Ultra-stretchable Multilayered Fiber with a Hollow-Monolith Structure

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Applications of Polymer, Composite, and Coating Materials

Ultra-stretchable Multilayered Fiber with a HollowMonolith Structure for High Performance Strain Sensor Jiachen Gao, Xiaozheng Wang, Wei Zhai, Hu Liu, Guoqiang Zheng, Kun Dai, Liwei Mi, Chuntai Liu, and Changyu Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11527 • Publication Date (Web): 18 Sep 2018 Downloaded from http://pubs.acs.org on September 18, 2018

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ACS Applied Materials & Interfaces

Ultra-stretchable Multilayered Fiber with a Hollow-Monolith Structure for High Performance Strain Sensor

Jiachen Gao, 1 Xiaozheng Wang, 1 Wei Zhai, 1 Hu Liu, 1 Guoqiang Zheng, 1 Kun Dai,* 1 Liwei Mi, 2 Chuntai Liu, 1 Changyu Shen 1

1

School of Materials Science and Engineering, The Key Laboratory of Material Processing and Mold of Ministry of Education, Zhengzhou University, Zhengzhou, 450001, P. R. China

2

Center for Advanced Materials Research, School of Materials and Chemical Engineering, Zhongyuan University of Technology, Zhengzhou, 450007, P. R. China

*E-mail: [email protected]

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ABSTRACT: As a crucial component of data terminal acquisition devices, flexible strain sensor has shown promising applications in numerous fields, such as healthcare, bodynet, intelligent traffic system and robotic system. For stretchable strain sensor, it remains a giant challenge to realize a fine balance of wide detection range and high sensitivity. Here, an electrically conductive carbon nanotubes (CNTs)/ thermoplastic polyurethane (TPU) fiber with a multilayered, hollow, and monolith structure, accompanying with high stretchability (up to 476% strain) and low density (about 0.46 g/cm3) is fabricated through a facile coaxial wet-spun assembly strategy. The as-prepared fibers with a designed independent sensitive zone and flexible supporting zone possess an ultralow percolation threshold (0.17 wt.%) and tunable size and structure. This structure endows the fiber with a good integration of adequate flexibility, suitable strength and high elongation at break for wearable electronics. The fiber, which is then assembled as a strain sensor, realizes the perfect combination of wide sensing range (>350% strain), high sensitivity (gauge factor (GF) =166.7 at 350% strain) and excellent working durability (>10,000 cycles). Our sensor could also detect small compressing deformations (0.35 %N-1 at 0.025~50 N) by capturing the resistance change of the fiber with superior stability. The highly stretchable, lightweight and multilayered fiber with the designed hollow-monolith structure provides a new route for the preparation of high performance wearable electronics.

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KEYWORDS: Strain sensor, fiber, Hollow, Monolith, wearable electronics, Nanocomposite

1. INTRODUCTION With the coming of the era of big data, high-performance flexible strain sensors have been urgently required in numerous data terminal acquisition fields, such as healthcare, wearable electronics, intelligent traffic system and robotic system.1-6 Strain sensors are based on various working mechanisms, including transistor,7 capacitive,8 piezoelectric,9 and piezoresistive effects,1 etc. Among them, flexible strain sensors by piezoresistive effect have attracted extensive research interests because of their large stretchability, tunable mechanical and electrical properties, and low energy consumption, etc. Although great progress has been achieved in this field, most of the reported flexible strain sensors still present two constraints: I) low sensitivity and II) narrow detection range, which undoubtedly limits their detection for large-scale physical signals.1,

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For instance,

Yamada et al. developed a stretchable strain sensors using thin films of aligned single-walled carbon nanotubes (SWCNTs).6 When stretched, gaps and islands were generated gradually and SWCNTs bundles could bridge the gaps in the film. It just likes a peeled string cheese. This stretchable strain sensor is capable of measuring strain as large as 280%, while with a gauge factor (GF, that reflects the sensitivity of strain sensors by the slope of the relative change of the electrical signal vs. applied strain) of only 0.82. It 3



is not conducive for this strain sensors with such a low sensitivity to capture a subtle deformation. For a highly sensitive strain sensor, the detection range always tends to be narrowed. Kang et al. developed an ultrahigh-sensitivity strain sensor with a GF up to 2000 based on nanoscale crack junctions inspired by spider, but the strain detection range is 0-2%.4 Strain ranges of human skin on knee, elbow and knuckle are up to 49%, 60% and 80%,2 respectively, it is obvious that the deformation exceeds the above strain detecting ranges of 2%. Flexible strain sensors with high sensitivity always need considerable structural deformations of conductive network when exposed to stress stimulus; whereas the sensors with wide sensing range should be morphologically intact upon large deformation to keep efficient electrons transport through conductive channels. Therefore, to our knowledge, it remains giant challenges to realize the effective combination of high sensitivity and wide detection range for a satisfactory flexible strain sensor. Many studies have shown that novel and rational microstructural design of flexible strain sensors is one of the most effective strategies to solve this dilemma.1-3, 11-13 Wang et al. designed a wearable strain sensor by using a carbonized silk georgette, which has a typical woven structure of alternating twisted yarns along both warp and weft directions.3 The carbonized textile was then encapsulated in polydimethylsiloxane (PDMS) as a wearable sensor, exhibiting good strain sensing performances (GF = 29.7 within 40% strain and 173.0 within 60-100% strain). Nevertheless, the carbonization will deteriorate the stretchability and flexibility to a certain extent, and it is bad for the interfacial 4


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combination between silk georgette and PDMS. Liu et al. prepared a strain sensor based on a fish scale-like microstructure, they obtained a high sensitivity (GF ranges from 16.2 to 150) and wide strain detection range (82%) by tuning the microstructure change in overlapping areas of neighboring graphene layers during stretching.5 Cheng et al. prepared a graphene-based strain sensor with a compression spring architecture, achieving the combination of high sensitivity (10 within 1% strain) and broad strain detecting range (100%), which is attributed to the change of winding angle and gap number generated in the fiber.1 Duan et al. reported an extrusion-processed carbon black -filled stretchable sensors based on ternary composites, which comprise of thermoplastic polyurethane (TPU) and olefin block copolymer as matrix. The stretchable sensors obtained a good balance between sensitivity and strain sensing range (GF of 9.1 at the strain of 0-4% and 2.3 × 103 at 190% strain) by constructing a brittle conductive network.14 On the other hand, flexible sensors with unique microstructure, such as the bi-sheath buckled structure,2 the layer-by-layer structure,11 the bionic hierarchical structures12 and crack structure,13 have been designed for realizing an excellent strain detecting performance, but these methods can still not overcome the huge challenge completely, and some methods involve the complication in fabrication process with low cost-effective and difficulty in scale production. It should be stressed that the structural engineering vs. design of the manufacturing conditions is also very important for the development of a high performance flexible strain sensor.14 In order to obtain a high-performance strain sensor, the multilayered structure and 5



