Highly Sensitive and Stretchable Polyurethane Fiber Strain Sensors

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23649−23658

Highly Sensitive and Stretchable Polyurethane Fiber Strain Sensors with Embedded Silver Nanowires Guan-Jun Zhu,† Peng-Gang Ren,*,†,‡ Han Guo,‡ Yan-Ling Jin,‡ Ding-Xiang Yan,*,§ and Zhong-Ming Li§ School of Materials Science and Engineering and ‡The Faculty of Printing, Packaging Engineering and Digital Media Technology, Xi’an University of Technology, Xi’an 710048, China § College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China Downloaded via UNIV OF SOUTHERN INDIANA on July 17, 2019 at 09:48:12 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Flexible strain sensors have attracted a great amount of attention for promising applications in next-generation artificially intelligent devices. However, it is difficult for conventional planar strain sensors to meet the requirements of miniature size and light weight for flexible electronics. Herein, a highly sensitive and stretchable fiber strain sensor with a millimeter diameter was innovatively fabricated by the capillary tube method to integrate silver nanowires (AgNWs) in polyurethane (PU) fibers. Scanning electron microscopy results demonstrate that AgNWs were embedded into the surface layer of PU fibers and formed completely conductive networks. The unique AgNW networks endow the PU/AgNW fibers with superior electrical conductivity of 3.1 S/cm, high elongation at break of 265%, wide response range of 43%, high gauge factor of 87.6 up to 22% strain, fast response time of 49 ms, and excellent reliability and stability. Such satisfactory stretchability and sensitivity is attributed to the combination of the highly stretchable PU matrix and the embedded architecture of the AgNW conductive network. Moreover, PU/AgNW fibers can be employed as wearable devices to detect various human motions and to drive light-emitting diodes at a lower voltage (2.7 V). KEYWORDS: strain sensor, embedded structure, silver nanowires, polyurethane fiber, human motion monitoring conductive polymer fiber (CPF), consisting of a polymer and a conductive filler, was considered the most promising material to solve the problem of sensor functional failure because of its good flexibility and recoverability of electrical conductivity.16−18 To ensure normal operation at low voltages, a large amount of conductive filler is usually added in the traditional CPF to achieve sufficient conductivity.19−21 This high content of conductive filler significantly deteriorates the stretchability of the polymer fiber. Although the CPF prepared by the coating method exhibits good stretchability and electrical conductivity at low filler contents because of the concentrated distribution of the conductive material on the fiber surface, the

1. INTRODUCTION Strain sensors with highly flexible and sensitive features have great promising applications in various fields, such as wearable display, electronic skins, human motion monitoring, and so forth.1−5 Compared with thin film-shaped and foam-based strain sensors,6−11 fiber sensors are an ideal choice for the new generation of electronic devices due to their unique characteristics of light weight, small size, excellent flexibility, recoverable deformation, and good knittability.12−14 The earliest fiber sensors were usually made by mixing metal wire with textile yarn fiber and were rigid and difficult to stretch or bend.15 The sensor functions of such fibers were often deteriorated or completely lost due to the breaking of the metal wire in practical application of continuous stretch and repeated bends. Therefore, it was inevitable to exploit a novel conductive fiber sensor with high ductility, sensitivity, and stability. The © 2019 American Chemical Society

Received: May 17, 2019 Accepted: June 7, 2019 Published: June 7, 2019 23649

DOI: 10.1021/acsami.9b08611 ACS Appl. Mater. Interfaces 2019, 11, 23649−23658

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic diagram for fabrication of the PU/AgNW fibers.

