A Stripline-Based Integrated Microfluidic-Microwave Module

energies

Article A Stripline-Based Integrated Microﬂuidic-Microwave Module

Laura Jasi ´nska 1,* , Krzysztof Szostak 2, Mateusz Czok 1 , Karol Malecha 1 and Piotr Słobodzian 2

1 Department of Microsystems, Faculty of Microsystem Electronics and Photonics, Wrocław University of Science and Technology, Wybrzeze˙ Wyspia´nskiego27, 50-370 Wrocław, Poland; [email protected] (M.C.); [email protected] (K.M.) 2 Telecommunications and Teleinformatics Department, Faculty of Electronics, Wrocław University of Science and Technology, Wybrzeze˙ Wyspia´nskiego27, 50-370 Wrocław, Poland; [email protected] (K.S.); [email protected] (P.S.) * Correspondence: [email protected]

Abstract: The paper presents the preliminary results on the development of an integrated stripline- based microwave-microfluidic module. The measurements were performed in a frequency range from 300 MHz up to 12 GHz, with the microchannel filled with three different test fluids—deionized water, the ethanol-water solution and pure ethanol. Due to the higher-than-expected losses in transmittance, the selected module was examined with use of the cross-sections taken along its length. The possible causes were highlighted and described. Likewise, the proposed areas of further investigations have been clearly described.

Keywords: LTCC; microwave; microﬂuidics; stripline

Citation: Jasińska,L.; Szostak, K.; 1. Introduction Czok, M.; Malecha, K.; Słobodzian, P. This paper presents the preliminary studies of the development process of an LTCC- A Stripline-Based Integrated based microfluidic-microwave device. The growing interest in microfluidic devices is Microfluidic-Microwave Module. driven by several factors. These include the low volume of liquids and reagents, small Energies 2021, 14, 2439. https:// dimensions and the possibility of combining the microfluidic components with a wide doi.org/10.3390/en14092439 range of systems. Most of these systems are called lab-on-chip (LOC) microsystems or micro-total-analysis-systems (µTAS) [1–5]. The application areas include micromixers, mi- Academic Editor: croreactors and more [6–14]. Such systems can be manufactured using various technologies, Paul Christodoulides which are strongly connected with the desired application. Many sensing applications are

Received: 31 March 2021 using microﬂuidic components. The method of detection involved can vary. The common Accepted: 23 April 2021 solutions are based on optical [15–17], electrochemical [18], acoustical [19] and electromag- Published: 25 April 2021 netic [20–24] components. Moreover, the microreactors designed for continuous, as well as for digital microﬂuidics can be found in the literature [25–27]. The last area typically

Publisher’s Note: MDPI stays neutral includes circuits that are characteristic of the microwave technique. The trend of a growing with regard to jurisdictional claims in number of publications describing microwave sensing devices is shown in Figure1. published maps and institutional afﬁl- iations.

Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Energies 2021, 14, 2439. https://doi.org/10.3390/en14092439 https://www.mdpi.com/journal/energies Energies 2021, 14, x FOR PEER REVIEW 2 of 10

