Core–Shell-Yarn-Based Triboelectric Nanogenerator Textiles as

Core−Shell-Yarn-Based Triboelectric Nanogenerator Textiles as Power Cloths Aifang Yu,†,§,⊥ Xiong Pu,†,§,⊥ Rongmei Wen,†,§ Mengmeng Liu,†,§ Tao Zhou,†,§ Ke Zhang,†,§ Yang Zhang,†,§ Junyi Zhai,*,†,§ Weiguo Hu,*,†,§ and Zhong Lin Wang*,†,‡,§ †

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Science, Beijing, 100083, China School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States § CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing, 100190, China

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‡

S Supporting Information *

ABSTRACT: Although textile-based triboelectric nanogenerators (TENGs) are highly promising because they scavenge energy from their working environment to sustainably power wearable/mobile electronics, the challenge of simultaneously possessing the qualities of cloth remains. In this work, we propose a strategy for TENG textiles as power cloths in which core−shell yarns with core conductive fibers as the electrode and artificial polymer fibers or natural fibrous materials tightly twined around core conductive fibers are applied as the building blocks. The resulting TENG textiles are comfortable, flexible, and fashionable, and their production processes are compatible with industrial, large-scale textile manufacturing. More importantly, the comfortable TENG textiles demonstrate excellent washability and tailorability and can be fully applied in further garment processing. TENG textiles worn under the arm or foot have also been demonstrated to scavenge various types of energy from human motion, such as patting, walking, and running. All of these merits of proposed TENG textiles for clothing uses suggest their great potentials for viable applications in wearable electronics or smart textiles in the near future. KEYWORDS: triboelectric nanogenerator, core−shell yarns, fibers, power cloths, textile

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commonly coated on conductive yarns or metal wires as triboelectrification layers.18,27,37 These modified yarns or wires were then woven into an energy-generating TENG textile. Another approach simply applied conductive and dielectric coatings on existing fabrics.14,20 Although these TENG textiles have demonstrated the capability of effectively scavenging various types of mechanical energy from human motions for powering small personal electronics, several key issues for clothing uses have been ignored, making them inapplicable for large-scale fabrication and real applications. For example, coatings of carbonaceous/conductive materials might detach from the yarn substrates during the washing process or after long-term usage; the manufacturing processes of metal and polymer coatings on common yarns could be expensive and complicated for manufacturability; naked metal wires may cause safety and comfort issues; and polymer films coated directly on 2D textile substrates will make the clothes unbreathable and unfashionable. Furthermore, TENG textiles should be compat-

earable energy-harvesting devices have been receiving extensive attention because of their potential to supply sustainable power for wearable electronic devices with energies harvested in situ from working environments or from human motions.1−8 These energy-harvesting devices alone, or integrated energy-harvesting−storage systems, could be an effective approach to solve the long-standing challenge of short operational time and frequent recharging of batteries/supercapacitors in limiting the progress of wearable electronics. Among different types of energy harvesters, the emerging triboelectric nanogenerator (TENG) is one of the most promising candidates for wearable devices due to its high power output, low cost, and versatility in structural design and material choices.9−30 Intrinsically soft and flexible textiles are ideal agents for the integration of various functional electronics/sensors and energy-harvesting modules.31−37 Therefore, textile-based TENGs have been reported successively with different structures and materials. Metal wires or common fabric yarns with conductive metals or carbonaceous material coatings have been typically utilized as the electrodes in TENGs.8,9,27,28 Further, dielectric polymer films (e.g., PDMS) have been © 2017 American Chemical Society

Received: October 24, 2017 Accepted: December 6, 2017 Published: December 6, 2017 12764

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ACS Nano ible with the design processes of fashionable garments, which therefore require metrics such as tailorability, ability to be sewn, and washability. The challenge of incorporating TENGs into garments while simultaneously possessing the qualities of common apparel exists, and the rational design of yarn building blocks and TENG textiles is still needed to address these issues. Weaving yarns is one of the oldest handicrafts. By this technique, long yarns for the textile industry can be created from commercial short fibers or yarns with supermechanical properties through well-designed weaving methods. In this work, we proposed a strategy for TENG textiles with the merits of cloths that are woven or knitted from core−shell yarns, which consist of conductive fibers as the core and commonly used cloth fibers as the shell by a commercially available weaving method. The sheath covering fibers function as the electrification layers, and the inner core fibers serve as the electrodes in TENGs. With the core−shell yarns, the abovementioned issues of naked metal wires or metal/carbonaceous materials/polymer coatings can be ideally overcome. In addition, materials possessing the qualities of cloths and energy harvesting can be simultaneously realized. The resulting TENG textiles exhibit the advantages of a garment fabric: lightweight, flexible, universally applicable in terms of fibrous material choices, and able to be scaled-up for fabrication by an industrial production line. Machine washing up to 120 times confirms the washing durability of the TENG textile. More importantly, unlike other TENG textiles reported to date, the TENG textiles using core−shell yarns in this work can be further processed by cutting and sewing for garment design. In addition to an output voltage/current higher than or comparable to previous studies, our TENG textile exhibited versatility in the structural design such that a single-electrode mode, contact-separation mode, and free-standing mode were all realized.

