- Email: [email protected]
Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles
Article
Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles Chao Ye, Jing Ren, Yanlei Wang, ..., Markus J. Buehler, David L. Kaplan, Shengjie Ling [email protected] (M.J.B.) [email protected] (D.L.K.) [email protected] (S.L.)
HIGHLIGHTS Conductive silk fibers were produced through a dip-coating strategy Structure, property, and functional merits of silk and carbon nanotubes are combined Conductive silk fibers can withstand automated manufacturing and high-intensity washing Functional diversity is achieved in conductive silk fiber-based smart textiles
Continuously spinnable conductive silk fibers (CSFs) were constructed by a facile dip-coating strategy. The resultant CSFs integrate the mechanical and functional merits of both silk and carbon nanotubes and can be directly woven into smart textiles using automated equipment. These CSF-based e-textiles show promising applications in wearable devices, human augmentation, healthcare monitoring, and human-machine interfaces.
Ye et al., Matter 1, 1–15 December 4, 2019 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.matt.2019.07.016
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Article
Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles Chao Ye,1 Jing Ren,1,2 Yanlei Wang,3 Wenwen Zhang,1,4 Cheng Qian,3 Jun Han,5 Chenxin Zhang,5 Kai Jin,6 Markus J. Buehler,6,* David L. Kaplan,7,* and Shengjie Ling1,8,*
SUMMARY
Progress and Potential
E-textiles are of interest in a variety of fields, such as wearable electronics and real-time healthcare monitoring. Critical challenges remain, related to design and fabrication techniques to meet the varied requirements for e-textiles. As an ancient and incomparable textile fiber, natural silk fibers can be considered to address these issues. However, processing challenges have thus far precluded the realization of electrically conducting natural silks. Here, we report a scalable dip-coating strategy to construct conductive silk fibers (CSFs). Natural silk fibers were functionalized by a tailor-made carbon nanotube (CNT) paint, which selectively etches the surface of the silk fibers without destroying the internal structure of the fibers. The CSFs maintained the properties of both the silks and CNTs, with high mechanical performance, super-hydrophobicity, solvent resistance, and thermal sensitivity. The CSFs can be automatically woven into fabrics, resulting in textiles sensitive to surrounding physical stimuli, including force, strain, temperature, and solvents.
Silk fiber and carbon nanotubes are selected to construct conductive silk fibers (CSFs) for e-textile applications. Using a scalable dip-coating strategy, we integrate the complementary advantages of both components in one hybrid fiber system. In such a system, silk fibers provide mechanical strength and toughness and carbon nanotubes contribute functions such as water repellency, solvent resistance, and thermal and electrical conductivity. The mechanical and functional merits of CSFs allow them to be woven into e-textiles using automated fabric machines. The resultant e-textiles can withstand automatic or highintensity ultrasonic washing. These CSF e-textiles can be used as wearable sensing platforms to detect and monitor surrounding physical and chemical signals, such as force, temperature, and solvents, showing promising applications for wearable devices, human augmentation, healthcare monitoring, and human-machine interfaces.
INTRODUCTION Smart textiles (also referred to as e-textiles) are fabrics with integrated electronic components, such as batteries, sensors, circuits, lights, displays, and even small computers.1–3 Such systems provide intelligent and wearable materials for realtime healthcare monitoring,4 motion sensing,5 portable communication,6 and human augmentation applications.7,8 As smart, wearable systems, e-textiles must meet the basic requirements of clothing, such as comfort, light weight, heat retention, air permeability, and good durability, but are also required to provide additional value to the wearer.9 For example, e-textiles are expected to guard against extreme environmental hazards, such as a drenching rain, solvent erosion, or radiation damage, while monitoring and regulating body temperature and providing wind resistance.10 E-textiles often require fibers that can be woven into a textile context, combined with electrical conductivity. Widely used conducting or semi-conducting materials are based on metallic threads, which tend to be rigid, brittle, and costly, hampering their use for lightweight, large-deformation fabrics.10 Therefore, stretchable and electronic polymer-/carbon-based fibers are being developed using polymer fibers that contain conducting nanomaterials, such as carbon nanotubes (CNTs) and graphene.1–3 However, it remains challenging to combine processing, mechanical, electrical, and structural advantages in a single fiber system (Figure 1 and Table S1).11–15 For instance,
Matter 1, 1–15, December 4, 2019 ª 2019 Elsevier Inc.
1
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 1. Performance Comparison of Five Important Properties for Reported Conductive Fibers, Single CSFs, and CSF Yarn in This Work The area of each hexagon represents the general performance of CSFs in this work and the five most promising fibers selected from Table S1. Each property is classified as grade 1–5. Specific standards for the grading are declared in Table S2.
poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)-based conductive fibers feature high conductivity but their mechanical performance remains unable to meet the requirements of automatic textile manufacturing.12,13,16,17 In contrast, silks are ancient textile fibers with unique features for smart textile applications. Specifically, silks are mechanically tougher than metallic and carbonic threads and several times tougher than Kevlar fibers.18,19 Silks also provide advantages of light weight, low cost, sustainability, durability, biocompatibility, and large-scale production.20–23 However, despite these useful features, silks are hampered in e-textile applications due to their insulating properties versus the required conductive features of e-textiles. To obtain conductive silk fibers (CSFs), researchers have developed a variety of methods to introduce conductive components (e.g., metal particles, graphene, CNTs) into silks,24,25 including wet-spinning,26 dry-spinning,27,28 or feeding silkworms and spiders with carbon nanomaterials.29,30 However, the conductive components in these hybrid fibers are usually insufficient to reach conductive percolation thresholds. Carbonization of silk textiles31 is another effective approach to create conductive textiles; however, the mechanical strength and toughness of natural silks are destroyed after high-temperature processing. Accordingly, a significant challenge remains to fabricate CSFs with superior mechanical and conducting properties to match the requirements for e-textiles.
1School
of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
2Xinhua
Hospital Affiliated to Shanghai Jiao Tong University School of Medicine, Shanghai 200092, China
3Beijing
Key Laboratory of Ionic Liquids Clean Process, CAS Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
4College
of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, China
5Quanzhou
In this work, we designed a low-cost and scalable strategy to dissolve and modify the surface of natural silks with CNTs. A hexafluoroisopropanol (HFIP)-stabilized CNT paint was developed to adhere to degummed natural silk fibers. Strong bonding between the CNT paint and the silk fiber surface was achieved, since HFIP can be used to controllably etch the silk fiber surface. The process results in silk fibers uniformly coated with CNTs, which provides an electrically conducting path, and thereby a self-sensing mechanism, for response to distinct physical stimuli to the CSF structure.32 Finally, we incorporate yarn-spinning and automatic weaving techniques to fabricate e-textiles. These CSF-based e-textiles integrate the advantages of both the silk fibers and the CNTs, with sensitivity to the body and environmental stimuli
2
Matter 1, 1–15, December 4, 2019
Institute of Equipment Manufacturing, Haixi Institute, Chinese Academy of Science, Quanzhou 362216, China
6Department
of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
7Department
of Biomedical Engineering, Tufts University, Medford, MA 02155, USA
8Lead
Contact
*Correspondence: [email protected] (M.J.B.), [email protected] (D.L.K.), [email protected] (S.L.) https://doi.org/10.1016/j.matt.2019.07.016
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 2. Preparation and Characterization of CSFs (A) Schematic diagram of the fabrication method of CSF yarns. (B) CSF yarns were collected by a cylinder and were used as wires to light a bulb. (C and D) SEM images of CSFs at different magnifications. (E) SEM image of a typical CSF yarn. (F) Photograph of CSFs washed in water.
such as force, strain, temperature, and solvents. Thus, such systems show promise for wearable devices, human augmentation, healthcare monitoring, and humanmachine interfaces.
RESULTS AND DISCUSSION Preparation of CNT Paint As a low-cost, rapid, user-friendly, and scalable technique, dip-coating has been widely used to produce functional fibers.32,33 However, this technique was rarely employed to functionalize natural silks,32 because it is difficult to adhere functional components onto natural fibers without destroying their structure and properties. Selection of an appropriate solvent was vital to solving this problem whereby solvent should not only disperse and stabilize the CNTs but also etch the surface of the silk fibers in a controllable fashion for attachment of the CNTs. HFIP was chosen, based on our previous studies showing that HFIP partially dissolved the Bombyx mori (B. mori) silk fiber surfaces.34 In an optimized protocol, a CNT/isopropanol/polyvinyl alcohol (PVA) suspension with a CNT concentration of 15 wt % was dispersed into the HFIP, resulting in a 1 wt % CNT paint. The homogeneous CNT paint was stable at room temperature for more than 6 months, showing no aggregation or sedimentation (Figure S1). Fabrication of CSF Yarns A two-step route was designed to obtain weavable CSF yarns longer than kilometerscale (Figure 2A). At the first step, degummed B. mori silk fibers were immersed in the CNT paint and incubated at 60 C for 2 days. During this process, the surface of B. mori silk fibers partially dissolved to form a sticky surface and allowed a firm CNT coating. The fibers were then pulled from the CNT paint and dried in a chemical
Matter 1, 1–15, December 4, 2019
3
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
hood. After this process, the silk fibers took on an all-black appearance and they could be used as a wire to light a bulb, due to the even coating of CNTs on the fiber surfaces (Figures 2B–2F). A long fiber configuration and an intact silk fiber section were observed by scanning electron microscopy (SEM) after the dip-coating process (Figure S2), which indicated that the internal structure of the silk fibers was not destroyed. By using this method, an elegant balance of important mechanical properties can be achieved with the CSF yarns, which is promising for the expected applications in textiles. The HFIP/CNT paint/silk system in this work was distinct from a previously observed HFIP/silk fiber system with the same mass ratios,34 whereby the silk fibers dissolved into centimeter-scale microfibrils. Thus, the addition of CNTs attenuated the impact of HFIP on fibroin solubility. To understand why this difference occurred, we utilized steered molecular dynamics (SMD) simulations to understand the energy barrier (EB) for HFIP molecules entering the CNTs (Figure 3A). To perform SMD simulations, we attached a spring with force constant of k = 200 M m1 to the HFIP molecule with movement into the CNTs at a constant velocity of 10 m s1. The force profile for the HFIP entering the CNTs and the associated EB are summarized in Figure 3B, where the negative value of force and EB imply that HFIP molecule can enter into the CNTs spontaneously. SMD simulations were also performed with different movement velocities (Figure S3), indicating that 10 m s1 was slow enough to obtain a convergence energy barrier. To address the impact of the CNT wall on the EB, we performed the diffusion of HFIP in different initial positions, along with the center axis of CNT (center channel), and near the wall of the inner CNT (edge channel). For the center channel, EB increased with diameter and approached zero, showing weaker impact on the diffusive process of HFIP. For the edge channel, EB approached a constant value of about 7.5 kcal mol1 (Figure 3C). An additional SMD simulation also indicated that the energy barrier of HFIP entering the CNT was significantly lower than that of moving along the outside of the CNT (Figure S4). Furthermore, we compared the ability of solvent to enter the CNT’s inner space for several organic solvents, including ethanol, formic acid, acetone, toluene, and HFIP (Figures 3D and 3E), which are considered to have an influence on the structure and properties of silks. The results show the diffusive ability in the order ethanol < formic acid < acetone < toluene < HFIP, determined by the interactions between the solvent molecules and the CNT, indicating that HFIP can enter into the multiwalled CNT most easily. Based on these analyses of free energy, HFIP diffused into the inner space spontaneously and preferred to transport through the inner wall of the CNTs, which agrees with the experimental results. At the second step, the CSFs were spun into CSF yarns with twisted helical structures using a yarn-spinning approach (Figure 2E). These CSFs and CSF yarns tolerated extensive washing in water. After continuous stirring in water for an hour, the CSFs were still black and the water was still clear (Figure 2F). It should be noted that CSFs are hydrophobic; the extreme washing conditions destroyed the CSF/water interface air layer and the water permeated into the inner layer of CSFs, while the CNTs still firmly integrated with the silks (Figure S5A). More importantly, there was no obvious change in the electrical performance of the washed yarns (163 G 52 S m1), which was 160 G 60 S m1 before washing (Figure S5B). SEM images reveal the tight association between the silk fiber core and the CNT coating layer after washing (Figure S5C), with no visible differences observed compared with these CSFs before washing. Automatic washing with soap (30-min water washing and 30-min dehydration, at room temperature; Figures S5D and S5E; Video S1) and high-intensity ultrasonic washing (20 kHz, 300 W, 1 h; Figure S5F)
4
Matter 1, 1–15, December 4, 2019
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 3. SMD Simulations to Capture the Energy Barrier for HFIP Molecules Entering the Inner Space of CNT (A) Atomic structures of CNT that defined a channel for HFIP diffusion in steered molecular dynamics (SMD) simulations. Here we use double-walled CNTs to represent the multiwalled CNT in the simulations. (B) Force applied to HFIP molecules when entering CNTs with different diameters. ‘‘Center’’ represents movement along the center axis of CNT, while ‘‘edge’’ represents movement near the inner surface of CNT. Inert is the schematic to clarify the center (red arrow) and edge (blue arrow), and two circles represent the two walls of CNT. (C) Energy barrier (E B ) for HFIP molecules entering into the CNT against the diameter, where the blue line and red line represent the limit E B for diffusing along the center channel and edge channel, respectively. (D) Force applied to different solvent molecules when entering into CNTs with 1.9 nm diameter. (E) Comparison of E B for different organic solvents entering the inner space of CNT, where the diameter of CNT is 1.9 nm for all cases.