hollow-monolith structure are considered to be constructed in microstructural design.15-18 The multilayered structure, such as the sheath-core structure, integrating various functions in different layers, endowing the materials with an ability of reacting to multiple stimulations simultaneously, has attracted extensive attention of researchers in recent years. Wang et al. successfully integrated biosensors, super-capacitors and strain gauges into conducting fibers with sheath-core structure to realize the purpose of the versatility for application.15 The porous structural materials, such as hollow-sphere,16 sponges17 and monolith19 always has the characteristics of light weight, outstanding mechanical flexibility and high compressibility. For example, Liu et al. fabricated a porous pressure sensor based on graphene/ TPU with a density of approximately 0.11 g/cm3 and a compression strain up to 90%.18 Currently, most of the flexible strain sensors are designed in the form of thin film and solid fiber. Here, fiber-shaped strain sensors with unique one-dimensional structure, which could be weaved into clothing discretionarily owing to their comfortability to the human bodies, showed an enormous advantage for application as flexible electronics,20 and it is easy to realize mass production for fiber-shaped strain sensors. In the present paper, electrically conductive fibers with a novel and tunable multilayer-hollow-monolith structure, containing a flexible supporting zone and sensitive zone, are fabricated through a facile coaxial wet-spun assembly strategy. The sensitive zone in the outer region is composed of carbon nanotubes (CNTs)/TPU porous monolith, and the flexible supporting zone in the inner is pure TPU monolith with a hollow 6


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structure in the core of the fiber. The morphology, the mechanical properties and conductive properties of the fiber were studied in detail. The results show that our fiber has tunable structure and a good integration of adequate flexibility, suitable strength and high elongation at break. These conductive fibers were then assembled as flexible strain sensors to realize the conjunction between high sensitivity and wide detection range. Besides, the fibers exhibit excellent pressure-sensitive properties owing to the novel multilayer-hollow-monolith structure. Given the outstanding performances as mentioned above, our fiber is expected to be a good candidate for high performance flexible electronics in data terminal acquisition fields, such as healthcare, wearable electronics, intelligent traffic system and robotic system.

2. EXPERIMENTAL SECTION 2.1 Materials and Chemicals. CNTs (model TNIM2, 151126) with the diameter between 8 and 15 nm, length of 30~50 µm and purity over 95 wt.% were provided by Chengdu Organic Chemicals Co. Ltd. TPU (Elastollan 1185A, ρ = 1.12 g/cm-3) was obtained from BASF Co. Ltd. Deionized water was bought from water purification system, Shanghai Merck Millipore Co., Ltd. DMF (≥99.5%, analytical reagent) was supplied by Tianjin Fuyu Fine Chemical Co., Ltd., China and was used as received without any further treatment.

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2.2 Preparation of Spinning Dope. For spinning dope towards the outer layer, the required amount of CNTs was mixed with 20 mL DMF and treated under ultrasonication (KQ-600DE, 540 W, Kunshan Ultrasonic instrument Co., Ltd.) for 10 min to disperse the CNTs homogenously. 5.0 g TPU was added into the mixture under vigorously stirring for 4 h. The mixture was treated under ultrasonication for another 1 h to disperse CNTs in TPU matrix uniformly. The mixture is then subjected to the secondary stirring for 30 min. For the preparation of spinning dope to the inner layer, 5.0 g TPU was dissolved in 20 mL DMF by vigorously stirring for 4 h. 2.3 Preparation of Coaxial Wet-Spun Fibers. The fibers were fabricated by a simple coaxial wet-spun assembly strategy. First, the outer layer spinning dope was transferred to an injection syringe connected with the outer channel of coaxial spinneret and the inner layer spinning dope was transferred to an injection syringe connected with the inner channel of the coaxial spinneret (the diameters of the needle at the outer layer and inner layer are 1.64 and 0.52 mm, respectively). The typical extruded velocities of inner and outer channels were set at 1.09 and 0.38 ml/min, respectively. Deionized water was used for the coagulating bath (temperature at 24 ± 3 °C), which was friendly with the environment. The two spinning dopes were injected into the coagulating bath simultaneously (the production rate of the fibers was about 27.9 cm/min) and stayed 30 min for fully coagulating. Here, the layer of fibers formed by the outer spinning dope was defined as the outer layer, and the layer of fibers formed by inner spinning dope was

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defined as the inner layer. The samples were then placed in a draught cupboard for 24 h at room temperature. Finally, the fibers were moved to an oven (DHG-9076A, Shanghai Jinghong laboratory equipment Co. Ltd., China) to evaporate the solvent completely at 80 °C for 12 h in air. 2.4 Preparation of Pressure Sensor. The fibers of TPU-8CNT@TPU (5 cm in length) were fixed on the PMMA substrate (70 × 70 × 2 mm3 in dimension) as the pressure sensor (Figure S1 and S2) for pressure sensing performance tests. 2.5 Mounting Electrodes. The conductive copper tapes were mounted on the ends of the fibers as electrodes with the help of silver paste for strain sensing tests in tension. The copper wire electrodes were mounted on the ends of the fibers as electrodes with the help of silver paste for pressure sensing tests. 2.6 Characterization. SEM (Zeiss EVO 18) was utilized to characterize the microscopic morphology of the samples. The outer and inner diameters (D and d, mm) of the samples were measured by using polarized optical microscope (POM, Olympus BX 61) images. The fibrous microstructures were shot on a POM (Leica DM400911). The stress vs. strain properties were tested on a universal tensile testing machine with a 100 N load cell (UTM2203, Shenzhen Suns Technology Stock Co. Ltd., China). The electrical resistances (R, Ω) were measured by a digit precision multi-meter (DMM4050, Tektronix, Inc.). The I-V curves were measured by an electrochemical workstation (RST5000, Suzhou Risetest Electronic Co., Ltd., China). The strain sensing performance was studied 9



by using the universal tensile testing machine coupled with the digit precision multi-meter and the sensing data was recorded on-line by a computer.

3. RESULTS AND DISCUSSION

Figure 1. Fabrication process and microstructure of the multilayer-hollow-monolith fiber.

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(a) Schematic illustration of the fabrication process of the fiber. (b) SEM image of the cross section of the TPU-8CNT@TPU, the inset shows the profile. SEM images of the cross section of TPU-8CNT@TPU located in (c) the outer layer, (d) the inner layer and (e) the interface between the outer and inner layer. (f) Schematic illustration of the structure of TPU-8CNT@TPU. (g) SEM image of the surface of TPU-8CNT@TPU, the inset is the corresponding higher magnification image. (h) Schematic illustration of the formation process of the hollow-monolith structure of the multilayered fiber. As shown in Figure 1a, a coaxial wet-spun assembly strategy was designed to fabricate fibers with the novel multilayer-hollow-monolith structure. Spinning dope of TPU/N, N-dimethylformamide (DMF) in inner channel and spinning dope of CNTs/TPU/DMF in outer channel were simultaneously injected into the coagulation bath (deionized water). The fibers were rapidly formed in the deionized water through the non-solvent induced phase separation (NIPS, a polymer is firstly dissolved in a solvent A to form a homogeneous solution, then a non-solvent (with similar solubility to the solvent A) was added into the solution as extractant to extract the solvent A, developing a two-phase structural mixture with the polymer as the continuous phase and the solvent A as the dispersed phase, finally the solvent A is removed to obtain a material with a certain porous structure21), then fibers were removed from the coagulation bath after 30 min and dried completely. The obtained fibers are composed of a black conductive outer layer doped with CNTs as sensitive zone and a white insulating inner layer constructed by pure TPU as the flexible supporting zone (Figure 1f and S2, the thickness ratio of outer layer to inner layer ranges from 1:3.4 to 1:6). Here，the fibers are denoted as TPU-xCNT@TPU,