Figure 1. Characteristics of the core−shell yarns. (a) Schematic illustration of the structure of the core−shell yarns. SEM images of the middle section (b) and end section (c) of the core−shell yarns. Resistance of the yarns when the probes touch the SS (d) and touch the SP (e). (f) Industrialized SP/SS core−shell yarns with different colors. (g) Woven fabric (17 cm × 18 cm) with a plain pattern. (h) Knitted fashionable fabric.

conductive cores. For typical fabrication of a core−shell yarn, 200 SP fibers were tightly twined around two parallel SS fibers, as confirmed by the scanning electron microscopy (SEM) images of the body (see Figure 1b) and the tip (see Figure 1c) of a core−shell yarn. The diameter is ∼30 μm for a single SS fiber, ∼20 μm for an SP fiber, and ∼300 μm for the final core− shell yarns. Although the SS core fiber is highly conductive (Figure 1d), the covering dielectric fibers function as an effective insulating layer (Figure 1e). The fabrication processes of the core−shell yarns are applicable to various textile fibers. Fibers of cotton, silk, polyester, etc., can be used as covering fibers as well; the inner core may also be copper fibers, aluminum fibers, or carbonaceous material fibers (carbon fibers, carbon nanotube fibers, or reduced graphene oxide fibers). When designing a TENG textile, the covering fibers should be selected according to the triboelectrical series; two materials with a large difference in the triboelectrical series table generate more electrostatic charge at the contacting interfaces.38 Furthermore, the softness, flexibility, fashion, and strength of the core−shell yarns can be tuned by the material, diameter, number, and twist of the core and shell fibers. For demonstration, a core−shell yarn in white color was also fabricated with SS fibers as the core and cotton fibers as the shell (see Figure S1b). More importantly, the fabrication of both core−shell yarns and core−shell-yarns-based TENGs is compatible with current industrial textile manufacturing practices, which has seldom been achieved by any existing TENG textile. Figure 1f exhibits two as-fabricated colorful SP spindles by an industrial machine, confirming the convenient mass production of the basic units of TENG fabrics. A piece of large-area TENG textile (17 cm × 18 cm) was woven with a plain weave through shuttle-flying

RESULTS AND DISCUSSION Electrification materials and conductive electrodes are two indispensable components in a TENG. Artificial polymer fibers (e.g., polyester, nylon, polyacrylic) and natural fibrous materials (e.g., cotton, silk, leather) can all function as electrification layers in a TENG textile. The difficulty lies in the incorporation of the proper conductive electrodes in a TENG textile to generate and guide the flux of electrostatically induced charges. Unlike previously reported methods utilizing metal coatings or naked metal wires, we adopted an approach that is commonly applied in textile engineering, i.e., covered core−shell yarns, where the covering fibers are twined around core fibers to fabricate core−shell yarns, which could possibly combine the advantages of each fibrous material. Here, core−shell yarns composed of stainless-steel (SS) fibers as the core and dielectric fibers as the sheath, as schematically illustrated in Figure 1a, have been employed as the building blocks for TENG textiles. The core−shell yarns can subsequently be woven or knitted into cloths where the inner SS fibers are connected together as electrodes and the covering fibers serve as the dielectric materials for triboelectrification. Techniques that are commonly used in textile engineering, such as weaving, knitting, embroidering, and braiding, can all be applied to the fabrication of TENG textiles. Another benefit of using the 1D yarns as the starting units is that they can be readily incorporated into complicated or fashionable garment designs. Figure S1a shows an optical photograph of the as-fabricated core−shell yarns with two differently colored (black and blue) polyurethane (or Spandex, SP) fibers as the covering sheath and SS fibers as the 12765

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Figure 2. (a) Schematic illustration of three typical working modes and weave texture charts of TENGs and photos of as-fabricated corresponding TENG textiles. (b) Typical output performances of the TENG textiles at these three working modes. (c) Variation of peak current density and power density versus the external loading resistance of the SP-yarns-based TENG textile working at single-electrode mode. (d) Output performance of the as-fabricated SP-yarns-based TENG textile at single-electrode mode interacting with various commercial fabrics.