further illustrated the stability of the CSF textiles under standard and extreme washing conditions. Mechanical Properties and Weaveability of CSFs The single CSF featured ductile mechanical behavior (Figures S6A and S6B). The tensile strength and strain were 633 G 168 MPa and 12% G 4%, respectively, which are equivalent to native silk fibers. This result also corresponds to the aforementioned structural observation, revealing that the modification process did not affect the internal structure of the silk fibers. The CSF yarns maintained the ductile mechanical
Matter 1, 1–15, December 4, 2019
5
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
behavior of CSFs; the breaking loading force of the yarn was almost equal to the sum of the breaking loading force that the single fiber within the CSF yarn could withstand (Figures S6C and S6D; Table S3). It is noteworthy that the fracture behavior of regular yarns consisting of short fibers can be significantly influenced by the degree of the twisted structure. This is because the slip between the short fibers would be restricted as the degree of twisting increases. However, the effect of the twisting process on CSF yarns consisting of long CSFs was different, according to both the simulation and experimental results (Figures S7 and S8; Tables S3 and S4). In this work, the CSFs are long and almost all of them extend continuously from one end to the other end of the yarn. Upon tensile loading, the major deformation of the yarn mainly contributes to the tension of each fiber. Other contributions, such as bending and inter-fiber slidings, are negligible. As a result, the tensile property of the yarns nearly reproduces that of each single silk fiber. The increased twisting process reduces the gap between the long CSFs, thereby decreasing the cross-sectional area of the CSF yarns, which leads to increased fracture stress and improved mechanical stability, but will not influence the general breaking loading force of the CSF yarns. The detailed simulation method can be found in Supplemental Information. Both values of mechanical properties of CSFs and CSF yarns were comparable with those of regenerated silk fibers (109 G 34 MPa and 14.0% G 4.9%, respectively)34 and natural silk fibers (600 MPa and 17%, respectively).18 Most natural and synthetic materials can be confined to a single trend line by using extensibility and Young’s modulus as variables in horizontal and vertical coordinates, respectively35 (Figure 4A). This tendency indicates that larger values of extensibility typically lead to lower values of Young’s modulus. For example, graphene and CNTs dominate high elastic modulus, while they are very brittle with limited fracture strain;36–38 silicone,39 by contrast, is elastically stretchable with limited modulus. However, CSF yarns synergistically integrated the advantages of high modulus from the CNTs and good extensibility from the silk fibers, leading to similar mechanical behavior with natural collagen fibers.40,41 The mechanical merits of these CSF yarns supported their functionalization and utility as versatile e-textiles through automatic weaving. Herein, the CSF yarns can be processed by using knitting or weaving machines because they fulfill the following requirements: (1) the fibers have sufficient continuous length to meet the processing requirements; (2) the conductive coating layer and the core fibers need to be stable during the machinery knitting or weaving operation; and (3) electrical conductivity needs to be preserved after the machine processing. The CSF yarns were woven into a fabric by a loom (Figures 4B and 4C) and into a subtle logo pattern using an automatic embroidery machine (Figure 4D and Video S2). SEM images of the CSF pattern (Figure 4E) revealed that the CSF yarns (yellow) were intact and bound well with the substrate fabric (blue). High-resolution SEM images (Figure 4F) further displayed the full coverage of CNTs on the CSF surface, indicating that the sewing machinery did no damage the microstructure of the CSF yarns. Water Repellency and Solvent Resistance of CSF E-Textiles Although bare silk fibers show hydrophilicity, hydrophobicity can be introduced by both weaving silk yarns into a silk textile with heterogeneous micro-nano surface structures or coating silk fibers with hydrophobic and chemically inert CNTs (Figure S9). The prepared CSF e-textiles revealed excellent hydrophobicity due to the additive effect of hydrophobicity and the chemical inertness of the CNT coatings, as well as heterogeneous micro-nano structure on the surface (Figure S10). Thus,
6
Matter 1, 1–15, December 4, 2019
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 4. Mechanical Properties and Knittability of CSF Yarns (A) Comparison of specific Young’s modulus and tensile strain with other materials. The Ashby plot was redrawn from Vatankhah-Varnosfaderani et al., 35 Where r is the mass density, E is Young’s modulus, and l max is strain at fracture. (B) A piece of e-textile cloth woven by CSF yarns. (C) SEM image of the surface of the e-textile in (B). (D) Photograph of an embroidery machine knitting the CSF yarns into a logo pattern. (E) SEM image of the CSF yarns on non-woven fabric substrate, magnified from (D) (yellow region, CSFs; blue region, substrate; false color was used to distinguish the CSFs and substrate). (F) SEM image of the CSFs in (E) at high magnification.
these e-textiles appeared excellent as water-repellent and solvent-resistance materials. When water was poured onto the e-textiles, it flowed across the surface without wetting the pattern (Figure 5A and Video S3). A water droplet could stand on a CSF yarn and remained in spherical morphology until complete evaporation (inset image in Figure S10A). Next, we estimated the solvent resistance of the e-textile patterns. Acetone, alcohols, toluene, formic acid, and HFIP were gradually dripped onto a CSF yarn-based e-textile. The substrate textile was corroded thoroughly by these solvents, but the logo remained intact (highlighted by the red box in Figure 5B). Fluorescence microscopy images confirmed that the solvents penetrated the gap between single CSFs (Figure S11). This penetration process led to the increased resistance of the e-textiles. The resistance increase of the e-textiles when ethanol or acetone was dripped on the materials is shown in Figure 5C. Different response speeds were also observed between ethanol and acetone: the yarn resistance change peaks of acetone were sharper than that of ethanol under the same experimental conditions. Different solvents usually have distinct evaporation rates; thus, these e-textiles have the potential to identify solvent exposure by assessing rates of changes in resistance, with caveats based on the concentration of exposure to be calibrated. Five different solvents were tested, and all of the solvents led to distinct changes in resistance response (Figure S12).
Matter 1, 1–15, December 4, 2019
7
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 5. Water Repellency and Solvent Responses of the CSF Yarn (A) Hydrophobicity of a shoe made of CSF textile. (B) Photograph of the logo pattern on a non-woven fabric substrate after solvents corrosion. The red dashed line indicates the corroded area, where the green background can be seen. (C) Resistance change of the CSF yarn in response to ethanol and acetone drops. (D) Resistance change of the CSF yarn in response to aqueous ethanol solution at different concentrations.
We also employed e-textiles as sensors for ethanol concentrations in aqueous ethanol solutions. Because of the hydrophobicity and ethanol affinity of CNTs, the resistance changed, directly related to the water/ethanol ratio in the solution dropped on the surface (Figure 5D). The e-textiles showed the most significant responses to pure ethanol, with a sharp resistance change within 40 s. In contrast, a 400-s plateau region was observed for 20% ethanol solution. Other water/ethanol ratios responded with progressive changes in resistance curves between that of pure ethanol and the 20% ethanol solution. To reveal the mechanism of detection, we conducted molecular dynamics simulations of the water/ethanol system with the e-textile. The resistance change of the e-textiles should be attributed to the diffusive process of ethanol. Hence, we calculated the self-diffusive coefficient of ethanol as a function of the concentration of ethanol. As presented in Figure S13, the diffusive ability is constrained as the concentration of ethanol increases. When the concentration of ethanol increases from 0% to 100%, the diffusive coefficient decreases by about 62%. The reduction of diffusion coefficient originates from the aggregation of the ethanol molecules in the water. The evolution of diffusive ability matches well with the experimental results. Temperature Sensitivity of E-Textiles CSF e-textiles were also sensitive to temperature changes due to thermal expansion and contraction of the CNT coating layer. The temperature of the e-textile pattern increased rapidly from 25 C to 63.6 C with a 30-s far-infrared illumination (Figure 6A and Video S4). In addition, heat-retention ability was improved by the increased thickness of the CSF pattern. For example, after the far-infrared illumination was
8
Matter 1, 1–15, December 4, 2019
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 6. Thermal Responses of the CSF Yarns (A) Thermal image of the logo pattern weaved by CSF yarns under far-infrared illumination. (B) Thermal image of an e-textile with a thicker ‘‘STU’’ pattern after 20 s after far-infrared illumination was removed. The inset image is the textile knitted by CSF yarns with a two-layer STU pattern. (C and D) Photograph (C) and thermal image (D) under far-infrared illumination of a finger-shaped pattern sewed by CSF yarns. (E) The relationship between temperature and resistance under cyclic heating/cooling processes. The red dots and blue dots represent the heating and cooling process, respectively.
removed, the thicker e-textile pattern (a two-layer region comprising the letters ‘‘STU’’) presented a slower cooling speed than the one-layer region (Figure 6B). Furthermore, we designed a precise hand pattern with a line width of 2 mm to estimate the spatial resolution of the site-specific heating effect on e-textiles (Figure 6C). A clear high-temperature pattern (108 C) was detected under the low-temperature substrate layer (29 C, Figure 6D), indicating that site-specific heating can be conducted with a resolution below 2 mm. In addition, during cyclic heating-cooling measurements (Figure 6E), the resistance of the e-textiles varied with the temperature changes due to thermal expansion and contraction. Repeated cycles between temperature and resistance were detected in the temperature range of 30 C–65 C. Nevertheless, the CSF yarns exhibited thermal hysteresis and inaccuracy (G4 C) in the high-temperature region (50 C–65 C) during repeated heating and cooling, due to the non-isotropic dispersion of CNTs within the CSF yarns. The hysteresis and inaccuracy phenomena also exist in other high-performance electroconductive temperature sensors.42–45
Matter 1, 1–15, December 4, 2019
9
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 7. Cyclic Loading-Unloading Test of the CSF Yarns (A) Photograph of the experimental platform for evaluating the resistance changes of CSF yarns under loading-unloading test. (B) Electrical resistance changes of the CSF yarns during cycle loading-unloading tests.
