electronics Article Microwave Humidity Sensor for Early Detection of Sweat and Urine Leakage Lijuan Su * , Paris Vélez , Pau Casacuberta , Jonathan Muñoz-Enano and Ferran Martín * CIMITEC, Departament d’Enginyeria Electrònica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain; paris.velez@uab.cat (P.V.); pau.casacuberta@uab.cat (P.C.); jonatan.munoz@uab.cat (J.M.-E.) * Correspondence: lijuan.su@uab.cat (L.S.); ferran.martin@uab.cat (F.M.) Abstract: A planar microwave sensor devoted to the detection of humidity in underwear and clothes in general is proposed. The ultimate goal of the sensor is to detect the presence of liquids in fabrics, which is of interest to aid patients who suffer from certain pathologies, such as hyperhidrosis and enuresis. The main target in the design of the sensor, considering the envisaged application, is simplicity. Thus, the sensor operates at a single frequency, and the working principle is the variation in the magnitude of the transmission coefficient of a matched line loaded with an open-ended quarter- wavelength sensing stub resonator. The stub, which must be in contact with the so-called fabric under test (FUT), generates a notch in the transmission coefficient with a resonance frequency that depends on the humidity level of the fabric. By designing the stub with a moderately high-quality factor, the variation in the resonance frequency causes a significant change in the magnitude level at the operating frequency, which is the resonance frequency when the sensing stub is loaded with the dry fabric, and the presence of liquid can be detected by means of an amplitude detector. A prototype device is proposed and experimentally validated. The measured change in the magnitude level by simply depositing one 50 µL drop of water in the FUT is roughly 25 dB. Keywords: planar microwave sensor; humidity sensor; microstrip technology; detection of sweat; detection of urine leakage; enuresis; hyperhidrosis Citation: Su, L.; Vélez, P.; Casacuberta, P.; Muñoz-Enano, J.; 1. Introduction Martín, F. Microwave Humidity Sensor for Early Detection of Sweat Planar microwave technology is of the highest interest for the implementation of a and Urine Leakage. Electronics 2023, wide variety of sensors [1,2]. Among their advantages, planar microwave sensors exhibit 12, 2276. https://doi.org/10.3390/ low cost and profile, and they can be implemented in many different types of substrates, electronics12102276 including rigid substrates (such as low-cost FR4 or low-loss microwave substrates), or flexible substrates (including polymeric substrates, organic substrates, e.g., paper or fabric, Academic Editor: J.-C. Chiao among others). Planar microwave sensors can be fabricated by means of either subtractive Received: 24 April 2023 (e.g., photoetching or milling) or additive (e.g., inkjet printing, screen printing, or 3D Revised: 12 May 2023 printing) processes. Moreover, planar microwave sensors are compatible with many other Accepted: 16 May 2023 technologies such as microfluidics, micromachining, textiles, etc., and can be equipped Published: 18 May 2023 with functional films that make these sensors of interest in applications as diverse as liquid sensing [3–7], bio-sensing [8,9], gas sensing [10–16], wearables [17–22], measurement of physical variables (such as temperature or ambient humidity [23–25]), etc. The dielectric characterization of materials, namely permittivity measurements [7], [26–28], represents Copyright: © 2023 by the authors. the archetypal application of planar microwave sensors. Microwave sensors are highly Licensee MDPI, Basel, Switzerland. sensitive to the complex permittivity of the material surrounding the sensing element, This article is an open access article typically implemented as a transmission line section or a planar resonator. Thus, the distributed under the terms and permittivity of such material, or any other variable related to it, can be retrieved. The conditions of the Creative Commons sensor to be developed in this paper is based on the change in the permittivity of fabric (the Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ fabric under test, FUT) caused by liquid absorption (sweat or urine in real environments). 