Planar Inverted-F Antenna (PIFA) Using Microfluidic Impedance Tuner

sensors

Article Planar Inverted-F Antenna (PIFA) Using Microﬂuidic Impedance Tuner

Minjae Lee and Sungjoon Lim * School of Electrical and Electronics Engineering, College of Engineering, Chung-Ang University, Seoul 156-756, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2-820-5827; Fax: +82-2-812-7431

Received: 1 August 2018; Accepted: 18 September 2018; Published: 20 September 2018

Abstract: This paper proposes a microfluidic impedance tuner that is applied to a planar inverted-F antenna (PIFA). The proposed microfluidic impedance tuner is designed while using a simple double-stub and the impedance is changed by tuning the stub length. In this work, the stub length can be tuned by injecting a liquid metal alloy to the microfluidic channels. Initially, the PIFA operates at 900 MHz with impedance matching of 50 Ω. The impedance is mismatched when a hand is placed close to the antenna. The mismatched impedance is matched to 50 Ω by injecting the liquid metal alloy. The antenna is fabricated on the FR-4 substrate, and the impedance tuner is fabricated on polydimethylsiloxane (PDMS). In order to inject the liquid metal alloy, a piezoelectric micropump and microprocessor are used in the measurement. At 900 MHz, the return loss is successfully tuned from 4.69 dB to 18.4 dB when a hand is placed 1 mm above the antenna.

Keywords: impedance tuner; matching network; microﬂuidic; planar inverted-F antenna (PIFA)

1. Introduction The presence of a mismatched impedance between circuits results in an overall degradation in circuit performance; impedance matching between circuits is therefore the basis of high frequency electronics. Antennas significantly influence the performance of the entire application, as their impedance values vary significantly even in operating environments having small changes. In particular, as parts of the human body get closer to the antenna, its performance suffers [1]. With the advances in the field of wireless communication systems, an increasing number of components are being incorporated in mobile handset design, reducing the space in which complex advanced antennas can be implemented into the handset. To solve this problem, the use of an impedance tuner is one of the representative methods of matching the antenna impedance. Impedance tuners are widely used to match impedances, such as antennas [2], power amplifier (PA) load-pull [3], and noise characterization [4], and so on. Conventional impedance tuners comprise mainly three different techniques, namely the single-stub [5], double-stub [6], and optimization methods [7]. Impedance tuning has been widely studied using active and passive components, such as microelectromechanical systems (MEMS) components or diode varactors [6,8]. While circuits with many switching elements can have many tuning points, tuning is obtained at the expense of higher cost and insertion loss. Microfluidic technology has been developed to carry out analysis, screening, and detection of very small quantities of biomaterial and chemical samples. It can be implemented at a less complexity and low cost. Especially, liquid metal in microfluidic channels have been studied for flexible or reconfigurable radio frequency (RF) components [9], such as filters [10], sensors [11,12], absorbers [13,14], antennas [15], and MEMS switch [16]. In this paper, we propose a microfluidic impedance tuner while using liquid metal as a switch. When compared to diode switches, the liquid metal switch provides advantages, such as wider

Sensors 2018, 18, 3176; doi:10.3390/s18103176 www.mdpi.com/journal/sensors Sensors 2018, 18, x FOR PEER REVIEW 2 of 9