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where x represents the mass loading level of CNTs in the outer layer. For example, TPU-8CNT@TPU represents the content of 8 wt.% CNTs in the outer layer of the fiber. Figure 1b-g illustrates the novel multilayer-hollow-monolith microstructure of TPU-8CNT@TPU. A cylinder-shaped hollow structure is observed obviously (Figure 1b), and both the outer and inner layers are filled with round or oval cell-like pores (the monolith structure, Figure 1c-d). The pores in the outer layer (2.96 ± 1.23 µm in diameter, Figure S3a) are much smaller in size than that in the inner layer (13.07 ± 3.40 µm in diameter，Figure S3b) because of the introduction of CNTs. Interestingly, a certain amount of holes with diameter of 4.47 ± 2.64 µm appeared on the cell walls of the outer and inner pores (Figure 1c-e). The surface of TPU-8CNT@TPU is rather rough (Figure 1g) and also has numerous tiny holes (0.87 ± 0.38 µm in diameter, Figure S3c). These results indicate that an interesting TPU skeleton with hierarchical monolith structure is constructed successfully in the fiber. In Figure 1b-e, it should be noted that the TPU-8CNT@TPU contains an inner layer (flexible supporting zone) composed of pure TPU skeleton and a hollow core, and the outer layer (sensitive zone) is the CNTs/TPU monolith. The inner and outer layer exhibit an excellent interfacial compatibility owing to the use of the same polymer TPU, which is assuredly beneficial to the mechanical properties (Figure 1e). Here, although the fibers with individual sheath-core,15 hollow22 or monolith structure23 have repeatedly been reported, the fibers which integrate a novel multi-layered, hollow, and monolith structure, namely multilayer-hollow-monolith structure, have rarely been designed in previous literatures. Here, TPU-0CNT@TPU was 12


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prepared for comparison, and it shows that this controlled fiber has no obvious outer and inner layer (Figure S2 and S4a1). It can be seen that many larger finger-like macro-voids distribute along the fiber radial direction (Figure S4a1), while few holes (diameter of 0.18 ± 0.069 µm, Figure S3e) are located at the surface of TPU-0CNT@TPU. This sample is obviously unable to meet our requirements for integrating different functional zones. The formation mechanism of the multilayer-hollow-monolith structure of the fiber is then investigated. When the spinning dope enters into the coagulation bath, the surface immediately generates a dense rigid layer (Figure S4a2, b2), accompanying with the dynamic interaction that the deionized water as coagulation bath diffuses into the fibers and DMF as solvent seeps from the fibers (the water is the good-solvent to DMF). Meanwhile, a solidification boundary develops between the solidified phase and the non-solidified phase of the fibers, moving from the outside to the inside of the fiber.24 The solidified phase of the fibers forms a solid shell and the tensile stress from the solidification shrinkage causes the non-solidified phase of the fiber close to the solid shell to form a hole structure (Figure 1h).22 The presence of CNTs will affect the compactness of TPU matrix in the outer layer, resulting in a faster solidification rate and more holes in the surface (Figure S4 (c1-c4), (d1-d4)). This also causes the fact that the outer layer of the fiber quickly forms a harder shell, promoting the hollow structure (Figure S4 a2, b2). The two-phase structure that the polymer as the continuous phase and deionized water as the dispersed phase develops in the solidified portion. The deionized water droplets migrate to the polymer continuous phase of the fibers and gradually develop into big droplets. 13



The pores and holes emerge at the location where the deionized water droplets are detained in the fibers after solidification, the monolith and hollow structure is thus created in the fiber (Figure 1h). The finger-like macro-voids generated in TPU-0CNT/TPU are attributed to the immediate convective type solvent exchange in solidification process.25 However, the presence of CNTs in the outer layer inhibits the convective type solvent exchange and suppresses the deionized water intrusion into the inner layer to a certain extent, which confines the formation of the macro-void.25

Figure 2. Size, mechanical properties, electrical properties and tensile sensing properties of the fiber. (a) Size of the fibers with different CNTs loadings from POM images. (b) Mechanical properties of the fibers as a function of CNTs loading. (c) Digital photograph 14


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of a pipa knot weaved by TPU-8CNT@TPU on a flower. (d) Volume conductivity as a function of CNTs loading in the fiber. The insert shows the log conductivity vs. log (m-mc). (e) SEM image for the CNTs network in the outer layer. (f) Change of R/R0 as a function of tension strain, the inset shows the changes of GF with different CNTs loadings in tension at a rate of 10 mm/min. (g) Comparison of GF and maximum sensing strain of TPU-8CNT@TPU with those of the counterparts reported in literature. (h) Optical images of sensitive zone of TPU-8CNT@TPU at different strains. Figure 2a shows the change of outer diameter (D) and the inner (d) of the fibers (Figure S2d) with different CNTs loadings. With increasing the CNTs loading, D and d of the fibers increased slowly from 2.02 ± 0.059 and 0.59 ± 0.06 mm of TPU-0CNT@TPU to 2.35 ± 0.077 and 0.84 ± 0.067 mm of TPU-6CNT@TPU, which are increased by 16.3% and 42.4%, respectively. This is because the addition of CNTs in the outer layer severely destroys the compactness of TPU matrix (Figure S4 (c1-c4), (d1-d4)), resulting in the faster formation of a harder outer shell and further promoting a larger size hollow structure (Figure S4 a2, b2). When the CNTs loading is beyond 6 wt.% (TPU-6CNT@TPU), both D and d level off. The structures of the fibers, can also be controlled by changing the extrusion rate ratio of the outer layer to the inner layer (r), which opens a fancy route to tune the microstructure (Figure S5). The fibers with varying r were fabricated and the results showed that the hollow structures of fibers disappeared when the r was 0 (Figure S5a), infinity (Figure S5b) or 100: 1 (Figure S5c). It was impossible to prepare high quality wet-spun fibers when the r reduced to 0.05: 1 (Figure S5d). The effect of the r on the fibrous structure is related to the solidification rate and

