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

Research Article www.acsami.org

Highly Sensitive Wearable Textile-Based Humidity Sensor Made of High-Strength, Single-Walled Carbon Nanotube/Poly(vinyl alcohol) 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 ‡ Simulation Team, Korea Institute of Ceramic Engineering & Technology, Jinju 52851, South Korea § Department of Mechanical Engineering, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: Textile-based humidity sensors can 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. Here, we report a wearable textile-based humidity sensor for the first time using high strength (∼750 MPa) and ultratough (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 2 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 40 s. We also demonstrated that our sensor can be used to monitor 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

■

INTRODUCTION

into textiles as electrodes and cotton as a hygroscopic material.19 Weremeczuk and coauthors 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 toward 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

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 multifunctional 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 electronictextiles 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 textile-based humidity sensor using conductive yarns woven © 2017 American Chemical Society

Received: September 29, 2016 Accepted: January 18, 2017 Published: January 18, 2017 4788

DOI: 10.1021/acsami.6b12448 ACS Appl. Mater. Interfaces 2017, 9, 4788−4797

Research Article

ACS Applied Materials & Interfaces

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 FE-SEM and TEM images show that the SWCNTs are oriented parallel to the filament axis.

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 nanocarbon 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 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 2 times that under dry conditions, and, as a result, the electrical resistance increases

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 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 impeditive20,23 and capacitive.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 multifunctional filaments able to satisfy special functional requirements is a key research issue in this field. Carbon nanomaterials, such as carbon nanotubes (CNTs) and graphene, are promising candidates for nanoreinforcements or fillers because of their unique electromechanical properties and high specific surface areas.27−29 CNT film or composite film has been studied to be used as humidity sensor.30,31 Previously, considerable efforts have focused on integrating nanocarbon materials into filaments. The fabrication of singlewalled 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 4789


Research Article


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 (a−d), (e−h), and (i−l) are 100, 20, 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 g) 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.

■

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 h. SDS micelles were formed on the surfaces of the SWCNTs, and a suspension of uniformly dispersed SWCNTs was obtained.52,53 PVA pellets were added to the SWCNT suspension and heated to 95 °C with sonication for

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 relatively short response time (40 s). We also demonstrate that the textile sensor based on the SWCNT/PVA filament can be used to monitor 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. 4790


Research Article


Figure 4. (a) SWCNT/PVA filaments with different SWCNT content were stitched onto a cotton cloth, swelled in boiling water for 5 min, 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−d) Optical images of the SWCNT/PVA filaments before swelling, after swelling in boiling water, and under dried condition with scale bars of 50, 100, and 50 μm, respectively.

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 SWCNT-reinforced composite filaments, single filament tensile tests were performed. Typical stress−strain curves and tensile strengths of SWCNT/PVA filaments with different SWCNT-to-PVA weight ratios are shown in Figure 3a and b, 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 Figure 2i−l. 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−l). The SWCNT agglomeration may adversely affect the load transfer between SWCNTs and PVA molecular chains. In contrast, the electrical conductivity decreases from ∼5 S cm−1 for the 1:1 SWCNT/PVA filament to 0.001 S cm−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-g weight was lifted by a tightly knotted 1:5 SWCNT/PVA filament, as shown in Figure 3c (Supporting Information Video S1). The energy-to-break value of our 1:5 SWCNT/PVA filament (4300 J g−1) determined from the

30 min. 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-toPVA weight ratios of 1:1, 1:5, 1:10, and 1:20 as shown in Figure 2a−d indicate good deformability of our filaments. Crosssection morphologies shown in Figure 2e−l 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. The acetone then evaporated during the winding 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 4791


Research Article


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.

To assess the full recovery of the interconnection between CNTs and PVA during swelling−deswelling, Fourier transform infrared spectroscopy (FTIR) has been applied to analyze the PVA and the 1:5 SWCNT/PVA filament (Figure 5a and b). Because of 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 Figure 6b and c, respectively. To select an appropriate SWCNT/PVA filament for a textilebased 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 Figure 6d and e. On the basis of the magnitude of the sensitivity shown on the ordinate axis, the 1:5 filament is substantially more sensitive than the 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 RH in an environmental chamber

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 min (Supporting Information Video S2). Photos of the stitched filament before, after swelling, and after drying are presented in Figure 4a. The corresponding filament morphologies were analyzed by an optical microscope, as shown in Figure 4b−d. 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 shape well after drying as shown in Figure 4c and d, although PVA is normally water-soluble, especially in boiling water. This finding indicates that interconnecting 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 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. 4792


