sensors

Article Printed Graphene, Nanotubes and Electrodes Comparison for and Structural Electronics Applications

Agnieszka Tabaczy´nska 1,*, Anna D ˛abrowska 2 and Marcin Słoma 3

1 OPTEX S.A., Oskara Kolberga 2, 26-300 Opoczno, Poland 2 Department of Personal Protective Equipment, Central Institute for Labour Protection–National Research Institute, Wierzbowa 48, 90-133 Lodz, Poland; [email protected] 3 Institute of Metrology and Biomedical Engineering, Faculty of Mechatronics, Warsaw University of Technology, Sw.´ Andrzeja Boboli 8, 02-525 Warsaw, Poland; [email protected] * Correspondence: [email protected]

Abstract: Due to the appearance of smart and wearable electronics, the need for electro- conductive textiles and electro-conductive paths on textiles has become clear. In this article the results of a test of developed textile electro-conductive paths obtained by applying the method of screen printing pastes containing silver and (graphene, nanotubes, graphite) are presented. Conducted research included analysis of the adhesion test, as well as evaluation of the surface resistance before and after the washing and bending cycles. Obtained results indicated that the samples with the content of carbon nanotubes 3% by weight in PMMA on substrate made of aramid fibers (surface mass of 260 g/m2) were characterized by the best adhesion and the best   resistance to washing and bending cycles. Such electro-conductive paths have potential to be used in smart clothing applications. Citation: Tabaczy´nska,A.; D ˛abrowska, A.; Słoma, M. Printed Graphene, Nanotubes and Silver Keywords: textile electronics; printed and structural electronics; electrically conductive textiles; Electrodes Comparison for Textile wearable electronics; graphene; nanoparticles; composites and Structural Electronics Applications. Sensors 2021, 21, 4038. https://doi.org/10.3390/s21124038 1. Introduction Academic Editors: The industry of smart textiles, sometimes called active, interactive and adaptive tex- Hyun-Joong Chung and Eduardo tiles (smart and intelligent textiles and clothing) [1–3] incorporated in garments, has been García Breijo extensively developed. Its history started back in the middle of last century when the first heated clothing was developed, followed by the discovery of shape-memory materi- Received: 31 March 2021 als; these together marked a new era of innovations in materials. These inventions were Accepted: 29 May 2021 followed some decades ago by the application of these materials for textile products, intro- Published: 11 June 2021 ducing to the market smart textiles [4]. Further developing these ideas, researchers started to work on the combination of smart and conductive textiles with electronic components. Publisher’s Note: MDPI stays neutral This way, intelligent wearable textile systems with embedded electrically conductive with regard to jurisdictional claims in published maps and institutional affil- materials were created and called e-textiles [2,5–8]. Application challenges for e-textiles iations. have been summarized by Wang et al. [9]. Authors indicated that due to their closeness to the human body, higher demands are placed on them such as low weight, small size and flexibility which are usually failed by 2D and 3D electronic devices. Komolafe et al. [10] ad- ditionally indicates that e-textiles should be durable and should keep the traditional textile properties that make them appropriate for clothing applications. Therefore, researchers Copyright: © 2021 by the authors. look for the solutions in which electrically conductive fibrous structures are introduced Licensee MDPI, Basel, Switzerland. into the textile product without causing a visual change [8]. This article is an open access article Special attention is paid to the development of e-textiles for monitoring of physical distributed under the terms and conditions of the Creative Commons and physiological parameters of human body that will not negatively influence user Attribution (CC BY) license (https:// comfort [11–14]. Therefore, in the few last years, several projects (e.g., i-Protect, PROETEX, creativecommons.org/licenses/by/ WearIT, My Heart, BioTex, Stella, Context, Ofseth) aimed at the development of smart 4.0/).

Sensors 2021, 21, 4038. https://doi.org/10.3390/s21124038 https://www.mdpi.com/journal/sensors Sensors 2021, 21, 4038 2 of 14

clothing with the function of monitoring the user and/or the environment were carried out within the national and European framework programs. From a textile fabrication process point of view, the challenge is to make as many connections and active or passive components as possible coated or embedded in the textile material. Therefore, to develop electro-conductive textiles several approaches are proposed, for example by integrating conductive fibres or , applying conductive coatings, or using conductive printable inks [4]. Le´snikowski[15] analysed the possibility to develop microstrip and coplanar textile signal lines with characteristic impedance not higher than 50 ohms, and proposed a solution in which electro-conductive fabric was sewn onto a non-conductive substrate. Moreover, he showed [16] that the influence of humidity on the impedence of the textile signal lines depends on the substrate material. A use of lock-stitch embroidery technique for the purpose of preparation of conductive tracks in fabricating e-textiles was considered by Zheng et al. [17]. Their research was focused on embroidery made of silver-coated polyamide on the . On the basis of the obtained results, authors indicated embroidery parameters that may be used for conductive tracks for smart clothing. The application potential of inkjet printing for manufacturing of electronic devices such as solar panels, sensors, and transistors was indicated by Beedasy [18]. Authors analysed state of the art in this field of electronics’ inkjet printing, indicating glass, paper and polymers as most common substrates. Among the polymers, mainly polyimide (PI), polyethylene terephthalate (PET), polyethylene napthalate (PEN) and polydimethylsilox- ane (PDMS) are mentioned. Shahariar et al. [19] emphasized that inkjet printing on textile substrate is a novel approach and very challenging in terms of ensuring required func- tionality with a use of this technology for the purpose of wearable electronic textiles. As the main issues to overcome, the following challenges are mentioned: blocking nozzles and achieving electrically conductive percolation on rough textile substrate. In the paper, authors presented the inkjet printing process on three kinds of substrates: knitted, woven and non-woven fabrics. The lowest sheet resistance was obtained in the case of which was equal to 0.2 Ω/sq, which predestines this as a solution for e-textile applications. Karim et al. [20] highlighted that inkjet printing for manufacturing of electro conductive fabrics is a very promising technique due to its excellent processing and environ- mental benefits, while limited ink stability and cost related to electro-conductive material (e.g., Ag) concentration in the ink are its bottlenecks. Authors have overcome those issues by means of optimization of composite and combination of a silver -based ink with a highly concentrated graphene dispersion. Stempien et al. [21] also noticed an issue of ink stability, which is particularly important in the case of textile substrates due to their roughness. Therefore, authors proposed a water-soluble ink with silver content together with simultaneous sintering at 90 ◦C, and when eight layers were printed on different fabrics (made of polyacrylonitrile, polypropylene, polyester and basalt fibres) a surface resistance at the level of 0.155 0.235 Ω/sq was achieved. Conductive inks are used in many applications, including electronics, computers and communications. These rigid substrates of PCBs (Printed Circuit Boards) are different from textiles; therefore different technologies have to be applied, such as screen printing [4]. The aim of this study is to improve the functional characteristics of intelligent clothing by replacing the rigid electrical connections (between sensors, measuring and control devices and transmission systems) with the developed flexible textile material, with highly electrically conductive paths, resistant to maintenance cycles. In this study, screen printing techniques have been used for flame-retardant textile materials to achieve the effect of electro-conductivity by covering them with electro-conductive pastes based on silver, graphene and carbon nanotubes. The use of screen printing techniques to provide electro- conductive paths on fabrics made of either aramid or modacrylic fibers that provides resistance to thermal factors of the clothing is a novelty in the proposed approach. According to the performed analysis of the state-of-the-art, up to now, only a few experiments considered a use of a screen printing method on flame-retardant fabrics. Sensors 2021, 21, 4038 3 of 14

