sensors Review

Review of Batteryless Wireless Sensors Using Additively Manufactured Microwave Resonators Muhammad Usman Memon

ID

and Sungjoon Lim *

ID

School of Electrical and Electronics Engineering, College of Engineering, Chung-Ang University, 221 Heukseok-dong, Dongjak-gu, Seoul 156-756, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2-820-5827 Received: 28 June 2017; Accepted: 7 September 2017; Published: 9 September 2017

Abstract: The significant improvements observed in the field of bulk-production of printed microchip technologies in the past decade have allowed the fabrication of microchip printing on numerous materials including organic and flexible substrates. Printed sensors and electronics are of significant interest owing to the fast and low-cost fabrication techniques used in their fabrication. The increasing amount of research and deployment of specially printed electronic sensors in a number of applications demonstrates the immense attention paid by researchers to this topic in the pursuit of achieving wider-scale electronics on different dielectric materials. Although there are many traditional methods for fabricating radio frequency (RF) components, they are time-consuming, expensive, complicated, and require more power for operation than additive fabrication methods. This paper serves as a summary/review of improvements made to the additive printing technologies. The article focuses on three recently developed printing methods for the fabrication of wireless sensors operating at microwave frequencies. The fabrication methods discussed include inkjet printing, three-dimensional (3D) printing, and screen printing. Keywords: inkjet printing; 3D printing; screen printing; RF sensors; wireless; RF resonators

1. Introduction Progress in the area of wireless communication is directed toward a nonstop improvement of device operations, ubiquity, and commercial viability of devices. Evolving research into millimeter wave (mm-wave) radio frequency (RF) communication operations at frequencies ranging 30–300 GHz is important for the advancement of such automation as gigabit wireless local area networks, automobile contact prevention, self-operational steering radar systems, and fine-quality beam-tuning image-processing equipment. The classic production methods for mm-wave structures comprise patterning, lithographic masking, and reproduction of materials that require the use of severe liquid materials, and are expensive. In order to enable the maintained assimilation and spread of rising mm-wave methodologies, attempts should be made to boost their adaptability and minimize the price of component manufacture. An additive electronic fabrication technique known as “inkjet printing” has been gathering immense attention for industrial uses as a greatly scalable, cost-effective, and above all, environment-friendly substitute to classical lamination-based fabrication methods [1]. Using thick/thin polymer-based and conductive nanoparticle-based ink materials, multiple layering structures for RF industry—e.g., fully printed transformers, inductors and capacitors [2–4]—have been realized, using inkjet-printing on stretchy materials. Through the refinement and characterization of these ink materials, multilayer RF structures operational in the millimeter-wave frequency range have been accomplished with the help of inkjet-printing manufacture on both solid and stretchy materials [5,6]. Conversely, numerous latest presentations of inkjet-printed millimeter-wave modules suffer from

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restrictions existing in the distinctive multiple layers of RF structure designs—e.g., material properties and uniform thinness of corporate laminating materials—along with difficulties in multiple layers of laminating material treatment (e.g., material alignment, bonding, and stacking) [7–10]. The upright assimilated, additive aspect of inkjet-printed structure in electronics allows the integration with structures of significant interest, such as system-on-chip modules, stretchable and wearable electronics, and conformable/reconfigurable/rollable configurations [11,12]. With the help of this integration, the versatility and efficiency of RF millimeter-wave systems has drastically increased by directly post-process depositing antennas into any solid or stretchable active circuit configuration. There are many additive manufacturing techniques such as gravure/dispenser printing, spray deposition, stereolithographic, polymer-jet fused deposition modeling, laminated object manufacturing, selective laser sintering, electronic beam melting containing liquid, solid and powder based processes [13]. Nevertheless, the most popular additive manufacturing techniques for microwave printed electronics are 3D printing, inkjet printing, and screen printing. Therefore, we review these three additive printing techniques for microwave resonator-based wireless sensor applications. In the case of creating the preferred designs for malleable electronic devices, many recent material processing methods have been proposed including lithographic [14,15], stencil printing [16–21], microchannel molding and coating, and space-filling techniques [22,23]. However, the aforementioned techniques are expensive, lack fabrication scalability, or employ complex procedures and, consequently, their widespread implementation has been restricted. Furthermore, three-dimensional (3D) printing does not suffer from these restrictions, and has started to become a mainstream additive production technique. This technique uses the mechanism of constructing complex-shaped objects layer-by-layer containing thorough digital outlines [24]. Further, 3D printing technology has many benefits, such as shorter fabrication time and a capability to construct multifarious configurations and shapes with multiple materials at the same time, which certainly is beyond the capabilities of conventional fabrication processes. The speedy evolution of 3D and four-dimensional (4D) (time-managing) printing machinery promises exclusive rewards in the formation of optimum non-orthogonal shapes, on-demand deposition of nanostructures and other RF modules, such as antennas, and multiple layers on stretchy hermetic RF suites for flexible electronics. However, there are noteworthy obstacles in the construction of fully 3D multiple layer configurations. One of these obstacles is the bridging or overhanging complications, where the materials are affected by gravity throughout the printing course. Consequently, most schemes attempt to circumvent extremely overhung 3D structures and approve printing electrical components over a surface, such as those described in [18,25]. To be precise, these systems belong to the 2.5D printing classification. The existing marketplace proposes three solutions for printing extreme overhang structures: zero support, solid support structures, and non-solid supporting structures. Zero support printing is in the early stages of development [26]. Consequently, many limitations in the selection of printing materials and geometry design still exist. Solid support structures are the most common solution for extreme overhang designs. However, printing in this manner wastes material. Moreover, this technique is time-consuming, and it is difficult to remove the support. Consequently, small marks, which cause dimensional inaccuracies, remain on the printed object. In the case of non-solid supporting structures, powder-based technologies such as selective laser sintering and direct metal laser sintering are extensively adopted printing methods for commercial use. These techniques can use the surrounding powder to hold the structure in place during printing; e.g., the 3DPandoras printer [27]. However, the powder can be tough to remove, particularly when printing nylon or metal. Other methods in early development, such as embedded 3D printing [28], are still not sufficiently developed to be used in fabrication, since the ink utilized requires a high resistivity (117 Ωcm) and the printing process is complicated. Screen printing is the most popular and mature technology for printed electronics, since it has been employed in the electronics industry to print metallic interconnects on printed circuit boards for a long period. This technique is faster and more versatile than other printing tools,

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since the fabrication process is simple, affordable, quick, and adaptable. Screen-printed devices can be reproduced by repeating a few steps, and an optimum operating envelope can be developed quickly [29–34]. The feasibility of screen printing for flexible electronics has been demonstrated through the production of many printed sensors, electronic devices, and circuits. For example, all-screen-printed thin-film transistors (TFTs) have been demonstrated in [33,35,36]. Screen printing has been used to develop organic light-emitting diodes, following the investigation of the fabrication process and parameters of the screen printing solution i.e., viscosity of the solution and mesh count of the screen [37]. Multilayer high-density flexible electronic circuits, connected to embedded passive and optical devices through micro via holes, have been realized using advanced screen printing processes [32]. Screen printing is also used for patterning to develop shadow masks for the fabrication of organic TFTs. Screen-printed electrical interconnects for a temperature sensor on a polyethylene terephthalate (PET) substrate have been reported in [38]. However, in this review article, a summary/review of the developments in only printing technologies such as inkjet, 3-D and screen printing for the fabrication of batteryless wireless sensors operating at microwave frequencies is presented. Easier processing stages, minimized material waste, high speed and cost-effective substances, and simple patterning procedures render printing tools very attractive for accurate, multi-layered, and cost-effective development [39]. These features of printed electronics have allowed researchers to explore new avenues for material processing and to develop sensors and systems on non-planar surfaces that are otherwise difficult to realize with the conventional wafer-based fabrication techniques. In this paper, we compare the current developments in the three aforementioned additive printing fabrication techniques, with respect to their fabrication time, power consumption, and complexity. The design and analysis of inkjet-printed sensors are presented in Section 2. Section 3 focuses on the construction of a 3D RF sensor and its technological advantages. Section 4 comprises a review of the latest screen-printed sensors. Section 5 compares the technologies presented and summarizes this review. 2. Inkjet Printed Sensors This section describes the advanced inkjet-printed batteryless RF sensor devices. Inkjet-printed wireless sensor systems for numerous future applications are introduced in terms of their environmental impact and performance as a sustainable technology. 2.1. Inkjet-Printing on Paper Material Inkjet-printing methods have many benefits for RF sensor fabrication. Inkjet printing technology is cost-effective and environment-friendly because no hazardous chemicals are used to wash away the unwanted metals on the surface of a substrate. In this technique, nanoparticle ink is deposited at the desired position. Consequently, there are no by-products because inkjet printing is an additive fabrication method. The advantages of this technology, such as fast fabrication and ease of mass production, also reduce the cost of inkjet-printed electronics. The electrical properties of inkjet-printed silver nanoparticles were thoroughly studied in [1]. Many microwave applications utilizing silver nanoparticles have been proposed in [40–42]. The conductivity of the inkjet printing silver nanoparticle inks is approximately 1.12 × 107 S/m, which is sufficiently high for microwave or millimeter-wave applications. Inkjet printing technology can be used to fabricate various electronic devices on many kinds of materials; consequently, it is possible to utilize an environment-friendly alternative, such as paper, as a substrate. Paper is a very attractive substrate in inkjet-printed electronics for agricultural applications. Paper is a low-cost, renewable, and inkjet printable material. Apart from being one of the cheapest materials in the world, paper decomposes completely in agricultural environments. Moreover, there are many kinds of paper, such as hydrophobic, porous, and translucent paper. The hydrophilic property of normal paper is useful for implementing humidity or rainfall sensors. These sensors are

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widely adopted because water monitoring is crucial in agriculture. The properties of paper that are useful for inkjet printing have been reported in many studies based on measurements using different Sensors 2017, 17, 2068techniques, such as ring resonator or T-resonator methods [1,40]. Paper has a reported 4 of 30 characterization dielectric constant (ε r ) of approximately 3.0 and a loss tangent (tan δ) of approximately 0.05–0.06. approximately 0.05–0.06. relatively loss issue of paper is not frequency a critical issue for radio frequency The relatively high loss ofThe paper is not high a critical for radio identification (RFID) or identification (RFID) or planar structures, which have a low Q-factor, because paper is very thin. planar structures, which have a low Q-factor, because paper is very thin. This high loss results inThis low high loss results in low interaction between the electric field (E-field) and paper substrate. interaction between the electric field (E-field) and paper substrate.

2.2. RFID Retro-Directive Transponders, Transponders,and andan anInkjet-Printed Inkjet-PrintedSensor SensorPlatform Platform 2.2. RFID Enabled Enabled Sensor, Sensor, Retro-Directive 2.2.1. RFID-Empowered Sensor RFID-empowered sensor devices have many benefits over state-of-the-art sensor components in ofcost-effectiveness cost-effectivenessand and ease of use. Typically, the price of antag RFID tag is small and the terms of ease of use. Typically, the price of an RFID is small and the structure structure is comparatively simple (reader Thus, it to is realize possibleantoRFID-empowered realize an RFIDis comparatively simple (reader and sensorand tag).sensor Thus,tag). it is possible empowered over a largefield agricultural field RFID at a low cost. RFID principles are also sensor devicesensor over adevice large agricultural at a low cost. principles are also appropriate for appropriate for current wireless sensor which are easy to implement [43]. current wireless sensor networks, whichnetworks, are easy to implement [43]. thissubsection subsection[44], [44],ananinkjet-printed inkjet-printed RFID-empowered sensor device for haptic and waterIn this RFID-empowered sensor device for haptic and water-level level recognition is proposed. The device contains two similar RFIDfor tags the ultra-high-frequency recognition is proposed. The device contains two similar RFID tags thefor ultra-high-frequency (UHF) (UHF)atband at approximately MHz. The proposed sensor is combined one oftags. the The RFID tags. band approximately 915 MHz.915 The proposed sensor is combined with one ofwith the RFID sensor The sensor is a meandering line with a self-resonant frequency of approximately 915 MHz. When is a meandering line with a self-resonant frequency of approximately 915 MHz. When a material with a material with a dielectric constant loss tangent different those air comes with dielectric constant and loss tangentand different from those of airfrom comes intoof contact withinto the contact meandering the meandering stripline,ofthe of the which component which in shifting of the stripline, the capacitance the capacitance component varies, resultsvaries, in shifting of results the resonating frequency resonating frequency of RFID an RFID the RFID tagsfrequencies, have identical frequencies, their of an RFID tag. Since the tagstag. haveSince identical resonant theirresonant unique IDs are returned at are returned a similar frequency they However, are activated by the tag reader. aunique similarIDs frequency values at when they are activatedvalues by the when tag reader. the resonant frequency However, the resonant frequency of the device antenna(Figure connected the sensor devicefrequency (Figure 1) when is moved of the antenna connected to the sensor 1) isto moved to a lower the to a lower the detector is in contact withinhuman skin Utilizing or liquid the (water this case). detector is frequency in contact when with human skin or liquid (water this case). tag in without the Utilizing tag without the sensor a reference, existence of an analyst can easily sensor forthe a reference, the existence of for an analyst can bethe easily determined. Similarly, thebelevel of determined. level of water can beofidentified becauseofthe of the influences capacitancethe of water can beSimilarly, identifiedthe because the variation the capacitance thevariation sensor device the sensor deviceofinfluences the resonant frequency of the RFID-empowered structure. A resonant frequency the RFID-empowered structure. A metamaterial-empowered resonating element metamaterial-empowered resonating crosstalk element between is attached to the is attached to the tags for overturning the two tags.tags for overturning crosstalk between the two tags.

