium chlo ide (NaCl) and suga [4], and he o he is based on DGBE (die hylene glycol monobu yl e he ) and he de e gen T i on X‑100 [6]. A pa ame ic s udy was pe ‑ o med o compa e he wo mix u es in e ms o ease o ab ica ion and he accu acy o hei pe mi i i y and conduc i i y. 2. An enna Design Biocompa ible ma e ials a e used o make an ennas implan able in any li ing biolog‑ ical issue wi hou igge ing any eac ion om he immune sys em. The an enna was he e o e ini ially designed based on he use o alumina (ε = 9.8; anδ= 10−3) as a sub‑ s a e and supe s a e [7], bu due o i s low a ailabili y, a simila , widely ma ke ed non‑ biocompa ible dielec ic was used solely o in i o measu emen s: he Roge s RO3010 (ε = 10.2; anδ= 2.2 ×10−3) wi h a hickness o 127 µm. I was selec ed o his design as i has compa able dielec ic pa ame e s o alumina. 2.1. Geome y The p oposed an enna design was ini ially based on he i s i e a ion o Koch’s ac al s uc u e, a syn he ic cu e, he shape o which main ains he same pa e n in each o i s pa s, o ming smalle sel ‑copies, simila ly o na u al ac al cu es, i.e., ee oo s. This shape is used o ill a de ined space wi h nume ous smalle sel ‑copies o minia u iza‑ ion, as shown in Figu e 1. This allows he designed an enna o inhe i ac al s uc u es’ compac ness and he mul i‑band beha io [8]. Elec onics 2022, 11, x FOR PEER REVIEW 3 o 15 2.4 GHz. To alida e simula ion esul s h ough measu emen s, a phan om imi a ing a biological issue is equi ed. Va ious me hods and mix u es exis o c ea e such phan oms, bu wo mix u es ound in he li e a u e we e a ma e o in e es . One is based on sodium chlo ide (NaCl) and suga [4], and he o he is based on DGBE (die hylene glycol mono- bu yl e he ) and he de e gen T i on X-100 [6]. A pa ame ic s udy was pe o med o compa e he wo mix u es in e ms o ease o ab ica ion and he accu acy o hei pe mi - i i y and conduc i i y. 2. An enna Design Biocompa ible ma e ials a e used o make an ennas implan able in any li ing biolog- ical issue wi hou igge ing any eac ion om he immune sys em. The an enna was he e o e ini ially designed based on he use o alumina (ε = 9.8; anδ = 10−3) as a subs a e and supe s a e [7], bu due o i s low a ailabili y, a simila , widely ma ke ed non-bio- compa ible dielec ic was used solely o in i o measu emen s: he Roge s RO3010 (ε = 10.2; anδ = 2.2 × 10−3) wi h a hickness o 127 μm. I was selec ed o his design as i has compa able dielec ic pa ame e s o alumina. 2.1. Geome y The p oposed an enna design was ini ially based on he i s i e a ion o Koch’s ac- al s uc u e, a syn he ic cu e, he shape o which main ains he same pa e n in each o i s pa s, o ming smalle sel -copies, simila ly o na u al ac al cu es, i.e., ee oo s. This shape is used o ill a de ined space wi h nume ous smalle sel -copies o minia u - iza ion, as shown in Figu e 1. This allows he designed an enna o inhe i ac al s uc- u es’ compac ness and he mul i-band beha io [8]. (a) (b) (c) Figu e 1. Koch ac al cu e: (a) no i e a ions applied o a segmen line; (b) i s i e a ion applied; (c) second i e a ion applied. The design app oach consis s in c ea ing and modi ying a p in ed dipole based on he i s i e a ion, as shown in Figu e 2a. The dipole is placed be ween a RO3010 127 μm hick subs a e and supe s a e. The la e is added o a oid di ec con ac be ween he adia ing elemen and he human body o a oid any possible