Highly Stretchable and Ultrasensitive Strain Sensor Based on

Nov 23, 2015 - Flexible, Highly Sensitive, and Wearable Pressure and Strain Sensors with Graphene Porous Network Structure. Yu Pang , He Tian , Luqi ...
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Highly Stretchable and Ultrasensitive Strain Sensor Based on Reduced Graphene Oxide Microtubes−Elastomer Composite Yongchao Tang, Zongbin Zhao,* Han Hu, Yang Liu, Xuzhen Wang, Shanke Zhou, and Jieshan Qiu* Carbon Research Laboratory, Liaoning Key Lab for Energy Materials and Chemical Engineering, State Key Lab of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning 116024, P. R. China S Supporting Information *

ABSTRACT: Strain sensors with excellent flexibility, stretchability, and sensitivity have attracted increasing interests. In this paper, a highly stretchable and ultrasensitive strain sensor based on reduced graphene oxide microtubes−elastomer is fabricated by a template induced assembly and followed a polymer coating process. The sensors can be stretched in excess of 50% of its original length, showing long-term durability and excellent selectivity to a specific strain under various disturbances. The sensitivity of this sensor is as high as 630 of gauge factor under 21.3% applied strain; more importantly, it can be easily modulated to accommodate diverse requirements. Implementation of the device for gauging muscle-induced strain in several biological systems shows reproducibility and different responses in the form of resistance or current change. The developed strain sensors show great application potential in fields of biomechanical systems, communications, and other related areas. KEYWORDS: reduced graphene oxide microtubes, elastomer, strain sensor, stretchability, sensitivity

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INTRODUCTION Flexible electronics has emerged as a very promising field and recently attracted significant attention.1−5 In particular, wearable, bendable, and stretchable strain sensors with high sensitivity have showcased substantial promise.6−10 Such strain sensors are often mounted on wearable equipment by integrating with power sources, such as supercapacitors,11−16 lithium ion batteries,17−22 and so on. These devices are advantageous over traditional ones in various applications, such as human body motion detection,23,24 sports performance monitoring,25,26 etc. High stretchability and outstanding sensitivity of strain sensors are significantly important for precise sensing of human body motion, such as joint movement, which usually results in ∼50% strain.23,24 Traditional strain sensors based on semiconductors or metal oxides showcase unsatisfied flexibility and low detection strain range (typically ∼5% strain), even though possessing a high sensitivity (∼1813 of gauge factor).27,28 Therefore, they are unsuitable to be applied in some fields wherein both large stretchable range and high sensitivity are required simultaneously. In order to fabricate high-performance strain sensors, researchers have made tremendous effort by using diverse nanomaterial composites, including noble metal nanowires and carbon nanomaterials, etc.23,29−34 In general, for obtaining substantial stretchability and high sensitivity, these nanomaterials are required to be compounded with elastomers © XXXX American Chemical Society

in order to integrate their respective merits. Moreover, the structures of such composites also play a vital role in enhancing stretchability and sensitivity of strain sensor. For example, owing to outstanding electrical conductivity, ductility, and mechanical strength, some noble metal nanomaterials are designed to elaborate architectures such as the Pt-coated polymeric nanofibers interlocking structure29 and Ag nanowires-coated elastomers30,34 for superior strain sensors. However, the fabrication of these structures is usually not scalable due to either their complex process or high cost. In addition, burgeoning carbon nanomaterials such as carbon nanotubes23,31,35 and graphene,24,32,33,36−39 have been considered as excellent raw materials of high-performance strain sensors, which is mainly related to their superior flexibility, high conductivity, and robust mechanical strength. For instance, by uniformly spraying carbon nanotubes dispersion on a substrate and then coating them with PDMS, as-prepared strain sensors show stretchability of as high as 280% strain but lower sensitivity (∼1.6 of gauge factor).23,31 Besides those based on graphene flakes-rubber composite, strain sensors with ultrahigh stretchability (∼800% strain) and fair sensitivity (∼35 of gauge factor) have been obtained.32 Furthermore, through combining Received: October 2, 2015 Accepted: November 23, 2015

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DOI: 10.1021/acsami.5b09314 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) Fabrication steps of RGO microtubes−elastomer strain sensor. (b, c) Digital photos of arbitrarily bended and twisted strain sensor, respectively. (d, e) Excellent stretchability (∼50% strain) of strain sensor. (f, g) SEM images with different magnification of the strain sensor (RGO@PDMS).

