This is the accepted version of the article: Vélez Rasero, Paris; Muñoz Enano, Jonathan; Grenier, Katia; [et al.]. «Split ring resonator-based microwave fluidic sensors for electrolyte concentration measurements». IEEE Sensors Journal, Vol. 19, no. 7 (April 2019), p. 2562- 2569. DOI 10.1109/JSEN.2018.2890089 This version is avaible at https://ddd.uab.cat/record/221242 under the terms of the license 1 Split Ring Resonator (SRR) based Microwave Fluidic Sensors for Electrolyte Concentration Measurements Paris Vélez, Member, IEEE, Jonathan Muñoz-Enano, Katia Grenier, Member, IEEE, Javier Mata-Contreras, David Dubuc, Member, IEEE, and Ferran Martín, Fellow, IEEE  blood and urine are sodium (Na+), potassium (K+), calcium Abstract— A differential microwave sensor, based on a pair of (Ca2+), chloride (Cl) and bicarbonate (HCO3−). Imbalances in uncoupled microstrip lines each one loaded with a split ring the concentration of certain ions in blood or urine (the so- resonator (SRR), is applied to the measurement of electrolyte called anion gap [2]) are indicative of certain pathologies. For concentration in deionized (DI) water. For that purpose, fluidic channels are added on top of the SRR gaps, the most sensitive instance, in urine the anion gap is defined as [Na +] + [K+]  parts of the structure. The operating principle is based on the [Cl], where [*] denotes concentration, and typical values in measurement of the cross-mode insertion loss, highly sensitive to urine samples not subjected to physiological pathologies small asymmetries caused by differences between the reference should be comprised between 0 and 33 mEq/L. Ion liquid and the liquid under test (LUT). In this work, the concentration in blood and urine are typically measured by reference liquid is pure DI water (the solvent), whereas the solution, DI water with electrolyte content, is injected to the LUT means of electrochemical systems, such as ion selective channel. The proposed sensor is able to detect electrolyte electrodes (ISE) [3], able to provide the individual concentrations as small as 0.25 g/L, with maximum sensitivity of concentration of specific ions, and consequently the anion gap. 0.033 (g/L)-1. The sensor is validated by measuring the Measuring the total concentration of ions does not determine concentration of three types of electrolytes, i.e., NaCl, KCl and the anion gap, but may be indicative of an overall excess or CaCl2. Finally, the sensor is applied to monitor variations of total defect of electrolytes, as compared to the typical levels found electrolyte concentration in urine samples. in “healthy” samples. Therefore determining the total Index Terms— Microwave sensors, differential sensors, electrolyte concentration in serum, plasma or urine through microfluidics, permittivity measurements, microstrip technology, real-time low-cost systems (as compared to chemical systems) split ring resonator (SRR), electrolyte concentration. may be useful as a complementary diagnosis method. Within this context, this paper is focused on the I. INTRODUCTION measurement of electrolyte concentration in DI water using lectrolytes are chemical substances that dissociate into microwaves. The sensor is based on the variations of the E cations and anions when dissolved in polar solvents, such as water, thereby modifying the electric and complex dielectric constant (mostly related to variations in the conductivity or loss tangent) of DI water experienced by the electromagnetic properties of the solvent. Particularly, presence of electrolytes dissolved in it. Such variations can be changes in the conductivity and loss factor (or loss tangent) of sensitively detected by means of a differential technique based the solvent are expected by the presence of electrolytes in it. on the measurement of the cross-mode insertion loss in a pair Therefore, electrolyte concentration can be measured by of microstrip lines loaded with split ring resonators (SRRs), means of methods or approaches sensitive to the conductivity and equipped with fluidic channels for liquid injection. The or loss factor of the solution, and particularly through main advantage of this resonator-based approach is the high microwave techniques. sensitivity and capability to detect small concentrations of Electrolytes are physiologically important since they play electrolytes in water. key roles in various vital functions, such as blood pH and Many sensing structures based on resonator-loaded lines pressure control, body hydration, nerve and muscle functions, have been proposed, and they can be categorized in three main etc. [1]. The most important primary ions of electrolytes in groups: (i) frequency variation sensors [4]-[13], (ii) coupling modulation sensors [14]-[22] and (iii) frequency-splitting This work was supported by MINECO-Spain (project TEC2016-75650-R), sensors [23]-[30]. In frequency variation sensors, the output Generalitat de Catalunya (projects 2017SGR-1159 and TECSPR15-1-0050), variable is the frequency position of either a notch or a peak in Institució Catalana de Recerca i Estudis Avançats (who awarded Ferran Martín), and by FEDER funds. the frequency response of a transmission line loaded with a P. Vélez, J. Muñoz-Enano and F. Martín are with GEMMA/CIMITEC, resonant element (sometimes, the notch depth, or the peak Departament d’Enginyeria Electrònica, Universitat Autònoma de Barcelona, magnitude, is also used as an additional output variable). 