Figure 1. 1. Inkjet-printed capacitive sensor sensor [44]. [44]. Figure Inkjet-printed capacitive

2.2.2. Retro-Directive Transponder for Sensing A retro-directive antenna array can re-transmit an interrogation signal to its source without any complicated computations [45]. The Van Atta topology is a widely used retro-directive antenna array topology owing to its simple structure and passive implementation [46]. The integration of a microfluidic sensor with a retro-directive antenna array was proposed in [47,48]. The resulting device is purely passive and has a self-steering capability. The self-steering capability results in strong

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2.2.2. Retro-Directive Transponder for Sensing A retro-directive antenna array can re-transmit an interrogation signal to its source without any complicated computations [45]. The Van Atta topology is a widely used retro-directive antenna array topology owing to its simple structure and passive implementation [46]. The integration of a microfluidic sensor with a retro-directive antenna array was proposed in [47,48]. The resulting device is purely Sensors 2017, 17, 2068passive and has a self-steering capability. The self-steering capability results in strong 5 of 30 system performance owing to the wide readable angle of the passive transponder. This property is particularly critical in radar cross-section cross-section (RCS)-based (RCS)-based backscattering communication applications, such as as passive passivewireless wirelesssensor sensorsystems. systems. example, back-scattered power of most passive ForFor example, the the back-scattered power of most passive RCSRCS-based wireless sensors depends onillumination the illumination Retro-directive antenna arrays be based wireless sensors depends on the angle.angle. Retro-directive antenna arrays can becan used used to improve the performance of the sensor because these antennaarrays arrayscan canreflect reflect near-identical to improve the performance of the sensor because these antenna powers to the interrogation dual-band, interrogation direction over a broad broad angle. angle. The proposed inkjet-printed, dual-band, substrate-integrated and microfluidic sensor are are shown in Figure 2 [47]. substrate-integratedwaveguide waveguideretro-directive retro-directivearray, array, and microfluidic sensor shown in Figure 2 The operation of this device suggests a potential application as a chipless RFID-enabled sensor tag [47]. The operation of this device suggests a potential application as a chipless RFID-enabled sensor operating at two different frequencies forfor temperature oror water tag operating at two different frequencies temperature waterquality qualitysensing. sensing.The The variation variation of the RCS of the microfluidic sensor can be measured over a broad range owing to the retro-directive transponder. transponder. The dual-band property of the retro-directive transponder results in the ability to sense two targets at two different different frequencies. frequencies.

(a)

(b)

Figure 2.2.(a) (a)Inkjet-printed, Inkjet-printed, dual-band, substrate-integrated waveguide retro-directive array on Figure dual-band, substrate-integrated waveguide retro-directive array on paper; paper; and (b) proposed microfluidic sensor [47]. and (b) proposed microfluidic sensor [47].

2.2.3. Inkjet-Printed Platform 2.2.3. Inkjet-Printed Sensor Sensor Platform A cost-efficient inkjet-printed sensor sensor module module for for agricultural A cost-efficient inkjet-printed agricultural applications applications was was recently recently proposed proposed in [49]. moisture content, water content of in [49]. The The sensor sensorplatform platformhas hasbeen beenimproved improvedtotodetect detectambient ambient moisture content, water content the soil, and rainfall because moisture sensing is a crucial aspect of farming. The block diagram of the of the soil, and rainfall because moisture sensing is a crucial aspect of farming. The block diagram system, whichwhich contains a leafasensor, soil soil humidity sensor, microcontroller unit, andand antenna, is of the system, contains leaf sensor, humidity sensor, microcontroller unit, antenna, shown in Figure 3a. The capacitance of the leaf sensor and soil humidity sensor differ based on the is shown in Figure 3a. The capacitance of the leaf sensor and soil humidity sensor differ based water and humidity of the of soil theorenvironment surrounding the sensor module. The on the content water content and humidity theorsoil the environment surrounding the sensor module. microcontroller detects variations in the capacitance of of thethe leaf sensor The microcontroller detects variations in the capacitance leaf sensorand andthe thesoil soilmoisture moisturesensor. sensor. The microcontroller processes the data collected from the sensor and broadcasts this information The microcontroller processes the data collected from the sensor and broadcasts this information through the the antenna. antenna. The to gather gather ambient ambient power power through The microcontroller microcontroller and and antenna antenna can can also also be be used used to information to to initialize initialize the the microcontroller microcontroller or or reduce reduce battery battery consumption consumption [50]. [50]. In contrast to to information In contrast traditional sensor modules, all passive elements are inkjet-printed on an eco-friendly paper substrate. traditional sensor modules, all passive elements are inkjet-printed on an eco-friendly paper substrate. Finally,dense densemonitoring monitoringofofrainfall rainfalland andsoil soilhumidity humidityover overlarge large agricultural fields is possible owing Finally, agricultural fields is possible owing to to the advantages of inkjet printing technology, as low fabrication cost ofand ease of mass the advantages of inkjet printing technology, such assuch low fabrication cost and ease mass production. production. An implementation of the sensor platform is shown in Figure 3b. The soil moisture sensor is buried in the ground to detect surface soil humidity. The leaf sensor, microcontroller, and antenna are visible. The uncovered constituents can be chemically layered with Parylene or silicone, if required, to extend the lifetime and protect the sensor platform.

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An implementation of the sensor platform is shown in Figure 3b. The soil moisture sensor is buried in the ground to detect surface soil humidity. The leaf sensor, microcontroller, and antenna are visible. The uncovered constituents can be chemically layered with Parylene or silicone, if required, to extend the lifetime and Sensors 2017, 17, 2068protect the sensor platform. 6 of 30

(a)

(b) Figure 3. 3. Inkjet-printed applications: (a) (a) block block diagram of system; and Figure Inkjet-printed sensor sensor platform platform for for agriculture agriculture applications: diagram of system; and (b) proposed proposed design design in in operation operation [49]. [49]. (b)

2.3. A A Fully Fully Inkjet-Printed Inkjet-Printed Wireless Wirelessand andChipless ChiplessSensor Sensorfor forCarbon CarbonDioxide Dioxide(CO (CO)2and ) andTemperature Temperature 2.3. Detection 2 Detection This subsection describes a printed CO2 and temperature sensor, which utilizes different industrial a printed CO2 or andsensor temperature sensor, which utilizes different inks.This Thesubsection sensitivity describes of a batteryless detector is the result of the removal of complex industrial inks. The sensitivity of a batteryless detector or sensor is Itthe result of the removal of single-walled/polymer carbon-nanotube (SWCNT) ink material [51]. was recently demonstrated complex single-walled/polymer carbon-nanotube (SWCNT) ink material [51]. It was recently that graphene sheets and carbon nanotubes (CNT) provide robust sensitivity to several vapors and demonstrated that graphene sheets and carbon nanotubes (CNT) provide robust sensitivity to several gaseous elements [52–55], mitigating alternative substantial limitations. Prior to this study [56], and gaseous elements [52–55], alternative substantial Priorimpairs to this avapors straightforward system verifying the mitigating sensitivity of the proposed device limitations. to smog, which study [56], a straightforward system verifying the sensitivity of the proposed device to smog, many material properties, was proposed. This section intends to evaluate the performance which of the impairs many material properties, was proposed. This section intends to evaluate the performance proposed sensor when subjected independently to temperature changes and CO2 . Furthermore, the proposed sensorregarding when subjected independently changes and 2. Furthermore, aoftangential approach the multiple layers of to thetemperature responsive substance hasCO been adopted for a tangential approach regarding the multiple layers of the responsive substance has been adopted for the improvement of the sensing performance. Many specimens are tested to evaluate the reliability thereproduction. improvementWith of the sensing performance. Many are tested to evaluate the reliability of regard to the selectiveness, thespecimens authors include the results of coating the first of reproduction. With regard to the selectiveness, the authors include the results of coating the first or topmost layer with a polymer-based ink for the sensitivity of the proposed sensor to CO 2 and or topmost layer with a polymer-based ink for the sensitivity of the proposed sensor to CO 2 and temperature. In the following subsection, we review the dimensions and design of a batteryless sensor temperature. In the subsection, we principles. review the dimensions and design of a batteryless device (see Figure 4), following and explain its operating sensor device (see Figure 4), and explain its operating principles.

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Figure 4. 4. Top Top view view and and side side view view of of the the fully fully inkjet-printed inkjet-printed dual-polarized dual-polarized sensor sensor device device fabricated fabricated Figure using three three different different inks inks [51]. [51]. using

2.3.1. Design 2.3.1. Design and and Principle Principle Radiation Mechanism of a Dual-Polarization Split-Ring Resonator Resonator (SRR) (SRR) The operational RFID chip sensor device is comparable to the operational functionality functionalityofofa abatteryless batteryless RFID chip sensor device is comparable toidea the of a microchip-empowered RFIDRFID sensor without a unified analogue-to-digital converter (ADC). The idea of a microchip-empowered sensor without a unified analogue-to-digital converter (ADC). observation of a dimensional criterion depends on the to the or conduction of a The observation of a dimensional criterion depends onchanges the changes topermittivity the permittivity or conduction susceptible material. These variations result in the changes of the RCS of the RFID tag with respect of a susceptible material. These variations result in the changes of the RCS of the RFID tag with the magnitude magnitude shifts of some peaks and the resonant frequency can to the frequency. Consequently, the be sensed in the working range of the RFID RFID tag. tag. The electromagnetic device provides two different types of feedbacks on an equilateral basis. The electromagnetic (EM) results at one polarization are to be utilized for extracting the detected data, whereas the results at another polarization are to be utilized as a reference point for the identification of codes and calibration parameters. A similar idea presentedinin[56] [56] smoke detection. Figure 5a,b shows currents both scatterers when was presented forfor smoke detection. Figure 5a,b shows currents in bothin scatterers when exposed exposed to a vertically and horizontally polarizedplane incident plane wave, respectively. From this to a vertically and horizontally polarized incident wave, respectively. From this illustration, illustration, it can bethat observed thatscattering only one element scatteringiselement at a particular polarization. it can be observed only one agitatedisatagitated a particular polarization. Further, Further, the EM response is isolated each scattering Thus, variations in the degree the EM response is isolated betweenbetween each scattering element.element. Thus, variations in the degree of the of the detecting scatterer do not influence the degree of the corresponding referenced scatterer. detecting scatterer do not influence the degree of the corresponding referenced scatterer. The physical sizes of the scattering elements have been adjusted to operate in the 2.4–2.5 GHz band. The SRRs are square-shaped with a lateral length of approximately 18 mm. The two arms of the SRR are are 66 mm mmapart; apart;the thevalue valueofof66mm mmisisselected selectedasasa compromise a compromise considering bandwidth considering thethe bandwidth of of resonance, size, and highest magnitude e-field current distribution. The gap SRR resonance, size, and thethe highest magnitude of of thethe e-field current distribution. The gap of of thethe SRR is is maintained 2 mm, allowing a low resistance, which is favorable formagnitude the magnitude the maintained at 2atmm, allowing a low stripstrip resistance, which is favorable for the of theofRCS. RCS. Undeniably, a slimmer strip dimension might in a degradation of the performance in Undeniably, a slimmer strip dimension might result in aresult degradation of the performance in terms of the terms of the conductivity of a strip lithographed using silver nanoparticle ink, comparatively smaller conductivity of a strip lithographed using silver nanoparticle ink, comparatively smaller conductivity conductivity than the printed-circuit-board etching of copper. Figure 2a,b demonstrates a distance of than the printed-circuit-board etching of copper. Figure 2a,b demonstrates a distance of 9 mm between 9 mm between the two scattering elements, allowing sufficient decoupling of the EM response the two scattering elements, allowing sufficient decoupling of the EM response (−15 dB isolation (−15 dB isolation between polarizations). A minor estrangement space might have bandwidth demanded higher between polarizations). A minor estrangement space might have demanded higher for the bandwidth for the resonating dips in one and the other polarizations, also an abridged isolation resonating dips in one and the other polarizations, also an abridged isolation of cross-polarization. of cross-polarization.

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(a)

(b) Figure 5. Electric-field distributions at 2.4 GHz for the dual-polarized SRR-based sensor: (a) for

Figure 5. Electric-field distributions at 2.4 GHz for the dual-polarized SRR-based sensor: (a) for vertically polarized excitation; and (b) for horizontally polarized excitation [51]. vertically polarized excitation; and (b) for horizontally polarized excitation [51].

CNT Loaded Scattering Element for Sensing

CNT Loaded Scattering Element for Sensing

A scattering element, which is denoted by “H” in Figure 4, will be used to detect information.