4.0/). Such change will modify the electrical characteristics of the sensitive element (capacitance, Electronics 2023, 12, 2276. https://doi.org/10.3390/electronics12102276 https://www.mdpi.com/journal/electronics Electronics 2023, 12, 2276 2 of 14 characteristic impedance, electrical length, etc.), which in turn will generate a variation in the output variable of the sensor (resonance frequency, quality factor, group delay, phase and/or magnitude of the reflection or transmission coefficient, etc.). Planar microwave sensors can be categorized according to the output variable as frequency- variation sensors [7–9,26–35], frequency-splitting sensors [4,22,36–39], magnitude-variation sensors (or coupling-modulation sensors [40–51]), and phase-variation sensors [52–69]. Nev- ertheless, in some cases, two combined output variables are used. For example, resonant sensors [7,26–28] are commonly used to determine the complex permittivity of materials. These sensors exploit variations in resonance frequency, which are primarily correlated with the dielectric constant of the material under test, as well as variations in the magni- tude of the notch or peak in the frequency response, which are primarily correlated with the loss tangent. While frequency-variation and frequency-splitting sensors are robust against electromagnetic interference (EMI) and noise due to their reliance on frequency measurements, these sensors require wideband sweeping interrogation signals for sensing, which must cover the output dynamic range. The need for such signals complicates the implementation of associated electronics in real-world environments. The solution to this issue is given by the so-called single-frequency sensors, fed by a harmonic (single-tone) interrogation signal. Coupling-modulation and phase-variation sensors belong to this class of sensors. Phase-variation sensors, additionally, are robust against EMI and noise (similar to frequency-variation sensors) and are very attractive for sensing. Nevertheless, retrieving the phase of either the reflection or the transmission coefficient in the operational environment (i.e., without the use of a vector network analyzer), in general, is not as simple as inferring the magnitude [70]. Magnitude measurements are more prone to the effects of EMI and noise, but the simplicity of the associated electronics is important in certain applications, in particular the intended application in this work (the detection of liquid ab- sorption in fabric). Note that a simple amplitude detector suffices to convert the magnitude of the transmission coefficient in a voltage variable (an easy measurable magnitude) [49–51]. Thus, in this present paper, we exploit the magnitude of the transmission coefficient at a single frequency for sensing, with an eye towards sensor simplicity. The sensing device proposed in this work is a resonant sensor, where the sensitive element is an open-ended quarter-wavelength stub resonator that must be in contact with the FUT and connected to a matched transmission line. The output variable is the magnitude of the transmission coefficient at the operating frequency, which is the resonance frequency of the open-ended stub when it is loaded with the dry FUT. This frequency is indicated by a notch in the transmission coefficient. When the FUT absorbs liquid (urine or sweat in a real scenario), the frequency response shifts downwards (because the effective dielectric constant “seen” by the stub increases), and the magnitude of the transmission coefficient at the operational frequency varies. Thus, liquid absorption by the fabric can be detected with this method. This work is organized as follows. Section 2 presents the proposed humidity sensor and the working principle in detail. The specific sensor design is the subject of Section 3, where validation at the simulation level is included. Experimental validation is carried out in Section 4 by considering fabric and different levels of water absorption. In Section 5, some comparisons are made between the sensor proposed in this work and those in the references. Finally, the main conclusions of this work are highlighted in Section 6. 2. The Proposed Sensor and Working Principle The topology and perspective view of the proposed sensor is depicted in Figure 1. It consists of a transmission line loaded with an open-ended quarter-wavelength stub res- onator, the sensitive element. The transmission line is matched to the reference impedance of the ports (Z0 = 50 Ω), namely its characteristic impedance is Zc = Z0 . The design vari- ables are the length and width of the sensitive stub. The length determines the resonance frequency (notch frequency) of the sensor, whereas the width is related to the characteristic impedance of the stub, Zs . In turn, Zs is the key parameter that determines the quality certain difficulty. Thus, a tradeoff is necessary. The working principle of the sensor is the shift