Highly Sensitive, Ultra-Stretchable Strain Sensors Prepared by

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Highly Sensitive, Ultra-Stretchable Strain Sensors Prepared by Pumping Hybrid Fillers of Carbon Nanotubes /Cellulose Nanocrystal into Electrospun Polyurethane Membranes Li Zhu, Xin Zhou, Yuhang Liu, and Qiang Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00136 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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ACS Applied Materials & Interfaces

Highly Sensitive, Ultra-Stretchable Strain Sensors Prepared by Pumping Hybrid Fillers of Carbon Nanotubes /Cellulose Nanocrystal into Electrospun Polyurethane Membranes Li Zhu, Xin Zhou, Yuhang Liu, Qiang Fu* College of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Cheng Du, P.R. China ABSTRACT: Advanced flexible strain sensors for human motion detection and other potential use have attracted great attention in recent years. However, the preparation of strain sensor with both high sensitivity and large workable strain range remains challenge. In this work, the carbon nanotubes (CNTs) suspensions with the assist of cellulose nanocrystals (CNC) were directly pumped into the porous electrospun polyurethanes (TPU) membranes through a simple filtration process to prepare the flexible strain sensors in one step. The sensitivity and workable strain range of the strain sensors are tunable through changing the mass ratios of CNTs/CNC and the total amount of hybrid fillers. With increasing of total amount of fillers, a change of fillers layer from droplet to completely continuous film was observed, resulting a sharp increase of strain sensitivity. By combining the ultra-elasticity of the TPU material and the high sensitivity of hybrid fillers, the strain sensor with large workable strain range (>500%) and high sensitivity (GF=321) was successfully prepared. Its applications in visual controlling and full-range human body motion detecting were demonstrated, showing its tremendous potential applications in future intelligent electronics. 1

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KEYWORDS: strain sensor, high sensitivity, high stretchablity, electrospun TPU membrane, crack. 1. INTRODUCTION Wearable and skin-mountable strain sensors which can transduce mechanical deformations into electrical signals have versatile potential applications in human motion detection, electronic skin, intelligent electronics etc.1-11 Since traditional rigid strain sensors based on semiconductor or metal materials cannot meet the needs of full-range strain sensing due to their small test range (usually less than 5%) and poor sensitivity (GF=2~5),12-13 flexible strain sensors have attracted a lot of attentions in recent years. Furthermore, the sensing signals of some sensors cannot change linearly with the applied strain, making the development of testing standards complicated.14-15 Therefore, various efforts have been made to prepare the flexible strain sensors with wide sensing range, high sensitivity, high linearity and mechanical stability to meet the growing demand.16-23 Currently, many research groups have proposed to use nanomaterials (CB, CNTs,6,

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graphene,25-26 metal nanowires,27 and metal nanoparticles28-29 etc.) as sensitive materials and choose soft polymers30-31 as stretchable support materials to prepare flexible strain sensors. The strain sensors are fabricated through several common processes including mixing methods,32-33 chemical synthesis methods,34 coating techniques.35-37 The mixing methods (containing solution-mixing and melt-mixing) where the polymers and the sensitive nanomaterials are uniformly mixed together to obtain nanocomposite structures are simple 2


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and scalable, but a lot of fillers are needed to form the internal conductive network and the good dispersion of the fillers in the matrix is also difficult to achieve. In another approach, chemical synthesis methods are utilized for direct growth of nanomaterials thin films which is relatively complicated and not facile to produce. As for the coating techniques, they are low-cost, enabling large-area and now often chosen to prepare the strain sensors. There are several different coating methods, such as transferring the prepared nanomaterial films to the substrate surface38 or directly spraying the suspension of fillers on it13, where the filler layer may be peeled off from the dense surface of substrate during stretching or the workable strain range is narrow. For example, Daeshik Kang et al.39 deposited a platinum (Pt) layer on top of a viscoelastic polymer to prepare the crack -based sensors with a gauge factor of over 2,000, but its workable strain range was only 2% since the sensing layer on the surface cracked quickly and was separated completely during stretching. On the other side, the strain sensor with large workable strain range (82%) has been prepared with a fish-scale-like graphene-sensing layer while the GF is only 16.2 within the strain range. 38It is hoped to find a method to make the sensor combining the high sensitivity and the high stretchablity at the same time. The easy separation between the zero-dimensional fillers, such as metal nanoparticles make the sensor more sensitive, but the thorough disconnection of the fillers makes the material prone to failure under the lower workable strain range. The poor interaction between polymer substrates and conductive filler layer hinders an effective stress transfer at interface thus reduce the sensitivity of strain sensor. Wang et al.40 reported the sensor fabricated by 3