Force and Strain Sensitivity of E-Textiles Benefiting from the loose and twisted structure, the CSF yarns were sensitive to external forces. When the CSF yarns were tightened by stretching, the contact area between single CSFs increased, thus reducing electrical resistance. Conversely, when the tensile force was removed, the CSF yarns loosened to their initial state and the resistance of CSF yarns returned to the starting value. Accordingly, we coupled a tensile machine with a wireless multimeter to record the electrical resistance changes in situ during cyclic loading-unloading (Figure 7A). The CSFs exhibited mechanical behavior similar to that of silks, which showed good durability (Figure S14). Neither plastic deformation nor degradation in strength occurred at a set strain of 5%, a value close to the fracture strain of many woven fabrics.46,47 In this process, the resistance varied synchronously with yarn deformation and remained stable after 40 cycles (Figure 7B). E-Textiles for Monitoring Human Activity We knitted the CSF yarns into kneecap- and glove-fitting materials to monitor largeand small-scale human body motions based on their resistance-strain response (Figures 8A and 8B). It is noted that bending and pressing have little impact on the motion strategy (Figure S15). Marching paces were then continuously recorded by measuring and quantifying the output electrical signals through the e-kneecaps. The resistance of the CSF yarns decreased rapidly when stretching occurred and varied synchronously with the leg bending and frequency of release. The response rate of e-kneecaps reached up to 100 ms, a distinct signal change of more than 10% with excellent repeatability (Video S5). Finally, e-gloves were prepared by sewing CSF yarns into glove fingers to sense hand motions (Figure 8C). Long-term durability of the e-gloves was tested by continuously winding-unwinding two fingers of the glove more than 200 times (Figures 8D– 8F). The e-glove exhibited stable electrical responses during 200 bending-releasing cycles of the index finger (black line) and middle finger (blue line), essential for practical applications. Furthermore, the e-gloves were sensitive to finger deformation and distinguished distinct finger gestures (Figures 8G and 8H). For example, this e-glove successfully monitored the complete process of hand grabbing. The corresponding changes in resistance for each finger could be recorded in real-time by computer (Figure 8G and Video S6). We further designed a more complex hand motion, a continuous change of hand gestures of ‘‘5-6-8-2-0-6-8-0-5,’’ to test the sensitivity of the e-glove system (Figure 8H). The resistance of each finger sensor changed synchronously with finger movement but remained stable from the starting to the
10
Matter 1, 1–15, December 4, 2019
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 8. CSF E-Textiles for Monitoring Human Activity (A) CSF yarns were sewn into an e-kneepad. The photographs show the deformation of a CSF yarn at different knee-bending angles during walking. (B) Resistance change of an e-kneepad during the wearer marching at different speeds. (C) Photograph of an e-glove made by sewing five CSF yarns into a glove, respectively. (D–F) Resistance change of the index finger (D, black line in F) and middle finger (E, blue line in F) under repeated winding and releasing process for more than 200 cycles. (G) Real-time monitoring of resistance changes of the five fingers when grabbing an object. (H) Resistance change of every finger under different hand gestures.
ending gesture of ‘‘5.’’ Meanwhile, no signal interference from adjacent fingers was detected in the process. All of these characteristics of e-gloves endow promising applications in digitalizing hand gestures and provide useful translation for hearingimpaired individuals and for robotics. Conclusion Silk fibers, as one of most abundant natural protein fibers, feature light weight, flexibility, toughness, and biocompatibility.22,23 By contrast, CNTs, as high-performance synthetic inorganic nanomaterials, are known for their extraordinary thermal and electrical conductivity, as well as ultrahigh tensile strength and elastic modulus.48,49 However, it remains a significant challenge to integrate the complementary advantages of both components into one material system. In this study, we demonstrated that direct functionalization of natural silks is a practical option to overcome this challenge. With this strategy, the structural and mechanical advantages of natural silk fibers were transformed into CSFs and could be woven into e-textiles using an
Matter 1, 1–15, December 4, 2019
11
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
automated fabric machine. Furthermore, CNTs, the functional component of the system, endowed additional merit to the e-textile system, such as water repellency, solvent resistance, and heat retention. All of these characteristics are critical for practical applications of e-textiles. More remarkably, the CSF e-textiles could be directly assembled into a wearable environmental sensing platform based on variations in electrical parameters, to detect and monitor surroundings and communicate the acquired physical and chemical signals, such as force, strain, temperature, and solvents. These e-textiles with integrated advantages of both silks and CNTs show promising applications in wearable devices, human augmentation, healthcare monitoring, and human-machine interfaces. Future research can focus on introducing other components to improve the specific response of hybrid CSFs to different stimuli, while the multifunctional integration design, data collection, correction, and analyzing can be processed by computer technology. For practical applications, CSF yarns can be woven into the outer or inner layer of clothing to avoid safety concerns from the direct contact of CNTs with human skin. At minimum, the main silk components are environmentally compatible and could be enzymatically degraded for the recovery and reuse of other components in the electronic fibers.
EXPERIMENTAL PROCEDURES Preparation of CNT Paint To prepare the CNT paint, 3 mL of 15 wt % multiwalled CNT solution (isopropanol as solvent and PVA as stabilizing agent, Chengdu Organic Chemicals, China) was mixed with 17 mL of HFIP in a sealed glass bottle with sufficient stirring. Preparation of CSF Yarns B. mori silkworm cocoon silk fibers were degummed by boiling in two 30-min changes of 0.5% (w/w) NaHCO3 solution. The degummed silk fibers were washed with distilled water and allowed to air dry at room temperature. Next, 1 g of degummed silk fibers was immersed in 20 mL CNT/HFIP paint and incubated in airtight containers at 60 C for 2 days. The CSFs were obtained after being pulled out within a few seconds and dried at 70 C for 4 h. The strong adhesion between CNTs and silk is due to the selective dissolution of silk surface by HFIP, which is determined by the incubation time, so there is little need to control the pull-out speed, unlike the conventional dip-coating method. All of these steps were conducted in a chemical hood with the necessary precautions, as HFIP is a toxic solvent. Finally, dozens of CSFs were arranged in parallel, followed by a yarn-spinning approach to obtain CSF yarns. The surface of the CSFs and the structure of the e-textiles made of CSF yarns were characterized by SEM (JEOL JSM-7800F) at an acceleration voltage of 5 kV. SMD Simulations All the simulations in this work were completed using a large-scale atomic/molecular massively parallel simulator.50 The time step for integrating Newtonian equations of motion is 0.5 fs, which is validated to ensure the energy conservation. The all-atom optimized potential for liquid simulations (OPLS-AA) potential and adaptive intermolecular reactive empirical bond order potential are employed for organic molecules (HFIP, ethanol, toluene, and formic acid)51 and CNT,52 respectively. The interaction between organic molecules and CNT includes van der Waals force, which is described via Lennard-Jones potential function and the Lorentz-Berthelot mixing rules and used to model the parameters, which are truncated at 1.2 nm. In the entire simulations, the organics molecule and CNT were kept at a constant temperature of 0.01 K to discover the intrinsic properties of the HFP/CNT system. After the system was run to equilibrium, SMD simulations were used to explore the free energy profiles for organic molecules entering the CNT with different diameters.
12
Matter 1, 1–15, December 4, 2019
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
For calculation of molecular diffusion coefficient, the size of the simulation box was 5.0 3 5.0 3 5.0 nm3. A periodic boundary condition was applied in the x, y, and z directions. The OPLS-AA potential51 was employed for ethanol and the TIP3P model was used for water. The SHAKE algorithm53 was applied for the stretching terms between oxygen and hydrogen atoms to avoid numerical integration of hydrogenrelated high-frequency vibrations that required a much shorter time step. The interactions between ethanol and water include van der Waals and electrostatic terms, and both interactions were truncated at 1.2 nm. The Lorentz-Berthelot mixing rules were used to model the parameters of ethanol and water. The long-range Coulombic interaction was computed using the particle-particle-particle-mesh algorithm.54 The time step for integrating Newtonian equations of motion was 1.0 fs, which has been validated to ensure the energy conservation. In the SMD simulations, water-ethanol solution was relaxed in the NPT ensemble for 5 ns with a temperature of T = 300 K and pressure of 1 bar along x, y, and z directions. The temperature and pressure were controlled by a Berendsen thermostat. After the system was equilibrated, the mean square displacement was calculated in the NVE ensemble for 5 ns, and the molecular diffusion coefficient D calculated by D E. D = lim jrðtÞ rð0Þj2 6t: t>N
Here, t is the diffusing time and hjrðtÞ rð0Þj2 i the ensemble average. Mechanical Testing of CSF Yarns The CSFs and CSF yarns were cut into segments for tensile tests. The segments with a length of 40 mm were fixed on a hard-cardboard frame. The hard-cardboard frame was mounted onto the testing machine (Instron 5966; Instron, Norwood, USA) and the side support of the frame was cut off, so all the force was transmitted through the fiber/yarn. Meanwhile, the initial length of the fiber was measured with a caliper at zero load point, when the samples are tight without any force exerted on it. The cross-sectional area was measured by cutting the fiber/yarn in liquid nitrogen directly using razor blades, followed by coating with a 5-nm-thick Au layer, and observed by SEM (JSM-7800MF). Pictures were estimated by ImageJ software (NIH). The diameter of CSF yarns was measured by optical microscope before the tensile test. All of the tensile measurements were carried out at 25 C and 50% relative humidity with a tensile speed of 2 mm/min. The resistance variation with cyclic loading-unloading was recorded by further connecting the yarns to a digital multimeter (CEM DT-9989). The cyclic loading-unloading tests were carried out in a cyclic tensile mode with a speed of 2 mm/min, and the maximum deformation in each cycle was 5%. Thermal and Solvent Response of CSF Yarns The CSF yarns were woven into a non-woven fabric substrate. A far-infrared lamp was then used for heating while the temperature of the CSF region was recorded by a thermal imager (DIS-45; Fluke, USA) and the electrical resistance of CSF region was recorded by a digital multimeter (CEM DT-9989) in situ at a time resolution of 1 s. The solvent response function was evaluated by dropping several solvents onto a 50-mm CSF yarn and the electrical resistance recorded by digital multimeter in situ at a time resolution of 1 s. E-Textiles for Monitoring Body Motion To monitor walking, we weaved the CSF yarns into a kneepad. The resistance of the CSF yarn on the e-kneepad was recorded by a digital multimeter (Keithley DMM6500) at 2-wire mode with a resolution of 100 ms. For monitoring of hand gestures, The CSF yarns were woven on the finger region of the glove connected to a
Matter 1, 1–15, December 4, 2019
13
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
2001-Scan card at 2-wire mode. The resistance changes of the five CSF yarns could then be recorded by digital multimeter at the same time with a resolution of 200 ms.
SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.matt. 2019.07.016.
ACKNOWLEDGMENTS We acknowledge the National Natural Science Foundation of China (no. U1832109, 51973116 and 21935002), Shanghai Pujiang Program (18PJ1408600), the National Natural Science Foundation of China (21808220), and the Beijing Natural Science Foundation (2184124), and the starting grant of ShanghaiTech University and Shanghai Sailing Program (17YF1411500) for support of this work. We also thank the NIH (U01EB014976) and the AFOSR (FA9550-11-1-0199 and FA9550-14-10015) for support of various aspects of the work. We thank the Analytical Instrumentation Center (AIC) and the Center for High-resolution Electron Microscopy (C-EM) at School of Physical Science and Technology, ShanghaiTech University, for supporting mechancial and SEM characterization.
AUTHOR CONTRIBUTIONS C.Y. and J.R. contributed equally to this work. S.L. and J.R. designed the study, analyzed the results, and wrote the manuscript. C.Y., W.Z., J.H., and C.Z. performed the experiments and analyzed the data. Y.W., C.Q., and K.J. performed simulations and theoretical calculation. M.J.B. and D.L.K. analyzed the data and wrote the manuscript.