Energies 2021, 14, 2439 2 of 10

FigureFigure 1. 1. TheThe growing growing number number of of publications publications in the in area the ofarea microwave of microwave sensors—based sensors—based on the on the ScopusScopus database database [ 28[28].]. Low temperature co-fired ceramics (LTCC) seems to be a suitable substrate for the systemsLow that temperature incorporate microwaveco-fired ceramics and microfluidic (LTCC) components. seems to Firstly, be a itsuitable is possible substrate to for the systemsmanufacture that variousincorporate conductive microwave paths which and can microflu act as aidic microwave components. circuit. Moreover, Firstly, it is possible these circuits can be manufactured directly on top of the structure, as well as inside the sub- to manufacture various conductive paths which can act as a microwave circuit. Moreover, strate. Therefore, it is possible to create both the microstrip- and stripline-based microwave thesecircuits circuits [29,30]. can Besides, be manufactured many examples ofdirectly the three-dimensional on top of the structures structure, placed as insidewell as inside the substrate.the LTCC Therefore, structure can it be is found possible in the to literature, create both i.e., microchannels,the microstrip- microchambers, and stripline-based mi- crowavemicromixers, circuits etc. [ 31[29,30].–33]. In oppositeBesides, to many various examples polymer-based of microfluidic-microwavethe three-dimensional structures placedstructures, inside due tothe the LTCC multilayer structure character can of the be mentioned found in technology, the literature, the LTCC i.e., allows microchannels, the mi- fabrication of monolithic microfluidic-microwave devices [34]. Furthermore—the unit cost crochambers,of manufacturing micromixers, the LTCC device etc. is [31–33]. much lower In inopposite comparison to va to therious technology polymer-ba basedsed microfluidic-microwaveon the glass/silicon structures, substrates. due Recently, to the some multilayer applications character of microfluidic-microwave of the mentioned technology, thedevices LTCC have allows appeared, the fabrication such as fluidic of sensors,monolithic plasma microfluidic-microwave density measurement sensor devices or [34]. Fur- thermore—themicrowave spectroscopy unit cost sensors of manufacturing [34–36]. the LTCC device is much lower in comparison The most common microfluidic-microwave devices are based on solutions with the touse the of technology microstrip or based coplanar on waveguides the glass/silicon (CPW) lines. substrates. Among other Recently, things, this some is dictated applications of microfluidic-microwaveby the type of substrate. devices Based on have the literature appeared, reports, such it mayas fluidic be concluded sensors, that theplasma density measurementstripline-based sensor circuits or integrated microwave with thespectroscopy microfluidic sensors components [34–36]. are rarely present. Nevertheless,The most the common use of such microfluidic systems would-microwave extend the areadevices of microfluidics, are based especiallyon solutions with the usein theof microstrip presence of harshor coplanar chemical waveguides reagents. Moreover, (CPW) the lines. risk ofAmong the influence other ofthings, the this is dic- background noises can be reduced with the use of a stripline instead of the microstrip line. tatedThe advantagesby the type of usingof substrate. integrated Based striplines on withthe literature microfluidic reports, components it may are asbe follows: concluded that the stripline-based• The LTCC is circuits suitable for integr fabricationated ofwith the monolithic the microfluidic microfluidic-microwave components devices— are rarely present. Nevertheless,both from the the perspectiveuse of such of systems fabricating would the conductive extend paths,the area as wellof microfluidics, as the three- especially in thedimensional presence of structures harsh chemical inside the reagents. structure. Moreover, the risk of the influence of the background• The noises LTCC can is stable be reduced in harsh environmentalwith the use conditions,of a stripline while instead conductive of the paths microstrip are line. The much more vulnerable. The possibility of burying them inside the structure signifi- advantagescantly improves of using the integrated sensor lifetime striplines in these with conditions. microfluidic components are as follows: • TheHowever, LTCC during is suitable the preceding for fabrication design and fabricationof the monolithic of such structures, microfluidic-microwave numerous de- challengesvices—both have been from encountered, the perspective which are mainlyof fabricating related to thethe process conductive of delivering paths, the as well as the microwavethree-dimensional signal directlyto structures the stripline. inside The fact the of structure. being placed inside the structure, with the presence of ground planes over the whole structure, strongly limits how the microwave • The LTCC is stable in harsh environmental conditions, while conductive paths are connectors can be fed. For this reason, the main novelty applied in the proposed device are connectorsmuch solderedmore vulnerable. directly to the The stripline. possibility of burying them inside the structure signifi- cantly improves the sensor lifetime in these conditions. However, during the preceding design and fabrication of such structures, numerous challenges have been encountered, which are mainly related to the process of delivering the microwave signal directly to the stripline. The fact of being placed inside the structure, with the presence of ground planes over the whole structure, strongly limits how the microwave connectors can be fed. For this reason, the main novelty applied in the proposed device are connectors soldered directly to the stripline.

