Screen Printing Carbon Nanotubes Textiles Antennas for Smart Wearables

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Article Screen Printing Carbon Nanotubes Textiles Antennas for Smart Wearables

Isidoro Ibanez Labiano 1 , Dilan Arslan 2 , Elif Ozden Yenigun 3,* , Amir Asadi 4, Hulya Cebeci 2,5 and Akram Alomainy 1

1 Department of EECS, Queen Mary University of London, London E1 4NS, UK; [email protected] (I.I.L.); [email protected] (A.A.) 2 Aerospace Research Center, Istanbul Technical University, 34469 Istanbul, Turkey; [email protected] (D.A.); [email protected] (H.C.) 3 School of Design, Textiles, Royal College of Art, London SW7 2EU, UK 4 Department of Engineering Technology & Industrial Distribution, Texas A&M University, College Station, TX 77843-3367, USA; [email protected] 5 Faculty of Aeronautics and Astronautics, Istanbul Technical University, 34467 Istanbul, Turkey * Correspondence: [email protected]; Tel.: +44-7464881454

Abstract: Electronic textiles have become a dynamic research ﬁeld in recent decades, attracting attention to smart wearables to develop and integrate electronic devices onto clothing. Combining traditional screen-printing techniques with novel nanocarbon-based inks offers seamless integration of ﬂexible and conformal antenna patterns onto fabric substrates with a minimum weight penalty and haptic disruption. In this study, two different fabric-based antenna designs called PICA and

LOOP were fabricated through a scalable screen-printing process by tuning the conductive ink formulations accompanied by cellulose nanocrystals. The printing process was controlled and monitored by revealing the relationship between the textiles’ nature and conducting nano-ink. The Citation: Ibanez Labiano, I.; Arslan, D.; Ozden Yenigun, E.; Asadi, A.; fabric prototypes were tested in dynamic environments mimicking complex real-life situations, such Cebeci, H.; Alomainy, A. Screen as being in proximity to a human body, and being affected by wrinkling, bending, and fabric care Printing Carbon Nanotubes Textiles such as washing or ironing. Both computational and experimental on-and-off-body antenna gain Antennas for Smart Wearables. results acknowledged the potential of tunable material systems complimenting traditional printing Sensors 2021, 21, 4934. https:// techniques for smart sensing technology as a plausible pathway for future wearables. doi.org/10.3390/s21144934 Keywords: e-textiles; wearables; screen printing; ﬂexible printed antennas; carbon nanotubes inks Academic Editor: Eduardo García Breijo

Received: 14 June 2021 1. Introduction Accepted: 13 July 2021 Published: 20 July 2021 Electronic textiles (e-textiles) that involve the combination of electronics and textiles introduce “smart” functions to textile products [1]. Electronic and sensory functions on the

Publisher’s Note: MDPI stays neutral flexible substrates, including fabrics, have been demanded in many wearables and Internet with regard to jurisdictional claims in of Things (IoT) applications for wireless sensor networks (WSN) with a strong acceleration published maps and institutional affil- embracing sensory features [2,3]. In the context of wearable communication, antennas are iations. a key component that adheres to specific requirements, including flexibility, conformality, being low profile, and lightweight [4]. Unlike conventional antennas, wearable antennas should be analysed in dynamic environments to simulate complex real-life situations, such as being in the proximity of a human body and under deformation of wrinkling, bending, or cyclic deformation such as washing and ironing. In the emerging field of Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. flexible antennas, either knitted or woven, different textile forms can withstand bending, This article is an open access article twisting, and stretching and promise longevity in critical components. Add-on textile distributed under the terms and processes, including printing [5,6] and lamination, can induce innovative surface features conditions of the Creative Commons and layers using conductive materials [7]. However, the success of final executions on Attribution (CC BY) license (https:// the textiles is limited by the compatibility of ink and intrinsic properties of a conductive creativecommons.org/licenses/by/ material such as conductivity, durability, susceptibility of washing, and humidity. Until 4.0/). now, various kinds of conductive inks with different fillers such as metal nanoparticles [8–11],

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polymers [12], carbon nanotubes [13], and/or organic metal complexes [14] have been developed for forming conductive patterns. In the aim of scalable processes for printed electronics, inkjet and screen printing techniques tuning the liquid phase inks with high efficiency, stability, low cost, and zero waste materials have been employed [15–17]. A recent technology advancement attracting much attention from the design and research community is inkjet printing. Despite its widespread use, inkjet printing has drawbacks and difficulties associated with ink stability, clogging of nozzles, and surface limitations, including rough and porous textile surfaces due to its low viscosity and long printing times [18,19]. Screen printing is the most common and straightforward conventional printing method, being relatively effortless, versatile, affordable, fast, and adaptable [15,18–20]. Screen printing is a making method that allows transferring the ink through a stencil design to a flat surface by using a squeegee and mesh screen usually made from silk or nylon. Several parameters are critical in producing high-quality patterns, such as squeegee shape, speed and pressure, snap-off distance (a gap between the screen and substrate), and mask clearance [21,22]. The formulated inks for screen printing are qualified with rheology analysis such as shear viscosity as a function of shear rate [15]. During printing, the ink with high viscosity should be deposited slower with the squeegee to allow sufficient deposition time for the ink to flow through the screen meshes. When the ink has low viscosity, the speed should be higher to prevent uncontrollable spreading [23]. Tseng et al. examined the rheological behaviours of nickel nanoparticles dispersed solvents, using various organic surfactants over a shear-rate range 100–103 s−1. The results demonstrated that nanoparticle viscosity reduced as much as 40–70% with the surfactants, and pseudoplastic flow behaviour was observed at the shear-rate regime often encountered in most screen printing processes [24]. Hong et al. demonstrated UV-curing conductive inks with 60 wt.% silver nanoflakes for screen printing on standard fabrics. The UV-curable conductive ink contains 24 wt.% polymers, and 10.8 wt.% diluents showed a viscosity of 176.29 Pa·s at a shear rate of 0.1 s−1. Ultra-high frequency radio-frequency identification (UHF RFID) tags were manu- factured using this UV-curable conductive ink by screen printing with a conductivity of 6.02 × 106 Sm–1 [25]. Despite reported promising performances, the metallic conductive inks cannot fully meet the haptic requirement of textiles since they become gradually oxi- dised and lose performance, bringing additional weight and thickness to the structure that disrupts comfort. Thus, the researchers have alternative material systems to fulfil textiles sensory values while adding electronic functions. In an attempt to create flexible sensors and antennas, different sp2 carbon assemblies, such as graphene and carbon nanotubes (CNTs), grasp the best compromise between features such as high electrical conductivity, high thermal stability, strong mechanical properties, high corrosion resistance, and low density [26,27]. CNTs with their unique characteristics, such as high intrinsic current mobility, electrical and thermal conductivity, mechanical stability, and low-cost production capabilities are of interest for electronic and bio applications [28–33]. Among non-metallic conductive materials, CNTs play a pivotal role, owing to the aforementioned characteristics that make them strong candidates for flexible devices, thus attracting attention in wearable antennas and sensors [34–36]. CNTs have been previously repurposed for wireless communication [37], including on-body applications [38], and layered up in the form of thin conductive films [39]. However, the challenges in scaling up processes lead the researchers to repurpose conventional textile printing techniques for mass production that can boost the manufacturing of e-textiles. Previous works reported CNTs laminated antennas on flexible substrates such as Kapton [37], paper [40], and polystyrene, whereas the contribution of fabric as a substrate was not navigated for microwave antennas, near-field (NF) communication, and RFID [41,42]. In printed electronics, the conductive inks containing binders serve to secure together the other components, such as granular powders or conductive fillers, to create a smooth and homogenous film deposited on top of a substrate. However, these binders need to be cured or fixed through Sensors 2021, 21, 4934 3 of 28

high-temperature processes, such as crosslinking, annealing, and sintering. These post- processes require precision not to induce deviation in material qualities due to applied heat, impacting final antenna performance [43]. Furthermore, the heat processing steps are not incompatible with other flexible substrates such as papers and natural textile materials. Therefore, carbon-based ink formulation requires understanding the material to develop binder-free or low-temperature processing solutions while retaining high conductivity. In an attempt to design sensory fabrics by CNTs, [44] screen printed fabrics could be created by incorporating aqueous CNTs ink. The results showed that in the first layer of deposition, the CNTs based fabric surface sheet resistance was 141.5 Ω/sq, then it was decreased to 50.75 Ω/sq by applying three layers. Despite the promising electronic function for wearable applications, the poor solubility of CNTs in water due to their hydrophobic surfaces and highly attractive inter-van der Waals interactions between CNTs makes the process under control [45]. Menon et al. proposed a room temperature curable conducting ink using multiwalled carbon nanotubes (MWCNTs) for a printable electronic application [46]. In another study, flexible substrates such as Mylar®, silicone rubber, and photo paper were explored as challenging with screen-printing. As a result of using 9 wt.% MWCNTs, the sheet resistance was reported in the range of 0.5–13 Ω/sq, whereas the use of a binder in their suspensions disrupted the electrical conduction network [47]. Despite the strong evidence of CNTs based inks in printable electronics applications, the paucity of existing data for developing water-based and binder-free inks could discourage researchers from designing their ink formulations. In an attempt to rediscover earth materials in electronic functions, carbon nanomaterials still require support to bring their superiority forward for scaled-up processes. Hence, the researchers have started seeking out nature-derived-non-petroleum binders to promote interfacial bonding. Among a broad range of biopolymers, cellulose, an abundant biopoly- mer, promises favourable properties such as low cost, biodegradability, biocompatibility, nontoxicity, and mechanical strength [48]. Cellulose nanocrystals (CNC) are extracted from natural cellulosic materials by controlled acid hydrolysis, in which surface sulphate groups enable excellent colloidal stability [49]. It has polar and nonpolar groups that could assist dispersion in both apolar and polar solvents. A deeper understanding of intermolecular interactions in ink formulations could avoid phase separation in large-scale processes. For instance, the nonpolar areas of cellulose nanocrystal can interact with hydrophobic CNTs and aid in bridging between two nanotubes [50], and highly stable CNTs dispersions were possible by wisely introducing CNC [51]. This study investigated the CNC-assisted MWCNTs and single-walled carbon nanotubes (SWCNTs) aqueous suspensions’ stability and dispersing capacity. MWCNTs/CNC aqueous suspension achieved its maximum dispersion yield of 60% at the MWCNTs concentration of 0.05 wt.%. In comparison, SWCNTs/CNC aqueous suspension reached its maximum dispersion yield of 80% at the SWCNTs concentration of 0.01 wt.% In another study, the optimal parameters of the CNTs/CNC dispersion for both SWCNTs and MWCNTs have been revealed [45]. Respectively, in SWCNTs/CNC (0.55/10 g/L: sonication time of 2.5 hours) and MWCNTs/CNC (0.33/10 g/L: sonication time of 1 hour) dispersions, the yield was reported as high as 72% and 80% with a power density of 0.7 W/mL. Previous works reported isolated challenges regarding using carbon-based inks, screen-printed techniques, and flexible substrates for on-body applications. This study attempts to develop flexible and lightweight CNTs based textile antennas that can extend the possibilities of current state-of-the-art wireless body-centric systems by promising a new sustainable material medium for e-textiles. The research embraces a holistic approach from behavioural decisions and understanding of the wearer’s priorities to environmental and economic concerns related to the textile screen printing processes. In each intervention point, we elaborated on the material and process requirements to offer a working communication channel in the range of microwave frequencies. The previous section of the article presents a complete introduction to the topic. Section2 will provide information regarding the materials and methods used to fabricate the prototypes. At the same time, Sensors 2021, 21, 4934 4 of 28

both numerical and experimental analyses have been carried out in Section3, closing with conclusions in Section4.

2. Materials and Methods 2.1. Materials CNTs were purchased from Sigma Aldrich (Sigma Aldrich, SouthWest NanoTech- nologies, Norman, Oklahoma, US) having a purity of ≥98%, an outer diameter: 10 nm, and length: 3–6 µm. CNCs were provided from CelluForce, Windsor, QC, Canada, with a diameter of 2.3–4.5 nm and an average length of 44–108 nm [52] and were mixed with deionized water (DI-H2O). Screen-printing was performed using a polyurethane squeegee and 26 × 32 cm wooden silk frame with mesh size 55 thread/cm (55T) and 90 thread/cm (90T). Two different woven fabrics of 100% cotton (CO) and 65% cotton–35% polyester (CO–PES) were used as the textile substrates for screen-printing of CNTs/CNC inks. The fabric patterns were plain weave for both CO and CO–PES. The mass of the fabrics was 116 g/sqm and 122 g/sqm for CO and CO–PES fabrics, respectively. All chemicals were used as received.

2.2. Preparation of CNTs/CNC Ink Dispersions

The CNTs and CNC dispersions were prepared by mixing CNC with DI-H2O at a weight ratio (wt.%) of 0.5, 1, and 1.5%. The CNC/DI-H2O dispersions were sonicated with a microtip sonication (Sonics VCX750) for 35 min at 20 kHz, 8 Watt, and 20% amplitude at 480 J/min. Then, CNTs were mixed with CNC aqueous dispersion to derive the ink composition, and this mixture will be named CNTs/CNC ink throughout this research paper. The CNTs/CNC ink formulations were prepared with a weight ratio of 1:1 (for 0.5, 1, 1.5 wt.% of CNC) and 1:2 (1 wt.% of CNC). CNTs/CNC inks were dispersed using a microtip sonicator for 1 h at the same aforementioned sonication process. No conventional dispersants and surfactants were employed for preparing CNTs/CNC inks, and all inks were used directly without any post-treatment. The details of CNTs/CNC ink formulations related to sample codes were given in Table1. Different concentrations were used for both planar inverted cone antennas (PICA) and LOOP designed antenna models, detailed in the antenna design section.

Table 1. CNTs/CNC ink compositions with related sample codes and printed antenna designs.

Sample Codes CNTs/CNC Ink Compositions Sonication (Hour) Antenna Designs CNTs/CNC—0.5:0.5 0.5 wt.% CNTs and 0.5 wt.% CNC 1.0 PICA and LOOP CNTs/CNC—1:1 1 wt.% CNTs and 1 wt.% CNC 1.0 PICA and LOOP CNTs/CNC—1.5:1.5 1.5 wt.% CNTs and 1.5 wt.% CNC 1.5 PICA CNTs/CNC—0.5:1 0.5 wt.% CNTs and 1 wt.% CNC 1.0 LOOP

During CNTs/CNC ink preparation, when the CNTs were at higher concentrations, increased sonication energy and longer time was applied for an efﬁcient dispersion [53]. Thus, the sonication time was increased to 1.5 hours for CNTs/CNC—1.5:1.5 compared to 1-hour sonication for all other compositions while keeping the frequency and amplitude the same. During sonication, the dispersion container was always ice-cooled to prevent excess heating resulting in instability of viscosity.

the same. During sonication, the dispersion container was always ice-cooled to prevent excess heating resulting in instability of viscosity.