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graphene woven fabrics grown by the CVD method with polymer, a strain sensor with ultrahigh sensitivity (∼103 of gauge factor) and smaller stretchability (∼6% strain) has been reported recently.24 Compared with the noble metal nanomaterials based strain sensor, the carbon nanomaterials based strain sensor displays a greater potential, because of their relatively convenient solution compatibility and low-cost production. Although many dedications have been made, the contradiction between stretchability and sensitivity has not been successfully resolved (Table S1 and Figure S1). In other words, fabricating a strain sensor, which simultaneously possesses improved stretchability (∼50% strain) and sensitivity (above 100 of gauge factor), remains a great challenge.37,40−42 In this work, by integrating the advantages of reduced graphene oxide (RGO) microtubes and elastomers, a highly stretchable and ultrasensitive novel strain sensor has been prepared. Herein, GO solution possesses favorable compatibility with hydrothermal-assisted template induced assembly, and the resulted RGO can enhance the conductivity of the RGO/elastomer composites, thus enabling the strain sensor with higher sensitivity.43 In comparison with previously reported graphene or RGO based strain sensor,24,33,37 our strain sensor possesses remarkable advantages, especially, larger stretchable range and higher sensitivity. In addition, the strain sensor can be fabricated cost-efficiently and scalably. Such properties can pave the way for potential applications of the strain sensor in electronic fitness instruction, intelligence transmission, human−machine interaction, and other related areas.

EXPERIMENTAL SECTION

Hydrothermal-Assisted Synchronous Reduction and Assembly of GO Sheets over Copper Mesh (Cu@RGO). Commercially available copper mesh (80 meshes, ca. 50 μm in diameter) was pretreated by acetone or hydrochloric acid/acetone solution (1/50 by volume ratio). Graphene oxide prepared by the modified Hummers method44 was sonicated for 15 min and then diluted to various concentrations. Typically, pretreated copper mesh with a length of 8 cm and width of 3 cm was set into a 50 mL autoclave, and then, 35 mL of GO solution was added with various concentrations, subsequently putting them into an oven and heat-treating at 180 °C for 12 h. The obtained product was carefully washed twice with DI water to eliminate loose RGO fragments and dried in an oven at 50 °C. Selective Coating of PDMS over Cu@RGO (Cu@RGO@ PDMS). As seen from Figure S2c, for simplifying the experiment steps, Cu@RGO was divided into three areas; only the protected area and etched area of the Cu@RGO were coated with PDMS. Typically, the solution of PDMS monomer, curing agent, and n-hexane was drawn with a dropper and then droped over the selected areas of Cu@ RGO. The mass ratio of n-hexane, PDMS monomer, and curing agent was set at 30:10:1. It is worth noting that the solution should be stirred beyond 30 min for sufficient mixing before use. Then, the selectively coated Cu@RGO was vacuum-dried at 120 °C for 2.5 h to induce the polymerization reaction of PDMS monomer, resulting in the formation of Cu@RGO@PDMS (Figure S2b). Selectively Etching of Copper Mesh in Cu@RGO@PDMS (RGO@PDMS). As seen from Figure S2c, only the etched area of Cu@ RGO@PDMS was etched. Typically, the selected area of Cu@RGO@ PDMS was submerged into FeCl3/HCl solution (0.5M:0.5M)45 for 12 h at room temperature, where the Cu mesh can be oxidized by Fe3+ to Cu2+ (2Fe3+ + Cu → 2Fe2+ + Cu2+) and dissolved into the solution. Afterward, the resulting material RGO@PDMS was carefully washed with DI water for several times to remove the remaining etching solution and dried in the air. B

DOI: 10.1021/acsami.5b09314 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Characterization. The materials are observed using an optical microscope (AXIO scope A1) and scanning electron microscopy (QUANTA 450). The Raman characterization is performed using a Raman microscope (DXR). The tensile test is carried out on a stretching machine (Instron 5943). The electromechanical properties are tested by a digital source meter (Keithley 2602 and Keithley 4200SCS).