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2. Materials and Methods The appearance of the microchannel inside the structure strongly affected the design- Energies 2021, 14, 2439 ing process of microwave modules. In conventional applications, the microwave3 of 10 circuits are fabricated using a homogeneous substrate. Subsequently, the characteristic geometry (width and length) is developed using analytical methods. However, the microchannels presence2. Materials requires and using Methods more sophisticated calculation methods, such as the finite element method The(FEM). appearance This method of the microchannel is based on inside the the calculation structure strongly of the affected approximate the design- solutions of partialing differential process of microwave equations. modules. The geometry In conventional of the applications, proposed the module microwave has circuits been developed are fabricated using a homogeneous substrate. Subsequently, the characteristic geometry using(width COMSOL and length) Multiphysics is developed (Radio using analytical Frequency methods. Module), However, which the microchannels is based on the afore- mentionedpresence computational requires using more method. sophisticated The calculationselection methods,of numerical such as modelling the ﬁnite element was driven by the appearancemethod (FEM). ofThis the methodmicrofluidic is based channel on the calculation near the stripline of the approximate circuit. The solutions simulations of were carriedpartial out differential in the frequency equations. domain The geometry in the of ra thenge proposed of 0.1–10 module GHz has with been 100 developed MHz steps. Both using COMSOL Multiphysics (Radio Frequency Module), which is based on the afore- the striplinementioned as computational well as the method.ground Theplanes selection were of modelled numerical modellingas perfect was conductors. driven by The exci- tationsthe appearancewere performed of the microﬂuidic using the channelcoaxial nearlump theed stripline port mode circuit. with The the simulations characteristic im- pedancewere carriedof 50 Ω out. Such in the frequencyexcitations domain were in chosen the range due of 0.1–10 to the GHz distance with 100 between MHz steps. the stripline and Boththe ground the stripline planes as well being as the shorter ground planesthan th weree wavelength modelled as perfectof the conductors. signal. The The simulations excitations were performed using the coaxial lumped port mode with the characteristic were carried out in several steps. First, the device with a stripline without a microchannel impedance of 50 Ω. Such excitations were chosen due to the distance between the stripline was andmodelled. the ground Subsequently, planes being shorter the microchannels than the wavelength with of thedifferent signal. Theparameters simulations were added and weremodelled. carried outThen in several the connectors steps. First, thewere device integr withated a stripline into the without model. a microchannel The deionized (DI) waterwas has modelled. been selected Subsequently, as the the test microchannels liquid. The with microchannel different parameters was were designed added andto be in close distancemodelled. (144 Thenµm—one the connectors layer of were the integratedLTCC) abov intoe the and model. below The the deionized stripline. (DI) In water the described has been selected as the test liquid. The microchannel was designed to be in close distance module,(144 µ them—one stripline layer ofwas the placed LTCC) above in the and middl belowe theof the stripline. substrate In the and described the microchannel module, was dividedthe stripline into two was parts placed that in thewere middle directly of the abov substratee and and below the microchannel the stripline was with divided a height equal to 213into µm. two The parts thickness that were directly of the above whole and module below the and stripline the height with a height of the equal microchannel to 213 were imposedµm. The bythickness the layered of the nature whole moduleof the LTCC and the mo heightdules. of the The microchannel geometry weremodel imposed of the designed by the layered nature of the LTCC modules. The geometry model of the designed structure structure is shown in Figure 2. is shown in Figure2.

(a) (b)

Figure 2.Figure The model 2. The modelof the of stripline the stripline device device designed designed and and simulated simula withted usewith of theuse COMSOL of the COMSOL Multiphysics Multiphysics software (a) and software (a) and the geometrythe geometry scheme scheme with with highlightedhighlighted the the parts parts which which are buried are bu insideried the inside structure the (structureb). (b).

The width and length of the modelled stripline were 0.9 and 50 mm, respectively. The obtainedThe width width, and length length and thicknessof the modelled of the LTCC stripline substrate werewere 10, 0.9 50 and 1.57 50 mm.mm, The respectively. The lengthobtained of the width, microchannel length placed and thickness directly above of andthe belowLTCC the substrate stripline waswere 45 10, mm. 50 After and 1.57 mm. The modelling,length of the nextmicrochannel stage was fabrication placed directly of the actual above device. and DuPontbelow the 951 ceramicstripline foil was 45 mm. After(DuPont modelling, (UK) Electronicthe next Materialsstage was Ltd., fabrication London Rd, of Stevenage, the actual United device. Kingdom) DuPont was 951 ceramic chosen as the substrate material, with a layer thickness of approximately 144 µm (layers foil (DuPont (UK) Electronic Materials Ltd., London Rd, Stevenage, United Kingdom) was 4 and 5) and 213 µm (layers 1–3 and 6–8) after co-ﬁring. The geometry of the individual chosenlayer as is the shown substrate in Figure material,3. with a layer thickness of approximately 144 µm (layers 4 and 5) and 213 µm (layers 1–3 and 6–8) after co-firing. The geometry of the individual layer is shown in Figure 3. The stripline was deposited on the fourth layer (l4 in Figure 4) using a screen printing method with the ESL903A silver conductive paste (sheet resistance ≤ 2 mΩ/□) with use of the Aurel VS 1520A screen printer (Aurel S.p. A, Modilgiana, Italy). The microchannels were fabricated using the LPKF ProtoLaser U laser system (λ = 355 nm, LPKF

