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High Sensitive Wearable Textile-based Humidity Sensor made of High-strength, Single-walled Carbon Nanotube (SWCNT)/Poly(Vinyl Alcohol) (PVA) Filaments Gengheng Zhou, Joon-Hyung Byun, Youngseok Oh, Byung-Mun Jung, HwaJin Cha, Dong Gi Seong, Moon-Kwang Um, Sangil Hyun, and Tsu-Wei Chou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12448 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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ACS Applied Materials & Interfaces

High Sensitive Wearable Textile-based Humidity Sensor made of High-strength, Singlewalled Carbon Nanotube (SWCNT)/Poly(Vinyl Alcohol) (PVA) Filaments Gengheng Zhou*†, Joon-Hyung Byun*†, Youngseok Oh*†, Byung-Mun Jung†, Hwa-Jin Cha†, Dong-Gi Seong†, Moon-Kwang Um†, Sangil Hyun‡, and Tsu-Wei Chou§ †

Composites Research Division, Korea Institute of Materials Science, 797 Changwondaero, Changwon, Gyeongnam, 51508, South Korea ‡ Analysis, Certification & Simulation Center Korea Institute of Ceramic Engineering & Technology, Jinju 52851, South Korea § Department of Mechanical Engineering University of Delaware, Newark, DE 19716, USA E-mail: [email protected], [email protected], [email protected]

Abstract Textile-based humidity sensors can be an important component of smart wearable electronictextiles and have potential applications in the management of wounds, bed-wetting, and skin pathologies or for microclimate control in clothing. Here, we report a wearable textile-based humidity sensor for the first time using high strength (~750 MPa) and ultra-tough (energy-tobreak, 4300 J g-1) SWCNT/PVA filaments via a wet-spinning process. The conductive SWCNT networks in the filaments can be modulated by adjusting the intertube distance by swelling the PVA molecular chains via the absorption of water molecules. The diameter of a SWCNT/PVA filament under wet conditions can be as much as two times that under dry conditions. The electrical resistance of a fiber sensor stitched onto a hydrophobic textile increases significantly (by more than 220 times) after water sprayed. Textile-based humidity sensors using a 1:5 weight ratio of SWCNT/PVA filaments showed high sensitivity in high relative humidity. The electrical resistance increases by more than 24 times in a short response time of 40s. We also demonstrated that our sensor can be used to monitor human sweating and water leakage on a high hydrophobic textile (contact angle of 115.5°). These smart textiles will pave a new way for the design of novel wearable sensors for monitoring blood leakage, sweat and underwear wetting. Keywords: Single-walled carbon nanotube, Composite fiber, Wet spinning, Washable textile, Humidity sensor 1 ACS Paragon Plus Environment


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Introduction Wearable textile-based electronics is a highly influential and rapidly emerging research field relating to the development of new classes of materials with novel functionalities, such as stretchability, flexibility, and conductivity. These multi-functional materials have unique applications and can be used in designs that were previously impossible using traditional technology.1 This new technology, which is commonly referred to as electronic-textiles, aims to improve humans’ quality of life using built-in electronic elements, such as power generators,2,3 batteries,4,5 supercapacitors,6-13 memory devices,14 actuators,15 and strain sensors.16-18 Textile-based humidity sensors could be an important component of smart wearable electronic-textiles and have potential applications in the management of wounds, bed-wetting, and skin pathologies or for microclimate control in clothing. To date, several approaches to transfer conventional capacitive, impeditive and resistive humidity sensors onto textiles have been developed. Pereira et al. demonstrated a textilebased humidity sensor using conductive yarns woven into textiles as electrodes and cotton as a hygroscopic material.19 Weremeczuk and co-authors used textiles as substrates onto which electrodes were printed and a sorption layer deposited.20 Kindeldei et al. printed humidity sensors on a polymer tape and subsequently wove the sensors into a textile structure.21,22 Connolly et al. developed a moisture monitoring system with textile-integrated sensors for wound healing assessment.23 Gao et al. deposited carbon nanotubes onto glass fiber surface to detect humidity.24 Kotov et al. coated a thin carbon nanotube film onto cotton threads to measure humidity and albumin in blood.25 Although these approaches represent significant progress towards fabricating textile-based humidity sensors, major challenges remain to be overcome. For example, the durability of electrodes prepared by printing, deposition and coating remains an important issue to be addressed. Damage on the surface of the coated conductive layer may decrease the sensitivity of sensor. In addition, the sensitivity of sensors made of conductive yarn electrodes depends strongly on the wettability of the textile substrate 2 ACS Paragon Plus Environment

