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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX

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Flexible and Lightweight Pressure Sensor Based on Carbon Nanotube/Thermoplastic Polyurethane-Aligned Conductive Foam with Superior Compressibility and Stability Wenju Huang,† Kun Dai,*,†,‡ Yue Zhai,† Hu Liu,† Pengfei Zhan,† Jiachen Gao,† Guoqiang Zheng,† Chuntai Liu,*,† and Changyu Shen† †

School of Materials Science and Engineering, The Key Laboratory of Material Processing and Mold of Ministry of Education, Zhengzhou University, Zhengzhou, Henan 450001, P. R. China ‡ State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, Sichuan 610065, P. R. China S Supporting Information *

ABSTRACT: Flexible and lightweight carbon nanotube (CNT)/ thermoplastic polyurethane (TPU) conductive foam with a novel aligned porous structure was fabricated. The density of the aligned porous material was as low as 0.123 g·cm−3. Homogeneous dispersion of CNTs was achieved through the skeleton of the foam, and an ultralow percolation threshold of 0.0023 vol % was obtained. Compared with the disordered foam, mechanical properties of the aligned foam were enhanced and the piezoresistive stability of the flexible foam was improved significantly. The compression strength of the aligned TPU foam increases by 30.7% at the strain of 50%, and the stress of the aligned foam is 22 times that of the disordered foam at the strain of 90%. Importantly, the resistance variation of the aligned foam shows a fascinating linear characteristic under the applied strain until 77%, which would benefit the application of the foam as a desired pressure sensor. During multiple cyclic compression-release measurements, the aligned conductive CNT/TPU foam represents excellent reversibility and reproducibility in terms of resistance. This nice capability benefits from the aligned porous structure composed of ladderlike cells along the orientation direction. Simultaneously, the human motion detections, such as walk, jump, squat, etc. were demonstrated by using our flexible pressure sensor. Because of the lightweight, flexibility, high compressibility, excellent reversibility, and reproducibility of the conductive aligned foam, the present study is capable of providing new insights into the fabrication of a high-performance pressure sensor. KEYWORDS: carbon nanotubes, thermoplastic polyurethane, piezoresistive material, nanocomposite, aligned foam

1. INTRODUCTION Pressure sensor has attracted plenty of researchers from both academia and industry. Among the materials, conductive polymer composites (CPCs), fabricated by dispersing conductive fillers into an insulating polymeric matrix, are considered as one of the most fascinating candidates owing to their flexibility, sensitivity, and wide response range.1−7 On the basis of these characteristics, CPC-based pressure sensors have been applied in health monitoring,2,8 motion detection,9−12 electronic skin field, etc.13,14 For these applications, properties such as lightweight and high compressibility need to be urgently addressed. Porous CPCs have been proved to be proper materials to fulfill these requirements owing to their high porosity and flexibility. On the other hand, piezoresistive behavior,2,15,16 i.e., the resistance variations against a mechanical stimulus, is considered as one of the most vital features for the pressure sensor. Yao et al.17 prepared a graphene-wrapped polyurethane (PU) sponge with fractured microstructure; they found that the © XXXX American Chemical Society

introduction of this structure enhanced the pressure sensitivity and reproducibility of the material over 10 000 cycle tests. Yin et al. fabricated a piezoresistive sensor based on Au-coated PU foam. High sensitivity and excellent stability were achieved on the basis of the fracture structure induced by acid treatment. In our previous work, carbon nanotube (CNT)/thermoplastic polyurethane (TPU) and graphene/TPU18 porous composites with lightweight feature and high compressibility have also been prepared and piezoresistive reproducibility for the two porous composites has been improved after several load-release cycle treatments. Although many outstanding performances of the porous pressure sensors have been reported, deficiencies still exist. For example, porous pressure sensors usually have obviously inferior recoverability,16 poor mechanical properties,19 and Received: November 7, 2017 Accepted: November 13, 2017 Published: November 13, 2017 A

