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Flexible, Cuttable and Self-Waterproof Bending Strain Sensors Using Microcracked Gold Nanofilms@Paper Substrate Xinqin Liao, Zheng Zhang, Qijie Liang, Qingliang Liao, and Yue Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12991 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

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ACS Applied Materials & Interfaces

Flexible, Cuttable and Self-Waterproof Bending Strain Sensors Using Microcracked Gold Nanofilms@Paper Substrate Xinqin Liao,1,‡ Zheng Zhang,1,‡ Qijie Liang,1 Qingliang Liao,1, * and Yue Zhang1, 2, * 1

State Key Laboratory for Advanced Metals and Materials, School of Materials Science and

Engineering, University of Science and Technology Beijing, Beijing 100083, China 2

The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, University

of Science and Technology Beijing, Beijing 100083, China Corresponding author, E-mail: [email protected]; [email protected]. KEYWORDS: flexible devices, paper-based electronics, self-waterproof sensors, surface microstructures, gold nanoparticles.

ABSTRACT: Rapid advances in functional sensing electronics place tremendous demands on innovation towards creative uses of versatile advanced materials and effective designs of device structures. Here, we first report a feasible and effective fabrication strategy to integrate commercial abrasive papers with microcracked gold (Au) nanofilms to construct cuttable and self-waterproof crack-based resistive bending strain sensors. Via introducing surface

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microstructures, the sensitivities of the bending strain sensors are greatly enhanced by 27 times than that of the sensors without surface microstructures, putting forward an alternative suggestion for other flexible electronics to improve their performances. Besides, the bending strain sensors also endow rapid response and relaxation time of 20 ms and ultrahigh stability of > 18 000 strain loading-unloading cycles in conjunction with flexibility and robustness. In addition, the concepts of cuttability and self-waterproofness (attain and even surpass IPX-7) of the bending strain sensors have been demonstrated. Due to the distinctive sensing properties, flexibility, cuttability, and self-waterproofness, the bending strain sensors are attractive and promising for wearable electronic devices and smart health monitoring system.

Introduction Strain sensors have acquired increasing interests in recent years, as tremendous demands for wearable electronic devices, artificial intelligences, smart health monitoring system, and earlywarning system.1 Noticeable progresses in the domain of strain sensors are contributed to development of versatile technologies and transduction mechanisms, such as resistance,2-18 capacitance,19-24 field-effect transistor (FET),25-28 piezoelectricity,29-33 and triboelectricity.34-36 Among these, resistive strain sensors have shown great potential applications in the designing of strain gauges, due to their simple device structures, easy read-out mechanism, and also relatively low energy consumption in operation. Diverse materials, including carbon materials,3 metal nanomaterials,8 conductive polymers,10 and semiconductor materials,11 etc., have been introduced in the literatures to help the progression of resistive strain sensors. Despite the rapid developments, there is little work that has been recommended for resistive strain sensors to effectively modulate the sensitivity until now.


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At the same time, more and more novel features have been presented, such as flexibility,2 stretchability,3 transparency,19 multifunction,13 self-healing,10 self-power,29 ultrathinness,6 and ultralightness,37 to enrich and extend the strain gauges. In general, for protecting sensing elements from water damage in using process, encapsulations seem to be an essential part of waterproof strain sensors.38 Nevertheless, it inevitably adds extra manufacturing steps and device cost, and its impact on sensing performances should also be considered. From ergonomic aspect, strain sensors should be capable of being reshaped or cut, unlike conventional block metal or rigid but brittle semiconductor counterparts, towards broadened applications ranging from wearable electronics, smart cloth, and custom components to do-it-yourself devices. For all that, strain sensors with both waterproofness and cuttability have not yet been explored. In this work, we first demonstrate flexible, cuttable, and self-waterproof resistive bending strain sensors, which are fabricated on abrasive papers and functionalized by microcracked gold (Au) nanofilms. Notably, the sensors can be efficiently prepared by employing a scalable process. Although the reversible-microcontact strain sensing mechanism used here has been introduced into crack-based resistive sensors, the effects of surface microstructures on sensitivities of the crack-based resistive bending strain gauges need to be further researched. Through experiments and theoretical models, we found that the sensitivities of the crack-based resistive bending strain sensors were significantly modulated by the surface microstructures. Furthermore, we have validated that the bending strain sensors still hold sensing performances even if shorn, punched or cut in half. Moreover, the bending strain sensors were investigated for wearable operations under air, humid, and wet conditions, which proved the potential for extending lifetime of the sensing elements.