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into which the electrodes are woven, which may limit these sensors’ applications.26 Moreover, the poor stretchability of glass fibers deposited by conductive layers may not be utilized in the sensors for wearable devices. Most of the mentioned textile sensors are resistive humidity sensors, 19,21,24-26 while others are impeditive, 20,23 and capacitive ones.22 To overcome the limitations of these sensors as discussed above, the state-of-the-art solution for textile-based humidity sensors is the development of strong smart filaments that can be used as humidity sensors. Similar to other wearable smart electronic-textiles, the development of such multi-functional filaments able to satisfy special functional requirements is a key research issue in this field. Carbon nano-materials, such as carbon nanotubes (CNTs) and graphene, are promising candidates for nano-reinforcements or fillers because of their unique electromechanical properties and high specific surface areas.27-29 CNT film or composite film have been studied to be used as humidity sensor. 30,31 Previously, considerable efforts have focused on integrating nano-carbon materials into filaments. The fabrication of single-walled CNT (SWCNT)-based pristine and composite filaments has been extensively explored by dry spinning,32-35 gel spinning,36,37 melt spinning,38 wet spinning,9,39-42 and even aerogel drawing.43,44 Graphene-based pristine and composite filaments have also been widely studied.45-49 The main applications of CNT and graphene filaments and their composites are wire-type supercapacitors,6-10 actuators,15 and strain sensors.16,17 Although substantial work has been conducted to develop textile-based humidity sensors, relatively less attention has been paid to creating humidity sensors using these nano-carbon materials. Currently, the greatest challenge hindering the creation of a state-of-the-art textile-based humidity senor is the ability to fabricate a strong and tough filament with appropriate electrical conductivity and sensitivity to water molecules. One-dimensional SWCNTs are ideal materials to create an electrical conductive network with a small amount of SWCNTs.50 In addition, the high strength of individual SWCNTs could be advantageous for obtaining 3 ACS Paragon Plus Environment


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high-strength materials with desired alignments.51 In contrast, highly tough polymers are widely used in modern society. Poly(vinyl alcohol) (PVA) is an environmentally friendly polymer that swells via water absorption, which implies that the PVA molecular chain can be altered by water molecules. This research takes advantage of SWCNTs’ electrical conductivity and strength to form conductive networks in a PVA matrix with excellent toughness and sensitivity to water molecules. Here, we report the development of wearable textile-based humidity sensors using ultrastrong SWCNT/PVA filaments fabricated via a wet-spinning process. The ultimate tensile strength of the 1:5 weight ratio of SWCNT/PVA filament was up to 750 MPa. The conductive SWCNT networks in the filaments can be modulated by adjusting the intertube distance by swelling the PVA molecular chains via the absorption of water molecules. The diameter of a SWCNT/PVA filament under wet conditions can be as much as two times that under dry conditions, and as a result, the electrical resistance increases significantly (by more than 220 times). Textile-based humidity sensors produced using a 1:5 weight ratio of SWCNT/PVA filaments show high sensitivity in high relative humidity (RH), and the electrical resistance increases more than 24 times in a relativly short response time (40s). We also demonstrate that the textile sensor based on the SWCNT/PVA filament can be used to monitor human sweating and water leakage on a high hydrophobic textile (contact angle of 115.5°). These smart textiles will pave a new way for the design of novel wearable sensors for monitoring blood leakage, sweat and underwear wetting. Results and Discussion

Fabrication and characterization of SWCNT/PVA filament Figure 1 shows a schematic of the fabrication of SWCNT/PVA filaments by the wet-spinning process. Highly purified SWCNTs were homogeneously dispersed in distilled water with sodium dodecyl sulfate (SDS) as a surfactant by sonication for 1 hour. SDS micelles were 4 ACS Paragon Plus Environment