2.3. Antenna Designs In this section, the two different antenna designs named planar inverted cone antenna (PICA) and LOOP are presented. Among two different designs, ultra-wideband (UWB) PICA antenna was selected due to the ability to provide appealing features such as high capacity with significant bandwidth signals, multi-path robustness, and low- Sensors 2021, 21, 4934 5 of 28 power requirement. UWB antennas have been extensively studied in wearable and healthcare applications. Although this piece of research does not cover time domain characterisation (group delay, normalized amplitude, fidelity factor, or power spectral den- tionsity), (group it provides delay, normalizedenough evidence amplitude, in terms fidelity of antenna factor, performance or power spectral when density),placed on/off it providesbody settings. enough LOOP evidence antenna in terms (also of called antenna a magnetic performance loop) promises when placed inherent on/off robustness body settings.when in LOOP proximity antenna to (alsothe human called a body magnetic due loop)to its promisessensitivity inherent to the robustnessmagnetic field when where in proximitythe human to the body human has an body important due to its impact sensitivity on th toe electric the magnetic field. These field where two designs the human reveal bodydifferent has an communication important impact channels on the for electric textile-b field.ased These antennas two at designs highly reveal dispersive different medi- communicationums such as the channels body. For for textile-basedeach design, antennastwo prototypes at highly with dispersive varying mediumsformulations such of as ink thewere body. fabricated For each design,by screen two printing prototypes of five with layers varying onto formulations the cotton derivative of ink were substrates. fabricated by screen printing of five layers onto the cotton derivative substrates. 2.3.1. PICA 2.3.1. PICA The first antenna model analysed consists of a UWB, with a radiation quarter-wave- lengthThe first(λ/4) antenna monopole model inanalysed the shape consists of a PICA of a UWB, and witha coplanar a radiation waveguide quarter-wavelength (CPW) as the (λ/4)feeding monopole technique. in the The shape antenna of a PICA geometry and a was coplanar determined waveguide based (CPW) on the as monopole the feeding disc technique.principles The [54], antenna thus an geometryoptimization was proce determinedss was completed based on to the decide monopole the antenna disc princi- dimen- ples [54], thus an optimization process was completed to decide the antenna dimensions as sions as depicted in Figure 1. UWB designs are used in different sensing applications like depicted in Figure1. UWB designs are used in different sensing applications like tempera- temperature, moisture, strain, microwave imaging, etc. [55]. The proposed antenna has ture, moisture, strain, microwave imaging, etc. [55]. The proposed antenna has been proven been proven to be a solid alternative to the off-the-shelves solutions for wearable devices. to be a solid alternative to the off-the-shelves solutions for wearable devices. The textile The textile feature offers structural advantages, such as lightness, washability, and drap- feature offers structural advantages, such as lightness, washability, and drapability [7]. ability [7].

Figure 1. (a) PICA antenna design CAD model with the dimensions in mm; Prototyped models: (b) Figure 1. a CNTs/CNC—1:1;( ) PICA antenna(c) CNTs/CNC—1.5:1.5. design CAD model with the dimensions in mm; Prototyped models: (b) CNTs/CNC—1:1; (c) CNTs/CNC—1.5:1.5.

2.3.2.2.3.2. LOOP LOOP A wearableA wearable loop loop antenna antenna structure structure is printedis printed on on a verya very thin thin non-grounded non-grounded 0.245 0.245 mmmm- thickthick textile textile cotton cotton fabric, fabric, where where the the loop loop antenna antenna is is of of size sizeL L× ×W W == 5050 mmmm ×× 3535 mm, mm, as asdepicted depicted in Figure2 2,, designed designed to to resonate resonate around around 2.45 2.45 GHz GHz (industrial, (industrial, scientiﬁc scientific and and medical,medical, ISM ISM band), band), to to meet meet the the criteria criteria of of sensing sensing and and healthcare healthcare applications. applications. The The actual actual looploop is Lpis Lp× ×Wp Wp= = 43.5 43.5 mm mm× ×25.1 25.1 mm,mm, respectively,respectively,and andthe thegap gapat atthe thefeeding feedingpoint point isis 1 1 mm. The printed trace width of 1 mm and 2 mm, previously studied, showed a stronger resonance frequency for 2 mm; thus, this width was chosen in this study [56].

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Sensors 2021, 21, 4934 6 of 28 mm. The printed trace width of 1 mm and 2 mm, previously studied, showed a stronger mm.resonance The printed frequency trace width for 2 mm; of 1 mmthus, and this 2 width mm, previouslywas chosen studied, in this study showed [56]. a stronger resonance frequency for 2 mm; thus, this width was chosen in this study [56].

Figure 2. (a) Top view of the modelled textile loop printed antenna along with its dimensions in Figure 2. (a) Top view of the modelled textile loop printed antenna along with its dimensions in mm; Figuremm; 2. Prototyped (a) Top view models: of the (b modelled) CNTs/CNC—0.5:1; textile loop ( cpr) CNTs/CNC—1:1.inted antenna along with its dimensions in mm;Prototyped Prototyped models: models: (b) CNTs/CNC—0.5:1;(b) CNTs/CNC—0.5:1; (c) CNTs/CNC—1:1.(c) CNTs/CNC—1:1. 2.4.2.4. Screen-Printing Screen-Printing of of PICA PICA and and LOOP LOOP Antennas Antennas onto onto Textiles Textiles 2.4. Screen-Printing of PICA and LOOP Antennas onto Textiles AA manual manual screen-printing screen-printing setup setup was was used used to to prepare prepare the the CNTs/CNC CNTs/CNC patterns patterns of of PICAPICAA andmanual and LOOP LOOP screen-printing antenna antenna designs designs setup on on thewas the woven usedwoven to fabrics fabrics prepare of of CO theCO and andCNTs/CNC CO–PES. CO–PES. First,patterns First, woven woven of PICAfabricsfabrics and were wereLOOP cut cut antenna by by referring referring designs to to computational oncomputational the woven modelsfabrics models of described describedCO and inCO–PES. in Section Section First, 2.3 2.3 and wovenand were were fabricsconditionedconditioned were cut at at 85by 85 ◦referring C°C for for 24 24 to hours. hours.computational All All fabrics fabrics models were were weighed describedweighed until untilin Section achieving achieving 2.3 aand a constant constant were conditionedweightweight before before at 85 the the °C screen-printing screen-printing for 24 hours. processAll process fabr toics to record wererecord weighed the the baseline baseline until weight weightachieving of of the thea unprintedconstant unprinted weightfabric.fabric. before Conductive Conductive the screen-printing CNTs/CNC CNTs/CNC inkprocessink was was to printed printed record onto ontothe baseline woven woven fabrics weightfabrics in ofin a thea layer-by-layer layer-by-layer unprinted fabric.fashionfashion Conductive and and presented presented CNTs/CNC schematically schematically ink was in in Figureprinted Figure3. 3. Afteronto After eachwoven each printing printingfabrics layer, in layer, a alllayer-by-layer all fabrics fabrics were were fashionweighedweighed and to topresented measure measure the schematically the total total ink ink penetration penetration in Figure for3. for After identifying identifying each printing the the related related layer, thickness thicknessall fabrics through throughwere weighedthethe area area to and andmeasure density density the calculations calculations total ink penetration [57 [57].]. for identifying the related thickness through the area and density calculations [57].

Figure 3. Schematics of screen printing of PICA and LOOP patterned textile antennas from CNTs/CNC formulations. FigureFigure 3. 3. SchematicsSchematics of of screen screen printing printing of of PICA PICA and and LOOP LOOP patterned patterned textile textile an antennastennas from from CNTs/CNC CNTs/CNC formulations. formulations. 2.4.1. Printing PICA Textile Antennas 2.4.1. Printing PICA Textile Antennas Conductive inks of CNTs/CNC—0.5:0.5, CNTs/CNC—1:1, and CNTs/CNC—— 2.4.1. Printing PICA Textile Antennas 1.5:1.5Conductive were prepared inks of initially CNTs/CNC—0.5 as described:0.5, ea rlierCNTs/CNC—1:1, onto CO fabric and only. CNTs/CNC—— To obtain compa- 1.5:1.5rableConductive were results, prepared each inks sample initially of CNTs/CNC—0.5:0.5, and as describedlayer were ea scrrlier CNTs/CNC—1:1,een onto printed CO underfabric and theonly. CNTs/CNC——1.5:1.5 same To obtainconditions. compa- First, were prepared initially as described earlier onto CO fabric only. To obtain comparable rable0.5 results, mL of ink each was sample dropped and intolayer the were screen screen printing printed frame under and the by same using conditions. the squeegee, First, and results, each sample and layer were screen printed under the same conditions. First, 0.5 mL 0.55 mL times of ink of layering was dropped were intoperformed the screen to achi printingeve a uniformframe and and by homogeneous using the squeegee, printing and pat- of ink was dropped into the screen printing frame and by using the squeegee, and 5 times 5 timestern ontoof layering CO fabric. were The performed structure to was achi heldeve 30 a uniformseconds onand the homogeneous CO fabric before printing being pat- lifted of layering were performed to achieve a uniform and homogeneous printing pattern onto ternoff onto and CO then fabric. cleaned The with structure distilled was water held 30 to secondsprevent onpores the fromCO fabric clogging. before Afterwards, being lifted the CO fabric. The structure was held 30 seconds on the CO fabric before being lifted off and offprinted and then pattern cleaned was with dried distilled at 85 water °C in to the prevent oven poresuntil fromthe moisture clogging. was Afterwards, removed, the then printedthen cleaned pattern with was distilled dried at water 85 °C to preventin the oven pores until from the clogging. moisture Afterwards, was removed, the printed then pattern was dried at 85 ◦C in the oven until the moisture was removed, then weighed.

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Under the same printing conditions, five layers of printing were deposited on the CO fabric using both formulated inks. Still, after the first layer, the amount of ink and the number of back-and-forth movements of the squeegee were gradually reduced to prevent over- spreading inks on the fabric. The details of printing process parameters are presented in Table2 for the number of layers, dispersion volume, and squeegee movements. To achieve a homogeneous printing pattern and finely tuned sharp corners and lines, a systematic squeegee movement is also controlled with the number of movements and correlated with the weight of the fabric as well. The weight of the printed fabrics was measured for every print layer to control the amount of total CNTs/CNC coating precisely. The PICA designed antenna prototypes with five layers of thickness were 25 ± 2.9 µm for CNTs/CNC—1:1 and 35 ± 2.1 µm for CNTs/CNC——1.5:1.5, respectively. Due to the non-homogeneous distribution of the ink, CNTs/CNC—0.5:0.5 composition did not result in a well-defined printing pattern for PICA antenna, which was attributed to the viscosity of the ink and higher water absorbance of CO fabric.

Table 2. Parameters of screen printing on CO fabric using CNTs/CNC—1:1 and CNTs/CNC—1.5:1.5.

Number of Layer Volume of Dispersion (mL) Number of Squeegee Movement 1 0.5 5 2 0.4 2 3 0.4 2 4 0.4 2 5 0.4 2

2.4.2. Printing LOOP Textile Antennas The LOOP antenna designs were prepared with similar process parameters and steps as explained in the printing PICA-designed textile antenna section. The LOOP antennas were printed onto CO and CO–PES fabrics. Within the printing experiences observed in PICA antennas onto CO fabrics, CO–PES as an alternative textile substrate with lower water absorbance was also used for LOOP antennas. CNTs/CNC inks with weight ratios of 1:1 and 0.5:1 were utilized to prepare LOOP model samples. Similar printing protocols for PICA-designed antennas were used to print LOOP antennas. The details of the number of layers, the volume of dispersions, and squeegee movement is presented in Table3. The volume of distribution was lowered gradually from the first to fifth layers to avoid the excess use of CNTs/CNC ink. The final prototypes had an average thickness of 14 ± 5.2 µm for CNTs/CNC—1:1 and 6.4 ± 0.7 µm for CNTs/CNC—0.5:1 printed LOOP antennas, respectively.

Table 3. Parameters of screen printing on CO–PES fabric using CNTs/CNC—1:1 and CNTs/CNC— 0.5:1.

Number of Layers The volume of Dispersion (mL) Number of Squeegee Movement 1 0.4 5 2 0.4 4 3 0.3 4 4 0.3 4 5 0.3 4

2.5. Rheological Behaviours Rheological measurements of CNTs/CNC-based inks were conducted using an ro- tational rheometer (TA Instruments, Discovery Hybrid Rheometer 2) equipped with a plate–plate geometry diameter of 25 mm. Measures of 0.5 mL volume inks with 25 mm diameter and 1 mm gap thickness were injected between aluminium plates and tested at 25 ◦C. First, strain sweep tests were performed to determine the linear viscoelastic region (LVR) and then the frequency sweep tests were carried out using a strain amplitude within Sensors 2021, 21, 4934 8 of 28

the linear viscoelastic regime. The complex viscosity was recorded as a function of angular frequency between 0.1 and 102 rad/s.

2.6. Electrical Conductivity Measurements The electrical conductivities of conductive surfaces were measured by a Four-Point Probe (FPP 470, Eltek Electronics, Istanbul, Turkey) at room temperature. For each sample, at least seven measurements were taken by varying sites on the surface; the mean conductivities are reported in Section 3.3.

2.7. Fabric Care: Washing, Ironing, and Comfort A continuous washing cycle was performed to evaluate the adhesion of CNTs printed layers onto only CO fabric with PICA antennas using the revised protocol based on BS EN ISO 6330 via a domestic washing machine (D1 6101 ES A, BEKO, Istanbul, Turkey) due to the lack of a standard protocol in assessing the wash durability of e-textiles. In this study, only CO fabrics were evaluated in terms of care performance. Both CNTs/CNC—1:1 and CNTs/CNC—1.5:1.5 printed onto CO fabrics were washed and ironed. The wash process was performed in a hand wash machine at 30 ± 3 ◦C using a detergent without an optical brightener and sulphate. Throughout the washing, the textile samples underwent an initial cold wash, three rinsing cycles, two draining cycles, and a dry spin (400 speed). Each printed fabric was washed four times. In between cycles, the surface resistance change of PICA antennas was measured before and after washing. According to the BS EN ISO 105-X11:1996 standard, ironing was then applied, and the resistance was measured after each washing/ironing cycle as described in Section 3.4. The weight penalty is one of the significant issues within wearable systems in so much as it has a negative impact on the final user’s experience. CNTs antenna save the lightweight feature in comparison with more dense materials such as traditional bulky metals. Similarly, other fabrication methods such as lamination use extra layers for bonding, adding a significant proportion to the final weight of the antenna. The CNTs PICA antenna fabricated has attained a total weight of 53 g, ~12% lighter than the previous design of multi-layer graphene and ~29% less weight with respect to a copper tape model of 75 g [7].