A element, which iscomposite denoted by “H” in Figure 4, will be used to detect information. Anscattering engraved patterning, which is SWCNT/poly(3,4-ethylenedioxythiophene) polystyrene sulfonatepatterning, (PEDOT:PSS) ink-based [57,58], is interleaved between the space/gap of an SRR, to sensitize An engraved which is composite SWCNT/poly(3,4-ethylenedioxythiophene) polystyrene it to changes in various electrical parameters [59]. According to earlier characterizing it is sulfonate (PEDOT:PSS) ink-based [57,58], is interleaved between the space/gap of anprocesses, SRR, to sensitize known that the conductivity and resolution of the deposit are the most sensitive parameters with it to changes in various electrical parameters [59]. According to earlier characterizing processes, regard to a temperature or gas variation. Consequently, a variable resistor can model the deposit. The it is known that the conductivity and resolution of the deposit are the most sensitive parameters impedance is at a maximum in the gap. Consequently, a strong deviation in the sensing response with regard to a temperature or gas variation. Consequently, a variable resistor can model the may be observed because in the case of CNT ink, the bridging resistance of the deposit has a high deposit. TheToimpedance is at a maximum the gap. Consequently, strong deviation value. increase the sensitivity of theinsensing device, the size ofa the sensitive area in hasthe to sensing be response may be observed because in the case of CNT ink, the bridging resistance of the deposit maximized. Conversely, to avoid canceling the dominant resonant frequency mode of the scattering has aelement, high value. To increase sensitivity of the sensing size theasensitive the authors could notthe deposit a large resisting pattern indevice, the SRRthe space. Forofsuch structure, area theberesisting lining can be modeledtoasavoid a resistor in parallel a circuitresonant of resonance. Thus, ifmode the of has to maximized. Conversely, canceling thewith dominant frequency bridge resistance is minimized, the quality (QF) of the resonance dip is reduced. Consequently, the scattering element, the authors could factor not deposit a large resisting pattern in the SRR space. a resonant peak cannot be detectedlining at lowcan bridging resistances. sensitive inserted into of For such a structure, the resisting be modeled as The a resistor instripline parallelis with a circuit the SRR gap, as illustrated in Figure 4, to prevent covering a majority of space. The aim was resonance. Thus, if the bridge resistance is minimized, the quality factor (QF) of the resonancetodip is determine the longest path covering the large area inside the space of an SRR. Further, the delicate reduced. Consequently, a resonant peak cannot be detected at low bridging resistances. The sensitive stripline is in the shape of a meander line. The ratio of the length and width of the route is selected in stripline is inserted into the SRR gap, as illustrated in Figure 4, to prevent covering a majority of such a way that minimal bridge resistance is accomplished with respect to the following research of space.sensitivity. The aim was to determine longest path covering large areaofinside theisspace of an SRR. Hence, a stripline the width of 0.75 mm with a the route length 54 mm employed. Further, the delicate stripline is in the shapeconnection of a meander line. of the length andthe width Furthermore, to ensure a healthy electrical between theThe SRRratio and sensitive stripline, of theSWCNT/PEDOT:PSS-based route is selected in suchstripline a way that minimal bridge resistance is accomplished with respect overlays with the silver stripline toward and adjacent to the a surface of 4.5 × 2Hence, mm2 (Figure 4). to thegap-space, followingwith research of area sensitivity. a stripline width of 0.75 mm with a route length of In order to determine the minimum bridge resistance required maximize the logarithmic and and 54 mm is employed. Furthermore, to ensure a healthy electrical to connection between the SRR linear RCS changes, the authors performed a parametric simulation using CST Microwave Studio sensitive stripline, the SWCNT/PEDOT:PSS-based stripline overlays with the silver stripline toward (CST MWS) by gap-space, controlling with the plate resistance susceptible stripline and adjacent to the a surface area of of the 4.5 × 2 mm2 (Figure 4).between 10 Ω/sq and 100,000 Ω/sq. They demonstrated a susceptible stripline area with zero thickness, which can be In order to determine the minimum bridge resistance required to maximize the logarithmic and termed as an ohmic area in CST software. linear RCS changes, the authors performed a parametric simulation using CST Microwave Studio The simulation results for the RCS responses are illustrated in Figure 6a,b for several plate (CST resistances, MWS) by controlling the plate resistance of the susceptible stripline between 10 Ω/sq for both horizontal and vertical polarizations, respectively. Figure 5a shows the and 100,000 Ω/sq. They demonstrated a susceptible stripline area with zero thickness, which can be termed decoupling among both the polarizations, since by modifying the plate resistance of the susceptible as andeposit, ohmic area in CST software. no response was observed in the vertical polarization result. Instead, a noteworthy deviation was observed in the magnitude of the horizontal polarization result. in Figure 6a,b for several plate The simulation results for the RCS responses are illustrated resistances, for both horizontal and vertical polarizations, respectively. Figure 5a shows the decoupling among both the polarizations, since by modifying the plate resistance of the susceptible deposit, no response was observed in the vertical polarization result. Instead, a noteworthy deviation was observed in the magnitude of the horizontal polarization result.

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(a) (a)

(b) (b)

Figure SimulatedRCS RCS responses responses as the plate resistance of the deposit for: (a)for: Figure 6. 6.Simulated asaafunction functionofof the plate resistance of SWCNT the SWCNT deposit the vertical polarization; and (b) the horizontal polarization [51]. Figure 6. Simulated RCS responses as a function of the plate resistance of the SWCNT deposit for: (a) (a) the vertical polarization; and (b) the horizontal polarization [51]. the vertical polarization; and (b) the horizontal polarization [51].

2.3.2. Description of the Measurement Setup 2.3.2. Description of the Measurement Setup 2.3.2. Description of the Measurement Setup The sensors shown in Figure 7 are inkjet-printed over a stretchable 50-μm-thick polyimide The sensors in Figure 7 are7 inkjet-printed over over aisstretchable 50-µm-thick coating The sensors shown in The Figure are inkjet-printed stretchable 50-μm-thick coating used asshown a specimen. permittivity of polyimide 3.5awith a tangential losspolyimide valuepolyimide of 0.0027. used as a specimen. The permittivity of polyimide is 3.5 with a tangential loss value of 0.0027. coating used utilized as a specimen. The permittivity polyimide 3.5 with a tangential of 0.0027. The authors the Dimatix DMP-2831ofinkjet printeris for the deposition ofloss the value material. They The authors utilized the DMP-2831 inkjet for the the deposition the material. They The authors utilized theDimatix Dimatix DMP-2831 inkjet printer for deposition ofofthe They used a silver ink known as Harima Nanopaste for printer generating higher conductivity inmaterial. the stripline. used a silver ink known as Harima Nanopaste for generating higher conductivity in the stripline. used a silver inka known as Harima Nanopaste fordpi generating higher conductivity in the stripline. Two layers with resolution of approximately 635 (dots per inch) were printed initially, followed Two with aprocess resolution ofofmin approximately dpi (dots (dots perinch) inch)were were printed initially, followed Two with a resolution approximately 635 initially, followed bylayers a layers sintering for 70 at 130 °C to635 achieve the per thickness of 2 printed μm. The plate resistance byby a sintering atat130 C to thicknessof of22μm. µm.The Theplate plateresistance resistance a sintering processfor for7070min min 130◦°C to achieve achieve the thickness achieved wasprocess approximately 0.5 Ω/sq. achieved was approximately achieved was approximately0.5 0.5Ω/sq. Ω/sq.

Figure 7. View of the fully inkjet-printed sensor on a polyimide material. The bottom figure contains an added see-through covering film on top of the deposit [51].The Figure View thefully fully inkjet-printed sensor onsusceptible polyimide material. figure contains Figure 7. 7. View ofof the inkjet-printed sensor on a polyimide material. Thebottom bottom figure contains added see-throughcovering coveringfilm filmon ontop topof ofthe the susceptible susceptible deposit an an added see-through deposit[51]. [51].

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Further, the authors used composite SWCNT/PEDOT:PSS conducting ink for the susceptible Sensors 2017, 17, 2068 [57,58]. The delicate material is inkjet-printed at a resolution of 169410dpi of 30after conductive stripline ◦ being cured at 30 C for 30 min. The process of sintering is not compulsory, and the ink dries rapidly Further, the authors used composite SWCNT/PEDOT:PSS conducting ink for the susceptible in the ambient atmosphere. conductive stripline [57,58]. The delicate material is inkjet-printed at a resolution of 1694 dpi after The authors of [51] produced nine models of the prototype illustrated in Figures 4 and 7. being cured at 30 °C for 30 min. The process of sintering is not compulsory, and the ink dries rapidly The in dimensions the susceptible strip-lines and the SRR were maintained exactly equal. However, the ambientofatmosphere. the number of layers the produced CNT-based stripline varied between two and four. The measurement The authors ofof[51] nine modelswas of the prototype illustrated in Figures 4 and 7. The process in Figure was utilized strip-lines for carrying CO airtight plastic rectangular dimensions of 8 the susceptible andout thethe SRR were maintained An exactly equal. However, the 2 experiment. box number sufficiently large the stripline sensing was device, as between shown two in Figure 8b, The wasmeasurement utilized as the of layers of to theenclose CNT-based varied and four. process in Figure 8 was utilized for carrying out the CO experiment. An airtight rectangular device-under-test chamber. The testing box contained an 2inlet and an outlet with plastic checked regulators to box sufficiently large to enclose the sensing device, as shown in Figure 8b, was utilized as the deviceavoid any additional composite vapor travelling backwards. Dry air (10% relative humidity) or any under-test chamber. The testing box contained an inlet and an outlet with checked regulators to avoid specific gas were injected into the testing box. According to a sensitivity research of CNTs implemented anynumerous additional composite vapor backwards. Dry air (10% relative humidity) or any specific in [52], gases—e.g., NOtravelling 2 , NH3 , and CO2 —can be detected. In this subsection, we focus on gas were injected into the testing box. According to a sensitivity research of CNTs implemented in the sensitivity of the SWCNT deposit for CO2 only. CO2 gas was injected using a quick hand-operated [52], numerous gases—e.g., NO2, NH3, and CO2—can be detected. In this subsection, we focus on the pump. Each injected amount soaks the purity of the gas inside the box at a degree of approximately sensitivity of the SWCNT deposit for CO2 only. CO2 gas was injected using a quick hand-operated 20,000 ppm. A HD37AB17D Delta Ohm probe recorded the purity of CO2 , and simultaneously pump. Each injected amount soaks the purity of the gas inside the box at a degree of approximately measured humidity and temperature. ETS-Lindgren 3164-04 dual-polarized ridged 20,000 the ppm. A HD37AB17D Delta OhmAn probe recorded the puritywideband of CO2, and simultaneously hornmeasured antenna,the with a gain between 9 dBi and 12 dBi and frequency in the range of 3 GHz to 6 GHz, humidity and temperature. An ETS-Lindgren 3164-04 wideband dual-polarized ridged washorn positioned cm afrom testing box. The of this antenna were Agilent antenna,20with gainthe between 9 dBi and 12 terminals dBi and frequency in the range ofjoined 3 GHz to to an 6 GHz, PNAwas E8358A vector positioned 20network cm from analyzer. the testing box. The terminals of this antenna were joined to an Agilent PNA E8358A vector network analyzer.

(b)

(a)

(c) Figure 8. Experimental process: (a) view of the antenna in front of the test chamber; (b) view of the Figure 8. Experimental process: (a) view of the antenna in front of the test chamber; (b) view of the test test chamber; and (c) description of the set-up for gas measurement [51]. chamber; and (c) description of the set-up for gas measurement [51].

This subsection describes a stretchable inkjet-printed batteryless sensing device, and evaluates This subsection describes a temperature stretchable inkjet-printed batteryless sensing device, and evaluates its sensitivity to CO 2 gas and [51]. An analysis to make this device reactive only for temperature alsotemperature presented. Wireless measurement the device, imperiled to a only CO2 for its sensitivity tovariation CO2 gasisand [51]. An analysis toofmake this device reactive purity of variation approximately ppm, displayed changes of 0.23ofdB 0.51 dB with and a temperature is also20,000 presented. Wireless measurement theand device, imperiled towithout a CO2 purity dielectric covering, respectively. The authors also observed variations of the magnitude of of approximately 20,000 ppm, displayed changes of 0.23 dB and 0.51 dB with and without a dielectric approximately 2 dB The in the changed prototypes over a temperature of 35 °C 65 °C. The top2 dB covering, respectively. authors also observed variations of the range magnitude of to approximately covering film in this research did not interfere with the temperature recordings of the sensing device. in the changed prototypes over a temperature range of 35 ◦ C to 65 ◦ C. The top covering film in this

research did not interfere with the temperature recordings of the sensing device.

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3. 3D Printed Sensors 3. 3D Printed Sensors 3D printers have been in use for approximately 35 years. With 3D printing, objects are 3D printers have been in usecomprehensive for approximately 35 years. With 3DMoreover, printing, objects are constructed constructed brick-by-brick with digital outlines [24]. 3D printing has been brick-by-brick with comprehensive digital outlines [24]. Moreover, 3D printing has been in utilized in fabrication. Free software available online and low-cost 3D printers and 3Dutilized printing fabrication. Free software available online and low-cost 3D printers and 3D printing supplies have supplies have resulted in the immense popularity of this trend. The benefits of 3D printing machinery resulted in the immense popularity of this trend. The benefits of 3D printing machinery include quick include quick fabrication and the ability to construct difficult structures from more than one material, fabrication the ability construct difficult structuresItfrom more than one material, which is a and limitation of thetoclassic fabrication techniques. has become evident that werewhich will beisaa limitation of the fabrication techniques. It has become evident that[60] wereSystem will be30 a proliferation proliferation ofclassic 3D-printed applications in the near future. A Hyrel 3D printer isof 3D-printed applications in the near future. A Hyrel [60] System 30 3D printer is illustrated Figure 9. illustrated in Figure 9. The printer utilizes Repetrel, a revised variety of the Repetier in controller The printer utilizes Repetrel, a revised variety of the Repetier controller software, which employed software, which employed the mutual slicing computer-aided drawing software, known the as mutual slicing computer-aided drawing software, known as Slic3r [61,62]. Slic3r [61,62].