08193 Bellaterra, Spain (e-mail: Ferran.Martin@uab.es). K. Grenier and D. Dubuc are with MH2F, LAAS-CNRS, 7th, Avenue du Sensors based on this approach have been applied to different Colonel Roche BP 54200 31031 Toulouse, Cedex 4, France (e-mail: types of measurements, but, particularly, they have been used Grenier@laas.fr). for dielectric characterization of solid [8]-[10] and liquid J. Mata-Contreras is with Departamento de Ingeniería de Comunicaciones, Universidad de Málaga, 29016 Málaga, Spain. [6],[12],[13] samples. Symmetry disruption is the physical 2 principle in coupling modulation sensors [31],[32]. In such hospital environments, related to extreme ion concentrations sensors, a transmission line is symmetrically loaded with a in such samples. symmetric resonator uncoupled to it. Such uncoupling is achieved by using resonant elements exhibiting a symmetry II. STRUCTURE OF THE PROPOSED SENSOR, SENSING PRINCIPLE plane of different electromagnetic sort than the symmetry AND FABRICATION plane of the line [31], [32]. However, by truncating symmetry The proposed differential sensor consists of two parts: (i) the (e.g., by means of a relative linear or angular displacement microwave structure, a FR4 substrate with a pair of uncoupled between the resonator and the line, or by means of an 50 Ω lines each one loaded with a SRR, and (ii) the fluidic asymmetric dielectric loading), line-to-resonator coupling is channels plus the mechanical accessories for liquid injection activated, thereby generating a notch in the frequency to the channels and for channel attachment to the substrate. response (with depth intimately related to the level of The topology of the microwave structure is depicted in Fig. 1, asymmetry). In such coupling modulation sensors the output where relevant dimensions are indicated. The dielectric variable is thus the notch depth, and they have been mostly constant and thickness of the considered substrate are r = 4.4 applied to the measurement of spatial variables [14]-[22],[33]. and h = 1.6 mm, respectively. The fluidic channels are made Finally, in frequency splitting sensors, a transmission line is of polydimethylsiloxane (PDMS), and are identical to those symmetrically loaded with a pair of (not necessarily used in [30]. Channel dimensions are designated by hch symmetric) resonators coupled to it. Under perfect symmetry, (height), wch (width) and lch (length). The mechanical parts a single notch at the fundamental resonance frequency of the consist of a polyether ether ketone (PEEK) structure, that resonant elements appears. However, if the resonators are accommodates the fluidic connectors (necessary for liquid asymmetrically unloaded, frequency splitting arises. In these injection to the channels), and screws, used to fix the PEEK- sensors, similar to differential sensors, the output variables are PDMS assembly on the substrate. In order to avoid substrate the differential frequency and the differential depth of the absorption, a dry film of clear polyester, with an estimated generated notches, and these frequency-splitting sensors have thickness of 50 µm and dielectric constant of 3.5, has been been mainly applied to differential permittivity measurements deposited on top of the SRRs. The top and lateral views of the [29],[30]. channel plus mechanical parts are depicted in Fig. 2. The The previous sensors exhibit, in general, acceptable photograph of the whole sensing structure is shown in Fig. 3. sensitivities and dynamic ranges, but they exhibit limited l resolution, not being able to detect small variations of the (a) variable of interest. This aspect is critical for the measurement g c w of solute concentration in highly diluted solutions. Therefore, P1 P2 for the accurate measurement of electrolyte content in DI LLT water solutions exhibiting concentrations in the range 1-10 g/L (b) or even below, as pursued in this paper, novel strategies, highly sensitive to small concentrations, are needed. The P3 P4 above-mentioned sensing approach, based on the measurement WLT of the cross-mode insertion loss in a pair of lines, each one loaded with a resonant element, provides the necessary sensitivity and resolution for the intended application. This Fig. 1. Topology of the microwave structure of the proposed sensor (a) and approach was first demonstrated in [34],[35] by considering zoom of resonator (b). Dimensions are (in mm): LLT = 76.5, WLT = 2.79, open complementary split ring resonators (OCSRRs) [36] as l = 24, c = 1, w = 8.17, g = 0.2. The ground plane is depicted in grey. loading resonators (NaCl was the considered electrolyte in [35]). In this paper, it is demonstrated that sensitivity can be (a) improved by replacing OCSRRs with SRRs. Moreover, sensor PEEK h2 validation is carried out by considering three electrolytes, i.e., hch PDMS h1 NaCl, KCl and CaCl2. An advantage of using SRRs instead of (b) OCSRRs as sensing element is the fact that SRRs do not require vias, hence easing fabrication and contributing to wf wch lch enhance resolution, intimately related to the “level” of symmetry achieved when the differential structure is loaded lf with identical loads in both resonators (approaching to “perfect symmetry” is more feasible if vias are not present). Fig. 2. Lateral (a) and top (b) views of mechanical and fluidic parts of the microwave sensor and relevant dimensions. Channel dimensions are: hch = 1.5 In the last part of the paper, it is demonstrated that the mm, lch = 26 mm, wch = 4.6 mm; other relevant dimensions are: lf = 46 