in the frequency response (magn of the transmission coefficient) when the FUT absorbs a certain liquid. This modifi dielectric constant of the FUT, and the resonance frequency of the stub shifts down (F Electronics 2023, 12, 2276 2). As a consequence, the magnitude of the transmission coefficient at 3fof0 (the 14 reso frequency of the stub loaded with the reference, dry FUT), varies (increases), and absorption is thereby detected. The output variable in the proposed sensor is, thu factor of the stub resonator. An extreme (i.e., very high or very low) quality factor should magnitude be avoided.ofThe thereason transmission coefficient is that if the at f0is quality factor , whereas the very small, input a soft variable variation is the die in the constant magnitude of of the theFUT and is intimately transmission coefficient atrelated to theoflevel the resonance of liquid the stub absorption loaded with the dry by the In the prototype to be discussed later, liquid absorption is achieved bytuning FUT (the reference FUT) is expected. By contrast, if the quality factor is very high, depositing to the resonance frequency of the stub when it is loaded with the dry FUT is not absent of drops on top of the FUT, with the result of a change in the dielectric constant of the certain difficulty. Thus, a tradeoff is necessary. (a) (b) Figure Figure1.1.Topology (a)and Topology (a) andperspective perspective view view (b)theofproposed (b) of the proposed sensor. sensor. The sensing The sensing region is region cated by the indicated dashed by the rectangle. dashed rectangle. The ground The ground plane plane inis(a) in (a) is depicted depicted in soft in soft grey grey color. color. The working principle of the sensor is the shift in the frequency response (magnitude of the transmission coefficient) when the FUT absorbs a certain liquid. This modifies the dielectric constant of the FUT, and the resonance frequency of the stub shifts down (Figure 2). As a consequence, the magnitude of the transmission coefficient at f 0 (the resonance frequency of the stub loaded with the reference, dry FUT), varies (increases), and liquid absorption is thereby detected. The output variable in the proposed sensor is, thus, the magnitude of the transmission coefficient at f 0 , whereas the input variable is the dielectric constant of the FUT and is intimately related to the level of liquid absorption by the FUT. In the prototype to be discussed later, liquid absorption is achieved by depositing water drops on top of the FUT, with the result of a change in the dielectric constant of the FUT. 12, x FOR PEER REVIEW 4 of 14 Electronics 2023, 12, 2276 4 of 14 Figure 2. Simulated Figureresults fromresults 2. Simulated ANSYS fromHFSS ANSYS simulator showing HFSS simulator the working showing the working principle principle of of the the hu- midity sensor. Note that the dry fabric is set with thickness 0.3 mm and dielectric constant 1.5; and humidity sensor. Note that the dry fabric is set with thickness 0.3 mm and dielectric constant 1.5; and the 1 drop and the 1 drop of 2 drops andliquid 2 dropsinofthe liquid in theare fabric fabric are emulated emulated by changingthe by changing the dielectric dielectric constant constantto 4 to 4 and 9, respectively, just for indictive purpose. The open-ended stub has characteristic and 9, respectively, just for indictive purpose. The open-ended stub has characteristic impedance impedance of of 70 Ω when it is empty (surrounded by air), and it resonates at 2 GHz. 70 Ω when it is empty (surrounded by air), and it resonates at 2 GHz. 3. Sensor Design 3. Sensor Design The proposed sensor is designed to exhibit a resonance frequency of 2 GHz when the stub is in contact with the air (bare sensor). The frequency of operation f is expected to 0 The proposed sensor be slightly is designed smaller since it should to exhibit be tunedatoresonance the resonance frequency frequency of 2 GHz of the when the stub loaded stub is in contact withwith the air(dry) the reference (bare sensor). FUT. The frequency Nevertheless, the dielectricofconstant operation f0 isisexpected of fabric expected toto be slightly smaller since be very it to close should that of thebe air, tuned to thef 0 resonance and, hence, should not vary frequency of the extremely from the stub loaded resonance frequency of the bare sensor. The considered substrate for sensor implementation is the with the reference (dry) FUT. Nevertheless, the dielectric constant of fabric is expected to Rogers 4003C substrate with a dielectric constant εr = 3.55, thickness h = 1.524 