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Experimental Section Device Fabrication: The Au nanofilm was deposited on the abrasive paper (3M Company) by employing the DC sputtering method. Then, a relatively large bending prestrain (~0.59%) was applied to the device. Finally, wire bonding by silver paste concluded the process. Characterization: The Abaqus soft was employed to numerically assess the tensile and compressive strain of the model. Field emission scanning electron microscope (FESEM, SUPRA55) was used to observe the morphology of materials. The electrical properties of the sensors were measured by using a semiconductor characterization system (Keithley 4200-SCS), a high-precision digital multimeter (Keithley DMM7510) and a standard digital multimeter (UNI-T UT39C). The surface morphology was carried out using atomic force microscopy (AFM, Nanoscope IIIa, Multimode). The roughness profiles of the smooth and abrasive papers were characterized by a surface profiler (Veeco Dektak 150). Results and Discussion The microcracked Au nanofilms were prepared on the abrasive papers by employing directcurrent (DC) sputtering method. Figure 1a shows the schematic illustration of the manufacturing process of the bending strain sensors (see Experimental Section for details). Briefly, Au particles were sputtered from Au target by ionized argon ions. Under the action of an electric field, the sputtered Au particles were deposited on the abrasive paper, and then formed Au nanoparticles film (Figure 1b). The morphology of the Au nanoparticles film is given in Figure 1c and Figure S1 (Supporting Information), where the Au nanoparticles with uniform size are clearly observed. In Figure S2 (Supporting Information), the substantial microcracks occurred at the peaks and valleys of the surfaces of abrasive papers after bending the substrates. The as-prepared bending


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strain sensors were flexible and easy to bend (Figure 1d), suggesting their potential applications in wearable electronics. For the sake of following performance comparisons, seven abrasive papers with different mesh number (800, 1 500, 2 000, 6 000, 8 000, 10 000, and 12 000 mesh) and a smooth paper served as the substrate materials of the strain sensors, respectively (Figure 1e). These kinds of the substrate materials were selected for the reason that they possessed not only the paper characteristics of intrinsically flexibility and cuttability but also specific surface microstructure and waterproofness. The scanning electron microscopy (SEM) images of the smooth and abrasive papers modified with the Au nanoparticles film are demonstrated in Figure 2, where the top-view images and 45° oblique-view images are provided. Figures 2a and b are the SEM images of the smooth paper, suggesting that this kind of substrate material is almost level in the micron level, not of obvious bump and crack. The surface morphology of the smooth paper is further presented in Figure S3a (Supporting Information), where the surface features obvious undulation only in the nano level. Figures 2c-p provide the microstructure and surface morphology images of the abrasive papers of different mesh numbers. In the series of the SEM top-view images (Figures 2c, e, g, i, k, m and o), it can be observed that the sizes of abrasives of the abrasive papers gradually reduce with the mesh number of the abrasive paper increasing. Indeed, the phenomenon is consistent with the definition of the mesh number of the abrasive paper that is the number of abrasives in per square inch. In other words, the abrasive number in per square gets more; the mesh number of the abrasive paper becomes larger. In order to fill more abrasive in per square, the abrasive would be taken after being smaller in the process of industrial preparation. Although the size of abrasive was related to the mesh number, the relationship between the surface microgroove depth and the mesh number seemed not to be definite. In the following experiments of electrical properties, it


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will be found that the sensitivities of these bending strain sensors are observably modulated by the surface microstructures. In Figures 2d, f, h, j, l, n, and p, the apparent bumps, which are abrasives, can be clearly observed. Specially, the surface microgroove depth of the abrasive paper of 1 500 mesh number was deeper than that of others. In addition, the surface microgroove depth of the abrasive paper of 8 000 mesh number was larger than that of the abrasive paper of 6 000 mesh number. The roughness profiles of the abrasive papers are characterized in Figures S3b-h (Supporting Information), where the obvious undulations are found in the micron level. Figures S4a and b show the roughness (Ra) and absolute average height (AAH) of the smooth and abrasive papers, respectively, further confirming above results. Figure 3a shows that the variations of resistance R of the bending strain sensors were relevant to the DC sputtering time. For all the samples, the resistances decreased monotonically with the DC sputtering time ranging from 30 s to 80 s, which are mainly caused by the increase of the deposited Au nanoparticles (Figure S5, Supporting Information). Noteworthiness is that the resistances of the bending strain sensors using smooth paper and abrasive paper are distinguishable. In simple terms, the resistance of strain sensor depended on how difficult electron movement is between cathode and anode. Unlike the smooth paper (Figures 2a and b), the abrasive paper endowed the rough surface in the micromorphology that obstructs the electron movement and prolongs the path between the electrodes. Besides, the multitudinous microcracks in the surfaces of the abrasive papers interdicted the directed migration of electrons. Accordingly, the resistance of the abrasive paper is relatively larger than that of the smooth paper at the same DC sputtering time. The current-voltage (I-V) curves for the smooth and abrasive paper-based bending strain sensors are presented in Figure S6 (Supporting Information) under differnent strains, respectively.