durability and repeatability cannot meet the operating requirements over a long working period due to the weak adhesion between the conductive material and the polymer surface.22−25 Interfacial debonding and exfoliation of conductive material from the fiber surface, which is a common occurrence, seriously degrades the sensing response under repeated stretching/releasing cycles. It is still challenging to achieve CPF with a good balance of comprehensive performance, including high sensitivity, outstanding stretchability, good conductivity, and excellent stability. Recently, the embedded structure has been seen as an effective strategy for obtaining excellent sensing performances of conductive polymer composites.6,26−28 In such a structure, the conductive filler is embedded into the surface layer of the polymer matrix or in a limited area, establishing a wellconnected conductive network with small amounts of conductive fillers because of the volume exclusion effect. Because of the low content and regional distribution of the fillers, the inherent high elasticity of the polymeric fiber is preserved and, therefore, results in a large response range and high sensitivity. Moreover, the embedded structure exhibits a stable combination of the conductive fillers and the polymer matrix, which endows the sensor with excellent durability and stability. Unfortunately, conventional embedded structures have some irreconcilable drawbacks. For example, a wellcontacted conductive network improves the stability and durability but reduces the sensitivity and stretchability. High sensitivity and stretchability may be achieved by using an

unstable conductive network but decreases in reversibility and stability inevitably follow. Therefore, for a strain sensor, it is particularly necessary to fabricate a conductive fiber with simultaneous high sensitivity, stretchability, excellent conductivity, and long-term working stability. The conductive fillers applied to the strain sensor normally include carbon-based fillers (carbon black, carbon nanotubes, graphene), metal nanowires (copper nanowires, silver nanowires (AgNWs)), and conducting polymers (hybrid nanocomposites).29−33 Because of their excellent conductivity and flexibility, AgNWs may be a very promising conductive filler for flexible fiber strain sensors. Polyurethane (PU) and polydimethylsiloxane are known for their high flexibility, elastic resilience, and good chemical durability, whose characteristics are highly valued in substrates for the sensor.33−36 Herein, we present a novel method of capillary glass tubes (CGT) to fabricate a conductive PU fiber sensor with embedded AgNWs. Because of the dense AgNW conductive networks in the surface layer of the PU fiber, the PU/AgNW fiber not only shows good electrical conductivity (3.1 S/cm) and high elongation at break (265%) but also exhibits high sensitivity, fast response (49 ms), wide response range, and excellent stability. Combined with other advantages such as light weight, small size, and excellent flexibility, the PU/AgNW fiber sensor presents promising applications in wearable smart fabrics and next-generation miniaturized electronics. 23650