mm, and be very close tolossthat of the factor tanδ air, and, With = 0.0022. hence, the fconsidered 0 should not vary extremely substrate, the width of fromthe 50-Ω thehost resonance line frequency of theis Wbare = 3.36sensor. mm. ForThe considered the stub, substrate let us consider for sensor three different implementation values of the characteristic is the impedance, Rogers 4003C substrate with Z = 25 Ω, s,1 a dielectric Z s,2 = 50 Ω, and constant εs,3Z = 70 Ω. The corresponding widths, r = 3.55, thickness h = 1.524 mm, and loss with the considered substrate, are Ws,1 = 9.04 mm, Ws,2 = 3.36 mm, and Ws,3 = 1.85 mm. We factor tanδ = 0.0022. With the considered substrate, the width of the 50-Ω host line is W = have adjusted the length of the open stub in all 3 cases in order to generate a notch (bare 3.36 mm. For sensor) the stub, let usasconsider at 2 GHz, indicated. The three different lengths are notvalues identical,ofsince the the characteristic imped- effective dielectric ance, Zs,1 = 25 Ω, Zs,2 =of50the constant Ω,stub anddepends Zs,3 = 70 on Ω. The corresponding its width, but the values arewidths, with very similar (ls,1the considered = 21.03 mm, l = 22.15 mm, and l = 22.45 mm). The responses substrate, are Ws,1 = 9.04 mm, Ws,2 = 3.36 mm, and Ws,3 = 1.85 mm. We have adjusted s,2 s,3 of the bare sensors (magnitude of the the reflection and transmission coefficient) for all three cases, inferred from full wave length of the open stub in all electromagnetic 3 casesusing simulation in order the ANSYSto generate HFSS 19.0 a notch (bare commercial sensor) software, at 2 GHz, are depicted as indicated. The lengths in Figure 3. Theare notofidentical, length the host line since theset has been effective to 50 mmdielectric constant in all cases, but of the this length doesstub not affect the magnitude of the reflection and transmission depends on its width, but the values are very similar (ls,1 = 21.03 mm, ls,2 = 22.15 mm, and coefficient, since the impedance of that line coincides with the reference impedance of the ports. The stop band bandwidth, ls,3 = 22.45 mm). The responses of the bare sensors (magnitude of the reflection and trans- or quality factor of the stub, varies with its characteristic impedance. According to the mission coefficient) previouslyformentioned all three tradeoff, cases, inferred we consider from that full wave the stub withelectromagnetic impedance Zs,3 = 70simulation Ω (and using the ANSYS widthHFSS Ws,3 = 19.0 1.85 mm)commercial is appropriate software, are depicted for our purposes in Figure since it exhibits 3. Thequality a moderate length of factor. Therefore, the sensor will be implemented by considering the host line has been set to 50 mm in all cases, but this length does not affect the magni- such stub impedance and width. tude of the reflection and transmission coefficient, since the impedance of that line coin- cides with the reference impedance of the ports. The stop band bandwidth, or quality fac- tor of the stub, varies with its characteristic impedance. According to the previously men- tioned tradeoff, we consider that the stub with impedance Zs,3 = 70 Ω (and width Ws,3 = 1.85 mm) is appropriate for our purposes since it exhibits a moderate quality factor. There- 12, x FOR PEER REVIEW 5 of 14 Electronics 2023, 12, 2276 5 of 14 Figure 3. Simulated frequency Figure 3. Simulatedresponse of the bare frequency response sensor of the bare forfordifferent sensor stub different stub impedances impedances (and (and widths). widths). Before experimental validation (left for the next section), let us anticipate the behavior of the sensor by electromagnetic simulation. For this purpose, let us consider that the Before experimental sensing region validation (leftafor is coated with the next material, hypothetical section),with let us anticipate a varying the constant, dielectric behav- ior of the sensorthat byemulates electromagnetic the FUT with simulation. Forofthis different levels liquidpurpose, absorption.letTheus consider consideredthat the thickness sensing region ofis that coated withis a0.3hypothetical material material,can mm since this thickness with a varyingtodielectric be comparable constant, the one of fabric, such as underwear or clothes made of cotton. We have varied that emulates the FUT with different levels of liquid absorption. The considered thickness the dielectric constant of the FUT between εFUT = 1 (the value of air and expected to be very close to the dielectric of that materialconstant is 0.3 mm since this thickness can be comparable to the one of fabric, such of