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formed on the surfaces of the SWCNTs, and a suspension of uniformly dispersed SWCNTs was obtained.52, 53 Then, PVA pellets were added to the SWCNT suspension and heated to 95 °C with sonication for 30 minutes. The PVA molecular chains can be stretched by the formation of associations between SDS and the PVA molecular chain,54 thereby improving the dispersion of SWCNTs in the aqueous PVA, as indicated in Figure 1a. The alignment of SWCNTs in composite materials is important for their tensile strength and electrical conductivity.55 The preferred alignment of SWCNTs can be realized by extruding the SWCNT suspension through a small spinneret during the wet-spinning process.56 To remove the SDS surfactant from the composite filaments, acetone was used as a coagulation solution because SDS is soluble in acetone, whereas PVA is not. The continuous SWCNT/PVA filament was collected on a paper drum, as shown in Figure 1b. SWCNTs that were well aligned along the filament axis were obtained, as confirmed by field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (TEM) (Figure 1c). Tightly knotted SWCNT/PVA filaments with SWCNT-to-PVA weight ratios of 1:1, 1:5, 1:10, and 1:20 as shown in Figures 2a)-2d) indicate good deformability of our filaments. Cross-section morphologies shown in Figures 2e)-2l) present the influence of SWCNT weight loading on the microstructure of the filaments. Local agglomeration of SWCNT observed in the 1:1 filament indicates that a high SWCNT loading (50 wt%) may decrease the filament strength. Much more uniform morphologies in the case of lower SWCNT weight loadings imply better mechanical properties of the filaments. A possible explanation for the formation of fiber morphology might be as follows. Our approach is a wet-dry phase inversion process. Once the SWCNT/PVA spinning suspension was extruded into the acetone coagulation solution, acetone diffused into the fiber. Simultaneously, water in the fiber diffused from the fiber into the acetone solution. During this process, SDS dissolved in the acetone. Then, the acetone evaporated during the winding 5 ACS Paragon Plus Environment


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up process and the fiber shrank and became dense. The fiber morphology is determined by the spinning suspension. In the case of higher SWCNT content, local SWCNT agglomerations could be formed and kept inside the fiber. While in the case of lower SWCNT content, however, much more uniform spinning suspension can be obtained and the morphology of the fiber is more uniform. It should be noted that the diffusion process could also have influence on the fiber morphology. Mechanical and electrical conductive performance To understand the influence of SWCNT content on the mechanical properties of the SWCNTreinforced composite filaments, single filament tensile tests were performed. Typical stressstrain curves and tensile strengths of SWCNT/PVA filaments with different SWCNT-to-PVA weight ratios are shown in Figures 3a and 3b, respectively. The static tensile strength increases from approximately 0.65 GPa for the 1:1 SWCNT/PVA filament to 1.16 GPa for the 1:20 SWCNT/PVA filament. Correspondingly, the maximum strain-to-failure increases from 4% to 160%. The decreased tensile strength and elongation exhibited by filaments with higher SWCNT content suggest that the local agglomeration of SWCNTs may have occurred in the filament because of the strong van der Waals force between adjacent SWCNTs. This phenomenon is in accordance with the SEM observations as shown in Figures 2i-2l. Serious CNT agglomeration was observed in the 1:1 SWCNT/PVA filament (Figure 2i). Much more uniform morphologies were observed in the filament with lower CNT loadings (Figure 2j-2l). The SWCNT agglomeration may adversely affect the load transfer between SWCNTs and PVA molecular chains. In contrast, the electrical conductivity decreases from ~5 Scm-1 for the 1:1 SWCNT/PVA filament to 0.001 Scm-1 for the 1:20 SWCNT/PVA filament. Thus, the mechanical and electrical conductive properties must be carefully balanced depending on the specific application. Filaments that can be stitched onto a textile fabric must have sufficient strength and toughness. To demonstrate this, a 200-gram weight was lifted by a tightly knotted 1:5 SWCNT/PVA filament, as shown in Figure 3c (Supporting Information Video, 6 ACS Paragon Plus Environment