Research Article


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 (a) 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 varying from 60% to 100%) of SWCNT/PVA fiber sensors as compared to the literature data for other film- and fiber-typed humidity senors. The numbers denote the reference number.

between 70%−95%, and the variation of sensor’s resistance was recorded simultaneously. The curves of RH versus time and the electrical resistance versus time were shown in Figure 6f. Evidently, the resistance varies rapidly with the RH with a relatively short time delay (∼40 s), 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 4793


Research Article


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 drying; the insets show the images of the sensor before water spray, after water spray, and after drying. (b) Quick response of resistance at the initial state (green area in (a) 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.

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. As 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. 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)

increasing and decreasing rate of the RH in the environmental chamber. The tensile strength and humidity sensitivity (with relative humidity varying from 60% to 100%) of our SWCNT/PVA fiber sensor as compared to the literature data for other filmand fiber-typed humidity senors are shown in Figue 6g. Clearly, the sensitivity of our textile-based 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 film-typed sensor.30,31,58,59 Importantly, our fiber sensor shows much higher tensile strength as compared to other sensors, which 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 presented in Figure 7a. The insets show the states of the sensor before water spray, after water spray, and after drying. Because of 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 s, 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 min 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). As 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 4794


Research Article


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. Next, 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 multimeter. The variation in the electrical resistance was recorded after every 1000 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 and 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 highresistance digital multimeter (Keithley 6517B). The cyclic test was performed by varying RH between 70% and 95%, while the electrical resistance was recorded simultaneously.

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 min) 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 min to reach a stable state in water leakage monitoring, the resistance increased 100% in a short time, within 10 s (Figure 6b). Because resistance increased evidently in a short time, the effective alarm of water leakage at early stage can be achieved.

■

CONCLUSION In summary, we fabricated strong and ultratough 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 40 s. We also demonstrated that by 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 wetspinning 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 blood leakage, human sweating, and underwear wetting.

■

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12448. A 200 g weight lifted by a knotted 1:5 SWCNT/PVA filament with a diameter of about 60 mm shows high strength and toughness of the fiber (AVI) Swelling behaviors of SWCNT/PVA filaments in boiling water (AVI) Demonstration of water leakage detection by a 1:5 SWNT/PVA filament sensor on a high hydrophobic textile (AVI)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

EXPERIMENTAL PROCEDURES

ORCID

Suspension Preparation and Wet Spinning. SDS (SigmaAldrich) 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 min. Next, the SWCNT powder (SA310, Nanosolution, Korea) (0.0125 g) was dispersed in the SDS solution by sonication for 1 h. Subsequently, poly(vinyl alcohol) (PVA, SigmaAldrich, 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 min. To obtain different SWCNT powder-to-PVA weight ratios, different amounts of PVA (0.0125, 0.0625, 0.125, 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. Next, 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.

Joon-Hyung Byun: 0000-0003-3528-9206 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS 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).

■

REFERENCES

(1) Stoppa, M.; Chiolerio, A. Wearable Electronics and Smart Textiles: A Critical Review. Sensors 2014, 14, 11957. (2) Kim, S. J.; We, J. H.; Cho, B. J. A Wearable Thermoelectric Generator Fabricated on A Glass Fabric. Energy Environ. Sci. 2014, 7, 1959−1965. (3) Seung, W.; Gupta, M. K.; Lee, K. Y.; Shin, K. S.; Lee, J. H.; Kim, T. Y.; Kim, S.; Lin, J.; Kim, J. H.; Kim, S. W. Nanopatterned TextileBased Wearable Triboelectric Nanogenerator. ACS Nano 2015, 9, 3501−3509. (4) Hu, L.; Cui, Y. Energy and Environmental Nanotechnology in Conductive Paper and Textiles. Energy Environ. Sci. 2012, 5, 6423− 6435. 4795