dry fabric) and εFUT = 21 (a relatively high value, as expected in situations as underwear orwhere clothes made liquid significant of cotton. We have absorption varied by the fabric the dielectric occurs). Note that constant of the FUT this maximum value between εFUT = of1 (the εFUT isvalue of airarbitrary, somewhat and expectedsince the to be very actual value close to environment) (in a real the dielectric constant is expected to depend on the specific liquid (sweat, urine, etc.). Nevertheless, of dry fabric) and εFUT = 21 (a relatively high value, as expected in situations where signif- it is a representative value providing a reasonable input dynamic range, useful to qualitatively predict the behavior of icant liquid absorption by the fabric occurs). Note that this maximum value of εFUT is some- the sensor. We have carried out the simulations parametrized by the loss tangent of the what arbitrary, FUT since the (by actual value considering values(in of a0.001, real0.01 environment) and 0.1). The is expected simulated to depend frequency on the responses are specific liquid (sweat, depicted urine, in Figure etc.). Nevertheless, 4, whereas it is a the Figure 5 depicts representative dependence ofvalue providing the magnitude a of the transmission coefficient reasonable input dynamic range, useful at f (2 GHz) with the dielectric constant of the 0 to qualitatively predict the behavior of the sensor. FUT for the different We have carriedconsidered values of the loss out the simulations tangent. According parametrized by thetoloss Figure 5, the transmission tangent of the FUT (by coefficient con- at f 0 varies significantly with εFUT for the 3 considered values of the loss tangent of the sidering values FUT. of 0.001, 0.01 and 0.1). The simulated frequency responses are depicted in Nevertheless, the variation (and hence the sensitivity) is more pronounced for small Figure 4, whereas valuesFigure of the5loss depicts tangent. theNotedependence of the magnitude also that the sensitivity for small valuesof theof εtransmission FUT degrades as coefficient at f0 (2 theGHz) with the loss tangent dielectric of the FUT increases. constant of the However, forFUT for the moderate anddifferent high values considered of εFUT , the sensitivity does not depend on the loss tangent of the FUT. values of the loss tangent. According to Figure 5, the transmission coefficient at f0 varies These simulation results reveal that the intended sensor can work to detect the presence of liquid absorption in fabric, an significantly with εFUT for the 3 considered values of the loss tangent of the FUT. Never- aspect to be discussed in the next section, where the sensor is experimentally validated. theless, the variation (and hence the sensitivity) is more pronounced for small values of the loss tangent. Note also that the sensitivity for small values of εFUT degrades as the loss tangent of the FUT increases. However, for moderate and high values of εFUT, the sensitiv- ity does not depend on the loss tangent of the FUT. These simulation results reveal that the intended sensor can work to detect the presence of liquid absorption in fabric, an as- pect to be discussed in the next section, where the sensor is experimentally validated. Electronics 2023, 12, x FOR PEER REVIEW 6 of 14 Electronics 2023, 12, 2276 6 of 14 (a) (b) (c) Figure 4. 4. Figure Simulated Simulatedfrequency frequency responses responses ofofthe thesensor sensor forfor different different values values ofdielectric of the the dielectric constant constant of the FUT of the FUT and andloss losstangent tangent of the FUT of the FUTset settoto0.001 0.001 (a),(a), 0.01 0.01 (b),(b), andand 0.1 (c). 0.1 (c). Electronics 2023, Electronics 2023, 12, 12, xx FOR FOR PEER PEER REVIEW REVIEW 77 of of 14 14 Electronics 2023, 12, 2276 7 of 14 Figure 5. Figure 5. Dependence Dependence Dependenceof ofofthe the magnitude themagnitude ofof of thethe the transmission transmission coefficient coefficient at ff00at at == 22f GHz GHz with the dielec- Figure magnitude transmission coefficient 0 = 2 with GHzthe dielec- with the tric constant tric constant dielectric of the FUT of theof constant FUT for the for FUT different different values values for different of the loss of theofloss values tangent thetangent of the LUT. of theofLUT. loss tangent the LUT. 4.Experimental 4. 