DOI: 10.1021/acsami.7b16975 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Fabrication process of the aligned CNT/TPU foam, (b) the schematic diagram of the directional-freezing device, and (c) the directional-freezing process. Polyester-based TPU (Elastollan 1185A) with the density of 1.12 g· cm−3 was bought from BASF Co. Ltd., China. 1,4-Dioxane was obtained from Zhiyuan Reagent Co., Ltd., Tianjin, China. All of the chemicals were used as received without further treatment. 2.2. Preparation of Samples. The preparation process of the aligned CPC foam is illustrated in Figure 1. Briefly, CNTs were dispersed in 1,4-dioxane under ultrasonication (SCIENTZ-II, 285 W, Ningbo Scientz Biotechnology Co. Ltd., China) to obtain a homogeneous suspension. The TPU particles were added into the suspension and stirred at 40 °C until dissolved completely. In this study, the ratio of the TPU solution was fixed at 5:100 (5 g TPU was dissolved in 100 mL 1,4-dioxane). The obtained CNT/TPU/1,4dioxane mixture was then collected into a glass tube. Several glass tubes were placed in a self-made device to achieve the unidirectional freeze (see Figure 1b). For the detailed process, a cylindrical metal block with a height of 10 cm was put in a thermal-insulated container with liquid nitrogen to a height of 8 cm. The glass tubes were placed on the surface of the metal block to ensure unidirectional thermal transmission in the tubes. To form a desirable aligned structure, the glass tube was insulated by polystyrene foam to prevent the solution from horizontal heat transfer. After 30 min of the freezing process, the frozen samples were immediately placed in a freeze-drier at −80 °C for 72 h under an atmospheric pressure less than 5 Pa to achieve the aligned porous structure. The aligned pure TPU foam was also prepared with the same method for comparison. Besides, the CNT/ TPU/1,4-dioxane solution was placed in a −25 °C freezer for 12 h to completely freeze and then put in a freeze-drier at the same conditions to obtain the disordered foam for comparison. More preparation details are available in our previous work.19 2.3. Characterization. The morphology study of samples was conducted by a field emission scanning electron microscope (JEOL JSM-7500F) under an accelerating voltage of 5 kV. The samples were fractured quickly after being immersed in liquid nitrogen for an hour. The newly formed surface was sputtered with a thin layer of platinum to avoid electrical charging. Transmission electron microscopy (TEM) was performed on FEI Tecnai F20, with an accelerating voltage of 200 kV. The open cell content of the sample was tested by using a Mercury Porosimeter (model AutoPore IV 9500). For the electrical property test, samples were cut into cylinders with a diameter of 15 mm and height of 10 mm. A precision digital resistor (model DMM4050; Tektronix) was used to measure the resistance. The electrical conductivity σ was calculated by eq 1

low mechanically durability, which would hinder the application of the materials as the end-use of pressure sensors. Consequently, it is still a great challenge to solve these problems mentioned above. Properties of porous foams are closely related to the morphology of the cells. Porous materials with an aligned structure20,21 have shown great potential applications in tissue engineering,22 separation,23 and environmental protection24 owing to their high stability, tunable porous structure, and enhanced mechanical properties along the aligned direction. The directional-freezing method has been utilized as an effective route to prepare porous materials with an aligned structure due to its convenience, low cost, and ease.25 For instance, Romeo et al.26 fabricated silver nanowire/poly(vinyl alcohol) (PVA)-ordered macroporous scaffolds using this method and they found that the silver nanowires were also highly oriented inside the macroporous PVA scaffolds. However, few researchers have paid their attention to the tunable piezoresistive behavior by constructing the aligned porous structure. In this article, to improve the piezoresistive recoverability and reproducibility of lightweight pressure sensors, the directionalfreezing method was adopted to fabricate an aligned porous CPC. One-dimensional CNT was chosen as the electrical conductive filler arising from its excellent mechanical properties, high electrical conductivities, and large aspect ratio. Thanks to the superior elasticity and flexibility, TPU was used as the polymer matrix to fabricate CPCs with high flexibility.27−30 Effects of the CNTs on electrical conductivity and cell morphologies were investigated. Mechanical and electrical properties of the aligned porous CNT/PTU foams were also studied. Stepwise and cyclic compressions were applied to evaluate the piezoresistive behavior and evolution of the aligned CNT/TPU porous composites.