2.8. Dielectric Characterization of the Fabric Substrate Dielectric properties of the substrate fabrics were characterized to carry out further numerical analyses. Two parameters, such as the dielectric constant (εr) and the dissipation factor (DF or tanδ), were measured to represent CO fabric’s dielectric characteristics; a resonant material characterization technique was employed [58,59]. A cavity perturba- tion method, by means of a split cylinder resonator for material characterization (Agilent 85072A), working at 10 GHz, was performed by using the Keysight material characterization software (N1500A-003 Materials Measurement Suite 2015). The textile sample was placed in the aperture of the cavity and measured, then rotated 90 degrees and measured again in the weft direction. For 100% cotton plain weave fabric, a dielectric constant of 1.58 and a dissipation factor of 0.02 were calculated, and for the CO–PES fabric, 1.62 εr and 0.018 DF was noted.

2.9. Antenna Measurement Considerations The fabric specimens were measured using a vector network analyser (VNA), PNA- L Agilent N5230C, and inside a mobile antenna electromagnetic compatibility (EMC)- screened anechoic chamber to examine the far-ﬁeld properties radiation patterns of the antenna under test (AUT). The chamber is ﬁtted with two open boundary quad-ridge horn antennas operating from 400 MHz to 6 GHz (ETS-Lindgren 3164-06) and from 0.8 to 12 GHz (Satimo QH800) allowing vertical and horizontal linear polarization measurements [60]. For wearable sensing applications, the performance of a wearable antenna when in proximity to the human body needs to be examined. During on-body numerical simulations, all electrical properties (εr, tanδ, conductivity (σ), and resistivity (ρ)) of human tissues, comprising: Sensors 2021, 21, x FOR PEER REVIEW 9 of 28

Sensors 2021, 21, 4934 GHz (Satimo QH800) allowing vertical and horizontal linear polarization measurements9 of 28 [60]. For wearable sensing applications, the performance of a wearable antenna when in proximity to the human body needs to be examined. During on-body numerical simulations, all electrical properties (εr, tanδ, conductivity (σ), and resistivity (ρ)) of human tis- dry skin, fat, and muscle have been considered from the available libraries in the CST sues, comprising: dry skin, fat, and muscle have been considered from the available li- microwave studio and literature [61], Table4. braries in the CST microwave studio and literature [61], Table 4. Table 4. Electromagnetic properties of human tissues tested at 2.45 GHz [61]. Table 4. Electromagnetic properties of human tissues tested at 2.45 GHz [61].

Tissue εr tanδ σ (S/m) ρ (Ω·m) Tissue εr tanδ σ (S/m) ρ (Ω·m) DryDry Skin Skin 38.007 38.007 0.283 0.283 1.464 1.464 0.683 0.683 Fat 5.280 0.145 0.105 9.568 Fat 5.280 0.145 0.105 9.568 Muscle 52.729 0.242 1.739 0.575 Muscle 52.729 0.242 1.739 0.575

TheThe antenna antenna under under test test (AUT) (AUT) was was carried carried out out by by placing placing the the device device on on the the phantom phantom model.model. A A human human torso torso phantom phantom was was ﬁlled filled with with a solution a solution that that mimics mimics the the electromagnetics electromagnet- propertiesics properties [7] of [7] the of dissipativethe dissipative human human body body as shownas shown in Figurein Figure4a. 4a. In In this this study, study, the the solution consisted of 79.7% of deionized 0.25% of water sodium chloride, 16% of Triton solution consisted of 79.7% of deionized 0.25% of water sodium chloride, 16% of Triton X- X-100 (polyethylene glycol mono phenyl ether), 4% of diethylene glycol butyl ether (DGBE), 100 (polyethylene glycol mono phenyl ether), 4% of diethylene glycol butyl ether (DGBE), 0.05% of boric acid, and was ﬁlled to the phantom in Figure4a. A 3D view of a numerical 0.05% of boric acid, and was filled to the phantom in Figure 4a. A 3D view of a numerical phantom model to characterize the performance of wearable antennas within proximity phantom model to characterize the performance of wearable antennas within proximity to a human body is sketched in Figure4b and the model consists of a three-layer block of to a human body is sketched in Figure 4b and the model consists of a three-layer block of 1 mm of dry skin, 3 mm of fat, and 40 mm muscle, using the data listed in Table4. 1 mm of dry skin, 3 mm of fat, and 40 mm muscle, using the data listed in Table 4.

FigureFigure 4. 4.(a ()a A) A real real image image of of the the human human torso torso phantom phantom model; model; (b ()b A) A 3D 3D view view of of a a layered layered numerical numerical representationrepresentation of of human human tissue. tissue.

TheThe radiation radiation efﬁciency efficiency of of an an antenna antenna (η ()η is) is explained explained as as the the fraction fraction of of the the radiated radiated powerpowerP radPradbetween between the the input input power powerP Pinin::

η = Prad/Pin = Prad/Prad + Ploss = Rrad/Rrad + Rloss, (1) η = Prad/Pin = Prad/Prad + Ploss = Rrad/Rrad + Rloss, (1)

Ploss is the lost power, and Rrad and Rloss are the radiated and loss resistance. Different P is the lost power, and R and R are the radiated and loss resistance. Different methodsloss could be used for measuringrad η,loss depending on the antenna design and size in the methods could be used for measuring η, depending on the antenna design and size in the study; the cylindrical wheeler cap technique was used [62–64], as displayed in Figure 5 by study; the cylindrical wheeler cap technique was used [62–64], as displayed in Figure5 by following the below equations for different antenna lengths. following the below equations for different antenna lengths. Short antennas (L ≤ λ/10, with L the antenna’s size and λ the operational wave- Short antennas (L ≤ λ/10, with L the antenna’s size and λ the operational wavelength): length): 2 2 2 η = [S ] − [S2 ] /12 − [S ] 2, (2) 11η wc= [S11wc] −11 [Sfs 11fs] /1 − [S1111fsfs] , (2)

ModerateModerate length length antennas: antennas:

η = 1 − ((1 − S11fs)(1 + S11wc)/(1 + S11fs)(1 − S11wc)), (3)

The radiation resistance is zero with the cap on, and the antenna reﬂection coefﬁcient S was measured and referred to as 11wc. With the cap off the radiation resistance is that of Sensors 2021, 21, x FOR PEER REVIEW 10 of 28

Sensors 2021, 21, 4934 10 of 28 η = 1 − ((1 − S11fs)(1 + S11wc)/(1 + S11fs)(1 − S11wc)), (3)

The radiation resistance is zero with the cap on, and the antenna reflection coefficient was measured and referred to as S11wc. With the cap off the radiation resistance is that of the antenna radiating into free space, and the antenna reﬂection coefﬁcient was measured the antenna radiating into free space, and the antenna reflection coefficient was measured and referred to as S11fs. and referred to as S11fs.

Figure 5. Laboratory model of a cylindrical wheeler cap: (a) Cap-off; (b) Cap-on. Figure 5. Laboratory model of a cylindrical wheeler cap: (a) Cap-off; (b) Cap-on.

3.3. Results Results and and Discussions Discussions 3.1.3.1. Stability Stability of of Printing Printing CNTs/CNC CNTs/CNC Dispersions Dispersions ShelfShelf life life of of CNTs-based CNTs-based inks inks determines determines the the practical practical use use in in large large scale scale applications applications suchsuch as as electronic electronic and and smart smart wearables. wearables. The The main main concern concern for for carbon-based carbon-based inks inks is is the the difficultydifficulty in maintainingin maintaining stability stability of the of suspension the suspension and agglomeration and agglomeration of particles of throughparticles time.through Several time. strategies Several strategies have been have employed been empl to increaseoyed to the increase stability the of stability the inks, of suchthe inks, as introducingsuch as introducing functional functional solvents to solvents maintain to effective maintain and effective constant and dispersion constant [dispersion65], applying [65], aapplying powerful sonicationa powerful process sonication [53], and/orprocess adding [53], and/or dispersing adding and dispersing stabilizing and agents stabilizing [66] as copolymers.agents [66] as When copolymers. screen-printing When screen-printin is in place forg creatingis in place sensory for creating surfaces, sensory the particlesurfaces, formation,the particle clogging formation, of meshes, clogging and of themeshes, aggregation and the of aggregation inks are major of inks process-related are major process- chal- lengesrelated to achievechallenges successful to achieve patterns. successful Efficient patte andrns. homogeneous Efficient and dispersion homogeneous of ink dispersion that can overcomeof ink that the can stability overcome issues the for inksstability could issues result for in successfulinks could printing result in and successful improved printing shelf life.and CNTs improved tend toshelf agglomerate life. CNTs in tend aqueous to agglom solutionserate duein aqueous to their solutions high surface due energy to their and high inadequatesurface energy chemical and affinityinadequate with chemical the dispersing affinity medium; with the thus, dispersing preventing medium; the formation thus, pre- ofventing large bundles the formation is difficult of large to control. bundles In thisis difficult work, theto control. colloidal In stability this work, of CNTs/CNC the colloidal with a weight ratio of 1:1, 1.5:1.5, and 0.5:1 was investigated by sedimentation tests, and stability of CNTs/CNC with a weight ratio of 1:1, 1.5:1.5, and 0.5:1 was investigated by photos of dispersions from 1 hour to 1 week (left to right)were presented in Figure6. sedimentation tests, and photos of dispersions from 1 hour to 1 week (left to right)were Additionally, the CNTs dispersion stability in DI-H2O was also explored and compared presented in Figure 6. Additionally, the CNTs dispersion stability in DI-H2O was also ex- with different compositions of CNTs/CNC inks. It is evident that CNTs/CNC exhibited plored and compared with different compositions of CNTs/CNC inks. It is evident that excellent stability even after a week, whereas the pristine CNTs have poor dispersion CNTs/CNC exhibited excellent stability even after a week, whereas the pristine CNTs stability in DI-H2O. Although not presented here, after several months, these inks were still have poor dispersion stability in DI-H2O. Although not presented here, after several in a good stable condition without any sedimentation. This was attributed to the fact that months, these inks were still in a good stable condition without any sedimentation. This the -OH groups of CNC, creating the hydrogen bonding with water molecules, enabled the was attributed to the fact that the -OH groups of CNC, creating the hydrogen bonding CNC to be effectively dispersed in an aqueous medium. Peculiarly, sulfuric acid-treated with water molecules, enabled the CNC to be effectively dispersed in an aqueous me- CNC presented here has negatively charged sulphate half-ester groups and promoted colloidaldium. Peculiarly, stability [67 sulfuric]. Additionally, acid-treated the CNC amphiphilic presented nature here ofhas CNC negatively with the charged nonpolar sul- groupsphate hadhalf-ester an affinity groups to interact and promoted with hydrophobic colloidal CNTs stability and finally[67]. Additionally, enabled the bridging the am- inphiphilic between nature nanotubes of CNC to create with athe network. nonpolar A similargroups observationhad an affinity was to performed interact with by Asadi hydro- etphobic al. for CNTs/CNCCNTs and finally dispersions enabled with the abridging weight ratioin between of 1:1, 1:2,nanotubes and 1:4 to maintaining create a network. their colloidalA similar stability observation up to was six months performed for reinforcingby Asadi etthe al. carbon-fibre-reinforced-polymerfor CNTs/CNC dispersions with a compositesweight ratio [52 of]. 1:1, 1:2, and 1:4 maintaining their colloidal stability up to six months for reinforcingMany carbon-based the carbon-fibre-reinforced-polymer inks were employed by composites hazardous solvents[52]. that were hard to manage for environmental concerns. The most prominent and widely used solvents were toxic as N, N-dimethylformamide (DMF) [68], absorbable through the skin as dimethyl sulfoxide (DMSO) [69], and carcinogenic as N-methyl-2-pyrrolidone (NMP) [70]. During the processing of solvent-based ink, the evaporation of these compounds causes a diversity Sensors 2021, 21, x FOR PEER REVIEW 11 of 28

Many carbon-based inks were employed by hazardous solvents that were hard to Sensors 2021, 21, 4934 manage for environmental concerns. The most prominent and widely used solvents11 ofwere 28 toxic as N, N-dimethylformamide (DMF) [68], absorbable through the skin as dimethyl sulfoxide (DMSO) [69], and carcinogenic as N-methyl-2-pyrrolidone (NMP) [70]. During the processing of solvent-based ink, the evaporation of these compounds causes a diver- of health, safety, and air contamination problems. However, the CNTs/CNC inks referred sity of health, safety, and air contamination problems. However, the CNTs/CNC inks re- to in this study were promoted to be one of the very few aqueous ink compositions without ferred to in this study were promoted to be one of the very few aqueous ink compositions any surfactant or dispersants and can be referred to as environmentally-friendly for the without any surfactant or dispersants and can be referred to as environmentally-friendly printing industry. for the printing industry.

Figure 6. Dispersion and stability state of the CNTs in water with and without CNC. The ink com- Figure 6. Dispersion and stability state of the CNTs in water with and without CNC. The ink positions are represented from CNTs/CNC—0.5:1; CNTs/CNC—1:1; CNTs/CNC—1.5:1.5, left to compositions are represented from CNTs/CNC—0.5:1; CNTs/CNC—1:1; CNTs/CNC—1.5:1.5, left right. to right.

3.2.3.2. Rheology Rheology of of CNTs/CNC CNTs/CNC Ink Ink and and the the Quality Quality of of Printed Printed Patterns Patterns ForFor a a successful successful print print pattern pattern created created using using screen screen printing, printing, especially especially when when thinner thinner lineslines and and sharper sharper edges edges were were in in place, place, the the rheology rheology of of the the ink ink plays plays a a critical critical role. role. Many Many factorsfactors such such as as the the solvent solvent system system and and solid solid content content influenced influenced the the dispersion dispersion degree degree of of particlesparticles in in ink, ink, resulting resulting in in different different rheological rheological behaviours. behaviours. Nowadays, Nowadays, inkjet inkjet printing printing isis one one of of the the methods methods widely widely used used in in printing printing electronic electronic circuitry. circuitry. The The rheology rheology of of the the designeddesigned inkink has has to to be be formulated formulated to to avoid avoid nozzle nozzle clogging clogging with with finely finely tuned tuned particle particle sizes.sizes. Thus, Thus, the the range range of of printable printable viscosities viscosities was was restricted restricted to to 1–10 1–10 mPa mPa·s·s and and the the particle particle sizesize must must be be smaller smaller than than the the nozzle nozzle output output diameter diameter [ 71[71].]. On On that that note, note, these these restrictions restrictions requirerequire lowlow solidsolid content and and finely finely tuned tuned particle particle sizes sizes which which may may result result in limited in limited ma- materialterial availability. availability. Additionally, Additionally, with with their their scalability scalability and andadequate adequate printing printing times times com- comparedpared to inkjet to inkjet printing, printing, screen-printing screen-printing could could lead leadthe way the waytowards towards printed printed smart smart wear- wearables.ables. Since Since high high solid solid content content is correlated is correlated with with the theincreased increased electrical electrical conductivity, conductivity, the theink ink formulation formulation with with high high solid solid content content reached reached to an to elevated an elevated viscosity viscosity regime regime of 10 of2 to 10102 6 tomPa·s 106 [71].mPa ·Thes [71 CNTs/CNC]. The CNTs/CNC ink compositions ink compositions with varying with weight varying ratios weight were studied ratios wereby monitoring studied by their monitoring viscosities their by viscositiesa plate rheometer by a plate and rheometer it was reported and it how was the reported CNTs’ howcontent the CNTs’and the content properties and theof CNC properties influenced of CNC the influencedrheological the profile rheological of corresponding profile of correspondinginks. Flow behaviour inks. Flow of CNTs/CNC behaviour ofinks CNTs/CNC with different inks withCNTs different and CNC CNTs concentrations and CNC concentrationswere tested at wereambient tested temperature. at ambient temperature.Figure 7 showed Figure the7 complexshowed theviscosity complex as viscositya function asof a functionfrequency of frequencyfor CNTs/CNC—0.5:0.5, for CNTs/CNC—0.5:0.5, CNTs/CNC—1:1, CNTs/CNC—1:1, CNTs/CNC—1.5:1.5, CNTs/CNC—1.5:1.5, and andCNTs/CNC—0.5:1 CNTs/CNC—0.5:1 inks inks as listed as listed in Table in Table 5. 5.