With core−shell yarns, typical structures of TENGs can all be realized in textiles, i.e., single-electrode mode, contactseparation mode, and free-standing mode, as schematically illustrated in Figure 2a, , and , respectively. For the single-electrode mode, a polyester textile with different materials as the covering fibers of the TENG textile exhibits motion relative to the TENG textile, which is connected to a ground through an external load; in the contact-separation mode, two TENG textiles with different shell yarns are connected through the external load; for the free-standing mode, two parallel parts of core−shell yarns woven in one single textile serve as the two electrodes in the TENGs, and a polyester textile with the same width of one electrode will have friction motion relative to the bottom TENG textile. For all three working modes, the energy of the relative mechanical motion will be converted into alternating currents. The

processing using the core−shell yarns as the warps, as shown in Figure 1g. Further fashion design using the TENG textile is also possible, as shown by the four sewn letters (“TENG”). Because the conductive SS is a fiber, the density of this TENG textile is low (approximately 330 g m−2). Given a human body with a surface area of approximately 1.44 m2 (0.9 m chest perimeter, 0.7 m upper body height, 0.45 m leg perimeter, and 0.9 m leg length), the weight of the TENG-textile-based garment is very light, only 475 g. Other textile fabrication techniques are also applicable. To demonstrate, a fashionable fabric was knitted with blue core−shell yarns and common green cotton yarns (see Figure 1h). The fabrication process of two kinds of TENGs demonstrates that both core−shell yarns and core− shell-yarns-based TENG textiles have good flexibility. Therefore, this rational design of the core−shell yarns shows promise for TENG textiles with excellent comfort and fashion. 12766

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Figure 3. Harvesting mechanical energy of different human motions by the TENG textiles. (a) Optical images of two pieces of TENG textiles woven from SP-fiber-based core−shell yarns sewn under the arm of a sweater and under the thenar of a sock. (b) OC voltage and SC current of the TENG textiles under different human motion, from left to right: patting under the arm, sliding under the arm, running under the foot, and walking under the foot. (c) Voltage curves of a capacitor (2 μF) charged by the TENG textiles under different human motions. (d) Digital watch driven by the TENG sock under running state.

μA of ISC. Figure 2c shows the variation in the peak current density and power density versus the external loading resistance of the SP-yarn-based TENG textile working in the singleelectrode mode. The current density decreases with increased external resistance, and the peak power density reaches ∼60 mW m−2 at a 200 MΩ external resistance, indicating that the asfabricated TENG textile is suitable as a constant current source. Furthermore, nearly all common textile materials can be applied in TENG textiles, since any two different dielectric materials can exhibit a triboelectrification. Therefore, for demonstration, a series of different fibrous materials (nylon (N), cotton (C), wool (W), polyester (T), and polyacrylonitrile (A)) have been tested with motion relative to an SP-yarn-based TENG textile at single-electrode mode, as shown in Figure 2d. For a single-electrode TENG textile, the short-circuit charge quantity (QSC) and VOC are related by QSC = CTENGVOC, where CTENG represents the inner capacitance of the TENGs and is constant, and QSC and VOC have the same trend.39,40 The variations in QSC and VOC coincide well with the tribo-series table, such that two materials with large differences in the triboseries produce a higher number of electrified charges and, therefore, a higher output. With these results, it is safe to anticipate that the TENG textile with the aid of core−shell yarns is universally applicable in terms of fabric materials, textile fabrication techniques, and textile design structures. These