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using ultrasonic method to attach graphene to TPU electrospinning film. The working mechanism of the sensor is mainly based on the tunneling theory, so the GF value is not high enough and the resistance increases exponentially with the increase of strain. Xu et al.41 obtained the homogeneous TPU/NFC@CNTs suspension and evaporated it at 80℃ for film formation. Since more filler is needed to form the conductive network through the solution method, it will raise the cost. Wang et al.42 fabricated the graphene/CNTs/CNCs-TPU sensor via transfer process. The workable strain range of the material is relatively narrow and the conductive layer is easy to peel off the matrix. In this work, carbon nanotubes (CNTs) with high aspect ratio have been considered as one of the most suitable conductive fillers for the preparation of stain sensors due to their outstanding combination of high electrical conductivity and excellent mechanical properties. Since the dispersion state of CNTs has a great influence on the final properties of the material 41but

CNTs prefer assembling together due to the strong van der Waals interaction, it is

important to improve the dispersion state of CNTs. It has been proved43-44 that cellulose nanocrystals (CNC) fabricated by the acid-catalyzed hydrolysis of cellulose exhibit hydrophilic and hydrophobic characteristics named amphipathy. The hydrophobic part of CNC may occur hydrophobic interactions with the hydrophobic surface of the CNTs, so the CNTs can be well dispersed with the assist of CNC. As for the polymer substrates, instead of smooth surface but porous polymer substrate is considered for better interaction between filler layer and polymer substrate. Chemical modification is the main method to increase the interaction between matrix and filler, such as low temperature plasma treatment and 4


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UV-induced grafting, but the treatment process is complex. By changing the matrix surface from dense to porous, the specific surface area and interaction of materials can be simply increased. More importantly, the filler network can be confined in the porous structure, which is very helpful for the strain sensitivity. Electrospinning45 is often used to prepare porous films and it can adjust fiber diameter, pore size and film thickness by changing working conditions. We take a different approach to directly pump the CNTs-CNC hybrid suspension into the porous electrospun TPU membrane through vacuum filtration. The fillers can fit tightly on the substrate and the total amount of fillers can be controlled by adjusting the volume of added hybrid suspension with the same concentration because all CNTs-CNC are captured on the TPU electrospun membrane during filtration. The prepared strain sensor owns ultra-stretchability attributing to the ultra-elasticity of the TPU material itself, and it is highly sensitive at the same time.

2.EXPERIMENTAL SECTION 2.1 Materials TPU (WHT-1570) with a density of 1.21 g/cm3 was purchased from Yantai Wanhua Pol yurethanes Co. Ltd, China. CNTs served as the conductive filler were produced by Nanocyl S.A. (Belgium). The cellulose material (Celish MFC KY100-S) obtained from Daicel Chemical Industries, Ltd, Japan was used to prepare the cellulose nanocrystals (CNC). Reagents such as THF and DMF used in relevant experiments were purchased from Chengdu 5



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Kelong Chemical Reagent Co. (China) and the deionized water was homemade by the laboratory. 2.2 Preparation of Electrospun TPU Membranes The TPU spinning solution consisting 16 wt% TPU and a mixture of DMA /THF with a mass ratio of 1:1 was prepared by magnetic stirring for 2 h at 65 oC. Next, the spinning solution was spun for 2 hours at a spinning voltage of 27 kV and a feed rate of 1 mL/h to obtain the electrospun TPU membranes. 2.3 Preparation of CNC and CNTs-CNC Hybrid Fillers CNC was fabricated by the acid-catalyzed hydrolysis of cellulose. First, 100 ml deionized water was used to dilute 100 ml concentrated sulfuric acid. Then the cellulose material (20 g) was mixed with the obtained sulfuric acid solution (200 mL, 62 wt%) and the mixture was stirred vigorously at 60 oC for 30 min. The suspension was poured into 200 mL deionized water to stop the reaction immediately. Then the obtained product was centrifuged (8000 r/min, 15 min) and washed with deionized water 3 times to collect all the soluble components. Dialysis was performed until the pH increased to 5-6, whereupon a well-dispersed CNC was obtained. The solid content of CNC was tested, and the CNC concentration was calculated to be about 4.42 mg/mL. For the CNTs-CNC hybrid fillers, CNTs was added into well-dispersed solution of CNC with mass ratios of 2:0, 2:1, 2:2, 2:3 where the concentration of CNTs was fixed at 0.25 mg/mL and the CNC solution was diluted to the corresponding concentration according to the 6