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Laser&Ελεχτρονιχσ, Garbsen, Germany). Thereafter, all layers were stacked together and laminated in the isostatic press with reduced pressure (compared to the 20 MPa recommended by the manufacturer), for 10 min at a temperature of 70 °C. The laminated layers were then formed into the desired shape with the aforementioned laser system. After- wards, the modules were co-fired in the Nabertherm 03/16 chamber furnace (Nabertherm GmbH, Lilienthal, Germany) with the maximum (peak) temperature of 880 °C. Subse- quently, the ground planes were deposited with the mentioned screen printing method using the ESL903D conductive paste with the sheet resistance ≤ 2 mΩ/□. The metallization was post-fired using the BTU QA 41-6-54 belt furnace with the maximum temperature of 850 °C. The final step of the fabrication process was the soldering of the SMA connectors and assembling the steel needles, which act as the inlet and the outlet of the microchannel. After fabrication, the module was examined for the possible presence of undesired elec- Energies 2021, 14, 2439 trical connections between the ground and the stripline with the use of the multimeter4 of 10 and the X-ray imaging method. Pictures of the manufactured module along with the X- ray images are shown in Figures 4 and 5.

(a) (b)

FigureFigure 3. 3. TheThe project project of ofthe the LTCC LTCC layers layers (a) and (a) and the stack the stack scheme scheme (b) designed (b) designed in the inCorelDraw the CorelDraw (white—parts (white—parts of a micro- of a channel,microchannel, grey—a grey—a silver conductive silver conductive path that path acts that asacts a stripline, as a stripline, blue—LTCC blue—LTCC substrate, substrate, dotted dottedrectangles—cut-outs rectangles—cut-outs made to mount the SMA connectors directly onto the stripline). made to mount the SMA connectors directly onto the stripline).

The stripline was deposited on the fourth layer (l4 in Figure4) using a screen printing method with the ESL903A silver conductive paste (sheet resistance ≤ 2 mΩ/) with use of the Aurel VS 1520A screen printer (Aurel S.p. A, Modilgiana, Italy). The microchannels were fabricated using the LPKF ProtoLaser U laser system (λ = 355 nm, LPKF Laser&Eλεχτρoνιχσ, Garbsen, Germany). Thereafter, all layers were stacked together and laminated in the isostatic press with reduced pressure (compared to the 20 MPa recommended by the manufacturer), for 10 min at a temperature of 70 ◦C. The laminated layers were then formed into the desired shape with the aforementioned laser system. Afterwards, the modules were co-fired in the Nabertherm 03/16 chamber furnace (Nabertherm GmbH, Lilienthal, Germany) with the maximum (peak) temperature of 880 ◦C. Subsequently, the ground planes were deposited with the mentioned screen printing method using the ≤ ESL903D conductive paste with the sheet resistance 2 mΩ/. The metallization was post-fired using the BTU QA 41-6-54 belt furnace with the maximum temperature of 850 ◦C. FigureThe final 4. The step picture of the of the fabrication developed process LTCC microfluidic-microwave was the soldering of modules the SMA with connectors steel needles and as inlet and outlet and soldered SMA connectors. assembling the steel needles, which act as the inlet and the outlet of the microchannel. After fabrication, the module was examined for the possible presence of undesired electrical The X-ray images indicate that the stripline of the LTCC module was placed directly connections between the ground and the stripline with the use of the multimeter and the in the middle of the substrate (Figure 5a,b) and no damages were observed. The darker X-ray imaging method. Pictures of the manufactured module along with the X-ray images areas are the microchannel that surrounds the stripline (Figure 5a) and the lighter parts are shown in Figures4 and5. The X-ray images indicate that the stripline of the LTCC module was placed directly in the middle of the substrate (Figure5a,b) and no damages were observed. The darker areas are the microchannel that surrounds the stripline (Figure5a) and the lighter parts are conductive materials, such as the previously mentioned stripline (Figure5b). The measurements of the scattering parameters were carried out after verifying the correct geometry of the module. Energies 2021, 14, x FOR PEER REVIEW 4 of 10