It is worth mentioning that this is an extremely important aspect in terms of potential applications in protective clothing for fire fighters. At the same time, the influence of the kind of textile substrate (defined by e.g., manufacturing technique, wave, square weight, raw material composition) on electrical conductivity was confirmed. An attempt to make an electro-conductive textile material by a screen printing method with the use of aramid fabric was made by Kazani [22] with a low surface resistance reaching 0.020–0.048 Ω/ (one of the lowest values from the samples considered by Kazani) depending on the ink used. However, in this work only woven aramid fabric was used, and after 20 washing cycles the surface resistance of those samples increased to 11–36 Ω/. Therefore, on the basis of these results it can be concluded that the use of this method is promising in terms of application into the flame retardant protective clothing; however, further research works are required in order to improve resistance to washing cycles. Moreover, taking into account the multi-layer structure of protective clothing for fire fighters, including a non-woven flame-retardant layer, the use of such a textile structure for screen printing with electro-conductive ink seems to be a novel research direction worth consideration. This technique seems to be also more appropriate for applications in protective clothing for fire fighters than embroidery due to the fact that in the case of embroidery, the electro- conductive thread goes through the fabric and as a consequence may transfer heat from the external environment close to the user’s body. Positive results of the conducted research can highly contribute to further progress in e-textiles for protective clothing applications.

2. Materials and Methods 2.1. Materials 2.1.1. Textile Materials In this study, three variants of woven textile substrates were selected. Their character- istics are presented in Table1.

Table 1. Characteristics of textile materials used as a substrate in the study.

Surface Mass, g/m2 No. Fabric Composition Thickness, mm (±5%) Non-woven made of aramid and paraaramid fibers 1 (/Nomex) laminated with PTFE membrane 140 g/m2 0.78 mm (Teflon) (white colored yellow). Fabric made of modacrylic fand fibers 2 320 g/m2 0.60 mm Composition: 54% Protex MA, 46% CO (color red) Fabric made of paraaramid/aramid fibers (Nomex 3 Comfort fabric Composition: 93% Nomex, Kevlar 7%) 260 g/m2 0.52 mm (colored blue).

2.1.2. Electrically Conductive Composites For the electrically conductive paths application of composite coatings in the form of a paste consisting of a polymer vehicle carrier and the functional phase were selected. The detailed description of the paste composition of the composites is described below. As a base polymer material, polymethacrylates (PMMA) and fluorovinylidene (PVDF) materials were used, previously applied for the printing of electronic circuits on elastic foils or glass substrates [14]. They were selected by the criteria of easy processing and low cost, which allows for the production of low-cost printed electronic circuits. These polymers allow the formation of composite pastes with a rheology suitable for applying layers with the screen printing technique. The resulting layers should have a specific electrical properties (sheet resistivity/resistance) and acceptable mechanical properties, allowing them to be flexible. In the study the following polymers were used: SensorsSensors2021 2021, 21, 21, 4038, x FOR PEER REVIEW 44 of of 14 15

• • organicorganic vehicle vehicle in in the the form form of of poly poly (methyl-methacrylate) (methyl-methacrylate) (PMMA) (PMMA) having having a molecular a molec- weightular weight of 350,000 of 350,000 mol supplied mol supplied by Sigma-Aldrich by Sigma-Aldrich dissolved dissolved in butyl in butyl carbitol carbitol acetate ac- (OKB)etate (OKB) with a with concentration a concentration of 8 wt.%; of 8 wt.%; •• LuxPrintLuxPrint DuPont DuPont 8155 8155 resin resin (available (available commercially), commercially), intended intended for for the the manufacturing manufactur- ofing composite of composite pastes pastes for screen for screen printing, printing, in particular in particular for the forcompositions the compositions compris- com- ingprising metal metal powders powders and and ceramics. ceramics. The The main ma componentin component of theof the fluoroelastomer fluoroelastomer is ais vinylidenea vinylidene (PVDF) (PVDF) dissolved dissolved in ethylin ethyl acetate acetate cellosolve cellosolve (OCE). (OCE). ForFor thethe functionalfunctional phase,phase, thethe followingfollowing electricallyelectrically conductiveconductiveparticles particles werewere selected:selected: silversilver particles,particles, graphite,graphite, graphenegraphene platelets,platelets, multi-walledmulti-walled carboncarbon nanotubes.nanotubes. Commer-Commer- ciallycially available available multi-walledmulti-walled carboncarbon nanotubesnanotubes (MWCNTs,(MWCNTs, CNTs)CNTs) from from CheapTubes CheapTubes were were used,used, whilewhile thethe graphenegraphene plateletsplatelets (GNPs)(GNPs) werewere supplied supplied and and prepared prepared at at the the Institute Institute of of ElectronicElectronic Materials Materials Technology Technology (ITME) (ITME) in in Warsaw. Warsaw. In In the the study, study, two two types types of theof the following follow- compositeing composite pastes pastes were were used: used: •• commerciallycommercially available available composite composite paste paste with with silver silver particles particles (L-121, (L-121, from from Institute Institute of of ElectronicsElectronics Materials Materials Technology, Technology, Warsaw, Warsaw, Poland) Poland) and graphiteand graphite paste paste (421 electrodag (421 elec- SS,trodag Acheson), SS, Acheson), •• developeddeveloped pastes pastes containing containing graphene graphene and and carbon carbon nanotubes. nanotubes. CompositeComposite withwith silver conductive conductive phase phase pastes pastes contained contained commercially commercially available available sil- silverver flakes flakes AX20LC AX20LC (Amepox) (Amepox) with with 99% 99% Ag Ag purity, purity, a amean mean flake flake size size of of 2–4 2–4 µm.µm. FigureFigure1 1shows shows an an image image taken taken using using SEM SEM (Scanning (Scanning Electron Electron Microscope), Microscope), presenting present- theing characteristic the characteristic dimensions dimensions of multi-walled of multi-walled carbon carbon nanotubes nanotubes used used in the in research. the research. The meanThe mean values values of the of characteristic the characteristic dimensions dimensions are: diameter are: diameter (10 ÷ 50) (10 nm, ÷ 50) a length nm, a of length 0.5 µm of to0.5 15 µmµm. to 15 µm.

FigureFigure 1. 1.Scanning Scanning ElectronElectron MicroscopyMicroscopy micrographmicrograph imageimage ofof usedused multi-walledmulti-walled carboncarbon nanotubes-nanotubes- AURIGAAURIGA Cross Cross Beam Beam Workstation Workstation (Carl (Carl Zeiss). Zeiss).

GrapheneGraphene flakesflakes werewere preparedprepared inin ITME ITME with with direct direct exfoliation exfoliation process. process. TheThe basebase materialmaterial in in form form of of graphite graphite flakes flakes (180–300 (180–300µm) µm) was was added added to the to the surfactant, surfactant, and and deionized deion- water.ized water. Then, Then, the processes the processes of intercalation of intercalatio expansion,n expansion, exfoliation, exfoliation, freezing freezing and freeze and drying freeze anddrying separation and separation of graphene of graphe flakesne was flakes carried was carried out. Figure out. 2Figure presents 2 presents the results the results of SEM of analysis.SEM analysis. Electrical Electrical properties properties of graphene of graphene flakes flakes are as are follows: as follows: • • resistanceresistance R Rss= = 0081 ΩΩ/□ •• conductivityconductivityδ δ= = 115,525115,525 S/mS/m •• carriercarrier mobility mobility 172 172 cm cm22/V/V∙·ss •• carriercarrier concentration concentration− −6.618 × 10101717 cmcm−2− 2 Obtained graphene nanoplatelets characteristic dimensions are: diameter approx. 15 microns thickness of less than 20 layers.