Figure 9. Hyrel System 30 3D printer [60]. Figure 9. Hyrel System 30 3D printer [60].

3.1. Novel Strain Sensor Based on 3D Printing Technology and 3D Antenna Design 3.1. Novel Strain Sensor Based on 3D Printing Technology and 3D Antenna Design The foremost 3D printed stretchable RF strain sensing device is discussed here [18]. The RF The foremost 3D printed stretchable RF strain sensing device is discussed here [18]. The RF response of NinjaFlex, the famous 3D printer material, was characterized. A 3D antenna was response of NinjaFlex, the famous 3D printer material, was characterized. A 3D antenna was analyzed analyzed and constructed using these materials and stretchable electrically conductive adhesives and constructed using these materials and stretchable electrically conductive adhesives (ECAs). These (ECAs). These materials hold immense promise for the upcoming 3D-printed RF solicitations, e.g., materials hold immense promise for the upcoming 3D-printed RF solicitations, e.g., wearable RF wearable RF components and flexible 3D sensing devices. The NinjaFlex filament was presented by components and flexible 3D sensing devices. The NinjaFlex filament was presented by Fenner Drives, Fenner Drives, Inc. in 2014 as one of the latest commercial 3D printing provisions [63]. NinjaFlex is a Inc. in 2014 as one of the latest commercial 3D printing provisions [63]. NinjaFlex is a kind of kind of thermoplastic elastomer (TPE) composed of thermoplastic and rubber [64]. The properties of thermoplastic elastomer (TPE) composed of thermoplastic and rubber [64]. The properties of TPEs TPEs hypothetically allow 3D printing to spread to various new domains, such as wearable antennas hypothetically allow 3D printing to spread to various new domains, such as wearable and RF and RF electronics, owing to their elasticity and higher flexibility. Following its antennas announcement, electronics, owing to their elasticity and higher flexibility. Following its announcement, NinjaFlex was NinjaFlex was used in a variety of assignments [65,66]. This part of the review discusses a batteryless used insensor a variety of assignments [65,66]. This part of and the review discusses a batteryless strain sensor strain based on 3D printing stretchable ECAs NinjaFlex. based on 3D printing stretchable ECAs and NinjaFlex. 3.1.1. 3D-Printed Strain Sensor Prototype 3.1.1. 3D-Printed Strain Sensor Prototype 3D Antenna Design 3D Antenna Design The dimensions of the dipole structure are shown in Figure 10. The dielectric material is a The dimensions of the dipole structure are shown in Figure 10. The dielectric material is a 30 mm × 30 mm × 30 mm hollow cube made of NinjaFlex (dielectric constant of 2.98, and tangential 30 mm × 30 mm × 30 mm hollow cube made of NinjaFlex (dielectric constant of 2.98, and tangential loss of 0.06 at 2.4 GHz). With two perpendicular arms, the dipole antenna is constructed using ECAs. loss of 0.06 at 2.4 GHz). With two perpendicular arms, the dipole antenna is constructed using ECAs. We can observe in Figure 10a that the dipole is positioned on the top exterior of the hollow cube and We can observe in Figure 10a that the dipole is positioned on the top exterior of the hollow cube and is is bent toward two additional planes on the sides. The feeding point of the dipole is at the midpoint bent toward two additional planes on the sides. The feeding point of the dipole is at the midpoint of the of the top exterior. The two arms extend to the edge of the top surface and bend along the other two top exterior. The two arms extend to the edge of the top surface and bend along the other two vertical vertical surfaces. A similar dipole array geometry was also presented recently [67]. This 3D structure surfaces. A similar dipole array geometry was also presented recently [67]. This 3D structure facilitates facilitates simple quantitatively analysis of the changes to antenna topology caused by strain. The

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simple quantitatively analysis of the changes to antenna topology caused by strain. The NinjaFlex NinjaFlex a hollow at its center. This structured design improves structure structure contains acontains hollow cube at itscube center. This structured design improves the qualitythe of quality printed NinjaFlex structure contains a hollow cube at its center. This structured design improves the quality of printed NinjaFlex, andeasy enables easy stretching of the dipoleon antenna on the frontas surface, NinjaFlex, and enables stretching of the partofofthe thepart dipole antenna the front surface, shown of printed NinjaFlex, and enables easy stretching of the part of the dipole antenna on the front surface, as 10. The directions which strain iscan applied can be observed Figure 10b. inshown Figure in 10.Figure The directions in which in strain is applied be observed in Figure in 10b. as shown in Figure 10. The directions in which strain is applied can be observed in Figure 10b.

(a) (a)

(b) (b)

Figure 10. (a) 3D antenna on aahollow cube; and (b) directions ininwhich strain isisapplied on the front Figure antenna hollow cube; and directions which strain applied front Figure 10.10. (a)(a) 3D3D antenna on on a hollow cube; and (b)(b) directions in which strain is applied on on thethe front and the back surfaces of the cube [18]. and the back surfaces of the cube [18]. and the back surfaces of the cube [18].

3D-Printed Strain Sensor Prototype 3D-Printed Strain Sensor Prototype 3D-Printed Strain Sensor Prototype First, the cube and antenna traces were 3D-printed using aaHyrel System 30 3D-printer and First, cube and antenna traces were 3D-printed using Hyrel System 3D-printer and First, thethe cube and antenna traces were 3D-printed using a Hyrel System 3030 3D-printer and NinjaFlex filament. Subsequently, the antenna traces were filled with the ECAs, based on a design in NinjaFlex filament. Subsequently, the antenna traces were filled with the ECAs, based on a design NinjaFlex filament. Subsequently, the antenna traces were filled with the ECAs, based on a design in ANSYS High Frequency Structure Simulator (HFSS). An impedance-matched balun was added in ANSYS Frequency Structure Simulator (HFSS). impedance-matched balun was added ANSYS HighHigh Frequency Structure Simulator (HFSS). An An impedance-matched balun was added between the two antenna traces to connect to aasub-miniature version AA(SMA) connector for insertion between two antenna traces connect sub-miniature version (SMA) connector insertion between thethe two antenna traces to to connect to atosub-miniature version A (SMA) connector forfor insertion loss tests, as shown in Figure 11. loss tests, shown Figure loss tests, as as shown in in Figure 11.11.

Figure 11. Ready-to-test 3D-printed strain sensor [18]. Figure Ready-to-test 3D-printed strain sensor [18]. Figure 11.11. Ready-to-test 3D-printed strain sensor [18].

3.1.2. Strain Experiment and Results 3.1.2. Strain Experiment and Results 3.1.2. Strain Experiment and Results Strain was applied to the front and rear faces ofofthe cube-shaped box (Figure 10b), and the change Strain was applied front and rear faces the cube-shaped box (Figure 10b), and change Strain was applied to to thethe front and rear faces of the cube-shaped box (Figure 10b), and thethe change in the resonant frequency of an antenna was observed. Two dissimilar intensities of strain were in the resonant frequency of an antenna was observed. Two dissimilar intensities of strain were applied in the resonant frequency an antenna was observed. Two dissimilar intensities of strain were applied to the box during experimentation. The measured results are illustrated by the solid lines in to the to box during experimentation. The measured results results are illustrated by the solid lines in lines Figure applied the box during experimentation. The measured are illustrated by the solid in12. Figure 12. We observed that the center frequencies shifted byand 30 MHz and 50 MHz, respectively, after We observed that the that center shifted by 30 MHz 50 MHz, after applying Figure 12. We observed thefrequencies center frequencies shifted by 30 MHz and 50respectively, MHz, respectively, after applying strain-1 and strain-2. Owing to the 3D structure of the box, the most noticeable alteration in applying strain-1 and strain-2. Owing to the 3D structure of the box, the most noticeable alteration in the length of the antenna is at the front. After applying a strain, this location of the structure stretches the length of the antenna is at the front. After applying a strain, this location of the structure stretches owing to the NinjaFlex. Consequently, the resonant frequency of the antenna is decreased as owing to the NinjaFlex. Consequently, the resonant frequency of the antenna is decreased as

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strain-1 and strain-2. Owing to the 3D structure of the box, the most noticeable alteration in the length Sensors 17, 2068 13 of 30 of the 2017, antenna is at the front. After applying a strain, this location of the structure stretches owing to the NinjaFlex. Consequently, the resonant frequency of the antenna is decreased as anticipated. anticipated. In [18], afor 3Duse antenna for sensor use as was a strain sensor planned using and constructed using In [18], a 3D antenna as a strain planned andwas constructed stretchable ECAs stretchable ECAs and NinjaFlex.offers This enormous technologyprospects offers enormous for RF future 3D printing and NinjaFlex. This technology for futureprospects 3D printing equipment, e.g., RF equipment, e.g., wearable RF devices and 3D stretchable sensor modules. wearable RF devices and 3D stretchable sensor modules.

the 3D-printed 3D-printed strain strain Figure 12. Results Results of of measurements measurements and and simulations simulations of of the the strain strain test test of the sensor [18]. [18].

3.2. Microfluidic Sensor Constructed Constructed on on aa Flexible Flexible Material Material of of Kapton Kapton for for Measurement Measurement of of Complex Complex 3.2. Microfluidic Sensor Liquid Materials Materials Permittivity of Different Liquid operating in the frequency range of 10–12 GHzGHz was A sensitive sensitive and andlow-cost low-costmicrofluidic microfluidicsensor sensor operating in the frequency range of 10–12 validated and proposed in a in previous study [68].[68]. This This sensor contains various dabsdabs coupled to a was validated and proposed a previous study sensor contains various coupled micro-strip (MS) line. The sensor is used for measuring and characterizing liquid chemicals, with to a micro-strip (MS) line. The sensor is used for measuring and characterizing liquid chemicals, applications in chemical laboratories and biological fields. The deviceThe is constructed on a stretching with applications in chemical laboratories and biological fields. device is constructed on Kapton material by utilizing printing technologies. With the helpWith of mathematical a stretching Kapton material electronic by utilizing electronic printing technologies. the help of calculations that describe that the describe characteristics of resonance, the variance complex mathematical calculations the characteristics of resonance, thebetween variance the between the permittivity of a testof and reference sample predicts thethe complex complex permittivity a testaand a reference sample predicts complexpermittivity permittivityof of different concentrations of sodium chloride (NaCl) water solutions. The estimated numbers for the imaginary and real portions of the complex permittivity present a continuous deviation with the purity of the NaCl-water solution. solution.Two Two of linear the linear zones correspond the real for part—one for minor NaCl-water of the zones correspond to the realtopart—one minor concentration concentration levels (0.5 M). Only one linear region levels (0.5 M). Only one linear region was achieved wasthe achieved for the imaginary part for the several concentration levelsAexamined. A close for imaginary part for the several concentration levels examined. close similarity is similarity observed is observed among theobtained outcomesfrom obtained from the model Cole–Cole and the experimental results among the outcomes the Cole-Cole andmodel the experimental results recorded. recorded. The recorded experimental results determine the sensitivity and practicality of the The recorded experimental results determine the sensitivity and practicality of the addressed device addressed devicemicro-quantity for characterizing liquid frequencies. chemical at Although microwave for characterizing liquid micro-quantity chemical at microwave the frequencies. investigated Although the investigated frequencies were approximately the same can be frequencies were approximately 10 GHz, the same approach can10 beGHz, implemented forapproach any frequency in implemented for any frequency in the microwave range. Furthermore, the proposed method can be the microwave range. Furthermore, the proposed method can be utilized to detect several new liquid utilized tothat detect several new liquid chemicals that possess a complex permittivity at the chemicals possess a complex permittivity at the same specifications of frequency using the same specifications of frequency using the same calibration. Although the proposed sensor is not common, calibration. Although the proposed sensor is not common, it can be used favorably in a variety of it can be used favorably in a variety of practical where the material under observation is practical experiments where the material under experiments observation is a mixture or a liquid chemical. a mixture or a liquid chemical. 3.2.1. Design and Fabrication of the Liquid Chemical Sensing Device Sensor Design Figure 13 shows the design of the proposed RF chemical sensing device, constructed on 0.13-mm-broad Kapton material in HFSS. As evident from the illustration, this batteryless sensing device works on various dabs coupled to an MS line. The 25-mm-long MS has a width of 0.3 mm, as

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3.2.1. Design and Fabrication of the Liquid Chemical Sensing Device Sensor Design Figure 13 shows the design of the proposed RF chemical sensing device, constructed on 0.13-mm-broad Kapton material in HFSS. As evident from the illustration, this batteryless sensing Sensors 2017, 17, 2068 of 30 device works on various dabs coupled to an MS line. The 25-mm-long MS has a width of 0.314mm, as shown in Figure 13c, matching a characteristic impedance of 50 Ω. The size or length of every dab is in Figure for 13c,amatching a characteristic impedance ofdabs 50 Ω.are The size orsymmetrically length of every dab is 4shown mm, improved 50-Ω impedance matching. Two of the located opposite 4 mm, a 50-Ω impedance the dabs areMS located opposite on eachimproved side of a for central dab, which is matching. situated atTwo the of center of the line. symmetrically The space between the on each side of a central dab, which is situated at the center of the MS line. The space between the dabs is fixed at 1.5 mm. This sensing scenario is designed to have a larger detection space, where the dabs is fixed at 1.5 mm. This sensing scenario is designed to have a larger detection space, where the E-field at resonance is also powerfully concentrated. A system is constructed for obtaining an accurate E-field at resonance is also A system is constructed obtaining accurate response in the X-band (8 powerfully to 12 GHz).concentrated. The measurement parameters of thefor bent area areanshown in response in the X-band (8 to 12 GHz). The measurement parameters of the bent area are shown in Figure 13d. The sensor contains two orthodox segments (5 mm each) and three curves. With regard to Figure 13d. The sensor contains two orthodox segments (5 mm each) and three curves. With regard the stab in the center, 9 mm of the substrate is bent at an angle of 180◦ , whereas the other two curves to the stab the in the 9 mm of the are substrate bentlengths at an angle of 180°, the ◦ . other two that match twocenter, orthodox segments curvedisfrom of 3 mm at an whereas angle of 90 curves that match the two orthodox segments are curved from lengths of 3 mm at an angle of 90°.