DECLARATION OF INTERESTS The authors declare no competing interests. Received: February 23, 2019 Revised: May 12, 2019 Accepted: July 22, 2019 Published: October 9, 2019
REFERENCES 1. Zeng, W., Shu, L., Li, Q., Chen, S., Wang, F., and Tao, X.M. (2014). Fiber-based wearable electronics: a review of materials, fabrication, devices, and applications. Adv. Mater. 26, 5310–5336. 2. Sang, H.J., Jimi, E., Yong-Hoon, K., and Kyu, P.S. (2018). Recent progress of textile-based wearable electronics: a comprehensive review of materials, devices, and applications. Small 14, 1703034. 3. Agcayazi, T., Chatterjee, K., Bozkurt, A., and Ghosh, T.K. (2018). Flexible interconnects for electronic textiles. Adv. Mater. Technol. 3, 1700277. 4. Coyle, S., and Diamond, D. (2016). 10. Medical applications of smart textiles. In Advances in Smart Medical Textiles, L. van Langenhove, ed. (Woodhead Publishing), pp. 215–237. 5. Liu, Z.F., Fang, S., Moura, F.A., Ding, J.N., Jiang, N., Di, J., Zhang, M., Lepro´, X., Galva˜o,
14
Matter 1, 1–15, December 4, 2019
D.S., Haines, C.S., et al. (2015). Hierarchically buckled sheath-core fibers for superelastic electronics, sensors, and muscles. Science 349, 400–404. 6. Rein, M., Favrod, V.D., Hou, C., Khudiyev, T., Stolyarov, A., Cox, J., Chung, C.C., Chhav, C., Ellis, M., Joannopoulos, J., et al. (2018). Diode fibres for fabric-based optical communications. Nature 560, 214–218. 7. Liu, M., Pu, X., Jiang, C., Liu, T., Huang, X., Chen, L., Du, C., Sun, J., Hu, W., and Wang, Z.L. (2017). Large-area all-textile pressure sensors for monitoring human motion and physiological signals. Adv. Mater. 29, 1703700. 8. Yandell, M.B., Quinlivan, B.T., Popov, D., Walsh, C., and Zelik, K.E. (2017). Physical interface dynamics alter how robotic exosuits augment human movement: implications for optimizing wearable assistive devices. J. Neuroeng. Rehabil. 14, 40–50.
9. Xu, J., Wang, S., Wang, G.J.N., Zhu, C., Luo, S., Jin, L., Gu, X., Chen, S., Feig, V.R., To, J.W.F., et al. (2017). Highly stretchable polymer semiconductor films through the nanoconfinement effect. Science 355, 59–64. 10. Schneegass, S., and Amft, O. (2017). Smart Textiles: Fundamentals, Design, and Interaction (Springer Publishing Company). 11. Granero, A.J., Razal, J.M., Wallace, G.G., Panhuis, M.I.H. (2010). Elastic conducting carbon nanotube-laden SIBS fibers. In: International Conference on Nanoscience and Nanotechnology, pp 80–83. https://doi.org/10. 1109/ICONN.2010.6045183. 12. Jalili, R., Razal, J.M., Innis, P.C., and Wallace, G.G. (2011). One-step wet-spinning process of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) fibers and the origin of higher electrical conductivity. Adv. Funct. Mater. 21, 3363–3370.
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
13. Jalili, R., Razal, J.M., and Wallace, G.G. (2012). Exploiting high quality PEDOT:PSS-SWNT composite formulations for wet-spinning multifunctional fibers. J. Mater. Chem. 22, 25174–25182. 14. Wang, C., Xia, K., Zhang, M., Jian, M., and Zhang, Y. (2017). An all-silk-derived dual-mode e-skin for simultaneous temperature-pressure detection. ACS Appl. Mater. Interfaces 9, 39484–39492. 15. Tang, Z., Jia, S., Wang, F., Bian, C., Chen, Y., Wang, Y., and Li, B. (2018). Highly stretchable core-sheath fibers via wet-spinning for wearable strain sensors. ACS Appl. Mater. Interfaces 10, 6624–6635. 16. Zhou, J., Li, E.Q., Li, R., Xu, X., Ventura, I.A., Moussawi, A., Anjum, D.H., Hedhili, M.N., Smilgies, D.M., Lubineau, G., et al. (2015). Semi-metallic, strong and stretchable wet-spun conjugated polymer microfibers. J. Mater. Chem. C 3, 2528–2538. 17. Jalili, R., Razal, J.M., and Wallace, G.G. (2013). Wet-spinning of PEDOT:PSS/functionalizedSWNTS composite: a facile route toward production of strong and highly conducting multifunctional fibers. Sci. Rep. 3, 3438–3445. 18. Yarger, J.L., Cherry, B.R., and van der Vaart, A. (2018). Uncovering the structure-function relationship in spider silk. Nat. Rev. Mater. 3, 18008–18018. 19. Keten, S., Xu, Z., Ihle, B., and Buehler, M.J. (2010). Nanoconfinement controls stiffness, strength and mechanical toughness of b-sheet crystals in silk. Nat. Mater. 9, 359–367. 20. Vepari, C., and Kaplan, D.L. (2007). Silk as a biomaterial. Prog. Polym. Sci. 32, 991–1007. 21. Hardy, J.G., and Scheibel, T.R. (2010). Composite materials based on silk proteins. Prog. Polym. Sci. 35, 1093–1115. 22. Ling, S., Kaplan, D.L., and Buehler, M.J. (2018). Nanofibrils in nature and materials engineering. Nat. Rev. Mater. 3, 18016–18030. 23. Ling, S., Chen, W., Fan, Y., Zheng, K., Jin, K., Yu, H., Buehler, M.J., and Kaplan, D.L. (2018). Biopolymer nanofibrils: structure, modeling, preparation, and applications. Prog. Polym. Sci. 85, 1–56. 24. Ling, S., Wang, Q., Zhang, D., Zhang, Y., Mu, X., Kaplan, D.L., and Buehler, M.J. (2018). Integration of stiff graphene and tough silk for the design and fabrication of versatile electronic materials. Adv. Funct. Mater. 28, 1705291. 25. Ayutsede, J., Gandhi, M., Sukigara, S., Ye, H., Hsu, C.M., Gogotsi, Y., and Ko, F. (2006). Carbon nanotube reinforced Bombyx mori silk nanofibers by the electrospinning process. Biomacromolecules 7, 208–214. 26. Hu, X., Li, J., and Bai, Y. (2017). Fabrication of high strength graphene/regenerated silk fibroin composite fibers by wet spinning. Mater. Lett. 194, 224–226. 27. Zhang, C., Zhang, Y., Shao, H., and Hu, X. (2016). Hybrid silk fibers dry-spun from
regenerated silk fibroin/graphene oxide aqueous solutions. ACS Appl. Mater. Interfaces 8, 3349–3358. 28. Zhang, C., Zhang, Y., Luo, J., Shi, J., Shao, H., and Hu, X. (2017). Microstructural evolution of regenerated silk fibroin/graphene oxide hybrid fibers under tensile deformation. RSC Adv. 7, 3108–3116. 29. Wang, Q., Wang, C., Zhang, M., Jian, M., and Zhang, Y. (2016). Feeding single-walled carbon nanotubes or graphene to silkworms for reinforced silk fibers. Nano Lett. 16, 6695–6700. 30. Emiliano, L., Federico, B., Francesco, B., Matteo, B., Simone, T., Giovanni, G., Andrea, C.F., and Nicola Maria, P. (2017). Spider silk reinforced by graphene or carbon nanotubes. 2D Mater. 4, 031013–031021. 31. Wang, C., Xia, K., Jian, M., Wang, H., Zhang, M., and Zhang, Y. (2017). Carbonized silk georgette as an ultrasensitive wearable strain sensor for full-range human activity monitoring. J. Mater. Chem. C 5, 7604–7611. 32. Steven, E., Saleh, W.R., Lebedev, V., Acquah, S.F.A., Laukhin, V., Alamo, R.G., and Brooks, J.S. (2013). Carbon nanotubes on a spider silk scaffold. Nat. Commun. 4, 2435–2442. 33. Zhang, W., Ye, C., Zheng, K., Zhong, J., Tang, Y., Fan, Y., Buehler, M.J., Ling, S., and Kaplan, D.L. (2018). Tensan silk-inspired hierarchical fibers for smart textile applications. ACS Nano 12, 6968–6977. 34. Ling, S., Qin, Z., Li, C., Huang, W., Kaplan, D.L., and Buehler, M.J. (2017). Polymorphic regenerated silk fibers assembled through bioinspired spinning. Nat. Commun. 8, 1387– 1398. 35. Vatankhah-Varnosfaderani, M., Daniel, W.F.M., Everhart, M.H., Pandya, A.A., Liang, H., Matyjaszewski, K., Dobrynin, A.V., and Sheiko, S.S. (2017). Mimicking biological stress-strain behaviour with synthetic elastomers. Nature 549, 497–501. 36. Lee, C., Wei, X., Kysar, J.W., and Hone, J. (2008). Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 321, 385–388. 37. Xie, S., Li, W., Pan, Z., Chang, B., and Sun, L. (2000). Mechanical and physical properties on carbon nanotube. J. Phys. Chem. Solids 61, 1153–1158. 38. Yu, M.F., Files, B.S., Arepalli, S., and Ruoff, R.S. (2000). Tensile loading of ropes of single wall carbon nanotubes and their mechanical properties. Phys. Rev. Lett. 84, 5552–5555. 39. Kenry, Yeo, J.C., and Lim, C.T. (2016). Emerging flexible and wearable physical sensing platforms for healthcare and biomedical applications. Microsyst. Nanoeng. 2, 16043–16062. 40. Muiznieks, L.D., and Keeley, F.W. (2013). Molecular assembly and mechanical properties of the extracellular matrix: a fibrous protein perspective. Biochim. Biophys. Acta 1832, 866–875.
41. Wenger, M.P.E., Bozec, L., Horton, M.A., and Mesquida, P. (2007). Mechanical properties of collagen fibrils. Biophys. J. 93, 1255–1263. 42. Jeon, J., Lee, H.B.R., and Bao, Z. (2013). Flexible wireless temperature sensors based on Ni microparticle-filled binary polymer composites. Adv. Mater. 25, 850–855. 43. Tomoyuki, Y., Yusuke, I., Yuki, T., Jonathan, R., Martin, K., Taylor, W., Kejia, Y., Kunihiko, M., Tomohiro, M., and Masaki, S. (2015). Ultraflexible, large-area, physiological temperature sensors for multipoint measurements. Proc. Natl. Acad. Sci. U S A 112, 14533–14538. 44. Gao, L., Zhang, Y., Malyarchuk, V., Jia, L., Jang, K.-I., Chad Webb, R., Fu, H., Shi, Y., Zhou, G., Shi, L., et al. (2014). Epidermal photonic devices for quantitative imaging of temperature and thermal transport characteristics of the skin. Nat. Commun. 5, 4938. 45. Trung, T.Q., Ramasundaram, S., Hwang, B.U., and Lee, N.E. (2016). An all-elastomeric transparent and stretchable temperature sensor for body-attachable wearable electronics. Adv. Mater. 28, 502–509. 46. Wang, L., Zhao, B., Wu, J., Chen, C., and Zhou, K. (2018). Experimental and numerical investigation on mechanical behaviors of woven fabric composites under off-axial loading. Int. J. Mech. Sci. 141, 157–167. 47. Chen, J., Chen, W., Zhou, H., Zhao, B., Wang, M., Sun, W., and He, S. (2018). Fracture failure analysis and bias tearing strength criterion for a laminated fabric. J. Ind. Text. 47, 1496–1527. 48. Yu, M.F., Lourie, O., Dyer, M.J., Moloni, K., Kelly, T.F., and Ruoff, R.S. (2000). Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load. Science 287, 637–640. 49. Lu, X., and Chen, Z. (2005). Curved piconjugation, aromaticity, and the related chemistry of small fullerenes (
Matter 1, 1–15, December 4, 2019
15
Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles Chao Ye, Jing Ren, Yanlei Wang, ..., Markus J. Buehler, David L. Kaplan, Shengjie Ling [email protected] (M.J.B.) [email protected] (D.L.K.) [email protected] (S.L.)