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the die swell effect of the fiber affected by the shear rate in the spinning channel. To construct the novel multilayer-hollow-monolith structure, the outer and inner layer extrusion rate ratio should be maintained within a certain range of 0.05:1 ~ 100:1 (Supporting Information 1 for details). Figures 2b displays the mechanical properties of the fibers with different CNTs loadings (typical stress-strain curves in Figure S6). The modulus of TPU-0CNT@TPU is only 1.44 ± 0.13 MPa, which is lower than the previously reported wet-spun pure PU fibers26 and commercially available polyurethane yarns27, showing the monolith characteristic. Interestingly, the modulus of the TPU-8CNT@TPU fiber is only 2.92 ± 0.45 MPa. Such a low modulus value of TPU-8CNT@TPU indicates that our fiber have satisfying softness and flexibility.27 Here, the strength of TPU-8CNT@TPU is significantly improved by 52.5%, compared with the TPU-0CNT@TPU. The TPU-4CNT@TPU shows the maximum elongation at break, up to 476 ± 32%. Clearly, the improvement of the strength and elongation at break is related to the strong interactions between polymer chains and CNTs in the outer layer (Figure S7), which is conducive to the CNTs dispersion and stress transfer.28-29 With further increasing the CNTs loading, the strength and elongation at break of the fibers deteriorate gradually. High loading of CNTs might lead to structure defects and more serious CNTs entanglement which limit the movement of TPU molecular chains when the fibers suffer stress loading, resulting in the reduction in fracture strain.29 Based on the above-mentioned results, our fibers realize an interesting integration of an adequate 16


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flexibility (low modulus), suitable strength and high elongation at break for flexible electronics. In addition, it is worth mentioning that the fibers show a rather low apparent density, such as 0.34 ± 0.0052 g/cm-3 for TPU-0CNT@TPU and 0.46 ± 0.013 g/cm-3 for TPU-8CNT@TPU (Table S1), demonstrating the light weight characteristic of the fibers based on the multilayer-hollow-monolith structure. The aforementioned features are shown by a pipa knot of TPU-8CNT@TPU in Figure 2c. Figure 2d shows the volume conductivity of the fibers as a function of CNTs content in the fiber (calculated by thermogravimetric analysis test, Figure S8 and Table S2). A classical percolation model is used to fit the data, which is defined as follows, = (m-mc)t, where σ represents volume conductivity of the fibers at a given CNTs content in the fibers (σ = L/RS, where L, R and S are the length, resistance and cross-sectional area of the fibers, respectively); m is mass content of CNTs; mc represents the percolation threshold of the fibers; t represents a critical resistance exponent related to the forming mechanism of the conductive network.29-30 A very low percolation threshold of 0.17 wt.% is obtained from the fitting curve, lower than the reported 1.6 wt.% for the SWCNTs /TPU composite fibers,26 0.98 wt.% by melt mixing and 0.39 wt.% by solution mixing of the

multi-walled

carbon

nanotubes

(MWCNTs)/styrene-butadiene-styrene

block

copolymer (SBS) composite films,31 which is attributed to our novel structure design. Our as-prepared flexible fibers with the multilayer-hollow-monolith structure obviously contain a designed flexible supporting zone and sensitive zone. As shown in Figure 2e, for the sensitive zone, the TPU matrix constructs a three-dimensional porous skeleton, in 17



which the percolation networks of CNTs are interconnected with each other (Figure S9). Interestingly, the sensitive zone only occupies approximate 23.3 vol.% of the whole fiber (Figure S2e), which greatly reduces the percolation threshold of composite fibers. Meanwhile, the fibers exhibit stable volt-ampere characteristics (Figure S10). The resistance of TPU-8CNT@TPU retains stable when the fibers suffer from ultrasonication and boiling in deionized water (100 °C) even for 60 min, showing outstanding durability (Figure S11). Figure 2f shows the dependence of the relative resistance (defined as R/R0, where R0 represents the initial resistance and R is the measured resistance) of the fibers with different CNTs loadings as a function of tensile strain. It should be noted that the R/R0 of all the fibers increases exponentially with the strain. In order to understand the relationship between the novel structure and strain-induced response, we traced the evolution of the microstructures at sensitive zone in tension with a maximum strain of 300% by the polarized optical microscope (POM) image. It shows that the TPU porous skeleton of the fibers at sensitive zone is elongated along the tensile direction, and shrinks in the direction perpendicular to the tensile direction (Figure 2h, the shrinkage strain can be theoretically calculated by the εx = -νεy, where εx is the transverse strain, εy is the axial strain, and ν is the Poisson's ratio of the fiber; when the fiber is stretched to εy along the tensile direction, -νεy will produce in the cross-direction of the fiber, that is, the wall thickness of the fiber shrinkages by νεy). This structural development damages the CNTs conductive paths in the porous skeleton along the tensile direction, while some new 18


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conductive paths are built along the direction perpendicular to the tension. The destruction and reconstruction of the conductive paths compete with each other during the whole tensile process. The change of the CNTs conductive network is weak under a small strain. When the strain increases further, the destruction of conduction paths along the tensile direction dominates in the competition. Meanwhile, a part of TPU porous skeleton is torn and some defects gradually evolved into larger size cracks and voids due to the increased strain, which further damages the CNTs conductive network, leading to the sharp rise of the resistance. Thus, the R/R0 of the fibers increases exponentially with strain. Interestingly, when the fibers are released to the initial position, the TPU porous skeleton of the fibers returns to the initial state. Here, GF (defined as (∆R/R0)/εT, where εT is the tensile strain) is utilized to evaluate the tensile response sensitivity. The fiber with low CNTs loading exhibits high sensitivity (Inset of Figure 2f). This is attributed to the fact that the lower CNTs loading leads to in-complete CNTs network at sensitive zone of the fibers, which is easier to be damaged under tensile strain. The sensitivity of all fibers increases monotonously towards the tensile strain, this is because the TPU porous skeleton at sensitive zone of the fiber suffers a greater deformation under a large strain, which results in a stronger damage of the CNTs conductive network. Although TPU-4CNT@TPU has excellent mechanical properties, TPU-8CNT@TPU shows an extremely wide strain detection range of over 350%, and a very high GF (2.51 at a low strain of 10% and 166.7 at 350% strain). Therefore, the TPU-8CNT@TPU sample was chosen as the testing samples in our subsequent experiment. Figure 2g 19



exhibits the main performance criteria of recently reported typical strain sensors as comparison (Table S3),1, 26, 32-41 demonstrating the superior performance of our fibers with the integration of wide detection range and high sensitivity.

Figure 3. Cyclic tensile sensing properties of the fibers. (a) Change in R/R0 of TPU-8CNT@TPU under 100% strain at a rate of 50 mm/min during 1-5 and 21-25 cycles. (b) Change in R/R0 of TPU-8CNT@TPU with different strains at a rate of 50 mm/min during 21-25 cycles. (c) The long-term working stability test of TPU-8CNT@TPU over 10,000 loading-unloading cycles under 100% strain, the insert shows the sensing curves in 4995-5000 cycles. (d) Change in R/R0 of TPU-8CNT@TPU under the 100% strain at a series of increasing strain rates of 10, 30, 50, 70 and 90 mm/min, respectively; before that, TPU-8CNT@TPU suffers a 50-cycle pre-straining under 100% strain to eliminate the influence of the initial drop of the resistance. 20