Laser&Ελεχτρονιχσ, Garbsen, Germany). Thereafter, all layers were stacked together and laminated in the isostatic press with reduced pressure (compared to the 20 MPa recommended by the manufacturer), for 10 min at a temperature of 70 °C. The laminated layers were then formed into the desired shape with the aforementioned laser system. After- wards, the modules were co-fired in the Nabertherm 03/16 chamber furnace (Nabertherm GmbH, Lilienthal, Germany) with the maximum (peak) temperature of 880 °C. Subse- quently, the ground planes were deposited with the mentioned screen printing method using the ESL903D conductive paste with the sheet resistance ≤ 2 mΩ/□. The metallization was post-fired using the BTU QA 41-6-54 belt furnace with the maximum temperature of 850 °C. The final step of the fabrication process was the soldering of the SMA connectors and assembling the steel needles, which act as the inlet and the outlet of the microchannel. After fabrication, the module was examined for the possible presence of undesired electrical connections between the ground and the stripline with the use of the multimeter and the X-ray imaging method. Pictures of the manufactured module along with the X- ray images are shown in Figures 4 and 5.

(a) (b)

Figure 3. The projectEnergies of2021 the, 14LTCC, 2439 layers (a) and the stack scheme (b) designed in the CorelDraw (white—parts of a micro- 5 of 10 channel, grey—a silver conductive path that acts as a stripline, blue—LTCC substrate, dotted rectangles—cut-outs made to mount the SMA connectors directly onto the stripline).

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are conductive materials, such as the previously mentioned stripline (Figure 5b). The Figure 4. The picture of the developed LTCC microfluidic-microwave modules with steel needles Figure 4. The picture of the developedmeasurements LTCC microﬂuidic-microwave of the scattering modules parameters with steel were needles carried as inlet out and after outlet verifying and the correct as inlet and outlet and soldered SMA connectors. soldered SMA connectors. geometry of the module. The X-ray images indicate that the stripline of the LTCC module was placed directly in the middle of the substrate (Figure 5a,b) and no damages were observed. The darker areas are the microchannel that surrounds the stripline (Figure 5a) and the lighter parts

(a) (b)

Figure 5. The X-rayFigure images 5. The of the X-ray fabricated images ofmodule—the the fabricated top module—theview (a) and topthe side view view (a) and (b). theThe side lighter view places (b). The show the conductive materialslighter (soldered places show connectors the conductive on Figure materials 5a,b and (soldered stripline connectorsplaced directly on Figure inside5a,b the and structure) stripline and placed the darker ones shows lowerdirectly substrate inside material the structure) density and(which the can darker be observed ones shows on lowerthe Figure substrate 5a). material density (which can be observed on the Figure5a). 3. Results 3. Results The stripline-based microfluidic-microwave module was evaluated using an N5230A The stripline-basedvector network microfluidic-microwave analyzer (VNA, Agilent, module Santa Clara, was evaluated California, using USA). an The N5230A measurements vector networkwere preceded analyzer by (VNA, short-open-load-through Agilent, Santa Clara, (S California,OLT) calibration USA). and The the measure- investigated fre- ments werequency preceded range by was short-open-load-through from 300 MHz to 12 GHz. (SOLT) As calibrationtest liquids, and deionized the investigated water, the ethanol- frequencywater range solution was from (with 300 the MHz volume to 12 concentratio GHz. As testn of liquids, ethanol deionized equal to 50%) water, and the the ethanol ethanol-waterwere solution selected. (with Such thean approach volume concentration was chosen to of check ethanol the equal possibility to 50%) of andextending the further ethanol wereresearch selected. in the Such area an of approach sensors. was chosen to check the possibility of extending further researchThe in theresults area of of the sensors. conducted experiment are presented in Figure 6a,b with solid lines The resultsfor the of first the module conducted andexperiment dashed lines are for presented the second in one. Figure The6a,b liquid with used solid during lines the mod- for the firstelling module of the and designed dashed module lines for was the deionized second one.water The with liquid a relative used permittivity during the of 81. This modellingis of the the reason designed why modulethe reflectance was deionized of the micr waterowave with circuit a relative is the permittivitylowest for the of mentioned 81. This isliquid. the reason For each why of the the reflectance measured of LTCC the microwave modules, circuitthe transmittance is the lowest of for the the DI water is mentionedhigher liquid. than For − each3 dB ofin thedifferent measured frequency LTCC ranges. modules, The the cut-off transmittance frequency ofof thethe DIfirst module water is higherwas around than − 93 GHz dB in (Figure different 6), while frequency the second ranges. one The was cut-off around frequency 8 GHz. The of the phenomena associated with such behaviour of the LTCC microwave planar circuits were described by Szostak et al. [37]. The observed increase of the losses (Figure 6b) for higher frequencies are typical for the water solutions. Moreover, the deformations of the stripline could have additionally strongly increased losses. Such deformations are present due to the nature of the manufacturing process of the LTCC modules and the use of screen-printing method. Therefore, the described area of the LTCC devices requires further research. However, as it can be seen, the tested structures are characterized by only a slight matching difference, due to the similar scattering characteristics. The possible reasons behind the variation of the scattering parameters of each module are discussed in the next section.