HIGHLIGHTS Conductive silk fibers were produced through a dip-coating strategy Structure, property, and functional merits of silk and carbon nanotubes are combined Conductive silk fibers can withstand automated manufacturing and high-intensity washing Functional diversity is achieved in conductive silk fiber-based smart textiles
Continuously spinnable conductive silk fibers (CSFs) were constructed by a facile dip-coating strategy. The resultant CSFs integrate the mechanical and functional merits of both silk and carbon nanotubes and can be directly woven into smart textiles using automated equipment. These CSF-based e-textiles show promising applications in wearable devices, human augmentation, healthcare monitoring, and human-machine interfaces.
Ye et al., Matter 1, 1–15 December 4, 2019 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.matt.2019.07.016
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Article
Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles Chao Ye,1 Jing Ren,1,2 Yanlei Wang,3 Wenwen Zhang,1,4 Cheng Qian,3 Jun Han,5 Chenxin Zhang,5 Kai Jin,6 Markus J. Buehler,6,* David L. Kaplan,7,* and Shengjie Ling1,8,*
SUMMARY
Progress and Potential
E-textiles are of interest in a variety of fields, such as wearable electronics and real-time healthcare monitoring. Critical challenges remain, related to design and fabrication techniques to meet the varied requirements for e-textiles. As an ancient and incomparable textile fiber, natural silk fibers can be considered to address these issues. However, processing challenges have thus far precluded the realization of electrically conducting natural silks. Here, we report a scalable dip-coating strategy to construct conductive silk fibers (CSFs). Natural silk fibers were functionalized by a tailor-made carbon nanotube (CNT) paint, which selectively etches the surface of the silk fibers without destroying the internal structure of the fibers. The CSFs maintained the properties of both the silks and CNTs, with high mechanical performance, super-hydrophobicity, solvent resistance, and thermal sensitivity. The CSFs can be automatically woven into fabrics, resulting in textiles sensitive to surrounding physical stimuli, including force, strain, temperature, and solvents.
Silk fiber and carbon nanotubes are selected to construct conductive silk fibers (CSFs) for e-textile applications. Using a scalable dip-coating strategy, we integrate the complementary advantages of both components in one hybrid fiber system. In such a system, silk fibers provide mechanical strength and toughness and carbon nanotubes contribute functions such as water repellency, solvent resistance, and thermal and electrical conductivity. The mechanical and functional merits of CSFs allow them to be woven into e-textiles using automated fabric machines. The resultant e-textiles can withstand automatic or highintensity ultrasonic washing. These CSF e-textiles can be used as wearable sensing platforms to detect and monitor surrounding physical and chemical signals, such as force, temperature, and solvents, showing promising applications for wearable devices, human augmentation, healthcare monitoring, and human-machine interfaces.
INTRODUCTION Smart textiles (also referred to as e-textiles) are fabrics with integrated electronic components, such as batteries, sensors, circuits, lights, displays, and even small computers.1–3 Such systems provide intelligent and wearable materials for realtime healthcare monitoring,4 motion sensing,5 portable communication,6 and human augmentation applications.7,8 As smart, wearable systems, e-textiles must meet the basic requirements of clothing, such as comfort, light weight, heat retention, air permeability, and good durability, but are also required to provide additional value to the wearer.9 For example, e-textiles are expected to guard against extreme environmental hazards, such as a drenching rain, solvent erosion, or radiation damage, while monitoring and regulating body temperature and providing wind resistance.10 E-textiles often require fibers that can be woven into a textile context, combined with electrical conductivity. Widely used conducting or semi-conducting materials are based on metallic threads, which tend to be rigid, brittle, and costly, hampering their use for lightweight, large-deformation fabrics.10 Therefore, stretchable and electronic polymer-/carbon-based fibers are being developed using polymer fibers that contain conducting nanomaterials, such as carbon nanotubes (CNTs) and graphene.1–3 However, it remains challenging to combine processing, mechanical, electrical, and structural advantages in a single fiber system (Figure 1 and Table S1).11–15 For instance,
Matter 1, 1–15, December 4, 2019 ª 2019 Elsevier Inc.
1
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 1. Performance Comparison of Five Important Properties for Reported Conductive Fibers, Single CSFs, and CSF Yarn in This Work The area of each hexagon represents the general performance of CSFs in this work and the five most promising fibers selected from Table S1. Each property is classified as grade 1–5. Specific standards for the grading are declared in Table S2.
poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)-based conductive fibers feature high conductivity but their mechanical performance remains unable to meet the requirements of automatic textile manufacturing.12,13,16,17 In contrast, silks are ancient textile fibers with unique features for smart textile applications. Specifically, silks are mechanically tougher than metallic and carbonic threads and several times tougher than Kevlar fibers.18,19 Silks also provide advantages of light weight, low cost, sustainability, durability, biocompatibility, and large-scale production.20–23 However, despite these useful features, silks are hampered in e-textile applications due to their insulating properties versus the required conductive features of e-textiles. To obtain conductive silk fibers (CSFs), researchers have developed a variety of methods to introduce conductive components (e.g., metal particles, graphene, CNTs) into silks,24,25 including wet-spinning,26 dry-spinning,27,28 or feeding silkworms and spiders with carbon nanomaterials.29,30 However, the conductive components in these hybrid fibers are usually insufficient to reach conductive percolation thresholds. Carbonization of silk textiles31 is another effective approach to create conductive textiles; however, the mechanical strength and toughness of natural silks are destroyed after high-temperature processing. Accordingly, a significant challenge remains to fabricate CSFs with superior mechanical and conducting properties to match the requirements for e-textiles.
1School
of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
2Xinhua
Hospital Affiliated to Shanghai Jiao Tong University School of Medicine, Shanghai 200092, China
3Beijing
Key Laboratory of Ionic Liquids Clean Process, CAS Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
4College
of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, China
5Quanzhou
In this work, we designed a low-cost and scalable strategy to dissolve and modify the surface of natural silks with CNTs. A hexafluoroisopropanol (HFIP)-stabilized CNT paint was developed to adhere to degummed natural silk fibers. Strong bonding between the CNT paint and the silk fiber surface was achieved, since HFIP can be used to controllably etch the silk fiber surface. The process results in silk fibers uniformly coated with CNTs, which provides an electrically conducting path, and thereby a self-sensing mechanism, for response to distinct physical stimuli to the CSF structure.32 Finally, we incorporate yarn-spinning and automatic weaving techniques to fabricate e-textiles. These CSF-based e-textiles integrate the advantages of both the silk fibers and the CNTs, with sensitivity to the body and environmental stimuli
2
Matter 1, 1–15, December 4, 2019
Institute of Equipment Manufacturing, Haixi Institute, Chinese Academy of Science, Quanzhou 362216, China
6Department
of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
7Department
of Biomedical Engineering, Tufts University, Medford, MA 02155, USA
8Lead
Contact
*Correspondence: [email protected] (M.J.B.), [email protected] (D.L.K.), [email protected] (S.L.) https://doi.org/10.1016/j.matt.2019.07.016
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 2. Preparation and Characterization of CSFs (A) Schematic diagram of the fabrication method of CSF yarns. (B) CSF yarns were collected by a cylinder and were used as wires to light a bulb. (C and D) SEM images of CSFs at different magnifications. (E) SEM image of a typical CSF yarn. (F) Photograph of CSFs washed in water.
such as force, strain, temperature, and solvents. Thus, such systems show promise for wearable devices, human augmentation, healthcare monitoring, and humanmachine interfaces.
RESULTS AND DISCUSSION Preparation of CNT Paint As a low-cost, rapid, user-friendly, and scalable technique, dip-coating has been widely used to produce functional fibers.32,33 However, this technique was rarely employed to functionalize natural silks,32 because it is difficult to adhere functional components onto natural fibers without destroying their structure and properties. Selection of an appropriate solvent was vital to solving this problem whereby solvent should not only disperse and stabilize the CNTs but also etch the surface of the silk fibers in a controllable fashion for attachment of the CNTs. HFIP was chosen, based on our previous studies showing that HFIP partially dissolved the Bombyx mori (B. mori) silk fiber surfaces.34 In an optimized protocol, a CNT/isopropanol/polyvinyl alcohol (PVA) suspension with a CNT concentration of 15 wt % was dispersed into the HFIP, resulting in a 1 wt % CNT paint. The homogeneous CNT paint was stable at room temperature for more than 6 months, showing no aggregation or sedimentation (Figure S1). Fabrication of CSF Yarns A two-step route was designed to obtain weavable CSF yarns longer than kilometerscale (Figure 2A). At the first step, degummed B. mori silk fibers were immersed in the CNT paint and incubated at 60 C for 2 days. During this process, the surface of B. mori silk fibers partially dissolved to form a sticky surface and allowed a firm CNT coating. The fibers were then pulled from the CNT paint and dried in a chemical
Matter 1, 1–15, December 4, 2019
3
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
hood. After this process, the silk fibers took on an all-black appearance and they could be used as a wire to light a bulb, due to the even coating of CNTs on the fiber surfaces (Figures 2B–2F). A long fiber configuration and an intact silk fiber section were observed by scanning electron microscopy (SEM) after the dip-coating process (Figure S2), which indicated that the internal structure of the silk fibers was not destroyed. By using this method, an elegant balance of important mechanical properties can be achieved with the CSF yarns, which is promising for the expected applications in textiles. The HFIP/CNT paint/silk system in this work was distinct from a previously observed HFIP/silk fiber system with the same mass ratios,34 whereby the silk fibers dissolved into centimeter-scale microfibrils. Thus, the addition of CNTs attenuated the impact of HFIP on fibroin solubility. To understand why this difference occurred, we utilized steered molecular dynamics (SMD) simulations to understand the energy barrier (EB) for HFIP molecules entering the CNTs (Figure 3A). To perform SMD simulations, we attached a spring with force constant of k = 200 M m1 to the HFIP molecule with movement into the CNTs at a constant velocity of 10 m s1. The force profile for the HFIP entering the CNTs and the associated EB are summarized in Figure 3B, where the negative value of force and EB imply that HFIP molecule can enter into the CNTs spontaneously. SMD simulations were also performed with different movement velocities (Figure S3), indicating that 10 m s1 was slow enough to obtain a convergence energy barrier. To address the impact of the CNT wall on the EB, we performed the diffusion of HFIP in different initial positions, along with the center axis of CNT (center channel), and near the wall of the inner CNT (edge channel). For the center channel, EB increased with diameter and approached zero, showing weaker impact on the diffusive process of HFIP. For the edge channel, EB approached a constant value of about 7.5 kcal mol1 (Figure 3C). An additional SMD simulation also indicated that the energy barrier of HFIP entering the CNT was significantly lower than that of moving along the outside of the CNT (Figure S4). Furthermore, we compared the ability of solvent to enter the CNT’s inner space for several organic solvents, including ethanol, formic acid, acetone, toluene, and HFIP (Figures 3D and 3E), which are considered to have an influence on the structure and properties of silks. The results show the diffusive ability in the order ethanol < formic acid < acetone < toluene < HFIP, determined by the interactions between the solvent molecules and the CNT, indicating that HFIP can enter into the multiwalled CNT most easily. Based on these analyses of free energy, HFIP diffused into the inner space spontaneously and preferred to transport through the inner wall of the CNTs, which agrees with the experimental results. At the second step, the CSFs were spun into CSF yarns with twisted helical structures using a yarn-spinning approach (Figure 2E). These CSFs and CSF yarns tolerated extensive washing in water. After continuous stirring in water for an hour, the CSFs were still black and the water was still clear (Figure 2F). It should be noted that CSFs are hydrophobic; the extreme washing conditions destroyed the CSF/water interface air layer and the water permeated into the inner layer of CSFs, while the CNTs still firmly integrated with the silks (Figure S5A). More importantly, there was no obvious change in the electrical performance of the washed yarns (163 G 52 S m1), which was 160 G 60 S m1 before washing (Figure S5B). SEM images reveal the tight association between the silk fiber core and the CNT coating layer after washing (Figure S5C), with no visible differences observed compared with these CSFs before washing. Automatic washing with soap (30-min water washing and 30-min dehydration, at room temperature; Figures S5D and S5E; Video S1) and high-intensity ultrasonic washing (20 kHz, 300 W, 1 h; Figure S5F)
4
Matter 1, 1–15, December 4, 2019
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 3. SMD Simulations to Capture the Energy Barrier for HFIP Molecules Entering the Inner Space of CNT (A) Atomic structures of CNT that defined a channel for HFIP diffusion in steered molecular dynamics (SMD) simulations. Here we use double-walled CNTs to represent the multiwalled CNT in the simulations. (B) Force applied to HFIP molecules when entering CNTs with different diameters. ‘‘Center’’ represents movement along the center axis of CNT, while ‘‘edge’’ represents movement near the inner surface of CNT. Inert is the schematic to clarify the center (red arrow) and edge (blue arrow), and two circles represent the two walls of CNT. (C) Energy barrier (E B ) for HFIP molecules entering into the CNT against the diameter, where the blue line and red line represent the limit E B for diffusing along the center channel and edge channel, respectively. (D) Force applied to different solvent molecules when entering into CNTs with 1.9 nm diameter. (E) Comparison of E B for different organic solvents entering the inner space of CNT, where the diameter of CNT is 1.9 nm for all cases.