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mass ratio. The mixture was predispersed for 30 min using a high-shear homogenizer (8000 r/min). Next, the ultrasonic cell crusher (100 Hz) was used for 60 min to obtain a uniform CNTs-CNC hybrid suspension. 2.4 Fabrication of TPU/CNTs-CNC Strain Sensors The prepared electrospun TPU membranes were cut into round sheets with a diameter of 6 cm. Due to its porous nature, it was fixed on a vacuum filter device to serve as the suction filter. The CNTs-CNC hybrid suspension was directly pumped onto the electrospun TPU membranes to prepare the TPU/CNTs-CNT composite material. All the CNTs-CNC hybrid fillers were drawn into the TPU membrane. It was peeled off and placed in an ordinary oven at 60

oC

for 6 hours to obtain the TPU/CNTs-CNC strain sensor. A series of

TPU/CNTs-CNC strain sensors with different proportion of CNC in the CNTs-CNC hybrid fillers and various obtained CNTs-CNC hybrid (both are 0.25 mg/ml) fillers loadings of 2.5, 5, 7.5, 10,15 mL were similarly prepared. All the prepared films were tailored into uniform samples with length of 25 mm and width of 5 mm for the subsequent tests. 2.5 Characterization Optical microscope (Olympus BX51) was employed to observe the surface micromorphology of TPU/CNTs-CNC strain sensors. Scanning electron microscopy (SEM; JEOL JSM-5900LV) was employed to observe the surface and cross-section microstructure of TPU/CNTs-CNC strain sensors. Transmission electron microscopy (TEM; JEOL JEM-100CX, Japan) was carried out to examine the morphology of the CNTs, CNC, and 7



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hybrid fillers of CNTs-CNC. The stress-strain curves of TPU/CNTs-CNC strain sensor and pure TPU membrane were recorded on an Instron 5567 universal testing machine. The electromechanical performance of the strain sensors was measured with a SANS CMT4000 universal testing machine at a constant rate of 5 mm/min and a Keithley 6487 picoammeter simultaneously at a voltage of 1 V.

3. RESULTS AND DISCUSSION 3.1 Preparation Process and Structure Characterization of the Strain Sensor. It illustrates the fabrication process of the TPU/CNTs-CNC strain sensors with a simple filtration method in Figure 1a (see details from experimental section above). Figure 1b shows the resultant TPU/CNTs-CNC strain sensor which is cut into spline of 25 mm×5 mm. The white surface of the spline indicates that the CNTs are almost completely pumped in the TPU membrane without leakage. As shown in Figure 1c, the morphology of TPU fiber is very uniform with the diameter of about 1um seen from the SEM image of the TPU membrane surface. The thickness of TPU membrane is about 250 μm (Figure S1) and the aperture of the porous membrane is about 2 μm (Figure S2). The SEM image in Figure 1d shows the surface morphology of the TPU membrane after the fillers are pumped in. The LED can emit light even when the sensor is twisted (Figure S3), indicating the initial resistance of our sensor is relatively small and the sensor is robust enough.

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Figure 1. (a) Illustration showing the fabricating process of a TPU/CNTs-CNC strain sensor. (b) Photograph of the resultant TPU/CNTs-CNC strain sensor. SEM pictures of TPU membrane (c) before and (d) after the CNTs-CNC are pumped in.