DOI: 10.1021/acsami.9b08611 ACS Appl. Mater. Interfaces 2019, 11, 23649−23658

Research Article

ACS Applied Materials & Interfaces

2. EXPERIMENTAL SECTION 2.1. Materials. A AgNW/isopropyl alcohol (IPA) suspension was purchased from Zhejiang Kechuang Advanced Materials Technology Co., Ltd. (Hangzhou, China). The average diameter and length of AgNWs are 30 nm and 15 μm, respectively. PU raw materials (Clear Flex 50), including urethane liquid rubber compound A (4,4′methylenedicyclohexyl diisocyanate) and B (phenylmercury neodecanoate) were purchased from Smooth-On, Inc. (Pennsylvania, USA). The A/B mix ratio is 1:2 by weight; more details on PU are shown in Figure S1 in the Supporting Information. Hydrofluoric acid (HF, 40%), used as an etching agent for CGT, was provided by Tianjin Tianli Chemical Reagents Ltd. (Tianjin, China). CGT with an internal diameter of 0.5 and 100 mm in length were supplied by Shanghai Xinpeng Glass Instrument Co., Ltd. (Shanghai, China). 2.2. Preparation of PU/AgNW Fibers. The fabrication procedure of the PU/AgNW fibers is schematically illustrated in Figure 1. Initially, one end of the CGT was immersed in a AgNW suspension (3 mg/mL) with the AgNW suspension drawn into the CGT by capillary action. Uniform and dense AgNW conductive networks were formed on the inner surface of the CGT after the suspension flowed out of the other end. The CGT containing the AgNW networks was then kept in a drying oven at 60 °C for 1 h to remove the residual IPA. Thus, stable AgNW networks with good adhesion were obtained on the inner wall of CGT. The network density of the AgNWs was adjusted by the coating cycles. The mass of the AgNW coating with various coating cycles is shown in Figure S2. Subsequently, the PU was drawn into the CGT by the negative pressure method and cured at 20 °C for 24 h. After removal of the external CGT using a glass etchant and rinsing with deionized water multiple times, the PU/AgNW fiber with embedded AgNW networks was obtained. The prepared PU/AgNW fibers with various coating cycles are abbreviated as PU/AgNW-x, where x represents the number of AgNW coating cycles. For comparison, pure PU fiber without AgNWs was also prepared using the same method. 2.3. Characterization and Measurement. The surface morphologies of the AgNW networks were observed using scanning electron microscopy (SEM, Inspect-F, FEI, Finland). The sample surface was sputtered with a thin layer of gold for better imaging. The resistance (R) of the PU/AgNW fibers was measured using an auto dc low-ohm meter (model: TH2512/A, Changzhou Tonghui Electronic Co., Ltd., China). The electric conductivity (σ) was calculated according to σ = L/R·S, where S and L are the cross-sectional area and length between the electrodes, respectively. Copper electrodes were attached to two end sections of the PU/AgNW fiber. The distance between the two electrodes was 20 mm. All samples were coated with silver paste to ensure good contact between the sample and the electrodes. During cyclic extensions, resistance was measured using an insulation resistance meter (model: TH2684, Changzhou Tonghui Electronic Co., Ltd, China). Tensile tests were conducted on a universal tensile testing machine (UTM2503, Shenzhen Sun Technology Stock Co., Ltd, China.) at a rate of 5 mm/min. The gauge factor (GF), which is frequently used to evaluate the sensitivity of a strain sensor, is estimated using the following formula: GF = ΔR/ εR0, where ΔR is the measured resistance (R) minus the initial resistance (R0), and ε is the applied strain. In human motion monitoring, PU/AgNW fibers were attached to the surface of the skin using tape to directly detect the change in relative resistance at various states of motion.

Figure 2. Surface SEM images of PU/AgNW fibers with different numbers of AgNW coating cycles. (a) 1 cycle, (b) 3 cycles, (c) 5 cycles, and (d) 7 cycles.

fiber and form perfect conductive networks. This demonstrates that migration and agglomeration of AgNWs do not occur during injection and curing of the PU. A relatively sparse but completely conductive network is observed with one AgNW coating (Figure 2a), indicating that an electrically conductive path is formed using only a small amount of AgNWs, often called the conductive percolation threshold (Figure S4). With an increase in the number of AgNW coating cycles, the overlap points between AgNWs increase, and the conductive network becomes more and more perfect (Figure 2b,c). As the number of AgNW coating cycles increases to 7, AgNWs come into close contact with each other and form a perfect and denser conductive network (Figure 2d). A closer inspection of the conductive network reveals that most of the AgNWs are embedded inside the PU matrix, and a small group of them is exposed to the surface of the PU (see the solid red arrows in the SEM images). Much of the interstitial space among the AgNWs will be easily occupied by the subsequently injected PU matrix due to the low viscosity of the PU raw materials. Thus, almost all AgNWs are embedded tightly into the cured PU, leading to strong resistance to exfoliation of the AgNW layer. Such an anchoring effect of the embedded structure endows the AgNW networks with a deformation identical to that of the PU matrix. Therefore, the prepared PU/AgNW fibers possess unique characteristics of recoverable deformation of the conductive network. This is of great importance for realizing excellent durability, reliability, and high sensitivity of the PU/AgNW fibers. 3.2. Mechanical Properties. Mechanical properties are important in the applications of strain sensors. The monotonic tensile tests of pure PU and PU/AgNW fibers with varying numbers of AgNW coating cycles are carried out, and the typical stress−strain curves at a tensile rate of 5 mm/min are depicted in Figure 3a. Overall, all fibers exhibited similar stress−strain behavior consisting of two distinct linear elastic deformations with a sharp increase in stress at strains below 22% and a slow increase of stress at subsequent strain levels. The stress−strain curves of all fibers almost coincide within the 22% strain, indicating that the effect of the addition of AgNWs on the mechanical properties of the PU fiber under low strain conditions is negligible. With the increase in the number of coating cycles, the tensile strength of the fiber improved slightly, whereas the elongation at break decreased slightly. Compared with that of pure PU fiber, the tensile strength of

