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Flexible and Transparent Strain Sensors with Embedded Multiwalled-Carbon-Nanotubes Meshes Bangbang Nie, Xiangming Li, Jinyou Shao, Xin Li, Hongmiao Tian, Duorui Wang, Qiang Zhang, and Bingheng Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12987 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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ACS Applied Materials & Interfaces

Flexible and Transparent Strain Sensors with Embedded Multiwalled-Carbon-Nanotubes Meshes Bangbang Nie, Xiangming Li, Jinyou Shao*, Xin Li, Hongmiao Tian, Duorui Wang Qiang Zhang, Bingheng Lu Micro- and Nano-technology Research Center, State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China *Correspondence to: [email protected]

Keywords: strain sensors; transparent sensors; high sensitivity; high stability; embedded multiwalled carbon nanotubes meshes ABSTRACT Strain sensors combining high sensitivity with good transparency and flexibility would be of great usefulness in smart wearable/flexible electronics. However, the fabrication of such strain sensors is still challenging. In this study, new strain sensors with

embedded

multiwalled

carbon

nanotubes

(MWCNTs)

meshes

in

polydimethylsiloxane (PDMS) films were designed and tested. The strain sensors showed elevated optical transparency of up to 87% and high sensitivity with a gauge factor of 1140 at a small strain of 8.75%. The gauge factors of the sensors were also found relatively stable since they did not obviously change after 2000 stretching/releasing cycles. The sensors were tested to detect motion in the human body, such as wrist bending, eye blink, mouth phonation and pulse, and the results were shown satisfactory. Furthermore, the fabrication of the strain sensor consisting of

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mechanically blading MWCNTs aqueous dispersions into microtrenches of prestructured PDMS films was straightforward, low-cost, and resulted in high-yield. All these features testify of the great potential of these sensors in future real applications.

1. INTRODUCTION Flexible strain sensors coupled with transparent characteristics are widely demanded in a variety of applications ranging from structural health monitoring1,2 and artificial skin3-11 to wearable devices12-20 and flexible touch screens/displays.21-23 Mechanical stretching of conductive materials results in resistance change. This mechanism is used by most strain sensors to detect elongation changes in an object. Based on this mechanism, different conductive thin films were explored to fabricate strain sensors. Metal foils or films with high conductivities can be employed as strain sensors but their non-transparent features and low-sensitivities limit their practical applications.24 Free-standing

composite

films,

containing

polymer

matrix

like

polydimethylsiloxane (PDMS) or polymethyl methacrylate (PMMA) combined with conductive carbon nano-materials like carbon nanotubes (CNTs) or graphene, have been reported as strain sensors with improved flexibility and sensitivity.25-31 However, their transparency features are still limited by the presence of carbon nanomaterials, randomly distributed in the matrix. Also, their initial conductivities, determining their sensitivities, are often much tricky by the percolation threshold of carbon nanomaterials.32


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The deposition of conductive materials, such as CNTs, graphene, silver nanowires onto flexible substrates like PDMS films is another commonly used strategy to fabricate strain sensors.12,33-42 The transparency features of the resulting flexible strain sensors were well improved by properly thinning the conductive films. However, it is still challenging to achieve strong interfacial adhesion between the conductive materials and substrates, often leading to degradation of stability over time. Graphene woven fabrics with conductive inter-connected graphene and optically transparent openings have recently been used as strain sensors with existing transparent and flexible characteristics.43-46 However, the line widths of graphene woven fabrics generated by means of atmospheric pressure chemical vapor deposition (CVD) on copper meshes followed by chemical etching of the copper core are typically in the order of 0.1 mm.43 Therefore, the graphene ribbons are still visible to the naked eyes, generally undesirable from aesthetic view of the human body during daily activities. Overall, the fabrication of strain sensors with integrated high transparency, elevated flexibility, superior sensitivity and relevant stability is still challenging. In this study, new transparent and flexible strain sensors with high sensitivities were fabricated by embedding multiwalled carbon nanotubes (MWCNTs) into interconnected microtrenches of PDMS films. The strain sensors showed high sensitivity with a gauge factor of 1140, and high cycling stability more than 2000 times without decline of the sensitivity. The transparency of the strain sensors reached up to 87%, and the meshes of MWCNTs were invisible to the naked eyes. The strains



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sensors showed superior transparency, flexibility and sensitivity features when compared to other reported state-of-the-arts strain sensors. The real applications of the proposed strain sensors for detecting activities in the human body, such as wrist motion, eye blink, speech and heartbeat were also tested, and the results were shown very promising.