TableTable 5. 5.Complex Complex viscosity viscosity values values at at different different angular angular frequencies. frequencies.

Complex Viscosity Viscosity (Pa (Pa/s)·s) InkInk Code Code 0.1 0.1 (rad/s) (rad/s) 1.0 1.0 (rad/s) (rad/s) 10 10 (rad/s) (rad/s) 100 100 (rad/s) (rad/s) CNTs/CNC—0.5:0.5 47.3 47.3± ±0.2 0.2 6.4 6.4± ±0.2 0.2 0.84 0.84± ±0.3 0.3 0.6 0.6± ±0.1 0.1 CNTs/CNC—1:1 141 ± 6.8 24.1 ± 8.5 3.05 ± 1.2 0.6 ± 0.2 CNTs/CNC—1.5:1.5 783 ± 7.1 132 ± 1.0 17.3 ± 0.2 2.2 ± 0.3 CNTs/CNC—0.5:1 81.6 ± 6.9 12.2 ± 0.7 1.6 ± 0.1 0.6 ± 0.1 Sensors 2021, 21, x FOR PEER REVIEW 12 of 28

CNTs/CNC—1:1 141 ± 6.8 24.1 ± 8.5 3.05 ± 1.2 0.6 ± 0.2 Sensors 2021, 21, 4934 12 of 28 CNTs/CNC—1.5:1.5 783 ± 7.1 132 ± 1.0 17.3 ± 0.2 2.2 ± 0.3 CNTs/CNC—0.5:1 81.6 ± 6.9 12.2 ± 0.7 1.6 ± 0.1 0.6 ± 0.1

FigureFigure 7.7.Complex Complex viscosityviscosity ofof CNTs/CNC-basedCNTs/CNC-based inksinks versusversus angularangular frequency.frequency.

− AllAll inksinks exhibitedexhibited a shear thinning thinning be behaviourhaviour at at a afrequency frequency regime regime of of 10 10−1 to1 10to2 10rad/s.2 rad/s. Hence, Hence, higher higher CNTs CNTs concentrations concentrations caused caused a viscosity a viscosity increase increase which which was was at- attributedtributed to to the the enhanced enhanced Van Van der der Waals Waals forc forceses between between the thenanotubes nanotubes [72]. [ 72All]. ink All com- ink compositionspositions showed showed a gradual a gradual decrease decrease in complex in complex viscosity viscosity since since shear shear forces forces assisted assisted the thebreaking breaking of ofCNTs CNTs clus clustersters and and aligned aligned particles particles along along the the flow flow direction; direction; consequently, slidingsliding ofof smallersmaller CNTsCNTs clustersclusters andand aa shear-thinningshear-thinning profileprofile couldcould bebe observedobserved [[73–75].73–75]. AsAs aa criticalcritical requirement,requirement, althoughalthough screen-printingscreen-printing inksinks hadhad aa higherhigher viscosityviscosity regimeregime comparedcompared toto inkjet printing inks, inks, still still the the viscosity viscosity should should be be low low enough enough to toenable enable the the ink inkto pass to pass through through the thescreen screen meshes meshes with with the thesqueegee squeegee but buthigh high enough enough to allow to allow the con- the conductiveductive ink ink to cover to cover the the overall overall pattern pattern [76]. [76 Th]. Thee variables variables of ofthe the printing printing process process such such as asmesh mesh size, size, squeegee squeegee movement movement should should be be well well correlated correlated with with the the complex complex viscosity viscosity of ofthe the CNTs/CNC CNTs/CNC formulated formulated inks inks that thatcause cause rheological rheological changes changes during during the flow. the In flow. manual In manualprocesses processes like screen like printing screen printing in this study, in this the study, control the on control the ink on injection the ink injection rate is not rate well- is notcontrolled well-controlled to match to with match complex with complexviscosity viscosityranges. Nevertheless, ranges. Nevertheless, the physical the character- physical characteristicsistics of screen-printed of screen-printed patterns patterns were also were obse alsorved observed to assess to the assess quality thequality of printing of print- as a ingmeasure. as a measure. Several trials Several by trialstuning by different tuning different ink compositions ink compositions and mesh and sizes mesh of the sizes print of the print frame were conducted to achieve sharp and clean corners and lines onto rough frame were conducted to achieve sharp and clean corners and lines onto rough surfaces. surfaces. CO and CO–PES fabrics were studied in terms of water affinity, where CO–PES CO and CO–PES fabrics were studied in terms of water affinity, where CO–PES fabrics fabrics presented a lower water absorbance than CO fabric. Initially, the mesh size of the presented a lower water absorbance than CO fabric. Initially, the mesh size of the print print frames such as 55T and 90T was selected by keeping mind of the lowest viscosity frames such as 55T and 90T was selected by keeping mind of the lowest viscosity ink com- ink composition as CNTs/CNC—0.5:0.5. As depicted in Figure S1, for both PICA and position as CNTs/CNC—0.5:0.5. As depicted in Figure S1, for both PICA and LOOP de- LOOP designs, 55T frames, which have larger pore sizes, were not compatible with the low signs, 55T frames, which have larger pore sizes, were not compatible with the low viscos- viscosity (47.3 ± 0.2 Pa·s at 0.1 rad/s) inks. ity (47.3 ± 0.2 Pa·s at 0.1 rad/s) inks. Additionally, the high water affinity of CO fabrics also caused the spread of the ink, Additionally, the high water affinity of CO fabrics also caused the spread of the ink, particularly around sharp corners and vertical lines, and damaged the quality. Figure particularly around sharp corners and vertical lines, and damaged the quality. Figure S2a S2a shows that the screen printing success on CO fabric was improved with very few scatteredshows that regions the screen on theprinting prints success using on a 90T CO framefabric was in a improved circular pattern. with very However, few scattered the sheetregions resistance on the prints of the using CNTs/CNC—0.5:0.5 a 90T frame in a ci patternrcular waspattern. very However, high as 75.5 the Ωsheet/sq resistance after five layersof the andCNTs/CNC—0.5:0.5 concluded as insufficient pattern was for an very antenna high as application. 75.5 Ω/sq Afterward,after five layers in an and attempt con- tocluded achieve as ainsufficient higher conductivity, for an antenna CNTs/CNC—1:1 application. Afterward, composition in was an attempt used to exploreto achieve the a higherhigher concentrationconductivity, ofCNTs/CNC—1:1 CNTs on the overall composit resistanceion was of theused printed to explore patterns. the higher con- centrationThe screen of CNTs printing on the process overall was resistance performed of the by printed a 55T frame patterns. based on the feedback of early investigations since a larger pore size would help more particles to accumulate and adhere onto the fabric surface effectively. Hence, CNTs/CNC—1:1 sheet resistance was measured as 45 Ω/sq after five layers without any loss in the print quality, as shown in Figure S2b. As a result, 55T was chosen as the optimal mesh size for formulated CNTs/CNC

ink compositions. For an effective antenna performance, the sheet resistance of patterned Sensors 2021, 21, x FOR PEER REVIEW 13 of 28

The screen printing process was performed by a 55T frame based on the feedback of early investigations since a larger pore size would help more particles to accumulate and adhere onto the fabric surface effectively. Hence, CNTs/CNC—1:1 sheet resistance was measured as 45 Ω/sq after five layers without any loss in the print quality, as shown in Figure S2b. As a result, 55T was chosen as the optimal mesh size for formulated CNTs/CNC ink compositions. For an effective antenna performance, the sheet resistance of patterned surfaces should be lower than the reported values. On that note, CNTs content was also increased in ink formulations, and the resistance change was monitored on printed PICA and LOOP antenna patterns by CNTs/CNC—1:1 and CNTs/CNC—1.5:1.5. When the CNTs concentration was increased up to 1.5 wt.%, the pores of the meshes were Sensors 2021, 21, 4934 relatively clogged, as presented in Figure 8b due to higher solid content than 1:1 ink13of com- 28 position. However, for both of the 1:1 and 1.5:1.5 ink compositions, the overall quality of the printed pattern was good, as shown in Figure 8a, even in fine details such as sharp corners and clean lines on CO fabric. When the ink viscosity was tuned by increasing the surfacesCNTs up should to 1 wt.%, be lower promising than the print reported quality values. was achieved On that note,on CO CNTs fabric, content as illustrated was also in increasedFigure 8c. in ink formulations, and the resistance change was monitored on printed PICA and LOOPAdditionally, antenna patternsthe CNTs/CNC—1:1 by CNTs/CNC—1:1 had a viscosity and CNTs/CNC—1.5:1.5. of 141 ± 6.8 Pa·s at When 0.1 rad/s, the CNTsshowed concentration adequate shear was increased flow properties up to 1.5 for wt.%, screen the printing pores of application the meshes with were comparable relatively clogged, as presented in Figure8b due to higher solid content than 1:1 ink composition. viscosities from others [76–78]. For CNTs/CNC—0.5:1, the viscosity was 81.6 ± 6.9 Pa·s at However, for both of the 1:1 and 1.5:1.5 ink compositions, the overall quality of the printed 0.1 rad/s exhibiting similar flow properties with CNTs/CNC—0.5:0.5 as well. In this vis- pattern was good, as shown in Figure8a, even in ﬁne details such as sharp corners and cosity regime, CO–PES fabrics were preferred as the textile substrates with lower water clean lines on CO fabric. When the ink viscosity was tuned by increasing the CNTs up to absorbency rather than employing CO fabric. Even though there were small casualties on 1 wt.%, promising print quality was achieved on CO fabric, as illustrated in Figure8c. the CO–PES fabric, homogeneous print quality was obtained in Figure 9.

Figure 8. (a) Optic microscope images of CNTs/CNC—1.5:1.5 ink printed PICA pattern on CO fabric Figure 8. (a) Optic microscope images of CNTs/CNC—1.5:1.5 ink printed PICA pattern on CO fabric from different points; (b) A real picture of clogging meshes after using CNTs/CNC—1.5:1.5 ink; (c) from different points; (b) A real picture of clogging meshes after using CNTs/CNC—1.5:1.5 ink; Real image of CNTs/CNC—1:1 ink printed PICA pattern on CO fabric; (d) Cross-sectional optic c d ( )microscope Real image images of CNTs/CNC—1:1 of CNTs/CNC—1.5:1.5 ink printed ink PICA printed pattern CO fabric. on CO fabric; ( ) Cross-sectional optic microscope images of CNTs/CNC—1.5:1.5 ink printed CO fabric.

Additionally, the CNTs/CNC—1:1 had a viscosity of 141 ± 6.8 Pa·s at 0.1 rad/s, showed adequate shear ﬂow properties for screen printing application with comparable viscosities from others [76–78]. For CNTs/CNC—0.5:1, the viscosity was 81.6 ± 6.9 Pa·s at 0.1 rad/s exhibiting similar ﬂow properties with CNTs/CNC—0.5:0.5 as well. In this viscosity regime, CO–PES fabrics were preferred as the textile substrates with lower water absorbency rather than employing CO fabric. Even though there were small casualties on the CO–PES fabric, homogeneous print quality was obtained in Figure9.

Sensors 2021, 21, 4934 14 of 28 Sensors 2021, 21, x FOR PEER REVIEW 14 of 28

Figure 9. LOOP pattern images: (a,b) CNTs/CNC—1:1 ink printed on CO–PES fabric; (c,d) Figure 9. LOOP pattern images: (a,b) CNTs/CNC—1:1 ink printed on CO–PES fabric; (c,d) CNTs/CNC—0.5:1 printed CO–PES fabric, where red circles pointed out irregularities associated CNTs/CNC—0.5:1with ink concentration. printed CO–PES fabric, where red circles pointed out irregularities associated with ink concentration.

3.3.3.3. ElectricalElectrical PropertiesProperties ofof Printed Printed Surfaces Surfaces ConsistentConsistent electrical electrical conduction conduction is is one one of of the the most most critical critical requirements requirements for anforeffective an effec- antennative antenna performance performance because because it impacts it impacts the return the return loss and loss radiation and radiation efficiency. efficiency. These materialThese material qualities qualities played played an essential an essential role in role producing in producing antenna antenna patterns patterns regardless regardless of theof the applied applied printing printing techniques. techniques. The The printed printed surface surface must must be be thick thick enough enough to to assure assure goodgood conductivity conductivity and and low low ohmic ohmic losses, losses, usually usually achieved achieved by by integrating integrating metallic metallic or or other other particlesparticles as as the the conductive conductive medium. medium. However,However, using using metallic metallic inks inks is is not not so so cost-effective cost-effective andand limits limits the the useuse inin textiletextile applicationsapplications byby bringingbringing additionaladditional costscosts andand oxidationoxidation issues.issues. TheThe thickness thickness associated associated withwith the the number number of of printed printed layers layers determines determines the the conductivity conductivity andand fabric fabric properties, properties, including including comfort, comfort, weight, weight, and and outcomes, outcomes, including including cost cost and and process pro- time.cess time. In this In study, this study, we examined we examined the sheet the resistancesheet resistance and electrical and electrical conductivity conductivity variation var- asiation a function as a function of the printed of the printed number number of layers of forlayers both for PICA both and PICA LOOP and patterns.LOOP patterns. AsAs discusseddiscussed inin detaildetail here,here, aa single single layer layer of of CNTs/CNC CNTs/CNC inkink waswas notnot sufficientsufficientto to achieveachieve the the required required low low sheet sheet resistivity; resistivity; thus, thus, stacking stacking multilayers multilayers together together and and doping doping themthem could could be be suggested suggested to to achieve achieve lower lower sheet sheet resistance. resistance. Figure Figure 10 10aa presents presents the the sheet sheet resistanceresistance of of CNTs/CNC CNTs/CNC 1:1 1:1 and and CNTs/CNC—1.5:1.5 CNTs/CNC—1.5:1.5 printedprinted PICA PICA pattern pattern on on CO CO fabrics fabrics decreaseddecreased byby ~95%~95% from 405 ±± 5858 ΩΩ/sq/sq to to22 22± 1.9± 1.9Ω/sqΩ and/sq andfrom from 233 ± 233 40.2± Ω40.2/sq toΩ /sq15 ± to2.0 15Ω/sq± 2.0 withΩ/sq an increase with an increasein the number in the of number layers offrom layers 1 to from 5, respectively. 1 to 5, respectively. The rule Theof thumb rule offor thumb decreasing for decreasing extrinsic sheet extrinsic resistance sheet resistancevalues as valueslow as as15 low± 2.0 as Ω 15/sq± is 2.0to increaseΩ/sq is the to increasethickness the of thickness CNTs using of CNTs layer-by-layer using layer-by-layer piling, and piling, doping and the doping CNTs can the CNTsallow canthe extrin- allow thesic extrinsicsheet resistance sheet resistance values to valuesbe reduced to be to reduced as low toas as15 low± 2.0 as Ω 15/sq.± After2.0 Ω the/sq. first After printed the firstlayer, printed the decrease layer, the in decreasesheet resistivity in sheet for resistivity both PICA for and both LOOP PICA designs and LOOP wasdesigns attributed was to attributedthe less void tothe content less void between content the betweenconductive the layer conductive and the layer fabric and surface the fabric and higher surface CNTs and higherdeposition CNTs [79,80]. deposition After [79 ,applying80]. After the applying second the layer, second when layer, the when interconnecting the interconnecting CNTs CNTsformed formed a conductive a conductive pathway pathway for forelectrons electrons that that promoted promoted a alarge large number ofof electronselectrons flowing,flowing, lower lower sheet sheet resistance resistance was was recorded. recorded. The The effect effect of theof the number number of layersof layers increasing, increas- resultinging, resulting in a decrease in a decrease in sheet in sheet resistance, resistan once, the on conduction the conduction mechanism mechanism of thin of films thin films was alsowas noted also noted [81–83 [81–83].].