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RESULTS AND DISCUSSION The schematic illustration of the fabrication process of the RGO microtubes−elastomer strain sensor is shown in Figure 1a. In a typical experiment, commercially available copper mesh was selected as a substrate to deposit the RGO. Through hydrothermal-assisted synchronous reduction and assembly,46,47 RGO was deposited on copper mesh (composed of copper wires with ca. 0.05 mm of diameter and ca. 95% of purity), resulting in the formation of RGO microtubes with the copper wire core (Cu@RGO). As shown in Figure S3a, the synchronous reduction and assembly of neutral graphene oxide sheets can be dramatically sped up under hydrothermal conditions. It is worth noting that the pretreatment of copper mesh with acid is the key to uniform assembly of RGO, because the removal of the surface oxide layer provides an active microenvironment for the redox reaction between copper mesh and GO sheets.47 Figure S4a demonstrates an intact RGO mesh-like structure obtained by using pretreated copper mesh. On the contrary, only a fragmentary structure was gained when using untreated copper mesh due to the existence of surface passivant (Figures S4b and S5).45 Through the droplet-coating method, PDMS monomers were coated over the Cu@RGO and then transformed into Cu@RGO@PDMS by curing processing. Eventually, copper in the Cu@RGO@PDMS was selectively etched to obtain the strain sensor (RGO@PDMS). As shown in Figure 1b−e, the strain sensor demonstrates excellent bendability, twistability, and substantial stretchability up to 50% strain, which significantly surpasses the conventional value (∼5% strain).27 Such excellent stretchability may be attributed to the excellent intrinsic elasticity of PDMS48,49 and the weaker van der Waals force among RGO sheets.33 Figure 1f presents SEM images of the cross-section of the strain sensor; a row of ordered microtube channels can be observed clearly (highlighted in red cycles). Figure 1g shows an amplified microtube channel, in which the interface between PDMS and RGO microtube can be obviously seen. Moreover, the excellent flexibility of the strain sensor can also be partly ascribed to its smaller total thickness of ca. 83 μm (Figure S4d), in which the synergistic effect between PDMS and RGO sheets can work well. Figure 2a−d illustrates the composition and elemental distributions of the RGO@PDMS. As shown from Figure 2a, the single microtube architecture in the strain sensor is composed of an RGO tube inner wall and PDMS film. The energy dispersive X-ray (EDX) spectrum taken from the rectangular area of Figure 2a mainly consists of three elements, including carbon, oxygen, and silicon. In Figure 2b−d, carbon, oxygen, and silicon show rational distribution. Such distribution is in good agreement with the hollow microtube channel structure of the strain sensor. The Raman spectra of PDMS, RGO@PDMS, and RGO are compared in Figure 2e. The pure PDMS primarily exhibits four peaks at 2964, 2906, 706, and 489 cm−1 consistent with the asymmetric and symmetric vibrations of CH3, the symmetric stretching of Si−C, and the symmetric stretching of Si−O−Si, while the RGO shows two

Figure 2. Elemental mapping and Raman spectra. (a) SEM image of RGO@PDMS. (b) Carbon, (c) oxygen, and (d) silicon mapping of RGO@PDMS in the rectangular area in part (a). (e) Raman spectra of RGO, RGO@PDMS, and PDMS.

remarkable peaks at 1349 and 1598 cm−1, corresponding to the D and G bands. In addition to the red shift of the G band from 1584 cm−1 for RGO to 1598 cm−1 for RGO@PDMS, the RGO@PDMS composite exhibits all bands of both PDMS and RGO at the same position, which confirms the uniform combination of RGO and PDMS.50 It has been reported that the red shift of the G band of carbon may be resulted from chemical doping of other elements on carbon materials.51−54 Herein, a 14 cm−1 shift in the G band may indicate the chemical doping of PDMS on the RGO sheets since the RGO could possess some basal plane for doping.50 In addition, to investigate the homogeneity of the RGO@ PDMS composite, Raman mapping was also performed. As can be seen in Figure S6b, the RGO@PDMS composite exhibits relatively uniform Raman intensity modes over a wide area, which verifies the homogeneous distribution of PDMS and RGO in the strain sensor. Moreover, the Raman spectra in Figure S6c further suggest the combination of PDMS and RGO microtube mesh-like structure. Such property of the composite can contribute to uniformly distribute the stress applied, which is beneficial to enhance the reproducibility and stability of the strain sensor. The working principle of the strain sensor is illustrated in Figure 3a. Theoretically, when the strain sensor is being stretched, the current of the test circuit can decrease correspondingly, due to the increase of contact resistance between the adjacent RGO sheets. In contrast, during the relaxing process of the strain sensor, the circuit current can increase gradually to the original level before stretching, since the RGO sheets can return to the original position. Therefore, the sensor can transmit the information on applied strain in the C