The obtained CNC solution is transparent and stable in deionized water (Figure 2a) due to the introducing of the sulfate charges onto the crystal surface during hydrolysis. Most pristine CNTs are precipitated at the bottom after standing for 24 h. However, by adding CNC into the suspension of CNTs (the mass ratio of CNTs-CNC is 1:1), the obtained mixture is homogeneous and remains stable for quite a long time. The UV−vis spectra of the three suspensions was carried out to investigate the effect of CNC on the dispersion state of CNTs and the result is shown in Figure 2b. The characteristic absorption peak of CNTs appears between 200 and 300 nm in the UV−vis region. It can be seen that there are no obvious characteristic absorption peaks in the curves of CNC and pristine CNTs suspension, indicating there are no individual CNTs suspended in the solution. However, the plot of 9



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CNTs-CNC suspension exhibits an obvious absorption peak between 200 and 300 nm, revealing abundant individual CNTs are obtained by adding CNC. TEM images were took to further investigate the microscopic morphology of CNC, CNTs, and CNTs-CNC. CNC is rod like (Figure 2c) and well dispersed with a cross-section of 3-20 nm and a length of hundreds of nanometers. Figure 2d shows the pristine CNTs are still in a state of entanglement. It can be seen from Figure 2e that individual carbon nanotube with approximate diameter of 10 nm and length of 1.5 μm is obtained by the assisting of CNC. CNC adheres to carbon nanotube and successfully separates the originally agglomerated carbon nanotubes. The results demonstrate CNC can steady disperse the CNTs which is nontoxic and environmentally friendly.

Figure 2. (a) Digital pictures of CNC, CNTs, and CNTs-CNC hybrid suspension after standing for 24 hours. (b) UV−vis absorption spectra of CNC, CNTs, and CNTs-CNC suspension. TEM images of (c) CNC, (d) CNTs,

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and (e) CNTs-CNC.

Figure 3 shows the morphological changes of the TPU/CNTs-CNC composite films surface with increasing CNTs loadings. When the volume of the CNTs-CNC suspension is below 10 mL (Figure 3a-c), most fillers is directly pumped on the TPU membranes to form a conductive network. With increasing of total amount of fillers, a change of fillers layer from droplet to completely continuous film was observed, resulting a sharp increase of strain sensitivity when the volume of the hybrid fillers increases to 10 mL (Figure 3d). In order to further observe the morphology of the surface fillers layer, SEM is conducted to characterize the cross sections of these strain sensors in Figure S4. It also can be demonstrated that the filler layer on the surface is formed with the increase of volume of hybrid fillers.

Figure 3. SEM images of the surface morphology of TPU/CNTs-CNC strain sensors with different CNTs-CNC

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loadings (both are 0.25mg/mL): (a) 2.5 mL, (b) 5 mL, (c) 7.5 mL, (d) 10 mL, (e) 15 mL. The scale bar in the images is 50 μm.

3.2 Stretching Sensitivity of the Strain Sensor. The response behavior in tensile process is one of the important criteria for evaluating the performance of strain sensors, including the workable strain range, the degree of sensitivity and linearity and so on. The effect of the mass ratio of CNTs and CNC on the performance of the sensors is first studied to choose the best proportion. Figure S5a shows the relative resistance variations change as a function of tensile strain in the stretching process with different mass ratios of CNTs and CNC (pure CNTs, 2:1, 2:2 and 2:3), where the concentration of CNTs is 0.25 mg/mL and the volume of the hybrid solution is 10 mL. The introduction of CNC as a compatibilizer of CNTs significantly increases the sensitivity of sensors and the sensitivity of the sensors raises with the increase of CNC mass ratio. When the mass ratio of CNTs and CNC reaches 2:3 as shown in Figure S5b, the maximum gauge factor (GF) of the sensor is 9460, but its workable strain range is only within 100% and the whole response process is nonlinear at the same time. Beyond this range, the resistance of the material increases rapidly with the increase of strain, and the irreversible damage of the conductive network causes the sensor to fail. Gauge factor (GF) given by GF=(ΔR/Ro)/ε can reflect the sensitivity of the strain sensor, where ΔR means the change from the initial resistance (Ro) at the corresponding strain (ε). Noting that the initial resistance of the material increases with the increase of CNC ratio as shown in Figure S6, but the change of resistance is much greater than the increasing of initial resistance, so the GF value of the material is 12