corresponding working mechanisms of the TENG textiles in the three modes are shown in Figure S2. Generally, two different fibers in contact will generate static charges at surfaces of opposite polarities, which will then induce movable charges in the core SS fibers. These induced free charges driven by the electrostatic potential can flow through the external circuit to generate electricity once the two textiles are in relative motion. The weave texture charts in Figure 2a show the detailed structures of the TENG textiles. Photos of the corresponding as-fabricated TENG textiles are also shown at the bottom of Figure 2a. For all TENG textiles, a plain weave was adopted, but satin and twill weaves are also applicable. In this study, SP fiber-based core−shell yarns and cotton fiber-based yarns were utilized to fabricate two types of TENG textiles for demonstration. The typical output performance of the TENG textiles in the three working modes is shown in Figure 2b. When the SP core−shell-yarns-based TENG textile (4.5 cm × 8 cm) exhibits motion relative to a common polyester textile in singleelectrode mode, an open-circuit voltage (VOC) of ∼75 V and short-circuit current (ISC) of ∼1.2 μA can be generated (Figure 2b). When SP-yarn-based and cotton-yarn-based TENG textiles (both 4.5 cm × 8 cm) are in contact-separation mode, the VOC and ISC are ∼19 V and ∼0.3 μA, respectively. The SP-yarnbased TENG textile, with relative friction with polyester yarns in free-standing mode, also generates ∼56 V of VOC and ∼1.0 12767

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Figure 4. Durability and stability of the core−shell-yarns-based TENG textile. (a) OC voltage and (b) resistance of the TENG textile washed up to 120 cycles under machine washing. (c) SEM images of the core−shell yarns unwashed and washed up to 120 times (white scale bar: 100 μm, black bar: 10 μm). (d) OC voltage of the TENG textile was continuously tested for 5 h to examine its stability, with the first 15 peaks displayed.

number of electrified charges generated from the closer contact of the material interface resulting from the larger pressure applied by the foot. Similarly, running induces a higher impact force between the TENG sock and the insole; thus, higher outputs are obtained, i.e., ∼125 V and ∼4 mA m−2 for the peak VOC and JSC, respectively. For practical applications, the electricity generated by the TENG textile would need to be stored to provide stable power for electronics or sensors. Therefore, a commercial capacitor (2.2 μF) was charged by two TENG textiles under these four motion conditions, as shown in Figure 3c. The charging rate was consistent with the output performances in Figure 3b. The charge accumulation slope was measured to be 0.28, 1.05, 7.30, and 8.80 μC min−1 for arm patting, arm sliding, walking, and running, respectively. The charging time was only ∼19 s for the TENG sock to charge the capacitor to 1 V under walking. When integrated with the capacitor, TENG textiles could be used as a power source to drive electronic devices, as demonstrated in Figure 3d. The power generated by the TENG sock under running was first used to charge a commercial capacitor, and then the stored electricity was used to drive a watch. The capacitor reached 4 V in ∼40 s, which could then continuously power the watch for ∼5 s. Although the stored electricity cannot continually drive the

attributes will greatly enhance the viability of an energyscavenging TENG textile in various applications. The versatility of our TENG textile in scavenging the energy from various types of human movements was also demonstrated, as shown in Figure 3. Two pieces of TENG textiles woven from SP-fiber-based core−shell yarns were sewn under the arm of a sweater and under the thenar of a sock, respectively, as shown in the two photos in Figure 3a. Thanks to the flexible and lightweight characteristics of the TENG textiles, the comfort of the original fabrics was not affected, and the difference in appearance between the TENG textiles and the common garments was negligible. The TENG textiles can be worn at nearly every position on the human body if there is relative motion between the TENG textiles or between a TENG textile and another common cloth or human skin. For demonstration, we tested the practical performance of these two TENG textiles both working under single-electrode conditions. When patting or sliding the arm, the W fibers of the sleeve experienced contact-separation motion between the TENG textiles, and while walking or running, the TENG textiles on the sock have motion relative to the insole. The generated VOC and ISC are shown in Figure 3b. The output performance of the TENG textile under the thenar is significantly higher than that under the arm, due to the higher 12768

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Figure 5. Tailorability of the as-fabricated TENG textiles. (a) Photograph and (b) output performance of a TENG textile before and after being cut by a scissors. (c) Photograph and (d) output performance of two pieces of TENG textiles before and after being sewn together.

watch, there is great potential for TENG textiles to increase the output through increasing the working area, optimizing the material and structure, and improving the storage cell. Washability is one of the most crucial factors for wearable garment fabrics. Therefore, for wearable TENG textiles, the washability determines its viability in real-life applications. To evaluate the washability, we washed the as-made TENG textile (effective size 3 cm × 7 cm) with a commercial machine as described in the Experimental Methods. The wefts and warps of the as-made TENG textiles are all SP-based core−shell yarns. The VOC values of the unwashed and washed devices were measured at a fixed tapping velocity of 5 cm s−1, as shown in Figure 4a. The peak VOC of the TENG textiles remained at ∼32 V, even after 120 washes, thus confirming the excellent washing durability of the TENG textile. The excellent washing durability is consistent with the results in Figure 4b,f. The resistance of SP yarns with a length of 8.8 m maintained 3.7 kΩ even after 120 cycles in a washing machine (Figure 4b). SEM images of the unwashed SP yarns after 10, 60, and 120 wash cycles are shown in Figure 4c. Unlike previously reported TENG textiles with poor washing durability due to the separation of the conductive and polymer material coatings, the washed SP-based core−shell yarns maintained their structural integrity when compared to the original, and the twining SP fibers still tightly twined the inner SS fibers without any observable damage after 120 washes. Moreover, the size of the as-made TENG textile did not change after 120 washes (Figure S3). Even for a common