2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. Multiwalled carbon nanotubes with diameter of 8−15 nm and length of 30−50 μm were purchased from Chengdu Organic Chemicals, Chinese Academy of Science. B


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Figure 2. Schematic of the setup for the piezoresistive behavior upon cyclic compression.

Figure 3. (a, b) Optical and scanning electron microscopy images of the virgin TPU foam and CNT/TPU foam; (c) different shapes of aligned conductive CNT/TPU foam; (d) the sample was bended by tweezers, the inset of (d) is the recovery state of the sample; (e) the lightweight foam standing on a flower; the CNT loading is 0.076 vol %; (f) TEM images of the aligned CNT/TPU foam with a CNT content of 0.049 vol %. σ=

4L πRd 2

where R is electrical resistance and L and d stand for the length and diameter of the samples, respectively.

(1) C


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Figure 4. (a) Volume conductivity of aligned CNT/TPU foam as a function of CNT loading; the insert shows the log conductivity vs log (m − mc); (b) comparison of compression stress−strain curves of aligned and disordered TPU foam; (c) compression stress−strain curves of aligned CNT/ TPU foam with different CNT contents; (d) compression strength of aligned CNT/TPU foam with different CNT contents. As for the mechanical performance test, cylindrical samples with a length of 10 mm and diameter of 15 mm were measured by an electrical universal testing machine (UTM 2203, Shenzhen Suns Technology Stock Co. Ltd., China) with a 100 N load. Samples were compressed to a strain with a constant speed of 5 mm·min−1. A schematic of the setup for piezoresistive measurements is illustrated in Figure 2. Aluminum plates, used as electrode, were placed at the two ends of the samples. Silver paste was used to encourage a good contact between aluminum plates and the sample. The loading rate was 5 mm·min−1 with a duration time of 10 s between every two adjacent cycles in the cyclic compression-release test. All tests were conducted at the room temperature. An electrochemical workstation (RST5200F; Suzhou Risetest Electronic Co. Ltd., China) was used to test the current−voltage (I−V) curves. The operating parameters are as follows: the quiescence time is 3 s, scanning rate is 0.05 V·s−1, and sampling interval is 0.01 V.

Figure 3b2. The addition of the CNTs breaks the regular porous morphology partly,32 and this phenomenon might be related to the increase of solution viscosity and some entanglements of CNTs in the solution. Nevertheless, the channels are still oriented obviously with a good interconnected structure. The cell size distribution of virgin TPU foam and aligned CNT/TPU foam are both from 8 to 18 μm (Figure S1). The mean sizes are 13.08 μm for virgin TPU foam and 13.15 μm for aligned CNT/TPU foam, respectively. In other words, the distinction between the mean size of the two foams is small; this might be related to the small additional amount of CNTs in the nanocomposite foam. Nevertheless, the cell size distribution in the range 10−16 μm of the aligned CNT/ TPU system turns to be more dispersive compared to that of the virgin TPU foam sample. The open cell content of the 0.049% CNT/TPU aligned foam sample is 82.4% by using the mercury intrusion method. The aligned porous samples with various shapes can be fabricated easily by using this strategy (Figure 3c). The samples can be curled more than 90° and then recover to its original shape without fracture (Figure 3d), suggesting that the aligned CNT/TPU foam has an excellent flexibility. In addition, owing to the existence of the porous structure, the density of samples is as low as 0.123 g·cm−3. The foam is so light that the porous composites with the CNT content of 0.049 vol % can stand on the top of flowers without bending the petal, as shown in Figure 3e.33 The distribution state of CNTs in the TPU cell wall is clearly shown in Figure 3f. It is found that the CNTs were distributed uniformly (see Figure 3f2,3f3). It is well known that superior compressibility and remarkable recoverability are significant for an excellent pressure sensor. As