2. EXPERIMENTAL SECTION

2.1. Materials and equipment The MWCNTs ink was commercially purchased from XFNANO, China, and dispersed in water. The concentration of MWCNTs was 10 wt %, the dispersant content of XFWDIS was 1.4wt%, and the average diameter and length of MWCNTs were 80 nm and 10-20 µm. The PDMS prepolymer and crosslinker (Sylgard 184) were purchased from Dow Corning, USA. The contact angles were measured by contact angle meter (dataphysics, Germany). The strains were provided by a linear motor, and the electrical currents were measured by a source meter (KEYSIGHT, B2912A, Precision Source/Measure Unit). The morphology details of Si templates and PDMS films were visualized by confocal laser scanning microscopy (CLSM) (OLS4000, Olympus) and scanning electron microscopy (SEM) (SU8010, Hitachi). 2.2. Fabrication of flexible micro-trenched PDMS films PDMS prepolymer and crosslinker were mixed at the weight ratio of 10:1 then stirred for about ten minutes. The mixture was drop casting onto Si templates with


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raised mesh-like structures, and then degassed by vacuum assistant. After curing on a hot plate at 80℃ for 6 hours, flexible PDMS thin films with microtrenches were obtained and peeled off from the Si templates. The Si templates were fabricated by conventional photolithography and dry etching processes. 2.3. Filling of MWCNTs into microtrenches of PDMS films The PDMS films were treated by oxygen plasma (300 W, 50 s) and became hydrophilic. A proper amount of the MWCNTs ink was drop cast onto the micro-trenched PDMS film. A doctor blade was used to wade and drag the MWCNTs ink forward-backward over the PDMS films. During the movement of the doctor blade, the dispersed MWCNTs filled into the microtrenches thanks to the capillary force acting on the dispersion in the microtrenches. Next, the MWCNTs in the microtrenches were dried at 60℃ on a hot plate for 5 min, and excess MWCNTs were removed from the PDMS film surfaces by ethanol-assisted scraping. Followed by sintering at 130℃ for 30min, flexible and transparent strain sensor films with embedded MWCNTs meshes were obtained. 2.4. Testing of the strain sensors The fabricated flexible and transparent strain sensors were attached to the test platform of a linear motor, which was used for controlling the strains. Silver paste was painted on both ends of the strain sensors and conductive tapes were used to link the silver paste to a source meter, which supplied the voltage to the strain sensors and collected the current flow. The application of a constant voltage to the strain sensors induces changes in the electrical current under different strains. The changes in



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resistance of the strain sensor could be calculated by the Ohm's law (R = U/I).

3. RESULTS AND DISCUSSION Figure 1 shows the fabrication process of the transparent and flexible strain sensors. A transparent and stretchable PDMS thin film with interconnected mesh-like microtrenches was fabricated by the micromolding process shown in Fig. 1(a). The procedure consisted of drop casting a liquid mixture containing PDMS prepolymer and a crosslinker onto a Si template with raised mesh-like structures, and then spun into a thin film of proper thickness (300 µm). After thermal curing of the PDMS deposition, the resulting film was peeled off from the Si template to yield a micro-trenched PDMS film with apparent flexibility and transparency. However, MWCNTs ink was difficult to drop on the surface of PDMS films on PDMS supports due to the hydrophobicity of PDMS (Fig. S1). After oxygen plasma treatment, the PDMS film became hydrophilic and the contact angle changed from 112.3°to 19.9°.Next, an ink containing MWCNTs was filled into the mesh-like microtrenches of the PDMS film using a doctor-blading process (Fig. 1(b)). The MWCNTs residues left on the PDMS surface can be scraped away by a clean wiper. Next, the PDMS film with embedded MWCNTs was heated at 130℃ for 30 min to remove any impurities left in the MWCNTs and result in strain sensors with embedded mesh-like MWCNTs with better conductivity. Since the formation of MWCNTs meshes did not require high-cost chemical vapor growth, time-consuming mixing processes or suffers from interfacial adhesion issues between the conductive materials and substrates, hence the proposed method should be low-cost and


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high-yield which should be suitable for mass production.