13.(a) (a)Design Design and dimensions of proposed the proposed filledaqueous with aqueous chemicals; (b) Figure 13. and dimensions of the sensorsensor filled with chemicals; (b) zoomed zoomed structure ofportion the bent portion highlighted in (a); of the structure the structure of the bent highlighted in (a); (c) top view(c)oftop the view structure showing theshowing dimensions; dimensions; and (d) measurements of 3D-printed Acrylonitrile Butadiene Styrene (ABS) and (d) measurements of 3D-printed Acrylonitrile Butadiene Styrene (ABS) plastic module [68].plastic module [68].

Sensor Fabrication Process Sensor Fabrication Process The device is constructed on 130-µm-thick Kapton material using inkjet technology. Kapton was Thefrom device is constructed onfor 130-μm-thick Kapton material inkjet technology. Kapton was bought Dupont Teijin films this research. The reason for using choosing Kapton material is its high bought from Dupont Teijin films for this research. The reason for choosing Kapton material is its high thermal stability that enables sintering over high temperatures. Before printing, the Kapton sheet thermal stability that enables sintering over temperatures. printing, the Kapton was was washed in acetone chemical, cleaned byhigh isopropyl alcohol, Before and completely dried usingsheet a flow of washed in acetone chemical, cleaned by isopropyl alcohol, and completely dried using a flow of nitrogen. An easily accessible nanoparticle ink from market, acquired from Sun Chemical (Suntronic nitrogen. An easily accessible nanoparticle ink from market, acquired from Sun Chemical (Suntronic EMD5714) company, was used as a conducting material. The ink material contains silver nanoparticles EMD5714) inside company, wasofused as glycerol, a conducting material. The ink concentration material contains silver distributed a blend ethanol, and ethanediol, at a 42% by weight. ® nanoparticles distributed inside a blend of ethanol, glycerol, and ethanediol, at a 42% concentration Dimatix printhead (Spectra SE-128AA) was used for the deposition fixed in a Ceraprinter X-Series ® by weight. Dimatix printheadsociety. (Spectra was used forby the fixed a Ceraprinter inkjet printer from Ceradrop TheSE-128AA) outlets were activated a deposition custom-made 57 in V vibration at a X-Series inkjet printer from Ceradrop society. The outlets were activated by a custom-made 57 V jet frequency speed of 2 kHz. The space between the outlets and printing material was fixed at 800 µm. ◦ vibration at a jet frequency speed of 2 kHz. The space between the outlets and printing material was The drop spacing was fixed to 38 µm. Sintering was performed for 45 min at 200 C for obtaining fixed at silver 800 μm. The drop spacing The was thickness fixed to 38 Sintering was performed for 45 min atas200 °C reliable tracker conductivity. ofμm. the final deposition material was as small 1 µm for obtaining reliable silver tracker conductivity. The thickness of the final deposition material was for the metal parts (ground and conductive line). These were the constraints chosen to obtain acceptable as small as 1 μm for the metal parts (ground and conductive line). These were the constraints chosen to obtain acceptable realization with respect to conductivity (δ ≈ 5 × 106 (S/m)) [69] while maintaining the physical and biochemical properties of the Kapton material unaffected. Figure 14a,b shows the pictures of the resonator prototype. Notably, the resolution of the conductive pattern formed by using inkjet-printing is acceptable and there were no wrinkles observed. The proposed

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realization with respect to conductivity (δ ≈ 5 × 106 (S/m)) [69] while maintaining the physical and biochemical properties of the Kapton material unaffected. Figure 14a,b shows the pictures of the resonator prototype. Notably, the resolution of the conductive pattern formed by using inkjet-printing is acceptable and there were no wrinkles observed. The proposed microwave/microfluidic device is attached to SMA connectors with an overall 50 Ω impedance matched at both the sides of the MS line as shown in Figure 14c. A 3D-printed mold was constructed using Acrylonitrile butadiene styrene Sensors 2017, 17, 2068 15 of 30 (ABS) material and hardened at an oven temperature of 125 ◦ C with measurements indistinguishable sides of the bent device avoiding seepage of the to tested as bent shown in Figure 14d. fromboth those obtained in Figure 13.forSilicone glue is attached bothchemicals, sides of the device for avoiding The Kapton material utilized here is easily on the 3D-printed mold andutilized conforms to firm seepage of the tested chemicals, as shown in curved Figure 14d. The Kapton material here is easily curves without the helpmold of mechanical equipment. curved on the 3D-printed and conforms to firm curves without the help of mechanical equipment.

Figure Photograph setofofresonators resonators on on Kapton micrograph of aofstub loaded Figure 14. 14. (a) (a) Photograph ofofa aset Kaptonsubstrate; substrate;(b) (b) micrograph a stub loaded along the MS line; (c) photograph of the sensor curved on a 3D-printed support; and (d) photograph along the MS line; (c) photograph of the sensor curved on a 3D-printed support; and (d) photograph of of the sensor under testing [68]. the sensor under testing [68].

3.2.2. Simulation and Experiment Validity

3.2.2. Simulation and Experiment Validity

As previously mentioned, a mixture of deionized (DI) water with various purity mixtures of As previously mentioned, a mixture of deionized water with various purityThe mixtures NaCl was examined to evaluate the sensitivity of the (DI) microwave/microfluidic device. S21 of NaCl was illustrated examinedintothis evaluate the measured sensitivityusing of the microwave/microfluidic device. The S21 spectrum paper was a vector network analyzer (PNA-X N5242A spectrum illustrated in this paper was measured using a vector network N5242A (10 MHz–26.5 GHz)). Figure 15a presents the measured spectrum of theanalyzer sensing (PNA-X device under (10 MHz–26.5 GHz)). Figure 15a presents the measured spectrum of the sensing device under experiment without and with the NaCl mixtures. In this study, the volume of the deposited solution was maintained (0.3 mL) for mixtures. all concentrations. Notably, recorded spectrum (S21) solution was experiment withoutconstant and with the NaCl In this study, thethe volume of the deposited repeatable than change in the point of the resonant dip and approximately was completely maintained constantwith (0.3less mL) for0.29% all concentrations. Notably, the recorded spectrum (S21 ) was 1.9% variation in accordance with 0.29% the magnitude thepoint attenuation dip. By considering the error completely repeatable with less than change inofthe of the resonant dip and approximately generated by the depositor, the variations are also expected to escalate in a manner undefined yet.error 1.9% variation in accordance with the magnitude of the attenuation dip. By considering the When there is nothing to examine (i.e., air is present on the sensor), the sensor has an insertion generated by the depositor, the variations are also expected to escalate in a manner undefined yet. resonant dip of 76 dB at 10.61 GHz. When pure DI water (C0) is injected, it shifts the resonant dip to When there is nothing to examine (i.e., air is present on the sensor), the sensor has an insertion 10.32 GHz with an insertion loss of 70 dB. Notably, with the increase in the purity of NaCl in the resonant dip of 76 dB at 10.61 GHz. When pure DI water (C0 ) is injected, it shifts the resonant dip mixture, the resonant dip shifts toward lower frequency values, and the bandwidth of the dips to 10.32 GHz(Figure with an insertion loss of 70expected dB. Notably, inthe thepurity purity NaCl increases 15b). This outcome was owing with to thethe factincrease that, when of of NaCl is in the mixture, dip shifts toward lower frequencyisvalues, and bandwidth of the increased,the theresonant real component of the complex permittivity observed to the decrease, resulting in adips increases (Figure 15b). This outcome was expected owing to the fact that, when the purity of NaCl variation in the resonant dip. Furthermore, the decrease in the magnitude of the attenuation dip as is increased, of the complex observed to decrease, resulting the puritythe of real NaClcomponent in the mixture increases is permittivity clearly relatedisto the increased magnitude of the in a imaginary of the complex permittivity [70–72]. many purity levels variation in thecomponent resonant dip. Furthermore, the decrease in theSimulations magnitudefor of the attenuation dip of as the NaCl were investigated manually using HFSSrelated computer toolincreased for detecting the subtlest areaimaginary of the purity of NaCl in the mixture increases is the clearly to the magnitude of the device and for examining the magnitude of electric field distribution inside the bent region of thewere component of the complex permittivity [70–72]. Simulations for many purity levels of NaCl device during the experiment. investigated manually using the HFSS computer tool for detecting the subtlest area of the device and

for examining the magnitude of electric field distribution inside the bent region of the device during the experiment.

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Figure 15. (a) Experimental parameters of the sensor recorded in the X-band as function Figure15. 15.(a) (a)Experimental ExperimentalSS21 S2121parameters parametersof ofthe thesensor sensorrecorded recordedin inthe theX-band X-bandas asaaafunction functionofof of Figure different concentrations of NaCl; and (b) zoom of (a), with selected concentrations to highlight the different concentrations of NaCl; and (b) zoom of (a), with selected concentrations to highlight the different concentrations of NaCl; and (b) zoom of (a), with selected concentrations to highlight variation in the position magnitude of attenuation when the variation inin the position ofofthe the resonant frequency and the the magnitude magnitudeof ofattenuation attenuationwhen whenthe the the variation the positionof theresonant resonantfrequency frequency and and the concentration increased [68]. concentration increased [68]. concentration isisis increased [68].

Asshown showninin Figure Figure 16, 16, the simulated using symmetry (the As the half-structures half-structures ofofthe thesensor sensorareare simulated using symmetry symmetric plane is illustrated in in Figure 13). The electric field distribution is is evaluated (the symmetric plane is illustrated Figure 13). The electric field distribution evaluatedatat1010GHz. GHz.In the case of C 0, it is demonstrated that the electric field is intensely gathered at the center dab of the In the case of C0 , it is demonstrated that the electric field is intensely gathered at the center dab of structure. Nevertheless, when the the purity is increased, the electric field distribution is gathered toward the structure. Nevertheless, when purity is increased, the electric field distribution is gathered the board of the of dab, the center dab, dab, which is near the MS similar to Cto 0. This toward the board thebut dab,not butaround not around the center which is near the line, MS line, similar C0 . result indicates that the center location close to the MS line is the most subtle with respect to the This result indicates that the center location close to the MS line is the most subtle with respect to the increment in the purity of the solution under test [68]. increment in the purity of the solution under test [68].

Figure different NaCl-water concentrations Figure16.16.Electric Electricfield field(E-field) (E-field)distribution distributionasasa afunction functionofof different NaCl-water concentrations calculated inside a symmetric half-structure of the sensor [68]. calculated inside a symmetric half-structure of the sensor [68].

cost-efficientmicrowave/microfluidic microwave/microfluidicsensor sensorthat thatcharacterizes characterizesthe thecomplex complexpermittivity permittivityofof AAcost-efficient aqueous solutions in an efficient and exact manner is designed, fabricated, and validated in the Xaqueous solutions in an efficient and exact manner is designed, fabricated, and validated in the X-Band. Band. This sensor isusing realized using inkjet-technology andon3D printing on a Kapton substrate. This sensor is realized inkjet-technology and 3D printing a Kapton substrate. Simulation and Simulation and experimental investigations of the device presented, and is observed thatare the experimental investigations of the device are presented, and itare is observed that theitresults obtained results obtained are consistent with the Cole–Cole model [68]. consistent with the Cole-Cole model [68]. 1