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Figure 2. Scanning Electron Microscopy micrograph image of used graphenegraphene platelets-AURIGAplatelets-AURIGA Cross Beam Workstation (Carl Zeiss). 2.1.3. Printed Samples Preparation Obtained graphene nanoplatelets characteristic dimensions are: diameter approx. 15 micronsIn the thickness study, theof less following than 20 marking layers. of the samples were applied: • X-the Roman numeral indicating the variant with respect to the chemical composition 2.1.3.of Printed the paste Samples (I to XV) Preparation • InY-Arabic the study, number the following means a seriesmarking of producedof the samples pastes were (for applied: the series in this study it is always 2), • X-the Roman numeral indicating the variant with respect to the chemical composi- • Z-Arabic number indicates one of three textile materials used as the substrate (e.g., 1, tion of the paste (I to XV) 2, 3). • Y-Arabic number means a series of produced pastes (for the series in this study it is alwaysBelow in2), Table 2, the sample characteristics of used variants of pastes are presented. • Z-Arabic number indicates one of three textile materials used as the substrate (e.g., Table 2. Characteristics of the samples. 1, 2, 3). Variant of Chemical Composition of the PasteBelow in The Table Percentage 2, the of Functionalsample Phasecharacteristics in the Paste of used variants Polymer of Vehiclepastes Paste are presented. I Silver flakes 34% PMMA in OKB, paste L121 (from ITME) II Silver flakes PVDF in OCE Table 2. Characteristics of the samples. Commercially available paste Electrodag 421 III Graphite flakes SS(Acheson) E421SS IVVariant of Graphen nanoplates GNP 5 wt.% PMMA in OKB V Graphen nanoplates GNP 5 wt.% PVDF in OCE VIChemical Multi-walledThe Percentage carbon nanotubes of Functional MWCNT 1 wt.% Phase PMMA in OKB VII Multi-walled carbon nanotubes MWCNT 3 wt.%Polymer PMMA Vehicle in OKB Paste VIIIComposition Multi-walled carbon nanotubesin the Paste MWCNT 5 wt.% PMMA in OKB IXof the Paste Graphen nanoplates GNP 7 wt.% PMMA in OKB X Graphen nanoplates GNP 10 wt.% PMMA in OKB XI Graphen nanoplates GNP 7 wt.%34% PMMA PVDF in in OKB, OCE paste L121 XIII Graphen nanoplatesSilver GNP flakes 10 wt.% PVDF in OCE XIII Multi-walled carbon nanotubes MWCNT 5 wt.% PVDF(from in ITME) OCE XIV Multi-walled carbon nanotubes MWCNT 1 wt.% PVDF in OCE XVII Multi-walled carbon nanotubes Silver flakes MWCNT 3 wt.% PVDF PVDF in in OCE OCE Commercially available paste III Graphite flakes Electrodag 421 SS(Acheson) E421SS Prepared composite pastes were deposited on the textile substrates with the screen- printingIV technique Graphen on an AMI nanoplates Presco 242GNP screen-printer 5 wt.% with 67T PMMA in polyester OKB screens. The printedV layers Graphen were cured nanoplates at 120 ◦ CGNP for one5 wt.% hour in a laboratory PVDF oven in OCE dryer. Multi-walled carbon nanotubes EachVI of the three selected textile materials was covered byPMMA fifteen in variants OKB of pastes with different compositionMWCNT described 1 inwt.% Table 2 (designated I-XV) in the model form shown in Figures3 and4. AsMulti-walled an example carbon designation, nanotubes IV. 2.1 means a sample where, on the textile VII PMMA in OKB no. 1 (non-woven fabric laminatedMWCNT 3 filmwt.% PTFE), a paste with a composition of 5 wt.% of graphene plateletsMulti-walled in PMMA was carbon applied. nanotubes Due to the structure of the fabrics it is very hard VIII PMMA in OKB to measure the thickness ofMWCNT printed layers.5 wt.% However, the thickness of the printed layers can be estimated to be around 10 µm, as observed for the layers printed with the same screen IX Graphen nanoplates GNP 7 wt.% PMMA in OKB on a rigid, flat substrate. X Graphen nanoplates GNP 10 wt.% PMMA in OKB XI Graphen nanoplates GNP 7 wt.% PVDF in OCE XII Graphen nanoplates GNP 10 wt.% PVDF in OCE Multi-walled carbon nanotubes XIII PVDF in OCE MWCNT 5 wt.%

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Multi-walled carbon nanotubes XIV Multi-walled carbon nanotubes PVDF in OCE XIV MWCNT 1 wt.% PVDF in OCE MWCNT 1 wt.% Multi-walled carbon nanotubes XV Multi-walled carbon nanotubes PVDF in OCE XV MWCNT 3 wt.% PVDF in OCE MWCNT 3 wt.% Prepared composite pastes were deposited on the textile substrates with the screen- Prepared composite pastes were deposited on the textile substrates with the screen- printing technique on an AMI Presco 242 screen-printer with 67T mesh polyester screens. printing technique on an AMI Presco 242 screen-printer with 67T mesh polyester screens. The printed layers were cured at 120 °C for one hour in a laboratory oven dryer. The printed layers were cured at 120 °C for one hour in a laboratory oven dryer. Each of the three selected textile materials was covered by fifteen variants of pastes Each of the three selected textile materials was covered by fifteen variants of pastes with different composition described in Table 2 (designated I-XV) in the model form with different composition described in Table 2 (designated I-XV) in the model form shown in Figures 3 and 4. As an example designation, IV. 2.1 means a sample where, on shown in Figures 3 and 4. As an example designation, IV. 2.1 means a sample where, on the textile no. 1 (non-woven fabric laminated film PTFE), a paste with a composition of 5 the textile no. 1 (non-woven fabric laminated film PTFE), a paste with a composition of 5 wt.% of graphene platelets in PMMA was applied. Due to the structure of the fabrics it is wt.% of graphene platelets in PMMA was applied. Due to the structure of the fabrics it is very hard to measure the thickness of printed layers. However, the thickness of the Sensors 2021, 21, 4038 very hard to measure the thickness of printed layers. However, the thickness of6 ofthe 14 printed layers can be estimated to be around 10 µm, as observed for the layers printed printed layers can be estimated to be around 10 µm, as observed for the layers printed with the same screen on a rigid, flat substrate. with the same screen on a rigid, flat substrate.

FigureFigure 3.3. GeneralGeneral viewview ofof the topology of electro-conductiveelectro-conductive printedprinted pathspaths forfor allall prepared samples–samples– Figurehere silver 3. General conductive view ofpaths. the topology of electro-conductive printed paths for all prepared samples– here silversilver conductiveconductive paths.paths.