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The response behaviors of TPU-8CNT@TPU under dynamic loading-unloading cycles are investigated to evaluate the sensing characteristic. As shown in Figure 3a, the R/R0 increases gradually with the increasing strain. A shoulder peak appears during the unloading process, this phenomenon is attributed to the competition between the destruction and reconstruction of CNTs conductive networks in the fiber.29 The amplitude of the response peaks decreases a little in the first several cycles and turns to be stable quickly (Figure S12), this is related to the structure arrangement between CNTs and TPU,31 the monolith structure and the entanglement structure evolution of the CNTs in the skeleton. An obvious hysteresis of the change of resistance was observed in the first loading-unloading cycle, which is mainly related to the viscoelasticity of TPU matrix and the unstable conductive network (Figure S13).42 Nevertheless, the resistance hysteresis gets weaker in the following cycles. It is generally considered that the resistance hysteresis of the sensor is closely related to the mechanical hysteresis.43 When stretched with different strains of 20%, 100%, 200% and 300% at the same strain rate of 50 mm/min, as shown in Figure 3b, TPU-8CNT@TPU exhibits similar response curves, confirming its reliability under cyclic loading-unloading in a wide detection range (cyclic response test in the strain of 2% and strain of 350% are further displayed in Figure S14a). With the help of the real-time high-resolution current-time curve (I-t), the response time of the fibers is calculated to be 167 ms (detailed in Supporting Information 2 and Figure S15), faster than reported CNTs-Ecoflex (332 ms)44 and Ag nanowires (AgNWs)-PDMS based strain sensors (200 ms)45, undoubtedly facilitating the accurate detection of 21



complex mechanical stimulations. TPU-8CNT@TPU also show excellent reproducibility in more than 10,000 loading-unloading tests under the tensile strain of 100% (Figure 3c), demonstrating a desirable long-term working life.

Figure 4. Pressure sensing properties of the fibers and theoretical analysis of sensing mechanism. (a) Digital photograph shows a fiber pressure sensor in test and a pressure plate (20 mm × 20 mm × 2 mm) is fixed to the upper press. (b) Changes in R/R0 and 22


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loading force for TPU-8CNT@TPU in loading. The insets are the enlarged image in stage 1, and the R/R0 of TPU-8CNT@TPU as a function of the loading force in stage 2 (experimental data (blue dots), theoretical data (red solid line)). (c) Change in R/R0 of TPU-8CNT@TPU under a 20 N loading force at a compress rate of 10 mm/min during 1-5 and 21-25 cycles. (d) Change in R/R0 of TPU-8CNT@TPU at a series of increasing loading forces from 3 to 40 N at a compress rate of 10 mm/min. The dependence of R/R0 on the deformation of TPU-8CNT@TPU under (e) tension and (f) pressure: experimental data (blue dots), theoretical data (red solid line), εc is the compressive strain. The effect of strain rate on strain-response behaviors of the flexible fibers is then investigated to fulfill the complex application in various conditions. As displayed in Figure 3d, it can be clearly seen that the response peaks rise gradually as the strain rate increases, which is attributed to a high mobility of molecular chains in TPU porous skeleton as a fast rate, as a result, CNTs conductive networks are violently damaged and have a shorter time to recover.29 The cyclic tensile tests with different strain of 20%, 100%, 200% and 300% at the same frequency of 0.014 Hz are also implemented (Figure S14b), exhibiting an excellent strain-sensing ability of TPU-8CNT@TPU under various detecting patterns. It is well known that the porous structure is often designed for preparing pressure sensing materials.18, 46 Except the excellent sensing performance towards the tensile stress, the flexible fiber with the novel multilayer-hollow-monolith structure, is expected to be a candidate for future pressure sensors. As shown in Figure 4a-b, it is observed that the dependence of R/R0-loading force of TPU-8CNT@TPU under loading force could be divided into two stages: the R/R0 decreases with increasing the loading forces from initial 23



0 to 0.025 N (stage 1); while the applied loading force reaches 0.025 N (stage 2), the R/R0 increases drastically with increasing the loading force. This is because the pressure sensing property of the fiber is generated from two competing mechanisms: I) the densification of monolith structure, which reduces the distance of adjacent CNTs and makes the CNTs network closer in the direction of loading force; II) the bending in both sides of the fiber and the increased deformation of the porous skeleton along the direction orthogonal to the loading force, which destructs the previous CNTs conductive paths and increase the average electron tunneling distance between CNTs (Figure S16).47 In stage 1, the densification of monolith structure plays a leading role in creating new conductive paths at sensitive zone. With increasing the loading force, the compression deformation causes slow increase of the gap and average electron tunneling distance among CNTs. As the applied loading force is increased to 0.025 N (stage 2), mechanism II becomes dominant, the R/R0 increases further. Specially, we obtain a good linearity of resistance-loading force dependence in stage 2 (Sp = 0.35 %N-1. SP = d(R/R0)/dF, where F is the loading force).48 Moreover, the fiber with the multilayer-hollow-monolith structure exhibits excellent repeatability under the cyclic loading force (Figure 4c) and good synchronism towards the stepwise loading forces (Figure 4d). Similar phenomena of shoulder peaks and resistance hysteresis are observed again (Figure 4c and S17). The compression response time is then calculated to be 115 ms (Supporting Information 2), which is shorter than that in tension.

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Figure 5. Real-time human motions and personal healthcare monitoring. Change in R/R0 of TPU-8CNT@TPU as strain sensors in detecting three types of human motions: (a) wrist bending, (b) knee bending, and (c) breathing; Change in R/R0 of TPU-8CNT@TPU as pressure sensors in detecting four types of human motion: (d) stepping, (e) tiptoeing, (f) jumping and (g) marching. Based on these sensing results, we found the fiber with the multilayer-hollow-monolith structure shows excellent sensitivity to both tension and compression stimulus. Interestingly, it exhibits similar response characteristics, such as the shape of response peaks and the short responsive time. Therefore, the response mechanisms of the two response patterns may have the intrinsic consistency, that is, the sensing behaviors of our

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fiber are mainly caused by the microstructure change of CNTs conductive network in the CNTs/TPU monolith of the sensitive zone. A mathematical model is then applied to explain the experimental phenomenon based on the tunneling theory. The total resistance R of the composite fibers is calculated as:49-51

R =( )( )exp() =

(1) (2)

where L is the number of particles forming one conductive pathway, N is the number of conductive pathways, h is the Plank’s constant, s is the smallest distance between adjacent conductive particles, a2 is the effective cross-section area, e is the electron charge, m is the electron mass and is the height of the potential barrier between the adjacent particles. When the tensile or compressing strain is applied, the separation s between adjacent conductive particles is altered linearly and proportionally with the applied strain ε, which can be calculated as follows: = (1 + C)

(3)

where s0 is the in initial distance between adjacent particles, and C is a constant. When the strain ε is applied, the change of the number of conductive pathways can be expressed by: =

!" ($%&'% &(% ) &*% + )

(4)

Where N0 is the initial number of conductive pathways, M, W, U, and V are constants. When the strain ε is applied, the change in R/R0 is given by: 26


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,

, = -. / 0123( − )5

(5)