3. RESULTS AND DISCUSSION 3.1. Morphology of the Sample. As shown in Figure 3, it is obvious that aligned virgin TPU foam and conductive CNT/ TPU foam were both prepared successfully using the directional-freezing method. Detailed morphologies and microstructures of aligned TPU foam and CNT/TPU foam are shown in Figure 3a1,a2 and Figure 3b1,b2. Because of the unidirectional solidification in the preparation process, porous structures are apparently different from that of the disordered foam in our previous work.19 The aligned foam prepared by virgin TPU, as shown in Figure 3a, presents a superb oriented microstructure. The pure TPU foam, with an average channel diameter of several micrometers, shows a ladderlike cell with an interconnected structure along the freezing direction.31 However, for CNT/TPU composite foam, the aligned porous structure is not as well as the pure TPU foam, as shown in D


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Figure 5. (a) Stress and R/R0 of CNT/TPU foam with 0.049 vol % CNTs as a function of strain; (b) a magnified view of figure (a) at the turning point.

at the strain of 90%. The pronounced enhancement of mechanical properties is mainly attributed to the difference in the foam density; higher mechanical properties (such as modulus, Figure S5) can enhance the capability of the composite to undergo obviously higher pressures.37 From the compressive deformation curve of the aligned foam, the stress− strain curves are divided into three stages:38 in stage 1 (strain from 0 to 10%), the compressive stress increased linearly with the strain, showing the linear elastic deformation.39 In stage 2 (strain from 10 to 77%), the strain increased obviously, whereas the stress increased slowly because a large number of pores was compressed gradually at this stage.40 In stage 3 (strain > 77%), the stress increased rapidly with the strain, indicating the porous structure was compacted. The compressive stress−strain curves of the aligned CNT/ TPU foam are displayed in Figure 4c, and the stress in 50% strain is defined as the compression strength,17 as shown in Figure 4d. The result reveals that the compression properties of the composites are promoted evidently with the increase of CNT loading.41 It is also found that adding more CNTs increased the modulus of the composites (see Figure S5). It is due to the reinforcing effect of CNTs and the phase separation behavior change of the polymer and dioxane with the addition of CNTs (it then results in the change of the density of the foam, Figure S5). For example, the compression strength (18.4 kPa) was enhanced by 39.4% with the addition of 0.049 vol % CNTs, compared to that of the pristine TPU foam (13.2 kPa). The improvement of compression strength is ascribed to the following reasons: the homogeneous dispersion of CNTs in TPU cell walls (see Figure 3b3,3f), the excellent mechanical properties of CNTs, and the good interaction between CNTs and the TPU matrix.19 3.3. Piezoresistive Sensing Behavior. Figure 5 shows the changes of stress and piezoresistive behavior with the increase of compressive strains. Usually, higher electrical conductivity, which means a fine state of the conductive network in CPCs, would lead to better stability in the piezoresistive behavior test. As shown in Figure 4a, the conductivity of the aligned conductive foam increases to 1.1 × 10−3 S·m−1 with addition of 0.049 vol % CNTs and no obvious increase of conductivity can be observed by further increasing the CNT loading. Considering the conductivity and flexibility of the conductive foam, the aligned TPU/CNT foam with 0.049 vol % CNTs was chosen to investigate the piezoresistive behaviors. During the

exhibited in Figure S2 and Movie S1, the aligned CNT/TPU foam can be easily compressed more than 90% and then quickly recover to its original height, showing excellent compressibility, flexibility, and recoverability. 3.2. Electrical Conductivity and Mechanical Properties. The electrical conductivity (σ) of the conductive CNT/ TPU foam, calculated by eq 1, as a function of the CNT loading is shown in Figure 4a. Obviously, an abrupt increase of 8 orders of magnitude in electrical conductivity is observed when the CNT loading is increased from 0 to 0.012 vol %, implying an insulator−conductor transition. This result can be explained by the classical percolation theory using eq 2 σ = σ0(m − mc )t