garment fabric, deformation after washing will disrupt its application, so the durability of our TENG textile is adequate for practical applications. The long-term cycling result is presented in Figure 4d with the electrical output over 5 h. To simulate a harsher environment, the tapping velocity is increased to 8 cm s−1. The OC voltage during the first and last 7 s remains almost constant, indicating the good reliability of the as-made TENG textile. The good durability and reliability of our TENG textiles are promising for their broad application in the wearable electronics field. In addition to their excellent washability and reliability, TENG textiles can also be tailored to realize complicated, fashionable garment designs. The tailorability endows the TENG with the ability and versatility to be designed into clothing of different styles and sizes. Figure 5 exhibits the tailorability of the as-fabricated TENG textiles. The TENG textile used in the tailoring experiment is a plain woven SPbased core−shell yarn with an effective size of 4.5 cm × 9.0 cm. The core−shell SP yarns (blue yarns) were used as wefts and cotton yarns (green yarns) as warps. The TENG textile can be cut by scissors, as demonstrated in Figure 5a. After being cut into two halves, each part maintains almost half of the original output (both the ISC and VOC), as displayed in Figure 5b. In single-electrode mode, the transferred QSC can be derived as QSC = σA/2, where σ is the static charge density generated at the contacting interface and A is the contacting area. ISC can be expressed as ISC = dQSC/dt.40,41 Therefore, the output of each 12769

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ACS Nano part becomes almost half of the original textile. It is worth noting that when the cut half of the blue TENG textile is sewn together with another black TENG textile with the same size (both woven from SP-based core−shell yarns), the electrical performances can almost be fully recovered, as shown in Figure 5c and d. Therefore, the TENG textile will not malfunction after being tailored, and further treatments, such as sewing, are applicable as well, enabling the TENG textile to have great potential for design and production of fashionable garments. In the next step, fashionable garments will be designed based on power cloths.

ASSOCIATED CONTENT

CONCLUSION

ORCID

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b07534. Additional information (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail (J.-Y. Zhai): [email protected]. *E-mail (W.-G. Hu): [email protected]. *E-mail (Z. L. Wang): [email protected].

Yang Zhang: 0000-0002-3002-4367 Junyi Zhai: 0000-0001-8900-4638 Weiguo Hu: 0000-0002-8614-0359 Zhong Lin Wang: 0000-0002-5530-0380

In conclusion, TENG textiles woven from core−shell yarns were successfully fabricated to scavenge various types of human motion energies as energy-harvesting cloths. The core−shell yarns, with core conductive fibers as electrodes and shell dielectric fibers as electrification layers, enabled the versatility of the core−shell-yarn-based TENG in material choices and structure designs. The TENG textiles, worn under the arm or foot, were also demonstrated to scavenge energy from various human motions, such as patting, walking, and running. The harvested energies can charge a commercial capacitor, which is then able to stably power a watch. More importantly, the core− shell-yarn-based TENGs are comfortable and fashionable, and their production processes are compatible with industrial largescale textile manufacturing. Washability and tailorability were further demonstrated, indicating that the TENG textiles are fully applicable for further processing of a garment. All these merits of the proposed TENG textiles as power cloths manifest their great potential for viable applications in wearable electronics or smart textiles in the near future.