Figure 1. The fabrication process of flexible and transparent strain sensors. (a) Illustration of micomolding of transparent and flexible PDMS film with mesh-like microtrenches. (b) Illustration of filling MWCNTs ink into the mesh-like microtrenches of PDMS film using a doctor-blading process. (c, d) SEM images of the strain sensor with MWCNTs embedded in the microtrenches. The inset curves (d) show the profiles of the microtrench before and after embedding the MWCNTs. (e) MWCNTs embedded into the



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microtrenches of 4-µm-width (Ⅰ), 2-µm-width (Ⅰ), 1.5-µm-width (Ⅰ) and 1-µm-width (Ⅰ). Snapshot of a strain sensor located on the logo of Xi’an Jiaotong University (f) or twisted by hands (g).

To obtain proper transparency for the strain sensors, the width of the non-transparent microtrenches region was kept to 10 µm or smaller (Fig. 1c and 1d), which was invisible to the naked eyes. Meanwhile, the size of the spacing between microtrenches was kept to 100 µm or more, which was tens of times larger than the microtrench width (Fig. 1c and 1d). When the microtrench width was invariant and the spacing was smaller, the transparent mesh openings would be smaller and transparency would be poor. Thus, a spacing of 100 µm was selected to fabricate the sensors to ensure good transparency. The experiments to test the minimum microtrench width for embedding MWCNTs were also performed and the results were shown in Fig. 1e. It can be seen that MWCNTs were easily embedded into microtrenches with 4-µm-width, 2-µm-width and 1.5-µm-width. However, filling into the microtrenches with 1-µm-width became challenging. Almost MWCNTs were on the surface of the microtrenches with 1-µm-width, which indicated that the capillary force could not fill WMCNTs into the microtrenches with width smaller than 1 µm. Therefore, MWCNTs could be well filled into microtrenches with width larger than 1.5 µm. Small microtrench (6 µm) and large spacing (100 µm) obtain relative large mesh opening, allowing the strain sensors to acquire better transparency, as shown in Fig. 1f. The flexibility of PDMS thin film coupled with the good mechanical interlock between the embedded MWCNTs and the microtrenched PDMS film resulted in


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relevant mechanical flexibility and stability (Fig. 1g). The strain sensors with microtrenches of 6-µm-width were fabricated as shown in Fig. 2. The Si template with raised mesh-like structures of 6-µm-width (Fig. 2a) was used to fabricate the microtrenched PDMS films. The CLSM image estimated the height of the raised structures to 7.5 µm (Fig. 2d). After spin and curing processes of the mixture containing the PDMS prepolymer and crosslinker, PDMS films with microtrenches of 6-µm-width were obtained (Fig. 2b and 2e). Using the doctor-blading process, the MWCNTs were embedded into the microtrenches of PDMS films (Fig. 2c). The comparison between microtrench depth obtained before (Fig. 2e) and after the doctor-blading process (Fig. 2f) indicated that MWCNTs filled up to the depth of 5 µm, resulting in strain sensors with microtrenches of 6-µm-width.



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Figure 2. SEM images and confocal laser scanning microscopy (CLSM) images of the Si

template (a, d), micro-trenched PDMS (b, e), and embedded MWCNTs in the mesh of microtrenches with 6-µm-width (c, f). Note the embedded cross profiles of the Si template and mcirotrenches before and after embedded MWCNTs.

The performances of the strain sensors with 6-µm-width are shown in Fig. 3. Typical responses of the strain sensors to different strains are gathered in Fig. 3(a). For instance, the change in normalized resistance (∆R/R0) of the strain sensor, measuring the ratio between the resistance change (∆R) to the initial resistance (R0), exhibited a value of 22% under a small strain of 1.25%. When the strain rose to about 10%, the ∆R/R0 significantly increased to 3300%. As anticipated, the ∆R/R0 generally


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increased with the strains of the sensor (Fig. 3b).