Research Article

ACS Applied Materials & Interfaces (5) Lee, Y. H.; Kim, J. S.; Noh, J.; Lee, I.; Kim, H. J.; Choi, S.; Seo, J.; Jeon, S.; Kim, T. S.; Lee, J. Y.; Choi, J. W. Wearable Textile Battery Rechargeable by Solar Energy. Nano Lett. 2013, 13, 5753−5761. (6) Lee, J. A.; Shin, M. K.; Kim, S. H.; Cho, H. U.; Spinks, G. M.; Wallace, G. G.; Lima, M. D.; Lepró, X.; Kozlov, M. E.; Baughman, R. H.; Kim, S. J. Ultrafast Charge and Discharge Biscrolled Yarn Supercapacitors For Textiles and Microdevices. Nat. Commun. 2013, 4, 1970. (7) Jost, K.; Stenger, D.; Perez, C. R.; McDonough, J. K.; Lian, K.; Gogotsi, Y.; Dion, G. Knitted and Screen Printed Carbon-fiber Supercapacitors for Applications in Wearable Electronics. Energy Environ. Sci. 2013, 6, 2698−2705. (8) Kou, L.; Huang, T.; Zheng, B.; Han, Y.; Zhao, X.; Gopalsamy, K.; Sun, H.; Gao, C. Coaxial Wet-spun Yarn Supercapacitors for Highenergy Density and Safe Wearable Electronics. Nat. Commun. 2014, 5, 3754. (9) Choi, C.; Kim, S. H.; Sim, H. J.; Lee, J. A.; Choi, A. Y.; Kim, Y. T.; Lepró, X.; Spinks, G. M.; Baughman, R. H.; Kim, S. J. Stretchable, Weavable Coiled Carbon Nanotube/MnO2/Polymer Fiber Solid-State Supercapacitors. Sci. Rep. 2015, 5, 9387. (10) Liu, L.; Yu, Y.; Yan, C.; Li, K.; Zheng, Z. Wearable EnergyDense and Power-Dense Supercapacitor Yarns Enabled by Scalable Graphene−Metallic Textile Composite Electrodes. Nat. Commun. 2015, 6, 7260. (11) Hu, L.; Pasta, M.; Mantia, F. L.; Cui, L. F.; Jeong, S.; Deshazer, H. D.; Choi, J. W.; Han, S. M.; Cui, Y. Stretchable, Porous, and Conductive Energy Textiles. Nano Lett. 2010, 10, 708−714. (12) Wang, X.; Jiang, K.; Shen, G. Flexible Fiber Energy Storage and Integrated Devicess: Recent Progress and Perspectives. Mater. Today 2015, 18, 265−272. (13) Xu, P.; Wei, B.; Cao, Z.; Zheng, J.; Gong, K.; Li, F.; Yu, J.; Li, Q.; Lu, W.; Byun, J. H.; Kim, B. S.; Yan, Y.; Chou, T. W. Stretchable Wire-Shaped Asymmetric Supercapacitors Based on Pristine and MnO2 Coated Carbon Nanotube Fibers. ACS Nano 2015, 9, 6088− 6096. (14) Sun, G.; Liu, J.; Zheng, L.; Huang, W.; Zhang, H. Preparation of Weavable, All-Carbon Fibers for Non-Volatile Memory Devices. Angew. Chem., Int. Ed. 2013, 52, 13351−13355. (15) Chen, P.; Xu, Y.; He, S.; Sun, X.; Pan, S.; Deng, J.; Chen, D.; Peng, H. Hierarchically Arranged Helical Fibre Actuators Driven by Solvents and Vapors. Nat. Nanotechnol. 2015, 10, 1077−1084. (16) Yamada, T.; Hayamizu, Y.; Yamamoto, Y.; Yomogida, Y.; LzadiNajafabadi, A.; Futaba, D. N.; Hata, K. A Stretchable Carbon Nanotube Strain Sensor for Human-Motion Detection. Nat. Nanotechnol. 2011, 6, 296−301. (17) Ge, J.; Sun, L.; Zhang, F. R.; Zhang, Y.; Shi, L. A.; Zhao, H. Y.; Zhu, H. W.; Jiang, H. L.; Yu, S. H. A Stretchable Electronic Fabric Artificial Skin with Pressure-, Lateral Strain-, and Flexion-Sensitive Properties. Adv. Mater. 2016, 28, 722−728. (18) Yang, T.; Wang, W.; Zhang, H.; Li, X.; Shi, J.; He, Y.; Zheng, Q.; Li, Z.; Zhu, H. Tactile Sensing System Based on Arrays of Graphene Woven Microfabrics: Electromechanical Behavior and Electronic Skin Application. ACS Nano 2015, 9, 10867−10875. (19) Pereira, T.; Silva, P.; Carvalho, H.; Carvalho, M. Textile Moisture Sensor Matrix for Monitoring of Disabled and Bed-rest Patient. Procedings of EUROCON. 2011, 1. (20) Weremczuk, J.; Tarapata, G.; Jachowicz, R. Humidity Sensor Printed on Textile with Use of Ink-Jet Technology. Procedia Eng. 2012, 47, 1366−1369. (21) Kinkeldei, T.; Zysset, C.; Cherenack, K. H.; Tröster, G. A Textile Integrated Sensor System for Monitoring Humidity and Temperature. Solid State Sens. Actuators Microsys. Conf. (Transducers) 2011, 1156−1159. (22) Kinkeldei, T.; Mattana, G.; Leuenberger, D.; Ataman, C.; Lopez, F. M.; Quintero, A. V.; Briand, D.; Nisato, G.; de Rooij, N. F.; Tröster, G. Feasibility of Printing Woven Humidity and Temperature Sensors for the Integration into Electronic Textiles. Adv. Sci. Technol. 2012, 80, 77−82.