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S1). The energy-to-break value of our 1:5 SWCNT/PVA filament (4300 J g-1) determined from the stress-strain curve is much higher than that of spider dragline silk (165 J g-1).57 Textile-based humidity sensors should have good deformability, including bending or stretching, for real applications, and the variation in the electrical resistance caused by mechanical deformation should be much smaller than that caused by humidity. To evaluate the influence of mechanical deformation on the electrical resistance of the SWCNT/PVA filament, a cyclic bending-extending test was performed and clearly demonstrated that the variation of the electrical resistance of the filament under bending (with different bending radii, as shown in Figure 3d) is reasonably small, even after a 10,000-cycle test (Figure 3e). These results indicate that the deformation of this textile-based sensor has little influence on its electrical resistance. Swelling behaviors To test their swelling behavior, SWCNT/PVA filaments with different SWCNT content were stitched onto a cotton cloth and then swelled in boiling water for 5 minutes (Supporting Information Video, S2). Photos of the stitched filament before, after swelling and after dry are presented in Figure 4a. The corresponding filament morphologies were analyzed by an optical microscope, as shown in Figures 4b, 4c and 4d. Clearly, the extent of water uptake increases with increasing PVA content in the filament. The filament diameters are expanded by 59% and 320% in the 1:1 and 1:20 SWCNT/PVA filaments, respectively. The volume increases of the 1:1 and 1:20 SWCNT/PVA filaments are 153% and 1760%, respectively. All filaments survived in the boiling water and kept in shape well after dry as shown in Figure 4c and 4d, although PVA is normally water soluble, especially in boiling water. This finding indicates that inter-connecting SWCNT networks form inside the PVA matrix. Additionally, the SWCNTs maintain the network structure during the PVA molecular swelling, while the intertube distance increases, resulting in increased filament volume. Therefore, the degree of SWCNT networking can be adjusted by the swelling of the PVA molecules, even under harsh 7 ACS Paragon Plus Environment


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conditions, such as boiling water. Thus, the electrical conductivity of the SWCNT/PVA filament can be modulated by humidity. Notably, the swelled filament recovers after being dried as shown in Figure 4a, indicating that our filament is washable. To assess the full recovery of the interconnection between CNTs and PVA during swellingdeswelling, Fourier transform infrared spectroscopy (FTIR) has been applied to analyze the PVA and the 1:5 SWCNT/PVA filament (Figure 5a and 5b). Due to the hydrogen bonding formed between PVA and water molecules, the intensity of C-H stretching indicated by arrows in Figure 5a decreased significantly after the pristine PVA was swollen and recovered once the swollen PVA was dried. This phenomenon indicates that the structure of PVA was fully recovered after deswelling. The situation of the 1:5 fiber is the same as that of PVA, which indicates a full recovery of the fiber structure after deswelling. Humidity sensing performance The investigation of the swelling behavior upon water absorption revealed that SWCNT/PVA filaments with an appropriate SWCNT content can be utilized as an effective textile-based humidity sensor. Because of SWCNT/PVA filaments’ high strength (~750 MPa) and superior toughness (4300 J g-1), textile-based sensors with different patterns can be designed by stitching the filaments onto the desired cloth substrates, as shown in Figure 6a. The dry and swollen states of one pattern of the textile-based sensor are presented in Figures 6b and 6c, respectively. To select an appropriate SWCNT/PVA filament for a textile-based humidity sensor, the humidity sensing behaviors of the 1:1 and 1:5 SWCNT/PVA filaments were determined. The 1:10 and 1:20 SWCNT/PVA filaments were not selected because of their relatively low electrical conductivities. The variations in the electrical resistance versus RH at different temperatures are plotted and shown in Figures 6d and 6e. Based on the magnitude of the sensitivity shown on the ordinate axis, the 1:5 filament is substantially more sensitive than the 8 ACS Paragon Plus Environment

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1:1 filament, especially under high-RH conditions. The electrical resistance of the 1:1 filament increases by only approximately 4 times as the RH increases from 60% to 100%. For the 1:5 filament, however, the electrical resistance increases by approximately 24 times. Notably, the electrical resistance of the filament sensor decreases as the temperature increases, possibly because the mobility of electrons increases at higher temperatures. The response time and reversibility of sensors are important parameters for practical applications. To test the performance of our sensor, we varied relative humidity (RH) in an environmental chamber between 70%-95% and the variation of sensor’s resistance was recorded simultaneously. The curves of RH vs time and the electrical resistance vs time were shown in Figure 6f. Evidently, the resistance varies rapidly with the RH with a relatively short time delay (~40s), which indicates that it takes a fairly short response time for our sensor to reach a stabilized state. The cyclic test result demonstrated that the resistance recovered as soon as the RH recovered, which demonstrates excellent reversibility of the sensor. It should be noted that the unsymmetry of the curve is due to the difference between the increasing and decreasing rate of the RH in the enviromental chamber. The tensile strength and humidity sensitivity (with relative humidity varies from 60% to 100%) of our SWCNT/PVA fiber sensor as compared with the literature data for other film and fiber-typed humidity senors is shown in Figue 6g. Clearly, the sensitivity of our textilebased sensor is about 70 times higher than that of polylactide (PLA)/CNT fiber-typed sensor.38 It can even be comparable to the sensitivity of CNT/polymer or graphene/PVA filmtyped sensor.30,31,58,59 Importantly, our fiber sensor shows much higher tensile strength as compared to other sensors, whcih makes it an excellent candidate for advanced textile-based humidity sensors. To examine the sensor’s practical performance on water leakage monitoring, the 1:5 filament was stitched onto a high hydrophobic substrate textile with a contact angle of 115.5°. The variation of electrical resistance after water spray on the textile was recorded as 9 ACS Paragon Plus Environment