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Minor variations of resistance (all lines are almost overlapped) against applied strains were found for the smooth paper-based bending strain sensor, while large variations of resistance were presented for the abrasive paper-based bending strain sensor. The resistance values of the devices were changed by the geometry of cracks under bending tension and compression modes. The bending tests were performed using 5 samples for each type of sensor by same fabrication method. Figures 3b and c are the typical response curves. Figure 3b shows the variations in normalized resistance (∆R/R0, where ∆R is the relative change in resistance and R0 is the resistance of the strain sensor under no strain) under different tensile strains. In order to evaluate the sensitivities of the bending strain sensors, gauge factor (GF) is defined as GF = (∆R/R0)/∆ɛ, where ∆ɛ is the change in applied strain.5 An approximately linear relationship between the normalized resistance and applied strain in the range of 0-0.59%, leading to a sensitivity value GF = 75.8 (inset in Figure 3b). In the contrastive case of compressive strain (Figure 3c), the matching value of GF is 10.7. The GFs are different mainly due to the fact that the newborn microcracks (Figure S7, Supporting Information) further make an impact on the change of the resistance value under bending tension mode. Figure 3d illustrates the GF variation of the smooth and abrasive paper-based bending strain sensors with varying DC sputtering time for the tensile strains. The reason for the GFs having non-monotonic dependence on mesh numbers will be discussed later. We noticed that the GF value (75.8) of the bending strain sensor (the mesh number of the abrasive paper is 1 500) is 27 times higher than that (2.8) of the smooth paper-based bending strain sensor at the same DC sputtering time of 30 s. In addition, for the compressive strains, the maximum GF value (10.6) of the abrasive paper-based bending strain sensor is almost 4 times higher than that of the smooth paper-based bending strain sensor (Figure 3e). The use of a smooth paper limits device performance, highlighting that the


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high sensitivity of the abrasive paper-based bending strain sensor results from its surface microstructure, as discussed later. Here the maximum magnitude of GF is higher than that of many other recently reported papers (see Supporting Information, Table S1 for details), and the comparison between the two types of paper-based bending strain sensors results that the surface microstructures can significantly modulate and further enhance the sensitivities of the crackbased resistive bending strain sensors. In the following sections, the abrasive paper-based (the mesh number is 1 500 and the DC sputtering time is 30 s) bending strain sensor was chosen as the testing sample unless otherwise specified. Figure 3f indicates a rapid response to both external strain loading and unloading with response and relaxation time of 20 ms. For dynamic strain measurements, we performed multiple loading–unloading tests under various strain loading conditions (Figure 3g). Large variations in resistance are obtained with high repeatability with a bending curvature radius of 8 mm when the bending strain sensor is loaded to produce up to 0.59% bending strain and unloaded back to 0% strain at a loading speed of 87 cycles per minute (Figure 3h). After > 18 000 strain loadingunloading cycles, the further response curve is provided inside the Figure 3h (the purple line). The resistance amplitude exhibited tiny change, which ismainly caused by the newborn microcracks (Figure S7, Supporting Information). The results reveal high repeatability, stability and durability of the bending strain sensor. To provide further insights into the strain-induced resistance change mechanism, the tensile and compressive strains of the bending strain sensors are simulated by using finite-element method (Figure 4). It needs to be reminded that the surface microstructure of the bending strain sensor is subjected to horizontal strain in the micron level, even though the bending strain is applied to the device (Figures 4a and e). Figures 4b-d illustrate the simulated effects of different