Figure 4. Views of the samples covered with paste of the composition of carbon nanotubes XIV 1 Figurewt.% PVDF 4.4. Views OCE ofof three thethe samples samplesfabrics: ( coveredcoveredA): material withwith 1, pastepaste (B): material ofof thethe compositioncomposition 2, (C): material ofof carboncarbon 3. nanotubesnanotubes XIVXIV 11 wt.%wt.% PVDFPVDF OCEOCE threethree fabrics:fabrics: ((AA):): materialmaterial 1,1, ((BB):): materialmaterial 2,2, ((CC):): materialmaterial 3.3. 2.2. Test Methods 2.2. Test MethodsMethods For the evaluation of developed paths, the following laboratory tests were performed: adhesion tests and surface resistance of new material, as well as after washing and bending cycles.

2.2.1. Adhesion Test For the evaluation of the adhesion of the printed layer to the fabric, the scotch tape test was performed. The tape was adhered to the surface of the sample and pressed with a finger, then allowed to stabilize for five minutes. In the next step the tape was torn from ◦ the surface at an angle of 180 with two speeds: v1 = 20 mm/s and v2 = 40 mm/s. The amount of deposited material on the surface of the adhesive tape, as well as the defects in the printed paths, were assessed. To evaluate the adhesion, a five-stage scale of 0–4 was used. The test method of rating adhesion by the tape test is described in ASTM D3359 [23].

2.2.2. Surface Resistance For each sample, the surface resistance of the printed path was measured. Measure- ments were made using a laboratory digital multimeter Agilent 34401Ain an ambient temperature of 21◦C and relative humidity of 45%. Due to the different width of electrically conductive paths, measurements were performed for each path and then the appropriate Sensors 2021, 21, 4038 7 of 14

conversion to sheet resistance was applied, with respect to the number of geometry squares in the measured topology (sheet resistance is expressed in Ω/ or Ω/sq units). Conversion to the sheet resistance is the most common approach in characterising electrical properties of conductive thin and thick films and other planar systems. The value of sheet resistance is not related to the scaling of the planar geometry, allowing the comparison of the electrical properties of different size layers and electrodes. The value of sheet resistance is calculated with the formula: ρ L L R = = R (1) t W s W where R is measured resistance, ρ is bulk resistivity, t is thickness of the layer, L is length, and W is width. This way, having measured resistance and knowing the planar geometry of the sample (length and width), we can calculate uniform sheet resistance by one square surface of the layer, regardless of the actual length and width of the measured sample:

W R = R (2) s L This approach eliminates the element of layer thickness, which is especially problem- atic to measure and estimate for the conductive lines printed on wavy textile substrate. For each sample, five measurements were performed. Then, the results for each sample were averaged, and the final result was the lowest resistance value for the measured samples. Differences in the resistance due to width of the electro-conductive paths were also taken into account.

2.2.3. Surface Resistance after Washing and Bending Cycles In order to assess how maintenance cycles affect the properties of the electrically conductive paths printed samples were subjected to the 10 washing cycles. Washing was performed in accordance with EN ISO 6330: 2012 [24]. Washing was performed by hand at 30 ◦C using a detergent containing no phosphate, no optical brightener and without enzymes. The obtained samples were also subjected to a series of bending cycles with the Sensors 2021, 21, x FOR PEER REVIEWfollowing number of cycles: 10, 20, 30, 50. After each series, the surface resistance8 wasof 15 measured. Bending cycles were performed by means of specially designed device presented in Figure5.

Figure 5. Views on the mechanical bendingbending cyclescycles atat differentdifferent stagesstages ofof sample’ssample’s bending.bending.

Final influenceinfluence of thethe washingwashing andand mechanicalmechanical bendingbending cyclescycles andand waswas evaluatedevaluated comparing thethe finalfinal value value of of resistance resistance (R (Rk)k) with with the the initial initial value value of of resistance resistance after after printing print- (ingR0). (R The0). The relative relative change change in resistance in resistance was was determined determined with with the Equation:the Equation: R − R ΔR= 0 k × 100% RR0 − Rk (3) ∆R = 0 × 100% (3) R0 3. Results 3.1. Adhesion Test Results Results of the adhesion tests are presented in Tables 3–5. The higher the value of the adhesion index, the worse the adhesion of the application to the substrate.

Table 3. Results of the adhesion test of samples with electro-conductive paste I (with silver in PMMA), II (with silver in PVDF) and III (with graphite in PMMA) on three textile substrates.

I. 2.1 I. 2.2 I. 2.3 II. 2.1 II. 2.2 II. 2.3 III. 2.1 III. 2.2 III. 2.3 No. of Sample PMMA PVDF PMMA v1 0 0 0 0 0 0 4 0 0 v2 4 2 2 4 1 1 4 1 1

Table 4. Results of the adhesion test of samples with electro-conductive paste with nanotubes con- tent in PMMA and PVDF on three textile substrates.

MWNT PMMA PVDF Nanotubes Adhesion Adhesion No. of No. of Content in Sample v1 v2 Sample v1 v2 Polymer VI. 2.1 0 4 XIV. 2.1 1 4 1% VI. 2.2 0 0 XIV. 2.2 0 0 VI. 2.3 0 1 XIV. 2.3 0 0 VII. 2.1 4 4 XV. 2.1 4 4 3% VII. 2.2 1 1 XV. 2.2 0 1 VII. 2.3 1 1 XV. 2.3 0 1 VIII. 2.1 4 4 XV. 2.1 3 3 5% VIII. 2.2 1 1 XV. 2.2 1 2 VIII. 2.3 1 1 XV. 2.3 1 2

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3. Results 3.1. Adhesion Test Results Results of the adhesion tests are presented in Tables3–5. The higher the value of the adhesion index, the worse the adhesion of the application to the substrate.

Table 3. Results of the adhesion test of samples with electro-conductive paste I (with silver in PMMA), II (with silver in PVDF) and III (with graphite in PMMA) on three textile substrates.

I. 2.1 I. 2.2 I. 2.3 II. 2.1 II. 2.2 II. 2.3 III. 2.1 III. 2.2 III. 2.3 No. of Sample PMMA PVDF PMMA

v1 0 0 0 0 0 0 4 0 0 v2 4 2 2 4 1 1 4 1 1

Table 4. Results of the adhesion test of samples with electro-conductive paste with nanotubes content in PMMA and PVDF on three textile substrates.

PMMA PVDF

MWNT Nanotubes Content in Polymer No. of Adhesion No. of Adhesion

Sample v1 v2 Sample v1 v2 VI. 2.1 0 4 XIV. 2.1 1 4

1% VI. 2.2 0 0 XIV. 2.2 0 0 VI. 2.3 0 1 XIV. 2.3 0 0 VII. 2.1 4 4 XV. 2.1 4 4

3% VII. 2.2 1 1 XV. 2.2 0 1 VII. 2.3 1 1 XV. 2.3 0 1 VIII. 2.1 4 4 XV. 2.1 3 3

5% VIII. 2.2 1 1 XV. 2.2 1 2 VIII. 2.3 1 1 XV. 2.3 1 2

Table 5. Results of the adhesion test of samples with electro-conductive paste with graphene flakes content in PMMA and PVDF on three textile substrates.