further illustrated the stability of the CSF textiles under standard and extreme washing conditions. Mechanical Properties and Weaveability of CSFs The single CSF featured ductile mechanical behavior (Figures S6A and S6B). The tensile strength and strain were 633 G 168 MPa and 12% G 4%, respectively, which are equivalent to native silk fibers. This result also corresponds to the aforementioned structural observation, revealing that the modification process did not affect the internal structure of the silk fibers. The CSF yarns maintained the ductile mechanical
Matter 1, 1–15, December 4, 2019
5
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
behavior of CSFs; the breaking loading force of the yarn was almost equal to the sum of the breaking loading force that the single fiber within the CSF yarn could withstand (Figures S6C and S6D; Table S3). It is noteworthy that the fracture behavior of regular yarns consisting of short fibers can be significantly influenced by the degree of the twisted structure. This is because the slip between the short fibers would be restricted as the degree of twisting increases. However, the effect of the twisting process on CSF yarns consisting of long CSFs was different, according to both the simulation and experimental results (Figures S7 and S8; Tables S3 and S4). In this work, the CSFs are long and almost all of them extend continuously from one end to the other end of the yarn. Upon tensile loading, the major deformation of the yarn mainly contributes to the tension of each fiber. Other contributions, such as bending and inter-fiber slidings, are negligible. As a result, the tensile property of the yarns nearly reproduces that of each single silk fiber. The increased twisting process reduces the gap between the long CSFs, thereby decreasing the cross-sectional area of the CSF yarns, which leads to increased fracture stress and improved mechanical stability, but will not influence the general breaking loading force of the CSF yarns. The detailed simulation method can be found in Supplemental Information. Both values of mechanical properties of CSFs and CSF yarns were comparable with those of regenerated silk fibers (109 G 34 MPa and 14.0% G 4.9%, respectively)34 and natural silk fibers (600 MPa and 17%, respectively).18 Most natural and synthetic materials can be confined to a single trend line by using extensibility and Young’s modulus as variables in horizontal and vertical coordinates, respectively35 (Figure 4A). This tendency indicates that larger values of extensibility typically lead to lower values of Young’s modulus. For example, graphene and CNTs dominate high elastic modulus, while they are very brittle with limited fracture strain;36–38 silicone,39 by contrast, is elastically stretchable with limited modulus. However, CSF yarns synergistically integrated the advantages of high modulus from the CNTs and good extensibility from the silk fibers, leading to similar mechanical behavior with natural collagen fibers.40,41 The mechanical merits of these CSF yarns supported their functionalization and utility as versatile e-textiles through automatic weaving. Herein, the CSF yarns can be processed by using knitting or weaving machines because they fulfill the following requirements: (1) the fibers have sufficient continuous length to meet the processing requirements; (2) the conductive coating layer and the core fibers need to be stable during the machinery knitting or weaving operation; and (3) electrical conductivity needs to be preserved after the machine processing. The CSF yarns were woven into a fabric by a loom (Figures 4B and 4C) and into a subtle logo pattern using an automatic embroidery machine (Figure 4D and Video S2). SEM images of the CSF pattern (Figure 4E) revealed that the CSF yarns (yellow) were intact and bound well with the substrate fabric (blue). High-resolution SEM images (Figure 4F) further displayed the full coverage of CNTs on the CSF surface, indicating that the sewing machinery did no damage the microstructure of the CSF yarns. Water Repellency and Solvent Resistance of CSF E-Textiles Although bare silk fibers show hydrophilicity, hydrophobicity can be introduced by both weaving silk yarns into a silk textile with heterogeneous micro-nano surface structures or coating silk fibers with hydrophobic and chemically inert CNTs (Figure S9). The prepared CSF e-textiles revealed excellent hydrophobicity due to the additive effect of hydrophobicity and the chemical inertness of the CNT coatings, as well as heterogeneous micro-nano structure on the surface (Figure S10). Thus,
6
Matter 1, 1–15, December 4, 2019
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 4. Mechanical Properties and Knittability of CSF Yarns (A) Comparison of specific Young’s modulus and tensile strain with other materials. The Ashby plot was redrawn from Vatankhah-Varnosfaderani et al., 35 Where r is the mass density, E is Young’s modulus, and l max is strain at fracture. (B) A piece of e-textile cloth woven by CSF yarns. (C) SEM image of the surface of the e-textile in (B). (D) Photograph of an embroidery machine knitting the CSF yarns into a logo pattern. (E) SEM image of the CSF yarns on non-woven fabric substrate, magnified from (D) (yellow region, CSFs; blue region, substrate; false color was used to distinguish the CSFs and substrate). (F) SEM image of the CSFs in (E) at high magnification.
these e-textiles appeared excellent as water-repellent and solvent-resistance materials. When water was poured onto the e-textiles, it flowed across the surface without wetting the pattern (Figure 5A and Video S3). A water droplet could stand on a CSF yarn and remained in spherical morphology until complete evaporation (inset image in Figure S10A). Next, we estimated the solvent resistance of the e-textile patterns. Acetone, alcohols, toluene, formic acid, and HFIP were gradually dripped onto a CSF yarn-based e-textile. The substrate textile was corroded thoroughly by these solvents, but the logo remained intact (highlighted by the red box in Figure 5B). Fluorescence microscopy images confirmed that the solvents penetrated the gap between single CSFs (Figure S11). This penetration process led to the increased resistance of the e-textiles. The resistance increase of the e-textiles when ethanol or acetone was dripped on the materials is shown in Figure 5C. Different response speeds were also observed between ethanol and acetone: the yarn resistance change peaks of acetone were sharper than that of ethanol under the same experimental conditions. Different solvents usually have distinct evaporation rates; thus, these e-textiles have the potential to identify solvent exposure by assessing rates of changes in resistance, with caveats based on the concentration of exposure to be calibrated. Five different solvents were tested, and all of the solvents led to distinct changes in resistance response (Figure S12).
Matter 1, 1–15, December 4, 2019
7
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 5. Water Repellency and Solvent Responses of the CSF Yarn (A) Hydrophobicity of a shoe made of CSF textile. (B) Photograph of the logo pattern on a non-woven fabric substrate after solvents corrosion. The red dashed line indicates the corroded area, where the green background can be seen. (C) Resistance change of the CSF yarn in response to ethanol and acetone drops. (D) Resistance change of the CSF yarn in response to aqueous ethanol solution at different concentrations.
We also employed e-textiles as sensors for ethanol concentrations in aqueous ethanol solutions. Because of the hydrophobicity and ethanol affinity of CNTs, the resistance changed, directly related to the water/ethanol ratio in the solution dropped on the surface (Figure 5D). The e-textiles showed the most significant responses to pure ethanol, with a sharp resistance change within 40 s. In contrast, a 400-s plateau region was observed for 20% ethanol solution. Other water/ethanol ratios responded with progressive changes in resistance curves between that of pure ethanol and the 20% ethanol solution. To reveal the mechanism of detection, we conducted molecular dynamics simulations of the water/ethanol system with the e-textile. The resistance change of the e-textiles should be attributed to the diffusive process of ethanol. Hence, we calculated the self-diffusive coefficient of ethanol as a function of the concentration of ethanol. As presented in Figure S13, the diffusive ability is constrained as the concentration of ethanol increases. When the concentration of ethanol increases from 0% to 100%, the diffusive coefficient decreases by about 62%. The reduction of diffusion coefficient originates from the aggregation of the ethanol molecules in the water. The evolution of diffusive ability matches well with the experimental results. Temperature Sensitivity of E-Textiles CSF e-textiles were also sensitive to temperature changes due to thermal expansion and contraction of the CNT coating layer. The temperature of the e-textile pattern increased rapidly from 25 C to 63.6 C with a 30-s far-infrared illumination (Figure 6A and Video S4). In addition, heat-retention ability was improved by the increased thickness of the CSF pattern. For example, after the far-infrared illumination was
8
Matter 1, 1–15, December 4, 2019
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 6. Thermal Responses of the CSF Yarns (A) Thermal image of the logo pattern weaved by CSF yarns under far-infrared illumination. (B) Thermal image of an e-textile with a thicker ‘‘STU’’ pattern after 20 s after far-infrared illumination was removed. The inset image is the textile knitted by CSF yarns with a two-layer STU pattern. (C and D) Photograph (C) and thermal image (D) under far-infrared illumination of a finger-shaped pattern sewed by CSF yarns. (E) The relationship between temperature and resistance under cyclic heating/cooling processes. The red dots and blue dots represent the heating and cooling process, respectively.
removed, the thicker e-textile pattern (a two-layer region comprising the letters ‘‘STU’’) presented a slower cooling speed than the one-layer region (Figure 6B). Furthermore, we designed a precise hand pattern with a line width of 2 mm to estimate the spatial resolution of the site-specific heating effect on e-textiles (Figure 6C). A clear high-temperature pattern (108 C) was detected under the low-temperature substrate layer (29 C, Figure 6D), indicating that site-specific heating can be conducted with a resolution below 2 mm. In addition, during cyclic heating-cooling measurements (Figure 6E), the resistance of the e-textiles varied with the temperature changes due to thermal expansion and contraction. Repeated cycles between temperature and resistance were detected in the temperature range of 30 C–65 C. Nevertheless, the CSF yarns exhibited thermal hysteresis and inaccuracy (G4 C) in the high-temperature region (50 C–65 C) during repeated heating and cooling, due to the non-isotropic dispersion of CNTs within the CSF yarns. The hysteresis and inaccuracy phenomena also exist in other high-performance electroconductive temperature sensors.42–45
Matter 1, 1–15, December 4, 2019
9
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 7. Cyclic Loading-Unloading Test of the CSF Yarns (A) Photograph of the experimental platform for evaluating the resistance changes of CSF yarns under loading-unloading test. (B) Electrical resistance changes of the CSF yarns during cycle loading-unloading tests.