Figure 3. (a) The responses of ∆R/R0 to typical strains. Note that the values of ∆R/R0

under different strains can recover to initial values after the strain is released. (b) The



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dependence of normalized resistance changes (∆R/R0) and GFs on the strains. (c) The SEM images of the sensor under the strain of 5%. The inset was locally enlarged SEM image. (d) The dependences of normalized resistance changes (∆R/R0) on strains of the PDMS support without microtrenches. The inset was SEM image of MWCNTs coated on the PDMS support without microtrenches. (e) The dependence of ∆R/R0 and GF on the stretching/releasing cycles at a constant strain of 7%. (f) Time-dependent response of ∆R/R0 at a typical strain of 7.5%. The inset curves suggest the response and recovery time to the strain. (g) Time-dependent response of ∆R/R0 at different frequencies.

In addition to ∆R/R0, the gauge factor (GF) representing changes in the slope of the relative resistance (∆R/R0)/ε, was also assessed to evaluate the sensitivity of the obtained strain sensors. The GF values of the strain sensors ranged from 16 to 330 when the strains increased from 1.25% to 10% (Fig. 3b). These values were significantly improved when compared to most previously reported values. For example, the GF values of metal-foil strain sensors varied typically from 1 to 5, and those of capacitive-type strain sensors were usually smaller than 1.47 The mechanism of the proposed sensors to the strains is caused by microcrack propagation in the microtrenches, as shown in Fig. 3c. This increase in resistance was caused by the formation of microcrack propagation of MWCNTs in microtrenches PMDS film. Before stretch of the strains, microcracks of MWCNTs were not obvious in microtrenches. When the strain sensor was stretched by external loads, the microcracks would extend according to the strain degree. This, in turn, would increase the electrical resistance of the sensor. The microcracks should form and increase in


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number under strains less than 10%. In subsequent cycles, the microcracks would regenerate and recover in the same positions under the stretching/ releasing processes of strains, which was confirmed by the SEM images under the strain of 5%, 10%, and release of the strains ( Fig. S2). Because MWCNTs were limited in the microtrenches of PDMS, the microcracks between MWCNTs should recover after the release of the strain, leading to a decline in resistance of the sensor until the initial value. Small strains and recovery of microcracks resulted in good hysteresis performance (Fig. S3). At strains over 10%, the microcracks resulted in deterioration of the conductive network of MWCNTs, and the current reached almost zero (Fig. S4). Therefore, the performances of the sensors were further investigated under strains of 10%. For comparison, the performance of the PDMS support without the microtrenches was evaluated. After oxygen plasma treatment, a uniform CNT film was spin coated on the PDMS film (Fig. S5). The SEM image of the CNT film estimated the height to about 1 um and the film appeared non-transparent. The resistance of the film was recorded as 2292 Ω, and the normalized resistance change (∆R/R0) was small at strains below 10% (Fig. 3d). Overall, because the MWCNTs were embedded within the microtrenches of PDMS films, the proposed strain sensors were easily fabricated without oxygen plasma treatment, resulting in sensors with high sensitivity than those of PDMS sensors without microtrenches. In real-life applications, the mechanical stability of the strain sensors is important as sensitivity where the sensors often endure thousands of stretching/releasing cycles. Strain sensors with embedded MWCNTs meshes often exhibit significantly improved



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mechanical stability when compared to other forms of strain sensors.5,12 Fig. 3e depicted that the sensitivity of the proposed sensors measured in terms of ∆R/R0 ranged from 524% to 562% during 2,000 stretching/releasing cycles at the elongation of 7%. The changing range of the proposed sensors appeared much smaller than the initially high sensitivity. Strain sensors based on conductive nanomaterials coated on elastic films usually result in unstable sensitivitiy.12,39,42 For example, strain sensors fabricated by transferring silver nanoparticles film onto PDMS sheet induced low and unstable sensitivity, which increased from 20% to 25% after 1000 stretching/releasing cycles at the strain of 10%.12 Also, strain sensors based on patterned graphene showed increased sensitivity but significantly declined from 7,000% to 5,000% after only 180 stretching/releasing cycles at the strain of 25%.5 By comparison, the mechanical stability of the proposed strain sensors may benefit from the embedded MWCNTs in the microtrenches, which mechanically interlocked with PDMS microtrenches. Because PDMS is flexible, the resulting sensors could be gently flexed thousands of cycles without deterioration of MWCNTs in microtrenches of PDMS film. The high sensitivity and good stability of the proposed strain sensors may be more useful in practical daily life applications. The response time of strain sensors is another important factor to evaluate for practical applications, such as real-time monitoring of human indicators. A 90% time constant is commonly used as the standard response time and recovery time values of strain sensors. Here, the sensors were cycled at the strain of 7.5% and the strains were applied at the speed of 15 mm s-1. The sensors were maintained 10 s under the strain