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4. Screen Printing Screen printing is a proven manufacturing technology that enables high-volume production at low cost. Thus, the main advantage of screen-printed RF sensors is the potential for low-cost and high-volume manufacturing. Practical implementation of screen printing technology for the fabrication of antennas began in the 1970s, when low-loss dielectrics arrived in the market. Screen printing technology offers the possibility for cost-optimized inline reel-to-reel manufacturing. Consequently, RF components can be thinner, lighter, more flexible, and cheaper than when fabricated using a conventional manufacturing process [73]. Screen printing is appropriate for fabricating electronics owing to the ability to produce patterned, thick layers from paste-like materials. This technique can produce conducting lines using inorganic materials (e.g., for circuit boards and antennas) and passive insulating layers, where the thickness of the layer is more important than the resolution. The characteristic throughput (50 m2 /h) and resolution (100 µm) are similar to those observed with inkjet printing. This versatile and comparatively simple method is used primarily for the fabrication of conductive and dielectric layers [74,75]. However, organic semiconductors, e.g., organic photovoltaic cells [76] and complete organic field-effect transistors [77], can be printed. 4.1. Novel Strain Sensor Based on 3D Printing Technology and 3D Antenna Design The design of a stretchable RF strain sensor fabricated using screen printing technology is suggested in [78]. The proposed sensing device is fabricated using a patch of half of the wavelength, which has a resonant frequency of 3.7 GHz. Its resonant frequency is determined by varying the size of the patch. Therefore, whenever the structure is stretched, it has a different resonant frequency. Polydimethylsiloxane (PDMS) was utilized as a substrate, since it is a stretchable and screen-printable surface. Dupont PE872 silver conductive ink (Dupont, NC, America) was utilized to produce a conducting structure with elasticity. The sensing operation is determined using full-wave computer simulations and experiments to be conducted on the fabricated prototype. After stretching, the resonant frequency of the device decreases to 3.43 GHz from 3.7 GHz, increasing the horizontal size by 7.8% and demonstrating a sensitivity of 3.43 × 107 Hz/1%. When the device is stretched in the vertical direction, there is no change in the resonant frequency. 4.1.1. Design of a Strain Sensor The construction of the sensing device is espoused from a rectangle resonator patch as shown in Figure 17. These four-sided patches are extensively utilized in RF resonators or structures containing resonator-based modules owing to their uncomplicated construction and ease of fabrication. In this particular research, the conductive patterning was generated using screen printing technology. The resistance of the surface of these conducting patterns was determined using the width-to-length ratio. The value of resistance for the silver conducting ink with stretching ability and the rectangle-shaped conductive patch are 0.64 Ω and 14.2 Ω, respectively. The diminished resistance on the surface of the rectangle-shaped patch is owing to the smaller width-to-length ratio. Figure 7 shows the dimensions of the strain sensor device. A coaxial transmission line feeding is utilized as an alternative to the classical MS line feeding. This modification is performed to ensure that the feed structure remains unchanged when the overall structure is stretched. Furthermore, PDMS is used here as a dielectric substantial or a substrate for the top conducting patch. The permittivity (ε r ) and tangential loss of PDMS were characterized using T-resonator process [79,80]. The resonant frequency ( f 0 ) of the rectangle-shaped conducting patch can be computed as [81,82]: f0 =

√

2 εe f f

c "

( L p + 0.824Hs

W

 #)

(ε e f f +0.3) Hsp +0.264 W  (ε e f f −0.258) Hsp +0.8

(1)

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



 11 ε r 𝜀+ + 1 1 ε r𝜀−−1  1 , , r (2) ε e f𝜀f𝑒𝑓𝑓 = = 𝑟 ++ 𝑟  (2)     22 22 hs 1 + 12 ℎp𝑠 ) √1 + 12 (W 𝑊𝑝 [ ] where ε e f f is the effective permittivity emanating from the fringing fields, c is the speed of light in where 𝜀𝑒𝑓𝑓 emanating fringingoffields, 𝑐 is the speed of and lightHins vacuum, Wpisisthe theeffective width ofpermittivity the rectangular patch, from L p is the length the rectangular patch, vacuum, 𝑊𝑝 is the width of the rectangular patch, The 𝐿𝑝 ispermittivity the length ofand thetangential rectangular patch, and 𝐻𝑠 is is the thickness of the dielectric material (PDMS). loss of the PDMS material were determined to bematerial 3.01 and(PDMS). 0.025, respectively. the thickness of the dielectric The permittivity and tangential loss of the PDMS The were lengthdetermined and widthto ofbe the rectangle-shaped conductive patch were selected to be 17.05 mm material 3.01 and 0.025, respectively. and 23.1 mm, respectively, to enable the sensor to have a resonantpatch frequency 3.7 GHz. Figure The length and width of the rectangle-shaped conductive were of selected to be 17.0518a,b mm displays respective real and portions inputa impedance, with various values of and 23.1the mm, respectively, toimaginary enable the sensoroftothehave resonant frequency of 3.7 GHz. D—the lengthwise distance of the coaxial feedimaginary to the edge of the rectangular It can with be observed Figure 18a,b displays the respective real and portions of the input patch. impedance, various in Figure that the resonant frequency and decrease in D. Thus, to realize values of18 D—the lengthwise distance of theimpedance coaxial feed to thewith edgethe of increase the rectangular patch. It can matched impedance of 18 50 that Ω, Dthe is determined to be exactly mm. ANSYS computer simulator (HFSS) be observed in Figure resonant frequency and 6.5 impedance decrease with the increase in D. is utilized for thematched full-wave simulation SMA Version connector was ANSYS also incorporated Thus, to realize impedance ofanalysis. 50 Ω, D isThe determined to beAexactly 6.5 mm. computer simulator (HFSS)of is the utilized for the simulation analysis. The SMAfrequency Version Ahighly connector was in the simulation structure, asfull-wave evident from Figure 17. If the resonant depends also inofthe the structure, as that evident from Figure 17. If will the resonant on theincorporated varied lengths thesimulation conductingofpatch, it is expected the operating frequency decrease frequency highly isdepends on the varied lengths of the conducting it is expected the when the device to be stretched vertically. Figure 19a,b presents patch, the simulation resultsthat of the operatingcoefficient frequency for willdifferent decreasevalues when of theLdevice is pto be stretched Originally, vertically. Figure 19a,b presents reflection , respectively. the ideal sensor had p and W simulation resultsofof reflection coefficient for different values of 𝐿frequency respectively. athe resonant frequency 3.7the GHz with a reflection coefficient of −25 dB. The operation 𝑝 and 𝑊𝑝 , of does not vary [83], as shown in Figure it is demonstrated Originally, thewhen ideal W sensor had a resonant frequency of 3.7 19a. GHzHowever, with a reflection coefficient in of p is changed Figure that the resonant frequency is reduced from 3.7𝑊 GHz to 3.43 GHz after stretching the device −25 dB.19b The frequency of operation does not vary when is changed [83], as shown in Figure 19a. 𝑝 by 7.8% in it the direction. Consequently, device canisbe effectively asto a However, is vertical demonstrated in Figure 19b that the proposed resonant frequency reduced fromutilized 3.7 GHz strain-sensing by the identifying variations resonant frequency. Therefore, tothe compute the 3.43 GHz afterresonator stretching device by 7.8% in in thethe vertical direction. Consequently, proposed elastic the strain utilized can be defined as: devicecapability, can be effectively as a strain-sensing resonator by identifying variations in the resonant frequency. Therefore, to compute the elastic capability, the strain can be defined as: ∆L (3) Strain = ∆𝐿 × 100(%) 𝑆𝑡𝑟𝑎𝑖𝑛 = Lo × 100(%) (3) 𝐿𝑜

Figure Dimensions of of the theaddressed addressedRF RFstrain strainsensor: sensor:(a)(a)top top view; perspective view; Figure 17. 17. Dimensions view; (b)(b) perspective view; andand (c) (c) side view. W = 17.05 mm, L = 23.1 mm, W = 40 mm, L = 50.1 mm, D = 6.5 mm, and p p s s side view. Wp = 17.05 mm, Lp = 23.1 mm, Ws = 40 mm, Ls = 50.1 mm, D = 6.5 mm, and Hs = 1.01 mm [78]. Hs = 1.01 mm [78].

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Figure 18. Simulated input impedance with D at different locations: (a) real part; and (b) imaginary Figure 18. Simulated input impedance with D at different locations: (a) real part; and (b) imaginary Figure 18. Simulated input impedance with D at different locations: (a) real part; and (b) imaginary part of impedance [78]. part of impedance [78]. part of impedance [78].

Figure 19. Simulated reflection coefficients with different patch: (a) widths; and (b) lengths [78]. Figure 19. Simulated reflection coefficients with different patch: (a) widths; and (b) lengths [78]. Figure 19. Simulated reflection coefficients with different patch: (a) widths; and (b) lengths [78].

4.1.2. Fabrication of the Strain Sensor 4.1.2. Fabrication of the Strain Sensor 4.1.2. PDMSFabrication of the Strain Sensor PDMS Figure 20 presents the method to fabricate in-house PDMS material. In Figure 20a, the desired PDMS Figure presents the method to PDMS Figure 20a,+the PDMS mold20has been constructed onfabricate a plasticin-house sheet using 3D material. printing In (Ultimaker2 3D desired printer Figure 20 presents the method to fabricate in-house PDMS material. In Figure 20a, the desired PDMS moldB.V, hasGeldermalsen, been constructed on a plastic sheet using Since 3D printing (Ultimaker2 + 3D (Ultimaker The Netherlands)) procedure. 3D production is faster andprinter easier PDMS mold hasGeldermalsen, been constructed on a plastic sheet using Since 3D printing (Ultimaker2 + 3D printer (Ultimaker B.V, The Netherlands)) procedure. 3D production is faster and easier than classic fabrication methods [84], it is extensively used. The thickness (Hs ), length (Ls ), and width (Ultimaker B.V, Geldermalsen, The Netherlands)) procedure. Since 3D production is faster andwidth easier than methods [84], it is 1.01 extensively used. thickness (Hs), length (Lsconstructing ), and (Ws ),classic of the fabrication flexible PDMS material were mm, 50.1, andThe 40 mm, respectively. After than classic fabrication methods [84], it is extensively used. The thickness (H s ), length (L s ), and width (W of thea flexible PDMS material 1.01 mm, a50.1, andagent 40 mm, Afterwith constructing thes),mold, liquid conformation of a were fraternization, curing withrespectively. base was created a ratio of (Ws), of the flexibleconformation PDMS material were 1.01 mm, 50.1, andagent 40 mm, respectively. After constructing the mold, a liquid of a fraternization, a curing with base was created with during a ratio 10:1. Subsequently, a void vacuum machine was utilized for the removal of air bubbles created the10:1. mold, a liquid conformation of a fraternization, a utilized curing agent with base was created withcreated a ratio of Subsequently, a void vacuum machine was for the removal of air bubbles ◦ the fraternization process. A dense liquid conformation was cured at 30 C for approximately 50 h, of 10:1. Subsequently, a void vacuum machine was utilized for the removal of air bubbles created during the◦ Cfraternization process. A also dense liquid conformation curedfor at 35 30 °C or at 110 for 40 min. The device underwent heat curingwas process minfor atapproximately a temperature during the fraternization process. A dense liquid conformation was curing cured atprocess 30 °C for 50 h, or at 110 °C for 40 min. The device also underwent heat forapproximately 35 USA) min at a ◦ of 75 C using a hot plate. Subsequently, a PDC-32G plasma cleaner (Harrick Plasma, NY, was 50 h, or at 110 °C for 40 min. The device also underwent heat curing process for 35 min at a temperature of 75plasma °C using a hot plate. Subsequently, a PDC-32G plasma cleanerwas (Harrick Plasma, used to perform treatment on the PDMS material. The plasma treatment performed for temperature of 75 °C using a hot plate. Subsequently, a PDC-32G plasma cleaner (Harrick Plasma, NY, USA.) was 20 used plasma treatment on the PDMS material. The plasma treatment was approximately s atto19perform W. NY, USA.) for wasapproximately used to perform treatment on the PDMS material. The plasma treatment was performed 20 plasma s at 19 W. performed for approximately 20 s at 19 W.

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Figure 20. PDMS fabrication process: (a) 3D-printed mold for the PDMS substrate; (b) mixing the Figure 20. PDMS fabrication process: (a) 3D-printed mold for the PDMS substrate; (b) mixing the base Figure 20. PDMS fabrication process: (a)PDMS 3D-printed mold for the PDMS substrate; (b) plasma mixing the base base and agent; (c)pouring pouring liquid PDMS into fabricated outline; (d) treatment and curing curing agent; (c) liquid into thethe fabricated outline; andand (d) plasma treatment and curing agent; (c) pouring liquid PDMS into the fabricated outline; and (d) plasma treatment processing [78].[78]. processing processing [78].

Screen Printing Screen Printing Screen Printing The silver printing method is shown in Figure As shown in Figure 21a,b, the The silver screenscreen printing method is shown in Figure 21. As21. shown in Figure 21a,b, the conducting The silver screen is shown in Figure As of shown in Figure conducting pattern forprinting the uppermethod rectangle-shaped patch and the 21. ground the structure were21a,b, screen-the pattern for the upper rectangle-shaped patch and the ground of the structure were screen-printed on conducting pattern for thePDMS upper material rectangle-shaped the ground the structure screenprinted on the flexible using thepatch silverand conductive ink of (Dupont PE872,were Bucheon, the flexible PDMS material using the silver conductive ink (Dupont PE872, Bucheon, Korea), which Korea), exhibitsPDMS elasticity. Daeyoung (Bucheon,ink Korea) produced theBucheon, screen printed onwhich the flexible material usingTechnology the silver Co. conductive (Dupont PE872, exhibits elasticity. Technology Co.a (Bucheon, Korea) produced screen printer printer used exhibits in Daeyoung this project. TheDaeyoung device has printing speed in the range of the 45–595 mm/s, aused Korea), which elasticity. Technology Co. (Bucheon, Korea) produced theand screen in printer this project. The device has a printing speed in the range of 45–595 mm/s, and a squeegee angle squeegee between 60°The and device 90°. Thehas 400awire count speed mesh of steel a mesh tension usedangle in this project. printing instainless the range ofwith 45–595 mm/s, and a ◦ and 90◦ . The 400 wire count mesh of stainless steel with a mesh tension of approximately between 60 of approximately 150 N60° was utilized. A mask of the pattern was placed the PDMS squeegee angle between and 90°. The 400 wire count mesh of created stainlessand steel with on a mesh tension onto which silver conducting nanoparticle inkplaced was using aonsqueegee. 150ofNmaterial was utilized. A of the pattern was created and on the PDMS material onto which approximately 150mask Nthe was utilized. A mask of the pattern wasscreen-printed created and placed the PDMS Figure 21c displays a fully fabricated and functional sensor. The rectangle-shaped conductive patch thematerial silver conducting nanoparticle ink was screen-printed using a squeegee. Figure 21c displays a fully onto which the silver conducting nanoparticle ink was screen-printed using a squeegee. resides on the top, and the bottom is also screen-printed as the top patch. It is necessary to cure the fabricated and functional sensor. The rectangle-shaped conductive patch resides on the top, and Figure 21c displays a fully fabricated and functional sensor. The rectangle-shaped conductive patchthe prototype for improving the on the area. Therefore, heat for sintering was bottom is on alsothe screen-printed as conduction the top patch. It isscreen-printed necessary to top curepatch. the prototype improving resides top, and the bottom is also screen-printed as the It is necessary to cure thethe performed in a vacuumed oven (ON-22GW) [85,86] for approximately 35 min at 90 °C. A hovel was conduction screen-printed area. Therefore, heat sintering area. was performed a vacuumed oven prototypeon forthe improving the conduction on the screen-printed Therefore, in heat sintering was subsequently pierced on the patch through to the bottom and an SMA connector pin was implanted ◦ performed in a vacuumed oven (ON-22GW) [85,86] for approximately 35 min at 90 °C. A hovel was (ON-22GW) [85,86] for approximately 35 min at 90 C. A hovel was subsequently pierced on the patch using silver glue epoxy (the inner conductor pin of SMA attached to the top patch; the outer part of subsequently piercedand on the through to the and an SMA connector pin was implanted through the bottom an patch SMA connector pinbottom was implanted using silver glue epoxy (the inner SMAtoconnected to the ground in the same manner). using silver epoxy (the inner conductor pinthe of outer SMA attached to the top patch;tothe partinofthe conductor pin glue of SMA attached to the top patch; part of SMA connected theouter ground SMA connected to the ground in the same manner). same manner).