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even bigger according to the formula that GF=(ΔR/Ro)/ε. The CNC can separate the conductive network formed by the entangled CNTs when it is located along the CNTs, enhancing the local inhomogeneity and giving the hybrid fillers layer of brittleness. This leads to the tiny cracks on the surface of the sensor after the process of pre-stretching and the cracks appear more obviously with the increase of CNC as shown in Figure S7. The CNC fragmentized the conductive network and resulted into highly sensitive strain sensing due to the reversible variation of the tiny cracks. Taking into account the comprehensive performance of sensors, we choose 2:2 of CNTs to CNC as the best condition for subsequent experiments. Then a series of TPU/CNTs-CNC strain sensors are prepared with different obtained CNTs-CNC hybrid fillers (both are 0.25 mg/mL) loadings of 2.5, 5, 7.5, 10, 15 and 20 mL to study the influence of fillers amount on the sensitivity of the sensors. Different from general sensors which sensitivity often decreases with the increase of the amount of fillers, the sensitivities of the TPU/CNTs-CNC sensors increase with the increase of CNTs-CNC loadings when the loadings are below 20 mL as shown in Figure 4a. It is worth noting that when the fillers volume is 10 mL, the sensitivity of material has a marked increase (GF is raised from 85 to 321) due to the formation of a complete conductive fillers layer as shown in Figure 3d. The sensor with the fillers volume of 15 mL owns the highest sensitivity and the digital photo of its surface at 400% tensile strains is shown in Figure S8. The initial resistance of the sensors after pre-stretching are shown in Figure 4b. When the filler is 2.5 mL, the surface of the material has not formed a complete conductive network, and the sensor 13



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owns the largest resistance value of 44705.8Ω. Then, the complete network is formed when the volume of filler is increased to 5 mL, and the resistance value of the sensor drops rapidly to 1181.3 Ω. After that, the resistance value does not change much with the increase of the filler content. When the fillers volume reaches 20 mL, the crack on the surface of the material is wider after pre-stretching causing the conductive network of the sensor irreversibly damaged, which makes the initial resistance rise and finally decreases the calculated GF value. Considering the stability of surface cracks in the process of stretching and releasing, the sensor with the fillers volume of 10 mL is selected as the best sample for overall performance. The sensitive properties of sensor with the fillers volume of 10 mL is further studied in detail. As shown in Figure 4a, the strain sensor owes excellent comprehensive performance, which possesses a tolerable strain of 500% and a high gauge factor (GF) up to 321 with resistance changing almost linearly. The resistivity of strain sensor increases gradually with increasing strain up to 500% at which point the test is stopped and the sensor is not broken. The stress-strain curves show the addition of the hybrid fillers do not deteriorate the performance of the matrix itself but increase the stretchability and tensile strength of the material instead (Figure S9). The fracture strain of the TPU membrane reaches 590%. After the fillers are added, the fracture strain even increases to 780%, and the tensile strength also significantly increases from 4.8 to 13.5 MPa, so the adding of fillers is beneficial to the mechanical properties of the sensor. Due to the hydrophilicity of CNC, the sensor may be sensitive to water/humidity according to the research46-48. But when we compared the sensor 14


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after drying in vacuum oven for 12 hours with untreated sample (Figure S10), we found that the humidity in the air had little effect on the performance of the sensor in our system. This is because there is no significant difference in humidity between the two materials. And the slight change in resistance of the material itself has little effect on the overall performance of the sensor after pre-stretching. Figure 4c illustrates that the performances of stretching and releasing within 100% strain are nearly reversible. Specifically, all the samples used for the test have been subjected to pre-stretching steps of five cycles of 100% strain at a rate of 5 mm/min to uniformly damage the filler layer on the sample surface and stabilize the conductive network on the internal TPU elastic skeleton, so the hysteresis of resistance cannot be seen during stretching and releasing. Despite this, we still recorded the electromechanical performance of the pre-stretching process. After pre-stretching, both the valley resistance and the peak resistance increase, and the amplitude tends to be stable after 5 cycles (Figure S11). There are irreversible cracks produced on the surface (Figure S12). Figure 4d shows the stable relative resistance variations of the sensor under cyclic stretching-releasing within strain of 20%, 50%, 100%, 200% with the same stretching speed. The variations in relative resistance increase with the strain increasing which are in great agreement with the results shown in Figure 4a, demonstrating the sensor can response reliably to the applied strain. In Figure 4e，relative resistance variations of the strain sensor are almost independent of frequency during the stretching-releasing cycles of 50% strain. Figure 4f record the overshoot behavior of the strain sensor at the speed of 180 mm/min. We know the overshoot values are dependent on the GFs of strain sensors, viscoelasticity of 15