PMMA PVDF Graphene Nanoplates Adhesion Adhesion Content in Polymer No. of No. of Sample v1 v2 Sample v1 v2 IV. 2.1 1 3 V. 2.1 3 3 5% IV. 2.2 3 3 V. 2.2 2 2 IV. 2.3 3 3 V. 2.3 2 3 IX. 2.1 2 3 IX. 2.1 3 3 7% IX. 2.2 2 3 IX. 2.2 1 2 IX. 2.3 2 2 IX. 2.3 2 3 X. 2.1 3 3 XII. 2.1 3 3 10% X. 2.2 2 2 XII. 2.2 1 2 X. 2.3 2 2 XII. 2.3 2 2

The study showed (Table3) that the applications of pastes I and II containing the silver particles in PMMA and PVDF respectively on the three textile substrates are resistant to tearing at slow rate of v1 = 20 mm/s. Lesser adhesion was observed for the samples evaluated at the speed v2 = 40 mm/s. The worst results of the adhesion test were observed for the pastes printed on substrate 1, having a smooth surface of PTFE film. For the graphite paste, the worst adhesion properties were also demonstrated for substrate 1 (adhesion value 4). For the other substrates (2 and 3) adhesion value was acceptable (0 and 1). The study showed (Table4) that the use of nanofillers in the form of multi-walled carbon nanotubes, 1 wt.%, 3 wt.% and 5 wt.%, respectively, does not have any negative Sensors 2021, 21, 4038 9 of 14

influence on the adhesion to the fabric substrate 2 and 3, and it is comparable with the results obtained for the pastes with the silver particles printed on substrate 1. In the study (Table5) it was observed that the use of 5 wt.%, 7 wt.% and 10 wt.%. of the graphene nanoplatelets in the paste showed inferior adhesion to the textile substrate in relation to the nanofiller in the form of nanotubes. In this case, there is also a tendency of higher adhesion to the textile substrate 2 and 3 than to the PTFE coated film on substrate 1. The best adhesion was observed for the pastes with nanotubes in PMMA and PVDF vehicles on substrates 2 and 3.

3.2. Surface Resistance Results In the Figure6 the results of measurements of the surface resistance for the pastes Sensors 2021, 21, x FOR PEER REVIEW 10 of 15 containing silver particles (I) and graphite (III) are presented. Pastes with the silver particles Sensors 2021, 21, x FOR PEER REVIEW(I) are treated as the reference samples for the developed paste containing carbon10 of nanotubes,15

and graphene.

FigureFigureFigure 6. 6.Results Results of of theof the thesurface surface surface resistance resistance resistance measurements measurements measurements for for pastes pastes for containing:pastes containing: containing: (A)–silver (A)–silver (micro-A)–silver microflakes micro- flakes and (B)–graphite microflakes, both in PMMA vehicle on three types of textile substrates. andflakes (B)–graphite and (B)–graphite microflakes, microflakes, both in PMMAboth in PMMA vehicle onvehi threecle on types three of types textile of substrates. textile substrates. The resistance of the paths (Figure 6) printed from pastes with silver in PMMA vehi- The resistance of the paths (Figure6) printed from pastes with silver in PMMA vehicle cle on Thesubstrate resistance 1 (non-woven) of the paths is 0.25 (Figure Ω/□ compared 6) printed with from 0.11 pastes Ω/□ onwith substrate silver in3 (thePMMA vehi- onwovencle substrate on fabric substrate 1of (non-woven) aramid 1 (non-woven) fibers). is 0.25For Ωtheis /0.25 pastescompared Ω/ □with compared graphite, with 0.11 with theΩ /values 0.11 on Ω substrateare/□ 22on Ωsubstrate/□ 3 for (the woven 3 (the fabricsubstratewoven of aramidfabric1 (nonwoven of fibers). aramid with For fiPTFEbers). the pastesmembrane) For withthe pastesand graphite, 28 Ωwith/□ thefor graphite, valuessubstrate are the 3 (aramid 22 valuesΩ/ fabric), forare substrate 22 Ω/□ for 1 (nonwovenrespectively.substrate 1 with These(nonwoven PTFE results membrane) representwith PTFE the and membrane) samp 28 lesΩ/ made for and with substrate 28 the Ω /use□ for 3 of (aramid commerciallysubstrate fabric), 3 (aramid avail- respectively. fabric), Theseablerespectively. paste, results therefore, representThese they results thecan representbe samples treated made asthe a sampreference. withles the made Obtained use with of commerciallyresults the use suggest of commercially availablethat the paste, avail- therefore,pasteable withpaste, theysilver therefore, canplates be allowed treatedthey can asfor be aobtaining reference.treated asmuch Obtaineda reference. lower surface results Obtained resistance suggest results thatthan suggest thein the paste that with the silvercasepaste of platesthewith paste silver allowed with plates graphite. for obtainingallowed for much obtaining lower much surface lower resistance surface than resistance in the casethan of in the the pastecaseIn withof the the Figure graphite. paste 7 withthe values graphite. of resistance measurements results for the pastes with dif- ferent content of carbon nanotubes in PMMA and PVDF are presented. InIn the the FigureFigure7 7 the the valuesvalues ofof resistance me measurementsasurements results results for for the the pastes pastes with with dif- differentferent content content of of carbon carbon nanotubes nanotubes in in PMMA PMMA and and PVDF PVDF are are presented. presented.

A B Figure 7. Test results of surface resistance measurements for the pastes with different wt.% content of carbon nanotubes (CNT) in: (A)–PMMA and (B)–PVDF vehicles, printed on substrates 1, 2 and 3. A B

FigureFigureIn 7.Figure 7.Test Test results8, results the values of of surface surface of resistance resistance resistance meas measurements measurementsurement results for for the forthe pastesthe pastes pastes with with with different different different wt.% wt.% content content content of graphene nanoplatelets in PMMA and PVDF are presented. ofof carbon carbon nanotubes nanotubes (CNT) (CNT) in: in: ( A(A)–PMMA)–PMMA and and ( B(B)–PVDF)–PVDF vehicles, vehicles, printed printed on on substrates substrates 1, 1, 2 2 and and 3. 3.

In Figure 8, the values of resistance measurement results for the pastes with different content of graphene nanoplatelets in PMMA and PVDF are presented.

Sensors 2021, 21, 4038 10 of 14

Sensors 2021, 21, x FOR PEER REVIEW 11 of 15 Sensors 2021, 21, x FOR PEER REVIEW In Figure8, the values of resistance measurement results for the pastes with different11 of 15

content of graphene nanoplatelets in PMMA and PVDF are presented.

A B B

FigureFigureFigure 8.8. 8. Test Test resultsresults results of of surfacesurface resistanceresistance measurements measurementsmeasurements for forfor the thethe pastes pastespastes with withwith different differentdifferent wt.% wt.%wt.% content contentcontent ofofof graphenegraphene graphene nanoplateletsnanoplatelets nanoplatelets inin AA–PMMA–PMMA and and B BB–PVDF,–PVDF,–PVDF, on onon substrates substratessubstrates 1, 1, 1,2 2and 2 and and 3. 3. 3.