4. ExperimentalValidation Experimental Validation Validation For For experimental validation, we For experimental validation, experimental validation, we have we have fabricated have fabricated the fabricated the sensor the sensor with sensor with dimensions with dimensions and dimensions and aaa and substrate substratecorresponding substrate correspondingtoto corresponding tothe thesimulations the simulations simulations ofof Figures of Figures Figures 4 and 44 and5. The and 5. The 5. sensor The sensor sensor waswasfabricated was fabricated fabricated by means by of means the of LPKF the LPKFH100 drilling H100 machine, drilling machine, available availablein our in by means of the LPKF H100 drilling machine, available in our laboratory (the photograph laboratory our laboratory (the photograph (the photograph is depicted is depicted in Figure in 6). Figure The 6). measured The measured frequency frequency response response of is depicted in Figure 6). The measured frequency response of the bare sensor and one of the of bare the sensor bare and sensor one and of onetheof sensors the loaded sensors with loaded the with reference the (dry) reference FUT, (dry) depicted FUT, in Figure depicted the sensors loaded with the reference (dry) FUT, depicted in Figure 7 and inferred by in7 and Figureinferred 7 and by means inferred of by the means meansKeysight of the of N5221A the Keysight Keysightvector N5221A N5221A network vectoranalyzer, vector networkreveal network thatreveal analyzer, analyzer, the dielectric reveal theconstant that the that dielectric dielectric ofconstant the dry constant FUT of the of is very the dry dry FUT similar FUT is to is very the one very similar of similar to air, to the as the one anticipated one of of air, air, as (note that as anticipated the anticipated (note responses (note that that theare very the responsessimilar). responses are are Indeed, very the notch similar). frequency Indeed, the of the notch sensor of frequency loaded the with loaded sensor the drywith FUTthe is slightly dry FUT inferior is to slightly very similar). Indeed, the notch frequency of the sensor loaded with the dry FUT is slightly the one of inferior the to the bareofsensors, the one one bareand the bare it is the sensors, andconsidered operatingoperatingfrequency of the sensor, inferior to of the sensors, and itit is is the the considered considered frequency operating frequency of the of the fsensor, 0 = 2 GHz.f0 = The 2 FUTThe GHz. is aFUT piece is of a cottonofunderwear, piece cotton which has underwear, whichbeenhascutbeen in a rectangular cut in a rec- sensor, f0 = 2 GHz. The FUT is a piece of cotton underwear, which has been cut in a rec- shape tangularto accommodate shape to it to the dimensions to accommodate accommodate of the sensing region. tangular shape itit to to the the dimensions dimensions of the of the sensing region. sensing region. Figure6. Figure Figure 6.Measurement 6. Measurementsetup Measurement setupof setup ofthe of thefabricated the fabricatedsensor. fabricated sensor. sensor. Next, we Next, we have have emulated emulated sweat sweat or or urine urine absorption absorption by by the the fabric fabric by by dropping dropping DI DI water water with aa pipette with pipette on on top top of of it. it. Specifically, Specifically, we we have have sequentially sequentially added added aa water water drop drop on on top top of the fabric and made the measurement of the frequency response after each of the fabric and made the measurement of the frequency response after each drip. The drip. The results, also included in Figure 7, reveal that, as the number of accumulated drops in the fabric increase, the frequency response progressively shifts down. The result is an increase of the magnitude of the transmission frequency at f0, |𝑆 | . The variation of |𝑆 | with the accumulated number of drops is depicted in Figure 8. (It follows a similar trend to that Electronics 2023, 12, 2276 of Figure 5). According to the results of Figure 8, the presence of water in the fabric can be 8 of 14 detected, and therefore the sensor is experimentally validated. (a) (b) Figure 7. Frequency responses of the bare sensor, sensor loaded with the reference (dry) FUT, and Figure sensor 7. Frequency loaded with the FUTresponses with water of the bare absorbed sensor, (indicated sensor with loaded the number with the of drops). reference (dry) FUT, and (a) trans- sensorcoefficient; mission loaded with the FUTcoefficient. (b) reflection with water absorbed Note (indicated that the volume withofthe of water onenumber drop from of the drops). (a) transmission pipette is 50 µL. coefficient; (b) reflection coefficient. Note that the volume of water of one drop from the pipette is 50 µL. Next, we have emulated sweat or urine absorption by the fabric by dropping