Highly Compressible and Sensitive Pressure Sensor under

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Highly Compressible and Sensitive Pressure Sensor under Large Strain Based on 3D Porous Reduced Graphene Oxide Fiber Fabrics in Wide Compression Strains Xiaoping Jiang, Zongling Ren, Yafei Fu, Yafeng Liu, Rui Zou, Guipeng Ji, Huiming Ning, Yuanqing Li, Jie Wen, H. Jerry Qi, Chaohe Xu, Shao-Yun Fu, Jianhui Qiu, and Ning Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11596 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Highly Compressible and Sensitive Pressure Sensor under Large Strain Based on 3D Porous Reduced Graphene Oxide Fiber Fabrics in Wide Compression Strains Xiaoping Jiang,

§†

Zongling Ren,

§†

Yafei Fu,§ Yafeng Liu,§ Rui Zou,§ Guipeng Ji,§ Huiming

Ning,§ Yuanqing Li,§ Jie Wen,§ H. Jerry Qi,⊥ Chaohe Xu,‡ * Shaoyun Fu,§ Jianhui Qiu,# and Ning Hu§∥* † These authors contributed equally to this work

§

College of Aerospace Engineering, Chongqing University, Chongqing, 400044, China

‡

MOE Key Laboratory of Low-grade Energy Utilization Technologies and Systems, CQU-NUS

Renewable Energy Materials & Devices Joint Laboratory, Chongqing University, Chongqing, 400044, China #

Department of Machine Intelligence and Systems Engineering, Faculty of Systems Science and

Technology, Akita Prefectural University, Yurihonjo, 015-0055, Japan

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

⊥

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The George W. Woodruff School of Mechanical Engineering, Renewable Bioproduct Institute,

Georgia Institute of Technology, Atlanta, Georgia 30332, United States ∥

The State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing

University, Chongqing, 400044, China

KEYWORDS: graphene fibers, pressure sensor, flexible electronics, interface, fiber fabrics


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ABSTRACT

The development of highly sensitive wearable and foldable pressure sensors is one of the central topics in artificial intelligence, human motion monitoring and healthcare monitors. However, current pressure sensors with high sensitivity and good durability in low, medium and high applied strains are rather limited. Herein, a flexible pressure sensor based on hierarchical 3D and porous reduced graphene oxide fiber fabrics (rGOFF) as the key sensing element is presented. The internal conductive structural network is formed by the rGO fibers which are mutually contacted by interfused or non-interfused fiber-to-fiber interfaces. Thanks to the unique structures, the sensor could deliver an excellent sensitivity from low to high applied strains (0.24% to 70.0%), a high gauge factor (1668.48) at an applied compression of 66.0%, a good durability in a wide range of frequencies, a low detection limit (1.17 Pa) and a ultrafast response time (30 ms). The dominated mechanism is that under compression, the slide of the graphene fibers through the PDMS matrix reduces the connection points between the fibers, causing a surge in electrical resistance. In addition, because graphene fibers are porous and defective, the change in geometry of the fibers also causes a change in the electrical resistance of the composite under compression. Furthermore, the interfused fiber-to-fiber interfaces could strengthen the mechanical stability under 0.01-1.0 Hz loadings and high applied strains, and the wrinkles on the surface of the rGO fibers increased the sensitivity under tiny loadings. In addition, the noninterfused fiber-to-fiber interfaces could produce a highly sensitive contact resistance, leading to a higher sensitivity at low applied strains.