3. RESULTS AND DISCUSSION 3.1. Morphology. Figure 2 shows the surface SEM images of the PU/AgNW fiber after different numbers of AgNW coating cycles. The panoramic SEM images of the PU/AgNW fibers at different magnifications are shown in Figure S3. The complete nanowire structure and the overlapping conductive networks of AgNWs indicate that the AgNWs are not damaged during the process of curing the PU and etching the CGT. The AgNWs are evenly embedded in the surface layer of the PU 23651

DOI: 10.1021/acsami.9b08611 ACS Appl. Mater. Interfaces 2019, 11, 23649−23658

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ACS Applied Materials & Interfaces

Figure 3. Mechanical properties of pure PU fiber and PU/AgNW fibers: (a) Representative stress−strain curves and (b) elongation at break and tensile strength. Digital images of the PU/AgNW-7 fiber at different states: (c) initial state, (d) stretched state, (e) bent state, and (f) twisted state.

PU/AgNW-7 only increased by 9.8% from 447 to 491 kPa, and the elongation at break decreased from 274 to 266%, an approximate 97% strain retention rate (Figure 3b). These results are attributed to the low content and regional distribution of the AgNWs in the PU fiber surface, which has little influence on the inherent structure of the PU matrix and, therefore, results in invariable mechanical properties of the PU fiber. Multiple cycles of the strain−stress curve indicate that the prepared PU/AgNW fiber exhibits excellent reliability of mechanical behavior during loading and unloading (5% strain, 10 mm/min rate) after 2500 cycles (Figure S5 in the Supporting Information). Moreover, the excellent stretchable, bendable, and twistable behavior (as shown in Figure 3c−f) reveals that the prepared PU/AgNW fibers have outstanding deformation ability, which paves the way for practical applications in large tensile strain-sensing and flexible electronic devices. 3.3. Electrically Conductive Property. The electric conductivity of PU/AgNW fibers with different numbers of AgNW coating cycles is demonstrated in Figure 4. The pure PU fiber exhibits obvious insulation characteristics with electrical conductivity of approximately 10−15 S/cm. A remarkable increase in electrical conductivity (5.2 × 10−5 S/ cm) by approximately 10 orders of magnitude was observed for the PU/AgNW composite fiber with only one AgNW coating cycle. Such superior electrical performance is attributed to the inherently high conductivity and complete network of AgNWs in the surface layer of the PU fiber. The electrical conductivity of the PU fibers increased with increasing number of AgNW coating cycles, which can be explained by the increasingly dense conductive networks and the formation of more new conductive pathways with the increase in overlapping AgNWs. These results are consistent with the SEM images in Figure 2. The electrical conductivity was observed to show almost no

Figure 4. Electric conductivity of PU/AgNW fibers with varying numbers of AgNW coating cycles.

enhancement when the AgNW coating cycles exceeded seven (see inset in Figure 4), demonstrating that a highly developed and stable conductive network was formed, and the increased AgNWs thereafter hardly affected the electrical conductivity. Therefore, the PU/AgNW-7 (3.1 S/cm) was chosen for investigation of the electromechanical properties of the fiber strain sensor. The stable and high electric conductivity of the PU/AgNW fibers enable them to be used as strain sensors that output a stable signal, which is another important parameter of the sensor. 3.4. Strain Sensing Behavior. The relationship between relative resistance (ΔR/R0) and strain of the PU/AgNW-7 fiber is shown in Figure 5a. The relative resistance of PU/ AgNW-7 increases with increasing strain. This may be attributed to the change in the contact conditions for the AgNW network, such as contact quantity, separation, and damage to contacts. A schematic of these changes in the 23652

DOI: 10.1021/acsami.9b08611 ACS Appl. Mater. Interfaces 2019, 11, 23649−23658

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ACS Applied Materials & Interfaces