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ﬁrst module was around 9 GHz (Figure6), while the second one was around 8 GHz. The Energies 2021, 14, x FOR PEER REVIEW 6 of 10 phenomena associated with such behaviour of the LTCC microwave planar circuits were described by Szostak et al. [37].

(a)

(b)

Figure 6. The reflectance S11 (aFigure) and transmittance 6. The reflectance S21 (b S11) of (thea) and first transmittance (module 1, m1) S21 and (b second) of the (module first (module 2, m2) 1, exemplary m1) and second module. The solid line shows (modulethe results 2, m2)of first exemplary module, module. when the The dashed solid linelines shows describe the the results second of first one. module, when the dashed lines describe the second one. 4. Discussion TheThe obtained observed results increase highlighted of the losses the mismatch (Figure6b) between for higher modules, frequencies which are can typical be ob- for servedthe water mostly solutions. in transmittance Moreover, and the reflectanc deformationse at frequencies of the stripline higher could than have 8 GHz additionally for each module.strongly Therefore, increased the losses. authors Such decided deformations to perform are the present cross-section due to the across nature the ofmodule the manu- (in 10facturing mm increments process ofalong the LTCCthe microchannel) modules and in the order use ofto screen-printingfind possible inaccuracies method. Therefore, in the structurethe described geometry. area Pictures of the LTCC of the devices cross-sections requires are further shown research. in Figure However, 7a–f. as it can be seen,The the cross-sections tested structures demonstrate are characterized that there by onlywere a several slight matching issues. Firstly, difference, a slight due todis- the placementsimilar scattering between characteristics. the microchannel The possibleand the reasonsstripline behind can be the observed variation (Figure of the scattering 7c,d,f). Secondly,parameters the ofdelamination each module of arethe discussed ceramic laye in thers (Figure next section. 7c,e,f) can be seen—small darker areas4. Discussion below the microchannel. Moreover, another small delamination is present in Figure 7a,b—on the left side of the stripline. Furthermore, the stripline is partially deformed (Fig- ure 7d).The The obtained aforementioned results highlighted facts probably the mismatchstrongly affected between the modules, electrical which parameters can be of observed mostly in transmittance and reflectance at frequencies higher than 8 GHz for each the substrate, which could influence the circuit behaviour by filtering the signal at higher module. Therefore, the authors decided to perform the cross-section across the module frequencies. Probably, the main cause of the delamination was the lower lamination pres- (in 10 mm increments along the microchannel) in order to find possible inaccuracies in the sure, which had been chosen instead of the recommended one in order to minimize the structure geometry. Pictures of the cross-sections are shown in Figure7a–f. risk of deformation of the microchannel geometry. The obtained results and the cross- sections of the modules indicate that there is a requirement for further research to achieve the optimal conditions throughout the LTCC fabrication process.