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INTRODUCTION Wearable and foldable pressure sensors have gained tremendous attention in recent decades owing to their considerable scientific and technological potentials in practical applications1, such as electronic skin2, human motion monitoring3,4, intelligent robot5,6, human-machine interfaces7, and portable healthcare monitors8-11. To date, various types of innovative pressure sensors with excellent sensitivity have been constructed according to mechanisms of piezoelectric12,13, capacitive14, optical15,16, triboelectric17 and resistive sensing18,19. Among these, the resistive-type pressure sensors appear to be the dominated ones in recent studies5,18,19. The resistive sensing characteristics depend on the resistance change of the components in the sensors, which is triggered by the changes of the sizes, or the resistance of the piezoresistive materials or the contact resistance of the unique structures under external stimuli20,21. However, the intrinsic piezoresistive materials such as metals and semiconductors are inherently stiff and brittle22, and hence they can only detect low strains within a few percent (the detection limit is ca. 5.0%). As a result, the conventional sensors in current wearable and foldable sensors with high sensitivity can only be applied in a narrow range of applied strains. Recently, many studies highlighted that nanostructured materials such as nanoparticles23, nanowires24,25, nanotubes26 and 2D graphene27 are promising building blocks for innovative pressure sensors with excellent performances28. Especially, 2D graphene with unique characteristics such as excellent electrical properties, mechanical strength and flexibility has been extensively studied for strain sensing applications29-33. However, 2D graphene layers without special macro- and/or micro-structural designs can only be stretched or compressed to a very limited extent (less than ca. 6.0%)34,35. Macroporous graphene monoliths with ultralow density, good electrical conductivity and excellent elasticity have been employed to improve the


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stretchablity of graphene to a level higher than 30.0%36, which still cannot fulfill the need for vigorous human-motion detection with the maximum strain exceeding 50.0%32. Chen et al. reported the fabrication of 3D CVD graphene foam embedded in polymer elastomers, which can be stretched to a level higher than 95.0% before mechanical fracture. However, the synthesis method for the 3D CVD graphene foam requires high-cost CVD techniques and time-consuming etching processes, greatly hindering its applications. Therefore, design of unique graphene architectures by facile, low-cost and scalable fabrication approaches is of growing interest. At present, many researches have applied fiber in piezoresistive sensors, and these sensors are designed

on

the

basis

of

the

relationship

between

fiber

contact

and

electrical

conductivity11,21,33,37. Graphene fiber fabric (GFF) is a newly-developed hierarchical 3D and porous graphene architecture38, which is composed of randomly oriented and interfused graphene fibers with strong interfiber interactions. The interfused networks endow the GFF with outstanding mechanical robustness, flexibility and electrical conductivity39, which make a promising resistive sensing material for pressure sensors with excellent sensitivity and a wide range of applied strains. There are many studies focused on the the relationship between porosity and piezoresistibility of different materials40,41,42,43. However, the related study on graphene fibers fabric is quite limited until now. Here, we demonstrated a flexible resistive pressure sensor based on hierarchical 3D and porous reduced graphene oxide fiber fabrics (rGOFF) filled with polydimethylsiloxane (PDMS) elastomer for monitoring the human motion, sphygmus, breathing and tiny strains. The designed rGOFF/PDMS sensor could deliver an excellent sensitivity from low applied strains (0.24%) to high applied strains (up to 70.0% of compression strain), a good durability in a wide range of frequencies, a very low detection limit of 1.17 Pa and an ultrafast response time (30 ms). The