In this paper, we propose a microfluidic impedance tuner while using liquid metal as a switch. When compared to diode switches, the liquid metal switch provides advantages, such as wider Sensors 2018, 18, 3176 2 of 9 switching range, high power capability, no bias network, and direct current (DC) power consumption, and less parasitic effect. Previously, a microfluidic impedance tuner with a double switchingstub structure range, has high been power proposed capability, [17,18] no where bias network, the liquid and metal direct is current continuously (DC) power injected consumption, into long andfluidic less channels. parasitic In effect. this Previously,work, the liquid a microfluidic metal is impedance used as atuner switch with in ashort double fluidic stub channels. structure hasTherefore, been proposed the injection [17,18 and] where extraction the liquid are faster, metal iseasier, continuously and reliable. injected In addition, into long the fluidic injection channels. and Inextraction this work, of theliquid liquid metal metal are is used controlled as a switch a inmicrocontroller short fluidic channels.and micropump Therefore, for the injectionpractical andimplementation. extraction are Finally, faster,easier, the proposed and reliable. microfluid In addition,ic impedance the injection tuner and extractionis tested with of liquid a planar metal areinverted-F controlled antenna a microcontroller (PIFA). and micropump for practical implementation. Finally, the proposed microfluidicThe proposed impedance microfluidic tuner is testedimpedance with atuner planar is inverted-Fdesigned using antenna a simple (PIFA). double-stub and the impedanceThe proposed is changed microfluidic by tuning impedancethe stub length. tuner Th ise designed stub length using can abe simple tuned double-stubby injecting anda liquid the impedancemetal alloy isto changedthe microfluidic by tuning channels. the stub The length. PIFA The with stub the length proposed can beimpedance tuned by tuner injecting is designed a liquid metalwhile alloyusing to circuit the microfluidic and full-wave channels. simulation. The PIFA Its performance with the proposed is numerica impedancelly and tuner experimentally is designed whiledemonstrated. using circuit The design and full-wave and fabrication simulation. processes Its performance are explained. is numerically and experimentally demonstrated. The design and fabrication processes are explained. 2. Design of PIFA and Microfluidic Impedance Tuner 2. Design of PIFA and Microfluidic Impedance Tuner 2.1. PIFA Design 2.1. PIFA Design First of all, the conventional PIFA is designed to resonate at 900 MHz using ANSYS high frequencyFirst of structure all, the conventional simulator (HFSS), PIFA is as designed shownto in resonate Figure at1 [19,20]. 900 MHz The using FR-4 ANSYS board high (size: frequency 44 × 82 2 structuremm2, thickness: simulator 1.6 (HFSS), mm) is as used shown as the in Figure antenna1[ 19 subs,20].trate. The Its FR-4 permittivity board (size: and 44 × loss82 mmtangent, thickness: are 4.2 1.6and mm) 0.02, is respectively. used as the antennaThe geometrical substrate. parameters Its permittivity are indicated and loss tangentin Figure are 1a 4.2 as andPa = 0.02,28.8, respectively.Pb = 14, Pc = The23, P geometricald = 2, Pe = 14, parameters Pf = 15, Pg = are 34, indicatedPh = 2, Pj = in 6, Figure Pk = 1,1 Taw as = P3,a and= 28.8, Tl = P60b =mm. 14, Its Pc =simulated 23, Pd = 2,measured Pe = 14, Pimpedancesf = 15, Pg = are 34, plotted Ph = 2, on Pj = Smith 6, Pk Chart,= 1, Tw as= shown 3, and in Tl Figure= 60 mm. 1b,c Its shows simulated the simulated measured and impedances measured arereturn plotted loses. on At Smith 900 MHz, Chart, the as shownsimulated inFigure and measured1b,c shows returns the simulated losses are and −30.88 measured dB and return −12.25 loses. dB, Atrespectively. 900 MHz, the simulated and measured returns losses are −30.88 dB and −12.25 dB, respectively.

(a)

(b) (c)

Figure 1. (a) Geometrical parameters of planar inverted-F antenna (PIFA); (b) Simulated and measured

input impedances on Smith Chart; and, (c) Simulated and measured return losses. Sensors 2018, 18, x FOR PEER REVIEW 3 of 9

Figure 1. (a) Geometrical parameters of planar inverted-F antenna (PIFA); (b) Simulated and measured input impedances on Smith Chart; and, (c) Simulated and measured return losses.

2.2. Impedance Measurement at Touched and Untouched States In order to design the impedance tuner, we measured input impedance of the PIFA under two scenarios of touched and untouched cases, as illustrated in Figure 2a. It is expected that the impedance is mismatched when the PIFA is touched by a hand. Before measuring the impedance, we fabricate the mockup using a three-dimensional (3D) printer with poly lactic acid (PLA) filament. As shown in Figure 2b, the size of the mockup is 48 × 82 × 7.6 mm3 and thickness is 1 mm. The Styrofoam is inserted to fix the antenna position with the SubMiniature version A (SMA) connector. Figure 2c,d show the measured input impedance and return loss of the PIFA with the mockup for touched and untouched states, respectively. At 900 MHz, the return loss of the untouched state is 12.25 dB. When the top of the mockup is touched by a hand, the return loss becomes 5.946 dB at 900 MHz. The measured impedances will be used for a microfluidic impedance tuner design.