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Figure 10c demonstrated the sheet resistance changes of LOOP design depending on the number of layers. The same trend was observed in the LOOP design; after a big jump in conductivity level, slight increase in conductivity was noted by increasing print layers. Utilizing CNTs/CNC—1:1 and CNTs/CNC—0.5:1 for LOOP patterns with 5 printed layers reduced the sheet resistance from 11.5 ± 1.5 kΩ/sq to 176 ± 37 Ω/sq and from 3.7 ± 2.7 kΩ/sq to 326 ± 78 Ω/sq, respectively. The penetration of ink into the fabric was affected by a range of material factors, such as the surface tension, viscosity of the ink, the volatility of ink, and the wettability of ink to the fabric [84]. All these parameters also impacted on ink deposition and surface roughness [85]. Thus, the difference in sheet resistance between the LOOP and PICA patterns could be attributed to the nature of the fabric surface and the low dispersion absorbency of CO–PES fabric compared to CO fabric. The electrical conductivity was calculated following the equation where σ, ρ, t are the electrical conductivity, resistivity, and thickness of layers, respectively.

ρ = (π/ln2)(V/I)t, (4)

σ = 1/ρ, (5) As shown in Figure 10b, the conductivity of printed PICA patterns increased with the number of layers from 12.3 ± 2.2 S/cm to 27.6 ± 2.4 S/cm up to the 4th layer and then slightly decreased to 20.4 ± 0.6 S/cm in CNTs/CNC—1:1. In CNTs/CNC—1.5:1.5 printed PICA pattern, the conductivity of printed designs was noted as 8.6 ± 2.0 S/cm at the ﬁrst layer, then reached to c.a 20 ± 1.4 S/cm. For CNTs/CNC—1:1, surface conductivities measured on LOOP patterns were recorded as 0.40 ± 0.20 S/cm, 2.1 ± 1.2 S/cm, 3.5 ± 0.3 S/cm, and 5.5 ± 0.7 S/cm up to the 4th layer, and then reduced to 5.2 ± 0.2 S/cm at the ﬁfth layer as shown in Figure 10d. The conductivity values obtained from the CNTs/CNC—0.5:1 were higher than the CNTs/CNC—1:1 dispersion that was 4.3 ± 0.7 S/cm, 5.4 ± 1.6 S/cm 7.1 ± 2.3 S/cm up to the 3rd layer respectively, and then decreased 6.2 ± 1.8 S/cm and 5.3 ± 1.1 S/cm in 4th and 5th layers, as shown in Figure 10d. The decrease in conductivity of both PICA and LOOP patterns after applying the 5th layer can be explained by the increment of thickness while the resistance remains relatively constant according to the formulation σ = 1/ρ. Liu et al. reported the electrical conductivity of a 15 wt.% carbon black-based ink printed sample as 2.15 × 104 S/m [86]. As in our study, they reported that the sheet resistance of printed patterns is in the tendency to decrease with the increase in pattern thickness, which causes a reduction in the electrical conductivity. In addition, as seen in Figure 10b,d, the electrical conductivity values of the samples obtained from the ink with less CNTs ratio were higher for both PICA and LOOP patterns. It was ascribed to the fact that the ink containing less solid content created a thinner layer for LOOP and PICA patterns. As mentioned in Sections 2.4.1 and 2.4.2, the average thickness of the CNTs/CNC—1:1 printed PICA pattern was 25 ± 2.9 µm, while the CNTs/CNC— 1.5:1.5 printed PICA pattern had 35 ± 2.1 µm. CNTs/CNC—0.5:1 and CNTs/CNC—1:1 printed LOOP sample average thicknesses were 6.4 ± 0.7 µm and 14 ± 5.2 µm, respectively. Consequently, the thickness of samples prepared with low solid content inks was low and according to Equations (4) and (5), when the thickness decreases, the electrical conductivity increases. It is important to note that due to lower water absorption of the CO–PES fabric, the water based inks were not penetrated into the fabric thoroughly as in the CO fabric. Hence, the thickness of the printed layer was thinner than in CO fabric samples [87]. As a result, the low solid content samples which were CNTs/CNC—0.5:1 for LOOP and CNTs/CNC—1:1 for PICA pattern had higher electrical conductivity compared to high solid content inks which were CNTs/CNC—1:1 for LOOP and CNTs/CNC—1.5:1.5 for PICA. Sensors 2021, 21, 4934 16 of 28 Sensors 2021, 21, x FOR PEER REVIEW 16 of 28

FigureFigure 10. Resistance and and electrical electrical conductivity conductivity of CNTs/CNC of CNTs/CNC printed printed samples samples concerning concerning the number the number of printed of printed layers; layers;(a,b) CNTs/CNC—1:1 (a,b) CNTs/CNC—1:1 and CNTs/CNC—1.5:1.5 and CNTs/CNC—1.5: printed1.5 printed PICA PICA designs, designs, (c,d )(c CNTs/CNC—0.5:1,d) CNTs/CNC—0.5:1 and and CNTs/CNC—1:1 CNTs/CNC— 1:1 printed LOOP antenna as a function of deposited conductive layers. printed LOOP antenna as a function of deposited conductive layers. 3.4. Fabric Care: Washing and Ironing Tests 3.4. Fabric Care: Washing and Ironing Tests In the context of wearable electronics, apart from intrinsic material properties includ- In the context of wearable electronics, apart from intrinsic material properties in- ing electrical conductivity, fabric qualities have a crucial role in final application. In this cluding electrical conductivity, fabric qualities have a crucial role in ﬁnal application. In study, the textiles antennas were tested in terms of washing and ironing stability. Figure 11 this study, the textiles antennas were tested in terms of washing and ironing stability. displays the deprivation in conductivity as a function of four wash cycles in 5 layers Figure 11 displays the deprivation in conductivity as a function of four wash cycles in CNTs/CNC—1:1 and CNTs/CNC—1.5:1.5 printed PICA samples. Even though the 5 layers CNTs/CNC—1:1 and CNTs/CNC—1.5:1.5 printed PICA samples. Even though washed samples still were able to conduct electrons, after four wash cycles, the surface the washed samples still were able to conduct electrons, after four wash cycles, the surface resistivity of CNTs/CNC—1:1 increased from 5.9 ± 0.4 Ω to 52.9 ± 17.3 Ω. The same de- resistivity of CNTs/CNC—1:1 increased from 5.9 ± 0.4 Ω to 52.9 ± 17.3 Ω. The same decrease in conductivity was also noted for CNTs/CNC—1.5:1.5, the resistivity was increase in conductivity was also noted for CNTs/CNC—1.5:1.5, the resistivity was increased creased from 3.3 ± 0.5 Ω to 112 ± 39 Ω. This distress after each wash cycle can be attributed from 3.3 ± 0.5 Ω to 112 ± 39 Ω. This distress after each wash cycle can be attributed to toaccumulation accumulation of surfactantsof surfactants through through the cracks the cracks on the on surface, the surface, and damaging and damaging layer bonding layer bondingand CNTs and conductive CNTs conductive pathway pathway due to mechanicaldue to mechanical friction friction [80]. The [80]. deterioration The deterioration of the ofprinted the printed layers layers causes causes an increase an increase in its in resistance its resistance to a to value. a value. Several Several studies studies reported reported a apossible possible enhancement enhancement of of electrical electrical conductivityconductivity withwith heatheat post treatment that eliminates thethe surfactants surfactants [88]. [88]. Thus, Thus, between between each each wash wash cycle, cycle, the thetextile textile antennas antennas were were ironed, ironed, but therebut there was wasno noticeable no noticeable change change noted. noted. In the In literature, the literature, the surfaces the surfaces of such of such samples samples are usuallyare usually coated coated with witha polymer a polymer [17] or [17 plastisol] or plastisol ink which ink which has PVC has particles PVC particles in it as in a itplas- as a ticizerplasticizer [89] [that89] thatmake make the thestructure structure more more durable durable in repeated in repeated wash. wash. This This study study reports reports a polymera polymer free free materials materials approach approach without without any any coatings; coatings; the the change change in in resistance resistance after after the the firstﬁrst wash wash only increased fromfrom 5.95.9± ± 0.4 toto 7.27.2 ±± 0.6 ΩΩ and from 3.23.2 ±± 0.4 to 5.1 ± 1.91.9 ΩΩ forfor CNTs/CNC—1:1CNTs/CNC—1:1 and and CNTs/CNC—1.5:1.5, CNTs/CNC—1.5:1.5, respectively. respectively.

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Figure 11. Resistance change after 4 washing and ironing cycles; refers to 5 layers CNTs/CNC—1:1 Figureand CNTs/CNC—1.5:1.5 11. Resistance change inks after screen 4 washing printed samples. and ironing cycles; refers to 5 layers CNTs/CNC—1:1 and CNTs/CNC—1.5:1.5 inks screen printed samples. 3.5. Characterisation of Antenna Parameters 3.5. CharacterisationTheir EM characteristics of Antenna areParameters analysed using simulated and measured data. Return lossTheir (S11), EM in combination characteristics with are far-field analysed properties using and simulated efficiency and values, measured demonstrates data. Return the lossresponsiveness (S11), in combination of these prototypes with far-field in wireless properties body and area efficiency networks values, (WBAN) demonstrates and wireless the responsivenesspersonal area networks of these prototypes (WPAN). in wireless body area networks (WBAN) and wireless personal area networks (WPAN). 3.5.1. Impedance Bandwidth 3.5.1. ImpedanceFigure 12 presents BandwidthS11 computational and experimental results for the PICA design in both scenarios, off- and on-body settings. Experimental measurements were carried out for bothFigure concentrations 12 presents of 1.0 S11 wt.% computational and 1.5 wt.%. and Results experimental suggest the results ultrawideband for the PICA behaviour design in bothof the scenarios, antenna off- with and a bandwidth on-body ofsettings. 8.25 GHz Experimental and 8.5 GHz measurements in the free-space were scenario, carried for out for1.0 both wt.% concentrations and 1.5 wt.% cases,of 1.0 respectively;wt.% and 1.5 while wt.%. for Results the on-body suggest case, the returnultrawideband losses less be- haviourthan 10 of dB the are antenna slightly narrowerwith a bandwidth in range, from of 8.25 3.5–11.5 GHz GHzand at8.5 lower GHz CNTsin the concentration free-space scenario,and widenedfor 1.0 wt.% from 2.5and to 1.5 11.5 wt.% GHz cases, by increased respectively; CNTs concentration. while for the Measurement on-body case, results return lossesdo not less possess than the10 dB multi-frequency are slightly narrower resonance in characteristic range, from due 3.5–11.5 to the gradualGHz atimpedance lower CNTs concentrationmatching across and thewidened frequency from band, 2.5 to so 11.5 the GHz resonance by increased effect revealed CNTs concentration. a traveling wave Meas- urementantenna results steadily. do Nevertheless, not possess thethe proposed multi-freq antennauency designresonance and materials,characteristic whether due free-to the gradualspace or impedance on-body, covermatching a wide across bandwidth the frequenc rangey that band, fulfils so the the resonance requirements effect of revealed many a wirelesstraveling standards, wave antenna including steadily. UWB Nevertheless, communications the [ 90proposed,91]. antenna design and materials, whether free-space or on-body, cover a wide bandwidth range that fulfils the requirements of many wireless standards, including UWB communications [90,91].

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Figure 12. Simulated vs. Measured S11 of CNTs/CNC PICA antennas in off- and on-body settings. Figure 12. Simulated vs. Measured S11 of CNTs/CNC PICA antennas in off- and on-body settings. Figure 12. Simulated vs. Measured S11 of CNTs/CNC PICA antennas in off- and on-body settings. Fabric-based devices are exposed to prolonged stresses of various kinds during their Fabric-basedFabric-based devices devices are are exposed exposed to to prolonged prolonged stresses stresses of of various various kinds kinds during during their their use and after-care, attaching themselves onto both irregularly shaped surfaces and the useuse and and after-care, after-care, attaching attaching themselves themselves onto onto both both irregularly irregularly shaped shaped surfaces surfaces and and the the body [92]. To examine bending endurance, the antennas were placed on two foam cylin- bodybody [92]. [92 ].To To examine examine bending bending endurance, endurance, the antennas werewere placedplaced on on two two foam foam cylinders cylinders (Rohacell) with εr close to 1 mimicking the air to consider the bending effect alone. ders(Rohacell) (Rohacell) with withε close εr close to 1to mimicking 1 mimicking the the air toair consider to consider the the bending bending effect effect alone. alone. Two Two cylinders of 70r and 150 mm radius were used to replicate a similar degree of con- Twocylinders cylinders of 70 of and 70 and 150 mm150 radiusmm radius were were used toused replicate to replicate a similar a similar degree degree of conformity of con- to formity to that of lower and upper human limbs [7]. Figure 13 shows the simulation and formitythat of to lower that andof lower upper and human upper limbs human [7]. limbs Figure [7]. 13 Figure shows 13 the shows simulation the simulation and measured and measured PICA design results for the two cylinders, with a slight detuning frequency in measuredPICA design PICA results design for results the two for cylinders, the two withcylinders, a slight with detuning a slight frequency detuning in frequency each bending in each bending ratio. Higher bending ratio causes lower resonance length, and therefore, eachratio. bending Higher ratio. bending Higher ratio bending causes lowerratio causes resonance lower length, resonance and therefore,length, and the therefore, resonance the resonance frequency increases as also noted in PICA designs in Figure 13. thefrequency resonance increases frequency as alsoincreases noted as in also PICA noted designs in PICA in Figure designs 13. in Figure 13.