unwan ed elec ical low in he su ounding issues, and o ensu e biocompa ibili y when using alumina. In he ig- u e, he s uc u e esona es a 2.4 GHz p io o de elopmen and op imiza ion. Figu e 2b,c show he modi ica ion p ocess o minia u iza ion while main aining he o e all leng h and wid h o he an enna. While he esul ing modi ied s uc u e esembles a spi al shape, mul iband beha io is obse ed in he i e a ions p esen ed in hese igu es. In Figu e 2d, he s uc u e is designed o each he MICS band and includes modi ica ions o be e ma ching while main aining simila beha io o i s p e ious i e a ions. (a) (b) Figu e 1. Koch ac al cu e: (a) no i e a ions applied o a segmen line; (b) i s i e a ion applied; (c) second i e a ion applied. The design app oach consis s in c ea ing and modi ying a p in ed dipole based on he i s i e a ion, as shown in Figu e 2a. The dipole is placed be ween a RO3010 127 µm hick subs a e and supe s a e. The la e is added o a oid di ec con ac be ween he adia ing elemen and he human body o a oid any possible unwan ed elec ical low in he su ounding issues, and o ensu e biocompa ibili y when using alumina. In he igu e, he s uc u e esona es a 2.4 GHz p io o de elopmen and op imiza ion. Figu e 2b,c show he modi ica ion p ocess o minia u iza ion while main aining he o e all leng h and wid h o he an enna. While he esul ing modi ied s uc u e esembles a spi al shape, mul iband beha io is obse ed in he i e a ions p esen ed in hese igu es. In Figu e 2d, he s uc u e is designed o each he MICS band and includes modi ica ions o be e ma ching while main aining simila beha io o i s p e ious i e a ions. The design p ocess is conduc ed as ollows: •Til he s uc u e’s elemen s o educe size while main aining compa able esonan equencies. Fo his pu pose, a p in ed ac al dipole is s udied a i s , as shown in Figu e 2a. •The s uc u e’s elemen s a e ex ended inwa ds, as shown in Figu e 2b,c. •The s uc u e is pa ame ically s udied o iden i y pa s whe e i esona es in he de‑ si ed bands. Fo be e ma ching, a pa ame ic s udy on di e en pa s o he an enna is conduc ed. The lowe side o he adia ing elemen is modi ied and pa ame ically op imized, as seen in Figu e 2d. •Fo u he minia u iza ion, one elemen is emo ed and is eplaced wi h a g ound plane a he bo om o he subs a e, cu ing he s uc u e’s wid h almos in hal , as shown in Figu e 2e. This esul s in a sligh deg ada ion in e ms o pe o mance. •Fo u he minia u iza ion, he an enna’s sides a e la ened and pa ame ically s ud‑ ied o ensu e compa able beha io o he ini ial design. Elec onics 2023,12, 1475 4 o 15 •A e all modi ica ions a e done, he s uc u e’s ma ching is enhanced again by c ea ‑ ing and uning wo pa s, “A” and “B”, as shown in Figu e 3, whe e: ◦A: is he space esona ing in he MICS band; ◦B: is he space esona ing in he ISM 2.4 GHz band. Elec onics 2022, 11, x. h ps://doi.o g/10.3390/xxxxx www.mdpi.com/jou nal/elec onics A icle 1 Modeling o a compac implan able dual band an enna o bio- 2 medical applica ions 3 4 (a) (b) 5 (c) (d) 6 7 (e) 8 Figu e 1. Di e en i e a ions ob ained h oughou he op imiza ion p ocess: (a) Koch 1s i e a ion 9 esona ing a 2.4 GHz (b) Modi ied Koch s uc u e (c) Ex ended modi ied Koch s uc u e o lowe 10 equency esonance (d) modi ied s uc u e o be e ma ching (e) single elemen s uc u e 11 Figu e 2. Di e en i e a ions ob ained h oughou he op imiza ion p ocess: (a) i