InInIn general, general, almost almost all all variantsvariants variants ofof of the the the past past pasteseses noted noted noted their their their highest highest highest value value value of surfaceof ofsurface surface re- re- resistancesistancesistance for for for substrate substrate substrate 2. 2. This This phenomenonphenomenon phenomenon is is isobserved observed observed under under under SEM SEM SEM inspections inspections inspections (Figure (Figure 9), 9 9),), wherewherewhere wewe we cancan can see seesee the thethe unevenuneven distributiondistribution of ofof the thethe paste pastepaste and andand its itsits discontinuity discontinuitydiscontinuity on on onthe thethe surface surfacesurface ofofof thethe the fabrics.fabrics. fabrics. ThisThis isisis duedue toto the highly highly uneven unevenuneven surface surfacesurface of ofof substrate substratesubstrate textile textiletextile 2, 2,which2, whichwhich has hashas the the the highest surface mass of the three substrates used, with the highest differences in the tex- highesthighest surfacesurface massmass of of the the three three substrates substrates used, used, with with the the highest highest differences differences in the in the texture tex- ture of the material. Such geometry of the material causes agglomeration of the applied ofture the of material. the material. Such Such geometry geometry of the of material the material causes causes agglomeration agglomeration of the of applied the applied paste paste in the hollow spaces between the weave of the fabric and creates its local disconti- inpaste the in hollow the hollow spaces spaces between between the weave the weave of the fabricof the and fabric creates and creates its local its discontinuity local disconti- of nuity of printed paths in the peeks of the weave of the fabric. The unusual results related printed paths in the peeks of the weave of the fabric. The unusual results related to the nuityto the of local printed increase paths of in the the sheet peeks resistivity of the weave for the of composite the fabric. pastes The unusualwith higher results concen- related local increase of the sheet resistivity for the composite pastes with higher concentration of totration the local of conductive increase of filler the is sheet due to resistivity the higher for concentration the composite of the pastes filler resultingwith higher in higher concen- conductive filler is due to the higher concentration of the filler resulting in higher viscosity trationviscosity of conductiveof the paste, filler which is mightdue to result the higher in uneven concentration printing on of the the wavy filler textile resulting substrate, in higher of the paste, which might result in uneven printing on the wavy textile substrate, creating viscositycreating oflocal the discontinuities paste, which might in the resultlayer or in reducing uneven printingits thickness. on the wavy textile substrate, localcreating discontinuities local discontinuities in the layer in the or reducing layer or reducing its thickness. its thickness.

Figure 9. Scanning Electron Microscopy micrograph image presenting the morphology of the sam- ple surface with printed graphene paste containing 10 wt.% GNP in PMMA organic vehicle. FigureFigure 9.9. ScanningScanning ElectronElectron Microscopy Microscopy micrograph micrograph image image presenting presenting the the morphology morphology of theof the sample sam- surfaceple3.3. surface Surface with with printedResistance printed graphene after graphene Washing paste paste containing and contai Bendingning 10 wt.% Cycles10 wt.% GNP GNP in PMMA in PMMA organic organic vehicle. vehicle. The aim of the study was to simulate the use of materials with electrically conductive 3.3.3.3. Surface Resistance afterafter WashingWashing andand BendingBending CyclesCycles tracks. The greater the index of change in resistance, the worse its resistance to either washingTheThe aimaim or bending ofof thethe study of the was electro-conductive to simulate thethe usepath. of For materials most variants with electrically a significant conductiveconductive influ- tracks.tracks.ence of The washing greater and the bending index cycles of changechange was observ inin resistance,reed,sistance, therefore, thethe only worseworse selected itsits resistanceresistance results are toto pre- eithereither washingwashingsented. or or bending bending of of the the electro-conductive electro-conductive path. path. For For most most variants variants a significant a significant influence influ- ofence washing ofOn washing the and basis bending and of bendingthe cyclesperformed cycles was observed, measurementswas observ therefore,ed,, therefore,it was only indicated selected only selected that results sample results are presented.VII. are 2.3 pre- sented.(withOn 3% the wt. basis nanotubes of the performed in PMMA) measurements, showed the lowest it was rate indicated (about 15%) that sampleof resistance VII.2.3 (withchangesOn 3% the after wt. basis nanotubesthe ofwashing the performed in cycles. PMMA) These measurements showed result thes are lowest, coherentit was rate indicated with (about observation that 15%) sample of that resistance VII. the 2.3 (with 3% wt. nanotubes in PMMA) showed the lowest rate (about 15%) of resistance changes after the washing cycles. These results are coherent with observation that the

Sensors 2021, 21, 4038 11 of 14 Sensors 2021, 21, x FOR PEER REVIEW 12 of 15

Sensors 2021, 21, x FOR PEER REVIEW 12 of 15 changes after the washing cycles. These results are coherent with observation that the samplesample VII. 2.3 2.3 is is characterized characterized by by the the best best ad adhesionhesion of ofthe the paste paste to the to the substrate substrate from from the the samples with 5% nanotube content. For the rest of the samples, the observed change samples with 5%sample nanotube VII. 2.3 content. is characterized For the byre stthe of best the ad samples,hesion of the the observed paste to the change substrate in from the insurface surface resistance resistancesamples was was withshaping shaping 5% nanotube between between content.25%—for 25%—for For the the samples samplesrest of the with with samples, 1% 1% content the observed of thethe change in nanotubes inin PMMAsurface to resistance 50%—for was the samplesshaping withbetween 5% content25%—for of thethe samplesnanotubes with in PMMA. 1% content of the InIn thethe casecase ofof thethenanotubes samplessamples in withwith PMMA thethe to 5%,5%, 50%—for 7%7% andand the 10%10% samples contentcontent with ofof thethe5% graphenecontentgraphene of the nanoplates,nanoplates, nanotubes aa in PMMA. higher raterate ofof changechangeIn the case ofof thethe of the surfacesurface samples resistanceresi withstance the waswas 5%, observed,observed, 7% and 10% whichwhich content exceededexceeded of the graphene 50%.50%. nanoplates, a The resultshigher ofof thethe rate changeschanges of change inin surfacesurface of the surface resistanceresistance resi ofstanceof thethe selectedselectedwas observed, samplessamples which areare presentedexceededpresented 50%. inin FiguresFigures 1010 andand 1111.The. results of the changes in surface resistance of the selected samples are presented in Figures 10 and 11.

Figure 10. Test results of the change in surface resistance after bending cycles–III. 2.3. Figure 10. Test resultsFigure of10. the Test change results in of surface the change resistance in surface after resistance bending after cycles–III. bending 2.3. cycles–III. 2.3.