Figure 13. Simulated vs. Measured S11 of CNTs/CNC PICA antennas under 70 and 150 mm bending FigureFigure 13. 13. SimulatedSimulated vs. vs. Measured Measured S11S 11of ofCNTs/CNC CNTs/CNC PICA PICA antennas antennas under under 70 70and and 150 150 mm mm bending bending scenarios. scenarios.scenarios.

FigureFigureFigure 14 14 14 shows shows shows the the the numerically numerically numerically computed computed computed an an anddd measured measured measured reflection reflection reﬂection coefficient coefficient coefﬁcient of of of the the the wearablewearablewearable LOOPLOOP LOOP antenna.antenna. antenna. AA A slightslight slight shiftshift shift towardstowards towards lowerlower lower frequenciesfrequencies frequencies isis is observedobserved observed forfor for bothboth both cases,cases,cases, with with with 2.41 2.41 2.41 GHz GHz GHz (simulated) (simulated) (simulated) and and and2.39 2.39 2.39GHz GHz GHz for for 0.5 0.5 for wt.% wt.% 0.5 (CNTs/CNC—0.5:1), wt.%(CNTs/CNC—0.5:1), (CNTs/CNC—0.5:1), and and 2.385 2.385 and GHzGHz2.385 11 wt.% GHzwt.% 1 (CNTs/CNC—1:1)(CNTs/CNC—1:1) wt.% (CNTs/CNC—1:1) (measured).(measured). (measured). LoweLowerr Lower returnreturn return lossloss valuesvalues lossvalues towardstowards towards lowerlower lower fre-fre- quenciesquenciesfrequencies are are calculated calculated are calculated in in the the in case case the caseof of on-bod on-bod of on-bodyyy measurements. measurements. measurements.

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Figure 14. Simulated vs. Measured S11 of CNTs/CNC LOOP antennas in off- and on-body settings. FigureFigure 14. 14. SimulatedSimulated vs. vs. Measured Measured SS1111 ofof CNTs/CNC CNTs/CNC LOOP LOOP antennas antennas in inoff- off- and and on-body on-body settings. settings. The same approach as in the previous example has been followed for this model to investigateTheThe same same the antenna’sapproach approach returnas as in in the theloss previousprevious performa example examplence under has has bending been been followed followed circumstances. for for this this Figuremodel model 15to to depictsinvestigateinvestigate the theeffects the antenna’s antenna’s of bending return return on loss loss the performa performancetextile wearablence under under loops bending bending prototyp circumstances. circumstances.es when bentFigure Figure on 15 15a foamdepictsdepicts cylinder. the the effects effects In the of bendingcasebending of the onon LOOP the the textile textile antenna wearable wearable design loops s,loops as prototypes depicted prototyp in whenes Figure when bent 15,bent on we a onfoam ob- a servedfoamcylinder. cylinder. that In the the Infrequencies case the ofcase the of LOOPare the shifted LOOP antenna towaantenna designs,rds lowerdesign as values. depicteds, as depicted This in Figurephenomena in Figure 15, we 15, is observed weassoci- ob- that the frequencies are shifted towards lower values. This phenomena is associated with atedserved with that its the resonance frequencies frequency are shifted which towa reliesrds on lower the perimeter values. This of the phenomena loop. Furthermore, is associ- its resonance frequency which relies on the perimeter of the loop. Furthermore, a slight aated slight with reduction its resonance in resonance frequency level which resulting relies from on the the perimeter disturbance of theof current loop. Furthermore, distribution reduction in resonance level resulting from the disturbance of current distribution while whilea slight on reduction bending inmode resonance was also level noted resulting [56]. from the disturbance of current distribution whileon bending on bending mode mode was alsowas notedalso noted [56]. [56].

Figure 15. Simulated vs. Measured S11 of CNTs/CNC LOOP antennas under 70 and 150 mm bending Figure 15. Simulated vs. Measured S11 of CNTs/CNC LOOP antennas under 70 and 150 mm bending scenarios.Figure 15. Simulated vs. Measured S11 of CNTs/CNC LOOP antennas under 70 and 150 mm bending scenarios.scenarios. 3.5.2.3.5.2. Radiation Radiation Pattern Pattern 3.5.2. Radiation Pattern InIn order order to tounderstand understand better better the performance the performance of the of antenna, the antenna, radiation radiation pattern patternmeas- urementsmeasurementsIn order were to takenunderstand were takeninside better insidethe anechoic the the performance anechoic chamber chamber ofdetailed the detailedantenna, in Section inradiation Section 2.9 to pattern2.9 evaluate to evaluate meas- the radiationurementsthe radiation propertieswere propertiestaken of inside the ofantenna the the anechoic antenna in free chamber inspace free and space detailed on andthe in human on Section the humanphantom. 2.9 to evaluate phantom. It defines the It theradiationdefines radiated theproperties power radiated by of poweran the antenna antenna by an as in antennaa functionfree space as of a andthe function arrivalon the of anglehuman the arrivalobserved phantom. angle in theIt observed defines antennas’thein radiated the far antennas’field. power Figure far by 16 field. an shows antenna Figure the as16CST-simu a shows function thelated of CST-simulated theradiation arrival patternsangle radiation observed (black) patterns inof thethe anten- (black)PICA antenna,nas’of the far PICAfield. along antenna,Figure with the16 along showsmeasurements with the theCST-simu measurements (red 1.0lated wt.% radiation CNTs/CNC—1:1 (red 1.0 patterns wt.% CNTs/CNC—1:1 (black) and blue of the 1.5 PICAwt.% and CNTs/CNC—1.5:1.5)antenna,blue 1.5 along wt.% with CNTs/CNC—1.5:1.5) the in the measurements off-body case. in (red the Thre off-body1.0e wt.%main frequenciesCNTs/CNC—1:1 case. Three along main and the frequencies operationblue 1.5 wt.% along fre- quencyCNTs/CNC—1.5:1.5)the operation band were frequency selected in the band off-bodyas 4, were7, and case. selected 10 Thre GHz.e asmain In 4,each 7,frequencies andfrequency, 10 GHz. along electric Inthe each operationfield frequency, E-plane fre- (quencyφelectric = 90°) band fieldand wereEmagnetic-plane selected (ϕ field= 90 as H◦ )4,-plane and 7, and magnetic cuts 10 (GHz.φ = field 0°) In Hwere each-plane shown.frequency, cuts (Theϕ = electric 0antenna◦) were field showed shown. E-plane Thean omnidirectional(φantenna = 90°) and showed magnetic pattern; an omnidirectional field this Hwas-plane expected, cuts pattern; (φ du = e this0°) to were wasits CPW expected,shown. design The due with antenna to no its backgroundCPW showed design an omnidirectionalwith no background pattern; plane this to was enhance expected, the antenna’sdue to its directivity.CPW design At with higher no frequencies background (7 and 10 GHz), the omnidirectional patterning was distorted due to the slight mismatch

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Sensors 2021, 21, 4934 20 of 28 plane to enhance the antenna’s directivity. At higher frequencies (7 and 10 GHz), the omnidirectional patterning was distorted due to the slight mismatch caused by the optimisa- causedtion of byantenna the optimisation dimensions of at antenna 3 GHz. dimensionsMeasured values at 3 GHz. are well Measured correlated values with are simula- well correlatedtions with with minimum simulations variation with and minimum a slight ri variationpple effect, and where a slight an rippleomnidirectional effect, where pattern an omnidirectionalcan be appreciated. pattern These can slight be appreciated. variations are These associated slight variations with experimental are associated deviations with experimentalcaused by the deviations measurement caused setup by theandmeasurement the flexible nature setup of and the the fabric ﬂexible antenna nature itself. of theThe fabricrotation antenna generated itself. physical The rotation tension generated on the flexible physical antenna tension and on theits SMA ﬂexible connector, antenna andthus, itsalso SMA caused connector, these variations. thus, also caused these variations.

(a) (b)

(e) (f)

FigureFigure 16. 16.Measured Measured vs. vs. simulated simulated radiation radiation pattern pattern of of the the graphene-based graphene-based textile textile PICA PICA antenna antenna in in free free space, space,E E-plane-plane cut cut atatϕ φ= = 90 90°◦ and andH H-plane-plane cut cut at atϕ φ= = 00°,◦, forfor thethe threethree mainmain frequencies:frequencies: ((aa,,bb)) 44 GHz;GHz; ((cc,,dd)) 77 GHz;GHz; ((ee,,ff)) 1010 GHz.GHz.

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ForFor on-body on-body characterisation, characterisation, the the phantom phantom previously previously described described was was employed employed in the in testthe campaign. test campaign. The 2D The radiation 2D radiation pattern pattern representation representation is depicted is depicted in Figure in Figure17. The 17. effect The ofeffect the humanof the human body’s body’s presence presence on the on antenna’s the antenna’s performance performance can be ca seen,n be seen, increasing increasing the front-backthe front-back ratio dueratio to due the to lack the of lack background of background plane in plane the design in the and design because and thebecause body the is abody dissipative is a dissipative medium that medium absorbs that the absorbs backward the emitted backward energy. emitted Figure energy. 17a–f Figure suggest 17a–f the physicalsuggest torsothe physical phantom torso absorbs phantom less energy absorbs than less the energy computed than numericalthe computed models. numerical This deviationmodels. This in behaviour deviation could in behaviour be due tocould the complexitybe due to the of reproducingcomplexity of the reproducing human body the andhuman its exact body representation. and its exact representation.

(a) (b)

(e) (f)

FigureFigure 17. 17.Measured Measured vs. vs. simulated simulated radiation radiation pattern pattern of of the the graphene-based graphene-based textile textile PICA PICA antenna antenna on on the the phantom, phantom,E E-plane-plane cutcut at atϕ φ= = 90 90°◦ and andH H-plane-plane cut cut at atϕ φ= = 00°,◦, forfor thethe threethree mainmain frequencies:frequencies: ((aa,,bb)) 44 GHz;GHz; ((cc,d,d)) 77 GHz;GHz; ((ee,f,f)) 1010 GHz.GHz.

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FigureFigure 18 1818 depicts depictsdepicts the thethe 2D 2D2D view viewview of ofof the thethe omnidirectional omnidirectionalomnidirectional radiation radiationradiation pattern patternpattern for for thefor the wearablethe wear-wear- loopableable antennalooploop antennaantenna when whenwhen simulating simulatingsimulating free-space free-spacefree-space conditions; conditions;conditions; it behaves itit behavesbehaves the same thethe as same asame short asas horizon- aa shortshort talhorizontalhorizontal dipole antenna dipoledipole antennaantenna (red 0.5 wt.%(red(red 0.50.5 CNTs/CNC—0.5:1 wt.%wt.% CNTs/CNC—0.5:1CNTs/CNC—0.5:1 and blue andand 1 wt.% blueblue CNTs/CNC—1:1). 11 wt.%wt.% CNTs/CNC—CNTs/CNC— As noted,1:1).1:1). AsAs the noted,noted, energy thethe is energyenergy radiated isis radiated inradiated an omnidirectional inin anan omnidirectionalomnidirectional pattern with patternpattern a distinctive withwith aa distinctivedistinctive toroid shape to-to- alongsideroidroid shapeshape with alongsidealongside two-lobe withwith opposite two-lobetwo-lobe radiation oppositeopposite directions radiationradiation directionsdirections that are separated thatthat areare separatedseparated 180◦ between 180°180° eachbetweenbetween other. eacheach Figure other.other. 19 FigureexhibitsFigure 1919 the exhibitsexhibits on-body thethe case on-bodyon-body for wearable casecase forfor applications; wearablewearable applications;applications; the robustness thethe ofrobustnessrobustness the wearable ofof thethe loop wearablewearable antenna looploop when antennaantenna close whenwhen to human closeclose tissuestoto humanhuman needs tissuestissues to be needsneeds maintained. toto bebe main-main- As previouslytained.tained. AsAs previouslypreviously addressed, addressed,addressed, at these frequencies, atat thesethese freqfreq theuencies,uencies, human thethe body humanhuman acts a bodybody reﬂector actsacts thataa reflectorreflector is capable thatthat ofisis capable enhancingcapable ofof enhancingenhancing the front–back thethe front–backfront–back ratio. ratio.ratio.

((aa)) ((bb)) FigureFigureFigure 18. 18.18. Measured MeasuredMeasured vs. vs.vs. simulated simulatedsimulated radiation radiationradiation pattern patternpattern of ofof the thethe graphene-based grapgraphene-basedhene-based textile textiletextile LOOP LOOPLOOP antenna antennaantenna in free inin freefree space, space,space,E-plane EE-plane-plane cut cut at φ = 90° and H-plane cut at φ = 0°. atcutϕ at= φ 90 =◦ 90°and andH-plane H-plane cut cut at ϕ at= φ 0 ◦=. 0°.

((aa)) ((bb)) Figure 19. Measured vs. simulated radiation pattern of the graphene-based textile LOOP antenna on the phantom, E-plane FigureFigure 19.19. Measured vs. simulated radiation pattern of the graphene-basedgraphene-based textile LOOP antenna on the phantom, E-plane-plane cutcut atat φφ == 90°90° andand HH-plane-plane cutcut atat φφ == 0°.0°. cut at ϕ = 90◦ and H-plane cut at ϕ = 0◦.

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3.5.3. Antenna Efficiency and Gain Efficiency measurements were taken using a wheeler cap and following its method for the three selected frequencies. An accurate comparison is not possible due to the lack of references of CNTs screen printed antennas onto textiles within the microwave range. Values are lower than values previously reported in the literature for CNTs films on polyimide substrate [37,39]. Fabric substrates deal with an order of magnitude higher in terms of dissipation factor; therefore, reducing the efficiency of the whole antenna. The final parameters for the PICA model are listed in Table6 and gain values for both ink formulations (simulated and measured) are noted in Table7.

Table 6. PICA efﬁciency simulations and measurements at 4, 7, and 10 GHz for both ink formulations.

Frequency (GHz) 4 7 10 CNTs/CNC—1:1 26 37 47 Simulated Efﬁciency CNTs/CNC—1.5:1.5 27 39 50 (%) CNTs/CNC—1:1 20 25 34 Measured CNTs/CNC—1.5:1.5 22 27 40

Table 7. PICA gain simulations and measurements at 4, 7, and 10 GHz for both ink formulations.