(23) McColl, D.; Cartlidge, B.; Connolly, P. Real-Time Monitoring of Moisture Levels in Wound Dressings in Vitro: An Experimental Study. Int. J. Surg. 2007, 5, 316−322. (24) Gao, S.; Zhuang, R.; Zhang, J.; Lin, J.; Mäder, E. Glass Fibers with Carbon Nanotube Networks as Multifunctional Sensors. Adv. Funct. Mater. 2010, 20, 1885−1893. (25) Shim, B. S.; Chen, W.; Doty, C.; Xu, C.; Kotov, N. A. Smart Electronic Yarns and Wearable Fabrics for Human Biomonitoring made by Carbon Nanotube Coating with Polyelectrolytes. Nano Lett. 2008, 8, 4151−4157. (26) Parkova, I.; Ziemele, I.; Vil ̧umsone, A. Fabric Selection for Textile Moisture Sensor Design. Mater. Sci.: Textile Clothing Technol. 2012, 7, 38−43. (27) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Carbon Nanotubes-The Route Toward Applications. Science 2002, 297, 787− 792. (28) Coleman, J. N.; Khan, U.; Blau, W. J.; Guńko, Y. K. Small But Strong: A Review of the Mechanical Properties of Carbon NanotubePolymer Composites. Carbon 2006, 44, 1624−1652. (29) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-Based Composite Materials. Nature 2006, 442, 282−286. (30) Liu, L.; Ye, X.; Wu, K.; Han, R.; Zhou, Z.; Cui, T. Humidity Sensitivity of Multi-Walled Carbon Nanotube Networks Deposited by Dielectrophoresis. Sensors 2009, 9, 1714−1721. (31) Fei, T.; Jiang, K.; Jiang, F.; Mu, R.; Zhang, T. Humidity Switching Properties of Sensors Based on Multiwalled Carbon Nanotubes/Polyvinyl Alcohol Composite Films. J. Appl. Polym. Sci. 2014, 39726, 1−7. (32) Behabtu, N.; Greena, M. J.; Pasquali, M. Carbon nanotubebased neat fibers. Nano Today 2008, 3, 24−34. (33) Zhang, X.; Li, Q.; Tu, Y.; Li, Y.; Coulter, J. Y.; Zheng, L.; Zhao, Y.; Jia, Q.; Peterson, D. E.; Zhu, Y. Strong Carbon-Nanotube Fibers Spun from Long Carbon-Nanotube Arrays. Small 2007, 3, 244−248. (34) Koziol, K.; Vilatela, J.; Moisala, A.; Motta, M.; Cunniff, P.; Sennett, M.; Windle, A. High-Performance Carbon Nanotube Fiber. Science 2007, 318, 1892−1895. (35) Zhang, M.; Atkinson, K. R.; Baughman, R. H. Multifunctional Carbon Nanotube Yarns by Downsizing an Ancient Technology. Science 2004, 306, 1359−1361. (36) Razal, J. M.; Coleman, J. N.; Muñoz, E.; Lund, B.; Gogotsi, Y.; Ye, H.; Collins, S.; Dalton, A. B.; Baughman, R. H. Arbitrarily Shaped Fiber Assemblies from Spun Carbon Nanotube Gel Fibers. Adv. Funct. Mater. 2007, 7, 2918−2924. (37) Minus, M. L.; Chae, H. G.; Kumar, S. Interfacial Crystallization in Gel-Spun Poly(vinyl alcohol)/Single-Wall Carbon Nanotube Composite Fibers. Macromol. Chem. Phys. 2009, 210, 1799−1808. (38) Devaux, E.; Aubry, C.; Campagne, C.; Rochery. PLA/Carbon Nanotubes Multifilament Yarns for Relative Humidity Textile Sensor. M. J. Eng. Fiber. Fabr. 2011, 6, 13−24. (39) Dalton, A. B.; Collins, S.; Razal, J.; Munoz, E.; Ebron, V. H.; Kim, B. G.; Coleman, J. N.; Ferraris, J. P.; Baughman, R. H. Continuous Carbon Nanotube Composite Fibers: Properties, Potential Applications, and Problems. J. Mater. Chem. 2004, 14, 1−3. (40) Ericson, L. M.; Fan, H.; Peng, H.; Davis, V. A.; Zhou, W.; Sulpizio, J.; Wang, Y.; Booker, R.; Vavro, J.; Guthy, C.; Parra-Vasquez, A. N. G.; Kim, M. J.; Ramesh, S.; Saini, R. K.; Kittrell, C.; Lavin, G.; Schmidt, H.; Adams, W. W.; Billups, W. E.; Pasquali, M.; Hwang, W. F.; Hauge, R. H.; Fischer, J. E.; Smalley, R. E. Macroscopic, Neat, Single-Walled Carbon Nanotube Fibers. Science 2004, 305, 1447− 1450. (41) Behabtu, N.; Young, C. C.; Tsentalovich, D. E.; Kleinerman, O.; Wang, X.; Ma, A. W. K.; Bengio, E. A.; ter Waarbeek, R. F.; de Jong, J. J.; Hoogerwerf, R. E.; Fairchild, S. B.; Ferguson, J. B.; Maruyama, B.; Kono, J.; Talmon, Y.; Cohen, Y.; Otto, M. J.; Pasquali, M. Strong, Light, Multifunctional Fibers of Carbon Nanotubes with Ultrahigh Conductivity. Science 2013, 339, 182−185. (42) Zhou, G.; Wang, Y. Q.; Byun, J. H.; Yi, J. W.; Yoon, S. S.; Cha, H. J.; Lee, J. U.; Oh, Y.; Jung, B. M.; Moon, H. J.; Chou, T. W. High4796