Figure 11. FigureTest results 11. Testof the results change of in the surface change resistance in surface after resistance bending cycles after for bending the pastes cycles with for different the pastes wt.% with content of Figure 11. Test results of the change in surface resistance after bending cycles for the pastes with different wt.% content of nanotubes in PMMA in A–1% wt. and B–3% wt., on substrate 3. nanotubes in PMMA in A–1%different wt. and wt.% B–3% content wt., on of substrate nanotubes 3. in PMMA in A–1% wt. and B–3% wt., on substrate 3. On the basis ofOn the the obtained basis of resultsthe obtained (Figures results 10 and (Figures 11), it 10 can and be 11), stated it can that be bendingstated that bending On the basis of the obtained results (Figures 10 and 11), it can be stated that bending has a negative influencehas a negative on the influence surface resistanceon the surface of the resistance tested samples. of the tested In the samples. case of theIn the III. case of the has a negative influence on the surface resistance of the tested samples. In the case of the 2.3 sample withIII. graphite 2.3 sample the changewith graphite in surface the resistancechange in aftersurface 50 bendingresistance cycles after reached50 bending cycles III. 2.3 sample with graphite the change in surface resistance after 50 bending cycles more than 20%.reached A similar more result than (close20%. A to similar 20%) wasresult obtained (close to in 20%) the was case obtained of VI. 2.3 in sample the case of VI. 2.3 reached more than 20%. A similar result (close to 20%) was obtained in the case of VI. 2.3 with 1% wt. nanotubessample with in PMMA. 1% wt. nanotubes The lowest in change PMMA. in The surface lowest resistance, change in that surface in addition resistance, that in sample with 1% wt. nanotubes in PMMA. The lowest change in surface resistance, that in was stable withaddition further was bending stable cycles,with further was obtainedbending cycles in the, was case obtained of VII. 2.3 in the with case 3% of wt. VII. 2.3 with nanotubesaddition was in PMMAstable3% wt. with andnanotubes further did not in bending exceed PMMA 6%. andcycles did, was not exceedobtained 6%. in the case of VII. 2.3 with 3% wt. nanotubes in PMMA and did not exceed 6%. 4. Discussion 4. Discussion 4. Discussion In the studies itIn wasthe studies found thatit was the found PMMA that vehiclethe PMMA allows vehicle for obtainingallows for obtaining layers with layers with a a lowerIn the surface studieslower resistance it surfacewas found than resistancePVDF that the than vehicle PMMA PVDF for vehicle vehicle pastes allows containingfor pastes for containing obtaining a comparable alayers comparable content with a content of oflower graphene surface nanoplatelets. resistancegraphene nanoplatelets.than An PVDF exception vehicle An isexception thefor pastepastes is with containingthe paste 10 wt.% with a ofcomparable 10 graphene wt.% of incontentgraphene PMMA of in PMMA printedgraphene on nanoplatelets. substrateprinted 3. on For substrateAn the exception paste 3. basedFor is the th on pastee PVDF,paste based with a decrease on 10 PVDF, wt.% in a theofdecrease graphene value in of the the in value surfacePMMA of the surface resistanceprinted on with substrateresistance increasing 3. For with contentthe increasing paste of based graphene content on PVDF, nanoplateletsof graphe a decreasene nanoplatelets was in observed.the value was of At observed. the the surface same At the same resistance with increasing content of graphene nanoplatelets was observed. At the same

Sensors 2021, 21, 4038 12 of 14

time, for pastes based on PMMA a decrease in the surface resistance with graphene content of 7 wt.% relative to the 5 wt.% was observed, with an unexpected increase in the value of resistance for 10 wt.% of graphene also observed. This is due to the higher viscosity of the 10 wt.% pastes negatively influencing the printed paths’ uniformity, and hence the conclusion is that for the paste with the graphene nanoplatelets the content of 7 wt.% is most preferred. Analyzing the results of resistance measurements of pastes with CNTs on substrates 1 and 3 it is noted that, except for the case with the content of 1 wt.% carbon nanotubes in the PMMA polymer, nanotubes in PMMA allow obtaining a lower surface resistance than the PVDF pastes with a comparable content of nanotubes. Application potential of screen-printed electro-conductive fabrics with CNTs was confirmed e.g., by Kruci´nska et al. [13]. Increasing the content of nanotubes in the polymer from 1 wt.% to 3 wt.% caused a decrease in resistance, with a very small increase of resistance for the 5 wt.% samples–also due to the higher viscosity negatively influencing the printing process, as observed for GNPs. It was found that the lowest resistance was obtained for substrate 3 and pastes with 3 wt.% of nanotubes in PMMA vehicle and on substrate 1 and pastes with 5 wt.% of nanotubes in PMMA vehicle, respectively. This result is in coherence with results obtained e.g., by Kazani et al. [1] who also analyzed the influence of various textile substrate on the surface resistance of the samples made with a use of the screen printing technique. The observed unusual increase of the sheet resistivity for the composite pastes with a higher concentration of carbon nanotubes (5 wt.%) and graphene nanoplatelets (10 wt.%) needs additional explanation. While a higher concentration of conductive filler should decrease the sheet resistance, it is at the same time negatively affecting the printing process by increasing the viscosity of the pastes. This results in uneven printing on the wavy textile substrate, creating local discontinuities in the layer or reducing its effective thickness. Such unfavourable phenomenon can be eliminated but needs additional steps in materials prepa- ration. Such an approach should be focused on the optimisation of the paste composition. The addition of viscosity modifiers or dispersing agents should influence the viscosity of the paste, tailoring the rheology for the printing process, even with a higher amount of nanofiller. Obtained this way, printed layers should be uniform and continuous even when printed on high roughness substrates, such as textiles. The analysis of the final results of the surface resistance measurements after washing cycles led to the conclusion that the best results were obtained for the pastes with CNTs in PMMA vehicle on substrate 3. For the sample with 3 wt.% of CNTs in PMMA on substrate 3, the lowest value of resistance change after washing cycles was observed which was equal to 15%. The obtained results correspond to the observations that the same paste has the best adhesion to the substrate. For the remaining samples, the results were 25% for 1 wt.% and 50% for 5 wt.% of the carbon nanotubes in PMMA, respectively. Our results confirmed that the resistance of the screen-printed electro-conductive layer to washing cycles is limited and requires further processing in order to avoid increasing of the surface resistance after such maintenance processing. This problem was observed also by other authors who look for protecting the electro-conductive layer by application of additional PU coating [1,22]. For most of the samples exposed to bending cycles, the change of the value of coeffi- cient of resistance after 10, 20, 30 and 50 cycles was high. For pastes with silver particles in the PMMA vehicle on three substrates, the values of resistance changed from 50% after 10 cycles to 85% after 50 cycles, respectively. These results indicate that this type of paste with silver particles on the textile substrates is not resistant to the regular use in textronics application in clothing, despite the lowest resistance just after printing. For pastes with graphite, the worst results of the surface resistance on samples before the pre-treatment were obtained for pastes applied on substrate 2, but at the same time the lowest change in resistance after 50 cycles of bending was observed for the paste with graphite applied on this substrate (about 23%). For pastes with graphene nanoplatelets the coefficient of Sensors 2021, 21, 4038 13 of 14

resistance changes were obtained from 10% up to 100% in the case of pastes with PVDF vehicle. In further research, a higher number of bending cycles should be considered.

5. Conclusions In the study, the developed electrically conductive paths printed on three types of textile substrates were presented. The following conclusions summarize our observations: • Geometry and the type of substrate affect the adhesion of the paste. The poorest adhesion to the substrate was observed for the smooth Teflon (PTFE) coating. The best adhesion was observed for the multi-walled carbon nanotubes in PMMA and PVDF vehicles on the substrates made of modacrylic (substrate 2) and aramid (substrate 3) fibers; • The more complex the substrate surface, the higher the values of surface resistance observed. It was also found that the highest resistance was observed for substrate 2 (made of modacrylic fibers with a surface mass of 320 g/m2) with the most developed surface; • For substrates 1 (non-woven fabric, laminated with a PTFE membrane) and 3 (aramid fibers), PMMA vehicle allows for obtaining layers with the lowest surface resistance, compared with PVDF for pastes of analogical content of graphene nanoplatelets; • For the PMMA-based pastes we observed a decrease in the resistance with an increase of the graphene content from 7 wt.% to the 5 wt.%, followed by the increase in the resistance for 10 wt.% samples, due to the higher viscosity of the 10 wt.% pastes influencing negatively the printed paths uniformity; • PMMA vehicle allows to obtain layers with a lower surface resistance than PVDF vehicle based pastes with comparable content of multi-walled carbon nanotubes; • The pastes containing multi-walled carbon nanotubes obtained the lowest resistance on substrate 3 (aramid fibers) with 3 wt.% of CNTs in PMMA vehicle, followed by paths printed on substrate 1 ( and laminated with a PTFE membrane) with 5 wt.% of CNTs in PMMA. Summarizing the obtained results, it was shown that the samples that are character- ized by the best adhesion and the best resistance to washing and bending cycles were the samples with the content of the carbon nanotubes 3% by weight in PMMA on substrate 3 (of aramid fibers, of a weight of 260 g/m2). Such materials can be effectively applicable in the advanced textronics applications, withstanding regular wearing and service conditions. The applications of printed paths on textile substrates also go beyond textronics applica- tions, allowing the preparation of pre-pregs for epoxy-fiber composites, therefore creating embedded structural electronics systems. Such composite structures can be adapted for automotive, aerospace and other advanced applications with embedded intelligent sensing, monitoring, heating and other implementations.