Author Contributions ⊥

A. Yu and X. Pu contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Key R & D Project from the Minister of Science and Technology, China (2016YFA0202703, 2016YFA0202704), NSFC 51503108, NSFC 51472056, the “Thousands Talents” program for pioneer researcher and his innovation team, China, and the Recruitment Program of Global Youth Experts, China. Thanks to Chao Yuan, Jinzong Kou, Ying Lei, and Yudong Liu for technical support. REFERENCES (1) Zhong, J. W.; Zhang, Y.; Zhong, Q. Z.; Hu, Q. Y.; Hu, B.; Wang, Z. L.; Zhou, J. Fiber-Based Generator for Wearable Electronics and Mobile Medication. ACS Nano 2014, 8, 6273−6280. (2) Xu, S.; Zhang, Y.; Cho, J.; Lee, J.; Huang, X.; Jia, L.; Fan, J. A.; Su, Y.; Su, J.; Zhang, H.; Cheng, H.; Lu, B.; Yu, C.; Chuang, C.; Kim, T. I.; Song, T.; Shigeta, K.; Kang, S.; Dagdeviren, C.; Petrov, I.; Braun, P. V.; Huang, Y.; Paik, U.; Rogers, J. A. Stretchablbatteries with Self-similar Serpentine Interconnects and Integrated Wireless Recharging Systems. Nat. Commun. 2013, 4, 1543. (3) Zhang, Z. T.; Yang, Z. B.; Wu, Z. W.; Guan, G. Z.; Pan, S. W.; Zhang, Y.; Li, H. P.; Deng, J.; Sun, B. Q.; Peng, H. S. Weaving Efficient Polymer Solar Cell Wires into Flexible Power Textiles. Adv. Energy Mater. 2014, 4, 1301750. (4) Yang, P. H.; Mai, W. J. Flexible Solid-state Electrochemical Supercapacitors. Nano Energy 2014, 8, 274−290. (5) Jung, S.; Lee, J.; Hyeon, T.; Lee, M.; Kim, D. H. Fabric-Based Integrated Energy Devices for Wearable Activity Monitors. Adv. Mater. 2014, 26, 6329−6334. (6) Kim, D. H.; Lu, N. S.; Ghaffari, R.; Kim, Y. S.; Lee, S. P.; Xu, L. Z.; Wu, J. A.; Kim, R. H.; Song, J. Z.; Liu, Z. J.; Viventi, J.; de Graff, B.; Elolampi, B.; Mansour, M.; Slepian, M. J.; Hwang, S.; Moss, J. D.; Won, S. M.; Huang, Y. G.; Litt, B.; Rogers, J. A. Materials for Multifunctional Balloon Catheters with Capabilities in Cardiac Electrophysiological Mapping and Ablation Therapy. Nat. Mater. 2011, 10, 316−323. (7) Lee, S. H.; Jeong, C. K.; Hwang, G. T.; Lee, K. J. Self-powered Flexible Inorganic Electronic System. Nano Energy 2015, 14, 111−125. (8) Dagdeviren, C.; Joe, P.; Tuzman, O. L.; Park, K.; Lee, K. J.; Shi, Y.; Huang, Y. G.; Rogers, J. A. Recent Progress in Flexible and Stretchable Piezoelectric Devices for Mechanical Energy Harvesting, Sensing and Actuation. Extreme Mechanics Letters 2016, 9, 269−281.

EXPERIMENTAL METHODS Measurements. For the washing test, the TENG textiles were put into a laundry bag, and the whole bag was put into a commercial laundering machine (XQB55-M1268, Whirlpool, Haier) with 5.5 kg fabrics as ballasts to perform delicate machine laundering cycles. The laundering machine was first filled with 4 gallons of water at 20 °C, and the agitator started to rotate at 119 strokes per min for ∼6 min. Rinsing of fabrics was then performed with another 4 gallons of water at 20 °C. A spinning procedure was finally carried out in order to remove excess water. The spinning speed was 700 rpm, and the whole spinning process lasted for ∼5 min. The whole washing process lasted for ∼40 min. The textile TENG was then hung dry in the air. The output short-current and open-circuit voltage of the TENG textiles were measured by a Stanford low-noise current preamplifier (model SR570) and a Keithley electrometer (Keithley 6514), respectively. The TENG textiles were attached to a linear motor to mimic human motions for the measurement. Fabrication of Core−Shell Yarns. The core−shell yarns were fabricated through a commercial machine. The fabrication process of core−shell yarns is described briefly: The SS fibers were stretched vertically with a certain degree of tensile force. The shell fibers were rotated around the core SS fiber at high speed and then were twisted together; meanwhile, the core fiber moved upward, dragging the rotating shell fibers. The twisted shell fibers twined around the core SS fiber into yarns using the reverse force generated in the process of high-speed rotation. Materials Characterization. Scanning electron microscopy was taken with a Hitachi SU8200. 12770

DOI: 10.1021/acsnano.7b07534 ACS Nano 2017, 11, 12764−12771

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DOI: 10.1021/acsnano.7b07534 ACS Nano 2017, 11, 12764−12771