Force and Strain Sensitivity of E-Textiles Benefiting from the loose and twisted structure, the CSF yarns were sensitive to external forces. When the CSF yarns were tightened by stretching, the contact area between single CSFs increased, thus reducing electrical resistance. Conversely, when the tensile force was removed, the CSF yarns loosened to their initial state and the resistance of CSF yarns returned to the starting value. Accordingly, we coupled a tensile machine with a wireless multimeter to record the electrical resistance changes in situ during cyclic loading-unloading (Figure 7A). The CSFs exhibited mechanical behavior similar to that of silks, which showed good durability (Figure S14). Neither plastic deformation nor degradation in strength occurred at a set strain of 5%, a value close to the fracture strain of many woven fabrics.46,47 In this process, the resistance varied synchronously with yarn deformation and remained stable after 40 cycles (Figure 7B). E-Textiles for Monitoring Human Activity We knitted the CSF yarns into kneecap- and glove-fitting materials to monitor largeand small-scale human body motions based on their resistance-strain response (Figures 8A and 8B). It is noted that bending and pressing have little impact on the motion strategy (Figure S15). Marching paces were then continuously recorded by measuring and quantifying the output electrical signals through the e-kneecaps. The resistance of the CSF yarns decreased rapidly when stretching occurred and varied synchronously with the leg bending and frequency of release. The response rate of e-kneecaps reached up to 100 ms, a distinct signal change of more than 10% with excellent repeatability (Video S5). Finally, e-gloves were prepared by sewing CSF yarns into glove fingers to sense hand motions (Figure 8C). Long-term durability of the e-gloves was tested by continuously winding-unwinding two fingers of the glove more than 200 times (Figures 8D– 8F). The e-glove exhibited stable electrical responses during 200 bending-releasing cycles of the index finger (black line) and middle finger (blue line), essential for practical applications. Furthermore, the e-gloves were sensitive to finger deformation and distinguished distinct finger gestures (Figures 8G and 8H). For example, this e-glove successfully monitored the complete process of hand grabbing. The corresponding changes in resistance for each finger could be recorded in real-time by computer (Figure 8G and Video S6). We further designed a more complex hand motion, a continuous change of hand gestures of ‘‘5-6-8-2-0-6-8-0-5,’’ to test the sensitivity of the e-glove system (Figure 8H). The resistance of each finger sensor changed synchronously with finger movement but remained stable from the starting to the
10
Matter 1, 1–15, December 4, 2019
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
Figure 8. CSF E-Textiles for Monitoring Human Activity (A) CSF yarns were sewn into an e-kneepad. The photographs show the deformation of a CSF yarn at different knee-bending angles during walking. (B) Resistance change of an e-kneepad during the wearer marching at different speeds. (C) Photograph of an e-glove made by sewing five CSF yarns into a glove, respectively. (D–F) Resistance change of the index finger (D, black line in F) and middle finger (E, blue line in F) under repeated winding and releasing process for more than 200 cycles. (G) Real-time monitoring of resistance changes of the five fingers when grabbing an object. (H) Resistance change of every finger under different hand gestures.
ending gesture of ‘‘5.’’ Meanwhile, no signal interference from adjacent fingers was detected in the process. All of these characteristics of e-gloves endow promising applications in digitalizing hand gestures and provide useful translation for hearingimpaired individuals and for robotics. Conclusion Silk fibers, as one of most abundant natural protein fibers, feature light weight, flexibility, toughness, and biocompatibility.22,23 By contrast, CNTs, as high-performance synthetic inorganic nanomaterials, are known for their extraordinary thermal and electrical conductivity, as well as ultrahigh tensile strength and elastic modulus.48,49 However, it remains a significant challenge to integrate the complementary advantages of both components into one material system. In this study, we demonstrated that direct functionalization of natural silks is a practical option to overcome this challenge. With this strategy, the structural and mechanical advantages of natural silk fibers were transformed into CSFs and could be woven into e-textiles using an
Matter 1, 1–15, December 4, 2019
11
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
automated fabric machine. Furthermore, CNTs, the functional component of the system, endowed additional merit to the e-textile system, such as water repellency, solvent resistance, and heat retention. All of these characteristics are critical for practical applications of e-textiles. More remarkably, the CSF e-textiles could be directly assembled into a wearable environmental sensing platform based on variations in electrical parameters, to detect and monitor surroundings and communicate the acquired physical and chemical signals, such as force, strain, temperature, and solvents. These e-textiles with integrated advantages of both silks and CNTs show promising applications in wearable devices, human augmentation, healthcare monitoring, and human-machine interfaces. Future research can focus on introducing other components to improve the specific response of hybrid CSFs to different stimuli, while the multifunctional integration design, data collection, correction, and analyzing can be processed by computer technology. For practical applications, CSF yarns can be woven into the outer or inner layer of clothing to avoid safety concerns from the direct contact of CNTs with human skin. At minimum, the main silk components are environmentally compatible and could be enzymatically degraded for the recovery and reuse of other components in the electronic fibers.
EXPERIMENTAL PROCEDURES Preparation of CNT Paint To prepare the CNT paint, 3 mL of 15 wt % multiwalled CNT solution (isopropanol as solvent and PVA as stabilizing agent, Chengdu Organic Chemicals, China) was mixed with 17 mL of HFIP in a sealed glass bottle with sufficient stirring. Preparation of CSF Yarns B. mori silkworm cocoon silk fibers were degummed by boiling in two 30-min changes of 0.5% (w/w) NaHCO3 solution. The degummed silk fibers were washed with distilled water and allowed to air dry at room temperature. Next, 1 g of degummed silk fibers was immersed in 20 mL CNT/HFIP paint and incubated in airtight containers at 60 C for 2 days. The CSFs were obtained after being pulled out within a few seconds and dried at 70 C for 4 h. The strong adhesion between CNTs and silk is due to the selective dissolution of silk surface by HFIP, which is determined by the incubation time, so there is little need to control the pull-out speed, unlike the conventional dip-coating method. All of these steps were conducted in a chemical hood with the necessary precautions, as HFIP is a toxic solvent. Finally, dozens of CSFs were arranged in parallel, followed by a yarn-spinning approach to obtain CSF yarns. The surface of the CSFs and the structure of the e-textiles made of CSF yarns were characterized by SEM (JEOL JSM-7800F) at an acceleration voltage of 5 kV. SMD Simulations All the simulations in this work were completed using a large-scale atomic/molecular massively parallel simulator.50 The time step for integrating Newtonian equations of motion is 0.5 fs, which is validated to ensure the energy conservation. The all-atom optimized potential for liquid simulations (OPLS-AA) potential and adaptive intermolecular reactive empirical bond order potential are employed for organic molecules (HFIP, ethanol, toluene, and formic acid)51 and CNT,52 respectively. The interaction between organic molecules and CNT includes van der Waals force, which is described via Lennard-Jones potential function and the Lorentz-Berthelot mixing rules and used to model the parameters, which are truncated at 1.2 nm. In the entire simulations, the organics molecule and CNT were kept at a constant temperature of 0.01 K to discover the intrinsic properties of the HFP/CNT system. After the system was run to equilibrium, SMD simulations were used to explore the free energy profiles for organic molecules entering the CNT with different diameters.
12
Matter 1, 1–15, December 4, 2019
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
For calculation of molecular diffusion coefficient, the size of the simulation box was 5.0 3 5.0 3 5.0 nm3. A periodic boundary condition was applied in the x, y, and z directions. The OPLS-AA potential51 was employed for ethanol and the TIP3P model was used for water. The SHAKE algorithm53 was applied for the stretching terms between oxygen and hydrogen atoms to avoid numerical integration of hydrogenrelated high-frequency vibrations that required a much shorter time step. The interactions between ethanol and water include van der Waals and electrostatic terms, and both interactions were truncated at 1.2 nm. The Lorentz-Berthelot mixing rules were used to model the parameters of ethanol and water. The long-range Coulombic interaction was computed using the particle-particle-particle-mesh algorithm.54 The time step for integrating Newtonian equations of motion was 1.0 fs, which has been validated to ensure the energy conservation. In the SMD simulations, water-ethanol solution was relaxed in the NPT ensemble for 5 ns with a temperature of T = 300 K and pressure of 1 bar along x, y, and z directions. The temperature and pressure were controlled by a Berendsen thermostat. After the system was equilibrated, the mean square displacement was calculated in the NVE ensemble for 5 ns, and the molecular diffusion coefficient D calculated by D E. D = lim jrðtÞ rð0Þj2 6t: t>N
Here, t is the diffusing time and hjrðtÞ rð0Þj2 i the ensemble average. Mechanical Testing of CSF Yarns The CSFs and CSF yarns were cut into segments for tensile tests. The segments with a length of 40 mm were fixed on a hard-cardboard frame. The hard-cardboard frame was mounted onto the testing machine (Instron 5966; Instron, Norwood, USA) and the side support of the frame was cut off, so all the force was transmitted through the fiber/yarn. Meanwhile, the initial length of the fiber was measured with a caliper at zero load point, when the samples are tight without any force exerted on it. The cross-sectional area was measured by cutting the fiber/yarn in liquid nitrogen directly using razor blades, followed by coating with a 5-nm-thick Au layer, and observed by SEM (JSM-7800MF). Pictures were estimated by ImageJ software (NIH). The diameter of CSF yarns was measured by optical microscope before the tensile test. All of the tensile measurements were carried out at 25 C and 50% relative humidity with a tensile speed of 2 mm/min. The resistance variation with cyclic loading-unloading was recorded by further connecting the yarns to a digital multimeter (CEM DT-9989). The cyclic loading-unloading tests were carried out in a cyclic tensile mode with a speed of 2 mm/min, and the maximum deformation in each cycle was 5%. Thermal and Solvent Response of CSF Yarns The CSF yarns were woven into a non-woven fabric substrate. A far-infrared lamp was then used for heating while the temperature of the CSF region was recorded by a thermal imager (DIS-45; Fluke, USA) and the electrical resistance of CSF region was recorded by a digital multimeter (CEM DT-9989) in situ at a time resolution of 1 s. The solvent response function was evaluated by dropping several solvents onto a 50-mm CSF yarn and the electrical resistance recorded by digital multimeter in situ at a time resolution of 1 s. E-Textiles for Monitoring Body Motion To monitor walking, we weaved the CSF yarns into a kneepad. The resistance of the CSF yarn on the e-kneepad was recorded by a digital multimeter (Keithley DMM6500) at 2-wire mode with a resolution of 100 ms. For monitoring of hand gestures, The CSF yarns were woven on the finger region of the glove connected to a
Matter 1, 1–15, December 4, 2019
13
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
2001-Scan card at 2-wire mode. The resistance changes of the five CSF yarns could then be recorded by digital multimeter at the same time with a resolution of 200 ms.
SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.matt. 2019.07.016.
ACKNOWLEDGMENTS We acknowledge the National Natural Science Foundation of China (no. U1832109, 51973116 and 21935002), Shanghai Pujiang Program (18PJ1408600), the National Natural Science Foundation of China (21808220), and the Beijing Natural Science Foundation (2184124), and the starting grant of ShanghaiTech University and Shanghai Sailing Program (17YF1411500) for support of this work. We also thank the NIH (U01EB014976) and the AFOSR (FA9550-11-1-0199 and FA9550-14-10015) for support of various aspects of the work. We thank the Analytical Instrumentation Center (AIC) and the Center for High-resolution Electron Microscopy (C-EM) at School of Physical Science and Technology, ShanghaiTech University, for supporting mechancial and SEM characterization.
AUTHOR CONTRIBUTIONS C.Y. and J.R. contributed equally to this work. S.L. and J.R. designed the study, analyzed the results, and wrote the manuscript. C.Y., W.Z., J.H., and C.Z. performed the experiments and analyzed the data. Y.W., C.Q., and K.J. performed simulations and theoretical calculation. M.J.B. and D.L.K. analyzed the data and wrote the manuscript.