DI water with a pipette on top of it. Specifically, we have sequentially added a water drop on top of the fabric and made the measurement of the frequency response after each drip. The results, also included in Figure 7, reveal that, as the number of accumulated drops in the fabric increase, the frequency response progressively shifts down. The result is an increase of the magnitude of the transmission frequency at f 0 , |S21 | f0 . The variation of |S21 | f0 with the accumulated number of drops is depicted in Figure 8. (It follows a similar trend to that Electronics 2023, 12, x FOR PEER REVIEW 9 of 14 of Figure 5). According to the results of Figure 8, the presence of water in the fabric can be detected, and therefore the sensor is experimentally validated. Figure 8.8.Dependence Figure of the of Dependence magnitude of the transmission the magnitude coefficient at f0 with of the transmission the number coefficient atoff 0cu-with the number of mulative water drops deposited on the FUT. cumulative water drops deposited on the FUT. 5. Discussion Let us next put in context the proposed sensor as compared to other approaches to detect/measure humidity. Let us start by mentioning that the sensor of this work is based on microwave technology. One of the advantages of microwaves for sensing is their in- herent wireless connectivity, which can be of interest within the framework of the internet of things (IoT), and particularly in the field of smart health, where retrieving information Electronics 2023, 12, 2276 9 of 14 5. Discussion Let us next put in context the proposed sensor as compared to other approaches to detect/measure humidity. Let us start by mentioning that the sensor of this work is based on microwave technology. One of the advantages of microwaves for sensing is their inherent wireless connectivity, which can be of interest within the framework of the internet of things (IoT), and particularly in the field of smart health, where retrieving information wirelessly is necessary in many cases. Microwave sensors are canonically permittivity sensors, and for this main reason the proposed sensor is based on permittivity variations of the FUT rather than on the variation of other variables, such as conductivity. The reported device is a proof- of-concept demonstration of the potential of the approach to detect the presence of liquid in fabric. We leave for future work the implementation of the humidity sensor directly on fabric, with the idea of replacing the rigid microwave substrate considered in this work with a textile substrate (that can be part of the underwear to detect urine leakage or the presence of sweat). It is also left for future work the implementation of the whole sensor, including the associated electronics for signal generation and for signal processing. In the prototype presented in this paper, the main interest has been focused on the microwave module of the sensor rather than on the associated electronics, and we have obtained the sensor response with a vector network analyzer. In real scenarios, such response can be inferred by means of an envelope (amplitude) detector connected to the output port of the host line, as has been demonstrated in several papers (e.g., in [51]). On the other hand, the feeding (harmonic) signal can be generated by means of a voltage-controlled oscillator (VCO) managed by a microcontroller. We should mention that, despite the fact that the proposed sensor detects the pres- ence of liquid in fabric, it cannot be considered a microwave liquid sensor. Liquid sen- sors are mainly focused on the characterization of the complex dielectric constant of a certain liquid under study [4–7] or on the determination of the composition of liq- uid mixtures [51,57,71–81]. Such sensors are typically implemented by means of fluidic channels [51,57,71–77]. However, sensing structures based on holders [7] or submersible devices [82–85] have also been reported. In the sensor reported in this paper, the level of hu- midity in the fabric due to absorption was detected. Thus, it can be considered a humidity sensor. There are several reported sensors devoted to the measurement of humidity [86–91] but mostly devoted to measuring ambient humidity. In most cases, such sensors utilize functional materials, e.g., Kapton [88], polyvinyl alcohol (PVA) [86], polyethylene tereph- thalate (PET) [87], etc., with the dielectric constant very sensitive to the humidity level, or nanostructures (e.g., silicon nanowires) [89], or halloysite nanotubes (HNTs) [90]. Nev- ertheless, examples of humidity sensors