(2)

where m represents the volume fraction of the conductive filler, mc is the percolation threshold, and t is the critical exponent and decided by the dimension of the conductive network.34 The value of mc = 0.0023 vol % and t = 3.64 is obtained by the best fitting of eq 2, as shown in Figure 4a. To the best of our knowledge, the percolation threshold of 0.0023 vol % is much lower than that in many reports about the conductive foam composites.16,19,32,35,36 However, the critical exponent 3.64 cannot be explained by the classical percolation theory. In general, the value of t from 1.1 to 1.3 and from 1.6 to 2.0 represents a two-dimensional and three-dimensional conductive network, respectively. In the current work, the special aligned porous structure affects the distribution of CNTs significantly, which might result in a relatively higher value of t.36 Compression tests were conducted on cylindrical samples and the effect of CNT content on the mechanical properties of the aligned CNT/TPU foams is shown in Figure 4b−d. Digital photographs of compression process of the aligned CNT/TPU foam are shown in Figure S3. Interestingly, the mechanical properties are affected obviously by the aligned foam structure in the porous material, as shown in Figures 4b and S4. Compared with the disordered TPU foam, the compression strength of the aligned TPU foam was enhanced by 30.7%, from 10.1 to 13.2 kPa, suggesting that the aligned porous structure can obviously improve the mechanical properties. Note that the stress of the aligned foam is higher than that of the disordered foam during the whole compression process, as shown in the inset in Figure 4b. In particular, the stress of the aligned foam is 22 times larger than that of the disordered foam E


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Figure 6. Rt/R0 of aligned porous materials with 0.049 vol % CNTs under compression strain: (a) under different compression speeds of 1, 3, 5, and 10 mm·min−1 at a strain of 30%; (b) step cycle test ranging from 5 to 50% (increased by 5% for each step).

Figure 7. Schematic diagram of the comparison on the reversibility process of the aligned and disordered conductive CNT/TPU foams.

could endow this aligned porous sample with potential application in the sensor field. In domain 2, however, it shows a different phenomenon. The stress increases significantly by about 1125% (from 0.92 MPa at 77% strain to 11.27 MPa at 85.6% strain), whereas the electrical resistance changes slower than that in the domain 1. As a consequence, the GF of the aligned CNT/TPU composites changes from 1.22 in domain 1 to 0.06 in domain 2. This behavior might be caused by the special aligned foam structure of the composite. In domain 1, the extrusion of the air plays a decisive role in the increasing strain, which leads to a large deformation and low stress, whereas in domain 2, the cell walls collapse and form an approximate solid state, which induces the rapid increase of stress. In this domain, however, electrical resistance caused by the change of the foam structure changes relatively gently. More concretely, the best fit line of domain 2 can be described by eq 5

compression process, it is evident that the electrical resistance decreases gradually with the increasing strain. There are two obvious domains with different stress and electrical resistance response behaviors in Figure 5, divided at the strain of 77% (Figure 5b). In domain 1, the stress increases slowly, only 0.92 MPa at 77% strain. At the same time, the pressure sensing responsivity Rt/R0 shows fantastic linearity along the strain until 77%. Here, the gauge factor (GF), defined as the ratio of relative changes in electrical resistance to the compression strain, is calculated by eq 326 GF =

d(R t /R 0) dε

(3)

In the equation, Rt and R0 refer to the resistance at the time t and the initial resistance ε stands for the strain.42 The GF in domain 1 was calculated to be 1.22. The best fit line in this domain can be described as an equation Rt = −1.22ε + 1.0 R0

Rt = −0.06ε + 0.015 R0

(4)

(5)

Comparing eq 5 with eq 4, it is obvious that the resistance changes slowly with a much smaller slope of −0.06, which is