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desirable controllability, which is beneficial to expand its application range. However, it should be noted that our result is somewhat different from the reports elsewhere, in which the gauge factor is reduced with the increase of GO concentration.33,37 Herein, the most possible reason proposed for this is the difference in fabrication methods. For example, percolative strain sensor can be fabricated by spray coating of graphene solution on a heated substrate, which primarily involves disordered deposition of graphene. In contrast, in our work, the RGO microtubes meshlike structure is mainly composed of orderly assembled RGO sheets by weak van der Waals force, in which the reaction between GO sheets and Cu element is a self-limited process. When the Cu wires are covered completely by the RGO sheets, the ordered assembly reaction can be finished;47 hereafter, the assembly of RGO sheets could be boosted hydrothermally with little reduction action of Cu wire. As can be seen from Figure S8b, the superfluous parts (highlighted in red) may result from hydrothermal assembly of GO sheets almost without intervention of Cu wire. In addition, only an incomplete RGO mesh-like structure can be obtained when using lower GO concentration (Figure S8a). Therefore, the RGO mesh-like structure prepared by using a lower GO concentration displays a higher intrinsic resistance. Such structure is unsuitable to be applied as strain sensor. On the contrary, the RGO microtubes mesh-like structure obtained by using a higher GO concentration possesses a lower intrinsic resistance; together with the easy-sliding property of adjacent RGO sheets, a higher sensitivity can be achieved. Furthermore, as can be seen from Figure 3c, the linearity of dynamic i−t curves gradually become worse with the increase of GO concentration. This may be caused by the incomplete incorporation between PDMS monomers and RGO sheets due to the difficulty in permeating PDMS monomer into thicker RGO layers (Figure S8b), which is harmful to the synergistic effect.31 For simplifying the analysis, strain sensor fabricated from GO with 0.1 mg mL−1 concentration was selected for subsequent various characterizations. Apart from the stability and reproducibility, the higher sensitivity and good linearity of strain sensor play an important role in precise sensing as well. As seen from Figure 4a,b, the relative resistance change versus strain curves demonstrate favorable linearity and negligible hysterisis under a smaller strain range (∼10.7% or ∼21.3%), which can enable excellent stability, reproducibility, and shorter response time with the strain sensor. Along with the increase of applied strain, particularly, when the range is in excess of approximately 20% strain, the linearity gradually generates some disordered points (Figure S9), which might be ascribed to somewhat irreversible damages of strain sensor microstructure resulting from the shear stress difference applied between PDMS and RGO sheets. Moreover, typically, I−V curves of the device under various applied strains strictly confirm to the Ohm’s law (Figure 4c). On the basis of i−t curves of the strain sensor, by a general calculation formula of gauge factor, GF = (ΔR/R0)/ε (herein, GF, ΔR, R0, and ε represents gauge factor, resistance change, original resistance, and strain applied, respectively),55 gauge factors of the strain sensor under 10.7%, 21.3%, 32.0%, and 42.7% strain were calculated. As seen from Figure 4d, the gauge factor first increases and then reduces with increased strain in the ∼42.7% strain range, which can be explained by the working principle of the strain sensor.33 Dependent on whether the synergistic effect between PDMS and RGO sheets works, the relative resistance change of the