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and another 10s when the strain was released. The stretching device indicated a response time of ~ 0.28 s and recovery time of ~ 0.35 s (Fig. 3f). Due to their fast response and recovery times, the sensors were able to detect strains with frequencies less than 0.5 Hz (Fig. 3g). Due to time-delay in stretching/releasing of strain by the testing platform, the real response of recovery times would be faster, thus useful for real-time detection of human motion. The line width of the MWCNTs meshes is an important parameter to evaluate since it influences both the sensitivity and transparency of strain sensors. Here, strain sensors with three different line widths (6 µm, 4 µm, 2 µm) were fabricated as predicated in Figs. 4 (a – c). As a result, strain sensors with 2 µm wide meshes exhibited obviously higher sensitivities under the same strains than those with wider meshes (Fig. 4d). For example, under the strain of 8.75%, the sensors with 2 µm wide meshes of MWCNTs showed superior sensitivity (∆R/R0) reaching up to 10000% or a gauge factor up to 1140. This value was significantly higher than gauge factors (0.06~0.82) of CNT/polymer composites of conventional metal foils or films (1~5),3 carbon black and polymer composites (~ 20),48 and nanowire/polystyrene hybrid films (100).49 In narrower microtrenches, smaller amounts of MWCNTs resulted in less connection between them, hence poor conductivity of the sensors as confirmed by CV (18 × 20 mm) (Fig. 4e). The resistance values of the strain sensors with microtrench widths of 2 µm, 4 µm and 6 µm were estimated to about 95 kΩ, 38 kΩ and 10 kΩ, respectively. MWCNTs could easily be filled into microtrenches of PDMS by the doctor-blading process, indicating that the strain sensors had good fabrication



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repeatability. However, it was nearly impossible to embed the same amount of MWCNTs in sensors under the same microtrenches. Therefore, the resistance values of the sensors ranged from 70-100 kΩ, 30-60 kΩ and 5-25 kΩ, for microtrench widths of 2µm, 4µm and 6µm, respectively. Thus, the responses of the sensors with microtrenches of the same width would differ, resulting in error bars in Fig. 4d. Under the same strain, relative large nonuniform deformation would generated in the thinner MWCNTs meshes, resulting in forming easily microcrack propagation in microtrenched PDMS film. Consequently, higher sensitivities were observed for strain sensors with thinner meshes of MWCNTs. In addition to sensitivity, the transparency also increased with thinner MWCNT meshes, as shown in Fig. 4(f). For example, the transparency increased from 78% for strain sensors with 6µm width MWCNTs meshes to 87% for those with 2µm width MWCNTs meshes. This resulted from the fact that thinner MWCNTs meshes shielded less insight light.

Figure 4. (a-c) SEM images of embedded MWCNTs meshes in PDMS film. Note the

different line widths of meshes as indicated in the insets. (d) ∆R/R0 response curves of strain sensors with different embedded MWCNTs meshes to strains from 1.25% to 10%.


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(e) The current-voltage curves of embedded MWCNTs conductive networks with different line widths. (f) The optical transmittances of strain sensors with different line widths of embedded MWCNTs meshes. Snapshots of a strain sensor located on the logo of Xi’an Jiaotong University are shown as insets.

In addition to mesh width, sizes of the mesh openings also influence both transparency and sensitivity of strain sensors with embedded MWCNTs meshes. Obviously, larger mesh transparent openings would increase the ratio of transparent to non-transparent areas of MWCNTs meshes with a given line width. Consequently, higher transparency was observed for strain sensors with larger mesh openings. For certain line widths of MWCNTs meshes, large openings also tended to result in less amounts of microtrenches in the PDMS and reduced nonuniform deformation, resulting in decreased possibility for microcrack propagation between MWCNTs. Hence, the sensitivity of the strain sensor may decrease as the size of mesh openings increased. Due to their high performances in sensitivity, transparency, stability and response speed, the proposed strain sensors with embedded MWCNTs meshes could well be potential for use in wearable devices and health diagnosis. Some real applications of the sensors in detecting human motions, such as movement of the eyes or mouth, bending in the wrist, and the heartbeat, were evaluated and the results are depicted in Fig. 5. The strain sensors were tightly attached to the surface of the human skin through double-sided adhesive. It can be seen that the attachment of strain sensors to the skin near the eyes or mouth resulted in clear detection of motion due to blinking in