2.3. Microfluidic Impedance Tuner Design In order to design the microfluidic impedance tuner, Keysight Advanced Design System (ADS) circuit simulator is used. As shown in Figure 3a, the double-stub matching network is designed by using the measured impedance of the PIFA at the touched state. At the untouched state, the 50-Ω Sensorstransmission2018, 18, 3176line is not connected to both two open stubs (TL4 and TL5). In order to connect3 of to 9 stubs, liquid metal is used. Electrical length of TL1, TL2, and TL3 are 0.02λ, 0.125λ, and 0.02λ, respectively. In addition, electrical length of TL4 and TL5 are 3.27λ and 0.75λ, respectively. 2.2.Characteristic Impedance Measurementimpedance of at all Touched transmission and Untouched lines is States50-Ω. Based on the ADS design, the impedance tunerIn is order designed to design in a the microstrip impedance line tuner, using we ANSYS measured HFSS. input Figure impedance 3b shows of the the PIFA layout under of twothe scenariosproposed ofimpedance touched and tuner untouched where two cases, open as illustratedstubs are not in Figure connected2a. It isto expected the 50-Ω that microstrip the impedance line. In isorder mismatched to minimize when the the space, PIFA the is touchedTL4 is realized by a hand. while Before using measuring a meander the line. impedance, The TL5 is we not fabricate shown thebecause mockup it will using be arealized three-dimensional using liquid (3D) metal. printer Figure with 3c polyshows lactic the acidmicrofluidic (PLA) filament. channel As which shown is inworking Figure 2asb, thea switch. size of When the mockup the channel is 48 × is82 filled× 7.6 with mm3 liquidand thickness metal alloy, is 1 mm. two The open Styrofoam stubs are is insertedconnected to to fix the the 50- antennaΩ microstrip position line. with Figure the SubMiniature3d shows the integrated version A microfluidic (SMA) connector. impedance Figure tuner.2c,d showThe geometrical the measured parameters input impedance of Figure 3b,c and are return Ls = loss 60, W ofs the= 40, PIFA Wa = with 1.51, the d = mockup 5, Gm = 1, for Lm touched = 20, Ca and= 7, untouchedCb = 8, Cc =states, 3, Lp = respectively. 55, and Wp At= 30 900 mm. MHz, The the overlap return lossarea ofbetween the untouched the microfluidic state is 12.25 channel dB. When and the topmicrostrip of the mockup lines are is used touched as a byreservoir a hand, for the the return liquid loss metal becomes alloy, 5.946 as illustrated dB at 900 in MHz. Figure The 3d. measured impedances will be used for a microfluidic impedance tuner design.

(a) (b)

Figure 2. (a) Measurement setup of PIFA at touched and untouched states; (b) Photograph of the 3D printed mockup with PIFA; (c) measured input impedances on Smith Chart at touched and untouched states; (d) measured reﬂection coefﬁcients at touched and untouched states.

2.3. Microfluidic Impedance Tuner Design In order to design the microfluidic impedance tuner, Keysight Advanced Design System (ADS) circuit simulator is used. As shown in Figure3a, the double-stub matching network is designed by using the measured impedance of the PIFA at the touched state. At the untouched state, the 50-Ω transmission line is not connected to both two open stubs (TL4 and TL5). In order to connect to stubs, liquid metal is used. Electrical length of TL1, TL2, and TL3 are 0.02λ, 0.125λ, and 0.02λ, respectively. In addition, electrical length of TL4 and TL5 are 3.27λ and 0.75λ, respectively. Characteristic impedance of all transmission lines is 50-Ω. Based on the ADS design, the impedance tuner is designed in a microstrip line using ANSYS HFSS. Figure3b shows the layout of the proposed impedance tuner where two open stubs are not connected to the 50-Ω microstrip line. In order to minimize the space, the TL4 is realized while using a meander line. The TL5 is not shown because it will be realized using liquid metal. Figure3c shows the microfluidic channel which is working as a switch. When the channel is filled with liquid metal alloy, two open stubs are connected to the 50-Ω microstrip line. Figure3d shows the integrated microfluidic impedance tuner. The geometrical parameters of Figure3b,c are Ls = 60, Ws = 40, Wa = 1.51, d = 5, Gm = 1, Lm = 20, Ca = 7, Cb = 8, Cc = 3, Lp = 55, and Wp = 30 mm. The overlap area between the microfluidic channel and the microstrip lines are used as a reservoir for the liquid metal alloy, as illustrated in Figure3d. Sensors 2018, 18, x FOR PEER REVIEW 4 of 9