The linearity of the sample is better than that in many reports (see the short review in the Supporting Information),14,43 and it F


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Figure 8. Cyclic compression-release measurement of piezoresistive behaviors of aligned conductive CNT/TPU foam at strain amplitudes of (a) 15%, (b) 30%, and (c) 60%, respectively; (d) I−V curve of the aligned CNT/TPU foam with 0.049 vol % CNTs in a compression-release cycle, the variations of the I−V curves in the compression and release processes at strain amplitudes of 0, 15, 30, and 45% were compared; (e) the resistance response of a cycle from 0 to 50% and then to 0% with an incremental step of 10%; (f) drift characteristics of the sensor under a constant strain at 60% for 1 h.

only 1/20 of the slope in domain 1 of −1.22. However, the superb linear behavior is still maintained in this domain. As for the linear variation in electrical resistance, the aligned structure is responsible for this excellent pressure sensing behavior. 3.4. Cyclic Compression. The effect of different compressive speeds on the response behavior of the aligned foams is shown in Figure 6a. The result suggests that different compressive speeds have little influence on the response behavior of our samples. Owing to the novel aligned porous

structure, the sample can recover even under a high compressive rate, which is meaningful for the sensing stability in practical application of a pressure sensor. Figure 6b shows the aligned porous materials with 0.049 vol % CNTs under stepped cyclic compression strain range from 5 to 50%, with an incremental step of 5% strain. Satisfyingly, the resistance can almost reach the original value when the applied strain was fully released, showing a nice pressure sensing reversibility. Compared with the aligned samples, an obvious G


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Figure 9. (a) Reproducibility test of the conductive CNT/TPU foam under repeated loading−unloading of 30% strain for 2000 cycles, (b) the 1− 10, 991−1000, and 1991−2000 compression-release cycles for the durability test, and (c) a circuit constructed with the aligned CNT/TPU foam. The brightness changes obviously with the compression and release of the conductive CNT/TPU foam.

related to the aligned porous structure and the interaction between filler and the TPU matrix. At the low and medium strains, the plastic deformation is feeble and the rearrangement of the fillers is also weak.45−47 When a large strain was applied, on one hand, the plastic deformation and hysteresis effect become more obvious owing to the viscoelasticity of the TPU matrix; the macromolecular chains entangled with the CNTs do not have adequate time to relax. On the other hand, the destruction of the imperfect structure can also slightly decrease with the increasing cycle number. After several compressionreleasing cycles, the evolution of the conductive networks reach the equilibrium state gradually15,44 and the electrical resistance becomes stable again. The stress−strain curves of the 1st, 10th, 20th, 30th, and 50th cycles are shown in Figure S6. After 10 cycles, the stress−strain curves are almost invariable, indicating that the porous structures become stable, which would lead to a good reversibility. The current−voltage curves under different strains and the recovery of lightweight CNT/TPU foam are shown in Figure 8d. Excellent linear behavior can be observed, which indicates that good Ohmic behavior of the samples is achieved regardless of various applied strains.48−50 Furthermore, it is worth noting that the slope of I−V curves increases gradually with the increase of strain, showing that the resistance decreases when a larger strain is applied, corresponding to the result in Figure 5. More intriguingly, the I−V curves of the applied strain and the released strain are almost coincident (Figure 8d), suggesting that the electrical resistance can return to the initial value well. This characteristic is vital for a material used as a satisfactory compression sensor. As shown in Figure 8e, the aligned pressure sensor was applied to the strain from 0 to 50% and then to 0% again with an incremental step of 10% and a residence time of 30 s. When increasing the strain form 0 to 50%, the sensing responsivity decreases gradually by the contact of the cell walls and the formation of new conductive paths. During the residence period, the variation of the sensing responsivity is very feeble.