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superior sensing properties can be ascribed to the following advantages: i) the rGO fiber has an intrinsic high mechanical strength44, which enhances the mechanical performance of the rGOFF/PDMS composite, further benefiting the durability of the sensor; ii) the good electrical conductivity of rGO fibers30 and the fiber-to-fiber interfacial contact (non-interfused) can produce a sensitive contact resistance, which leads to a higher sensitivity at low applied strains; iii) the interfused fiber-to-fiber interfaces not only strengthen the mechanical stability under 0.01-1.0 Hz loadings and high applied strains, but also increase the sensitivity under tiny loadings owing to the rich wrinkles on the surface of rGO fibers; iv) the loose 3D structure results in an outstanding piezoresistive ability under high applied strains. The fabricating process is facile and convenient compared with many other high-sensitivity sensor manufacturing processes. As a proof of concept, the as-fabricated sensor could be employed to monitor the motion of fingers and arms, breathing during exercise, dancing of robots, pulses and tiny pressures. EXPERIMENTAL SECTION Preparation of rGOFF: GO used in this work was prepared via an improved Hummers method45,46. Briefly, 1.0 g graphite (80 mesh) and 8.5 g CrO3 were added into 7.0 mL concentrated HCl solution, which was stirred for 2 h under the ambient condition. The treated graphite powders were washed with DI water several times to remove the CrO3 residue. After dried in a vacuum oven, the products were immersed into 40 mL H2O2 (30%) at room temperature for another 20 h and then filtrated. The product was immersed into 200 mL concentrated sulfuric acid for 10 min and subsequently filtrated to remove sulfuric acid to get the chemically expanded graphite. Next, 2.0 g KMnO4 was carefully added into 40 mL concentrated sulfuric acid incubated in an ice water bath and stirred for 30 min until a green solution was


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formed. The chemically expanded graphite was introduced to the KMnO4-acid solution, and the solution was incubated at 35oC for 4 h. The gray black mixture was filtered through a 100-mesh metal filter, and then added into a solution composed of 100 mL DI water and 5 mL H2O2. Finally, the product was washed repeatedly with DI water, and sonicated for 1 h to produce the GO solution with a concentration of 12 mg/ml. The rGOFF was prepared by a combination of wetting-spinning and vacuum filtration assembly technique. Typically, a GO solution (12 mg/ml) was injected into a rotating ethyl acetate coagulation bath with an injection speed of 0.15 ml/min. The obtained long GO fiber was quickly cut with scissors to further obtain the short GO fiber with a length of about 4 ~ 6 mm. A wet GO fiber fabric was collected through a vacuum filtration assembly of the short GO fibers. After drying at room temperature for more than 15 h, the GO fiber fabrics were transferred into a 60 oC vacuum oven for 3 h to completely remove the residual ethyl acetate. Finally, the fully dried GO fiber fabrics (GOFF) were reduced to rGOFF via reacting with hydrazine hydrate vapor at 95 oC for 12 h. Preparation of the rGOFF/PDMS Composite Based Pressure Sensor: The rGOFF was firstly put into a plastic petri dish. The PDMS base and curer were mixed with a weight ratio of 10:1 and poured into the dish until the rGOFF was fully covered. The petri dish was then degassed in a vacuum oven for 10 min and dried at 40 oC for 12 h. The obtained rGOFF/PDMS composites were cut into pieces with proper sizes, electrically connected to an Al foil by silver conducting resins, and further dried at 60 oC for 3 h to solidify the silver glue. Characterization: X-ray diffraction (XRD) was used to analyze the crystal structure of GO, which was carried out on a Rigaku D/max 2500 (Rigaku, Takatsuki, Japan). Scanning electron


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microscopy (SEM) was performed using a Zeiss AURIGA FIB (Zeiss, Jena, Germany). X-ray photoelectron spectroscopy (XPS) was characterized on a KRATOS AXIS DLD spectrometer (Kratos, Japan). Raman spectrum was performed on HORIBA LabRAM HR Evolution with a 532 nm excitation length (HORIBA, Fukuoka, Japan). The weight percentage of oxygen functional groups was analyzed by using the thermogravimetric analysis (TGA) on NETZSCH STA449C (NETZSCH, Germany). The conductivity of the sample was tested on a digital multimeter (VICTOR 86E) (KEYSIGHT, Palo Alto, USA). To investigate the sensitivity, the rGOFF/PDMS sensor was fixed between the two fixtures of the universal testing machine (UTM, Shimadzu, EZ-LX), while each electrode of the sensor was connected with the electrode of a high-resolution digital multimeter (KEYSIGHT 34465A) (KEYSIGHT, Palo Alto, USA).