Figure 21. Silver screen printing process: (a) side view of screen printing process; (b) top view of screen printing process; and (c) picture of the fabricated prototype [78].

Figure Silver screen printing process: view of screen printing process; (b) view top view of Figure 21.21. Silver screen printing process: (a) (a) sideside view of screen printing process; (b) top of screen screenprocess; printing and process; and (c)of picture of the fabricated prototype printing (c) picture the fabricated prototype [78]. [78].

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4.1.3. Experimental Results 4.1.3. Experimental Results The specimen of the proposed RF sensor is shown in Figure 21c. Further, an Anritsu MS2038C vector network analyzer Japan) was for21c. the Further, measurement of theMS2038C reflection The specimen of the (Anritsu, proposedKanagawa, RF sensor is shown in used Figure an Anritsu coefficients of the RF strain sensor. The measurement results coefficients recorded vector network analyzer (Anritsu, Kanagawa, Japan) was used of forthe thereflection measurement of the reflection for the strain sensor were compared with the corresponding simulation results in Figure coefficients of the RF strain sensor. The measurement results of the reflection coefficients recorded22a. for Originally, the non-stretched device resonant frequency of 3.7 results GHz with reflection the strain sensor were compared withhas theacorresponding simulation in Figure 22a. coefficients Originally, of −27 dB. The recorded results and the computer-simulated results are consistent the non-stretched devicemeasurement has a resonant frequency of 3.7 GHz with reflection coefficients of −27with dB. eachrecorded other. The repeatability test isand carried out for ensuring theresults reliability of results and shown in The measurement results the computer-simulated are consistent withiseach other. Figure 22b. The graph recorded reflection coefficientofafter every 10, 15, in and 20 cycles. The repeatability test isshows carriedthe out for ensuring the reliability results and1,is5,shown Figure 22b. Onegraph cycle represents the relaxed state after stretching theevery strain1,sensor. It can in Figure The shows the recorded reflection coefficient after 5, 10, 15, andbe 20observed cycles. One cycle 22b that the resonant not change untilsensor. 10 cycles of stretching andinrelaxation have represents the relaxedfrequency state after does stretching the strain It can be observed Figure 22b thatbeen the performed. The resonant frequency changes slightly 3.67 andand 3.68relaxation GHz afterhave 15 and 20 performed. repetitions, resonant frequency does not change until 10 cycles of to stretching been respectively. The resonant frequency changes slightly to 3.67 and 3.68 GHz after 15 and 20 repetitions, respectively.

Figure Figure22. 22.(a) (a)Simulated Simulated and and measured measured reflection reflectioncoefficients coefficientsof ofthe theproposed proposedpatch patchresonator resonatorbefore before stretching; stretching;and and(b) (b)repeatability repeatabilitytest testof ofthe thefabricated fabricatedstrain strainsensor sensor[78]. [78].

Furthermore,the thereflection reflection coefficients recorded for different strain scenarios. The Furthermore, coefficients werewere recorded for different strain scenarios. The specimen specimen was stretched along the vertical and horizontal axes; this can be observed in Figure 23. was stretched along the vertical and horizontal axes; this can be observed in Figure 23. Figure 23a Figure 23a illustrates the recorded reflection coefficient results when the device is stretched along the illustrates the recorded reflection coefficient results when the device is stretched along the vertical axis. axis. Itexpected was already from the simulation resultscoefficients, of reflection observed Itvertical was already fromexpected the simulation results of reflection ascoefficients, observed inas Figure 19a, in Figure 19a, that the resonant frequency does not vary with the stretch being applied. Nevertheless, that the resonant frequency does not vary with the stretch being applied. Nevertheless, the impedance the impedance the becomes coaxial feedhole becomes the stretch. Moreover, Figure varies, since the varies, coaxial since feedhole larger during the larger stretch.during Moreover, Figure 23b demonstrates 23b demonstrates that the recorded reflection coefficient wasstretch changed as applied the stretch washorizontal applied in that the recorded reflection coefficient was changed as the was in the the horizontal direction. Compared to the reflection coefficient results of the simulation shown in direction. Compared to the reflection coefficient results of the simulation shown in Figure 19b, Figure 19b, the resonantvaried frequency from 3.7 GHz 3.44aGHz, after aofstretching of applied 7.82% was the resonant frequency fromvaried 3.7 GHz to 3.44 GHz,toafter stretching 7.82% was to applied to the sensor. The strain of 7.82% corresponds to a 1.85 mm increase in length, and it was the sensor. The strain of 7.82% corresponds to a 1.85 mm increase in length, and it was selected by selected by maintaining the strength mechanical strength of the of PDMS material. maintaining the mechanical of the flexibility of flexibility PDMS material. The relationship relationship between between the the resonant resonant frequency frequencyand andthe thestrain strainalongside alongside the thevertical vertical direction direction The (width) and horizontal direction (length) is graphically shown in Figure 24a,b, respectively. As stated (width) and horizontal direction (length) is graphically shown in Figure 24a,b, respectively. As stated before, the frequency does not change when a strain is applied in the vertical direction; nonetheless, before, the frequency does not change when a strain is applied in the vertical direction; nonetheless, linearly proportional to strain the strain applied along the horizontal evident24b. in ititisislinearly proportional to the applied along the horizontal direction,direction, as evidentas in Figure Figure 24b. The sensor calibrate/fitting curve is defined as y = −0.0343x + 3.7. Consequently, the The sensor calibrate/fitting curve is defined as y = −0.0343x + 3.7. Consequently, the sensitivity of sensitivity of RF the straining proposedsensor RF straining 3.443 × 107 Hz/percentage. Tablethe 1 confirms the the proposed is 3.443sensor × 107 is Hz/percentage. Table 1 confirms significance significance of this work, and the proposed sensor is compared to the other strain sensors recently of this work, and the proposed sensor is compared to the other strain sensors recently developed. developed. The RF strain sensor exhibits longer andowing straintoscale owing to its The proposed RFproposed strain sensor exhibits longer flexibility andflexibility strain scale its stretch-proof stretch-proof conductive inks and flexibility. conductive inks and flexibility.

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Figure 23. 23. Measured Measured reflection reflection coefficients coefficients at at different different patch: (a) (a) widths; and (b) lengths [78]. Figure Figure 23. Measured reflection coefficients at different patch: (a) widths; and (b) lengths [78].

Figure 24. Relation between the Figure the resonant resonant frequency frequency and and the the strain strain along along the: the: (a) (a) x-direction x-direction (width;) (width); Figure 24. 24. Relation Relation between between the resonant frequency and the strain along the: (a) x-direction (width;) and (b) y-direction (length) [78]. and and (b) (b) y-direction y-direction (length) (length) [78]. [78]. Table 1. Comparison of RF strain sensors [78]. Table 1. Comparison of RF strain sensors [78]. [78] [87] [88] [89] [90] [87] [88] [89] [89] [90] [90] [78] [78] [87] [88] Substrate PDMS Kapton tape Si Duroid 5880 Kapton Substrate PDMS Kapton Kaptontape tape SiSi Duroid 58805880 KaptonKapton Substrate PDMS Duroid Conductive Material Au Au Au Cu Cu/Al Conductive Material Au Au Au Au Conductive Material Au Au Cu Cu Cu/Al Cu/Al Maximum Strain (%) N/A N/A 0.2 0.2 1 Maximum Strain (%) 7.8 7.8 N/A N/A 1 Maximum Strain (%) 7.8 N/A N/A 0.2 1 Strain Gauge * (%) * (%) 7.3 7.3 5.69 0.21 Strain Gauge 5.69 0.21 0.14 0.14 2.35 2.35 Strain Gauge * (%) 5.69 0.21 0.14 5 2.35 3.62 Resonant Frequency (GHz) 3.7 7.3 12.3 0.4742 Resonant Frequency (GHz) 3.7 12.3 0.4742 5 3.62 ∆ f Resonant Frequency (GHz) * Strain 3.7 Gauge =12.3 × 100(0.4742 5 3.62 %) ∆𝑓f * 𝑆𝑡𝑟𝑎𝑖𝑛 𝐺𝑎𝑢𝑔𝑒 = ∆𝑓0⁄𝑓 × 100(%) * 𝑆𝑡𝑟𝑎𝑖𝑛 𝐺𝑎𝑢𝑔𝑒 = ⁄𝑓0 × 100(%) 0

In this this subsection, subsection, an an RF RF strain strain sensor sensor using using screen screen printing printing with with stretch-proof stretch-proof silver silver conductive conductive In In this subsection, an RF strain sensor using screen printing with stretch-proof silver conductive ink over over a flexible PDMS material is presented. A A rectangle-shaped rectangle-shaped top top patch patch was was considered considered and and ink ink over a flexible PDMS material is presented. A rectangle-shaped top patch was considered and utilized for RF strain detection. The sensing operation considers the variation in the frequency of utilized for RF detection. The sensing operation considers the variation in the frequency of utilized for RF strain detection. The sensing operation considers the variation in the frequency of operation after is applied along the vertical and horizontal dimensions of the sensing operation aftera astrain strain is applied along the vertical and horizontal dimensions of theresonator. sensing operation after a strain is applied along the vertical and horizontal dimensions of the sensing The practicality of the sensor wassensor also verified comparing both simulation and recorded results. resonator. The practicality of the was alsoby verified by comparing both simulation and recorded resonator. The practicality of the sensor was also verified by comparing both simulation and recorded results. results. 4.2. Flexible Screen Printed Biosensor with High-Q Microwave Resonator for Rapid and Sensitive Detection of Glucose 4.2. Flexible Screen Printed Biosensor with High-Q Microwave Resonator for Rapid and Sensitive Detection 4.2. Flexible Screen Printed Biosensor with High-Q Microwave Resonator for Rapid and Sensitive Detection of Glucose This section describes a sensitive and fast moderator-free glucose biosensor based on the of Glucose phenomenon of an RF batteryless resonator realized using circular bent T-shaped identical impedance This section describes a sensitive and fast moderator-free glucose biosensor based on the This section a sensitive moderator-free glucose biosensor based on has the resonators printeddescribes using screen printingand on afast stretchy polyethylene material. Since the device phenomenon of an RF batteryless resonator realized using circular bent T-shaped identical phenomenon of an RF batteryless resonator realized using circular bent T-shaped identical impedance resonators printed using screen printing on a stretchy polyethylene material. Since the impedance resonators printed using screen printing on a stretchy polyethylene material. Since the

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a high QF of 166, the proposed glucose biosensor has a sensitivity of 72 MHz/(mg·mL−1 ) and an ultralow detection limit of 0.0167 µM at a central frequency of 11.8 GHz, within the linear detection Sensors 2017, 17, 2068 23 of 30 range of 1–5 mg/mL. Moreover, the fair dependency of the loaded QF (QL ), propagation constant −1) (γ), reflection (S11 ), and device impedance (Z) on the of glucose facilitates adequate device hascoefficient a high QF of 166, the proposed glucose biosensor has levels a sensitivity of 72 MHz/(mg∙mL and an ultralow detection of 0.0167sensor. μM at a central frequency of 11.8 GHz, within the linear multidimensional sensing by limit the glucose detection range of 1–5 mg/mL. Moreover, the fair dependency of the loaded QF (QL), propagation

4.2.1.constant Designing and Construction Sensor (γ), reflection coefficientof (Sthe 11), and device impedance (Z) on the levels of glucose facilitates adequate multidimensional sensing by the glucose sensor.