Figure 2. (a) Measurement setup of PIFA at touched and untouched states; (b) Photograph of the 3D printed mockup with PIFA; (c) measured input impedances on Smith Chart at touched and untouched states; (d) measured reflection coefficients at touched and untouched states.

The side view of the proposed microfluidic impedance tuner is shown in Figure 3e. The microstrip lines of Figure 3b is realized on a Rogers RT/Duroid 5880 (thickness: 0.51 mm). The microfluidic channels of Figure 3c is realized on polydimethylsiloxane (PDMS). Two substrates are bonded using the adhesive film with a thickness of 0.05 mm. In order to connect a tube to the microfluidic channel, the diameter of the inlet hole is set to 1.2 mm. The tubes will be connected to Sensorsthe micropump.2018, 18, 3176 Figure 4 shows the simulated and measured impedances of the proposed4 of 9 impedance tuner at 900 MHz.

(a)

(b) (c)

(d)

(e)

Figure 3.3. ((a)) MeasurementMeasurement setupsetup ofof PIFAPIFA atat touchedtouched andand untoucheduntouched states;states; ((bb)) PhotographPhotograph ofof thethe 3D3D printedprinted mockupmockup with with PIFA; PIFA; (c) measured(c) measured input input impedances impedances on Smith on Chart Smith at touchedChart at and touched untouched and states;untouched (d) measured states; (d) reflection measured coefficients reflection atcoefficients touched andat touched untouched and states;untouched and, (states;e) Side and, view (e of) Side the proposedview of the microfluidic proposed microfluidic impedance tuner.impedance tuner.

The side view of the proposed microfluidic impedance tuner is shown in Figure3e. The microstrip lines of Figure3b is realized on a Rogers RT/Duroid 5880 (thickness: 0.51 mm). The microfluidic channels of Figure3c is realized on polydimethylsiloxane (PDMS). Two substrates are bonded using the adhesive film with a thickness of 0.05 mm. In order to connect a tube to the microfluidic channel, the diameter of the inlet hole is set to 1.2 mm. The tubes will be connected to the micropump. Figure4 shows the simulated and measured impedances of the proposed impedance tuner at 900 MHz. Sensors 2018,, 18,, 3176x FOR PEER REVIEW 55 ofof 99 Sensors 2018, 18, x FOR PEER REVIEW 5 of 9

Figure 4. Simulated and measured impedances of the proposed impedance tuner from 100 MHz to Figure1000 MHz. 4.4. Simulated and measured impedances of the proposed impedance tuner from 100 MHz to 10001000 MHz.MHz.