DECLARATION OF INTERESTS The authors declare no competing interests. Received: February 23, 2019 Revised: May 12, 2019 Accepted: July 22, 2019 Published: October 9, 2019
REFERENCES 1. Zeng, W., Shu, L., Li, Q., Chen, S., Wang, F., and Tao, X.M. (2014). Fiber-based wearable electronics: a review of materials, fabrication, devices, and applications. Adv. Mater. 26, 5310–5336. 2. Sang, H.J., Jimi, E., Yong-Hoon, K., and Kyu, P.S. (2018). Recent progress of textile-based wearable electronics: a comprehensive review of materials, devices, and applications. Small 14, 1703034. 3. Agcayazi, T., Chatterjee, K., Bozkurt, A., and Ghosh, T.K. (2018). Flexible interconnects for electronic textiles. Adv. Mater. Technol. 3, 1700277. 4. Coyle, S., and Diamond, D. (2016). 10. Medical applications of smart textiles. In Advances in Smart Medical Textiles, L. van Langenhove, ed. (Woodhead Publishing), pp. 215–237. 5. Liu, Z.F., Fang, S., Moura, F.A., Ding, J.N., Jiang, N., Di, J., Zhang, M., Lepro´, X., Galva˜o,
14
Matter 1, 1–15, December 4, 2019
D.S., Haines, C.S., et al. (2015). Hierarchically buckled sheath-core fibers for superelastic electronics, sensors, and muscles. Science 349, 400–404. 6. Rein, M., Favrod, V.D., Hou, C., Khudiyev, T., Stolyarov, A., Cox, J., Chung, C.C., Chhav, C., Ellis, M., Joannopoulos, J., et al. (2018). Diode fibres for fabric-based optical communications. Nature 560, 214–218. 7. Liu, M., Pu, X., Jiang, C., Liu, T., Huang, X., Chen, L., Du, C., Sun, J., Hu, W., and Wang, Z.L. (2017). Large-area all-textile pressure sensors for monitoring human motion and physiological signals. Adv. Mater. 29, 1703700. 8. Yandell, M.B., Quinlivan, B.T., Popov, D., Walsh, C., and Zelik, K.E. (2017). Physical interface dynamics alter how robotic exosuits augment human movement: implications for optimizing wearable assistive devices. J. Neuroeng. Rehabil. 14, 40–50.
9. Xu, J., Wang, S., Wang, G.J.N., Zhu, C., Luo, S., Jin, L., Gu, X., Chen, S., Feig, V.R., To, J.W.F., et al. (2017). Highly stretchable polymer semiconductor films through the nanoconfinement effect. Science 355, 59–64. 10. Schneegass, S., and Amft, O. (2017). Smart Textiles: Fundamentals, Design, and Interaction (Springer Publishing Company). 11. Granero, A.J., Razal, J.M., Wallace, G.G., Panhuis, M.I.H. (2010). Elastic conducting carbon nanotube-laden SIBS fibers. In: International Conference on Nanoscience and Nanotechnology, pp 80–83. https://doi.org/10. 1109/ICONN.2010.6045183. 12. Jalili, R., Razal, J.M., Innis, P.C., and Wallace, G.G. (2011). One-step wet-spinning process of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) fibers and the origin of higher electrical conductivity. Adv. Funct. Mater. 21, 3363–3370.
Please cite this article in press as: Ye et al., Design and Fabrication of Silk Templated Electronic Yarns and Applications in Multifunctional Textiles, Matter (2019), https://doi.org/10.1016/j.matt.2019.07.016
13. Jalili, R., Razal, J.M., and Wallace, G.G. (2012). Exploiting high quality PEDOT:PSS-SWNT composite formulations for wet-spinning multifunctional fibers. J. Mater. Chem. 22, 25174–25182. 14. Wang, C., Xia, K., Zhang, M., Jian, M., and Zhang, Y. (2017). An all-silk-derived dual-mode e-skin for simultaneous temperature-pressure detection. ACS Appl. Mater. Interfaces 9, 39484–39492. 15. Tang, Z., Jia, S., Wang, F., Bian, C., Chen, Y., Wang, Y., and Li, B. (2018). Highly stretchable core-sheath fibers via wet-spinning for wearable strain sensors. ACS Appl. Mater. Interfaces 10, 6624–6635. 16. Zhou, J., Li, E.Q., Li, R., Xu, X., Ventura, I.A., Moussawi, A., Anjum, D.H., Hedhili, M.N., Smilgies, D.M., Lubineau, G., et al. (2015). Semi-metallic, strong and stretchable wet-spun conjugated polymer microfibers. J. Mater. Chem. C 3, 2528–2538. 17. Jalili, R., Razal, J.M., and Wallace, G.G. (2013). Wet-spinning of PEDOT:PSS/functionalizedSWNTS composite: a facile route toward production of strong and highly conducting multifunctional fibers. Sci. Rep. 3, 3438–3445. 18. Yarger, J.L., Cherry, B.R., and van der Vaart, A. (2018). Uncovering the structure-function relationship in spider silk. Nat. Rev. Mater. 3, 18008–18018. 19. Keten, S., Xu, Z., Ihle, B., and Buehler, M.J. (2010). Nanoconfinement controls stiffness, strength and mechanical toughness of b-sheet crystals in silk. Nat. Mater. 9, 359–367. 20. Vepari, C., and Kaplan, D.L. (2007). Silk as a biomaterial. Prog. Polym. Sci. 32, 991–1007. 21. Hardy, J.G., and Scheibel, T.R. (2010). Composite materials based on silk proteins. Prog. Polym. Sci. 35, 1093–1115. 22. Ling, S., Kaplan, D.L., and Buehler, M.J. (2018). Nanofibrils in nature and materials engineering. Nat. Rev. Mater. 3, 18016–18030. 23. Ling, S., Chen, W., Fan, Y., Zheng, K., Jin, K., Yu, H., Buehler, M.J., and Kaplan, D.L. (2018). Biopolymer nanofibrils: structure, modeling, preparation, and applications. Prog. Polym. Sci. 85, 1–56. 24. Ling, S., Wang, Q., Zhang, D., Zhang, Y., Mu, X., Kaplan, D.L., and Buehler, M.J. (2018). Integration of stiff graphene and tough silk for the design and fabrication of versatile electronic materials. Adv. Funct. Mater. 28, 1705291. 25. Ayutsede, J., Gandhi, M., Sukigara, S., Ye, H., Hsu, C.M., Gogotsi, Y., and Ko, F. (2006). Carbon nanotube reinforced Bombyx mori silk nanofibers by the electrospinning process. Biomacromolecules 7, 208–214. 26. Hu, X., Li, J., and Bai, Y. (2017). Fabrication of high strength graphene/regenerated silk fibroin composite fibers by wet spinning. Mater. Lett. 194, 224–226. 27. Zhang, C., Zhang, Y., Shao, H., and Hu, X. (2016). Hybrid silk fibers dry-spun from
regenerated silk fibroin/graphene oxide aqueous solutions. ACS Appl. Mater. Interfaces 8, 3349–3358. 28. Zhang, C., Zhang, Y., Luo, J., Shi, J., Shao, H., and Hu, X. (2017). Microstructural evolution of regenerated silk fibroin/graphene oxide hybrid fibers under tensile deformation. RSC Adv. 7, 3108–3116. 29. Wang, Q., Wang, C., Zhang, M., Jian, M., and Zhang, Y. (2016). Feeding single-walled carbon nanotubes or graphene to silkworms for reinforced silk fibers. Nano Lett. 16, 6695–6700. 30. Emiliano, L., Federico, B., Francesco, B., Matteo, B., Simone, T., Giovanni, G., Andrea, C.F., and Nicola Maria, P. (2017). Spider silk reinforced by graphene or carbon nanotubes. 2D Mater. 4, 031013–031021. 31. Wang, C., Xia, K., Jian, M., Wang, H., Zhang, M., and Zhang, Y. (2017). Carbonized silk georgette as an ultrasensitive wearable strain sensor for full-range human activity monitoring. J. Mater. Chem. C 5, 7604–7611. 32. Steven, E., Saleh, W.R., Lebedev, V., Acquah, S.F.A., Laukhin, V., Alamo, R.G., and Brooks, J.S. (2013). Carbon nanotubes on a spider silk scaffold. Nat. Commun. 4, 2435–2442. 33. Zhang, W., Ye, C., Zheng, K., Zhong, J., Tang, Y., Fan, Y., Buehler, M.J., Ling, S., and Kaplan, D.L. (2018). Tensan silk-inspired hierarchical fibers for smart textile applications. ACS Nano 12, 6968–6977. 34. Ling, S., Qin, Z., Li, C., Huang, W., Kaplan, D.L., and Buehler, M.J. (2017). Polymorphic regenerated silk fibers assembled through bioinspired spinning. Nat. Commun. 8, 1387– 1398. 35. Vatankhah-Varnosfaderani, M., Daniel, W.F.M., Everhart, M.H., Pandya, A.A., Liang, H., Matyjaszewski, K., Dobrynin, A.V., and Sheiko, S.S. (2017). Mimicking biological stress-strain behaviour with synthetic elastomers. Nature 549, 497–501. 36. Lee, C., Wei, X., Kysar, J.W., and Hone, J. (2008). Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 321, 385–388. 37. Xie, S., Li, W., Pan, Z., Chang, B., and Sun, L. (2000). Mechanical and physical properties on carbon nanotube. J. Phys. Chem. Solids 61, 1153–1158. 38. Yu, M.F., Files, B.S., Arepalli, S., and Ruoff, R.S. (2000). Tensile loading of ropes of single wall carbon nanotubes and their mechanical properties. Phys. Rev. Lett. 84, 5552–5555. 39. Kenry, Yeo, J.C., and Lim, C.T. (2016). Emerging flexible and wearable physical sensing platforms for healthcare and biomedical applications. Microsyst. Nanoeng. 2, 16043–16062. 40. Muiznieks, L.D., and Keeley, F.W. (2013). Molecular assembly and mechanical properties of the extracellular matrix: a fibrous protein perspective. Biochim. Biophys. Acta 1832, 866–875.
41. Wenger, M.P.E., Bozec, L., Horton, M.A., and Mesquida, P. (2007). Mechanical properties of collagen fibrils. Biophys. J. 93, 1255–1263. 42. Jeon, J., Lee, H.B.R., and Bao, Z. (2013). Flexible wireless temperature sensors based on Ni microparticle-filled binary polymer composites. Adv. Mater. 25, 850–855. 43. Tomoyuki, Y., Yusuke, I., Yuki, T., Jonathan, R., Martin, K., Taylor, W., Kejia, Y., Kunihiko, M., Tomohiro, M., and Masaki, S. (2015). Ultraflexible, large-area, physiological temperature sensors for multipoint measurements. Proc. Natl. Acad. Sci. U S A 112, 14533–14538. 44. Gao, L., Zhang, Y., Malyarchuk, V., Jia, L., Jang, K.-I., Chad Webb, R., Fu, H., Shi, Y., Zhou, G., Shi, L., et al. (2014). Epidermal photonic devices for quantitative imaging of temperature and thermal transport characteristics of the skin. Nat. Commun. 5, 4938. 45. Trung, T.Q., Ramasundaram, S., Hwang, B.U., and Lee, N.E. (2016). An all-elastomeric transparent and stretchable temperature sensor for body-attachable wearable electronics. Adv. Mater. 28, 502–509. 46. Wang, L., Zhao, B., Wu, J., Chen, C., and Zhou, K. (2018). Experimental and numerical investigation on mechanical behaviors of woven fabric composites under off-axial loading. Int. J. Mech. Sci. 141, 157–167. 47. Chen, J., Chen, W., Zhou, H., Zhao, B., Wang, M., Sun, W., and He, S. (2018). Fracture failure analysis and bias tearing strength criterion for a laminated fabric. J. Ind. Text. 47, 1496–1527. 48. Yu, M.F., Lourie, O., Dyer, M.J., Moloni, K., Kelly, T.F., and Ruoff, R.S. (2000). Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load. Science 287, 637–640. 49. Lu, X., and Chen, Z. (2005). Curved piconjugation, aromaticity, and the related chemistry of small fullerenes (
Matter 1, 1–15, December 4, 2019
15