Research Article

ACS Applied Materials & Interfaces Strength Single-Walled Carbon Nanotube/Permalloy Nanoparticle/ Poly(vinyl alcohol) Multifunctional Nanocomposite Fiber. ACS Nano 2015, 9, 11414−11421. (43) Li, Y. L.; Kinloch, I. A.; Windle, A. H. Direct Spinning of Carbon Nanotube Fibers from Chemical Vapor Deposition Synthesis. Science 2004, 304, 276−278. (44) Alemán, B.; Reguero, V.; Mas, B.; Vilatela, J. J. Strong Carbon Nanotube Fibers by Drawing Inspiration from Polymer Fiber Spinning. ACS Nano 2015, 9, 7392−7398. (45) Dong, Z.; Jiang, C.; Cheng, H.; Zhao, Y.; Shi, G.; Jiang, L.; Qu, L. Facile Fabrication of Light, Flexible and Multifunctional Graphene Fibers. Adv. Mater. 2012, 24, 1856−1861. (46) Huang, G.; Hou, C.; Shao, Y.; Wang, H.; Zhang, Q.; Zhu, M. Highly Strong and Elastic Graphene Fibres Prepared from Universal Graphene Oxide Precursors. Sci. Rep. 2014, 4, 4248. (47) Cheng, H.; Hu, Y.; Zhao, F.; Dong, Z.; Wang, Y.; Chen, N.; Zhang, Z.; Qu, L. Moisture-Activated Torsional Graphene-Fiber Motor. Adv. Mater. 2014, 26, 1−5. (48) Jalili, R.; Aboutalebi, S. H.; Esrafilzadeh, D.; Shepherd, R. L.; Chen, J.; Aminorroaya-Yamini, S.; Konstantinov, K.; Minett, A. I.; Razal, J. M.; Wallace, G. G. Scalable One-Step Wet-Spinning of Graphene Fibers and Yarns from Liquid Crystalline Dispersions of Graphene Oxide: Towards Multifunctional Textiles. Adv. Funct. Mater. 2013, 23, 5345−5354. (49) Meng, F.; Lu, W.; Li, Q.; Byun, J. H.; Oh, Y.; Chou, T. W. Graphene-Based Fibers: A Review. Adv. Mater. 2015, 27, 5113−5131. (50) Hu, L.; Hecht, D. S.; Grüner, G. Percolation in Transparent and Conducting Carbon Nanotube Networks. Nano Lett. 2004, 4, 2513− 2517. (51) Young, K.; Blighe, F. M.; Vilatela, J. J.; Windle, A. H.; Kinloch, I. A.; Deng, L.; Young, R. J.; Coleman, J. N. Strong Dependence of Mechanical Properties on Fiber Diameter for Polymer-Nanotube Composite Fibers: Differentiating Defect from Orientation Effects. ACS Nano 2010, 4, 6989−6997. (52) Vaisman, L.; Wagner, H. D.; Marom, G. The Role of Surfactants in Dispersion of Carbon Nanotubes. Adv. Colloid Interface Sci. 2006, 128, 37−46. (53) Duan, W. H.; Wang, Q.; Collins, F. Dispersion of Carbon Nanotubes with SDS Surfactants: A Study from A Binding Energy Perspective. Chem. Sci. 2011, 2, 1407−1413. (54) Ramirez, J. C.; Herrera-Ordoneza, J.; Gonzalez, V. A. Kinetics of Styrene Minisuspension Polymerization Using a Mixture PVA-SDS as Stablizer. Polymer 2006, 47, 3336−3343. (55) Wang, Q.; Dai, J.; Li, W.; Wei, Z.; Jiang, J. The Effects of CNT Alignment on Electrical Conductivity and Mechanical Properties of SWCNT/Epoxy Nanocomposites. Compos. Sci. Technol. 