Multifunctional Wearable Electronic Textiles Using Cotton Fibers with

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Multifunctional wearable electronic textiles using cotton fibers with polypyrrole and carbon nanotubes Ravi M.A. P. Lima, José Jarib Alcaraz - Espinoza, Fernando A. G. da Silva Jr., and Helinando P. de Oliveira ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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ACS Applied Materials & Interfaces

Multifunctional wearable electronic textiles using cotton fibers with polypyrrole and carbon nanotubes Ravi M. A. P. Lima, Jose Jarib Alcaraz-Espinoza, Fernando A. G. da Silva Jr., and Helinando P. de Oliveira* Institute of Materials Science, Federal University of São Francisco Valley, 48920-310, Juazeiro, BA, Brazil.

Abstract Multifunctional wearable electronic textiles based on interfacial polymerization of polypyrrole on carbon nanotubes/cotton fibers offer advantages of simple and low-cost materials that incorporate bactericidal, good electrochemical performance and electrical heating properties. The high conductivity of doped polypyrrole/CNT composite provides textiles that reaches temperature in order of 70 °C with field of 5V/cm, superior electrochemical performance applied as electrodes of supercapacitor prototypes, reaching capacitance in order of 30 Fg-1 and strong bactericidal activity against Staphylococcus aureus. The combination of these properties can be explored in smart devices for heat and microbial treatment on different parts of body, with incorporated storage of energy on textiles.

Keywords: Supercapacitors, bactericidal, joule heating component, polypyrrole, textile. *Corresponding author: [email protected] Phone: +55(74)21027644

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Introduction The electronic wearable technology has created a revolution in our daily life, making possible to use portable devices for monitoring sport activities, communication, or healthcare

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. Nonetheless, the current devices are rigid structures that are not always

comfortable for the user: as ideal electronic wearable device would be as comfortable and adjustable as the used clothes. Based on that premise, an important trend in the last years has been the development of new materials on commercial fabrics or yarns

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.

Pioneer works in that area have shown that is possible to modify such textiles by incorporating electroactive nanomaterials that later are used for tailoring textile supercapacitors6-8, batteries9-10, Joule heaters 5, among others11-15. Recent works have shown electronic textile devices based on modified cotton fabrics and yarns trough dip-coating, in situ chemical process, screen-printing, and carbonization 5, 16. The most used materials include carbon nanotubes (CNTs), graphene (Gr), metal oxides, metallic nanoparticles, and intrinsic conducting polymers (ICPs), integrated as composites or alone. In that set of materials, ICPs stand out because of their electronic properties, electrochemical activity, stability, mechanical response, light weight, flexibility and low cost 17-18. One of the most attractive aspects of ICPs is their versatile nature, as they can be applied in a plethora of powerful applications as batteries, supercapacitors, antibacterial agents, sensors, anticorrosion inks, pollutants adsorbents, thermoelectric, among others 19-20. Particular interest is focused on polypyrrole (PPy), as this ICP presents a combination of high electrical conductivity, environmental stability, redox properties, high availability and low cost of monomers and oxidants. Among other characteristics, PPy can be synthetized trough several polymerization methodologies as chemical, electrochemical polymerization, vapor deposition and interfacial polymerization,


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producing each method a material with particular characteristics. Compared with polyaniline (PAn) or poly-3,4-ethylenedioxythiophene (PEDOT), PPy possess a higher degree of flexibility

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and greater mass density

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that is reflected in a higher

performance with a small volume. Nonetheless, PPy suffers from poor solubility and agglomeration issues when nanostructured, so it is interesting to develop methodologies for depositing high conductive PPy directly on flexible substrates that can simplify the process and reduce costs. A special application of textiles modified with ICPs that recently has gained relevance is the wearable Joule heaters, as this technology is important for athletic rehabilitation, heat therapy and joint pain relief

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. The optimization in the integration of ICPs on

textiles is an interesting alternative to the use of conventional metal wires (such as copper or silver-based materials) that presents some drawbacks as heavy weight, rigidity, low degree of flexibility, harsh chemical reactions, oxidation, and inappropriate integration with textiles (uncomfortable for contact with skin). One of the major challenges to produce an appropriate PPy textile for Joule heaters relies on a good electrical conductivity. With this aim, the most of polymerization techniques in combination with the available textiles fails due to the diameter of fibers and surface area. Recently, new polymerization techniques such as vapor chemical deposition have shown a great potential for PEDOT, nonetheless, this procedure requires multiple steps in order to attain a desirable conductivity. In that regard, another approach such as interfacial polymerization (IP) seem attractive and scarcely explored for this application. One of the advantages of IP is the possibility of developing a controlled micromorphology of PPy and high doping level, in contrast to the dense growth of PPy prepared by in situ polymerization, introducing some drawbacks such as low ion


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accessibility, that results in low electrical conductivity and electrochemical performance. Qi et al. produced a free-standing PPy trough the IP methodology with an outstanding conductivity because of the morphology and high doped levels which is an attractive alternative to bulk polymerization methods26. Peshoria and Narula 27 reported enhanced pseudocapacitive behavior for polypyrrole prepared via IP while de Oliveira et al. compared the electrochemical behavior of supercapacitors prepared by different methods, confirming the best performance for IP-based devices 28. In addition to the polymerization method, another relevant aspect that has shown potential for enhancing the conductivity of ICPs is the interaction of them with carbon nanotubes (CNTs). These carbon nanomaterials offer high inherent electrical and thermal conductivity, flexibility and mechanical stability. The effect of the inclusion of CNTs in ICPs could act as a doping agent (if functionalized it favors the electron transfer process). Some examples of this synergy effect can be found in the application of ICP-CNTs composites for supercapacitors with improved electrochemical capabilities (because of the association of pseudo capacitance of conducting polymers and electrical double layer capacitance of carbon derivatives) 28-30. For all the above, in this research we have produced a new flexible, wearable and multifunctional material based on an electrical conductive cotton yarn with CNT incorporated through a dip and dry coating in combination with the IP of PPy. The produced material was analyzed from SEM and Raman spectroscopy, while electrical and electrochemical behavior were previously explored for the development of prototypes of integrated Joule heater and supercapacitor. As a third application that is almost not explored in wearable electronics, we studied the bactericidal activity of the


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produced yarns based on previous expertise of our group about bactericidal activity of PPy 31-32.