3. Fabrication and Measurement Result 3. Fabrication and Measurement Result 3.1. Fabrication of MicrofluidicMicrofluidic Channel 3.1. Fabrication of Microfluidic Channel Figure 55 showsshows thethe fabricationfabrication process of the microfluidic microfluidic channel. channel. The The microfluidic microfluidic channel channel is Figure 5 shows the fabrication process of the microfluidic channel. The microfluidic channel is isfabricated fabricated inside inside a PDMS a PDMS (Shielding (Shielding Solutions Solutions Ltd., Ltd., Braintree, Braintree, UK) UK)while while using using laser laseretching. etching. The fabricated inside a PDMS (Shielding Solutions Ltd., Braintree, UK) while using laser etching. The Theadhesive adhesive film film(Adhesives (Adhesives Research, Research, Glen Glen Rock, Rock, PA, PA, USA) USA) is isalso also laser laser etched etched as as the the same pattern adhesive film (Adhesives Research, Glen Rock, PA, USA) is also laser etched as the same pattern with PDMSPDMS inin order order to to contact contact the the liquid liquid metal metal on the on copperthe coppe patternr pattern of Duroid of Duroid substrate. substrate. The surface The with PDMS in order to contact the liquid metal on the copper pattern of Duroid substrate. The ofsurface the liquid of the metal liquid can metal be solidified can be solidified when exposed when to exposed the air/oxygen. to the air/oxygen. Because these Because oxide these layers oxide tend surface of the liquid metal can be solidified when exposed to the air/oxygen. Because these oxide tolayers remain tend to to the remain microfluidic to the microfluidic channel as residue, channel this as oxidationresidue, this problem oxidation must problem be solved must for repeatablebe solved layers tend to remain to the microfluidic channel as residue, this oxidation problem must be solved uses.for repeatable In this work, uses. inIn orderthis work, to solve in order the oxidation to solve the problem, oxidation the problem, microfluidic the microfluidic channel is pre-treated channel is for repeatable uses. In this work, in order to solve the oxidation problem, the microfluidic channel is withpre-treated Nafion with solution. Nafion When solution. hydrochloric When hydrochloric acid (HCL) acid solution (HCL) is solution poured, is Nafion poured, absorbs Nafion the absorbs HCL pre-treated with Nafion solution. When hydrochloric acid (HCL) solution is poured, Nafion absorbs solution.the HCL solution. Thus, the Thus, oxidation the oxidation problemcan problem be solved can bybe solved pushing by off pushing the liquid off metalthe liquid in the metal channel in the as the HCL solution. Thus, the oxidation problem can be solved by pushing off the liquid metal in the thechannel residue. as the residue. channel as the residue.

Figure 5. Fabrication process of the polydimethylsiloxane (PDMS) and adhesive film.film. Figure 5. Fabrication process of the polydimethylsiloxane (PDMS) and adhesive film. 3.2. Microcontroller and Micropump 3.2. Microcontroller and Micropump 3.2. MicrocontrollerBefore injecting and the Micropump liquid metal alloy, the fabricated impedance tuner sample is shown in Before injecting the liquid metal alloy, the fabricated impedance tuner sample is shown in Figure6a. EGaIn (Eutectic gallium–indium) is used as the liquid metal alloy. In order to inject and FigureBefore 6a. EGaIn injecting (Eutectic the liquid gallium–indium) metal alloy, is the used fa bricatedas the liquid impedance metal alloy.tuner In sample order tois injectshown and in extract the liquid metal alloy into the microfluidic channels, an Mp6 micropump and MP6-QuadEVA extractFigure 6a.the EGaIn liquid (Eutectic metal gallium–indium)alloy into the ismicrofluidic used as the liquidchannels, metal analloy. Mp6 In ordermicropump to inject and microcontroller (Bartels Mikrotechnik GmbH, Dortmund, Germany) are used, as shown in Figure6b. MP6-QuadEVAextract the liquid microcontroller metal alloy (Bartels into theMikrotechnik microfluidic GmbH, channels, Dortmund, an Mp6Germany) micropump are used, and as Because a micropump can pass though only one way, two micropumps are required for injecting and shownMP6-QuadEVA in Figure microcontroller 6b. Because a micropump(Bartels Mikrotechnik can pass though GmbH, only Dortmund, one way, Germany) two micropumps are used, are as extracting ways of each microfluidic channel. In addition, the pump performance drops when the requiredshown in forFigure injecting 6b. Because and extracting a micropump ways ofcan each pass microfluidic though only channel. one way, In two addition, micropumps the pump are performancerequired for dropsinjecting when and the extracting liquid metal ways alloy of each is passed microfluidic through channel. this pump. In addition,Therefore, the we pump used performance drops when the liquid metal alloy is passed through this pump. Therefore, we used

Sensors 2018, 18, 3176 6 of 9 Sensors 2018, 18, x FOR PEER REVIEW 6 of 9 fourliquid pumps metal alloyto prevent is passed the throughliquid metal this pump.alloy from Therefore, passing we through used four the pumps pump. to Finally, prevent total the liquid four micropumpsmetal alloy from are used passing in this through work. theEach pump. micropump Finally, is total connected four micropumps to the microcontroller are used in with this tubes. work. Each micropump is connected to the microcontroller with tubes.