Author Contributions: Conceptualization, A.T.; methodology, A.T. and M.S.; validation, A.T. and M.S.; investigation, A.T. and M.S.; writing—original draft preparation, A.T., A.D. and M.S.; writing— review and editing, A.T., A.D. and M.S.; supervision, A.T. All authors have read and agreed to the published version of the manuscript. Funding: The paper has been based on the results of Phase III of the National Programme “Safety and working conditions improvement”, funded in the years 2014–2016 in the area of research and development works by the Ministry of Science and Higher Education/The National Centre for Research and Development. The Programme coordinator: Central Institute for Labour Protection– National Research Institute. The paper has been also supported by Foundation for Polish Science (project no. First TEAM/2016-1/7) and Statutory funds of Institute of Metrology and Biomedical Engineering (Faculty of Mechatronics at the Warsaw University of Technology). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Sensors 2021, 21, 4038 14 of 14

Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: Authors would like to thank J. Weremczuk from The Institute of Electronic System from the Warsaw University of Technology for his support in surface resistance testing. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Kazani, I.; Hertleer, C.; De Mey, G.; Schwarz, A.; Guxho, G.; Van Langenhove, L. Electrical Conductive Textiles obtained by Screen Printing. Fibres Text. East. Eur. 2012, 20, 57–63. 2. Le´snikowski,J. Modelowanie Tekstylnych Linii Sygnałowych do Zastosowa´nw Tekstronice; Politechnika Łódzka Zeszyty Naukowe: Cracow, Poland, 2013; Nr 1167. 3. Mattila, H.R. Intelligent Textiles and Clothing; Woodhead Publishing Limited: Cambridge, UK, 2010. 4. Michalak, M.; Kruci´nska,I.; Surma, B. Textronic Textile Product. Fibres Text. East. Eur. 2006, 14, 5. 5. T˛esiorowski, Ł.; Gniotek, K. Radiowa Transmisja Sygnałów w Systemie Tekstronicznym. Przegl ˛adTelekomunikacyjny Wiadomo´sci Telekomunikacyjne 2008, 81, 1048–1051. 6. Skrzetuska, E. Trendy rozwojowe w tekstronice-Rozwi ˛azaniatekstroniczne dla ochrony zdrowia. Przegl ˛adElektrotechniczny 2014, r90, 34–40. 7. Bendkowska, W. Tekstylia inteligentne—Przegl ˛adzastosowa´n.Cz˛e´s´cI: Tekstylia reguluj ˛acemikroklimat odziezy.˙ Przegl ˛ad Włókienniczy Technik Włókienniczy 2002, 8, 9–13. 8. Walter, M.; Eilebrecht, W.; Wartzek, T.; Leonhardt, S. The smart car seat: Personalised monitoring of vital signs in automotive applications. Pers. Ubiquit. Comput. 2011, 15, 707–715. [CrossRef] 9. Wang, L.; Fu, X.; He, J.; Shi, X.; Chen, T.; Chen, P.; Wang, B.; Peng, H. Application challenges in fiber and textile electronics. Adv. Mater. 2020, 32, 1901971. [CrossRef][PubMed] 10. Komolafe, A.; Torah, R.; Wei, Y.; Nunes-Matos, H.; Li, M.; Hardy, D.; Dias, T.; Tudor, M.; Beeby, S. Integrating flexible filament circuits for e-textile applications. Adv. Mater. Technol. 2019, 4, 1900176. [CrossRef] 11. Coyle, S.; Morris, D.; Lau, K.-T.; Diamond, D.; Moyna, N. Textile-based wearable sensors for assisting sports performance. Wearable and Implantable Body Sensor Networks. Int. Workshop 2009, 307–311. [CrossRef] 12. Scilingo, E.P.; Gemignani, A.; Paradiso, R.; Taccini, N.; Ghelarducci, B.; Danilo, R. Performance evaluation of sensing fabrics for monitoring physiological and biomechanical variables. IEEE Trans. Inf. Technol. Biomed. 2005, 9, 345–352. [CrossRef][PubMed] 13. Kruci´nska,S.E.; Urbaniak-Domagała, W. Prototypes of Carbon Nanotube-Based Textile Sensors Manufactured by the Screen Printing Method. FIBRES Text. East. Eur. 2012, 20, 79–83. 14. Słoma, M.; Jakubowska, M. Screen Printed Polymer Paste with Carbon Nanotubes for Printed Electronics Application, in: Thick Films: Properties, Technology and Applications; Panzini Michael: Flushing, NY, USA, 2012; pp. 237–260. ISBN 978-1-61470-384-6. 15. Le´snikowski,J. Analysis of characteristic impedance of microstrip and coplanar textile signal lines. J. Text. Inst. 2020.[CrossRef] 16. Le´snikowski,J. Effect of temperature and humidity on the transmission properties of textile signal lines. J. Text. Inst. 2020, 111, 604–610. [CrossRef] 17. Zheng, Y.; Jin, L.; Qi, J.; Liu, Z.; Xu, L.; Hayes, S.; Gill, S.; Li, Y. Performance evaluation of conductive tracks in fabricating e-textiles by lock-stitch embroidery. J. Ind. Text. 2020.[CrossRef] 18. Beedasy, V.; Smith, P.J. Printed Electronics as Prepared by Inkjet Printing. Materials 2020, 13, 704. [CrossRef][PubMed] 19. Shahariar, H.; Kim, I.; Soewardiman, H.; Jur, J.S. Inkjet Printing of Reactive Silver Ink on Textiles. ACS Appl. Mater. Interfaces 2019, 11, 6208–6216. [CrossRef] 20. Karim, N.; Afroj, S.; Tan, S.; Novosolev, K.S.; Yeates, S.G. All Inkjet-Printed Graphene-Silver Composite Ink on Textiles for Highly Conductive Wearable Electronics Applications. Nat. Sci. . 2019, 9, 8035. [CrossRef][PubMed] 21. Stempien, Z.; Rybicki, E.; Rybicki, T.; Lesnikowski, J. Inkjet-printing deposition of silver electro-conductive layers on textile substrates at low sintering temperature by using an aqueous silver ions=containing ink for textronic applications. Sens. Actuators B Chem. 2016, 224, 714–725. [CrossRef] 22. Kazani, I. Study of Screen-Printed Electroconductive Textile Materials; Ghent University Zwijnaarde: Ghent, Belgium, 2012. 23. ASTM D3359-17 Standard Test Method for Rating Adhesion by Tape Test. Available online: https://www.astm.org/Standards/ D3359 (accessed on 31 March 2021). 24. ISO 6330:2012 Textiles—Domestic Washing and Drying Procedures for Textile Testing. Available online: https://www.iso.org/ standard/43044.html (accessed on 31 March 2021).