The substitution of equation (3) and (4) into equation (5): ,

,

= (1 + C)0123(M + 7C) + W + U + V 5

(6)

where, E = ; . The theoretical model shows ideal agreement with our experimental data, as shown in Figure 4e-f, indicating that both the tensile and compressing sensing properties of the fibers stem from the same initial mechanism (Table S4 for fitting parameters). The fibers with the multilayer-hollow-monolith structure, possessing excellent mechanical properties, wide detection range and high sensitivity to mechanical stimulus are fabricated successfully in this paper, which makes it possible to monitor human motions and personal healthcare. When TPU-8CNT@TPU was fixed on different joints of a volunteer, such as the wrist and knee, a series of response signals were successfully captured instantaneously with the motions of the volunteer (Figure 5a-b and movie S1). Moreover, TPU-8CNT@TPU could discriminate the response signals from various joint motions. For example, the response signals for extending/flexing of the wrist and knee are significantly different (Figure 5a-b). Breathing patterns of human as important physiological signals are closely associated with pathologic patterns.52 Here, the as-prepared fiber was then connected with a ribbon to assemble a belt breathing sensor with the aim of monitoring the personal healthcare and giving timely assistance. In Figure 5c, the belt breathing sensor was worn over clothing at the chest and successfully captured a breathing pattern of the volunteer with healthy physical condition. Given the

27



excellent pressure-sensitive properties, the TPU-8CNT@TPU was also fixed on the sole of a volunteer to detect human motions, including stepping, tiptoeing, jumping and marching (Figure 5d-g and movie S2), and the corresponding response signs were all detected exactly. Most interestingly, the step number and frequency of marching of the volunteer could be calculated by analyzing the response signals (Figure 5g, 22 steps per minute forward or back).

Figure 6. Real-time traffic condition monitoring. (a) Digital photograph showing TPU-8CNT@TPU assembled into an intelligent traffic system (the length of the track is 2.3 m). (b) Online monitoring in R/R0 of TPU-8CNT@TPU when the car passes through the track. The aforementioned excellent detection ability of the fibers motivates the following investigation of their applicability as terminal data acquisition devices in the field of intelligent traffic system. In Figure 6a, TPU-8CNT@TPU was laid on a toy track to monitor vehicle travelling. When a car (34.98 g, inset of Figure 6a) passes through the track, the fiber immediately responds to the pressure from the little car (Movie S3). Based on the analysis in Figure 6b, we could deduce that the car orbits the track 5 times in 18.01 s and the speed is 2.3 km/h. These results further demonstrate the flexible and 28


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high-performance wet-spun fibers with the novel multilayer-hollow-monolith structure could be used as new-generation strain sensors in the field of intelligent traffic system in the future.

4. CONCLUSIONS In summary, a stretchable fiber with a novel and tunable multilayer-hollow-monolith structure, containing a conductive outer layer as sensitive zone and insulating inner layer as flexible supporting zone, are fabricated on a large scale through a facile coaxial wet-spun assembly strategy. The flexible supporting zone was composed of pure TPU monolith with a hollow core, and the sensitive zone is CNTs/TPU monolith. The as-prepared fibers with low density (0.46 g/cm3 for TPU-8CNT@TPU) and nice durability, possess an ultralow percolation threshold (0.17 wt.%). The novel and tunable multilayer-hollow-monolith structure endows the fibers with good mechanical properties of adequate flexibility, suitable strength and high elongation at break. The strain sensing performances of the fiber were then evaluated in detail, fine combination of wide detection range (>350% strain), high sensitivity (GF = 166.7 at 350% strain) and excellent stability (>10,000 cycles) have been realized simultaneously. Specially, the fibers could detect small compressing deformations with good linearity of resistance-loading force dependence (0.35 %N-1 at 0.025-50 N). The excellent sensing performances are attributed to the change of conductive CNTs networks at the sensitive

29



zone, caused by the structural evolution of the CNTs/TPU monolith. We also assembled the fibers into data terminal acquisition devices to monitor the human motions, personal healthcare and traffic congestion, and our fiber sensor could immediately respond to the stress/deformation stimuli with nice sensing durability. Our electrically conductive fibers with multilayer-hollow-monolith structure prepared by this simple method open a new route for the high-performance flexible strain sensor in healthcare, bodynet, and intelligent traffic system.

Associated Content Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Figures S1-S17 about the fiber morphology tuning and sensing properties are included. Tables S1-S4 about the performance parameters of the fibers and the summary of maximum detection range and GF for typical flexible strain sensors reported in recent years, etc., are included. The fiber assembled as strain sensors in real-time detecting three types of human motions: a) wrist bending, b) knee bending, and c) breathing is shown in Movie S1 (AVI); The fiber assembled as pressure sensors in detecting four types of human motion: d) stepping, e) tiptoeing, f) jumping and g) marching is shown in Movie

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S2 (AVI); The fiber assembled into an intelligent traffic system in online monitoring vehicle travelling is shown in Movie S3 (AVI).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

ORCID Kun Dai: 0000-0002-9877-8552

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support of this work by National Natural Science Foundation (Contract Number: 51773183, 51603193, 11572290, 11432003), National Natural Science Foundation of China-Henan Province Joint Funds (Contract number: U1604253).

REFERENCES (1) Cheng, Y.; Wang, R.; Sun, J.; Gao, L., A Stretchable and Highly Sensitive Graphene-Based Fiber for Sensing Tensile Strain, Bending, and Torsion. Adv. Mater. 2015, 27, 7365-7371. 31



(2) Wang, R.; Jiang, N.; Su, J.; Yin, Q.; Zhang, Y.; Liu, Z.; Lin, H.; Moura, F. A.; Yuan, N.; Roth, S.; Rome, R. S.; Ovalle-Robles, R.; Inoue, K.; Yin, S.; Fang, S.; Wang, W.; Ding, J.; Shi, L.; Baughman, R. H.; Liu, Z., A Bi-Sheath Fiber Sensor for Giant Tensile and Torsional Displacements. Adv. Funct. Mater. 2017, 27, 1702134. (3) Wang, C.; Xia, K.; Jian, M.; Wang, H.; Zhang, M.; Zhang, Y., Carbonized Silk Georgette as An Ultrasensitive Wearable Strain Sensor for Full-Range Human Activity Monitoring. J. Mater. Chem. C 2017, 5, 7604-7611. (4) Kang, D.; Pikhitsa, P. V.; Choi, Y. W.; Lee, C.; Shin, S. S.; Piao, L.; Park, B.; Suh, K. Y.; Kim, T. I.; Choi, M., Ultrasensitive Mechanical Crack-Based Sensor Inspired by the Spider Sensory System. Nature 2014, 516, 222-226. (5) Liu, Q.; Chen, J.; Li, Y.; Shi, G., High-Performance Strain Sensors with Fish-Scale-Like Graphene-Sensing Layers for Full-Range Detection of Human Motions. ACS Nano 2016, 10, 7901-7906. (6) Yamada, T.; Hayamizu, Y.; Yamamoto, Y.; Yomogida, Y.; Izadi-Najafabadi, A.; Futaba, D. N.; Hata, K., A Stretchable Carbon Nanotube Strain Sensor for Human-Motion Detection. Nat. Nanotechnol. 2011, 6, 296-301. (7) Schwartz, G.; Tee, B. C.; Mei, J.; Appleton, A. L.; Kim, D. H.; Wang, H.; Bao, Z., Flexible Polymer Transistors with High Pressure Sensitivity for Application in Electronic Skin and Health Monitoring. Nat. Commun. 2013, 4, 1859. (8) Cai, L.; Song, L.; Luan, P.; Zhang, Q.; Zhang, N.; Gao, Q.; Zhao, D.; Zhang, X.; Tu, M.; Yang, F.; Zhou, W.; Fan, Q.; Luo, J.; Zhou, W.; Ajayan, P. M.; Xie, S., 32