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presented in Figure 7a. The insets show the states of the sensor befor water spray, after water spray, and after dry. Due to the high hydrophobicity of the textile, small water beads formed on the textile with only a few beads in contact with the fiber sensor. However, the sensor’s resistance increased 100% within 10 seconds and it kept increasing as time passed because more water molecules were absorbed in the fiber (Figure 7b). The resistance of the sensor increased more than 220 times after about 3 minutes as it reached a stabilized state. Once the textile was dried, the resistance returned to its original value, which shows a perfect reversibility of our sensor (Supporting Information Video S3). Compared to the other sensors, in which the sensitivity is strongly dependent on the hydrophilicity of the substrate textile woven by conductive yarns,26 our sensor shows an excellent performance on the hydrophobic textile. Discussion on mechanisms The mechanism underlying the change in the resistance with humidity can be explained in terms of the filament composition as follows. In the filament, SWCNTs form conductive networks, and PVA molecules act as a relatively low electrical conductive matrix coated on the surface of the SWCNTs. When the filaments are exposed to moisture, water molecules are adsorbed on the filament surface, and then, they are absorbed into the filament by PVA molecules. The PVA molecules become partially swollen, the intertube distance increases, and the conductive networks are simultaneously modulated.60-62 The 1:1 and 1:5 SWCNT/PVA filaments differ in their SWCNT and PVA contents. The experimental results indicate that 16.7 wt% of the SWCNTs in the 1:5 filament form a stable conductive network. Compared to the 50 wt% PVA in the 1:1 filament, the 83.3 wt% PVA in the 1:5 filament absorbs substantially more water molecules. Thus, the conductive network is more effectively modulated, and the humidity sensitivity is increased.

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Generally, the SWCNTs in the fiber were coated with a layer of PVA molecule and the thickness of the PVA layer can be modified by the content of SWCNTs and PVA. A percolation threshold of the CNT content, which increases with the increasing degree of CNT alignment,63,64 must be surpassed to form conductive networks in polymer composites. The intertube tunneling effect is the dominant mechanism affecting the electrical resistance of composites when the CNT content is near the percolation threshold region.60, 65 The lower sensitivity of the 1:1 SWCNT/PVA filament implies that the SWCNT content is much higher than the percolation threshold and that, thus, the resistance is not dominated by the intertube tunneling effect. However, the rate of change of the resistance of the 1:5 SWCNT/PVA sensor increases significantly in high humidity (RH>90%), implying that the swelling of PVA expands the intertube distance and that the relative SWCNT volume fraction approaches the percolation threshold. Thus, the sensitivity of the 1:5 SWCNT/PVA sensor under high RH condition is governed by the intertube tunneling effect. It should be mentioned that the proton effect, in which the protons produced by water molecules serve as conducting carriers in graphene/PVA films,59 is negligible in our filament sensor. Because our sensor employs SWCNT networks in a PVA matrix, the proton effect is not significant relative to the influence of filament swelling, and the sensitivity is much improved. This is also confirmed by the increase in the resistance of the filaments as the RH increases. This trend is in contrast to the proton effect, which causes the resistance to decrease as RH increases. Interestingly, the increase in electrical resistance of our sensor under high RH conditions is much smaller (~24 times) than that under wet condition (~220 times). This phenomenon indicates that water molecules could only penetrate into a shallow depth of the fiber and then a steady state would prevail when the fiber is under humid condition. Under wet condition, however, the sensor’s resistance increased more than 220 times and a relatively longer time (3 minutes) was necessary to reach a stabilized state (Figure 6a). Because more water was absorbed, the fiber diameter increased significantly in this case. Although it took 3 minutes to 11 ACS Paragon Plus Environment