devoted to detecting the presence of liquids in fabric have also been reported. In these cases, functional materials are not needed, since, in general, fabrics absorb liquids, generating a significant change in the permittivity. One example is the sensor reported in [22], implemented in a fabric substrate. The functionality of that sensor was demonstrated in [22]. The main advantage of the sensor of Figure 6, over the sensor presented in [22], is operation at a single frequency, contrary to the sensor in [22], based on frequency splitting. Moreover, the sensor of Figure 6 is very simple, being based on a stub, the sensing element, loading a host transmission line. In [92,93], sensors devoted to detecting urine incontinence, based on changes in the conductivity of an interdigital electrode, were proposed. Capacitive sensors integrated in underwear with embroidered textile technology were proposed in [94,95]. The sensitive elements of these sensors [92–95] are based on interdigital conductive patterns, which occupy a much larger area and are more complex to fabricate than the sensitive element in this work, i.e., merely an open stub with quarter wavelength. In addition, in [92], the minimum amount of humidity (liquid) of the sensor that could be detected was 0.1 mL, corresponding to 11.71 MΩ, compared with 15.52 MΩ when it was dry. While in this work, the minimum amount of humidity that could be measured was 50 µL, corresponding to –12 dB at 2 GHz (operating frequency), compared with –36 dB at 2 GHz when it was dry. Although the output variables are different, but the sensor in this work is superior for the detection of a tiny amount of liquid Electronics 2023, 12, 2276 10 of 14 absorption, given the fact that, in [94,95] the aim was to detect leakage levels when the diaper was full. 6. Conclusions In conclusion, it has been experimentally demonstrated that humidity in fabric (par- ticularly, the cotton of underwear clothes) can be detected by means of a simple planar microwave sensor consisting in a microstrip line loaded with an open-ended quarter- wavelength resonant stub, the sensitive element. For simplicity, it has been considered that the output variable is the magnitude of the transmission coefficient measured at a specific frequency, i.e., the resonance frequency of the stub when it is coated with the dry fabric. Thus, the sensor works at a single frequency, an important aspect for operation in real environments, where vector network analyzers used at the laboratory level should be replaced with signal generators and detectors (in the considered structure, a narrow band voltage-controlled oscillator and an amplitude detector would suffice). According to the obtained experimental results, a single drop (with an estimated volume of 50 µL) gener- ates a magnitude variation in the transmission coefficient of the sensor at the operating frequency of roughly 25 dB, more than enough to detect the presence of small amounts of water in fabric. Thus, with these results, it can be envisaged that the reported sensor can be applied to the detection of sweet in fabric or urine leakage in underwear. In future studies, the focus will be the direct implementation of the sensor in a fabric substrate, which is more compatible with the intended application. Author Contributions: Conceptualization, F.M.; funding acquisition, F.M.; methodology, L.S. and P.V.; project administration, F.M.; software, L.S. and P.V.; supervision, F.M.; validation, L.S., P.V., P.C. and J.M.-E.; writing—original draft, L.S. and F.M.; writing—review and editing, L.S., P.V., P.C., J.M.-E. and F.M. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by MCIN/AEI 10.13039/501100011033, Spain, through projects PID2019-103904RB-I00 (ERDF European Union) and PDC2021-121085-I00 (European Union Next Generation EU/PRTR), by Generalitat de Catalunya through project 2021SGR-00192, and by ICREA. P. Casacuberta acknowledges the Ministerio de Universidades, Spain, for the FPU grant (Ayudas para la formación de profesorado universitario), ref. FPU20/05700. L. Su acknowledges the Juan de la Cierva Program for their support through the project IJC2019-040786-I. Data Availability Statement: This article contains the necessary data to verify the results (circuit di- mensions and substrate used, as well as setup conditions). Text-based files containing the S-parameter data obtained in simulations and measurements can be requested to the corresponding author. Conflicts of Interest: The authors declare no conflict of interest. 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