Figure 3. (a) Working principle of strain sensor. Blue arrows indicate the strain direction applied. The downward red arrow represents the decrease in test circuit current when the strain sensor is stretched, while the upward red arrow represents the increase in circuit current when the strain sensor is relaxed. (b, c) I−V curve and i−t curve of various strain sensors fabricated by using 0.025, 0.050, 0.075. 0.100, 0.150, and 0.200 mg mL−1 GO, respectively. Gauge factor varies with increased GO concentration (the inset). (d, e) i−t curve of the strain sensor under a larger strain range from ∼10.7% to ∼42.7%. The blue and green arrows represent the stretching and relaxing, respectively.

form of current or resistance change. Owing to the adoption of the selective etching strategy of copper, the performance test of the strain sensor becomes relatively simple without using conductive silver paste (Figure S2). When a constant voltage of ∼2 V is applied to the device, the applied strain can be detected in the form of current or resistance change, which is attributed to the changed contact resistance among RGO sheets coated by PDMS during its stretching and relaxing. Figures 3d,e and S7 present excellent stability and reproducibility of the sensor under a larger strain range from ∼10.7% to ∼42.7%, which are of vital importance in precise sensing. The blue and green arrows are employed to indicate the stretching and relaxing process, respectively. In a typical electromechanical performance test, when a constant voltage is applied, the test circuit current gradually reduces along with the stretching but increases along with the relaxing, which completely accords with the proposed working principle.33,55 Interestingly, gauge factor of the strain sensor can be effectively modulated by tailoring the GO concentration to accommodate different sensitivity requirements exhibited in Figure 3c.24,33,56,57 As seen from Figure 3b, all the I−V curves under 10% strain confirm to Ohm’s law; typically, resistance of the strain sensor is reduced when increasing the GO concentration. Moreover, the dynamic i−t curves in Figure 3c illustrate that relative resistance change increases with GO concentration. More importantly, gauge factor, which is usually used to evaluate sensitivity of strain sensor, also increases with the enlargement of GO concentration (the inset). Such properties suggest that the fabrication of strain sensor possesses D

DOI: 10.1021/acsami.5b09314 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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sensor possesses excellent selectivity to a specific strain. As seen from Figure 5a−c, relative resistance change of the strain sensor

Figure 4. (a, b) Relative resistance change versus strain curves of the strain sensor under various strains. Black and red arrows represent stretching and releasing, respectively. (c) I−V curves of the strain sensor under various strains. (d) Gauge factor distribution of the strain sensor under various strains.

Figure 5. (a−c) Selectivity test of the strain sensor under five bendings of 0−180°, five twistings of 0−90°, and an external temperature of 30.5−45 °C, respectively. (d) Selectivity test of the strain sensor under 1, 2, and 3 mm downward strains resulting from pressure-induced stretching, respectively.

strain sensor will increase with the enlarged strain until reaching its maximum, in which PDMS and RGO sheets of the strain sensor will possess the most advantageous integration; once exceeding the critical value, it will reduce due to the generation of somewhat irreversible damages. Most importantly, the device reaches its highest sensitivity, 630 of gauge factor, under 21.7% strain, even though under 42.7% strain, its sensitivity remains as high as 150 of gauge factor, which is far beyond the traditional ones reported previously.30,32 It is worth mentioning that a sensor fabricated by using 0.1 mg mL−1 GO solution is employed for all tests in Figures 4a,b and S9. Typically, these tests are conducted in the order from smaller to larger strains, in which the gauge factor is different under various stretchable ranges. Similar results can be seen in other reports,29,58 which are determined by the combination change between RGO sheets and PDMS monomers under different strains. Besides, under larger strain, the sensor unavoidably suffers from a certain uneven stretching due to the Poisson effect. Therefore, a sensor can display different gauge factors under various strains. This point can also be seen from Figure S10. Interestingly, such nonuniformly stretching induced by the Poisson effect in the strain sensor exhibits a negligible effect on the reproducibility and stability of the strain senor. As can be seen in Figures S11 and S12, the relative resistance change sensor still retains a good reproducibility and stability over 500 cycles under 25% and even 50% strain. Thus, the strain sensor can be used effectively under ∼50% strain and holds a great promise in the field of human motion detection. Besides, for examining the durability of the strain sensor fabricated by using GO solution with higher concentration, the strain sensor fabricated by using 0.15 mg mL−1 GO solution has been tested and shown in Figure S13. As can be seen, although retaining relatively favorable stability, the strain sensor shows unsatisfied reproducibility under 25% strain over 500 cycles. Such result might be derived from the uneven combination between RGO microtubes and PDMS, since the PDMS is difficult to be permeated into the superfluous RGO agglomeration (Figure S8b). In practical application of the strain sensor, selectivity also should be taken into consideration. Surprisingly, the strain