Frequency (GHz) 4 7 10 CNTs/CNC—1:1 −6.28 −3.82 −1.96 Simulated Efﬁciency CNTs/CNC—1.5:1.5 −6.10 −3.59 −1.70 (%) CNTs/CNC—1:1 −7.40 −5.52 −3.37 Measured CNTs/CNC—1.5:1.5 −7.00 −5.19 −2.60

Regarding the LOOP design, its radiation efficiency simulated is 65%, while the measured value with the wheeler cap is ~10% at 2.4 GHz; despite this discrepancy, this outcome remains close to reported values of compact antennas for wearable sensor networks [93] onto flexible textile substrates. High-efficiency textile devices are a significant challenge within fabric-based electronics [94]. Nevertheless, the PICA with a CPW-fed printer demonstrated good coherency between the numerical estimation and measured data. For both ink formulations, the results suggested an operational bandwidth of at least 8 GHz (3.5–11.5 GHz) for both in free-space and on-phantom settings, ensuring the ultra-wideband mode for these prototypes. Two bending scenarios were considered to address some of the hurdles of a highly dynamic environment, in the context of the body. Numerical and experimental analyses were carried out to determine the impact of being near the human body on the antenna’s features such as S-parameters and radiation compared to the off-body scenario. While an omnidirectional radiation pattern was observed in the free-space scenario due to its co-planar structure, an increased front-to-back (F/B) was recorded in the on-body case due to the human body absorbing the energy emitted by the antenna. When loop designs were tested as an antenna, for prospect use in wearable and healthcare applications, S11 measured values slightly shifted towards lower frequencies, 20 and 25 MHz in CNTs/CNC—0.5:1 and CNTs/CNC—1:1. Simulated and measured return loss and far-field parameters for both scenarios, off- and on-body, were reasonably correlated. The antenna performance under bending for both PICA and LOOP designs revealed only a slight frequency detuning in the response that could be negligible to the UWB model, but it has to be considered for the resonance frequency of the LOOP. Low efficiency was noted within range for wireless communications. Thus, through material intervention, improving radiation efficiency could lead the way forward to tackle major challenges for CNTs and textile-based antennas. Previous works reported CNTs compounds on different substrates such as paper, photo paper, or rubber [46], but the tunability and adaptability of textile substrates have not been fully explored [3]. Contrary to traditional metallic material solutions, the intrinsically Sensors 2021, 21, 4934 24 of 28

ﬂexible CNTs specimens promise a high degree of movement freedom and elongation with a moderate electrical conductivity. Manipulating different textiles substrates could enable full integration of technology into the fabrics, boosting the development of smart clothing. Nonetheless, translating nano-level superior properties of CNTs into the macro-scale still needs to be altered for efﬁcient communication channels. It is still important to note that CNC played a critical role in ensuring good quality of sensory surfaces printing with a trade-off in electrical conductivity.

4. Conclusions The electronics industry urges us to find sustainable material solutions that can replace some metallic components that are hardly resourced, recycled, or repurposed when coupled with other mediums such as natural or synthetic textiles. New electronic solutions should ground both on practical and environmentally friendly making and sustainable materials avoiding toxic substances. Combination of redundant CNC with a carbon allotrope, CNTs, can promise unique features such as adaptability, washability, and degradation to the electronic elements for sensing and communication in the context of smart wearables. This study is looking into new material led approaches for wearable communication. Within the purpose of understanding these new mediums and techniques, two CNTs/CNC antenna designs, such as LOOP and PICA, screen printed onto the flexible textile-based substrates, were studied. Through the successful formulation of water-based screen printing conductive inks with CNTs and CNC, we showed a facile approach that does not rely on inks with binder, solvent exchange, or requiring high-temperature heat post-processes annealing. Thus, on that note, these formulated inks can be applicable to other textiles printing processes by tailoring the solution viscosity and be used to work with a wide variety of flexible substrates including textiles with roughness. The work presented here strategically merged the new and nanoscopic forms of CNTs and cellulose, with a very established printing technique, screen-printing method to develop a new generation of textile-based wearable antennas. Inherent advantages of fabric features such as lightness, adaptability, and washability were noted along with the antenna measurements both on and off-body settings.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/s21144934/s1, Figure S1: Optic microscope images of CNTs/CNC-0.5:0.5 ink printed onto CO fabric using 55T frame, revealing printing defects on (a) PICA; (b) LOOP designs. Figure S2: Optic microscope images of CNTs/CNC printed ink onto CO fabric with 5 layers: (a) by using 90T frame with 0.5:0.5 weight ratios of CNTs/CNC; (b) by using 55T frame with 1:1 wt.% weight ratios of CNTs/CNC. Author Contributions: Conceptualization, I.I.L., H.C., and E.O.Y.; methodology, I.I.L., H.C., A.A. (Amir Asadi), and E.O.Y.; software, I.I.L.; validation, I.I.L., D.A., and H.C.; investigation, I.I.L., D.A., and H.C.; resources, H.C., A.A. (Amir Asadi), and A.A. (Akram Alomainy); data curation, I.I.L. and D.A.; writing—original draft preparation, I.I.L. and D.A.; writing—review and editing, H.C. and E.O.Y.; visualization, I.I.L. and D.A.; supervision, H.C., E.O.Y., and A.A. (Akram Alomainy); funding acquisition, E.O.Y. All authors have read and agreed to the published version of the manuscript. Funding: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 796640. Alomainy would like to acknowledge funding from the Engineering and Physical Sciences Research Council (EPSRC), UK, for the PAMBAYESIAN EP/P009964/1 project. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: Isidoro would like to acknowledge the School of Electronic Engineering and Computer Science, Queen Mary University of London. Sensors 2021, 21, 4934 25 of 28

Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Shyamkumar, P.; Rai, P.; Oh, S.; Ramasamy, M.; Harbaugh, R.E.; Varadan, V. Wearable Wireless Cardiovascular Monitoring Using Textile-Based Nanosensor and Nanomaterial Systems. Electronics 2014, 3, 504–520. [CrossRef] 2. Ibanez-Labiano, I.; Alomainy, A. Dielectric Characterization of Non-Conductive Fabrics for Temperature Sensing through Resonating Antenna Structures. Materials 2020, 13, 1271. [CrossRef][PubMed] 3. Kirtania, S.G.; Elger, A.W.; Hasan, M.R.; Wisniewska, A.; Sekhar, K.; Karacolak, T.; Sekhar, P.K. Flexible Antennas: A Review. Micromachines 2020, 11, 847. [CrossRef] 4. Ali, S.M.; Sovuthy, C.; Imran, M.A.; Socheatra, S.; Abbasi, Q.H.; Abidin, Z.Z. Recent Advances of Wearable Antennas in Materials, Fabrication Methods, Designs, and Their Applications: State-of-the-Art. Micromachines 2020, 11, 888. [CrossRef][PubMed] 5. Cao, R.; Pu, X.; Du, X.; Yang, W.; Wang, J.; Guo, H.; Zhao, S.; Yuan, Z.; Zhang, C.; Li, C.; et al. Screen-Printed Washable Electronic Textiles as Self-Powered Touch/Gesture Tribo-Sensors for Intelligent Human–Machine Interaction. ACS Nano 2018, 12, 5190–5196. [CrossRef] 6. Huang, Q.; Zhu, Y. Printing Conductive Nanomaterials for Flexible and Stretchable Electronics: A Review of Materials, Processes, and Applications. Adv. Mater. Technol. 2019, 4.[CrossRef] 7. Ibanez-Labiano, I.; Ergoktas, M.S.; Kocabas, C.; Toomey, A.; Alomainy, A.; Ozden-Yenigun, E. Graphene-based soft wearable antennas. Appl. Mater. Today 2020, 20.[CrossRef] 8. Li, W.; Cen, Q.; Li, W.; Zhao, Z.; Yang, W.; Li, Y.; Chen, M.; Yang, G.; Yang, J. A green method for synthesizing novel nanoparticles and their application in flexible conductive patterns. J. Mater. 2020, 6, 300–307. [CrossRef] 9. Huang, Q.; Al-Milaji, K.N.; Zhao, H. Inkjet Printing of Silver Nanowires for Stretchable Heaters. ACS Appl. Nano Mater. 2018, 1, 4528–4536. [CrossRef] 10. Liang, J.; Tong, K.; Pei, Q. A Water-Based Silver-Nanowire Screen-Print Ink for the Fabrication of Stretchable Conductors and Wearable Thin-Film Transistors. Adv. Mater. 2016, 28, 5986–5996. [CrossRef] 11. Matsushisa, N.; Kaltenbrunner, M.; Yokota, T.; Jinno, H.; Kuribara, K.; Sekitani, T.; Someya, T. Printable elastic conductors with a high conductivity for electronic textile applications. Nat. Commun. 2015, 6, 7461. [CrossRef][PubMed] 12. Basak, I.; Nowicki, G.; Ruttens, B.; Desta, D.; Prooth, J.; Jose, M.; Nagels, S.; Boyen, H.-G.; D’Haen, J.; Buntinx, M.; et al. Inkjet Printing of PEDOT:PSS Based Conductive Patterns for 3D Forming Applications. Polymers 2020, 12, 2915. [CrossRef][PubMed] 13. Aziz, A.; Bazbouz, M.B.; Welland, M.E. Double-Walled Carbon Nanotubes Ink for High-Conductivity Flexible Electrodes. ACS Appl. Nano Mater. 2020, 3, 9385–9392. [CrossRef] 14. Li, J.; Zhang, X.; Liu, X.; Liang, Q.; Liao, G.; Tang, Z.; Shi, T. Conductivity and foldability enhancement of Ag patterns formed by PVAc modified Ag complex inks with low-temperature and rapid sintering. Mater. Des. 2020, 185, 108255. [CrossRef] 15. Tran, T.S.; Dutta, N.K.; Choudhury, N.R. Graphene inks for printed flexible electronics: Graphene dispersions, ink formulations, printing techniques and applications. Adv. Colloid Interface Sci. 2018, 261, 41–61. [CrossRef] 16. Abutarboush, H.F.; Shamim, A. A Reconfigurable Inkjet-Printed Antenna on Paper Substrate for Wireless Applications. IEEE Antennas Wirel. Propag. Lett. 2018, 17, 1648–1651. [CrossRef] 17. Scarpello, M.L.; Kazani, I.; Hertleer, C.; Rogier, H.; Vande Ginste, D. Stability and efficiency of screen-printed wearable and washable antennas. IEEE Antennas Wirel. Propag. Lett. 2012, 11, 838–841. [CrossRef] 18. Radman, H.; Maghrebi, M.; Baniadam, M. Inkjet printing of carbon nanotubes (CNTs) with a binary surfactant mixture: The effect of the nonionic surfactant on the uniformity of the printed surface. Diam. Relat. Mater. 2019.[CrossRef] 19. Karim, N.; Afroj, S.; Tan, S.; Novoselov, K.S.; Yeates, S.G. All Inkjet-Printed Graphene-Silver Composite Ink on Textiles for Highly Conductive Wearable Electronics Applications. Sci. Rep. 2019, 9, 8035. [CrossRef][PubMed] 20. Zavanalli, N.; Yeo, W.H. Advances in Screen Printing of Conductive Nanomaterials for Stretchable Electronics. ACS Omega 2021, 6, 9344–9351. [CrossRef][PubMed] 21. Somalu, M.R.; Muchtar, A.; Daud, W.R.W.; Brandon, N.P. Screen-printing inks for the fabrication of solid oxide fuel cell films: A review. Renew. Sustain. Energy Rev. 2017, 75, 426–439. [CrossRef] 22. Zhong, Z.W.; Tang, R.W.L.; Chen, S.H.; Shan, X.C. A study of screen printing of stretchable circuits on polyurethane substrates. Microsyst. Technol. 2019, 25, 339–350. [CrossRef] 23. Tuomaala, T.; Tuhkala, T.; Törmänen, M.; Määttä, H.; Lehtonen, A. Basics of Screen Printing for Printable and Flexible Electronics. Oulun Ammattikorkeakoulun Tutkimus ja Kehitystyön Julkaisut 2019, 39, ePooki. Available online: http://urn.fi/urn:nbn:fi-fe2019050 614470 (accessed on 23 April 2021). 24. Tseng, W.J.; Chen, C.N. Dispersion and rheology of nickel nanoparticle inks. J. Mater. Sci. 2006, 41, 1213–1219. [CrossRef] 25. Hong, H.; Hu, J.; Yan, X. UV Curable Conductive Ink for the Fabrication of Textile-Based Conductive Circuits and Wearable UHF RFID Tags. ACS Appl. Mater. Interfaces 2019, 11, 27318–27326. [CrossRef] 26. Dresselhaus, M.; Dresselhaus, C.; Eklund, P. Science of Fullerenes and Carbon Nanotubes, 1st ed.; Academic Press: Cambridge, MA, USA, 1995. 27. Han, J.; Lee, J.-Y.; Lee, J.; Yeo, J.-S. Highly stretchable and reliable, transparent and conductive entangled graphene mesh networks. Adv. Mater. 2018, 30, 1704626. [CrossRef][PubMed] Sensors 2021, 21, 4934 26 of 28