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surface microstructures models under same tensile strain. In Figures 4b and c, the mesh numbers of the two models are same while the AAHs are different. It can be observed that the larger strain is generated at the valley, where the microcracks occur, of the model of the higher AAH. The larger strain was, the more microcracks were generated, and then the wider microcrack was (Figure S7, Supporting Information). The microcracks formations and deformations accommodated strain, while holding percolating pathways. Then, the resistance was greater changed, leading to higher sensitivity of the strain sensor. Figures 4c and d show strain models of bending strain sensors with same AAHs but different mesh numbers. As the mesh number increasing, the valley suffers from relatively small tensile strain, bringing about small resistance variation. Thus, this type of the bending strain sensor with larger mesh number endowed lower sensitivity. In the contrastive case (Figures 4f-h), the models were applied by same compressive strain. Compared Figure 4f with Figure 4g, it can be observed that the larger compressive strain is concentrated at the valley of the bending strain sensor with the higher AAH. Subsequently, the resistance was more decreased, giving rise to higher sensitivity of the bending strain sensor. In Figures 4g and h, the AAHs of the two models are same while the mesh numbers are different. As the mesh number increasing, the valley was subjected to relatively small compressive strain, making small resistance variation. Accordingly, the strain sensor with large mesh number will show low sensitivity. In brief, the simulated results pave a viable way for further improving the sensitivities of crack-based resistive bending strain sensors by modulating their surface microstructures. Furthermore, it can be known that the gauge factors are obviously related to the mesh number (roughness, Ra) and absolute average height (AAH). In Figures 3d and e, the gauge factors have non-monotonic dependence on mesh numbers. From Figures S4a and b (Supporting Information), it can be found that the AAHs have also non-monotonic dependence


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on mesh numbers and the change tendency is similar with that between gauge factors and mesh numbers. Thus, it can be inferred that the influence of AAH on gauge factor is greater than that of mesh number. To investigate the cuttability of the bending strain sensors, Figure 5a illustrates the variations in normalized resistance of the bending strain sensors, which are original, side-cut, punctured and transected devices, respectively, under the same strain loading-unloading tests. All the sidecut, punctured and transected bending strain sensors maintain the high signal-to-noise ratio (measured under ~0.3% strain) of the original sensor. The cuttability of the bending strain sensors is mainly attributed to the paper substrate. The bending strain sensors preserving high strain sensing properties can be attributed to the merit of resistive sensing mechanism. It is known that the variation in normalized resistance of resistive strain sensor is closely related to the geometrical factor [(1 + 2ν)*ɛ, where ν is the Poisson’s ratio] and the variation in normalized resistivity (∆ρ/ρ0, where ∆ρ is the variation resistivity and ρ0 is the initial resistivity). The relationship can be expressed as ∆R/R0 = (1 + 2ν)*ɛ +∆ρ/ρ0.39 According to the prior experimental results, the effect of the geometrical factor (GF ≈ 2) on the variation in normalized resistance of the bending strain sensor is insignificant. Moreover, the variation in normalized resistivity is irrelevant to the shape of the bending strain sensor. So the reshaped bending strain sensors can still keep high strain sensing properties. As a proof-of-concept implementation, the waterproofness of the bending strain sensors is examined (Figure 5b). Note that the bending strain sensors are not modified by any extra packaging processing. First, we used the bending strain sensor with the DC sputtering time of 60 s to tentatively estimate the waterproofness. Figure 5b and Video S1 (Supporting Information) show that the bending strain sensor is submerged by pure water. Before and during the process,


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the basic resistance of the device was still original. It is due to the fact that the pure water does not undermine or erode the component materials (abrasive paper and Au nanoparticles film) of the device. Moreover, the pure water does not lead to delamination of the Au nanoparticles film or change the geometry and density of cracks, and then not further make an impact on the sensing mechanism of device. Figure S8 (Supporting Information) shows the waterproofness of the device can last above 24 hours. Moreover, Figure S9 (Supporting Information) reveals that the waterproof grade of the device attains and even surpasses IPX-7. The flexibility of the sensor endows the device suitability for bio-monitoring. It can be used to consistently respond to the rapid powering and slow relaxing of arm muscle (Figure S10 and Video S2, Supporting Information). In this work, we further investigated the behaviors of the bending strain sensor for detecting the bending-stretching motion of a finger under spraying condition. As displayed in Figure 5c and Video S3 (Supporting Information), we attached the bending strain sensor onto a finger with adhesive tapes to perform wearable detection. The change in resistance was expressed as normalized values against bending-stretching motion of the finger. The results are primarily due to a part (as inside red line circles in Figure 5c) of the sensor under a relaxed or compressive bending state without regarding to the trembling of the finger. In the first half of the experiment, the bending strain sensor responded rapidly and repeatedly in a normal environment. Notably, the high signal-to-noise ratios were well maintained and the normalized resistance amplitude exhibited negligible changes even when the pure water was sprayed against the bending strain sensor. The results indicate that the bending strain sensors feature portability, self-waterproofness and high robustness, and can be used to steady monitor the various strains regardless of air, humid or wet condition.