is 7.7%, 16.8%, and 7.1% under five bendings of 0−180°, five twistings of 0−90°, and an external temperature of 30.5−45 °C, respectively. The relative resistance changes are small enough and thus hardly disturbs the precise sensing under even only 5.4% strain, which can result in 30% of relative resistance change. Theoretically, the strain sensor can work well, provided that relative resistance change resulted from undesirable disturbances is less than the detected corresponding value. Such properties make the strain sensor suitable to be mounted on certain wearable equipment, exhibiting superior performance under some special conditions. Moreover, the strain sensor also demonstrates excellent selectivity in various pressure-induced stretching strains; herein, the direction of applying force is perpendicular to the basal plane of the strain sensor. As seen from Figure 5d, i−t curves under 1, 2, and 3 mm downward strains resulting from pressure-induced stretching illustrate an excellent selectivity and reproducibility, which is hopeful for integrating the strain sensor onto wearable equipment as a touch monitor. For exploring its potential applications, the strain sensor was fixed on a medical tape by double faced glue and then clung on the human body to detect physical motion. According to the reported results,23 the strain resulting from the human wrist bending is ca. 30%, while there was 0.1−0.2% strain for Adam’s apple motion,24 representing two different application ranges of the strain sensor. As seen from Figure 6a, the i−t curve demonstrates a favorable stability and reproducibility under five wrist bendings or stretchings of the tester, illustrating the strain sensor might be potentially applied as fashionable electronic fitness instructor. Moreover, interestingly, when clung on the Adam’s apple of a tester, the strain sensor also can recognize different speeches stably and reproducibly such as “hello” or “DUT”, regardless of speaking softly (left) or loudly (right) (Figure 6b). Thus, the strain sensor can be potentially applied in intelligence transfer. Apart from high sensitivity, the excellent sensing performance of the strain sensor also can be partly E

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built mesh-like structure. Therefore, high sensitivity of the strain sensor can be explained from three different perspectives. First of all, the superlubricity property between overconnected flexible RGO sheets (the friction is nearly zero) contributes to readily slide the adjacent RGO sheets, resulting in a greater relative resistance change under a smaller strain.60 As seen from other graphene based strain sensors reported elsewhere,24,32,58,61 the high sensitivity usually can be obtained (gauge factor varies from tens to hundreds). In contrast, within traditional sensors consisting of rigid structure units, such as the Ag NWs,30 the absence of superior flexibility and superlubricity would increase the friction between structure units (single Ag NW), which is disadvantageous to enhance the sensitivity (specifically, the gauge factor range is 4−16.6). Additionally, the hollow microtubes structure composed of easily slided RGO sheets can effectively reduce the shear stress located at the intersecting surface, which facilitates separation from the adjacent structure units (RGO sheets), thus enhancing the sensitivity of strain sensor. On the contrary, it is not difficult to imagine that solid counterparts composed of overconnected RGO sheets possess bigger shear stress due to the friction accumulation. Obviously, such architecture is not desirable for a higher sensitivity. Finally, the unique RGO microtubes meshlike structure can effectively distribute the shear stress in various directions (such as the X or Y axis), which is greatly beneficial to enhance the sensitivity (typically, several hundreds of gauge factor).24,62 In comparison, the structure units in percolating film based strain sensors can be separated thoroughly from each other only under larger strains.23,33 Consequently, such architectures greatly restrict their sensitivity performance (usually up to tens of gauge factor). Besides, the strain sensors can display higher sensitivity under larger strains, which is attributed the synergistic effect of RGO sheets and PDMS.23,56As mentioned above, the friction between RGO sheets is nearly zero; therefore, the resistance force preventing the separation from structure units in our strain sensors is mainly derived from the friction between RGO sheet and PDMS.62 Such properties are remarkably different from other traditional metal based strain sensors, in which friction can result from structure units or between units and PDMS.30,62 Therefore, these strain sensors display lower sensitivity under larger strains. On the contrary, some metal oxides based strain sensors show higher sensitivity though their stretchable ranges are smaller.27,63 The higher sensitivity of such strain sensors can be derived from the piezoelectric effect, which is different from the work mechanism of our strain sensors. The smaller stretchability can be associated with the flexibility absence of metal oxides and bigger friction between structure units and PDMS. Moreover, the ultrathin (ca. 83 μm) structure of strain sensor is also beneficial to improve its reproducibility due to the weakening negative effect of plastic deformation of PDMS on synergistic action between RGO sheets and PDMS and enhance the stretchability of the strain sensor fabricated.23