Figure 5. Plots of (a) relative resistance change vs strain for PU/AgNW-7 fiber strain sensors. (b) Schematic illustration showing the conductive networks of the PU/AgNW fiber strain sensor at the unstretched state and stretched state, in which the PU fiber is shown as the green cylinder and the AgNWs are shown as solid yellow lines; the AgNW contact joints are indicated by dashed white circles. (c) Multicycle test of PU/AgNW fiber sensors under 5% strain at a rate of 10 mm/min for 2500 cycles.

range are attributed to the special embedded structure. AgNWs are fixed to the fiber surface and can only be deformed together with the PU fiber, which contributes to the high and stable GF at full range. The stability and durability of the PU/AgNW fibers were evaluated. As shown in Figure 5c, during 2500 stretching and releasing cycles under 5% strain at 10 mm/min, the relative resistance output was nearly invariable except for the initial few cycles because of the permanent destruction of the partial conductive network. The conductive network maintained a stable state after the process of destruction and reconstruction for several cycles. The relative resistance generally reached a constant value at the maximum strain and returned to the initial value after the strain was removed in each cycle. This indicates that the prepared PU/AgNW fibers exhibit remarkable cycling stability and repeatability. All these properties originate from the unique conductive network with the strong interaction between the AgNWs and the PU fiber. More specifically, the novel embedded structure of the conductive network endows the PU/AgNW fiber with excellent recovery capability. Additionally, the slight resistance hysteresis during stretching/release can be attributed to the viscoelasticity of the PU matrix (Figure S6). The curves of relative resistance and strain versus time between 0 and 10% strain are plotted in Figure 6a. The relative resistance follows the strain closely, and the relative resistance curve is almost exactly matched with the cyclic strain curve during cyclic stretching. The relative resistance curve is almost symmetrical, and the relative resistance recovers to its initial state when the applied strain is released to 0, indicating a reversible and reproducible change of resistance during the strain cycle. In addition, the relative resistance increases and decreases quickly in both the stretching and releasing processes. The instant change of relative resistance with

AgNW conductive network with applied strain is illustrated in Figure 5b. A typical correlation between relative resistance and strain up to 43% indicates a wide strain-sensing range and good reliability of the PU/AgNW fiber as a sensor. More importantly, the relative resistance is almost proportional to the strain up to 22%. This is attributed to the poor contact between the AgNWs in the linear elastic region. As the strain increases, the AgNWs gradually separate because they must maintain a deformation identical to that of the PU matrix, resulting in a reduction in the quantity of the AgNW contact joints. Therefore, the relative resistance increases constantly. It is worth noting that an abrupt increase in relative resistance appears at strains larger than 35%, and the maximum value approaches 2 × 104% at the 43% strain. This can be explained by damage to or even the complete destruction of the AgNW network in a large deformation. The unique piezoresistive behavior of PU/AgNW fibers can effectively identify the external strain when used as a sensor. To further explore the strain-sensing performance of PU/ AgNW fibers, GF is introduced to represent the sensitivity of the strain sensor. As shown in Figure 5a, the plot is divided into three stages (A, B, C). In stage A (up to 22% strain), the relative resistance increases linearly with increasing strain. The AgNW conductive network exhibits good recoverability and multicycle stability, demonstrating that the PU/AgNW fiber can act as a sensor to achieve stable detection. The GF in this stage was calculated to be 87.6, which is higher than that reported in many other studies.3,23,34,37−39 However, the GF increases rapidly to 519.3 in the B stage (22−39% strain) and to 3.05 × 103 in the C stage (over 39% strain), indicating high sensitivity of the PU/AgNW fiber under large strain. In these stages, the AgNW conductive network encounters more serious destruction than in the A stage. The high sensitivity under both small and large strain as well as the large sensing 23653

DOI: 10.1021/acsami.9b08611 ACS Appl. Mater. Interfaces 2019, 11, 23649−23658

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ACS Applied Materials & Interfaces