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the eyes and mouth opening/closing (Fig. 5a and 5b). The ∆R/R0 responses were typically more than 110%, suggesting the high sensitivity of the proposed strain sensors in detecting motion in eyes and mouth. The response aroused from the movement of the muscles, which led to deformation in the strain sensor attached to the skin.

Figure 5. Time-dependent response of ∆R/R0 to the movements in skins near the blinking

eyes (a), opening/closing mouth (b), bending wrist (c), and beating wrist artery (d). The insets are snapshots of the corresponding human body movements.

When the strain sensors were attached to the skin near the wrist, bending in the wrist could clearly be detected with different responses to various bending degrees of the wrist (Fig. 5c). For example, ∆R/R0 responses exhibited high values of up to 243% at bending angles of ~ 15°, and this increased to 418% at bending angles of ~ 30°.


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Furthermore, when the strain sensors were attached to skin near the beating wrist artery, heartbeat frequency was accurately monitored by the sensors to yield 67 beats per min. This value was similar to that obtained by commercial instruments (Fig. 5d and Fig. S6). A video of the real-time pulse monitoring was provided in the supporting information. When water drop fell on the surface of PDMS, as depicted in Fig. S7, the resistance of the strain increased. However, the response changed by only less than 10%. On sweaty skin, the response to human motions may be down a little bit. Nevertheless, the sensors also can detect human motions clearly. Therefore, the proposed strain sensors with embedded MWCNTs meshes can accurately detect heartbeats and thus have great future potential applications in wearable devices. 4. CONCLUSIONS Novel strain sensors with embedded MWCNTs meshes in PDMS films by means of the doctor-blading process were demonstrated. The strain sensors exhibited superior features

in

terms

of

sensitivity,

transparency,

flexibility,

stability,

and

response/recovery. Thinning the line width of the embedded MWCNTs meshes could improve both the sensitivity and transparency of the strain sensors. Properly designing the opening sizes of MWCNTs meshes could also improve the overall performances of the sensors. Furthermore, the fabrication of such strain sensors, consisting of blading MWCNTs ink into microtrenches to yield flexible and transparent PDMS films, should be low-cost with high-yield, hence suitable for mass production. The proposed strain sensors with embedded MWCNTs meshes may pave a novel way to wearable electronic devices detecting human motions as demonstrated in the last



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section.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website.

Figure S1: The contact angle of a PDMS film before oxygen plasma treatment; Figure S2. The SEM images of the MWCNTs under the strain of 5% (a), under the strain of 10% (b), under release of strains (c); Figure S3. The hysteresis behavior of strain sensors with microtrenches of 6-µm-width; Figure S4. The current change of the strain sensor under the strain of 15%; Figure S5. (a) The picture of CNT film coated on the PDMS support without the microtrenches after oxygen plasma treatment.(b) The resistance measured by I-V curve for MWCNTs coating on the PDMS support without microtrenches. The inset showed the height of MWCNTs; Figure S6. Heartbeats of the human body could be detected by the proposed strain sensors (I) in comparison with a commercial instrument (II), which showed the same heart rate of 67 times per min; Figure S7. (a) The response of the strain sensor under a strain of 5% before a drop of water falling on the strain sensor. (b) The resistance change of the sensor when a drop of water fell on the strain sensor. (c) The response under a strain of 5% after a drop of water falling on the strain sensor. (PDF) Movie of the real-time pulse monitoring by the strain sensor (MOV)


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Notes. The authors declare no competing ﬁnancial interest.

Acknowledgements

B.N. and X.L. contributed equally to this study. This work was financially supported by the Major Research Plan of NSFC on Nanomanufacturing (Grant Number: 91323303), National key research and development projects (2017YFB1102900), NSFC Funds (Grant Numbers: 51522508 and 51505372), and China Postdoctoral Science Foundation (Grant Numbers: 2016T90905 and 2015M570824).

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