s Koch i e a ion esona ing a 2.4 GHz; (b) modi ied Koch s uc u e; (c) ex ended modi ied Koch s uc u e o lowe equency esonance; (d) modi ied s uc u e o be e ma ching; (e) single‑elemen s uc u e. Region B is shaped in o a ec angle. I s dimensions a e de ined ollowing a pa ame ic s udy o bes ma ching. In he MICS band, he enhancemen is ob ained by applying se e al ex ensions a he ips o egion A. Region 3 is whe e he e lec ion coe icien is enhanced. Table 1shows he di e en an enna dimensions’ alues: O e all, he p oposed an enna has a olume o 64.65 mm3, and dimensions equi alen o (2.3% ×3.6% ×0.03%) λa 400 MHz and (14.01% ×12.1% ×0.2%) λa 2.4 GHz. 2.2. Simula ion The an enna was simula ed using he Ansys High F equency Simula ion Sys em— HFSS—in a single laye ‑mimicking en i onmen o acili a e he measu emen and alida‑ ion p ocedu e, as shown in Figu e 4. Elec onics 2023,12, 1475 5 o 15 Elec onics 2022, 11, x FOR PEER REVIEW 2 o 3 12 (a) 13 14 (b) 15 Figu e 2. Final p oposed s uc u e: (a) Face iew (b) Side iew wi h moun ed U.FL connec o 16 17 18 19 20 (a) (b) Figu e 3. An enna simula ion condi ions (a) in a homogenous skin mimicking phan om (b) wi h a 21 he U.FL connec o 22 23 e l=17.2 mm w=14.8 mm e=200 mm e e w l e Radia ion box An enna U.FL connec o Figu e 3. Final p oposed s uc u e: (a) ace iew; (b) side iew wi h moun ed U.FL connec o . Table 1. P oposed an enna’s dimensions. Pa ame e Value (mm) L 17.2 W 14.8 Dielec ic hickness 0.127 Sl 0.7 Sw 1.6 Sd1 1.8 Sd2 1 Lm1 9 Lm2 4.1 Lm3 5.3 Lm4 7.5 Lm5 3.9 Lm6 7.4 Lm7 1.9 Lmw 0.1 Lio 0.3 Wio 0.1 Le1 3.2 Le2 2 Wex 11.6 Lma 0.1 Lia 4.5 Wia 10.4 Elec onics 2023,12, 1475 6 o 15 Elec onics 2022, 11, x FOR PEER REVIEW 2 o 3 12 (a) 13 14 (b) 15 Figu e 2. Final p oposed s uc u e: (a) Face iew (b) Side iew wi h moun ed U.FL connec o 16 17 18 19 20 (a) (b) Figu e 3. An enna simula ion condi ions (a) in a homogenous skin mimicking phan om (b) wi h a 21 he U.FL connec o 22 23 e l=17.2 mm w=14.8 mm e=200 mm e e w l e Radia ion box An enna U.FL connec o Figu e 4. An enna simula ion condi ions (a) in a homogenous skin‑mimicking phan om (b) wi h he U.FL connec o . The simula ion was ca ied ou in wo con igu a ions. In Figu e 4a, he an enna is subme ged in a 200 ×200 ×200 mm3cube illed wi h a skin mimicking phan om, as i is in ended o be placed subcu aneously in he a m 2 mm below he skin. The i s simula‑ ion phases we e pe o med in he con igu a ion p esen ed in Figu e 4a. Due o simula o ’s limi a ions, he an enna was exci ed using an in e nal ideal po . To conside he use o an ex e nal connec o , including i s dimensions, he second con igu a ion, shown in Figu e 4b, was used. I consis ed o pu ing a 17.2 ×14.8 ×200 mm3skin mimicking phan om abo e he an enna o simula e placemen o a comme cially a ailable U.FL connec o as an ex e ‑ nal po in o de o ake in o accoun i s e ec s du ing simula ion. The U.FL connec o was used because o i s small dimensions and o simpli y connec ion be ween he an enna and he de ices. Human skin is cha ac e ized by ce ain elec ical pa ame e s, including pe mi i i y and conduc i i y. They depend on he equencies o he signals used. In he MICS and ISM 2.4 GHz bands, hese pa ame e s a e ε = 46.787, σ= 0.68807 S/m, and ε = 38.063, σ= 1.4407 S/m, espec i ely [9]. The simula ed S11 coe icien o he an enna p esen ed in Figu e 4b con igu a ion is shown in Figu e 5a,b. The simula