Figure 6. (a) Curves of relative resistance (blue) and strain (green) vs time between 0 and 10% strain at a tensile rate of 5 mm/min. (b) Response time of the PU/AgNW fiber strain sensor. (c) Relative resistance variation vs cyclic tensile strain of 5, 10, 15, and 20% at a rate of 5 mm/min. (d) Relative resistance response of a cycle from 0 to 30% and then to 0% with an incremental step of 5%. (e) Relative resistance change under cyclic stretching-releasing with a strain of 10% at tensile rates of 1, 3, 5, 7, and 9 mm/min. (f) Current−voltage curves under different strains (0, 5, 10, and 15%) for the PU/AgNW fiber strain sensor.

varying strain and the synchronization of the curves indicated that the PU/AgNW fiber has a fast response to external strain, which ensures synchronic monitoring to stretch strains. As depicted in Figure 6b, an additional test was carried out to evaluate the response time under tiny stretching (2% strain) with a high rate (1000 mm/min); the PU/AgNW fiber shows fast response/relaxation time (49 and 99 ms). The fast response of the fiber can be ascribed to the desirable embedded conductive network structure in which AgNWs combined with PU and the deformation of PU drove the immediate change in the AgNW network. The strain-sensing performance of the fiber was investigated by real-time monitoring of the relative resistance when loading under different strains. Figure 6c shows the relative resistance of PU/AgNW fibers at different maximum strains (5, 10, 15, and 20%) under cyclic stretching release at a rate of 5 mm/

min. The peak of relative resistance increases with increasing strain due to the increased separation between AgNWs and the destruction of partial conductive networks. These distinguishing signals in response to different strains make the PU/AgNW fibers capable of detecting and differentiating various movements. As shown in Figure 6d, the PU/AgNW fiber was applied to a strain from 0 to 30% and then to 0% again with an incremental step of 5% and a residence time of 30 s. When increasing the strain from 0 to 30%, the relative resistance increases gradually because of the poor contact and separation of the AgNWs. During the residence period, the variation of the sensing responsivity is very feeble. The relative resistance then recovers because of the reconstruction of the AgNW contact joints when the strain is released from 30 to 0%, and it can almost return to the original value due to good elasticity of the PU 23654

DOI: 10.1021/acsami.9b08611 ACS Appl. Mater. Interfaces 2019, 11, 23649−23658

Research Article

ACS Applied Materials & Interfaces

Figure 7. Digital images showing the PU/AgNW fiber strain sensors and brightness of the LED at the (a) initial state, (b) stretched state, and (c) recovered state.

Figure 8. Detection of various human motions using PU/AgNW strain sensors. Responsive curves of a PU/AgNW strain sensor on (a) the finger, (b) the wrist, and (c) the elbow under cyclic bending. Monitoring of small strains caused by muscle movement for functions of the cheek, chin, and forehead. The corresponding response signals caused by (d) pouting, (e) smiling, and (f) frowning. (g) Photograph of the PU/AgNW strain sensor attached to the throat. Signals showing the tiny movements caused by (h) deep breathing and (i) swallowing.

under the same strain demonstrates the good stability and reliability of PU/AgNW fibers, which ensures the application of PU/AgNW fibers in the detection of various motions. It is noteworthy that the shape of relative resistance varied with each other at different tensile rates. The higher the tensile rate, the narrower and sharper the waveform. By analyzing the distinguishing waveform of the relative resistance, both the deformation quantity and tensile rate can be accurately detected. Typical current−voltage (I−V) curves of PU/AgNW fibers under various tensile strains are shown in Figure 6f. Excellent

matrix and introduction of the novel embedded structure. In this test, the continuous and consistent response curves suggest that the PU/AgNW fiber has high stretchability and excellent stability and can be applied to the monitoring of both tiny and large motions. Figure 6e exhibits the effect of tensile rate on the strainsensing property of PU/AgNW fibers under the same strain (10% strain). The peak of relative resistance is almost the same at different tensile rates, indicating that the change in relative resistance is independent of tensile rate under the same strain. The constant relative resistance output to different tensile rates 23655