residual resistance was observed for disordered porous CNT/ TPU CPCs at the end of each cycle in our previous work.18,19 The origins are discussed as follows: first, for the disordered foam, as discussed in our previous research, there are some imperfect structures18,19 and these structures would be destroyed gradually in a cyclic compression process, resulting in poor reversibility, whereas in the present study, the microstructure of the aligned foam is good (see Figure 3b1− b3). Second, owing to the aligned porous structure, the strength of the cell walls was enhanced obviously, which is beneficial to the sensing reversibility of the aligned foam. Third, the modulus of aligned CNT/TPU foam (25.2 kPa) is larger than that of the disordered CNT/TPU foam (20.4 kPa) with the same CNT content of 0.049% due to the difference in the density (the densities are 0.123 and 0.097 g·cm−3, respectively, Figure S5). In other words, the aligned CNT/TPU foam has a better capability to resist the deformation during compression, causing a good pressure sensing reversibility (Figure 7). To study the influence of different strain amplitudes on the stability,44 cyclic compression-release tests with different strain amplitudes, including the low (15%, Figure 8a), the medium (30%, Figure 8b), and the high strain amplitude (60%, Figure 8c), were applied on the aligned foams at a compressive rate of 5 mm·min−1. For this test, the sample was compressed to the corresponding strain and then released to ε = 0, followed by keeping this strain for 10 s, and the cycle was repeated 49 more times. It is worth noting that the response of electrical resistance presents exceptional stability at the low and medium strain amplitude even at the beginning stage. It has a little decline at the high strain amplitude for the first 10 cycles owing to the viscoelasticity of the polymer matrix (still less obvious than that of the disordered CNT/TPU foam19). After several cycles, they also become very stable. For the disordered samples, the electrical resistance turned to be lower values compared with the starting value at the end of each cycle, especially for the first 10 cycles, showing an inferior reversibility. We believe that this phenomenon is closely H


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Figure 10. Monitoring of human motion using the aligned CNT/TPU foam. Resistance change to the motions of (a) jump and (b) walk; photographs of (c) arm bending, (e) standing on tiptoe, and (g) squatting monitored by using the aligned CNT/TPU foam and the corresponding resistance sensing signals of (d) arm bending, (f) stand on tiptoe and (h) squat, respectively. I


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4. CONCLUSIONS Flexible carbon nanotube (CNT)/thermoplastic polyurethane (TPU) conductive foam with an aligned porous structure was fabricated using a directional-freezing method. A low density of 0.123 g·cm−3, an ultralow percolation threshold of 0.0023 vol %, and a very high compressibility of more than 90% have been achieved synchronously. The lightweight and flexible CNT/ TPU foam was endowed with enhanced mechanical property and superior piezoresistive behavior by its inherent aligned porous structure. In particular, the resistance responses exhibited wonderful linear characteristics with the applied strain up to 77%. The aligned porous structure consisting of ladderlike cells with interconnected walls along the orientation direction could recover well in the release process, resulting in desirable piezoresistive behaviors. A nice reproducibility of the piezoresistive behaviors has been obtained, which was proved by multiple cyclic compression-release measurements. A lamp was illumined in a circuit assembled with the flexible conductive foam to demonstrate its potential as a pressure sensor. The capabilities of the aligned CNT/TPU foam in human motion detection were also evaluated; an excellent reliability has been achieved in monitoring the motions of jump, walk, arm bending, tiptoe, and squat. On the basis of the results, our aligned CNT/TPU foam is believed to be a promising candidate for the application as high-performance pressure sensor due to its flexibility, lightweight, high compressibility, good linearity, and excellent reversibility and reproducibility.