RESULTS AND DISCUSSION The fabrication process of rGOFF and the rGOFF/PDMS piezoresistive sensor are schematically illustrated in Figure 1a. Firstly, a modified Hummers method was employed to prepare GO, which is mostly single-layered with a ultra-large lateral size up to dozens of micron, as verified in the AFM, SEM and XRD images (Figure S1, S2 & S3). Via a common liquid crystal spinning approach, the concentrated GO solution continuously self-assembled into long GO fibers (Figure 1b & 1c). After cutting into short fibers, vacuum filtration and drying, we obtained a selfstanding and macroscopic porous GOFF as depicted in Figure 1b-1f. Here, a further hydrazine vapor reduction process was employed to reduce GOFF, during which the diameter of the fabric stayed nearly the same, while the color changed from yellow brown to black, indicating the successful removal of the oxygen functional groups in GO (Figure 1g). Meanwhile, the


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macroscopic porous structure of the fabric was well preserved after chemical reduction, which can greatly facilitate the subsequent infiltration of the elastomer matrix to form the rGOFF/PDMS composite (Figure 1h). As a result, a well-defined rGOFF/PDMS sensor was successfully prepared as shown in Figure 1i. GOFF and rGOFF were characterized by scanning electron microscopy (SEM) as shown in Figure 2. The top-view SEM images of GOFF clearly show that GOFF had a macroporous structure with a pore size of dozens of micrometers, and each fiber displayed a rough surface. The diameters of the GO fibers are 35-45 μm. On closer observation, it can be seen that the GO fibers in GOFF mainly linked with each other in two types, crosslinking with the fused GO at the fiber-fiber interfaces and direct contacting, as depicted in Figure 2a-2c. The main reason for this structure is that during the vacuum filtration process, GO fibers are stacked into a filter cake, and the fibers are overlapped with each other. During the drying process, some hydrogen bonds are formed between the fibers. After chemical reduction, most of the fibers stacked tightly together, and these two types of interface contacts were well preserved (Figure 2d-2f). Noteworthy, the rGO fibers in rGOFF became smoother, with the diameter increasing to 80-100 μm, 44.4% larger than that of the GO fibers. The apparent expansion of the diameter from the GO fibers to the rGO fibers was benefitted from the chemical leavening effect that was induced by the hydrazine hydrate vapor reduction, during which the gases generated by decomposition of oxygencontaining functional groups triggered the formation of numerous pores in the tightly packed GO structure47. Here, such a large expansion of the fiber diameters made the interface contact more tightly, which is highly beneficial for the development of pressure sensors with high sensitivity from low to high applied strains.


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Figure 1. (a) The simplified process for the preparation of the rGOFF/PDMS sensor. The optical photos of (b, c) long GO fibers, (d, e) short GO fibers, (f) GOFF, (g) rGOFF, (h) rGOFF/PDMS composite, (i) rGOFF/PDMS sensor.


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Figure 2. SEM images of (a-c) GOFF, and (d-f) rGOFF at different scales.

We further performed TG, X-ray photoelectron spectroscopy (XPS) and Raman tests to evaluate the reduction effect of the rGO fibers. Figure 3a clearly shows the TGA curves of GOFF and rGOFF. After heated to 800 oC in the nitrogen atmosphere, GOFF and rGOFF remained 35.37 wt% and 83.16 wt% of the original weights, respectively, indicating that most of the oxygen functional groups were removed. The XPS results also led to a similar conclusion as shown in the survey spectra (Figure S5) and high-resolution C 1s spectra in Figure 3b. The peak of C1s at 284.8 eV is attributed to C-C bonding (sp2 carbon) in rGO48,49. The peaks centered at 286.2, 287.0, and 289 are attributed to C-O, C=O, and O-C=O, respectively50. Clearly, the C=O peak of rGOFF is disappeared as seen in Figure 3b. The oxygen concentration of GOFF was 23.64 at.%, while it decreased to 7.44 at.% in rGOFF. The C/O ratio changed from 3.23 for GOFF to 12.44 for rGOFF, indicating a deep reduction via hydrazine vapor reduction. The Raman spectra of