ed esul s indica e ha in he MICS band, he pa ame e S11 has a alue o −32.18 dB a 402.5 MHz. The equency band in which he S11 coe icien is less han −10 dB uns om 395 o 409 MHz (a bandwid h o 14 MHz). In he ISM 2.4 GHz band, he an enna exhibi s ela i ely wide‑band beha io . The S11 emains below −10 dB a 2.33 o 2.51 GHz (a bandwid h o 180 MHz), and a minimum e lec ion coe icien equal o −19.65 dB is ob ained a 2.42 GHz. A pa ame ic s udy was pe o med in case he e was a misma ch on any o hese bands o any eason. Samples o he modi ied an ennas wi h di e en dimensions a e p esen ed in Figu e 6a–d, and he esul ing S11 pa ame e s a e shown in Figu e 7a,b o he MICS and ISM 2.4 GHz bands, espec i ely. Figu e 7a shows ha di e en alues o he “Lm7” pa ame e esiding in he adia ing egion o he MICS band con ibu e o he shi ing o he esonan equency, wi hou deg ading he impedance ma ching o he an enna. Figu e 7b shows a simila beha io in he ISM band, whe e he “Wex ” pa ame e helps in shi ing he esonan equency wi hou s ongly a ‑ ec ing he impedance ma ching. The shi ing in e ms o equency in one band does no a ec he o he , allowing one o pe o m equency‑independen uning. The equency shi ing sensi i i y ob ained by uning he men ioned pa ame e s is no he same: in he MICS band, shi ing he esonan equency om 402.5 MHz o 390 MHz equi es a a i‑ a ion o 4.8 mm in he “Lm7” pa ame e , whe eas in he ISM 2.4 GHz band, i akes only 0.5 mm o di e ence o shi he esonan equency om 2.42 o 2.51 GHz. Region B is he e o e mo e ulne able o s uc u al modi ica ions. Elec onics 2023,12, 1475 7 o 15 Elec onics 2022, 11, x FOR PEER REVIEW 7 o 16 (a) (b) Figu e 5. Simula ed e lec ion coe icien o he an enna in he (a) MICS band and (b) ISM 2.4 GHz band. The simula ed esul s indica e ha in he MICS band, he pa ame e S11 has a alue o −32.18 dB a 402.5 MHz. The equency band in which he S11 coe icien is less han −10 dB uns om 395 o 409 MHz (a bandwid h o 14 MHz). In he ISM 2.4 GHz band, he an enna exhibi s ela i ely wide-band beha io . The S11 emains below −10 dB a 2.33 o 2.51 GHz (a bandwid h o 180 MHz), and a minimum e lec ion coe icien equal o −19.65 dB is ob ained a 2.42 GHz. A pa ame ic s udy was pe o med in case he e was a mis- ma ch on any o hese bands o any eason. Samples o he modi ied an ennas wi h di - e en dimensions a e p esen ed in Figu e 6a–d, and he esul ing S11 pa ame e s a e shown in Figu e 7a,b o he MICS and ISM 2.4 GHz bands, espec i ely. Figu e 7a shows ha di e en alues o he “Lm7” pa ame e esiding in he adia ing egion o he MICS band con ibu e o he shi ing o he esonan equency, wi hou deg ading he imped- ance ma ching o he an enna. Figu e 7b shows a simila beha io in he ISM band, whe e he “Wex ” pa ame e helps in shi ing he esonan equency wi hou s ongly a ec ing he impedance ma ching. The shi ing in e ms o equency in one band does no a ec he o he , allowing one o pe o m equency-independen uning. The equency shi ing sensi i i y ob ained by uning he men ioned pa ame e s is no he same: in he MICS band, shi ing he esonan equency om 402.5 MHz o 390 MHz equi es a a ia ion o 4.8 mm in he “Lm7” pa ame e , whe eas in he ISM 2.4 GHz band, i akes only 0.5 mm o di e ence o shi he esonan equency om 2.42 o 2.51 GHz. Region B is he e o e mo e ulne able o s uc u al modi ica ions. Lm7 = 1.9 mm Lm7 = 6.7 mm Figu e 5. Simula ed e lec ion coe icien o he an enna in he (a) MICS band and (b) ISM 2.4 GHz band. Elec onics 2022, 11, x FOR PEER REVIEW 3 o 3 24 25 (a) (b) 26 27 (c) (d) Figu e 4. P oposed s uc u e wi h di e en pa ame e a ia ions: (a) Lm7= 1.9 mm (b) Lm7= 6.7 mm 28 (c) Wex =11.6 mm (d) Wex = 11.1 mm 29 30 Lm7=1.9mm Lm7=6.7mm Wex =11.6mm Wex =11.1mm Figu e 6. P oposed s uc u e wi h di e en pa ame e a ia ions: (a) Lm7 = 1.9 mm, (b) Lm7 = 6.7 mm, (c) Wex = 11.6 mm, (d) Wex = 11.1 mm. Elec onics 2023,12, 1475 8 o 15 Elec onics 2022, 11, x FOR PEER REVIEW 8 o 16 (a) (b) (c) (d) Figu e 6. P oposed s uc u e wi h di e en pa ame e a ia ions: (a) Lm7= 1.9 mm, (b) Lm7= 6.7 mm, (c) Wex = 11.6 mm, (d) Wex = 11.1 mm. (a) (b) Figu e 7. Re lec ion coe icien o he an enna wi h a ia ions in he esonan equency in he (a) MICS band and (b) ISM band. The simula ed adia ion pa e ns o he s uc u e in bo h bands a e shown in Figu e 8a,b. Wex = 11.6 mm Wex = 11.1 mm Figu e 7. Re lec ion coe icien o he an enna wi h a ia ions in he esonan equency in he (a) MICS band and (b) ISM band. The simula ed adia ion pa e ns o he s uc u e in bo h bands a e shown in Figu e 8a,b. Elec onics 2022, 11, x FOR PEER REVIEW 8 o 15 (c) (d) Figu e 6. P oposed s uc u e wi h di e en pa ame e a ia ions: (a) Lm7 = 1.9 mm, (b) Lm7 = 6.7 mm, (c) Wex = 11.6 mm, (d) Wex = 11.1 mm. (a) (b) Figu e 7. Re lec ion coe icien o he an enna wi h a ia ions in he esonan equency in he (a) MICS band and (b) ISM band. The simula ed adia ion pa e ns o he s uc u e in bo h bands a e shown in Figu e 8a,b. (a) (b) Figu e 8. Simula ed adia ion pa e n in (a) he MICS band and (b) he ISM band. Wex = 11.6 mm Wex = 11.1 mm Figu e 8. Simula ed adia ion pa e n in (a) he MICS band and (b) he ISM band. The an enna exhibi s quasi‑omnidi ec ional adia ion pa e ns in bo h MICS and ISM 2.4 GHz bands. Gain alues each −42.97 and −19.82 dBi in hese bands, espec i ely. The ob ained low gain alues a e due o he low p o ile o he an enna and he lossy en i on‑ men i is placed in. Elec onics 2023,12, 1475 9 o 15 3. Tissue‑Mimicking Phan om Cha ac e iza ion To alida e he a ious esul s ob ained by simula ion, measu emen s mus be ca ied ou ei he in i o o in i o. Fo he la e , he en i onmen simula ing eal condi ions mus imi a e he biological issues o in e es . The an enna is designed o ope a e a a dep h o 2 mm below he skin o he human a m. Thus, a phan om mimicking human skin was equi ed. In he li e a u e, a a ie y o mix u es exis , wo o which a e he suga /NaCl and T i on X‑100/DGBE/NaCl o mulas. A pa ame ic s udy was pe o med o es ablish he alidi y and accu acy o hese o mulas. Fo he s udy conduc ed in his wo k, he Keysigh 85070E open‑ended coaxial p obe was used o cha ac e iza ion, and he con igu a ion is p esen ed in Figu e 9. Elec onics 2022, 11, x FOR PEER REVIEW 9 o 15 The an enna exhibi s quasi-omnidi ec ional adia ion pa e ns in bo h MICS and ISM 2.4 GHz bands. Gain alues each −42.97 and −19.82 dBi in hese bands, espec i ely. The ob ained low gain alues a e due o he low p o ile o he an enna and he lossy en i on- men i is placed in. 3. Tissue-Mimicking Phan om Cha ac e iza ion To alida e he a ious esul s ob ained by simula ion, measu emen s mus be ca - ied ou ei he in i o o in i o. Fo he la e , he en i onmen simula ing eal condi ions mus imi a e he biological issues o in e es . The an enna is designed o ope a e a a dep h o 2 mm below he skin o he human a m. Thus, a phan om mimicking human skin was equi ed. In he li e a u e, a a ie y o mix u es exis , wo o which a e he suga /NaCl and T i on X-100/DGBE/NaCl o mulas. A pa ame ic s udy was pe o med o es ablish he alidi y and accu acy o hese o mulas. Fo he s udy conduc ed in his wo k, he Keysigh 85070E open-ended coaxial p obe was used o cha ac e iza ion, and he con ig- u a ion is p esen ed in Figu e 9. Figu e 9. Phan om cha ac e iza ion se up (Keysigh E5071C VNA and Keysigh 85070E dielec ic p obe ki ). 