Page 32 of 40

Page 33 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Super-Stretchable, Transparent Carbon Nanotube-Based Capacitive Strain Sensors for Human Motion Detection. Sci. Rep. 2013, 3, 3048. (9) Zhou, J.; Gu, Y.; Fei, P.; Mai, W.; Gao, Y.; Yang, R.; Bao, G.; Wang, Z. L., Flexible Piezotronic Strain Sensor. Nano. Lett. 2008, 8, 3035-3040. (10) Amjadi, M.; Kyung, K.-U.; Park, I.; Sitti, M., Stretchable, Skin-Mountable, and Wearable Strain Sensors and Their Potential Applications: A Review. Adv. Funct. Mater. 2016, 26, 1678-1698. (11) Wu, X.; Han, Y.; Zhang, X.; Lu, C., Highly Sensitive, Stretchable, and Wash-Durable Strain Sensor Based on Ultrathin Conductive Layer@Polyurethane Yarn for Tiny Motion Monitoring. ACS Appl. Mater. Interfaces 2016, 8, 9936-9945. (12) Jian, M.; Xia, K.; Wang, Q.; Yin, Z.; Wang, H.; Wang, C.; Xie, H.; Zhang, M.; Zhang, Y., Flexible and Highly Sensitive Pressure Sensors Based on Bionic Hierarchical Structures. Adv. Funct. Mater. 2017, 27, 1606066. (13) Park, B.; Kim, J.; Kang, D.; Jeong, C.; Kim, K. S.; Kim, J. U.; Yoo, P. J.; Kim, T. I., Dramatically Enhanced Mechanosensitivity and Signal-to-Noise Ratio of Nanoscale Crack-Based Sensors: Effect of Crack Depth. Adv. Mater. 2016, 28, 8130-8137. (14) Duan, L.; D'Hooge D, R.; Spoerk, M.; Cornillie, P.; Cardon, L., Facile and Low-Cost Route for Sensitive Stretchable Sensors by Controlling Kinetic and Thermodynamic Conductive Network Regulating Strategies. ACS Appl. Mater. Interfaces 2018, 10, 22678-22691. (15) Wang, H.; Liu, Z.; Ding, J.; Lepro, X.; Fang, S.; Jiang, N.; Yuan, N.; Wang, R.; Yin, 33



Q.; Lv, W.; Liu, Z.; Zhang, M.; Ovalle-Robles, R.; Inoue, K.; Yin, S.; Baughman, R. H., Downsized Sheath-Core Conducting Fibers for Weavable Superelastic Wires, Biosensors, Supercapacitors, and Strain Sensors. Adv. Mater. 2016, 28, 4998-5007. (16) Pan, L.; Chortos, A.; Yu, G.; Wang, Y.; Isaacson, S.; Allen, R.; Shi, Y.; Dauskardt, R.; Bao, Z., An Ultra-Sensitive Resistive Pressure Sensor Based on Hollow-Sphere Microstructure Induced Elasticity in Conducting Polymer Film. Nat. Commun. 2014, 5, 3002. (17) Huang, Y.; He, X.; Gao, L.; Wang, Y.; Liu, C.; Liu, P., Pressure-Sensitive Carbon Black/Graphene Nanoplatelets-Silicone Rubber Hybrid Conductive Composites Based on A Three-Dimensional Polydopamine-Modified Polyurethane Sponge. J. Mater. Sci.: Mater. Electron. 2017, 28, 9495-9504. (18) Liu, H.; Dong, M.; Huang, W.; Gao, J.; Dai, K.; Guo, J.; Zheng, G.; Liu, C.; Shen, C.; Guo, Z., Lightweight Conductive Graphene/Thermoplastic Polyurethane Foams with Ultrahigh Compressibility for Piezoresistive Sensing. J. Mater. Chem. C 2017, 5, 73-83. (19) Svec, F., Porous Polymer Monoliths: Amazingly Wide Variety of Techniques Enabling Their Preparation. J. Chromatogr. A 2010, 1217, 902-924. (20) Wei, Y.; Chen, S.; Yuan, X.; Wang, P.; Liu, L., Multiscale Wrinkled Microstructures for Piezoresistive Fibers. Adv. Funct. Mater. 2016, 26, 5078-5085. (21) Li, X.; Han, Y., Tunable Wavelength Antireflective Film by Non-Solvent-Induced Phase Separation of Amphiphilic Block Copolymer Micelle Solution. J. Mater. Chem. 2011, 21, 18024-18033. 34


Page 34 of 40

Page 35 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Jia, Z.; Lu, C.; Liu, Y.; Zhou, P.; Wang, L., Lignin/Polyacrylonitrile Composite Hollow Fibers Prepared by Wet-Spinning Method. ACS Sustainable Chem. Eng. 2016, 4, 2838-2842. (23) Sun, Z.; Fan, C.; Tang, X.; Zhao, J.; Song, Y.; Shao, Z.; Xu, L., Characterization and Antibacterial Properties of Porous Fibers Containing Silver Ions. Appl. Surf. Sci. 2016, 387, 828-838. (24) Paul, D. R., Diffusion During the Coagulation Step of Wet‐Spinning. J. Appl. Polym. Sci. 1968, 12, 383-402. (25) Sukitpaneenit, P.; Chung, T.-S., Molecular Elucidation of Morphology and Mechanical Properties of PVDF Hollow Fiber Membranes from Aspects of Phase Inversion, Crystallization and Rheology. J. Membr. Sci. 2009, 340, 192-205. (26) Seyedin, S.; Razal, J. M.; Innis, P. C.; Wallace, G. G., A Facile Approach to Spinning Multifunctional Conductive Elastomer Fibres with Nanocarbon Fillers. Smart Mater. Struct. 2016, 25, 035015. (27) Li, X.; Hua, T.; Xu, B., Electromechanical Properties of A Yarn Strain Sensor with Graphene-Sheath/Polyurethane-Core. Carbon 2017, 118, 686-698. (28) Spitalsky, Z.; Tasis, D.; Papagelis, K.; Galiotis, C., Carbon Nanotube–Polymer Composites: Chemistry, Processing, Mechanical and Electrical Properties. Prog. Polym. Sci. 2010, 35, 357-401. (29) Guo, Z.; Liu, H.; Li, Y.; Dai, K.; Zheng, G.; Liu, C.; Shen, C.; Yan, X.; Guo, J., Electrically Conductive Thermoplastic Elastomer Nanocomposites at Ultralow Graphene 35



Loading Levels for Strain Sensor Applications. J. Mater. Chem. C 2015, 4, 157-166. (30) Huang, J.-C., Carbon Black Filled Conducting Polymers and Polymer Blends. Adv. Polym. Technol. 2002, 21, 299-313. (31) Duan, L.; Fu, S.; Deng, H.; Zhang, Q.; Wang, K.; Chen, F.; Fu, Q., The Resistivity-Strain Behavior of Conductive Polymer Composites: Stability and Sensitivity. J. Mater. Chem. A 2014, 2, 17085-17098. (32) Roh, E.; Hwang, B. U.; Kim, D.; Kim, B. Y.; Lee, N. E., Stretchable, Transparent, Ultra-Sensitive and Patchable Strain Sensor for Human-Machine Interfaces Comprising a Nanohybrid of Carbon Nanotubes and Conductive Elastomers. ACS Nano 2015, 9, 6252-6261. (33) Liu, N.; Fang, G.; Wan, J.; Zhou, H.; Long, H.; Zhao, X., Electrospun PEDOT:PSS–PVA Nanofiber Based Ultrahigh-Strain Sensors with Controllable Electrical Conductivity. J. Mater. Chem. 2011, 21, 18962-18966. (34) Slobodian, P.; Riha, P.; Benlikaya, R.; Svoboda, P.; Petras, D., A Flexible Multifunctional Sensor Based on Carbon Nanotube/Polyurethane Composite. IEEE Sens. J. 2013, 13, 4045-4048. (35) Yan, C.; Wang, J.; Kang, W.; Cui, M.; Wang, X.; Foo, C. Y.; Chee, K. J.; Lee, P. S., Highly Stretchable Piezoresistive Graphene-Nanocellulose Nanopaper for Strain Sensors. Adv. Mater. 2014, 26, 2022-7. (36) Wang, D. Y.; Tao, L. Q.; Liu, Y.; Zhang, T. Y.; Pang, Y.; Wang, Q.; Jiang, S.; Yang, Y.; Ren, T. L., High Performance Flexible Strain Sensor Based on Self-Locked Overlapping 36


Page 36 of 40

Page 37 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphene Sheets. Nanoscale 2016, 8, 20090-20095. (37) Wang, C.; Li, X.; Gao, E.; Jian, M.; Xia, K.; Wang, Q.; Xu, Z.; Ren, T.; Zhang, Y., Carbonized Silk Fabric for Ultrastretchable, Highly Sensitive, and Wearable Strain Sensors. Adv. Mater. 2016, 28, 6640-6648. (38) Li, C.; Cui, Y. L.; Tian, G. L.; Shu, Y.; Wang, X. F.; Tian, H.; Yang, Y.; Wei, F.; Ren, T. L., Flexible CNT-Array Double Helices Strain Sensor with High Stretchability for Motion Capture. Sci. Rep. 2015, 5, 15554. (39) Wang, Z.; Huang, Y.; Sun, J.; Huang, Y.; Hu, H.; Jiang, R.; Gai, W.; Li, G.; Zhi, C., Polyurethane/Cotton/Carbon Nanotubes Core-Spun Yarn as High Reliability Stretchable Strain Sensor for Human Motion Detection. ACS Appl. Mater. Interfaces 2016, 8, 24837-24843. (40) Park, J. J.; Hyun, W. J.; Mun, S. C.; Park, Y. T.; Park, O. O., Highly Stretchable and Wearable Graphene Strain Sensors with Controllable Sensitivity for Human Motion Monitoring. ACS Appl. Mater. Interfaces 2015, 7, 6317-6324. (41) Shin, M. K.; Oh, J.; Lima, M.; Kozlov, M. E.; Kim, S. J.; Baughman, R. H., Elastomeric Conductive Composites Based on Carbon Nanotube Forests. Adv. Mater. 2010, 22, 2663-2667. (42) Tang, Z.; Jia, S.; Wang, F.; Bian, C.; Chen, Y.; Wang, Y.; Li, B., Highly Stretchable Core-Sheath Fibers via Wet-Spinning for Wearable Strain Sensors. ACS Appl. Mater. Interfaces 2018, 10, 6624-6635. (43) Wang, X.; Meng, S.; Tebyetekerwa, M.; Li, Y.; Pionteck, J.; Sun, B.; Qin, Z.; Zhu, 37



M., Highly Sensitive and Stretchable Piezoresistive Strain Sensor Based on Conductive Poly(styrene-butadiene-styrene)/Few Layer Graphene Composite Fiber. Compos. Part A-Appl. S. 2018, 105, 291-299. (44) Amjadi, M.; Yoon, Y. J.; Park, I., Ultra-Stretchable and Skin-Mountable Strain Sensors Using Carbon Nanotubes-Ecoflex Nanocomposites. Nanotechnology 2015, 26, 375501. (45) Amjadi, M.; Pichitpajongkit, A.; Lee, S.; Ryu, S.; Park, I., Highly Stretchable and Sensitive Strain Sensor Based on Silver Nanowire-Elastomer Nanocomposite. ACS Nano 2014, 8, 5154-5163. (46) Wang, H.; Lu, W.; Di, J.; Li, D.; Zhang, X.; Li, M.; Zhang, Z.; Zheng, L.; Li, Q., Ultra-Lightweight and Highly Adaptive All-Carbon Elastic Conductors with Stable Electrical Resistance. Adv. Funct. Mater. 2017, 27, 1606220. (47) Lu, J. R.; Weng, W. G.; Chen, X. F.; Wu, D. J.; Wu, C. L.; Chen, G. H., Piezoresistive Materials from Directed Shear ‐ Induced Assembly of Graphite Nanosheets in Polyethylene. Adv. Funct. Mater. 2005, 15, 1358-1363. (48) Ge, J.; Sun, L.; Zhang, F. R.; Zhang, Y.; Shi, L. A.; Zhao, H. Y.; Zhu, H. W.; Jiang, H. L.; Yu, S. H., A Stretchable Electronic Fabric Artificial Skin with Pressure-, Lateral Strain-, and Flexion-Sensitive Properties. Adv. Mater. 2016, 28, 722-728. (49) Zhang, X. W.; Pan, Y.; Zheng, Q.; Yi, X. S., Time Dependence of Piezoresistance for the Conductor-Filled Polymer Composites. J. Polym. Sci. Pol. Phys. 2000, 38, 2739–2749. 38


Page 38 of 40

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(50) Zheng, Y.; Li, Y.; Li, Z.; Wang, Y.; Dai, K.; Zheng, G.; Liu, C.; Shen, C., The Effect of Filler Dimensionality on the Electromechanical Performance of Polydimethylsiloxane Based Conductive Nanocomposites for Flexible Strain Sensors. Compos. Sci. Technol. 2017, 139, 64-73. (51) Chen, L.; Chen, G. H.; Lu, L., Piezoresistive Behavior Study on Finger-Sensing Silicone Rubber/Graphite Nanosheet Nanocomposites. Adv. Funct. Mater. 2007, 17, 898-904. (52) Wijdicks, E. F. M., Biot’s Breathing. J. Neurol., Neurosurg. Psychiatry 2007, 78, 512.

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Ultra-stretchable Multilayered Fiber with a Hollow-Monolith Structure

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