mm, proposed sensor may be useful to detect variations of total ion wf = 12.6 mm, h1 = 3 mm, and h2 = 9 mm. concentration in biological samples, particularly urine. This may be of interest for real-time monitoring of potential dysfunctions or anomalies, e.g., during disease treatment in 3 and the LUT liquid, which should provide a very small value of S21DC, may be obscured by S21DCBAL. Therefore, an accurate and rigorous sensor fabrication process, providing small S21DCBAL, is demanded for resolution optimization. III. EXPERIMENTAL VALIDATION The fabricated sensor has been validated by considering DI water solutions with different types of electrolytes, particularly, NaCl, KCl and CaCl2. These electrolytes have been considered since they are present in blood and urine and they play key roles in several vital functions. Fig. 3. Photograph of the proposed sensor including the microwave structure, The first set of experiments has been done by injecting DI the fluidic channels (plus mechanical accessories) and connectors. water solutions with different concentrations of NaCl in the LUT channel. In all the cases, pure DI water has been injected The working principle of the proposed differential sensor is in the REF channel. The measured cross-mode insertion loss symmetry disruption, caused by an asymmetric dielectric load for the different NaCl concentrations is depicted in Fig. 4. in the SRRs. Thus, in a sensing operation, the reference liquid Note that the cross-mode insertion loss resulting when the is injected in one of the channels (designated as REF channel), LUT channel is filled with pure DI water (balanced loading) is whereas the liquid under test (LUT) is injected in the other also included. S21DCBAL, expressed in dB, is found to be 38.95 channel (LUT channel). The output variable in the proposed dB, which is a very reasonable value. NaCl concentrations as sensor is the cross-mode insertion loss (or cross-mode small as 0.25 g/L can be perfectly resolved by virtue of the transmission coefficient), highly sensitive to small small value of S21DCBAL. By increasing the electrolyte asymmetries and, hence, very convenient to detect small concentration, the cross-mode transmission coefficient (or differences between the REF and the LUT liquids. This is insertion loss) also increases, as expected. In Fig. 5, we have necessary in order to measure small concentrations of depicted the dependence of the maximum value of the cross- electrolytes in DI water (the purpose of this work). Thus, the mode transmission coefficient, S21DCmax, with the considered REF liquid will be pure DI water (the solvent), concentration of NaCl, i.e., [NaCl]. For small concentrations, whereas the LUT is the DI water solution with electrolyte the sensitivity, i.e., the variation of S21DCmax with [NaCl], is content. very high, and it progressively decreases as [NaCl] increases. According to the port definition of Fig. 1, the cross-mode The maximum value of the sensitivity is found to be 0.033 transmission coefficient is given by [37] (g/L)1, i.e., higher than the maximum sensitivity of the 𝐷𝐶 𝑆21 1 = (𝑆21 − 𝑆43 ) (1) sensors proposed in [35], based on a similar approach, but 2 using OCSRRs as sensing elements (in [35] the sensors were This result is valid as long as the SRR-loaded lines are applied to the measurement of NaCl concentration, as well). uncoupled. If the structure exhibits perfect symmetry, it From the results of Fig. 5, we have obtained the calibration follows that S21DC = 0 (note that the cross-mode insertion loss, curve, from which the concentration of NaCl can be inferred i.e., the cross-mode transmission coefficient expressed in dB, by measuring S21DCmax. Such curve, with a correlation should be ideally  under perfect symmetry). However, if coefficient of R2 = 0.99966, is symmetry is disrupted, as occurs by injecting pure DI water in 𝑆𝐷𝐶 21 ) 𝑆 𝐷𝐶 ( ( 21 ) [𝑁𝑎𝐶𝑙]𝑔/𝐿 = −0.43 + 0.45𝑒 0.03649 −5 0.0091 the REF channel and the electrolyte solution in the LUT + 2 ∙ 10 𝑒 (2) channel, the cross-mode insertion loss should take finite values, and the maximum value should be indicative of the -15 60 g/L level of asymmetry. Therefore, the maximum level of the 40 g/L 20 g/L -20 cross-mode insertion loss is considered the output variable in 10 g/L 5 g/L the proposed sensing structure. -25 2.5 g/L |S21 | (dB) In practice, perfect symmetry is not achievable, as long as 1 g/L 0.5 g/L sensor fabrication involves several (“imperfect”) processes, -30 0.25 g/L ref. DC including substrate etching, dry film deposition, and mechanical fabrication of the PDMS-PEEK assembly. Thus, -35 certain level of cross-mode insertion loss is expected even by loading the REF and LUT channels with identical liquids. Let -40 us designate the maximum of the modulus of the cross-mode -45 transmission coefficient experimentally found with balanced 0.7 0.8 0.9 1.0 1.1 1.2 loads as S21DCBAL. It is important to minimize S21DCBAL as Frequency (GHz) much as possible, since it has direct impact on sensor Fig. 4. Cross-mode insertion loss for different values of NaCl concentration. resolution. Note that tiny differences between the REF liquid 4 0.14 0.14 0.12 0.12 |S21 |max(linear) |S21 |max(linear) 0.10 0.10 0.08 0.08 0.06 0.06 DC DC 0.04 0.04 Measurement 1 Measurement 1 0.02 Measurement 2 0.02 Measurement 2 Curve fitting Curve fitting 0.00 0.00 0 10 20 30 40 50 60 0 10 20 30 40 50 60 [NaCl] g/L [KCl] g/L Fig. 5. Variation of S21DCmax with [NaCl]. Fig. 7. Variation of S21DCmax with [KCl]. Repetitiveness of the results is important for the practical use Finally, we have prepared several solutions of CaCl2 in DI of the proposed sensors. Therefore, we have repeated the water, and we have also measured the cross-mode insertion previous measurements by subsequently injecting in the LUT loss, as well as S21DCmax, for the different solute channel the DI water solutions with the NaCl concentrations concentrations. The results are depicted in Figs. 8 and 9. The indicated in Fig. 5. The results, also included in Fig. 5 and calibration curve, with R2 = 0.99905, in this case is identified as Measurement 2, reveal that measurements are 𝑆𝐷𝐶 21 ) 𝑆𝐷𝐶 21 ) ( ( repetitive by virtue of the small differences with the first [𝐶𝑎𝐶𝑙]𝑔/𝐿 = −1.76 + 1.29𝑒 0.04221 + 3 ∙ 10 𝑒 −6 0.00848 (4) experimental campaign (Measurement 1). In the second set of experiments, KCl has been considered as and the maximum sensitivity is 0.021 (g/L)-1 solute. The variation of the cross-mode insertion loss and -15 60 g/L S21DCmax with KCl concentration are depicted in Figs. 6 and 7, 40 g/L respectively. Repetitiveness of the results is also demonstrated -20 20 g/L 10 g/L (see Fig. 7). In this case, the calibration curve, with 5 g/L R2 = 0.99931, has been found to be 2.5 g/L |S21 | (dB) -25 1 g/L 𝑆𝐷𝐶 (3) ( 21 ) 0.5 g/L 0.02347 [𝐾𝐶𝑙]𝑔/𝐿 = −0.11 + 0.14𝑒 0.25 g/L DC -30 ref. and the maximum sensitivity is 0.032 (g/L)-1 -15 60 g/L -35 40 g/L -20 20 g/L 10 g/L -40 5 g/L -25 2.5 g/L 0.7 0.8 0.9 1.0 1.1 1.2 |S21 | (dB) 1 g/L Frequency (GHz) 0.5 g/L -30 0.25 g/L Fig. 8. Cross-mode insertion loss for different values of CaCl2 concentration. DC ref. -35 0.14 -40 0.12 |S21 |max(linear) 0.10 -45 0.7 0.8 0.9 1.0 1.1 1.2 0.08 Frequency (GHz) 0.06 DC Fig. 6. Cross-mode insertion loss for different values of KCl concentration. 0.04 Measurement 1 0.02 Measurement 2 Curve fitting 0.00 0 10 20 30 40 50 60 [CaCl2] g/L Fig. 9. Variation of S21DCmax with [CaCl2]. 5 IV. DISCUSSION AND COMPARISON TO OTHER FLUIDIC TABLE I COMPARISON OF VARIOUS MICROWAVE FLUIDIC SENSORS FOR SENSORS FOR SOLUTE CONCENTRATION MEASUREMENTS SOLUTE CONCENTRATION MEASUREMENT IN LIQUID SOLUTIONS Note that the dependence of S21DCmax with the electrolyte Ref. Max. sensitivity Resolution Dynamic (dB/g/L) (g/L) range (g/L) concentration is not identical for KCl, NaCl and CaCl2, but [35] 4.3 0.25 80 similar. The relatively high sensitivity of the reported sensors [38] 0.005 2 10 is in part because the presence of electrolytes in DI water [39] 0.003 1 300 modifies substantially its conductivity (or loss tangent), [40] 1.75 1.5 5.5 [41] 0.017 10 150 provided the electrolyte molecules dissociate into anions and [42] 0.003 5 300 cations when dissolved in water. For a certain electrolyte [43], Glucose 0.055 1 100 concentration, ion concentration is determined by the [43], NaCl 1.609 0.5 100 molecular mass of the electrolyte. Therefore, since the This work, NaCl 12.27 0.25 60 conductivity depends on ion concentration, we cannot expect identical dependences of the cross-mode insertion loss with V. CHARACTERIZATION OF URINE SAMPLES the electrolyte concentration for the different electrolytes To further validate the proposed sensor, we have considered. Moreover, conductivity depends also on ion characterized several horse urine samples, with different levels mobility, also related to the atomic mass. Indeed, at of electrolyte concentrations (Na+, K+ and Cl), which have intermediate concentrations (between 2.5 g/L and 20 g/L), the been externally measured by means of an ISE electrochemical electrolyte that produces higher values of the cross-mode system (the electrolyte concentration measurements and the insertion loss is NaCl, which is coherent with the low value of urine samples have been provided to us by the Biochemistry the atomic mass of Na, as compared to K and Ca. However, and Molecular Biology Department of Universitat Autònoma the differences are not significant. On the other hand, for very de Barcelona). From the measured values of the individual small values of the electrolyte concentration (< 2.5 g/L), the electrolyte concentrations (i.e., Na+, K+ and Cl) for each accuracy is expected to be limited, and for large values (> 20 sample, expressed in mEq/L, we have obtained the total g/L), S21DCmax exhibits a quasi-saturation effect, and the concentrations by a simple addition. Note that the total concentration in mEq/L is proportional to the total ion dependence of the conductivity on ion concentration and concentration, a quantity of interest as it may be indicative of mobility may be more complex. These facts may explain that certain pathologies if certain limits are surpassed. The real- at low and high electrolyte concentrations there is not a clear time monitoring of possible variations in the total ion dependence of the cross-mode insertion loss with the ion concentration in urine (or even in blood) may be also of concentration and atomic mass. Nevertheless, for a given interest to detect possible dysfunctions or anomalies, e.g., electrolyte, the very good correlation coefficient that results by during disease treatment in hospital environments. Note that curve fitting, plus the repetitiveness of the results, reveals that with the proposed sensor system, ion concentration variations the corresponding calibration curve is useful to determine its as compared to a reference sample can be easily measured. concentration. The urine samples provided to us exhibit the total ion Besides the work [35], focused on the measurement of NaCl concentrations indicated in the right hand side column of Fig. concentration in DI water by using a pair of OCSRR-loaded 10. By considering as reference liquid the urine sample with lines, other papers report solute concentration measurements the highest level of ion concentration (615.8 mEq/L), the in liquid solutions. In [38], the considered solute is also NaCl, cross-mode insertion loss that results by injecting the different whereas in [39]-[42], the proposed sensors are used for the samples to the LUT channel are those depicted in Fig. 10. It measurement of glucose concentration in liquid solutions. should be mentioned that for monitoring changes in total ion Finally, in [43], both NaCl and glucose concentration concentration with time in a real scenario, the reference measurements in DI water are reported. Table I compares sample should be, obviously, the urine at the beginning of the these sensors in terms of maximum sensitivity, resolution and test campaign. In our case, we have (arbitrarily) considered dynamic range. It is remarkable the sensor reported in [40], that the reference sample is the one with 615.8 mEq/L ion with a very good value of the sensitivity, taking into account concentration, as mentioned above. that the considered solute is glucose, a substance that does not The variation of S21DCmax with the total ion concentration is depicted in Fig. 11. The calibration curve, with R2 = 0.99422, modify the water conductivity (or loss factor) as electrolytes is (the total ion concentration has been designated as [*]) do. If we focus on the sensors devoted to NaCl concentration −𝑆𝐷𝐶 −𝑆𝐷𝐶 measurements, we conclude that the sensor proposed in this ( 21 ) 0.037 ( 21 ) 0.0036 paper provides the best combination of sensitivity and [∗]𝑚𝐸𝑞/𝐿 = −46.62 + 517.26𝑒 + 428.68𝑒 (5) resolution. Moreover, sensor fabrication is simple (only a and the maximum sensitivity has been found to be 0.00058 single metal layer is etched) and vias are not required. (mEq/L)1. From these results, we can envisage the application Therefore, the reported differential sensing structure, based on of this sensing principle to monitor electrolyte concentration SRRs is very promising for the measurement of electrolyte variations in biological samples (e.g., urine and blood) in a content in very diluted liquid solutions. real scenario. 6 -25 dielectric constant), as compared to a reference. In the last part 615.8 mEq/L 434.7 mEq/L of the paper, the sensor has been applied to the -30 357.1 mEq/L characterization of urine samples with different levels of ion 348.9 mEq/L concentrations, and the potential of the approach for real-time -35 315.3 mEq/L 297.1 mEq/L monitoring of total ion concentration in biological samples in |S21 | dB -40 281.3 mEq/L a real scenario, has been pointed out. 244.2 mEq/L DC 232 mEq/L -45 118.2 mEq/L ACKNOWLEDGMENTS 106.8 mEq/L -50 The authors are grateful to Anna M. Bassols, from the Biochemistry and Molecular Biology Department of -55 Universitat Autònoma de Barcelona, for providing us the urine 0.6 0.7 0.8 0.9 1.0 1.1 samples. Frequency(GHz) Fig. 10. Cross-mode insertion loss for different values of total ion REFERENCES concentration in urine samples. [1] M. A. Hogan, Fluids, Electrolytes, and Acid-Base Balance with Nursing Reviews and Rationales (4th Edition), Pearson Reviews & Rationales 0.0500 Series, Pearson, 2017. Measurement [2] M. S. Oh and H. J. Carroll, “The anion gap,” N. Engl. J. Med., vol. Curve fitting 297(15), pp. 814-817 Oct. 1977. 0.0375 [3] R. P. Buck, “Ion selective electrodes”, Anal. Chem., vol. 48 (5), pp 23– 39, 1976. |S21 | (lineal) [4] M. Puentes, C. Weiß, M. Schüßler, and R. Jakoby, “Sensor array based on split ring resonators for analysis of organic tissues,” in IEEE MTT-S 0.0250 Int. Microw. Symp., Baltimore, MD, USA, Jun. 2011, pp. 1−4. [5] M. Puentes, Planar Metamaterial Based Microwave Sensor Arrays for DC Biomedical Analysis and Treatment, Springer, Heidelberg, Germany, 0.0125 2014. [6] A. Ebrahimi, W. Withayachumnankul, S. Al-Sarawi, D. Abbott, “High- sensitivity metamaterial-inspired sensor for microfluidic dielectric characterization,” IEEE Sensors J., vol. 14, no. 5, pp. 1345−1351, May 0.0000 2014 100 200 300 400 500 600 [7] M. Schueler, C. Mandel, M. Puentes, and R. Jakoby, “Metamaterial Total Ion Concentration (mEq/L) inspired microwave sensors,” IEEE Microw. Mag., vol. 13, no. 2, pp. 57–68, Mar. 2012. Fig. 11. Variation of S21DCmax with total ion concentration in urine. [8] M. S. Boybay and O. M. Ramahi, “Material characterization using complementary split-ring resonators,” IEEE Trans. Instrum. Meas., vol. 61, no. 11, pp. 3039−3046, Nov. 2012. VI. CONCLUSIONS [9] C.-S. Lee and C.-L. Yang, “Complementary split-ring resonators for measuring dielectric constants and loss tangents,” IEEE Microw. In summary, novel differential microwave fluidic sensors for Wireless Compon. Lett., vol. 24, no. 8, pp. 563−565, Aug. 2014. electrolyte concentration measurements in diluted aqueous [10] C.-L. Yang, C.-S. Lee, K.-W. Chen, and K.