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GOFF and rGOFF also demonstrated a noticeable reduction extent (Figure S4). The ID/IG ratio, an index of the structural defects, changed from 1.518 to 1.300 after reduction, indicating a dramatic decrease of the defect concentration and the role of defect reparation. The tensile strength of the as-formed GO fibers reached 33.49 MPa with an ultimate fracture strain of 3.60%. However, after reduction, the tensile strength of rGO fiber was reduced to 18.47 MPa with an ultimate fracture strain of 2.43% (Figure 3c) due to the brittleness of the rGO fibers and the increase of the diameter. The resulted electric conductivity of rGOFF was 1.13 S/m (Figure 3e), almost 105 times of that of GOFF (1.51 × 10−5 S/m). The good conductivity of rGOFF led to the high conductivity of the rGOFF/PDMS composites (0.12 S/m), further demonstrating a promising potential in pressure sensor applications51. GOFF is made up of GO fibers, so it has poor electrical conductivity. After chemical reduction, some of the oxygen-containing functional groups of the GO fiber are lost ， so the conductivity of GOFF becomes better. Immersion in PDMS will lead to poor contact between the fibers, especially for non-molten fibers, so the electrical conductivity of the composites will be reduced to some extent. In order to fabricate the pressure sensor, a self-standing and macroporous rGOFF was firstly impregnated with the mixture of PDMS base and curer; after solidification, the rGOFF/PDMS sensor was obtained by cutting the composite into pieces with a size of 25 mm (length) × 12 mm (width) × 3 mm (thickness), which were connected with external circuits via Al foils and silver conducting resins, as shown in Figure 1h & 1i. The microstructures of the as-formed rGOFF/PDMS composites are displayed in Figure 3d. The elastomer matrix was successfully filled into the macroporous rGOFF with a tight interfacial contact and uniform fiber distribution. Similar to previous CNT/polymer composites52,53, the preformed rGO fiber to fiber interfacial contact, the partial infused fiber/fiber interface and the macroporous structures could greatly


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promote the piezoresistive response sensitivity in the pressure sensor. These unique structural characteristics will endow a great mechanical performance of the rGOFF/PDMS composite. As demonstrated, the tensile stress (σ) as a function of strain (ε) of the composite is shown in Figure 3f. Clearly, the rGOFF/PDMS composite could recover to its original state even under 70.0% of the compressive strains. These results indicate that the pressure sensor based on the rGOFF/PDMS composites can work in a wide range of compressed applied strains.

Figure 3. (a) Thermogravimetric analysis (TGA) profiles of GOFF and rGOFF. (b) C 1s region in XPS spectra of GOFF and rGOFF. (c) Typical strain-stress curves of single GO fiber (GOF) and single reducing GO fiber (rGOF). In the test, the length of the fiber is 5 mm. (d) Crosssectional SEM image of the rGOFF/PDMS composite. (e) Comparison of the electrical conductivity of GOFF, rGOFF, and rGOFF/PDMS. (f) Typical cyclic tensile strain−stress curve of the rGOFF/PDMS composite.


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Figure 4. (a) Relative changes of the resistance with compressive strains of the rGOFF/PDMS sensor. (b) Gauge factors in different applied strain levels. (c) The natural logarithm result derived from b. (d) Changes of the resistance and strain with time delay in 0.0%-15.0% strain unloading-loading tests. (e) Multiple cycles of changes in resistance with different applied compressive strains. (f) Change in resistance with 30.0% of compressive strains in different frequencies. (g) Response and relaxation property and (h) the cyclic stability test results under a repeated applied strain of 20.0%.