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reach a stable state in water leakage monitoring, the resistance increased 100% in a short time, within 10s (Figure 6b). Since resistance increased evidently in a short time, effective alarm of water leakage at early stage can be achieved. Conclusion In summary, we fabricated strong and ultra-tough SWCNT/PVA filaments (tensile strength as high as 750 MPa) with high sensitivity to humidity, which can be used in designing textilebased humidity sensors. The SWCNT electrical conductive network can be adjusted by changing the SWCNT content in the filament. Using an appropriate SWCNT-to-PVA ratio, a highly sensitive textile-based humidity sensor was obtained, which exhibited an electrical resistance increase of more than 24 times under high RH (RH=100%) relative to that under low RH (RH=60%), and a short response time of 40s. We also demonstrated that using the fabricated SWCNT/PVA filament a textile-based sensor can be designed to monitor human sweating and water leakage on a high hydrophobic textile. Our sensor also showed excellent reversibility under high humidity and wet conditions. We have utilized the wet-spinning process for the fiber fabriation, which is cost-effective and industrially scalable. These smart filaments will facilitate the design of novel wearable humidity sensors to monitor bloodleakage, human sweating and underwear-wetting. Experimental Procedures Suspension preparation and wet spinning: SDS (Sigma-Aldrich) was used as a surfactant to obtain a uniformly dispersed SWCNT/PVA suspension. The SDS-to-SWCNT powder weight ratio was optimized to 8:1. First, SDS (0.1 g) was dissolved in distilled water (10 ml) by sonication for 2 minutes. Then, the SWCNT powder (SA310, Nanosolution, Korea) (0.0125 g) was dispersed in the SDS solution by sonication for 1 hr. Subsequently, poly(vinyl alcohol) (PVA, Sigma-Aldrich, molecular weight: 146,000~186,000; 99+% hydrolyzed) particles with the desired weight were added to the prepared solution and heated to 95 °C under sonication for 30 minutes. To obtain different SWCNT powder-to-PVA weight ratios, different amounts 12 ACS Paragon Plus Environment

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of PVA (0.0125 g, 0.0625 g, 0.125 g, and 0.25 g) were added to suspensions that had been previously prepared with the same amount of SWCNT (0.0125 g) and distilled water (10 ml) listed above. The SWCNT powder-to-PVA weight ratios of the prepared suspensions were 1:1, 1:5, 1:10, and 1:20, respectively. Finally, the prepared suspension was extruded into a coagulation solution (acetone) by a syringe through a spinneret with a diameter of 290 µm. The SDS surfactant was removed by acetone during the process. Then, a continuous filament was collected on a paper drum. Characterization: Optical microscopy (Nikon Eclipse Lv150N) was performed to observe the morphology of the SWCNT/ PVA filament. The arrangement of SWCNTs inside the filament was determined by a FE-SEM (JSM-6700F) and a high-resolution TEM (JEOL 2100F) operated at 200 kV. A thin-slice TEM sample was cut from the filament by a focused ion beam with an operating voltage of 5 keV. Tensile test: Stress-strain curves of the composite filament were obtained using a TA Instruments Q800 Dynamic Mechanical Analyzer (DMA). Detailed information can be found elsewhere.66 Bending-extending test: First, a SWCNT/PVA filament with a length of 10 cm was stitched onto a cotton cloth. Then, the cotton cloth with the SWCNT/PVA filament was fixed on a cyclic bending-extending stage by clamping the two ends. Next, the two ends of the filament were connected to a high-resistance digital multi-meter. The variation in the electrical resistance was recorded after every 1,000 cycles. Humidity sensing test: Textile-based humidity sensors fabricated using 1:1 and 1:5 SWCNT/PVA filaments were installed in an environmental chamber (TH-ME100 Temperature & Humidity Chamber) in which the RH was controlled to measure their sensitivity to humidity. The RH was varied in a stepwise manner, and the increments of the sensor resistance were measured in situ using a high-resistance digital multi-meter (Keithley



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6517B). Cyclic test was performed by varying RH between 70% and 95% while electrical resistance was recorded simultaneously. Supporting Information Videos on swelling behaviors of SWCNT/PVA filaments, demonstration of high strength and toughness of the filament and water leakage detect test.

Acknowledgements This research was supported by a Global Research Lab Program, Basic Science Research Program through the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2007-00017 and NRF-2015R1C1A02037139), and the Principal Research Program in the Korea Institute of Materials Science (KIMS).