-Z. Chen, “Noncontact solutions, based on pairs of uncoupled lines loaded with split measurement of complex permittivity and thickness by using planar ring resonators (SRRs), have been reported. The working resonators,” IEEE Trans. Microw. Theory Techn., vol. 64, no.1, pp. 247−257, Jan. 2016. principle of the sensor is the variation of the cross-mode [11] L. Su, J. Mata-Contreras, P. Vélez and F. Martín, "Estimation of insertion loss generated by an asymmetric dielectric load of conductive losses in complementary split ring resonator (CSRR) loading the SRRs. Thus, by loading the fluidic channel of one of the an embedded microstrip line and applications", IEEE MTT-S SRRs with the reference liquid (the solvent), and the other International Microwave Symposium (IMS'17), Honolulu, Hawaii, June 2017. fluidic channel with the liquid under test (the solution), it [12] L. Su, J. Mata-Contreras, P. Vélez, F. Martín, "Estimation of the follows that solute concentration determines the cross-mode complex permittivity of liquids by means of complementary split ring insertion loss level, which is used as output variable. Sensor resonator (CSRR) loaded transmission lines", 2017 IEEE MTT-S validation has been carried out by considering three types of International Microwave Workshop Series on Advanced Materials and Processes (IMWS-AMP 2017), Pavia, Italy, 20-22 Sep. 2017. electrolytes: NaCl, KCl and CaCl2. Repetitiveness of the [13] L. Su, J. Mata-Contreras, P. Vélez, A. Fernández-Prieto and F. Martín, results has been demonstrated, and it has been found that the “Analytical method to estimate the complex permittivity of oil dependence of the cross-mode insertion loss with electrolyte Samples”, Sensors, 18(4), paper 984, 2018. content does not experience significant variations when the [14] J. Naqui, M. Durán-Sindreu and F. Martín, “Novel sensors based on the symmetry properties of split ring resonators (SRRs),” Sensors, vol 11, different electrolytes are considered. The combination of pp. 7545−7553, 2011. sensitivity and resolution of the reported sensor for NaCl [15] J. Naqui, M. Durán-Sindreu, and F. Martín, “Alignment and position concentration measurements in DI water, as compared to those sensors based on split ring resonators,” Sensors, vol. 12, pp. of other sensors (also focused on NaCl measurements), has 11790−11797, 2012. [16] A.K. Horestani, C. Fumeaux, S.F. Al-Sarawi, and D. Abbott, been found to be very competitive. With these high “Displacement sensor based on diamond-shaped tapered split ring sensitivities and small resolutions, these differential sensors resonator,” IEEE Sens. J., vol. 13, pp. 1153–1160, 2013. are of general interest for measuring small solute [17] A.K. Horestani, D. Abbott, and C. Fumeaux, “Rotation sensor based on concentrations in liquids, as well as to detect changes in the horn-shaped split ring resonator,” IEEE Sens. J., vol. 13, pp. 3014–3015, 2013. properties of liquids (related to variations in the complex 7 [18] J. Naqui and F. Martín, “Transmission lines loaded with bisymmetric [36] A. Vélez, F. Aznar, J. Bonache, M. C. Velázquez-Ahumada, J. Martel, resonators and their application to angular displacement and velocity F. Martín, "Open complementary split ring resonators (OCSRRs) and sensors,” IEEE Trans. Microw. Theory Techn., vol. 61, no. 12, pp. their application to wideband CPW band pass filters", IEEE Microw. 4700–4713, Dec. 2013. Wireless Compon. Lett., vol. 19, no. 4, pp. 197-199, Apr. 2009. [19] J. Naqui and F. Martín, “Angular displacement and velocity sensors [37] F. Martín, L. Zhu, J. Hong and F. Medina, Balanced Microwave Filters, based on electric-LC (ELC) loaded microstrip lines,” IEEE Sensors J., Wiley-IEEE Press, Hoboken, NJ, 2018. vol. 14, no. 4, pp. 939–940, Apr. 2014. [38] A. Babajanyan, J. Kim, S. Kim, K. Lee, and B. Friedman, “Sodium [20] A.K. Horestani, J. Naqui, D. Abbott, C. Fumeaux, and F. Martín, “Two- chloride sensing by using a near-field microwave microprobe,” Appl. dimensional displacement and alignment sensor based on reflection Phys. Lett., vol. 89, no. 18, p. 183504, 2006. coefficients of open microstrip lines loaded with split ring resonators,” [39] A. Bababjanyan, H. Melikyan, S. Kim, J. Kim, K. Lee, and B. Friedman, Elec. Lett., vol. 50, pp. 620–622, Apr. 2014. “Real-time noninvasive measurement of glucose concentration using a [21] J. Naqui and F. Martín, “Microwave sensors based on symmetry microwave biosensor,” J. Sensors, vol. 2010, 2010. properties of resonator-loaded transmission lines: a review,” Journal of [40] S. Kim, H. Melikyan, J. Kim, A. Babajanyan, J.-H. Lee, L. Enkhtur, B. Sensors, vol. 2015, Article ID 741853, 10 pages, 2015. Friedman, and K. Lee, “Noninvasive in vitro measurement of pig-blood [22] J. Naqui, J. Coromina, A. Karami-Horestani, C. Fumeaux, and F. D-glucose by using a microwave cavity sensor,” Diabetes Res. Clin. Martín, “Angular displacement and velocity sensors based on coplanar Pract., vol. 96, no. 3, pp. 379–384, 2012. waveguides (CPWs) loaded with S-shaped split ring resonator (S-SRR),” [41] N. Sharafadinzadeh, M. Abdolrazzaghi1, M. Daneshmand, “Highly Sensors, vol. 15, pp. 9628–9650, 2015. sensitive microwave split ring resonator sensor using gap extension for [23] A. K. Horestani, J. Naqui, Z. Shaterian, D. Abbott, C. Fumeaux, and F. glucose sensing”, 2017 IEEE MTT-S International Microwave Martín, “Two-dimensional alignment and displacement sensor based on Workshop Series on Advanced Materials and Processes (IMWS-AMP movable broadside-coupled split ring resonators,” Sensors and Actuators 2017), Pavia, Italy, 20-22 Sep. 2017. A, vol. 210, pp. 18–24, Apr. 2014. [42] J. Kim, A. Babajanyan, A. Hovsepyan, K. Lee and B. Friedman, [24] J. Naqui, C. Damm, A. Wiens, R. Jakoby, L. Su, and F. Martín, “Microwave dielectric resonator biosensor for aqueous glucose “Transmission lines loaded with pairs of magnetically coupled stepped solution”, Rev. Sci. Instrum., vol. 