(a) (b)

Figure 6. (a) Photograph of sample microfluidic impedance tuner; and, (b) with the microcontroller Figureand micropump. 6. (a) Photograph of sample microfluidic impedance tuner; and, (b) with the microcontroller and micropump. 3.3. Measurement Result 3.3. MeasurementIn order to experimentally Result demonstrate the proposed idea, the microfluidic impedance tuner is connectedIn order to theto experimentally PIFA with the mockup.demonstrate We measured the proposed the S-parametersidea, the microfluid of the fullic impedance prototype withtuner the is connectedPIFA in the to mockup the PIFA and with the the microfluidic mockup. impedanceWe measur tunered the with S-parameters the micropumps of the full and prototype microprocessor with thein four PIFA steps. in Asthe a firstmockup step, theand impedance the microfluidic of the full impedance prototype istuner measured with whenthe micropumps the mockup isand not microprocessortouched, and liquid in four metal steps. alloy As isa first not injected.step, the Theimpedance measured of the return full lossprototype at 880 is MHz measured is 10.288 when dB, theasshown mockup in is Figure not touched,7a. As a secondand liquid step, metal the impedance alloy is not of injected. the full prototypeThe measured is measured return loss when at the880 MHzmockup is 10.288 is touched, dB, as and shown liquid in metalFigure alloy 7a. As is nota se injected.cond step, The the measured impedance return of the loss full at prototype 880 MHz is measured3.784 dB, as when shown the in mockup Figure7 b.is Astouched, a third and step, liquid the impedance metal alloy of is the not full injected. prototype The is measured measured return when lossthe mockupat 880 MHz is touched, is 3.784 and dB, liquid as shown metal in alloy Figure is injected. 7b. As Thea third measured step, the return impedance loss at 880of the MHz full is prototype17.104 dB, asis shownmeasured in Figure when7 c.the As mockup a fourth step,is touched, the impedance and liquid of the metal full prototypealloy is injected. is measured The measuredwhen the mockupreturn loss is untouchedat 880 MHz and is liquid 17.104 metal dB, alloyas shown is extracted. in Figure The 7c. measured As a fourth return step, loss the at impedance880 MHz is of 11.333 the full dB, prototype as shown inis measured Figure7d. when the mockup is untouched and liquid metal alloy is extracted. The measured return loss at 880 MHz is 11.333 dB, as shown in Figure 7d. 3.4. Instill Rate Although fluidically tuning capability provides merits of less complexity and no DC bias network, low tuning speed can limit its applications. Therefore, it is meaningful to discuss the injection speed for the microfluidic channel. In order to measure the injection speed, we measured the instill rate, which is an indicator of how quickly the proposed impedance tuner can respond. Figure8 shows the measurement setup. The liquid metal alloy is injected to the microfluidic channel with a 100-mm length and a 0.51-mm width. It takes 1–1.5 s to completely fill the liquid metal alloy into the microfluidic channel. Therefore, the instill rate is 66.7–100 mm/s.

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Figure 7. Measurement results of the full prototype with the PIFA and microfluidic tuner in the mockup: (a) not touched mockup, and not injecting liquid metal alloy; (b) touched mockup, and not injecting liquid metal alloy; (c) touched mockup, and injecting liquid metal alloy; and, (d) not touched mockup, and extracting liquid metal alloy.