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polymers, and strain rate. The higher the sensitivity of the sensor, the greater the variation of relative resistance during the stress relaxing of polymers, so the obvious overshooting behavior at the maximum strain (100%) confirms that the sensitivity of this sensor is relatively high. As for the dynamic durability, Figure 4g shows the excellent repeatable response of the strain sensor to 1000 times cyclic loading within 100% strain at a speed of 180 mm/min, indicating the sensor can be reversibly restored to its original state due to the high elastic behavior of CNTs. Considering the strain sensors possesses a desirable integration of such high sensitivity, linearity and ultra-large sensing range, its comprehensive performance is excellent in the current research work arranged in Table 1, broadening its applications in detecting full-range human motions.

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Figure 4. (a) Relative resistance variations change as a function of tensile strain with different hybrid fillers loadings of 2.5, 5, 7.5, 10, 15，20 mL. (b) The initial resistance of sensors with different CNTs-CNC loadings. (c) Resistance for strain of 100% during the stretching-releasing cycle. (d) Relative resistance variations under 20%，50%，100%, 200% cyclic strains. (e) Relative resistance variations under cyclic loading-unloading with a strain of 50% at frequency of 002, 0.04, and 0.08 Hz. (f) Relative resistance variation showing the low creep of the strain sensor when being held at different strains. (g) Relative resistance variation under repeated loading and unloading of 100% strain for 1000 cycles, showing the stability and durability of the sensor.

Table 1.

GFs Reported in the Literature.

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P3HT-NF: poly(3-hexylthiophene-2,5-diyl) nano-fibril; FCMS: fragmented carbonized melamine sponges.

3.3 Working Mechanism of the TPU-CNTs/CNC Strain Sensors In order to illustrate the role of porous TPU membrane in the sensing system, we compared the difference of sensing performance between the two sensors with dense and porous TPU membranes. TPU solution is poured into the film frame and dried to inform the dense TPU membrane and dense TPU/CNTs-CNC strain sensor is prepared by further pouring CNTs-CNC hybrid fillers onto the dense TPU film. Porous TPU/CNTs-CNC owns a higher linearity than dense TPU/CNTs-CNC in Figure 5a. When the strain is less than 350%, the sensitivity of porous TPU/CNTs-CNC is greater than that of dense TPU/CNTs-CNC and when the applied strain exceeds 350%, the sensitivity of porous TPU/CNTs-CNC is less than that of dense TPU/CNTs-CNC. Because of the large specific area of porous TPU membrane, the interaction force between fillers and matrix is stronger. The matrix deformation can easily drive the separation of fillers on the porous films when the strain is less than 350%, so the sensitivity and linearity are higher. When the strain exceeds 350%, the resistance of dense TPU/CNTs-CNC increases sharply, while that of porous TPU/CNTs-CNC still increases linearly. The fillers layer on the surface of the dense or porous TPU membrane is completely separated when the strain reaches a certain level. Figure 5b shows the dense surface of TPU membrane below the crack and there are almost no conductive paths, while Figure 5c shows that there are still conductive networks formed by fillers in the internal fibres below the crack, so that the whole material will not completely fail. 18


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Figure 5. (a) Sensing performance of dense TPU/CNTs-CNC and Porous/CNTs-CNC strain sensors. SEM images of (b) dense TPU/CNTs-CNC and (c) porous TPU/CNTs-CNC at the strain of 100%.