28. Dürkop, T.; Kim, B.M.; Fuhrer, M.S. Properties and applications of high-mobility semiconducting nanotubes. J. Phys. Condens. Matter 2004, 16, R553. [CrossRef] 29. Gupta, N.; Gupta, S.; Sharma, S. Carbon nanotubes: Synthesis, properties and engineering applications. Carbon Lett. 2019, 29, 419–447. [CrossRef] 30. Kholghi Eshkalak, S.; Chinnappan, A.; Jayathilaka, W.A.D.M.; Khatibzadeh, M.; Kowsari, E.; Ramakrishna, S. A review on inkjet printing of CNT composites for smart applications. Appl. Mater. Today 2017, 9, 372–386. [CrossRef] 31. Guo, Y.; Ruan, K.; Yang, X.; Ma, T.; Kong, J.; Wu, N.; Zhang, J.; Gu, J.; Guo, Z. Constructing fully carbon-based fillers with a hierarchical structure to fabricate highly thermally conductive polyimide nanocomposites. J. Mater. Chem. C 2019, 7, 7035–7044. [CrossRef] 32. Avouris, P.; Chen, Z.; Perebeinos, V. Carbon-based electronics. Nat. Nanotechnol. 2007, 10, 605–615. [CrossRef] 33. Lu, F.; Gu, L.; Meziani, M.J.; Luo, P.G.; Wang, X.; Veca, L.M.; Cao, L.; Sun, Y. Advances in bioapplications of carbon nanotubes. Adv. Mater. 2009, 21, 139–152. [CrossRef] 34. Mast, D. The future of carbon nanotubes in wireless applications. In Proceedings of the 2009 Antenna Systems/Short-Range Wireless Symposium, Philadelphia, PA, USA, 1–2 September 2009. 35. Misak, H.E.; Mall, S. Time-dependent electrical properties of carbon nanotube yarns. New Carbon Mater. 2015, 30, 207–213. [CrossRef] 36. Tsentalovich, D.E.; Headrick, R.J.; Mirri, F.; Hao, J.; Behabtu, N.; Young, C.C.; Pasquali, M. Influence of carbon nanotube characteristics on macroscopic fiber properties. ACS Appl. Mater. Interfaces 2017, 9, 36189–36198. [CrossRef][PubMed] 37. Bengio, E.A.; Senic, D.; Taylor, L.W.; Headrick, J.R.; King, M.; Chen, P.; Little, C.A.; Ladbury, J.; Long, J.C.; Holloway, C.L.; et al. Carbon nanotube thin film patch antennas for wireless communications. Appl. Phys. Lett. 2019, 114, 203102. [CrossRef] 38. Mansor, M.M.; Rahim, S.K.A.; Hashim, U. A CPW-fed 2.45 GHz wearable antenna using conductive nanomaterials for on-body applications. In Proceedings of the IEEE REGION 10 SYMPOSIUM, Kuala Lumpur, Malaysia, 14–16 April 2014; pp. 240–243. [CrossRef] 39. Mehdipour, A.; Rosca, I.D.; Sebak, A.; Trueman, C.W.; Hoa, S.V. Carbon Nanotube Composites for Wideband Millimeter-Wave Antenna Applications. IEEE Trans. Antennas Propag. 2011, 59, 3572–3578. [CrossRef] 40. Cheng, G.; Wu, Y.; Li, B. CNT-based conformal antenna design suitable for inkjet printing. In Proceedings of the 2017 International Applied Computational Electromagnetics Society Symposium (ACES), Suzhou, China, 1–4 August 2017; pp. 1–2. 41. Bais, A.; Ojha, S.S. Design and development of UWB antenna using CNT composite for RFID applications. In Proceedings of the 2016 Symposium on Colossal Data Analysis and Networking (CDAN), Indore, India, 18–19 March 2016; pp. 1–5. [CrossRef] 42. Gammoudi, I.; Aissa, B.; Nedil, M.; Abdallah, M.M. CNT-RFID passive tag antenna for gas sensing in underground mine. In Proceedings of the 2015 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, Vancouver, BC, Canada, 19–24 July 2015; pp. 1758–1759. [CrossRef] 43. Sa’don, S.N.H.; Jamaluddin, M.H.; Kamarudin, M.R.; Ahmad, F.; Dahlan, S.H. A 5G graphene antenna produced by screen printing method. Indones. J. Electr. Eng. Comput. Sci. 2019, 15, 950–955. [CrossRef] 44. Sadi, M.S.; Pan, J.; Xu, A.; Cheng, D.; Cai, G.; Wang, X. Direct dip-coating of carbon nanotubes onto polydopamine-templated cotton fabrics for wearable applications. Cellulose 2019, 26, 7569–7579. [CrossRef] 45. Mougel, J.B.; Adda, C.; Bertoncini, P.; Capron, I.; Cathala, B.; Chauvet, O. Highly efficient and predictable noncovalent dispersion of single-walled and multiwalled carbon nanotubes by cellulose nanocrystals. J. Phys. Chem. C 2016, 120, 22694–22701. [CrossRef] 46. Menon, H.; Aiswarya, R.; Surendran, K.P. Screen printable MWCNT inks for printed electronics. RSC Adv. 2017, 70, 44076–44081. [CrossRef] 47. Wang, H.; Na, C. Binder-Free Carbon Nanotube Electrode for Electrochemical Removal of Chromium. ACS Appl. Mater. Interfaces 2014, 6, 22. [CrossRef][PubMed] 48. Moon, R.J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J. Cellulose nanomaterials review: Structure, properties and nanocomposites. Chem. Soc. Rev. 2011, 40.[CrossRef][PubMed] 49. Miyashiro, D.; Hamano, R.; Umemura, K. A review of applications using mixed materials of cellulose, nanocellulose and carbon nanotubes. Nanomaterials 2020, 10, 186. [CrossRef] 50. Olivier, C.; Moreau, C.; Bertoncini, P.; Bizol, H.; Chauvet, O.; Cathala, B. Cellulose Nanocrystal-Assisted Dispersion of Luminescent Single-Walled Carbon Nanotubes for Layer-by-Layer Assembled Hybrid Thin Films. Langmuir 2012, 28, 12463–12471. [CrossRef] [PubMed] 51. Jiang, M.; Seney, R.; Bayliss, P.C.; Kitchens, C.L. Carbon nanotube and cellulose nanocrystal hybrid films. Molecules 2019, 24, 2662. [CrossRef] 52. Shariatnia, S.; Kumar, A.V.; Kaynan, O.; Asadi, A. Hybrid Cellulose Nanocrystal-Bonded Carbon Nanotubes/Carbon Fiber Polymer Composites for Structural Applications. ACS Appl. Nano Mater. 2020, 3, 5421–5436. [CrossRef] 53. Yu, J.; Grossiord, N.; Koning, C.E.; Loos, J. Controlling the dispersion of multi-wall carbon nanotubes in aqueous surfactant solution. Carbon 2007, 45, 618–623. [CrossRef] 54. Liang, J.; Guo, L.; Chiau, C.C.; Chen, X.; Parini, C.G. Study of CPW-fed circular disc monopole antenna for ultra wideband applications. IEEE Proc. Microwav. Antennas Propag. 2005, 152, 520–526. [CrossRef] 55. Thomson, D.; Card, D.; Bridges, G. RF cavity passive wireless sensors with time-domain gating-based interrogation for shm of civil structures. IEEE Sens. 2009, 9, 1430–1438. [CrossRef] Sensors 2021, 21, 4934 27 of 28

56. Bait-Suwailam, M.M.; Labiano, I.I.; Alomainy, A. Impedance Enhancement of Textile Grounded Loop Antenna Using High- Impedance Surface (HIS) for Healthcare Applications. Sensors 2020, 20, 3809. [CrossRef][PubMed] 57. Beamish, D. Coating thickness measurement. Met. Finish. 2010, 108, 379–385. [CrossRef] 58. Henson, H.; Sexton, J.L. Microwave Electronics; John Wiley & Sons: Chichester, UK, 2004; p. 142. 59. Lesnikowski, J. Dielectric permittivity measurement methods of textile substrate of textile transmission lines. Prz. Elektrotech. 2012, 88, 148–151. 60. Antennas & Electromagnetics Research Group. Queen Mary University of London. Available online: http://antennas.eecs.qmul. ac.uk/facilities/antenna-measurement-laboratory/ (accessed on 23 April 2021). 61. Andreuccetti, D.; Fossi, R.; Petrucci, C. An Internet resource for the calculation of the dielectric properties of body tissues in the frequency range 10 Hz - 100 GHz. IFAC-CNR, Florence (Italy), 1997. Based on Data Published by C. Gabriel et al. in 1996. Available online: http://niremf.ifac.cnr.it/tissprop/ (accessed on 18 July 2021). 62. Wheeler, H.A. The Radiansphere around a Small Antenna. Proc. IRE 1959, 47, 1325–1331. [CrossRef] 63. McKinzie, W.A. Modified Wheeler Cap Method for Measuring Antenna Efficiency. In Proceedings of the IEEE Antennas and Propagation Society International Symposium, Montreal, QC, Canada, 13–18 July 1997; pp. 542–545. 64. Johnston, R.H.; McRory, J.G. An Improved Small Antenna Radiation-Efficiency Measurement Method. IEEE Antennas Propag. Mag. 1998, 40, 40–48. [CrossRef] 65. Etminan, N.; Yoosefian, M.; Raissi, H.; Hakimi, M. Solvent effects on the stability and the electronic properties of histidine/Pd- doped single-walled carbon nanotube biosensor. J. Mol. Liq. 2016, 214, 313–318. [CrossRef] 66. Kosmala, A.; Wright, R.; Zhang, Q.; Kirby, P. Synthesis of silver nano particles and fabrication of aqueous Ag inks for inkjet printing. Mater. Chem. Phys. 2011, 129, 1075–1080. [CrossRef] 67. Foster, E.J.; Moon, R.J.; Agarwal, U.P.; Bortner, M.J.; Bras, J.; Espinosa, S.C.; Chan, K.J.; Clift, M.J.D.; Cranston, E.D.; Eichhorn, S.J.; et al. Current characterization methods for cellulose nanomaterials. Chem. Soc. Rev. 2018, 47, 2609–2679. [CrossRef] 68. Teerapanich, P.; Myint, M.T.Z.; Joseph, C.M.; Hornyak, G.L.; Dutta, J. Development and Improvement of Carbon Nanotube-Based Ammonia Gas Sensors Using Ink-Jet Printed Interdigitated Electrodes. IEEE Trans. Nanotechnol. 2013, 12, 255–262. [CrossRef] 69. Angelo, P.D.; Farnood, R.R. Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) Inkjet Inks Doped with Carbon Nanotubes and a Polar Solvent: The Effect of Formulation and Adhesion on Conductivity. J. Adhes. Sci. Technol. 2012, 24, 643–659. [CrossRef] 70. Wang, L.; Loh, K.J. Wearable carbon nanotube-based fabric sensors for monitoring human physiological performance. Smart Mater. Struct. 2017, 26, 055018. [CrossRef] 71. Sopeña, P.; Fernández-Pradas, J.M.; Serra, P. Laser-induced forward transfer of conductive screen-printing inks. Appl. Surf. Sci. 2020, 507, 145047. [CrossRef] 72. Jabbari, F.; Rajabpour, A.; Saedodin, S. Viscosity of carbon nanotube/water nanofluid. J. Therm. Anal. Calorim. 2019, 135, 1787–1796. [CrossRef] 73. Ma, A.W.K.; Yearsley, K.M.; Chinesta, F.; Mackley, M.R. A review of the microstructure and rheology of carbon nanotube suspensions. Proc. Inst. Mech. Eng. Part N J. Nanoeng. Nanosyst. 2008, 222, 71–94. [CrossRef] 74. Talebizadehsardari, P.; Shahsavar, A.; Toghraie, D.; Barnoon, P. An experimental investigation for study the rheological behavior of water–carbon nanotube/magnetite nanofluid subjected to a magnetic field. Phys. A Stat. Mech. Its Appl. 2019, 534, 122129. [CrossRef] 75. Kaynan, O.; Yıldız, A.; Bozkurt, Y.E.; Ozden-Yenigun, E.; Cebeci, H. Electrically conductive high-performance thermoplastic filaments for fused filament fabrication. Compos. Struct. 2020, 237, 111930. [CrossRef] 76. Hong, H.; Jiyong, H.; Moon, K.-S.; Yan, X.; Wong, C.-P. Rheological properties and screen printability of UV curable conductive ink for flexible and washable E-textiles. J. Mater. Sci. Technol. 2021, 67, 145–155. [CrossRef] 77. Qi, X.; Ha, H.; Hwang, B.; Lim, S. Printability of the Screen-Printed Strain Sensor with Carbon Black/Silver Paste for Sensitive Wearable Electronics. Appl. Sci. 2020, 10, 6983. [CrossRef] 78. Faddoul, R.; Reverdy-Bruas, N.; Blayo, A. Formulation and screen printing of water based conductive flake silver pastes onto green ceramic tapes for electronic applications. Mater. Sci. Eng. B 2012, 177, 1053–1066. [CrossRef] 79. Nazmul Karim, N.; Afroj, S.; Tan, S.; He, P.; Fernando, A.; Carr, C.; Novoselov, N.S. Scalable Production of Graphene-Based Wearable E-Textiles. ACS Nano 2017, 11, 12266–12275. [CrossRef][PubMed] 80. Islam, R.; Khair, N.; Ahmed, D.M.; Shahariar, H. Fabrication of low cost and scalable carbon-based conductive ink for E-textile applications. Mater. Today Commun. 2019, 19, 32–38. [CrossRef] 81. Lee, S.W.; Kim, B.-S.; Chen, S.; Horn, Y.S.; Hammond, P.T. Layer-by-Layer Assembly of All Carbon Nanotube Ultrathin Films for Electrochemical Applications. J. Am. Chem. Soc. 2009, 131, 671–679. [CrossRef][PubMed] 82. Abdelhalim, A.; Abdellah, A.; Scarpa, G.; Lugli, P. Fabrication of carbon nanotube thin films on flexible substrates by spray deposition and transfer printing. Carbon 2013, 61, 72–79. [CrossRef] 83. Kim, M.S.; Kim, M.; Son, S.; Cho, S.Y.; Lee, S.; Won, D.K.; Ryu, J.; Bae, I.; Kim, H.M.; Kim, K.B. Sheet Resistance Analysis of Interface-Engineered Multilayer Graphene: Mobility Versus Sheet Carrier Concentration. ACS Appl. Mater. Interface 2020, 12, 30932–30940. [CrossRef] 84. Hou, X.; Chen, G.; Xing, T.; Wei, Z. Reactive ink formulated with various alcohols for improved properties and printing quality onto cotton fabrics. J. Eng. Fibers Fabr. 2019, 14.[CrossRef] Sensors 2021, 21, 4934 28 of 28

85. Karaguzel, B.; Merritt, C.R.; Kang, T.; Wilson, J.M.; Nagle, H.T.; Grant, E.; Pourdeyhimi, B. Utility of nonwovens in the production of integrated electrical circuits via printing conductive inks. J. Text. Inst. 2008, 99, 1. [CrossRef] 86. Liu, L.; Shen, Z.; Zhang, X.; Ma, H. Highly conductive graphene/carbon black screen printing inks for flexible electronics. J. Colloid Interface Sci. 2021, 582 Pt A, 12–21. [CrossRef] 87. Jhanji, Y.; Gupta, D.; Kothari, V.K. Moisture management properties of plated knit structures with varying fiber types. J. Text. Inst. 2014, 106, 663–673. [CrossRef] 88. Park, J.G.; Smithyman, J.; Lin, C.Y.; Cooke, A.; Kismarahardja, W.; Li., S.; Richard Liang, R.; Brooks, J.S.; Zhang, C.; Wang, B. Effects of surfactants and alignment on the physical properties of single-walled carbon nanotube buckypaper. J. Appl. Phys. 2009, 106, 104310. [CrossRef] 89. Stanˇcić,M.; Kasikovic, N.; Novaković,D.; Dojcinovic, I.; Vladić,G.; Dragic, M.R. The influence of washing treatment on screen printed textile substrates. Tekstil ve Konfeksiyon. 2014, 24, 96–104. 90. Oppermann, I.; Hamalainen, M.; Linatti, J. UWB Theory and Applications; John Wiley & Sons: Chichester, UK, 2004; ISBN 9780-470-86917-8. 91. Alomainy, A.; Hao, Y.; Parini, C.G.; Hall, P.S. Comparison Between Two Different Antennas for UWB On-Body Propagation Measurements. IEEE Antennas Wirel. Propag. Lett. 2005, 4, 31–34. [CrossRef] 92. Yang, J.C.; Mun, J.; Kwon, S.Y.; Park, S.; Bao, Z.; Park, S. Electronic Skin: Recent Progress and Future Prospects for Skin-Attachable Devices for Health Monitoring, Robotics, and Prosthetics. Adv. Mater. 2019, 31, 1904765. [CrossRef] 93. Atanasova, G.; Atanasov, N. Small Antennas for Wearable Sensor Networks: Impact of the Electromagnetic Properties of the Textiles on Antenna Performance. Sensors 2020, 20, 5157. [CrossRef] 94. Wagih, M.; Weddell, A.S.; Beeby, S. Overcoming the Efficiency Barrier of Textile Antennas: A Transmission Lines Approach. Proceedings 2019, 32, 2018. [CrossRef]