The resistance then recovers because of the separation of the contact porous cell walls when the strain is released from 50 to 0%, and it can almost return to the original value owing to the good elasticity of the TPU matrix and the introduction of the novel aligned structure. In this test, no obvious decline is observed, indicating that the sensing signal could maintain a nice stability. Furthermore, the drift characteristic of the pressure sensor was studied by applying a constant strain at 60% for 1 h. Here, the “drift characteristics” means the relative change of resistance under a certain strain during a period of time.11 In our research, only about 5% drift of the resistance is observed by applying a constant strain of 60% for 1 h, indicating that the pressure sensors are suitable for long-term use. It is universally acknowledged that durability and reproducibility are crucial indications in practical application for pressure sensors.17,51 To study the cyclic durability and reproducibility of the flexible aligned CNT/TPU foam, the pressure sensor was subjected to 2000 compression-release cycles under the strain of 30%, as shown in Figure 9a,b. It is evident that even after 2000 cycles, the resistance of aligned CNT/TPU foam can also maintain favorable repeatability and no obvious variation was observed. This result implies the long lifetime and nice reliability of our aligned conductive CNT/ TPU foam. A simple circuit was constructed with the aligned CNT/TPU foam to test the compression performance, as shown in Figure 9c, Movie S2, and Figure S7. A light emitting diode (LED) lamp was illumined in a circuit assembled with our aligned conductive CNT/TPU foam. Interestingly, the brightness changed with the compression and release processes of the aligned CNT/TPU foam. More concretely, the lamp became brighter when the aligned foam was compressed and became dark again when it was released. This sample device can reveal the potential application of this lightweight and flexible aligned foam as a pressure sensor in motion detection, environment and stress monitoring, etc. 3.5. Human Motion Monitoring. To demonstrate the capabilities of the aligned CNT/TPU foam in human motion detection, the composites were attached to a different place, a volunteer. The aligned foam was adhered to the sole of a shoe to detect the jump and walk motion (Figure 10a,b). When jumping or walking, the sample was compressed and the resistance declined; when legs were lifted, the sample was released and the resistance returned to the original value. Furthermore, the sample was fixed on the elbow to detect arm bending (Figure 10c). Figure 10d shows the sensing performance of the flexible pressure sensor during the arm bending. The resistance declined with the applied deformation and immediately recovered to its initial value when the deformation was released. Here, our CNT/TPU sensor shows excellent long-time stability, high response rate, and excellent durability. Figure 10e,f shows the response of the pressure sensor with the tiptoe movement. It is worth noting that the resistance response is very stable and the shape of the response curves is obviously different from that of the other motions. In addition, the pressure sensor was attached beside the knee to test the resistance change when squatting; good stability and fast response characteristics were also achieved, as shown in Figure 10g,h. Some online human motion detections are shown in Movie S3. All of the results show that our aligned CNT/ TPU foam has excellent reliability in human motion detection.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b16975. Cell size and size distribution of the (a) virgin TPU foam (b) CNT/TPU foam, Figure S1; digital photographs of the compression and release process of aligned conductive CNT/TPU foam, Figure S2; compression process of aligned CNT/TPU foam in mechanical properties test with the CNT content of 0.0048 vol %, Figure S3; compression strength of disordered and aligned TPU foam, Figure S4; modulus and density of samples with different morphologies and CNT content, Figure S5; compressibility of aligned CNT/TPU foam at a strain of 30% for 1st, 20th, 50th, and 100th cycles, Figure S6; resistance variation with the compression and release processes of the conductive CNT/TPU foam, Figure S7 (PDF) Compression and release processes of the aligned CNT/ TPU foam, Movie S1 (AVI) LED lamp illumined in a circuit assembled with our aligned conductive CNT/TPU foam to test the piezoresistive performance, Movie S2 (AVI) Human motion detections, Movie S3 (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.D.). *E-mail: [email protected] (C.L.). ORCID

Kun Dai: 0000-0002-9877-8552 Notes

The authors declare no competing financial interest. J


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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support for this work by the National Natural Science Foundation of China (contract numbers 51603193, 51773183, 11572290, and 11432003), the National Natural Science Foundation of China, Henan Province Joint Funds (contract number U1604253), the China Postdoctoral Science Foundation (contract number 2015M580637 and 2016T90675), Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (contract number sklpme2016-4-21), and the Special Science Foundation for Excellent Youth Scholars of Zhengzhou University (contract number 1421320041).

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