79, paper 086107, 2008. impedance resonators (SIRs): modeling and application to microwave [43] P. Vélez, J. Mata-Contreras, D. Dubuc, K. Grenier, and F. Martín, sensors,” IEEE MTT-S Int. Microwave Symp., Tampa, FL, USA, June “Solute Concentration Measurements in Diluted Solutions by means of 2014, pp. 1–4. Split Ring Resonators”, Proc. 48th Europ. Microw. Conf., Madrid, [25] L. Su, J. Naqui, J. Mata-Contreras, and F. Martín “Modeling Spain, 23-28 Sep. 2018. metamaterial transmission lines loaded with pairs of coupled split ring resonators,” IEEE Ant. Wireless Propag. Lett., vol. 14, pp. 68–71, 2015. [26] L. Su, J. Naqui, J. Mata, F. Martín, “Dual-band epsilon-negative (ENG) Paris Vélez (S’10–M’14) was born in Barcelona,Spain, transmission line metamaterials based on microstrip lines loaded with in 1982. He received the degree in telecommunications pairs of coupled complementary split ring resonators (CSRRs): engineering, specializing in electronics, the Electronics modeling, analysis and applications”, 9th International Congress on Engineering degree, and the Ph.D. degree in electrical Advanced Electromagnetic Materials in Microwaves and Optics, engineering from the Universitat Autònoma de Metamaterials 2015, Oxford, UK, Sep., 7-12, 2015. Barcelona, Barcelona, in 2008, 2010, and 2014, [27] L. Su, J. Naqui, J. Mata-Contreras, P. Vélez, F. Martín, “Transmission respectively. His Ph.D. thesis concerned common mode line metamaterials based on pairs of coupled split ring resonators (SRRs) suppression differential microwave circuits based on and complementary split ring resonators (CSRR): a comparison to the metamaterial concepts and semilumped resonators. He light of the lumped element equivalent circuits”, International is currently involved in the subjects related to Conference on Electromagnetics for Advanced Applications, ICEAA metamaterials sensors for fluidics detection at LAAS-CNRS through a 2015, Torino, Italy, 7-11 Sep. 2015. TECNIOSpring fellowship cofounded by the Mari Curie program. His current [28] L. Su, J. Naqui, J. Mata-Contreras, and F. Martín, “Modeling and research interests include the miniaturization of passive circuits applications of metamaterial transmission lines loaded with pairs of RF/microwave and sensors-based metamaterials. coupled complementary split ring resonators (CSRRs),” IEEE Ant. Dr. Vélez was the recipient of a predoctoral teaching and research Wireless Propag. Lett., vol. 15, pp. 154–157, 2016. fellowship by the Spanish Government from 2011 to 2014. He is a Reviewer [29] L. Su, J. Mata-Contreras, J. Naqui, and F. Martín, “Splitter/combiner for the IEEE TRANSACTIONS ON MICROWAVE THEORY AND microstrip sections loaded with pairs of complementary split ring TECHNIQUES and for other journals. resonators (CSRRs): modeling and optimization for differential sensing applications,” IEEE Trans. Microw. Theory Techn., vol. 64, pp. 4362- Jonathan Muñoz-Enano was born in Mollet del Vallès 4370, Dec. 2016. (Barcelona) Barcelona, Spain, in 1994. He received the [30] P. Vélez, L. Su, K. Grenier, J. Mata-Contreras, D. Dubuc, and F. Martín, Bachelor's Degree in Electronic Telecommunications "Microwave microfluidic sensor based on a microstrip splitter/combiner Engineering in 2016 and the Master's Degree in configuration and split ring resonators (SRR) for dielectric Telecommunications Engineering in 2018, both at the characterization of liquids", IEEE Sensors J., vol. 17, pp. 6589-6598, Autonomous University of Barcelona (UAB). Actually, Aug. 2017. he is working in the same university in the elaboration [31] F. Martín, Artificial Transmission Lines for RF and Microwave of his PhD, which is focused on the development of Applications, John Wiley, Hoboken, NJ, 2015. microwave sensors based on metamaterials concepts for [32] J. Naqui, Symmetry Properties in Transmission Lines Loaded with the dielectric characterization of materials and biosensors. Electrically Small Resonators: Circuit Modeling and Applications, Springer, Heidelberg, Germany, 2016. Katia Grenier (S'99, M'03) received her M.S. and [33] J. Mata-Contreras, C. Herrojo, and F. Martín, "Application of split ring Ph.D. degrees in electrical engineering from the resonator (SRR) loaded transmission lines to the design of angular University of Toulouse, Toulouse, France, in 1997 and displacement and velocity sensors for space applications", IEEE Trans. 2000, respectively. She was engaged in Microw. Theory Techn., vol. 65, pp. 4450-4460, Nov. 2017. microelectromechanical systems (MEMS) circuits on [34] P. Vélez, J. Mata-Contreras, L. Su, D. Dubuc, K. Grenier, F. Martín, silicon. She was a Postdoctoral Fellow at Agere Systems "Modeling and analysis of pairs of open complementary split ring (Bell Labs). In 2001, she joined the Laboratory of resonators (OCSRRs) for differential permittivity sensing", 2017 IEEE Analysis and Architecture of System of the National MTT-S International Microwave Workshop Series on Advanced Scientific Research Center (LAAS-CNRS), Toulouse, Materials and Processes (IMWS-AMP 2017), Pavia, Italy, 20-22 Sep. France. From 2007 to 2009, she was with the 2017. Laboratory for Integrated Micromechatronic Systems [35] P. Vélez, K. Grenier, J. Mata-Contreras, D. Dubuc, and F. Martín, CNRS (LIMMS-CNRS)/Institute of Industrial Science (IIS), Universtity of “Highly-sensitive microwave sensors based on open complementary Tokyo, Tokyo, Japan, where she was engaged in launching research activities split ring resonators (OCSRRs) for