3.4. Instill Rate Although fluidically tuning capability provides merits of less complexity and no DC bias network, low tuning speed can limit its applications. Therefore, it is meaningful to discuss the injection speed for the microfluidic channel. In order to measure the injection speed, we measured the instill rate, which is an indicator of how quickly the proposed impedance tuner can respond. Figure 7. Measurement results of the full prototype with the PIFA and microﬂuidic tuner in the mockup: FigureFigure 8 shows 7. Measurement the measurement results setup.of the Thefull prototypeliquid metal with alloy the PIFAis injected and microfluidic to the microfluidic tuner in channelthe (a) not touched mockup, and not injecting liquid metal alloy; (b) touched mockup, and not injecting with mockup:a 100-mm (a) lengthnot touched and amockup, 0.51-mm and width. not in jectingIt takes liquid 1–1.5 metal s to alloy;completely (b) touched fill the mockup, liquid and metal not alloy liquid metal alloy; (c) touched mockup, and injecting liquid metal alloy; and, (d) not touched mockup, into theinjecting microfluidic liquid metal channel. alloy; Therefore, (c) touched the mockup, instill rate and is injecting 66.7–100 liquid mm/s. metal alloy; and, (d) not and extracting liquid metal alloy. touched mockup, and extracting liquid metal alloy.

Figure 8.8.Test Test setup setup to measureto measure instill instill rate of rate the proposedof the proposed microﬂuidic microfluid channelic with channel the micropumps with the andmicropumps microprocessor. and microprocessor.

4. Conclusions In this work, we proposed a PIFA with the microfluidic impedance tuner. The proposed impedance tuner is based on double-stub matching circuit and microfluidic channel. The microfluidic impedance tuner is designed while using Keysight ADS and ANSYS HFSS from measured S-parameters of the PIFA with the mockup when the PIFA with the mockup is touched and untouched. When the PIFA is touched by a hand, the mismatched impedance is matched to 50-Ω by injecting the liquid metal alloy to the microfluidic impedance tuner. Therefore, the return losses at 880 MHz can be Figure 8. Test setup to measure instill rate of the proposed microfluidic channel with the micropumps and microprocessor.

Sensors 2018, 18, 3176 8 of 9 kept as 10.288 dB and 17.104 dB for the untouched and touched states, respectively. The proposed prototype is successfully tested by using the micropumps and microprocessor. The instill rate of the micropumps and microprocessor is measured as 66.7–100 mm/s with the microfluidic channel. Although the microfluidic switch shows slower switching speed than semiconductor switches, it is useful for applications not requiring fast switching speed. Therefore, the proposed microfluidic impedance tuner can be potentially used for the antenna impedance tuner and millimeter-wave tuning components with more advanced microfluidic technology.