3.1. Suga /NaCl Mix u e The composi ion o his o mula is based on widely a ailable componen s: suga and an elec oly e, which in his case was egula sal , sodium chlo ide (NaCl), mixed in de- ionized wa e . The desi ed dielec ic pa ame e s can be achie ed by adjus ing hese com- ponen s. Pe mi i i y is dependen on he quan i y o suga added in o he mix u e, whe eas he elec oly e helps wi h adjus ing he conduc i i y. Howe e , as seen in [4], a highe equencies, and pa icula ly in he ISM 2.4 GHz band, he o mula ob ained shows ha al hough he pe mi i i y can be accu a ely con olled, he conduc i i y inc eases d as ically wi hou he use o NaCl, exceeding he desi ed alues uncon ollably. Fo he ISM 2.4 GHz band, a ial was ca ied ou o analyze he dielec ic p ope ies o a mix u e comp ised o 100 mg o suga dissol ed in 100 mL o deionized wa e . The ob ained e- sul s a e shown in Figu e 10. Figu e 9. Phan om cha ac e iza ion se up (Keysigh E5071C VNA and Keysigh 85070E dielec ic p obe ki ). 3.1. Suga /NaCl Mix u e The composi ion o his o mula is based on widely a ailable componen s: suga and an elec oly e, which in his case was egula sal , sodium chlo ide (NaCl), mixed in deion‑ ized wa e . The desi ed dielec ic pa ame e s can be achie ed by adjus ing hese compo‑ nen s. Pe mi i i y is dependen on he quan i y o suga added in o he mix u e, whe eas he elec oly e helps wi h adjus ing he conduc i i y. Howe e , as seen in [4], a highe equencies, and pa icula ly in he ISM 2.4 GHz band, he o mula ob ained shows ha al‑ hough he pe mi i i y can be accu a ely con olled, he conduc i i y inc eases d as ically wi hou he use o NaCl, exceeding he desi ed alues uncon ollably. Fo he ISM 2.4 GHz band, a ial was ca ied ou o analyze he dielec ic p ope ies o a mix u e comp ised o 100 mg o suga dissol ed in 100 mL o deionized wa e . The ob ained esul s a e shown in Figu e 10. As can be seen in Figu e 10a,b, bo h pe mi i i y and conduc i i y inc ease p opo ‑ ionally o he amoun o added suga wi hou adding sal . Wi h he addi ion o 100 mg o suga , he dielec ic pa ame e s ob ained we e ε = 38.08 and σ= 2.24 S/m, hough he e was an o e shoo in he desi ed conduc i i y, as expec ed. Mo eo e , aside om he inabili y o con ol a c ucial pa ame e , he dissolu ion o suga is slow because o i s high concen a‑ ion. This makes his o mula un eliable o making phan oms o high‑ equency bands. 3.2. T i on X‑100/DGBE Mix u e This o mula equi es essen ially wo comme cially a ailable ing edien s: T i on X‑ 100 and DGBE. These allow mixing a liquid wi h he desi ed dielec ic pa ame e s o hu‑ man issues in he bands o in e es [6]. Addi ional ing edien s a e used [10], bu o he phan oms equi ed o alida e he pe o mance o he p esen ed an enna, only NaCl was used. Figu es 11–14 show a ious pa ame ic s udies a he MICS and ISM 2.4 GHz bands wi h di e en amoun s o ing edien s added o 100 mL o deionized wa e .