DOI: 10.1021/acsami.9b08611 ACS Appl. Mater. Interfaces 2019, 11, 23649−23658

Research Article

ACS Applied Materials & Interfaces

distinguished by the PU/AgNW fiber strain sensor. In addition, the PU/AgNW fiber strain sensor could also be attached to the elbow (Figure 8c and Video S4 in the Supporting Information) to detect and discriminate the motions of extending/flexing. The bending of the elbow is precisely tracked by monitoring the relative change in the resistance. The response curve of the elbow bending has a characteristic double peak, which is caused by the special joint structure of the elbow. These results demonstrate that the PU/ AgNW fiber strain sensor has the capability to accurately detect some subtle changes in the monitoring of large-scale movement. Apart from the detection of large motions, the PU/AgNW fiber strain sensor is qualified for monitoring tiny movements, benefiting from its high sensitivity and piezoresistive performance under small-scale tensile strains. The PU/AgNW fiber strain sensors were attached to the cheek, chin, and forehead to monitor the tiny muscle movements induced by expression, as shown in Figure 8d−f. The resistance change of the sensors, corresponding to the stretching of facial muscle induced by cheek bulging (Figure 8d and Video S5 in the Supporting Information), smiling (Figure 8e), and frowning (Figure 8f and Video S6 in the Supporting Information) could be precisely recorded. Especially in Figure 8f, the relative resistance caused by the forehead moving upward is different from the relative resistance caused by the forehead moving downward. It is noteworthy that signal patterns varied when different expressions were made because each expression resulted in different motions of the facial muscle. The significant differences between these relative resistance signals reveal a good possibility for this sensor to act as an expression recognition device. To further demonstrate the capabilities of this flexible sensor in the monitoring of subtle motions, a PU/AgNW fiber was mounted on the neck to noninvasively monitor the epidermis and muscle movement near the throat (Figure 8g). When the tester took a deep breath, the motions of the epidermis around the throat resulted in unique relative resistance patterns (Figure 8h and Video S7 in the Supporting Information), which were different from the signal patterns caused by swallowing (Figure 8i). The difference of the relative resistance signals between them can be used to distinguish the motions near the throat, which makes it possible for this sensor to monitor and distinguish human physiological activities. Based on the above results, it can be seen that the prepared PU/AgNW fiber exhibits outstanding stability and repeatability in the monitoring of human motions, which is attributed to the high sensitivity and wide sensing range of the AgNW network embedded in the surface of a flexible PU fiber.

linear correlation between voltage and current indicates that good Ohmic behavior of the PU/AgNW fiber is achieved regardless of the various applied strains. Furthermore, it is worth noting that the slope of the I−V curve decreases gradually with an increase in strain, revealing that the relative resistance increases when a larger strain is applied. This is consistent with the result in Figure 5a. To visually demonstrate the electrical resistance change behavior of the PU/AgNW fibers during the stretching process, one light-emitting diode (LED) and the as-fabricated fiber (PU/AgNW-7) were connected to a simple circuit (Figure 7; the operating voltage was 2.7 V). The LED was lit successfully and shone brightly without stretching (Video S1, Supporting Information). This indicated that the PU/AgNW-7 fiber has high conductivity and can work at low voltage (Figure 7a). When the fiber was stretched, the brightness of the LED decreased (Figure 7b). This intuitively indicates that the electrical resistance of the PU/AgNW fibers increased during stretching. The light intensity of the LED increased during release (Figure 7c), and the final brightness was the same as the initial, indicating that the deformation and damage of the conductive network can be recovered during the release process. These demonstrations confirm that the flexible and stretchable PU/AgNW fiber strain sensor displayed a good balance of sensitivity, conductivity, and durability, making it a potential competitor for low voltage-driven wearable sensors. 3.5. Human Motion Monitoring. PU/AgNW fiber exhibits the merits of excellent flexibility, high sensitivity, fast response, durability, and broad working range, which are very suitable for full-range monitoring of human activities. The applications of PU/AgNW fiber strain sensors in human activity monitoring are demonstrated in Figure 8. The human activities to be monitored are divided into large motions (joint bending) and subtle motions (muscle movement). To facilitate monitoring of human motions, the sensors were fixed to the skin to monitor various motions. In Figure 8a, the repeated bending and straightening of a finger induce the sensor with typical relative resistance variations. The relative resistance of the sensor increases during the bending and returns to the original value after the index finger joint restored to its original status (Video S2, Supporting Information). The sensor showed an almost synchronous response to the motion state of the index finger joint. The waveform and amplitude of the relative resistance are precisely recorded and have a one-to-one correspondence with the rate and degree of the bending of joints, respectively. Close inspection of the relative resistance in Figure 8a shows that the response curves to the bending of the same joint are not exactly the same, which implies that the PU/AgNW fiber sensor is very sensitive and can detect tiny differences between the two bending processes. Figure 8b illustrates the strain-sensing behavior of a PU/AgNW fiber in wrist flexion monitoring. As the wrist flexion changes, the relative resistance corresponding to the state of motion of the wrist flexion is accurately recorded in real time (Video S3, Supporting Information). Compared with the finger, the bending deformation of the wrist is lower, so a smaller change of relative resistance is obtained (approximately 1800 for the wrist and 6500 for the finger). In addition, the bending speed of the finger is faster than that of the wrist, so the change rate of the finger (approximately 12 s for one cycle) is also faster than that of wrist (approximately 15 s for one cycle). Based on the clearly different patterns of the deformation response, the motions of index fingers and wrist joints could be easily