Author Contributions: S.L. conceived the idea; M.L. performed the design, fabrication, and measurement and wrote the manuscript. S.L. revised the manuscript. Funding: This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2018R1A4A1023826). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Boyle, K.R.; Yuan, Y.; Ligthart, L.P. Analysis of mobile phone antenna impedance variations with user proximity. IEEE Trans. Antennas Propag. 2007, 55, 364–372. [CrossRef] 2. Zhou, Z.; Melde, K.L. Frequency agility of broadband antennas integrated with a reconfigurable RF impedance tuner. IEEE Antennas Wirel. Propag. Lett. 2007, 6, 56–59. [CrossRef] 3. Bae, Y.; Kim, U.; Kim, J. A Programmable impedance tuner with finite SWRs for load-pull measurement of handset power amplifiers. IEEE Microw. Wirel. Compon. Lett. 2015, 25, 268–270. [CrossRef] 4. Bouvot, S.; Carlos, J.; Goncalves, A.; Bossuet, A.; Quémerais, T.; Ducournau, G.; Danneville, F.; Gloria, D.; Monnet, J. A 140 GHz to 160 GHz active impedance tuner for in-situ noise characterization in BiCMOS 55 nm. In Proceedings of the 2017 IEEE International Symposium on Radio-Frequency Integration Technology (RFIT), Seoul, South Korea, 30 August–1 September 2017; pp. 153–155. 5. Vähä-Heikkilä, T.; Varis, J.; Tuovinen, J.; Rebeiz, G.M. A 20-50 GHz RF MEMS single-stub impedance tuner. IEEE Microw. Wirel. Compon. Lett. 2005, 15, 205–207. [CrossRef] 6. Quaglia, R.; Andersson, C.M.; Fager, C.; Pirola, M. A double stub impedance tuner with SiC diode varactors. In Proceedings of the Asia-Pacific Microwave Conference 2011, Melbourne, VIC, Australia, 5–8 December 2011; pp. 267–270. 7. Gu, Q.; De Luis, J.R.; Morris, A.S.; Hilbert, J. An analytical algorithm for Pi-network impedance tuners. IEEE Trans. Circ. Syst. I Regul. Pap. 2011, 58, 2894–2905. [CrossRef] 8. Van Bezooijen, A.; De Jongh, M.A.; Chanlo, C.; Ruijs, L.C.H.; Van Straten, F.; Mahmoudi, R.; Van Roermund, A.H.M. A GSM/EDGE/WCDMA adaptive series-LC matching network using RF-MEMS switches. IEEE J. Solid-State Circ. 2008, 43, 2259–2268. [CrossRef] 9. Tabor, C.; Champion, M.; Cumby, B. Materials for liquid RF electronics long term operation of gallium liquid metal alloys in reconfigurable RF applications. In Proceedings of the 2017 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, San Diego, CA, USA, 9–14 July 2017; pp. 409–410. [CrossRef] 10. Park, E.; Lim, D.; Lim, S. Dual-band band-pass filter with fixed low band and fluidically-tunable high band. Sensors 2017, 17, 1884. [CrossRef][PubMed] 11. Memon, M.U.; Lim, S. Microfluidic high-Q circular substrate-integrated waveguide (SIW) cavity for radio frequency (RF) chemical liquid sensing. Sensors 2018, 18, 143. [CrossRef][PubMed] 12. Ali, M.M.; Narakathu, B.B.; Emamian, S.; Chlaihawi, A.A.; Aljanabi, F.; Maddipatla, D.; Bazuin, B.J.; Atashbar, M.Z. Eutectic Ga-In liquid metal based flexible capacitive pressure sensor. In Proceedings of the 2016 IEEE SENSORS, Orlando, FL, USA, 30 October–3 November 2016; pp. 1–3. 13. Nguyen, T.T.; Lim, S. Bandwidth-enhanced and Wide-angle-of-incidence Metamaterial Absorber using a Hybrid Unit Cell. Sci. Rep. 2017, 7, 1–11. [CrossRef][PubMed] 14. Lim, D.; Lim, S. Liquid-metal-fluidically switchable metasurface for broadband and polarization-insensitive absorption. IEEE Access 2018, 6, 40854–40859. [CrossRef] 15. Kelley, M.; Huff, G.; Lawrence, B.; Koo, C.; Li, S.; Han, A.; McQuilken, H. Frequency reconfigurable patch antenna using liquid metal as switching mechanism. Electron. Lett. 2013, 49, 1370–1371. [CrossRef] Sensors 2018, 18, 3176 9 of 9

16. Chen, C.H.; Whalen, J.; Peroulis, D. Non-toxic liquid-metal 2-100 GHz MEMS switch. In Proceedings of the 2007 IEEE/MTT-S International Microwave Symposium, Honolulu, HI, USA, 3–8 June 2007; pp. 363–366. 17. Morishita, A.M.; Dang, J.H.; Gough, R.C.; Ohta, A.T.; Shiroma, W.A. A tunable amplifier using reconfigurable liquid-metal double-stub tuners. In Proceedings of the 2015 Texas Symposium on Wireless and Microwave Circuits and Systems (WMCS), Waco, TX, USA, 23–24 April 2015; pp. 1–4. 18. Lei, B.J.; Hu, W.; Ohta, A.T.; Shiroma, W.A. A liquid-metal reconfigurable double-stub tuner. In Proceedings of the 2012 IEEE/MTT-S International Microwave Symposium Digest, Montreal, QC, Canada, 17–22 June 2012; pp. 1–3. 19. Yun, J.; Choi, J. Low-Profile Planar Inverted-F Antenna for Ultrawideband Applications. J. Electr. Eng. Sci. 2016, 16, 235–240. [CrossRef] 20. Yousaf, J.; Jung, H.; Kim, K.; Nah, W. Design, Analysis, and Equivalent Circuit Modeling of Dual Band PIFA Using a Stub for Performance Enhancement. J. Electr. Eng. Sci. 2016, 16, 169–181. [CrossRef]

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