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Figure 1. Schematic of the wet-spinning process used to produce the SWCNT/PVA composite filament: a) Well-dispersed SWCNTs in PVA solution were obtained using SDS as a surfactant. b) The prepared SWCNT/PVA suspension was extruded into an acetone coagulation solution, and a continuous SWCNT/PVA filament was collected on a paper drum. The surfactant was removed by acetone during the spinning process. c) High-resolution FESEM and TEM images show that the SWCNTs are oriented parallel to the filament axis.


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Figure 2. SEM images of SWCNT/PVA filaments. a)-d) Tightly knotted SWCNT/PVA filaments with SWCNT-to-PVA weight ratios of 1:1, 1:5, 1:10, and 1:20. e)-l) Low and high magnified cross-section morphologies of the filaments. The scale bars in Figures a)-d), Figures e)-h) and Figures i)-l) are 100 µm, 20 µm and 1 µm, respectively.

Figure 3. a) Stress-strain curves of SWCNT/PVA filaments with SWCNT-to-PVA weight ratios of 1:1, 1:5, 1:10, and 1:20. b) Static tensile strength of SWCNT/PVA filaments with different SWCNT content. c) A weight (200 gram) was lifted using a knotted SWCNT/PVA filament with a diameter of 60 µm. d) A 1:5 SWCNT/PVA filament was stitched onto a cotton cloth and fixed on a cyclic bending-extending stage. A cyclic bending-extending test was performed with a minimum bending radius of 2 mm. An extremely small resistance change was observed in different bending states. e) Variation in the measured relative resistance during the cyclic bending-extending test. 23 ACS Paragon Plus Environment


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Figure 4. a) SWCNT/PVA filaments with different SWCNT content were stitched onto a cotton cloth, swelled in boiling water for 5 minutes and dried. The hand-written numbers were removed after immersion in boiling water, which demonstrates the washability of our fiber on the substrate cloth. b), c) and d) Optical images of the SWCNT/PVA filaments before swelling, after swelling in boiling water, and dried condition with scale bars of 50, 100 and 50µm, respectively.

Figure 5. FTIR spectra of PVA and 1:5 fiber under different conditions. a) spectra of pristine PVA, swollen PVA and dried PVA. b) spectra of SWCNT, pristine 1:5 fiber, swollen fiber and dried fiber.


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Figure 6. a) The fabricated SWCNT/PVA filaments used to stitch different patterns onto a cotton cloth. b) Magnified image of one type of pattern shown in Figure 6a indicated by a blue dashed line. c) Photo of the SWCNT/PVA pattern swelled in boiling water. d) and e) Variations in the electrical resistance of the SWCNT/PVA filaments with SWCNT-to-PVA weight ratios of 1:1 and 1:5 under different RH conditions. f) Resistance reversibility and response time of the 1:5 SWCNT/PVA fiber sensor at 25 C°. g) The tensile strength and humidity sensitvity (with relative humidity varies from 60% to 100%) of SWCNT/PVA fiber sensors as compared with the literature data for other film and fiber-typed humidity senors. The numbers denote the reference number.



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Figure 7. a) Resistance reversibility of the 1:5 SWCNT/PVA fiber sensor stitched onto a high hydrophobic textile (contact angle of 115.5 C°) after water spray and dry; the insets show the images of the sensor before water spray, after water spray and after dry. b) Quick response of resistance at the initial state (green area in Figure 7a) after water spray; the insets show the water beads formed on the hydrophobic textile and the measured contact angle between water and the substrate textile.


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Keyword: Single-walled carbon nanotube, Composite fiber, Wet spinning, Washable textile, Humidity sensor Gengheng Zhou*, Joon-Hyung Byun*, Youngseok Oh*, Byung-Mun Jung, Hwa-Jin Cha, Dong-Gi Seong, Moon-Kwang Um, Sangil Hyun, and Tsu-Wei Chou Title High Sensitive Wearable Textile-based Humidity Sensor made of High-strength, Singlewalled Carbon Nanotube (SWCNT)/Poly(Vinyl Alcohol) (PVA) Filaments

ToC figure



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To develop textile-based humidity sensors, ultra tough and strong SWCNT/PVA composite fiber is faricated using a wet spinning process. The conductive SWCNT networks in the filaments can be modulated by swelling of PVA molecular chains via water molecule absorption. Using the fabricated filament, a high sensitive textile-based sensor with excellent reversibility was achieved to monitor human sweating or water leakage. 332x260mm (72 x 72 DPI)

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Highly Sensitive Wearable Textile-Based Humidity ... - ACS Publications

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