4. CONCLUSIONS In summary, we developed an innovative strategy to fabricate a PU fiber strain sensor with embedded AgNWs using the capillary tube method. The uniformly embedded AgNWs on the PU fiber surface connected with each other and formed a well-developed conductive network. The unique embedded conductive network endowed the PU/AgNW fiber with a superior electrical conductivity of 3.1 S/cm, high break elongation of 265%, wide sensing range of up to 43% strain, high sensitivity (GF of 87.6 up to 22% strain), fast response time of 49 ms, and excellent reliability and stability (multicycle test of 2500 cycles). Both large and subtle human motions were accurately monitored in real time by the PU/AgNW fiber 23656

DOI: 10.1021/acsami.9b08611 ACS Appl. Mater. Interfaces 2019, 11, 23649−23658

Research Article

ACS Applied Materials & Interfaces

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sensor. The results indicated that our strain sensors have wide application prospects in smart wearable devices. This study provides a practical and meaningful strategy to fabricate fiber strain sensors with excellent comprehensive performance for human motion monitoring.

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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.9b08611. Technical overview of PU; TGA curves of pure PU fiber and PU/AgNW-7 fiber; panoramic SEM images of PU/ AgNW-7 fibers at different magnifications; log σ versus log (n − nc) plots based on a power law relation, where n is the AgNW coating cycle and nc is the percolation threshold; and multiple cycles of the strain−stress curve (PDF) Visual demonstration of electrical resistance change behavior of the PU/AgNW fibers during the stretching process (MP4) Human motion monitoring (MP4) Strain-sensing behavior of a PU/AgNW fiber in wrist flexion monitoring (MP4) PU/AgNW fiber strain sensor attached to the elbow to detect and discriminate the motions of extending/flexing (MP4) Resistance change of the sensors induced by cheek bulging (MP4) Resistance change of the sensors induced by frowning (MP4) PU/AgNW fiber mounted on the neck to noninvasively monitor the epidermis and muscle movement near the throat (MP4)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.-G.R.). *E-mail: [email protected] (D.-X.Y.). ORCID

Ding-Xiang Yan: 0000-0002-9563-2910 Zhong-Ming Li: 0000-0001-7203-1453 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science Foundation of China (grant nos. 51573147, 21706208, 51773167, and 21704070).

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REFERENCES

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DOI: 10.1021/acsami.9b08611 ACS Appl. Mater. Interfaces 2019, 11, 23649−23658

Research Article

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DOI: 10.1021/acsami.9b08611 ACS Appl. Mater. Interfaces 2019, 11, 23649−23658