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(a) (b)

(e) (f)

Figure 7. PicturesFigure 7. Picturesof the cross-section of the cross-section of the of the proposed proposed mo moduledule inin 10 10 mm mm increments increments along along the microchannel the microchannel (a–f). The (a–f). The images wereimages obtained were obtained using usingthe Leica the Leica DM4000 DM4000 M M optical optical microscope. microscope. The blackThe black parts are parts the microchannelare the microchannel sections and thesections and the thin linethin in line the in middle the middle of ofthe the picture picture is thethe stripline stripline (co-ﬁred (co-fired silver silver paste). paste).

The cross-sections demonstrate that there were several issues. Firstly, a slight displace- Nevertheless,ment between the it microchannel was observed and the that stripline the sca canttering be observed parameters (Figure7c,d,f). of the Secondly, tested module were thenot delamination prone to changes of the ceramic in the layers arrangement (Figure7c,e,f) of can cables be seen—small or the position darker areasof the below module itself. Accordingthe microchannel. to the measurements Moreover, another of smallmodules delamination based on is present a microstrip in Figure 7linea,b—on (and the with com- binedleft stripline side of the and stripline. microstrip Furthermore, line parts), the stripline the improvement is partially deformed of the re (Figurepeatability7d). The and stabil- aforementioned facts probably strongly affected the electrical parameters of the substrate, ity of measurements is evident. The comparison between the results of various liquids indicates the potential of using the presented module in the sensing area of microfluidic systems in the future. Visible changes of S11 and S21 parameters in a wide range of frequency with the fluid change in the microchannel can be observed.

5. Conclusions The LTCC stripline-based microfluidic module was developed, manufactured and measured. In contrary to the solutions found in the literature, a waveguide in the form of a stripline (buried inside a ceramic structure) with the presence of a microchannel was

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which could influence the circuit behaviour by filtering the signal at higher frequencies. Probably, the main cause of the delamination was the lower lamination pressure, which had been chosen instead of the recommended one in order to minimize the risk of deformation of the microchannel geometry. The obtained results and the cross-sections of the modules indicate that there is a requirement for further research to achieve the optimal conditions throughout the LTCC fabrication process. Nevertheless, it was observed that the scattering parameters of the tested module were not prone to changes in the arrangement of cables or the position of the module itself. According to the measurements of modules based on a microstrip line (and with combined stripline and microstrip line parts), the improvement of the repeatability and stability of measurements is evident. The comparison between the results of various liquids indicates the potential of using the presented module in the sensing area of microfluidic systems in the future. Visible changes of S11 and S21 parameters in a wide range of frequency with the fluid change in the microchannel can be observed.

5. Conclusions The LTCC stripline-based microﬂuidic module was developed, manufactured and measured. In contrary to the solutions found in the literature, a waveguide in the form of a stripline (buried inside a ceramic structure) with the presence of a microchannel was chosen. The fabrication process was preceded by the numerical simulations utilising the COMSOL Multiphysics software. The modelled and tested frequency range included the most commonly used frequencies in the telecommunication industry—2.4 GHz and 5.1 GHz. Therefore, the components for the further development of the proposed solution are available and cost-effective. The module was examined in the presence of several liquids—deionized water, the ethanol-water solution with the ethanol volume concentration of 50% and the pure anhy- drous ethanol. The changes of the scattering parameters were observed for every single liquid. Thus, it shows the potential of further project development towards dielectric sensing. However, the proposed module requires additional investigations due to the appearance of geometry deviations. The selection of the LTCC is determined by the characteristics of the aforementioned material. Among them, the appropriate dielectric properties, high-temperature stability, possibility of simple micromechanical processing can be mentioned. Therefore, the LTCC technology appears as an attractive alternative to other technologies that are chosen to fabricate the microwave-microﬂuidic structures.

Author Contributions: Conceptualization, L.J. and K.S.; methodology, L.J., K.S. and M.C.; validation, K.M. and P.S.; investigation, L.J., K.S. and M.C.; resources, L.J.; data curation, L.J., K.S. and M.C.; writing—original draft preparation, L.J.; writing—review and editing, K.S., M.C., K.M. and P.S., visualization, L.J. and M.C.; supervision, K.M. and P.S.; project administration, K.M.; funding acquisition, K.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Wrocław University of Science and Technology (statutory activity). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest. Energies 2021, 14, 2439 9 of 10

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