The quasi-static compressive loading tests were carried out to evaluate the strain sensing capability of the as-designed rGOFF/PDMS pressure sensor. The relative change of the resistance (RCR, ∆𝑅 𝑅0 = (𝑅 ― 𝑅0) 𝑅0 , where R0 is the initial resistance) increased continuously with the applied strain (ε), as shown in Figure 4a. The RCR increased rapidly to as high as > 14000 after being applied with a 72.0% strain; even in a relatively low strain of 20.0%~30.0%, the values of RCR were still in the range of 100~300. This demonstrates that the rGOFF/PDMS pressure sensor can deliver an excellent performance in sensor applications. Generally, the sensitivity of the pressure sensor is calculated by the gauge factor (GF = ∆𝑅 𝑅0∆𝜀, where ∆𝜀 is the applied strain), as depicted in Figure 4b. In this work, the GF reached a very high level of 1668.48 when a ~66.0% strain was loaded, higher than most GFs reported in previous studies, e.g., the works23,24,26,27,29-32,34-36,,54-56, indicating it has a superior piezoresistive sensitivity. This is because the resistance of rGOFF/PDMS composites is closely related to the number of connection points formed by graphene fibers in the PDMS matrix. Under compression, the slide of the graphene fibers through the PDMS matrix reduces the connection


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points between the fibers, causing a surge in electrical resistance. In addition, the change in geometry of the fibers also causes a change in the electrical resistance of the composite under compression due to the porous and defective nature of graphene fibers. Despite the high sensitivity, the GF-strain curve showed a non-linear characteristic on closer inspection due to the tunneling effect among neighboring rGO fibers55,56. Notably, many previous studies reported that the GF-strain curves are linear within a relatively low strain range, for example < 30.0%. However, it is not true when performing a careful observation in the figures. In this work, the curve was essentially linear in the strain ranges of less than 6.0% and 6.0% to 10.0% (the inset in Figure 4b); however, the nonlinearity became apparent at applied strains of beyond 10.0%. Actually, the non-linear feature of the sensitivity may cause deviation in further practical scenes57. In order to avoid the deficiency, it is valuable to express the experimental data using natural logarithms as shown in the following equation56,58-60: ln

( ) = ln (𝑅 ― 𝑅0) ― ln 𝑅0 Δ𝑅 𝑅0

(1)

As depicted in Figure 4c, according to the characteristics of the curve, the change of the relative resistance was linearized by this transformation. As can been seen, the fitting was almost a straight line when the applied strain was greater than 6.5%, indicating that our designed rGOFF/PDMS pressure sensor could deliver superior piezoresistive performances in a wide range of applied strains. The excellent performance may be benefitted from the loose integral 3D structure, unique fiber-to-fiber interfacial contact, good electric conductivity and mechanical stability of the rGOFF. Undoubtedly, the outstanding sensitivity makes the sensor more competitive in practical applications59.


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To fully evaluate the performance, the behavior of the rGOFF/PDMS sensor under 0.0%-15.0% strain unloading-loading with a holding time of 6 s was recorded, as shown in Figure 4d. The RCR response almost occurred simultaneously with the applied strain loaded. During the holding period, clear RCR degradation was observed according to stress relaxation. The RCR did not return to zero after each cycle, which is a normal phenomenon in the piezoresistive sensor by using the PDMS as the elastomer because of the internal stress relaxation61,62. The RCR responses under compressive loading-unloading cycles with different applied strains are presented in Figure 4e. Clearly, the variation of RCR synchronized with the change of the applied strains. Again, the rGOFF/PDMS composites demonstrated an excellent sensitivity. The RCR values were 8.91%, 19.96%, 69.90% and 204.46% under the compressive strain of 5.0%, 10.0%, 20.0% and 30.0%, respectively, with no obvious drops during the loading-unloading cycles. This result is better than most of other same type sensors61-64.The reason is that the interfused fiber-to-fiber interfaces and the preformed continuous networks could decrease the relaxation of the composites62. The response curves at the frequency of 0.01 Hz to 1.0 Hz are also recorded in Figure 4f. The applied train was fixed as 30.0%. The RCR peaks were 233.0%, 257.0%, and 360.0% at the frequency of 0.01 Hz, 0.1 Hz and 1.0 Hz, respectively. It can be seen that the amplitude of RCR under high frequency is significantly higher than that under low frequency. This is because the stress applied on the sensor with fixed strain under high frequency is significantly higher than that under low frequency53. At the same time, the response time was calculated to be around 30 ms, as seen in Figure 4g, indicating a very rapid response time of our sensor. The response time is less than most results reported in similar studies65-71. The durability of the pressure sensors is another key issue for their practical applications. As exhibited in Figure 4h, the RCR value of