dielectric characterization and solute on microwave-based biosensors. Her research interests in LAAS-CNRS are concentration measurement in liquids”, IEEE Access, to be published. now focused on the development of fluidic-based microsystems, notably for 8 biological and medical applications at the cellular and molecular levels. Dr. Prof. Martín is a member of the IEEE Microwave Theory and Techniques Grenier is a member of the IEEE MTT-10 Technical Committee on Biological Society (IEEE MTT-S), member of the European Microwave Association effect and medical applications of RF and microwave of the IEEE Microwave (EuMA), and member of the Institution of Electronics and Technology (IET). Theory and Techniques Society. He is reviewer of dozens of journals, including IEEE journals such as IEEE Transactions on Microwave Theory and Techniques, IEEE Microwave and Wireless Components Letters, and IEEE Sensors Journal, among others, and Javier Mata-Contreras was born in 1976 in Málaga he serves as member of the Editorial Board of IET Microwaves, Antennas and (Spain). He received the Ingeniería de Propagation and International Journal of RF and Microwave Computer-Aided Telecomunicación Degree from the Universidad de Engineering. He is also a member of the Technical Committees of the Málaga (UMA) in 2000 and the PhD degree from the European Microwave Conference (EuMC) and International Congress on same university in 2010, with the Thesis “Distributed Advanced Electromagnetic Materials in Microwaves and Optics Amplifiers and Mixers with Transmission Lines based (Metamaterials). Among his distinctions, Ferran Martín has received the 2006 on Metamaterials”. In 2000, he joined the UMA Duran Farell Prize for Technological Research, he holds the Parc de Recerca Department of Ingeniería de Comunicaciones UMA as UAB – Santander Technology Transfer Chair, and he has been the recipient of Assistant Professor. He is currently working at two ICREA ACADEMIA Awards (calls 2008 and 2013). He is Fellow of the CIMITEC and the Universitat Autònoma de IEEE since 2012 and Fellow of the IET since 2016. Barcelona as Visitant Professor. His research interests include active and passive microwave devices and active distributed circuits based on metamaterials, among others. David Dubuc (S'99, M'03) received the Agregation degree from the Ecole Normale Supérieure de Cachan, Paris, France, in 1996, and the M.S. and Ph.D. degrees in electrical engineering from the University of Toulouse, Toulouse, France, in 1997 and 2001, respectively. From 2002 to 2013, he was an Associate Professor with the University of Toulouse, and a Researcher with the Laboratory of Analysis and Architecture of System part of National Scientific Research Center (LAAS-CNRS), Toulouse, France. From 2007 to 2009, he was a Visiting Senior Researcher with the Laboratory of Integrated Micromechatronic Systems (LIMMS-CNRS)/Institute of Industrial Science (IIS), University of Tokyo, Tokyo, Japan. Since 2013, he is Professor with the University of Toulouse. His research interests include the development of microwave circuits integrated due to microtechnologies and their application to wireless telecommunication and biology. Ferran Martín (M’04-SM’08-F’12) was born in Barakaldo, Spain, in 1965. He received the B.S. degree in physics and Ph.D. degree from the Universitat Autònoma de Barcelona (UAB), Barcelona, Spain, in 1988 and 1992, respectively. From 1994 to 2006 he was Associate Professor in Electronics at the Departament d’Enginyeria Electrònica (Universitat Autònoma de Barcelona), and since 2007 he is Full Professor of Electronics. His research activity has been very broad, including the modelling and simulation of electron devices for high frequency applications, millimeter wave and THz generation systems, the application of electromagnetic bandgaps to microwave and millimeter wave circuits, and the application of metamaterial concepts to the miniaturization and optimization of microwave circuits and antennas. He is now very active in the development of microwave sensors for dielectric characterization and motion control, and also in the topic of chipless-RFID. He is the head of the Microwave Engineering, Metamaterials and Antennas Group (GEMMA Group) at UAB, and director of CIMITEC, a research Center on Metamaterials supported by TECNIO (Generalitat de Catalunya). He has organized several international events related to metamaterials, including Workshops at the IEEE International Microwave Symposium (years 2005 and 2007) and European Microwave Conference (years 2009, 2015, 2017, and 2018), and the Fifth International Congress on Advanced Electromagnetic Materials in Microwaves and Optics (Metamaterials 2011), where he has acted as chair of the Local Organizing Committee. He has acted as Guest Editor in several Special Issues, mainly related to Metamaterials, in various International Journals. He has authored and co-authored over 580 technical conference, letter, journal papers and book chapters, he is co-author of the book on Metamaterials entitled Metamaterials with Negative Parameters: Theory, Design and Microwave Applications (John Wiley & Sons Inc. 2008), author of the book Artificial Transmission Lines for RF and Microwave Applications (John Wiley & Sons Inc. 2015), co-editor of the book Balanced Microwave filters (John Wiley & Sons Inc. and IEEE-Press 2018), and he has generated 19 PhDs. Ferran Martín has filed several patents on metamaterials and has headed several Development Contracts.