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Conclusions In summary, the flexible, cuttable, and self-waterproof (attain and even surpass IPX-7) resistive bending strain sensors based on microcracked Au nanofilms@abrasive papers have been firstly realized by the DC sputtering method. The surface microstructures have been introduced to considerably enhance the sensitivities of the bending strain sensors, demonstrated by the experiments and theoretical models. Via introducing surface microstructures, the sensitivities of the bending strain sensors were greatly enhanced by 27 times than that of the sensors without surface microstructures, putting forward an alternative suggestion for other flexible electronics to improve their performances. The bending strain sensors also possessed ultrahigh repeatability, stability and durability of >18 000 strain loading-unloading cycles and rapid response and relaxation time of 20 ms. Moreover, the reshaped bending strain sensors still keep high strain sensing properties of the original ones. Interestingly, the high signal-to-noise ratios of the bending strain sensors were well maintained to perform the wearable detections no matter under air, humid or wet condition without any other modifications. With the high performance, flexibility, cuttability, and self-waterproofness, the bending strain sensors are suitable for practical applications, such as for monitoring various human motions. ASSOCIATED CONTENT Supporting Information. Atomic force microscope images, FESEM images, roughness profiles, roughness and absolute average height, the relationship between thickness of sputtered Au film and sputtering time, current-voltage curves, comparison of the recently reported sensors, basic resistance, and photographs of the sensor attached to arm skin supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *Address correspondence to: [email protected]; [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes Conflict of Interest: The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program of China (No. 2016YFA0202701), the Major National Scientific Research Projects (No. 2013CB932602), the Program of Introducing Talents of Discipline to Universities (No. B14003), National Natural Science Foundation of China (No. 51672026, 51527802, 51232001, 51372020), Beijing Municipal Science & Technology Commission, and the State Key Laboratory for Advanced Metals and Materials(No. 2016Z-23). REFERENCES 1.

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Figure 1. (a) Schematic diagram of the manufacturing process of the bending strain sensor. (b) Model of the prepared bending strain sensor. (c) FESEM image of the Au nanoparticles film. (d) Photograph showing the flexibility of the bending strain sensor. (e) The optical images of the bending strain sensors based on the different abrasive papers and smooth paper.


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Figure 2. (a, c, e, g, i, k, m, o) top-view images and (b, d, f, h, j, l, n, p) 45° oblique-view images of the smooth paper and different abrasive papers.


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Figure 3. (a) Resistance as functions of the DC sputtering time. Variations in normalized resistance of the bending strain sensor under (b) tensile and (c) compressive strains. Insets show the relationship between the normalized resistance and applied strain. GF variations of the smooth and abrasive paper-based bending strain sensors with varying DC sputtering time for the (d) tensile and (e) compressive strains. (f) Time response of the bending strain sensor applied tensile and compression strain. (g) Various strain sensing responses of the bending strain sensor under different strain loading conditions. (h) Variations in resistance of the bending strain sensor for > 18 000 strain loading-unloading cycles at a loading speed of 87 cycles per minute.


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Figure 4. Simulation effects of different surface microstructures models under the same (a-d) tensile and (e-h) compressive strains.


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Figure 5. (a) Variations in normalized resistance of the original, side-cut, punctured, and transected bending strain sensors under the same strain loading-unloading tests. Insets are the photographs of different bending strain sensors. (b) Basic resistance of bending strain sensor before and after submerged into pure water. (c) Finger bending detections of the bending strain sensor before and during sprayed with pure water. Insets are the photographs of the wearable detections. The red line circle shows the deformations of the bending strain sensor.


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Flexible, Cuttable, and Self-Waterproof Bending ... - ACS Publications

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