Two circular-folded T-shaped uniform impedance resonators (TSUIRs) joined with parallel-coupled feeding lines for the construction of biosensor device over a PET complement are 4.2.1. Designing and Construction of the Sensor clearly illustrated in Figure 25a. The condition for the resonance of TSUIR is to be disclosed as an Two circular-folded T-shaped uniform impedance resonators (TSUIRs) joined with paralleladjustment to the condition of resonance for the two sections of a stepped-impedance resonator, with coupled feeding lines for the construction of biosensor device over a PET complement are clearly Zin =illustrated 0 and Z1 in =Z [91]: 2 as below Figure 25a. The condition for the resonance of TSUIR is to be disclosed as an adjustment to the condition of resonance for the two sections of a stepped-impedance resonator, with Zin = 0 and Z2 tan θ1 tan θ2 = = 1. Z1 = Z2 as below [91]:

Z1

tan 𝜃 tan 𝜃 =

𝑍2

= 1.

(4)

(4)

2 Figure 25a depicts two sections of 1 the resonator with electrical lengths of θ1 and θ2 . 𝑍1 The measurements of the structure were optimized for resonance the fundamental Figure 25a depicts two sections of the resonator with electricalatlengths of 𝜃1 and 𝜃2 .frequency The of 11.8 GHz. This selection of frequency was relevant for glucose detection since the changes measurements of the structure were optimized for resonance at the fundamental frequency of 11.8in the dielectric of this region are significant the glucose–water solution purity in GHz. constant This selection of frequency washighly relevant for glucosetodetection since the changes in thewith dielectric constant of regions this region arelower highlyfrequency significant [92]. to theThe glucose–water with purityresonator in contrastcompels to contrast to the with geometry solution of the biosensing the regions with lower frequency [92]. The geometry of the biosensing resonator compels the the coupling gap (S) to affect the coupling coefficient significantly, which consequently affects the QF gap[93]. (S) to affect the which consequently QF of of thecoupling resonator Hence, thecoupling couplingcoefficient gap was significantly, designated as the sensing regionaffects of thethe biosensor, as the resonator [93]. Hence, the coupling gap was designated as the sensing region of the biosensor, demonstrated in Figure 25b. The size of the coupling gap was optimized to 0.2 mm in order toas realize demonstrated in Figure 25b. The size of the coupling gap was optimized to 0.2 mm in order to realize a high QF of 166. Figure 25c shows a snapshot of the sensor prototype and compares the measurement a high QF of 166. Figure 25c shows a snapshot of the sensor prototype and compares the measurement and simulation results of S parameter of the resonator. The recorded fundamental frequency was and simulation results of11S11 parameter of the resonator. The recorded fundamental frequency was reduced by 50 MHz. The quality whichwas wasascribed ascribed a combination of bending reduced by 50 MHz. The qualityfactor factorwas was reduced, reduced, which to atocombination of bending loss, loss, dielectric lossloss of the substrate, dimensionaccuracy. accuracy. Figure demonstrates dielectric of the substrate,and andphysical physical dimension Figure 25d25d demonstrates the the corresponding design of of thethe device glucosesection, section,expressed expressed LT and RT , which corresponding design deviceloaded loaded with with a glucose by by LT and RT, which denote the source inductanceand andresistive resistive loss loss of the respectively. LR and CR CR denote the source inductance the load loadfeed feedcoupler, coupler, respectively. LR and represent the inductance and capacitanceof ofthe the resonator, resonator, respectively. represent the inductance and capacitance respectively.

Figure 25. Proposed flexibleglucose glucosebiosensor: biosensor: (a) ofof thethe resonator fabricated on a on PETa PET Figure 25. Proposed flexible (a)3D 3Dlayout layout resonator fabricated substrate; schematic viewof of the the sensing of of thethe resonator with with a glucose–water solution;solution; (c) substrate; (b) (b) schematic view sensingregion region resonator a glucose–water simulated and and measured measured SS11, ,including a aphotograph of of thethe fabricated resonator; and (d) the (c) simulated including photograph fabricated resonator; and (d) the 11 equivalent circuit of the proposed biosensing resonator [94]. equivalent circuit of the proposed biosensing resonator [94].

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Lc and Cc denote the inductance and capacitance, respectively, owing to the magnetic and electric coupling of the resonators with the load and the source, and are dependent on the glucose-level inside the testing sample. A Smart 3S screen printer was used to print the intended designs on a 0.245-mm-thick PET material using pg 007 silver ink (PURU, Seoul, Korea) diluted with ethylene glycol. The PET dielectric material has a permittivity of 3.1 and tangential loss of 0.0324. The deposited silver nanoparticle ink has a thickness of 1 µm. The printed patterns are placed in a heating chamber for 5 min at 150 ◦ C to dry them and to increase the conductivity. 4.2.2. Preparation of Testing Samples and Measurements A standard solution of glucose consisting of a combination of DI water (Millipore TM) and D-glucose powder (SIGMA, life science, GC, St. Louis, MO, USA) was prepared at the concentrations of 1, 2, 3, 4, and 5 mg/mL. Subsequently, 2 µL of these liquid mixtures were positioned on the surface of the sensor using a micropipette. The reflection coefficients were recorded at frequencies ranging 1–15 GHz using an Agilent 8510C vector network analyzer. The testers were positioned over the detecting area of the biosensor after every 2 s. 4.2.3. Detection using S-Parameters The shifts in the central frequency, indicated by the peak value of S11 of the five samples of glucose–DI water mixtures examined, are shown in Figure 26a. The fundamental frequencies of the sensor were 10.81 and 11.09 GHz for glucose samples with the maximum and minimum concentrations of 5 and 1 mg/mL, respectively. Therefore, the fundamental frequency of the sensor was observed to further reduce when the purity of the glucose liquid was reduced. However, for the rest of the glucose samples, the fundamental frequency increased from 10.81 GHz as the concentration of the glucose was increased. This behavior is caused by the interaction between aqueous glucose and the electromagnetic coupling among the resonators and feeding line. This interaction appears to be dependent on the increase in the permittivity of the glucose composition, as a result of the decrease in glucose concentration [95]. A regression analysis reveals a good linear correlation (r2 = 0.9993) between the glucose concentration and shift in central frequency. The linear equation is expressed as follows: y = 0.071x + 10.725,

(5)

where y and x represent the central frequency and concentration of glucose, respectively. Therefore, the sensor exhibited a sensitivity of 71 MHz/(mg·mL−1 ) for glucose–water solution. According to the optimization study and the associated calibration plot (see Figure 26b), the detection limit of the assay for a signal-to-noise ratio (S/N) of 3 was calculated as 0.0167 µmol of glucose in a 2 µL sample, as outlined in [96]. The S-parameters for each sample were measured four times. Although the points deviated from the central frequency, as shown by the error bars, there was no overlapping between each purity solution. Thus, the observation confirmed that the experiment is repeatable with the same phenomenon. Figure 26c shows the changes occurring in the loaded QF (QL ) and reflection coefficient (S11 ) of the sensor for glucose testing samples of different purity levels. S11 was maximized at −28.1 and −14.9 dB for glucose compositions of 1 and 5 mg/mL, respectively. There is a negative correlation between QL and the concentration of glucose. This relationship is expected owing to the positive correlation between the loss factor and concentration of glucose.

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Figure 26.Detection Detectionof ofglucose glucose level level using using measured S-parameters: (a) shift ofofcentral frequency; (b) Figure measured S-parameters: shift central frequency; Figure 26. 26. Detection of glucose level using measured S-parameters: (a)(a) shift of central frequency; (b) linearly fitted central frequency and actual central frequency with error bar; and (c) variation of (b) linearly fitted central frequency and actualcentral centralfrequency frequencywith witherror errorbar; bar; and and (c) (c) variation linearly fitted central frequency and actual variation of of loaded quality factor (Q L ) and return loss (S 11 ) with variation in glucose concentration [94]. loaded ) with variation in glucose concentration [94]. loaded quality quality factor factor (Q (QLL)) and and return return loss loss (S (S11 11) with variation in glucose concentration [94].

4.2.4. Detecting through Derived Parameters 4.2.4. 4.2.4. Detecting through through Derived Derived Parameters Parameters The propagationconstant constant (γ) and impedance (Z) were obtained from therecorded recorded reflection The The propagation propagation constant (γ) and impedance (Z) were obtained from the the recorded reflection reflection coefficients of the glucose testers, using the approach described in [97]. The propagation constant coefficients coefficients of of the the glucose glucose testers, testers, using using the the approach approach described described in in [97]. [97]. The The propagation propagation constant constant increased with changes in glucose purities, from approximately 11.51 GHz to 12.51 GHz, as shown increased 11.51 GHz to to 12.51 GHz, as as shown in increased with withchanges changesininglucose glucosepurities, purities,from fromapproximately approximately 11.51 GHz 12.51 GHz, shown in Figure 27a. Dips in resonance, which have a positive correlation between the frequency and Figure 27a.27a. DipsDips in resonance, whichwhich have ahave positive correlation between the frequency and glucose in Figure in resonance, a positive correlation between the frequency and glucose concentration, are observed. Dips in resonant impedance are also observed. These dips occur concentration, are observed. Dips inDips resonant impedance are also These dipsdips occur at glucose concentration, are observed. in resonant impedance are observed. also observed. These occur at dissimilar frequencies for varying glucose concentrations. The frequencies of these dips also have dissimilar frequencies for varying glucose concentrations. The frequencies of these dips also have a at dissimilar frequencies for varying glucose concentrations. The frequencies of these dips also have a positive correlation with glucose concentration, as shown in Figure 27b. positive correlation with glucose concentration, as as shown in in Figure 27b. a positive correlation with glucose concentration, shown Figure 27b.

(a) (a)

(b) (b)

Figure 27.Detection Detectionofofglucose glucoselevel level using using derived derived parameters: parameters: (a) propagation constant (γ); and (b) Figure propagation constant Figure 27. 27. Detection of glucose level using derived parameters: (a)(a) propagation constant (γ);(γ); andand (b) impedance (Z) [94]. (b) impedance [94]. impedance (Z)(Z) [94].

This subsection presents a stretchable biosensor using screen printing technology as a high QF This subsection subsection presents a stretchable stretchable biosensor biosensor using screen printing printing technology as a high high QF QF This RF batteryless resonator recognized for moderator-less sensing of glucose levels. Two circular-folded RF batteryless resonator recognized for moderator-less sensing of glucose levels. Two circular-folded RF batteryless resonator recognized for moderator-less sensing circular-folded T-shaped uniform impedance resonators were joined with parallel-coupled feeding lines on PET T-shapeduniform uniformimpedance impedance resonators were joined parallel-coupled feeding lines PET T-shaped resonators were joined withwith parallel-coupled feeding lines on PETon sheets. sheets. Based on the central frequency shifts, the projected sensor device confirmed a highly sensitive sheets. Based on the central frequency shifts, the projected sensor device confirmed a highly sensitive Based on the central frequency shifts, the projected sensor device confirmed a highly sensitive and and quick sensing mechanism of glucose with a significantly lower sensing limit. and quick sensing mechanism of glucose a significantly sensing quick sensing mechanism of glucose with with a significantly lowerlower sensing limit.limit. 5. Summary 5. 5. Summary Printed RFmicroelectronics microelectronics is anevolving evolving zoneofofinvestigation investigation withlarger larger commercial Printed Printed RF RF microelectronics is an an evolving zone zone of investigation with with larger commercial commercial expectations owing toitsits capability to sidestep conventional inflexible and expensive silicon-based expectations expectations owing to to its capability capability to to sidestep sidestep conventional conventional inflexible inflexible and and expensive expensive silicon-based silicon-based circuits to fabricate different types and shapes of components on bendable materials using highcircuits types andand shapes of components on bendable materials using high-quality circuitstotofabricate fabricatedifferent different types shapes of components on bendable materials using highquality printer methods. For the three additive techniques mentioned in this review, inkjet-printing printer For theFor three techniques mentioned in thisinreview, inkjet-printing is an quality methods. printer methods. theadditive three additive techniques mentioned this review, inkjet-printing is an alluring process for manufacturing electronic components owing to the negligible waste alluring processprocess for manufacturing electronic components owing toowing the negligible waste produced is an alluring for manufacturing electronic components to the negligible waste produced and its effectiveness at handling some expensive materials. Inkjet printing of the produced and its effectiveness at handling some expensive materials. Inkjet printing of the conducting forerunner materials, typically conductive nanoparticles or metallic–organic facilities, is conducting forerunner materials, typically conductive nanoparticles or metallic–organic facilities, is

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and its effectiveness at handling some expensive materials. Inkjet printing of the conducting forerunner materials, typically conductive nanoparticles or metallic–organic facilities, is employed as a comparatively faster method that can effectively handle roll-2-roll (R2R) manufacture. However, the sintering process in this fabrication, which is necessary to purify the patterns containing conductive inks, involves times longer than 20 min or higher temperatures (>200 ◦ C). Specially, the higher sintering schedules are not scalable for R2R manufacturing. For instance, a web-speed of 1 m/s with a sintering time of 35 min corresponds to the production line required to be at least 1.9 km long. However, screen printing techniques are suitable for bulk production. Further, 3D printed structures, such as origami, are gaining interest owing to their ease of fabrication, which was previously an issue with the technique owing to the support structures required for fabrication. Some of the recent works involve combination of inkjet and screen printing in the development of batteryless sensors [98–101]. For this review, we have discussed and compared the recent advances in the three popular printing fabrication techniques with respect to their fabrication time, power consumption, and complexity. The focus was on the additive manufacturing of batteryless RF sensors and the advantages of these fabrication techniques for sensors perspective. Acknowledgments: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No.2014R1A2A1A11050010). Author Contributions: Muhammad Usman Memon wrote this review article. Sungjoon Lim contributed to revision of the manuscript. Conflicts of Interest: The authors declare no conflicts of interest.

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