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the rGOFF/PDMS sensor delivered a high stability within 1200 loading-unloading cycles at a frequency of 0.1 Hz, in which the RCR peak was as high as 92.0% under an applied compressive strain of 20.0%, and a reduction of less than 10.0% of the initial value was achieved after 1200 cycles. Even after 1200 cycles, the RCR peak still reserved 64.0% of the initial value at a frequency of 0.1 Hz. Further increase the applied strain to 50% in Figure S6, the constructed rGOFF/PDMS sensor could still work very stable even after 1200 continuous cycles. The remarkable durability and stability could be attributed to the fascinating 3D porous structures and continuous conducting networks constructed via the rGO fibers, the interfused fiber-to-fiber interfaces, as well as incorporated with the flexibility of the PDMS matrix65. We also studied the detection limits of the tiny pressure. As displayed in Figure S7a, there were significant differences in the height among the four response peaks of 22.7 Pa, 45.3 Pa, 79.3 Pa and 215.2 Pa, indicating that the designed rGOFF/PDMS sensor is very prominent at measuring small stresses. In order to determine the minimal detection pressure of our sensor, we measured the response of RCR in the pressure range of 0.0 Pa~200.0 Pa as shown in Figure S7b & S7c. The curve is almost linear, again indicating that the sensor could deliver excellent sensing property under very small pressure. Significantly, the minimal detection pressure is 1.17 Pa as verified in the marked circle in Figure S7c, better than most previously reported flexible sensors24,64,68-70,72,73. The excellent small pressure detection capability is a great advantage for flexible and compressible sensors. We further employed small-load objects (rice or millet) to evaluate its sensitivity, as seen in Figure S7d. Surprisingly, our sensor could detect an ultralight loading even as tiny as the weight of a millet (6.6 mg), displaying the best comprehensive sensing property among those sensors reported to date (Table S1)19,24,67,68,74,75.


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The flexible rGOFF/PDMS sensors were further employed to characterize various human motions, such as the bending of fingers and arms, sphygmus, breathing, dancing of robots and very tiny pressures. As shown in Figure 5a, the flexible strain sensor was fixed onto the index finger to record the bending-relaxation cycles with a bending angle from 0o to 90o. The detected RCR during the bending motion was clearly observed76. By altering the bending angles, the values of RCR increased or decreased simultaneously during bending or relaxation of the finger, respectively. The same phenomena were observed by attaching the sensor on the arm; the difference was the RCR value, which was bigger than that for the finger due to the larger applied strain, as shown in Figure 5b. In addition, the sensor worked well in detecting breathing during exercise and sphygmus (Figure 5c & 5d). We used double-sided tape and art tape to hold the sensor in the breathing position inside the mask. After the tester jogged, we began to test the breathing signal, which were clearly displayed on the computer by attaching the sensors to a high-resolution digital multimeter and computer. The number of pulses detected by the sensor was 68 per min, which matches well with that under normal adult physiology. The flexible sensor could also be mounted to the sole of the robot foot to monitor the dancing, as can be seen in the supplementary video. The rGOFF/PDMS sensor could detect the dancing signal immediately after the dancing began. Besides of the applications of human and robot motion detection, our sensor is also ideal to work as a sensor switch due to the large resistant change during pressure loading and releasing. As shown in Figure S8, the green LED light turn dark immediately when the pressure is applied, and lights up again when pressure is released. Taken together, our specially designed flexible rGOFF/PDMS pressure sensor can be applied in various application fields, from human motion, psychological health to human-machine interaction as well as tiny strain detecting, owing to its excellent sensitivity and durability, which were caused


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by the excellent flexibility, good conductivity, 3D continuous and stable conducting networks constructed by the rGO fibers via fiber-fiber interfusion.