Materials and Methods Materials Multi-walled carbon nanotubes (MWCNTs), pyrrole, ethanol, anhydrous ferric chloride, triton X-100, sodium dodecyl sulfate (SDS), dodecyl benzene sodium sulfonate (SDBS) and camphorsulfonic acid (CSA) were purchased from Sigma-Aldrich and used as received. Acetone (Vetec, Brazil), hydrochloric acid (Química Moderna, Brazil) and hexane (Synth, Brazil) were also used as received. Pyrrole was distilled twice under reduced pressure twice before each experiment. All solutions were made using deionized water. Treatment of cotton yarn for CNT and polypyrrole coating Cotton threads with 0.5 mm diameter and 12 cm of length were previously treated and cleaned as follows: the samples were immersed in Triton X-100 aqueous solution and rinsed with deionized water to remove residues and dried in an oven at 90 °C. Then, the samples were immersed in alcohol for 10 min and dried in an oven at 90 °C for 5 min for complete solvent elimination. After, the material was immersed in acetone under sonication for 5 min and dried at 90 °C for 5 min. The overall process is repeated three times.

Chemical functionalization of carbon nanotubes Hydrophilic functional groups, such as carboxylic acid and hydroxyl, were incorporated into MWCNTs, as follows: 250 mL of an acidic solution of H2SO4/HNO3 (3:1) received


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2 g of MWCNTs that was kept under stirring for 5 h at 130 °C under reflux to avoid solvent evaporation. After this, the solution was cooled to room temperature

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. The

resulting material was filtered and rinsed several times with milli-Q water to neutralize the pH. The powder was dried in an oven at 60 °C for 24 h.

CNT-coating cotton yarn procedure (CNT samples)

The mother solution of carbon nanotubes was prepared as follows: 100 mg of functionalized carbon nanotubes

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and 100 mg of SDBS were dispersed in 100 mL of

milli-Q water and kept under sonication until the complete dispersion of carbon nanotubes in solution (negligible amount of CNT aggregates in solution). The pretreated cotton yarn was immersed into the resulting solution and kept under sonication for 15 min. The solvent removal was established in an oven (100 °C). This process was repeated four times and resulted in a good covering degree of CNT on textiles.

Chemical polymerization of polypyrrole on cotton yarn (PPy sample)

Two different solutions were prepared: pretreated cotton yarn and 17.5 µL of pyrrole were immersed in 12.5 mL of milli-Q water for 40 min under stirring in an orbital shaker, for impregnation of pyrrole on the fibers. A second solution was prepared from dispersion of 0.0406 g of FeCl in 12.5 mL of aqueous solution of HCl (1 M). The solution was magnetically stirred for 30 min. Then, the FeCl3 aqueous solution was dropwise into the PPy solution. The mixture was continuously stirred for 24 h, allowing the polymerization of PPy on fibers. The resulting textiles were washed in alcohol and


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water to remove unattached grains and residual monomers. The samples were dried under ambient conditions.

Chemical polymerization of polypyrrole on CNT-coated cotton yarn (CNT-PPy sample)

The CNT-PPy samples were prepared according the combination of two previously described methods: after the impregnation of fibers with CNT, the material is applied as template for standard chemical polymerization of PPy.

Interfacial polymerization of doped PPy on cotton yarn (I-PPy sample) and on CNT-coated cotton yarn (CNT-I-PPy sample) The CNT-coated cotton yarn was coated with doped polypyrrole as follows: an “oil phase” solution was prepared with the inclusion of 50 µL of pyrrole in 3 mL of hexane. After dispersion, the solution (under rest) was kept at 3 °C. The second solution “water phase” was prepared with the inclusion of 0.251 g of camphorsulfonic acid in 3 mL of water and 0.175 g of ferric chloride, corresponding to 8 g/L of monomer concentration. The solution is kept under intense stirring for 40 min at 70 °C until the complete dispersion of components. Pre-treated fibers were immersed in water phase solution and kept at 3 °C. The oil phase was slowly dropwise in water phase, allowing the formation of a two-phase system, an adequate environment for interfacial polymerization of polypyrrole that takes place at 3 °C for 12 h. The resulting samples were washed with deionized water for removal of monomers and non-attached aggregates. The removal of water from fibers was established in an oven at 60 °C for 1 h.


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Anionic laundering procedure In order to evaluate the adhesion degree of additives on textiles, it was performed the repetition of laundering procedure in which an aqueous solution of anionic surfactant sodium dodecyl sulfate (20 mM) was applied for fibers washing. The samples were immersed in solution, and mechanically stirred for 5 min. In the following step, each sample was immersed in 50 mL of deionized water and then stirred for 5 min and dried to eliminate residues of surfactant. The current-voltage curves were performed in the interval of -1 V to 1 V after each laundering process, which were repeated five times. Characterization Techniques SEM images were acquired in a scanning electron microscopy (Vega 3XM Tescan at accelerating voltage of 5 kV) with deposition of a thin layer of gold on fibers surface. Raman spectra were performed in a Raman spectrometer (LabRAM Aramis – Horiba Jobin Yvon) in the range of 500-2000 cm-1 with excitation at 532 nm from a He-Ne laser. Current-voltage curves were acquired using a DC Power Supply HY3003-3 (Polyterm) and a multimeter ET-2402A (Minipa). Electrical assays were carried out in a 2-point configuration in which samples were fixed between two metal tips disposed at 1 cm of distance and connected with a power source and an ammeter. A voltmeter was disposed in parallel with metal tips, allowing that I-V curves could be acquired simultaneously. The temperature on fiber was acquired from a thermal camera model E6 (Flir) disposed at fixed distance of 30 cm from experimental apparatus (power source and sample) under continuous capture. The room temperature was kept fixed during experiment with the previous calibration of thermal camera to avoid fluctuations in measured data. Electrochemical characterization was carried out at room temperature in an Autolab PGSTAT 302N (Metrohm, Switzerland) using a three-electrode system with reference


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electrode of Ag/AgCl (saturated KCl), counter electrode of platinum wire and working electrode of 3 mm of fibers (pristine and coated ones) immersed in an electrolyte – phosphoric acid (85%) in water (1:10).

Bactericidal assays Gram positive Staphylococcus aureus (S. aureus ATCC 25923) were kept in agar at 4 °C before assays. The serial dilution procedure was performed from a mother solution and the turbidity in solution was controlled to reach a value of 0.5 in the McFarland scale. Aliquots of 1000 µL from mother solution were removed (108 CFU) and added to 9 mL of saline solution, for the first cycle of dilution. Successive dilutions were performed to reach the 104 CFU value. These solutions received 7-cm of samples (CNT, PPy, CNT-PPy, I-PPy and CNT-I-PPy) that remained in contact with solution for 4 h. After the treatment, 100 µL of solution from each tube were inoculated in Petri dish containing the nutritive media for growth of viable bacteria at 37 °C for 24 h 35-37. The counting obtained from the plate count agar technique determined the number of remaining viable bacteria given in colony forming unities (CFU), characterizing the adequate quantification of bactericidal activity of composites in comparison with control experiments. The agar-disk diffusion test was performed as follows: aliquots of 10 µL were collected from mother solution and inoculated into Agar Muller-Hinton. Small rings (length of 7 cm) of coated cotton yarns (CNT, PPy, CNT-PPy, I-PPy and CNT-I-PPy samples) were disposed on Petri dishes and incubated at 37 °C for 24 h. Fabrication of Symmetrical Supercapacitors The feasibility of using the PPy/CNT – coated cotton fibers as supercapacitors electrodes was evaluated from production of metal-free structures composed by


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symmetric assembly of parallel electrodes (15 mm-length) separated by a thin layer of polyvinyl alcohol/ phosphoric acid (85%) in water (1:10) – PVA/H3PO4 layer, providing a prototype of flexible supercapacitor that was tested at different current densities and scan rate for determination of energy and power density. The schematic view of preparation process is summarized in Fig. 1, as follows: the step of CNT incorporation is followed by interfacial polymerization of CSA-doped polypyrrole. The resulting samples are applied as Joule heaters, bactericidal agents and electrodes for supercapacitors. The configuration of supercapacitor, previously described, is composed by two parallel fibers embedded in a thin layer of PVA/H3PO4 layer with separation of electrodes in order of 500 µm.

Results and discussion Morphology analysis We selected a commercial cotton yarn as a template for the deposition of electroactive components in order to obtain an electrical multifunctional material in combination with the excellent properties of textiles. Nonetheless, we focused on a thread and not on a fabric because a thread simplifies its incorporation in common clothes by sewing it in strategic points. First, the morphology of the cotton yarns was investigated by scanning electron microscopy. As one can observe in Fig. 2a, the cotton yarn possesses an average diameter of 500 µm (see the inset) and it is composed of multiple individual cotton fibrils with a ribbon structure and an average diameter of (13 ± 3) µm. In the next step, the incorporation of MWCNTs by a dip and dry method assisted by ultrasonic bath was achieved without disrupting the micromorphology of the yarn as observed in the Fig.


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2b. The only substantial change experimented by the yarn occurred at the surface of the cotton fibrils, as the MWCNTs formed a continuous interconnected network along the cotton fibrils producing an increase in the surface roughness. The reason of the adsorption of MWCNTs resides in the pretreatment of MWCNTs in acid solution (oxidant agents) which conferred several functional groups38 such as carboxylic, hydroxyl and epoxy capable of interacting by hydrogen bond with the hydroxyl groups from the poly-D glucose chains of cotton microfibrils39. After the inclusion of MWCNTs in the cotton yarns, we polymerized PPy in them by two different methodologies, an in situ chemical polymerization and an interfacial chemical synthesis. In Fig. 2c, it is presented the cotton yarn modified with PPy by the in situ polymerization method, where is possible to observe a homogeneous deposition along the cotton fibrils with the presence of small clusters. On the other hand, the in situ polymerization of PPy on the yarns previously modified with MWCNTs (Fig. 2d) presented a denser PPy coating when compared with the PPy on pristine cotton yarns. In order to understand that, it is necessary to consider two facts, first the surface enhancement of the yarns product of the MWCNTs inclusion and their chemical activity. As we discussed above, the MWCNTs offer the possibility of forming hydrogen bonds, in this case with Py but in addition, they can interact with Py via π-π stacking due to the nature aromaticity of the MWCNTs. Furthermore, as the polymerization of Py involves the release of electrons, it is possible that PPy oligomers and MWCNTs form a charge transfer complex during the polymerization which also favors the PPy deposition. It is presented in Fig. 2e the resulting cotton yarn from the interfacial polymerization method; the fibrils exhibited a rougher surface than in previous cases. The cotton fibrils presented a coating conformed of small particles, which corresponds to the typical


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morphology exhibited by PPy. The yarns modified with MWCNTs (Fig. 2f) presented the same morphology as in the previous case but a dense coating due to the described effect of MWCNTs in the PPy polymerization. The resulting increase in the amount of PPy as the change in morphology can be ascribed to the interfacial polymerization method in both cases. In that regard, contrary to the bulk polymerization, the formation of PPy occurs at the interface between water-hexane and not at the bulk. In particular, in our system, the cotton yarn acted as part of the aqueous phase, due to its hydrophilicity, absorbing by capillarity the major part of the FeCl3-camphor sulfonic aqueous solution. Thus, when the organic phase, containing the Py, is added an interface is created with the cotton yarn, occurring the PPy polymerization at the fibrils. Moreover, when compared with the in situ polymerization this process produced a scarce or null amount of PPy at the water interface, being more punctual and consequently increasing the effective mass of PPy at the yarns. Raman analysis Once the morphology has been revealed by SEM, we have to corroborate the inclusion of MWCNTs in the yarn, the synthesis of PPy and its structural changes associated to the methodologies employed. As a tool for this task, we employed Raman spectroscopy, as this technique is nondestructive and provides the vibrational fingerprint of materials which allows to identify and characterize them. It is presented in Fig. 3 the Raman spectra of all samples. The corresponding spectrum for the cotton yarn modified with the MWCNTs (Fig. 3a) exhibits three bands at 1335 cm-1, 1572 cm-1 and 1590 cm-1 which corresponds to the band D, G and D´ of the MWCNTs, respectively. The D band is related to the structural disorder from the amorphous carbon, while the G band corresponds to the tangential in plane stretching vibrations of the C-C bonds within the graphene sheets40. In the case of the MWCNTs as they were functionalized the D´ band


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appeared indicating a disorder degree, structural defects, or intercalation of chemical species between the graphitic walls38. We normalized the spectra in relation to the D band and calculated the ratio ID/IG, as this parameter is used to evaluate the disorder density of the tube walls41, revealing a value of 1.7 which corroborates that the MWCNTs possess a considerable amount of defects due to the functionalization treatment. Thus, through this analysis we confirmed the successful inclusion of the MWCNT in the cotton yarn and revealed the characteristics of them. The curves b, c, d and e at Fig. 3 correspond to the cotton yarns with PPy. In all of them, it is possible to identify the Raman fingerprint of PPy with subtle modifications due to the different synthesis and conditions of the employed procedure. All the spectra presents bands at 919 cm-1 (ring deformation associated with dication (bipolaron)), 960 cm-1 (ring in plane deformation associated with cation (polaron), 1032 cm-1 (symmetrical C-H in plane bending associated with bipolaron and N-H in plane deformation associated with radical cation), 1050 cm-1 (associated to neutral species, absent in PPy), 1240 cm-1 (antisymmetric in plane bending), 1321 cm-1 (C-C in ring and CC inter-ring stretching), 1364 cm-1 (antisymmetric in ring C-N stretching), 1395 cm-1 (C-C and C-N stretching absent in PPy), 1480 cm-1 (skeletal band; C=C and C-N stretching) and 1564 cm-1 (corresponding to in ring and C-C inter ring stretching - this band is an overlap from bands arising from polaron and bipolaron). It is possible to observe that the spectra for both PPy-CNT and CNT-I-PPy present a small displacement to lower wavenumber due to a π-π interaction between the benzene rings from the CNTs and the aromatic ring of pyrrole42-43.To identify the main differences of the samples, the ratio from two pairs of bands (917/959 and 1561/1472) was compared for each synthesis. The ratio of intensity from bands at 917/959 is related to bipolaron and polaron, respectively and indicates the degree of doping34 while the ratio from


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1561/1472 indicates the relative conjugation length33,

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. The ratio of conjugation

(1561/1472) for PPy, CNT-PPy, I-PPy and CNT-I-PPy revealed values of 2.9, 3.3, 3.2 and 3.3, respectively. The above relation indicated that the conjugation length was maximum for the samples incorporating MWCNTs and the minimum for PPy. Considering the degree of doping the ratio bipolaron/polaron revealed values of 0.85, 0.9, 0.9 and 1.11 for PPy, CNT-PPy, I-PPy and CNT-I-PPy, respectively. From that information, we can conclude that the PPy sample presented the most disordered structure and a low amount of charge carriers (bipolarons) while the sample I-PPy presented more equilibrated characteristics with a high amount of bipolarons and a considerable level of conjugation comparable to those with MWCNTs. Electrical characterization and application of composites as electrical heaters The electrical conductivity of the modified yarns is essential for the proposed application. In Fig. 4, it is presented the resulting I-V curves where all the samples presented a linear correlation between applied voltage and current, indicating an ohmic behavior. The slope increased in the following order, PPy, CNT, CNT-PPy, I-PPy, and CNT-I-PPy yarns which revealed conductivity levels summarized in Table 1. The lower value of conductivity is observed for samples prepared by standard method – in situ polymerization of PPy. The electrical conductivity of the samples composed with PPy are related to molecular or structural and macroscopic characteristics. Particularly, PPy cotton yarns exhibited the lowest degree of conjugation and doping (as showed by Raman) that associated with a low compactness of the PPy coating (as observed by the SEM images) contributed to restrict the current transport, generating a poor electrical conductor. Before considering the electrical characteristics of samples composed of MWCNT and PPy, it is necessary to explain the conductivity of the MWCNT. In these yarns, the MWCNTs (adsorbed in each cycle) create electrical paths until reach the


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percolation threshold. The MWCNTs yarns presented a higher conductivity than the PPY yarns but not as high as that presented by cotton yarns with the previously reported single wall carbon nanotubes (SWCNTs)39. MWCNTs are larger, rigid and less electrical conductive 2, 44 than SWCNTs which impedes a more dispersed network in the cotton fibrils, consequently a higher electrical resistance. Nonetheless, MWCNTs are cheaper than SWCNT and present an adequate conductivity. The CNT-PPy yarns presented a considerable enhancement in their electrical conductivity. The effect of MWCNTs on the polymerization improved the conjugation of PPy (as observed by Raman spectrum) and a denser coating as showed in SEM images. The nanotubes in this case acted as an additive that facilitates the chargetransfer process, compensating the defects of PPy. The I-PPy yarn presented an outstanding electrical conductivity comparable to the CNT-PPy, which is a clear consequence of the interfacial polymerization process. As discussed before, the I-PPy displayed a high degree of conjugation and doping, product of syntheses conditions as dopant, reaction time and temperature. Qi et al

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described that the reason of high

conductivity resides in the selection of an appropriate solvent for Py, low monomer concentration and the use of a sulfonic acid (in our case CSA). In this system, the CSA seems to be a key component as this acted as a surfactant at the interface because of its amphiphilic nature what allow an easy incorporation as Py slowly diffuses and polymerized, gradually incorporating more CSA and consequently increasing the doping level. Finally, as expected the interfacial polymerization of doped polypyrrole on the cotton yarn templates covered by carbon nanotubes results in the most conductive structure (10 x the conductivity level of I-PPy) characterizing a promising candidate for efficient


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application as electrical heaters, because of mutual doping effects of CSA/ CNT and rich morphology provided by interfacial polymerization. It is worth mentioning that different strategies have been made to produce highly conductive fabrics. The comparison with data reported in the literature (summarized in Table 1) reveals that low conductivity of PPy on cotton fabrics (0.69 mScm-1) represents an important point to be addressed. Xu et al.45 explored the in situ polymerization of polypyrrole using CuO nanoparticles as templates reaching conductivity of 10 Scm-1. In our case the best conductivity was observed for CNT-IPPy (10.44 Scm-1), characterizing a promising and competitive system for application in electronic devices. Before actual applications, it is necessary to explore the retention of the electrical properties upon the most diverse mechanical efforts. The durability of the CNT-I-PPy under the repeated action application of a minimum/ maximum voltage (5V) – Fig. 5a reveals that maximum and minimum in temperature values are regularly reached, indicating that negligible process of degradation is observed for successive and extreme condition of operation. In addition, we twisted, bended and rolled the CNT-I-PPy yarns for several cycles and measured their electrical properties. In Fig. 5b, it is observed that the initial electrical response of the CNT-I-PPy (normal state) remains even after bended and twisted. The same response is observed after constant cycles of mechanical efforts (400 cycles of bending – Fig. 5c) with only a slight variation in the resistance of samples, confirming robustness. In addition to this response, the attachment of MWCNTs and PPy on cotton yarn surface represents another important aspect for its successive use of wearable devices. In order to verify the reuse capability of textiles, we performed assays in which the CNT-I-PPy yarns were electrical characterized after cycles of anionic laundering. The results in Fig. 5d confirm that not important changes


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occurred, only a slight decrease in conductivity after the first washing cycle, indicating the possible removal of residues from fibers. Despite this initial observation, the following washing steps are accomplished by a negligible variation in the electrical response of fibers, confirming that electrical activity of fibers is maintained after successive washing procedures. Once assessed the characteristics of the produced yarns, we characterized them as a Joule heater. An important parameter is the dissipative power (P), as this is related to the heat generated by the energy loses of a charge moving across a resistor. The P is directly proportional to the voltage (V) and current (I) – P=VI. Then, it is possible to infer that the degree of conductivity of sample presents a direct correspondence with the maximum temperature achievable in each yarn produced. As expected, the lower variation in temperature is observed for the samples with higher resistance (PPy and MWCNT) – see Fig. 6a. The influence of doping level and additives (provided by the interfacial polymerization and MWCNT) affects the maximum achieved value of temperature for a corresponding value of voltage. The sample CNT-I-PPy reaches 75 °C with a field in the order of 5 V/cm, characterizing a strong advantage for this sample. The comparison of the slopes in the curves of temperature versus power density (Fig. 6b) confirms that it would be required a higher electrical field for all other samples to reach a temperature in the order of 75 °C. The lowest value of slope is observed for sample CNT-I-PPy that requires a low power to achieve a high excursion in temperature variation, as required for Joule heaters. The time-dependent response of temperature on CNT-I-PPy as a function of transition of voltage from 0V to a fixed value on sample is shown in Fig. 6c while the corresponding projection in the XZ-axis can be seen in Fig. 6d. The higher voltage is


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associated with higher variation in temperature and consequently the characteristic time required to reach the equilibrium temperature. Based on our results, it is possible to identify the characteristic time (required time to reach 63% of complete variation in temperature) of 16 s at applied voltage of 3.5 V to achieve 75 °C, confirming that CNTI-PPy represents a promising candidate for application as Joule heating component. To have a better idea of the significance of our results, we have to consider for example the work of Kaynak and Hakansson 46 that reported the use of polypyrrole-coated PETlycra fabrics. The samples were chemically synthesized and doped with antraquinone-2sulfonic acid (AQSA) and reached 40.55 °C, at an applied voltage of 24 V for 5 minutes. In the corresponding system reported by Maity et al.

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, it was observed a

maximum temperature of 42.4 °C on PPy-based fabrics at an applied voltage of 5 V for 5 minutes. The incorporation of PPy/PVA-co-PE on PET substrate

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returned samples

with a good conductivity (1 Scm-1) and temperature of 80 °C with a characteristic growth time constant of 41.3 s. In order to evaluate the effective action of fibers as a component of wearable device, we introduced the CNT-I-PPy cotton yarn sample on the index finger of a cotton knitted hand glove (see Fig. 7a). The terminals of the sewing fiber were connected to a DC voltage source (12 V). The thermal images (shown in Figs. 7b and 7c) reveal the activity of electrical heater on specific region (index finger): in the absence of external excitation (V= 0 V), the temperature on hand surface is homogenously distributed with value in order of 34 °C. The response of the electrical heater (connected to DC source of 12 V) is observed from the increase in the temperature on index finger (as depicted by the thermal image) that reaches values in order of 43 °C while other fingers and hand remain in the previous temperature. This experiment has showed the simplicity of


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incorporate the produced yarns for some important applications in wearable devices that refers to the controlled variation of temperature in specific regions of body. Supercapacitors Assays It is desirable that conductive textiles possess multiple capabilities, allowing their implementation for several tasks in wearable electronics. One of the most important is the energy storage, as devices for this task could harvest the generated energy by the motion of user and supply the required energy for incorporated sensors, heaters or displays49. Among, the most important energy storage devices supercapacitors stand out because of their high durability, power densities and fast charge-discharge process. A particular and interesting combination of materials for supercapacitors are the conducting polymers and carbon allotropes (carbon nanotubes and graphene) that has been currently applied in the development of electrodes for supercapacitors

28-29, 50-56

.

Predominantly, CNT-PPy composites introduce an enriching synergy derived from the properties of both components: the fast diffusion rate of charges (polymeric chains properties) and surface limited processes of CNT, associating the pseudocapacitance of PPy with electrical double layer capacitance (EDLC) of carbon allotropes. In order to evaluate the influence of composition of the proposed conductive yarns as electrodes for supercapacitors, we first studied their electrochemical properties in a three-electrode configuration in an acidic solution (H3PO4, 1M). The cyclic voltammetry curves of electrodes at a fixed scan rate of 50 mVs-1 are showed in Fig. 8a. The area enclosed by these curves is proportional to the specific capacitance of the samples: a higher area in curves reveal a better electrochemical performance for the resulting materials.


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As expected, poor electrical properties (high resistance) of CNT and PPy samples returned negligible area in voltammograms, characterizing the disadvantages of isolated components as electrodes for supercapacitors. However, as previously observed, the interaction of PPy and CNT returns improved electrical properties. In correspondence, the electrochemical response is improved and an increase in the area on voltammogram confirms the promising conditions for application of materials as electrodes. The optimization in the electrochemical performance was observed for samples prepared by interfacial polymerization. As we can see, a strong increase in the areal capacitance value is observed for the I-PPy samples while the interaction of doped PPy with the MWCNT reinforces the performance observed by pristine I-PPy. The quantification of these processes (corresponding linear capacitance) is shown in Fig. 8b. These values of linear capacitance of samples were obtained according Eq. 1 57,

=

. 1 (2. . .△ )

where i is the current of the voltammogram, s is the scan rate (mV.s-1), l is the length of electrode (cm), and ∆V is the potential window. The best performance was observed for sample CNT-I-PPy (45 Fm-1) which is 2 x superior than corresponding capacitance of I-PPy and 12 x than capacitance of CNTPPy sample, revealing the importance of interfacial polymerization of PPy on electrochemical performance of the resulting material. Due to the superior performance of CNT-I-PPy sample, it was performed a more complete scan rate in voltammograms (from 5 mVs-1 to 200 mVs-1) - see Fig. 8c - in order to calculate the specific capacitance. The results in Fig. 8d confirm that resulting


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capacitance reaches a competitive value for application as cotton-based wearable supercapacitor. Based on these findings, we constructed symmetric supercapacitors considering CNT-IPPy samples and an acidic polymeric electrolyte (PVA-H3PO4) according to the scheme shown in Fig. 1. Galvanostatic charge-discharge curves were evaluated at different current densities, as shown in Fig. 9a. Non-ideal straight line is observed in all curves, in response of faradaic mechanism due to the redox reactions in conducting layer. Cyclic voltammograms shown in Fig. 9b reveal that at low scan rate the curves are square shaped while distortions are observed at increasing scan rate. It is due to the increase of voltage drop at high sweep rate in consequence of variable counter-ion migration in small pore channel of polymer structure. As a consequence, the electrical potential in the bulk of electrodes is not constant, leading to a variable charging/ discharging current and dispersion of the resulting capacitance, a typical characteristic of faradaic-dominant behavior systems. The specific capacitance value was determined from the galvanostatic charge-discharge according to Eq. 2 57-59 =

2. . . 2 .

where V is the potential (V) from the IR drop to zero, I is the applied current of discharge (in A), m is the active mass of electrode (CNT-I-PPy layer), and Darea (in V s) is the area under the discharge curve. As expected, the curve in Fig. 9c confirms that specific capacitance varies inversely with the charging current, reaching a maximum value in order of 30 Fg-1. The Ragone plot for corresponding sample was obtained from calculus of energy and power density, as follows:


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the energy density was calculated from data of Fig. 9a and it is estimated by Eq. 3 =

. ! ∗#$$

Eq. 3

where E is the energy density (Whg-1), V is the potential, and Csp is the specific capacitance of material. the power density (Eq. 4) was calculated as %= where ∆t is the discharge time.

3600. . 4 △)

The maximum value of energy and power density (Fig. 9d) are 2.63 mWhg-1 and 11.33 mWg-1, respectively. In terms of cyclability, the device was cycled under a current of 1.5 mA. The capacitive retention is displayed in Fig. 9e. As shown, the capacitive retention tends to be increased in the first 500 cycles due to the self-activation process in which new pathways are created for current circulation at interfaces 60. Successive cycles of chargedischarge provoke the reduction in the capacitive retention to values in order of 80% after 2000 cycles of use, characterizing an important advantage in comparison with polymer-based supercapacitors. Electrical Impedance Spectroscopy (EIS) measurements were carried out to evaluate the influence of reuses on electrochemical behavior of the electrode-electrolyte interface of supercapacitor. For this, we compared the response of the same sample in Nyquist plot before the use and after 2000 cycles of use. Both responses are characterized in Nyquist plots (Fig. 9f) by depressed semicircles superposed by a linear branch at low frequency region related to the confined diffusion process in the electrolyte. The most appropriate equivalent circuit (shown in the inset of Fig. 9f) was used to fit the impedance curves: R0 represents the electrolyte resistance


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while the constant phase element (Y0) characterizes the double layer capacitance, the charge transfer resistance is represented by R1 and the Warburg element (Y1) represents diffusive processes. The line in each curve represents the best fitting curves obtained with corresponding parameters shown in Table 2. The electrolyte resistance and charge transfer resistance for as-prepared samples were lower than those of the cycled samples, characterizing the degradation after successive reuses, due to the elevation in voltage drop and degradation of conducting paths for charge circulation/ accumulation. The reduction in admittances Y0 and Y1 in association with reduction of parameter n confirms the non-uniform distribution of current along changeable sites. These results confirm that reduction in capacitive retention is a consequence of progressive structural modification at electrode/ electrolyte interface of supercapacitor. Antibacterial activity As wearable electronic textiles are in direct contact with the skin and experiment several changes in temperature, humidity and contact, they are prone to attach bacteria and favor diseases or bad odors. In that regard, most of the antibacterial additives imply the use of metal particles which is not a favorable condition because of the possible leaching and subsequent contamination of the environment. Recently, we have proposed the use of PPy as a metal free antibacterial agent whose bactericidal activity is attributed to the positive charges along the backbone chains (polaron and bipolaron), that attract bacteria, provoking the death by disrupting the cell wall 11, 14, 61-62. Thus, the doping level, the type of counter ion and morphology32 are directly related to the antibacterial effect of the resulting material. So, it is expected a strong activity of the highly doped samples produced by the interfacial polymerization on the bactericidal activity.


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The results from the diffusion halo experiments (shown in Fig. 10a) revealed a negligible halo, as observed for samples prepared with CNT while more pronounced ones are observed for CNT-PPy samples, indicating the diffusion of some species (Cl-) that contribute to its bactericidal activity in association with positively charged species on polymeric chains. The remaining colony-forming unit (CFU) due to the action of different composites is expressed in Fig. 10b. As expected, the control experiment – absence of bactericidal agent returned an uncountable number of colonies. The incorporation of modified cotton yarns inhibits the bacterial growth in the following order (CFU number): CNT> PPy> CNT-PPy> CNT-I-PPy>I-PPy. The most important result was observed for the sample I-PPy that reached a 100% bacterial (S. aureus) reduction, revealing that doping level observed in the interfacial polymerization produced a promising system for antibacterial activity. These results are in concordance with the observations of Varesano et al, that reported the production and application of PPy-coated cotton fabrics (by in situ chemical polymerization) against S. aureus. The authors observed that doped PPy (with dicyclohexyl sulfosuccinate sodium salt) reaches a complete inhibition of S. aureus (bacterial reduction of 100% in samples with concentration of monomers above 2 gL-1) 14. We attribute the effectivity of I-PPy to a low rate polymerization that allowed the gradual incorporation of CSA, affecting the doping level. In addition, the granular structure of I-PPy offered a higher available surface area for bacteria adhesion and killing, which confers negligible CFU for I-PPy samples, at monomer concentration (8 gL-1) above the critical value observed in the literature.


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The combination of these results (and respective applications) corroborates that CNT-IPPy composite represents a promising system that associates effective electrical heating with energy storage properties and good bactericidal activity.

Conclusion A new multifunctional conducting yarn was produced on commercial cotton yarn by the combination of MWCNTs and the subsequent interfacial polymerization of PPy. The CNT-I-PPy demonstrated a high conductivity and excellent retention of their conducting properties even after diverse mechanical efforts and laundering. The outstanding characteristics of the CNT-I-PPy, allowed their use as a joule heater that can be easily incorporated in strategic parts of the clothes for electronic wearable devices. In comparison with some of the reported joule heaters based on PPy, our system was more efficient, i.e. it required less potential to achieve a higher temperature (75 °C). Due to the pseudocapacitance/ electrical double layer capacitance (PPy and MWCNT properties) and high electrical conductivity, we studied the performance of the components as electrodes and fabricated a supercapacitor device that showed a capacitance, energy density and power of 30 Fg-1, 2.63 mWhg-1 and 11.33 mWg-1, respectively. Finally, as part of the electronic textiles, we demonstrate the antibacterial capability of PPy-MWCNT against bacteria S. aureus. For all of the above, this composite is positioned as a multifunctional platform that can play a key role in tailoring new electronic devices for smart textronics, sensors and devices for healthcare.

Acknowledgment This work was partially supported by the Brazilian agencies FINEP (project 04.13.0042.00), CAPES, FAPESB, FACEPE (project APQ-0980-1.05/14) and CNPq


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(project 301238/2013-8). José. J. Alcaraz-Espinoza would like to thank CNPq and FACEPE for a post-doctoral fellowship (DCR-0019-1.05/16).

Figure Captions Figure 1 – Schematic view of preparation steps of wearable devices (interfacial polymerization) and corresponding application as heating component, bactericidal agent and supercapacitor.

Figure 2 - SEM images of (a) textile, (b) CNT, (c) PPy, (d) CNT-PPy, (e) I-PPy and (f) CNT-IPPy. Figure 3 – Raman spectrum of samples: (a) CNT, (b) PPy, (c) CNT-PPy, (d) I-PPy and (e) CNT-I-PPy. Figure 4 – Current – voltage curves of samples: textile, CNT, PPy, CNT-PPy, I-PPy and CNTI-PPy. Figure 5 – (a) Temperature of CNT-I-PPy sample as function of successive on-off voltage (5 V) cycles; (b) I-V curve of CNT-I-PPy sample under specific mechanical deformations; (c)


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Response of resistance after continuous bending processes and (d) The electrical response of CNT-I-PPy sample after complete laundering cycles.

Figure 6 – (a) Influence of voltage and composition of fiber on temperature of different composites, (b) dependence of temperature with power density for different composites, (c) dependence of temperature of fiber CNT-I-PPy as function of voltage and time of external excitation (on and off conditions) and (d) projection of the results on the XZ axis – dependence of temperature with time. Figure 7 – (a) Scheme of disposition of device on index finger of cotton knitted hand gloves, (b) temperature of glove under control condition and (c) temperature of glove under external excitation of 12 V on index finger. Figure 8 – (a) Comparison of cyclic voltammograms for different samples disposed on textiles at scan rate of 50 mVs-1; (b) Calculated linear capacitance from CV curves; (c) Cyclic voltammograms of CNT-I-PPy sample at different scan rate and (d) Calculated linear capacitance for sample CNT-I-PPy as function of scan rate.

Figure 9 –Electrochemical characterization curves for CNT-I-PPy supercapacitor: (a) GCD curves at different current density; (b) comparison of CV curves at different scan rates; (c) calculated specific capacitance at different current density; (d) Ragone plot; (e) capacitance retention at 1.5 mA and (f) Nyquist plots for as-prepared supercapacitor and after 2000 cycles of use.

Figure 10 – (a) Inhibition halo images of samples against S. aureus and (b) remaining CFU of S. aureus after treatment with different composites.


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(60) Lu, X.; Zheng, D.; Zhai, T.; Liu, Z.; Huang, Y.; Xie, S.; Tong, Y. Facile synthesis of large-area manganese oxide nanorod arrays as a high-performance electrochemical supercapacitor. Energy & Environmental Science 2011, 4 (8), 2915-2921. (61) El Jaouhari, A.; El Asbahani, A.; Bouabdallaoui, M.; Aouzal, Z.; Filotás, D.; Bazzaoui, E.; Nagy, L.; Nagy, G.; Bazzaoui, M.; Albourine, A. Corrosion resistance and antibacterial activity of electrosynthesized polypyrrole. Synthetic Metals 2017, 226, 1524. (62) Varesano, A.; Aluigi, A.; Florio, L.; Fabris, R. Multifunctional cotton fabrics. Synthetic Metals 2009, 159 (11), 1082-1089, DOI: 10.1016/j.synthmet.2009.01.036. (63) Lu, M.; Xie, R.; Liu, Z.; Zhao, Z.; Xu, H.; Mao, Z. Enhancement in electrical conductive property of polypyrrole‐coated cotton fabrics using cationic surfactant. Journal of Applied Polymer Science 2016, 133 (32), 43601 (6 pages). (64) Cetiner, S. Dielectric and morphological studies of nanostructured polypyrrolecoated cotton fabrics. Textile Research Journal 2014, 84 (14), 1463-1475.

Table 1. Comparison of conductivity of PPy/ CNT – based samples with those of other reported samples in the literature

Material PPy

Conductivity (Scm-1) 0.69x10-3

CNT

0.15

CNT-PPy

1.17

Ref This work This work This work


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I-PPy

1.18

CNT-I-PPy

10.44

PPy+CuO PPy+lignosulfonate PPy+CTAB PPy(Fecl3+AQSA)

10.00 3.03 5.85 1.5x10-3

This work This work 45 56 63 64

Table 2. Fitted parameters from equivalent circuit (inset of Figure 9f) describing the aging behavior of PPy-I-CNT.

Number of cycles 0

69.8

28.2

Y0 (mMho) 7.4

2000

123

71,8

4.9

R0 (Ω)

R1 (Ω)


n 0.455

Y1 (mMho) 16.3

0.352

11.7

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TOC graphic


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Figure 1 – Schematic view of preparation steps of wearable devices (interfacial polymerization) and corresponding application as heating component, bactericidal agent and supercapacitor. 338x190mm (96 x 96 DPI)


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Figure 2 - SEM images of (a) textile, (b) CNT, (c) PPy, (d) CNT-PPy, (e) I-PPy and (f) CNT-I-PPy. 280x177mm (96 x 96 DPI)



Figure 3 – Raman spectrum of samples: (a) CNT, (b) PPy, (c) CNT-PPy, (d) I-PPy and (e) CNT-I-PPy. 279x215mm (300 x 300 DPI)


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Figure 4 – Current – voltage curves of samples: textile, CNT, PPy, CNT-PPy, I-PPy and CNT-I-PPy. 271x207mm (150 x 150 DPI)



Figure 5 – (a) Temperature of CNT-I-PPy sample as function of successive on-off voltage (5 V) cycles; (b) IV curve of CNT-I-PPy sample under specific mechanical deformations; (c) Response of resistance after continuous bending processes; (d) The electrical response of CNT-I-PPy sample after complete laundering cycles. 225x178mm (96 x 96 DPI)


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Figure 6 – (a) Influence of voltage and composition of fiber on temperature of different composites, (b) dependence of temperature with power density for different composites, (c) dependence of temperature of fiber CNT-I-PPy as function of voltage and time of external excitation (on and off conditions) and (d) projection of the results on the XZ axis – dependence of temperature with time. 236x171mm (96 x 96 DPI)



Figure 7 – (a) Scheme of disposition of device on index finger of cotton knitted hand gloves, (b) temperature of glove under control condition, (c) temperature of glove under external excitation of 12 V on index finger. 339x84mm (96 x 96 DPI)


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Figure 8 – (a) Comparison of cyclic voltammograms for different samples disposed on textiles at scan rate of 50 mVs-1; (b) Calculated linear capacitance from CV curves; (c) Cyclic voltammograms of CNT-I-PPy sample at different scan rate; (d) Calculated linear capacitance for sample CNT-I-PPy as function of scan rate. 232x172mm (96 x 96 DPI)



Figure 9 – Electrochemical characterization curves for CNT-I-PPy supercapacitor: (a) GCD curves at different current density; (b) Comparison of CV curves at different scan rates; (c) calculated specific capacitance at different current density; (d) Ragone plot; (e) Capacitance retention at 1.5 mA and (f) Nyquist plots for asprepared supercapacitor and after 2000 cycles of use. 327x164mm (96 x 96 DPI)


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Figure 10 – (a) Inhibition halo images of samples against S. aureus and (b) remaining CFU of S. aureus after treatment with different composites. 135x161mm (96 x 96 DPI)


Multifunctional Wearable Electronic Textiles Using Cotton Fibers with

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