To further understand the working mechanism of the TPU-CNTs/CNC strain sensor during stretching, the sensor with 2.5 mL and 10 mL CNTs-CNC (both are 0.25mg/mL) hybrid solution pumped in is applied to a series of strains in x-direction and the evolution of their surface macroscopic morphology is recorded by the optical microscope. As mentioned above, the sensors have been subjected to pre-stretching, so these photos just show the evolution in the subsequent stretching process. When the added volume is 2.5 mL, most of the fillers are directly pumped into the porous membrane and the surface of the sensor mainly shows the appearance of the TPU fibers with hybrid fillers fixed on it in Figure 6a. The macroscopic morphology has no obvious change in the process of stretching. TPU fibers are elongated slowly as the strain increases, which leads to the gradual separation of fillers attached to TPU fibers. With the destruction of conductive network, the resistance of materials increases gradually. However, when the volume of fillers reaches to 10 mL, the surface morphology of the sensor varies greatly during stretching. As shown in Figure 6b, 19



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the number of cracks as well as the width of the gaps on the outer surface of the TPU membrane increase with the loading of tensile strain. With the generation and increase of cracks, conductive network on the surface is gradually destroyed and the connection nodes are reduced, leading to a rapid increase in resistance. This part is similar to crack-based sensors, contributing to the high sensitivity of the TPU/CNTs-CNC strain sensor and partial monotonically increase of the resistance.

Figure 6. Optical microscope photos of strain sensors with the volume of CNTs-CNC of (a) 2.5 mL and (b) 10 mL during loading of 100% strain, showing the evolution of the surface morphology during stretching.

Under tensile strain of 100%, the surface structures of sensors with 0 mL, 2.5 mL, 5 mL, 7.5 mL, 10 mL, 15 mL CNTs-CNC hybrid fillers pumped in are observed by SEM in Figure 7. Because the pore size of the TPU membrane is not completely uniform, the fillers are easier to be pumped in the larger pore. When the volume of the fillers is less than 10 mL 20


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in Figure 7a-d, the sensors have not been completely covered by the filers, forming the droplets morphology. In the tension state, the stress is first transferred to the TPU membrane and then to the filler droplets on it, which makes the sensitivity not high enough. Otherwise, due to the different distribution positions of droplets, the initiation positions of cracks on each droplets are different. Numerous droplets produce a lot of cracks, which lead to the destruction of conductive network. The produced cracks are tiny and the adjacent cracks may form new lapping so that some new conductive paths are reformed, counteracting part of the increase in resistance. When the volume of the fillers reaches 10 mL, the continuous film is formed on the surface of the TPU membrane. Unlike before, the tensile stress acts on both TPU membrane and fillers film when continuous film is formed, which increases the sensitivity of the material significantly. It can be understood that the droplets before are connected into one piece, and the tiny cracks originally located in different positions are also connected together to form a wider crack in a straight line. And with the increase of fillers volume, the cracks become wider in Figure 7e-f, which leads to the complete separation of fillers. The complete destruction of conductive network results the rapid increase of resistance. To gain a clear understanding of the surface evolution of strain sensors, schematic illustration is proposed in Figure 8. The different volume of fillers results in two kinds of morphology of droplet and continuous film on the surface of sensors, which affects the sensing performance of it in tension process.

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Figure 7. SEM images of the surface structures of sensors with (a) 0 mL, (b) 2.5 mL, (c) 5 mL, (d) 7.5 mL, (e) 10 mL, (f) 15 mL CNTs-CNC hybrid fillers pumped in under tensile strain of 100%.

Figure 8. Schematic illustration for the working mechanism of TPU/CNTs-CNC strain sensors.

3.4 Applications of the TPU-CNTs/CNC Strain Sensor

There are numerous potential applications of strain sensors, such as for human motion detection, human-machine interfaces, entertainment technology and even for visual 22


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controlling. Figure 9 shows the potential application for visual controlling as the electrical resistance of the sensor changes monotonically with the strain. The sensor and LED indicator were collected in series to the 10 V power supplied by a Keithley 6487 picoammeter. LED can emit light normally when the sensor is in the original state, indicating the sensor still has certain conductivity in the unstretched state. As expected, the electrical resistance is modified by the applied strain and the brightness of the LED is modulated by the strain sensor which becomes darker with the increase of strain. These results show the potential application of the strain sensor in an intelligent visual-control system.

Figure 9. Visual controlling by using the TPU/CNTs-CNC strain sensor. (a-c) The different brightness of a LED indicator under different applied strain.