2008, 68, 1644−1648. (56) Zhang, S.; Koziol, K. K. K.; Kinloch, I. A.; Windle, A. H. Macroscopic Fibers of Well-Aligned Carbon Nanotubes by Wet Spinning. Small 2008, 4, 1217−1222. (57) Vollrath, F.; Knight, D. P. Liquid Cystalline Spinning of Spider Silk. Nature 2001, 410, 541−548. (58) Roppolo, I.; Chiappone, A.; Boqqione, L.; Castellino, M.; Bejtka, K.; Pirri, C. F.; Sangermano, M.; Chilerio, A. Self-standing Polymer-functionalized Reduced Graphene Oxide Papers Obtained via a UV-process. RSC Adv. 2015, 5, 95805−95812. (59) Hwang, S. H.; Kang, D.; Ruoff, R. S.; Shin, H. S.; Park, Y. B. Poly(Vinyl Alcohol) Reinforced and Toughened with Poly(dopamine)-Treated Graphene Oxide, and Its Use for Humidity Sensing. ACS Nano 2014, 8, 6739−6747. (60) Li, C.; Thostenson, E. T.; Chou, T. W. Dominant Role of Tunneling Resistance in The Electrical Conductivity of Carbon Nanotube-Based Composites. Appl. Phys. Lett. 2007, 91, 223114. (61) Li, J.; Kim, J. K. Percolation Threshold of Conducting Polymer Composites Containing 3D Randomly Distributed Graphite Nanoplatelets. Compos. Sci. Technol. 2007, 67, 2114−2120. (62) Ji, W. F.; Chang, K. C.; Lai, M. C.; Li, C. W.; Hsu, S. C.; Chuang, T. L.; Yeh, J. M.; Liu, W. R. Preparation and Comparison of the Physical Properties of PMMA/Thermally Reduced Graphene

Oxides Composites with Different Carboxylic Group Content of Thermally Reduced Graphene Oxides. Composites, Part A 2014, 61, 108−114. (63) Du, F.; Fischer, J. E.; Winey, K. I. Effect of Nanotube Alignment on Percolation Conductivity in Carbon Nanotube/Polymer Composites. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 121404. (64) Zeng, X.; Xu, X.; Shenai, P. M.; Kovalev, E.; Baudot, C.; Mathews, N.; Zhao, Y. Characteristics of The Electrical Percolation in Carbon Nanotubes/Polymer Nanocomposites. J. Phys. Chem. C 2011, 115, 21685−21690. (65) Kymakis, E.; Amaratunga, G. A. J. Electrical Properties of Singlewall Carbon Nanotube-polymer Composite Films. J. Appl. Phys. 2006, 99, 084302. (66) Zhou, G.; Byun, J. H.; Lee, S. B.; Yi, J. W.; Lee, W.; Lee, S. K.; Kim, B. S.; Park, J. K.; Lee, S. G.; He, L. Nano Structural Analysis on Stiffening Phenomena of PAN-Based Carbon Fibers During Tensile Deformation. Carbon 2014, 76, 232−239.

4797