Figure 5. Application of wearable devices. Relative changes in resistance versus time for (a) finger bending, (b) arm bending, (c) breathing during exercise, and (d) sphygmus.

CONCLUSIONS In summary, we firstly demonstrated a flexible resistive pressure sensor based on hierarchical 3D porous rGOFF and PDMS elastomer to monitoring human motion, sphygmus, breathing and


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minor strains. The internal conductive structural network is formed by the rGO fibers which are mutually contacted by interfused or non-interfused fiber-to-fiber interfaces, further endowing good conductivity and unique structural characters. Finally, the as-designed pressure sensor can provide applied strain from low to high (0.24% to 70.0%), with an ultrahigh gauge factor up to 1686.48 at an applied compression of 66.0%, a fast response time of 30 ms, a wide frequency range (0.01-1.0 Hz), great stability and durability, and a limit of detection of 1.17 Pa, which could be used to detect a millet as light as 6.6 mg. The dominated mechanism is that, under compression, the slide of the graphene fibers through the PDMS matrix reduces the connection points between the fibers, causing a surge in electrical resistance. In addition, because graphene fibers are porous and defective, the change in geometry of the fibers also causes a change in the electrical resistance of the composite under compression. The sensor could also be employed in wearable electronic devices for monitoring the motion of fingers and arms, breathing during exercise and dancing of robots.

ASSOCIATED CONTENT Supporting Information. The PDF file includes: fig. S1. Scanning Electron Microscope (SEM) images of GO. fig. S2. Atomic Force Microscope (AFM) images of GO sheets. fig. S3. X-ray diffraction (XRD) pattern of GO. fig. S4. Raman spectra of GO fiber fabric (GOFF) and reducing graphene fiber fabric (RGOF). fig. S5. XPS survey spectra of the GOFF and rGOFF. fig. S6. Long cycle testing under high strain level of 50%. fig. S7. The RGOFF/PDMS sensor responds to small loadings. fig. S8.Under pressure, the sensor resistance increases so that the light bulb darkens significantly.


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table S1. Comparison of the main parameters of the pressure sensor in this work and the previous work. Other Supplementary Materials for this manuscript includes the following: movie S1. The spinning of graphene oxide fibers. movie S2. Response of the RGOFF/PDMS sensor to robot motions.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (C. Xu); [email protected] (N. Hu) Author Contributions X.P.J, Z.L.R, C.H.X and N. H. conceived the idea, wrote the manuscript; C.H.X and N. H. provided financial support through grant application. Y.F.F., Y.F.L., R.Z., G.P.J, H.M.N. and Y.Q.L. helped with materials synthesis, device fabrication and data analysis. X.P.J., Z.L.R. and C.H.X. performed TEM, SEM and image analysis. All authors participated in discussing and writing the manuscript, read and approved the final manuscript. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 11632004, 21503025, U1864208), Key Program for International Science and Technology Cooperation Projects of Ministry of Science and Technology of China (No. 2016YFE0125900), Fundamental Research Funds for the Central Universities (No. 106112016CDJZR325520),


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Chongqing

Research

Program

of

Basic

Research

and

Frontier

Technology

(NO.

cstc2016jcyjA1059, cstc2017jcyjBX0063), China Postdoctoral Science（2018M633316）.

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TOC Figure


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Highly Compressible and Sensitive Pressure Sensor under

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