As for the monitoring of full-range human body motions, it has been demonstrated that the sensor can be used on detecting the phonation, respiration, and finger bending. Since the GF of the strain sensor reaches 112 even for the small strain of 0-1%, we can use it to recognize the subtle strain deformation of human body. Figure 10a shows the excellent performance of the strain sensor for phonation measurement. Volunteers were instructed to 23



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read a, b, c, d, e, f in turn, and the resulting signals were recorded. It is found that the signals detected during the repetition are almost the same, indicating the strain sensor can accurately detect such tiny movement of phonation. As shown in Figure 10b, the strain sensor can record the chest undulations with repeating respiration. In Figure 10c, we fixed the strain sensor on an index finger to see how it response to the bending of the finger. When we bend our fingers to a certain degree and keep it for a while, the electrical resistance also increases correspondingly and then remains stable. To demonstrate the strain sensor owns ability of monitoring large deformation of human body, the sensor was attached to the joint of the arm in Figure 10d to observe the arm's expansion and contraction and it was fixed onto a knee in Figure 10e to detect and monitor bending and extending of the knee joint. As shown in Figure 10f, the sensor also can be attached to the neck to detect the amplitude of the head. These applications have demonstrated that the requirements of full-range human body motions detection are satisfactory for our TPU/CNTs-CNC strain sensor due to the high sensitivity and stretchability.

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Figure 10. Monitoring of human motion using the TPU/CNTs-CNC strain sensors. Response to motions of (a) phonation, (b) respiration, (c) finger bending, (d) arm expansion, (e) knee bending, (f) neck bending.

4. CONCLUSIONS In conclusion, we reported a TPU/CNTs-CNC strain sensor through a simple filtration 25



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process. The sensitivity and workable strain range of the strain sensors are tunable through changing the mass ratio of CNTs/ CNC and the total amount of hybrid fillers. The existence of CNC can assist in dispersing the CNTs bundles and improving the sensitivity of strain sensors during stretching by reducing the connection points of the conductive network. When the volume of the fillers is less than 10 ml, the sensors exhibit the droplets morphology. And when the volume of the fillers reaches 10ml, the continuous film is formed on the surface of the TPU membrane. With the increase of fillers volume, the cracks at certain strain become wider, leading to the complete separation of fillers and resulting the rapid increase of resistance.The strain sensor owns large workable strain range (>500%) duo to the ultra-elasticity of the TPU material itself and exhibits high sensitivity (GF =321) at the same time. It is demonstrated that the strain sensor can be used in visual controlling and full-range human body motions detecting such as subtle (phonation, respiration) and large-scale (bending of finger) human body motions measurements. Here the introduction of CNC as a green compatibilizer and the used suction filtration method to prepare the strain sensor in one step may provide new ideas for the preparation of strain sensors with excellent comprehensive performance. ■ASSOCIATED CONTENT Supporting Information. Supplementary data related to this article is available free of charge via the Internet at http://pubs.acs.org.

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SEM image of the section of the electrospun TPU membrane; pore size distribution of the TPU membranes; photograph of the sensor connected with a light-emitting diode; SEM images of the cross sections of TPU/CNTs-CNC strain sensors with different CNTs-CNC loadings; relative resistance variations change as a function of tensile strain with different mass ratios of CNTs and CNC; photographs of the surface of the sensors at strain of 400% with different CNTs-CNC hybrid fillers loadings of 5, 10 and 15 ml; typical stress-strain curves of pure TPU membrane and TPU/CNTs-CNC strain sensor; resistance for the pre-stretching; SEM images of the surface of the TPU/CNTs-CNC strain sensor after been pre-stretched, showing the irreversible cracks produced on the surface. ■AUTHOR INFORMATION Corresponding author E-mail: [email protected] (Q. Fu), Tel./Fax: +86 28 8546 1795. Notes The authors declare no competing financial interest.

■ACKNOWLEDGMENT We express our sincere thanks to the National Natural Science Foundation of China for financial support (51721091). ■REFERENCES (1) Yuan, W.; Zhou, Q.; Li, Y.; Shi, G. Small and Light Strain Sensors Based on Graphene Coated Human Hairs. Nanoscale 2015, 7 (39), 16361-16365. 27



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Highly Sensitive, Ultra-Stretchable Strain Sensors Prepared by

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