Figure 6. (a) i−t curve of the strain sensor under five wrist bendings or stretchings. (b) i−t curve of the strain sensor when the tester is speaking differently: softly (left) or loudly (right). (c) Durability test of the strain sensor under 13.68% strain for 500 cycles. (d) i−t curve of the strain sensor extracted from the yellow part in (c).

attributed to its outstanding flexibility, which can make the strain sensor follow every detail of human motion. Excellent durability of the strain sensor is a very important indicator in practical application, which can remarkably reduce its use cost and enlarge its popularity.33 As seen from Figure 6c, after 500 cycles of stretching−relaxing under 13.68% strain, the strain sensor remains favorably stable. Figure 6d represents a random episode extracted from Figure 6c (yellow part); amazingly, the i−t curve shows excellent stability and reproducibility. As such, the long-life i−t curve under the 3 mm strain resulting from the pressure-induced stretching− relaxing also displays a favorable reproducibility and stability (Figure S14). Such properties make the strain sensor hold great application potential in many fields due its cost-effectiveness. To the best of our knowledge, so far, three different kinds of mechanisms of a graphene based strain sensor have been proposed.55 One considers that the applied strain can open the band gap of pristine graphene, inducing the resistance change. Usually, the strain sensor based on this mechanism has very little stretchability and lower sensitivity, because pristine graphene has no prominent resistance change even when stretched up to 6% strain.59 Another one is based on the tunelling effect between adjacent graphene sheets.36 Although such a strain sensor can show higher sensitivity (∼320 of GF) under a lower strain (∼0.3% strain), the poorer stretchability and complex fabrication procedure limit its application range. The last one deems that resistance change mainly derives from the variation of an overconnected area between graphene sheets under strain.33 Hererin, the work mechanism of our strain sensor can be classified as the last one based on the designed RGO microtubes mesh-like structure. In comparison with the percolating film-like structure or micro/nanowires based strain sensors,33,37,58 the RGO microtubes mesh-like structure obtained by hydrothermal-assisted synchronous reduction and assembly of RGO sheets displays almost 1 order of magnitude higher gauge factor and other superior performances. Herein, the strain sensor is a typical hierarchical structure involving overconnected RGO sheets, subsequently constructed RGO microtubes, and eventually

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CONCLUSION In summary, a highly stretchable and ultrasensitive strain sensor based on RGO microtubes−elastomer composite has been demonstrated. Combining the unique RGO microtubes meshlike architecture, easy-sliding of RGO sheets, and eminent elasticity of PDMS, the strain sensor demonstrates the following important merits. First, it displays a larger detection range (∼50%) and higher sensitivity (∼630 of gauge factor) F

DOI: 10.1021/acsami.5b09314 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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than traditional ones, which makes it more suitable to precisely detect physical motion under larger strain. Moreover, it exhibits outstanding selectivity to a specific strain under bending of 180°, twisting of 90°, external temperature of 30−63.5 °C, and different downward pressure-induced stretching strains, which enables it to be mounted on wearable equipment to adapt to some harsh sensing conditions. More importantly, its gauge factor can be effectively tailored by using the GO with diverse concentrations to accommodate various sensing requirements. Eventually, its excellent durability and cost-efficient and scalable fabrication procedure also offer substantial opportunities for its popularization. The aforementioned properties enable the strain sensor to have great application potential in electronic fitness instruction, intelligence transmission, human−machine interaction, and other related areas.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09314. Materials, general measurements, control experiments, additional table, and figures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (No. 21361162004). REFERENCES

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DOI: 10.1021/acsami.5b09314 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.5b09314 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX