Flexible and High Performance Piezoresistive Pressure Sensors

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Flexible and High Performance Piezoresistive Pressure Sensors based on Hierarchical Flower-shaped SnSe2 Nanoplates weiwei Li, Ke He, Daoshu Zhang, Nianci Li, Yuxin Hou, Guanming Cheng, Weimin Li, Fan Sui, Yang Dai, Hailin Luo, Ye Feng, Lei Wei, Wenjie Li, Guo-Hua Zhong, Ming Chen, and Chunlei Yang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00147 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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Flexible and High Performance Piezoresistive Pressure Sensors based on Hierarchical Flower-shaped SnSe2 Nanoplates

Weiwei Li,†,‡ Ke He,†,‡ Daoshu Zhang,†,‡ Nianci Li,†,§ Yuxin Hou,† Guanming Cheng,† Weimin Li,† Fan Sui†, Yang Dai†, Hailin Luo†, Ye Feng†, Lei Wei||, Wenjie Li,† Guohua Zhong,† Ming Chen,*,† Chunlei Yang*,†

Center for Information Photonics and Energy Materials, Shenzhen Institutes of

†

Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, People’s Republic of China Department of Nano Science and Technology Institute, University of Science and

‡

Technology of China, Suzhou 215123, People’s Republic of China School of Computer and Control Engineering, University of Chinese Academy of

§

Sciences, Beijing 100049, People’s Republic of China School of Electrical and Electronic Engineering, Nanyang Technological University,

||

50 Nanyang Avenue, 639798, Singapore *email:

[email protected]; [email protected]

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Abstract: Flexible piezoresistive pressure sensors featuring high sensitivity, wide operating pressure range and short response time are required urgently due to the rapid development of smart devices and artificial intelligence. Herein, a high-performance flexible piezoresistive pressure sensor based on naturally formed hierarchical flowershaped SnSe2 nanoplates and conical frustum-like structured polydimethylsiloxane (PDMS) is demonstrated. The micropatterned PDMS/Au and SnSe2 nanoplates/Au interdigital electrodes are exploited as the top and down part of the sensor, respectively. Benefiting from abundant contact sites and sufficient roughness provided by the SnSe2 nanoplates, the proposed sensing devices exhibit significantly enhanced sensitivity as high as 433.22 kPa-1, when compared with conventional configuration (planar Au film as

the

bottom

interdigital

electrodes).

The

resulting

pressure

sensor

(PDMS/Au/Au/SnSe2) also presents wide operating pressure range (0~38.4 kPa), lower limit of detection (~0.82 Pa), fast response time (~90 μs), and long-term cycle stability (>4000 cycles). Therefore, it shows a great potential in various applications, such as the detection of the magnitude and distribution of the loaded pressure, as well as the monitoring of the human physiological signals.

Keywords: sensitivity, response time, pressure sensors, micropatterned structures, SnSe2 nanoplates.

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INTRODUCTION Pressure sensor is an inevitable electronic device which enables the efficient conversion from the mechanical stimuli to the electronic signals, with applications in soft robotics, smart devices and electronic skin1-4. Recently, varied technologies have been developed for pressure sensing including capacitive sensing, piezoelectric sensing and piezoresistive sensing4-16. Among these technologies, the piezo-resistivity sensing is more promising and widely used due to the facile design, low manufacturing cost, good signal repeatability and high sensitivity. By far, a large variety of materials (including two-dimensional transition metal dichalcogenide nanosheets, graphene, nanowires, polymer nanofibers) and structures have been used in high-sensitivity piezoresistive pressure sensors17-29. For a typical piezoresistive pressure sensor, the sensitivity and operating pressure range is determined by the structure change (or variation of the contact area). Introducing micropatterned structures is a useful strategy to increase the performance of the pressure sensors, which result in a larger change of the contact area, while comparing with the pressure sensors with planar structures. Creating nanosized or microsized patterns on elastic polymer (e.g. patterned PDMS) is a common way to form highly sensitive pressure sensors. For example, it is found that PDMS structures with micropyramids, micro-hemispheres, micro-semicylinders and microhump patterns (via sandpaper mold) can improve sensitivity of the sensors30-32. In these pressure sensors, these micro-patterned PDMS will deform under external pressure and a large change of the contact surface area is realized. The whole working process of these pressure

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sensors can be briefly illustrated as follows: “surface contact” to “surface saturated”. In our previous work33, we reported a pressure sensor based on “rough-rough” structure (micropatterned PDMS/Ag/rough polyimide (PI)/Au interdigitial electrodes) and the assembled flexible, wearable device shows high sensitivity (259.32 kPa-1). Introducing the “rough-rough” configuration (touchpoint control in both the top and down structure of the pressure sensor) was shown to be the key factor33. And the working process of the “rough-rough” pressure sensors can be concluded as: “point contact” to “point saturated” to “surface contact” to “surface saturated”. However, the preparation of rough PI/Au interdigitial electrodes requires very complex process33. In our daily life, there are many natural and well-adapted structures, which also inspired us to design pressure sensor devices34-38. Wang et al34 fabricated a pressure sensor inspired by the silk textiles and demonstrated a high sensitivity of 1.8 kPa-1. Su et al proposed a pressure sensing device based on micropatterned PDMS molded from mimosa leaves35. Inspired by the wrinkles on surface of human skin, Pan et al36 use the skin-like wrinkled rGO film to prepare a pressure sensor which exhibits high sensitivity of 178 kPa-1. Kang et al proposed a pressure sensing device based on nanoporous polymer composite37. Yu et al proposed a piezoresistive pressure sensor featuring polyurethane sponge with MXene sheets coating38. However, piezoresistive pressure sensors with the merits of high sensitivity, wide response range and low detection limit is still the biggest challenge. Herein, we demonstrated an approach to realize a high performance piezo-resistive pressure sensor that employs the naturally formed, flower-shaped SnSe2 nanoplates/Au

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as the bottom part and the conical frustum-like structured PDMS/Au as the top part. The proposed configuration possess abundant, hierarchical, sensitive sites (touchpoints), contributing to the pressure sensor with high sensitivity (433.22 kPa-1), wide operating pressure range (0~38.4 kPa), ultrafast response (~90 μs), ultralow limit of detection (~0.82 Pa) and long-term cycling stability (>4000 cycles). Furthermore, the fabricated sensing devices are capable of detecting human wrist pulse. In addition, large-area sensor arrays are successfully fabricated and applied in detection of the intensity and distribution of external pressure.

RESULTS AND DISCUSSION The key fabrication process of the PDMS/Au/Au/SnSe2 (PAAS) pressure sensor are schematically shown in Figure 1a. A simple lithography and casting process was used to transfer the micropatterns to the PDMS mold, followed by the Au film deposition. The bottom part of the pressure sensor (SnSe2/Au) is prepared via coevaporation of SnSe2 nanoplates39, the deposition of Au film and laser scribing process (more details can be found in the Experiments Section). Figure 1b-d, Figure S1 and Figure S2 are the characterization of the obtained SnSe2 film under optical microscope and scanning electron microscopy (SEM), showing the continuous, uniform and naturally formed flower-shaped SnSe2 nanoflakes in a large-area of 10 cm × 10 cm. Figure 1e-g and Figure S3 are the SEM and AFM images of the microstructured PDMS, showing the conical frustum-like characteristics. Figure 1h-j depicts the optical microscopy image of the SnSe2/Au interdigital electrodes via laser scribing process.

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The width of a single interdigital is ~120 μm, the interval between two neighboring interdigital electrodes is ~350 μm. The crystallinity and composition of SnSe2 nanoplates on glass substrate are also investigated by XRD, Raman spectroscopy and EDS. Figure 2a depicts the X-ray pattern of SnSe2 nanoplate. The notable peaks confirmed the formation of hexagonal structure of SnSe240. The Raman spectrocopy of the SnSe2 nanoplates is shown in Figure 2b. There are two significant Raman peaks at 114.5 and 184.7 cm-1. The peak in lower wavenumber is assigned to the Eg mode (corresponding to non-degenerate inplane vibrations) and the A1g mode (corresponding to out-of-plane vibrations)

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is

observed at the higher wavenumber peak. The inset of Figure 2b shows the schematic of the Raman modes. Moreover, the atomic ratio Sn to Se is close to 1:2, as shown in Figure 2c-d. Under the external pressure, the top conical frustum-like structured PDMS and the bottom nature-inspired SnSe2 microstructures with rich sharp peaks provide abundant contact sites and sufficient roughness, which in principle can boost the performance of the pressure sensing devices. The current-pressure response of the sensors is evaluated using a platform including a Keithley-2400 and a computer-controlled stepping motor, as shown in Figure 3a. Figure 3b plots the representative response curves of the pressure sensors with planar Au sample and microstructured SnSe2/Au sample. We define the pressure sensors’ sensitivity as S = ((ΔI/I0)/ΔP), where ΔI is the change in current (IP-I0), IP is the current under applied pressure, I0 is the current under the minimum load and ΔP is the change in applied pressure. The sensitivity S is 11.52 (0.07) kPa-1 and 433.22 (2.91)

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kPa-1 in the low (high)-pressure range for sensors with PDMS/Au/Au (PAA) and PAAS configurations. The low and high pressure region for PAAS configuration is 0-2.4 kPa and 2.4-38.4 kPa, respectively. Compared with PAA configuration, the PAAS configuration provides the higher sensitivity and wider operating pressure range. This large increase in the sensitivity of the PAAS configuration over the PAA configuration can be attributed to the introduction of height variations for the bottom SnSe2/Au interdigital electrodes (as shown in Figure 3c and Figure 3d). The peak-to-peak roughness of the SnSe2 film is 515 nm, which is much higher than the value (4.5 nm) of the Au film (Figure 3d). As a result, in the PAA configuration, the current change is ascribed to the deformation of the micropatterned PDMS during the mechanical loading process, resulting in the increment of the contact surface area of each individual micropatterned PDMS. However, in the PAAS configuration, the initial contact is formed by the highest microstructures on the top PDMS and the tips of the SnSe2 nanoplates, thus resulting in relatively high initial resistance. Under the external pressure, the highest PDMS microstructures begin to deform, while the other lower PDMS microstructures start to touch the SnSe2 nanoplates. Thus, the contact touchpoints significantly increase and in turn decreases the contact resistance of the sensor, finally contributing to the high sensitivity features. Generally speaking, the high sensitivity feature of our PAAS configuration in the low pressure range (0-2.4 kPa) is ascribed to the great variation of the number of contact points. In our PAAS configuration, the Rtotal can be simply expressed as Rtotal= Rsingle/n + Rfilm

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(1)

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where Rsingle is the resistance of a single contact point, n is the number of the contact points, Rfilm is thin film resistance between the contact points. In the initial state, there is only a small amount of contact points, thus resulting to the high initial resistance. Under loading, the rougher the bottom structure, the more the number of the formed contact points, hence, the greater the change of Rsingle/n (Rtotal). We believe that our work provides a new route to fabricate high sensitivity piezoresistive pressure sensors. It is worth noting that the PAAS sensors’ sensitivity is higher than the pressure sensor in our previous work33. This is because the previous sensing device (PDMS/Ag/Au/PI) is under further encapsulation (dropping mixed PDMS liquid around the back of PDMS, see Methods in our previous work33), that is, more touchpoints are formed in the initial state while in this work, the initial current (10 μA) is much lower than that of PDMS/Ag/Au/PI device. This is because the initial pressure is only originated from the PDMS weight. In this case, the effect of packaging process on the performance of the PAA and PAAS configuration is averted. Stability under cyclic deformations is a basic requirement for the pressure sensor. Therefore, we test the durability of the PAAS sensing devices. The results are shown in Figure 4a, no obvious current degeneration is found in the whole cyclic process (>4000 cycles), indicating that the pressure sensors are very reliable and robust.

Fig. 4b

exhibits the consecutive tests of the PAAS pressure sensor with various on/off pressure: 40 Pa, 160 Pa, 4000 Pa and 12000 Pa. The current increases rapidly after the pressure applied and recovers to initial level with removing the pressure. Also, the relative changes in resistance precisely follow the variation of the external pressures from 40

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Pa to 12000 Pa, showing the great stability and reproducibility of the PAAS pressure sensor under different pressures regardless of the scale of applied pressure. Figure 4c is the current-voltage (I-V) curves of the PAAS pressure sensor under different pressures from 30 Pa to 1200 Pa. All curves show good linearity in the voltage from -5 V to 5 V, implying its good ohmic behavior. Therefore, the sensitivity of the PAAS pressure sensor is independent with working voltage. In additional, the slope of the IV curve increases with increasing applied external pressure, showing that the resistance decrease under increasing external pressure. The PAAS pressure sensor also showed rapid response (~70 μs) and recovery time (~90 μs), as shown in Figure 4d. The fast response speed is superior to many reported literature results, as shown in Table S1. Benefiting from the ultrahigh sensitivity, the response of PAAS pressure sensor to a fragment of glutinous rice with a low pressure of 0.82 Pa is shown in Figure 4e. As far as we know, this detection limit is lower than most reported results (See Table S1 in the supporting information). Moreover, our PAAS pressure sensor possesses the ability to resolve the magnitude and spatial distribution of different complex objects. Figure 5a and 5b show the schematic illustration of the designed 3×3 and 4×4 pixel arrays of SnSe2/Au interdigital electrodes. Figures 5c-f and Figure S4 show the demonstrations with a right-angle-type hexagon wrench and different complex objects (screw, nut, prill, et. al.), respectively, indicating that the sensor array can perceive the load shape, load pressure and realize the differentiation of compressed local regions. In addition, our PAAS pressure sensors are capable of detecting physiological signals directly (e.g. wrist pulse). For demonstration, one pressure sensor was tightly attached

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onto a human wrist for testing the real-time monitoring of pulse beat (Fig. 5g). Figure 5h shows the measured pulse pressure waveform. The pulse signals are stable, and several important peaks (percussion, tidal and diastolic peaks) can be clearly detected.

CONCLUSIONS In summary, we have demonstrated a piezo-resistive pressure sensor based on hierarchical flower-shaped SnSe2 nanoplates. The fabricated PAAS pressure sensors exhibited high sensitivity, wide operating pressure range, low limit of detection, and a rapid response/recovery time. Moreover, the feasibility of fabricated PAAS pressure sensors in human wrist pulse detection has been demonstrated. In addition, large-area integration of these PAAS pressure sensors can be easily fabricated for mapping and identifying the spatial pressure distribution. These outstanding features make our pressure sensors suitable for many practical applications in soft robotics, smart devices and electronic skin applications.

METHODS Preparation of micropatterned PDMS/Au. In the first step, the microstructure template was prepared by photolithography on the glass (PI) substrate. In the second step, the microstructure template was filled with

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polydimethylsiloxane by the spin-coating, and then the microstructure was reproduced on the PDMS film. In the third step, in order to separate the microstructured PDMS from the substrate, the photoresist was dissolved in acetone solution. Finally, Cr/Au (1 nm/ 100 nm) was deposited on the PDMS microstructure by thermal evaporation. More detailed processes for preparations of micropatterned PDMS can be seen from our previous work.33 Preparation of flower-shaped SnSe2 nanoplates/Au interdigital electrode. Coevaporation process is employed to deposit flower-shaped SnSe2 nanoplates. The co-evaporation time last for 20 min and the heating temperature for Sn, Se sources and the glass substrate were 1100 oC, 235 oC and 250 oC, respectively, followed by the deposition of Cr/Au (1 nm/100 nm) via thermal evaporation and laser scribing process. Characterization and Measurement. The morphology and the elements contents of SnSe2 nanoplates was characterized using SEM and EDS (ZEISS SUPPA55 equipped with Oxford EDS detector). Raman characterization of SnSe2 nanoplates were characterized using Witec CRM200 system. For XRD, Rigaku D/ Max-2500 diffractometer was used, the scanning rate is 5o min-1. Agilent 5500 was used to for the AFM-image. The current-time-pressure characteristics were determined using Keithley 2400, the pressure meter (DS2-5N, Dongguan Zhiqu Precision Instruments Co., Ltd.) and the oscilloscope (LUCK-3, digital storage oscilloscope, Chengdu Rongte instrument Co., Ltd) ASSOCIATED CONTENT

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ACKNOWLEDGMENTS This work was partially supported by the Shenzhen Basic Research Grant: JCYJ20150925163313898, JCYJ20170413153246713, JCYJ20180507182431967, the National Nature Science Foundation of China (11804354, 61574157, 61774164). The authors are also grateful for the support of the Singapore Ministry of Education Academic Research Fund Tier 2(MOE2015-T2-1-066 and MOE2015-T2-2-010), Singapore Ministry of Education Academic Research Fund Tier 1 (RG85/16), and Nanyang Technological University (Start-up grant M4081515: Lei Wei).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website Additional information regarding OM/AFM characterization of SnSe2 nanoplates on glass (glass/PI) substrate, micropatterned PDMS, summary of pressure sensors. (PDF)

AUTHOR INFORMATION Corresponding Authors * email: [email protected]. * email: [email protected].

Author Contributions W. Li, K. He and D. Zhang contributed equally to this work.

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(34) Cao, Y.; Li, T.; Gu, Y.; Luo, H.; Wang, S.; Zhang, T. Fingerprint Inspired Flexible Tactile Sensor for Accurately Discerning Surface Texture. Small 2018, 14, 1703902. (35) Su, B.; Gong, S.; Ma, Z.; Yap, L. W.; Cheng, W. Mimosa-inspired Design of a Flexible Pressure Sensor With Touch Sensitivity. Small 2015, 16, 1886. (36) Jia, J; Huang, G; Deng, J; Pan, K. Skin-inspired Flexible and High-sensitivity Pressure Sensors Based on rGO Films with Continuous-gradient Wrinkles. Nanoscale 2018, DOI: 10.1039/C8NR08503J. (37) Li, J.; Orrego, S.; Pan, J.; Pei, H.; Sung, K. Ultrasensitive, Flexible, and Low-cost Nanoporous Piezoresistive Composites for Tactile Pressure Sensing. Nanoscale 2019, 11, 2779-2786. (38) Li, X.; Li, Y.; Li, X.; Song, D.; Min, P.; Hu, C.; Zhang, H.; Koratkar, N.; Yu, Z.; Highly sensitive, Reliable and Flexible Piezoresistive Pressure Sensors Featuring Polyurethane Sponge Coated with MXene Sheets. J. Colloid Interface Sci. 2019, 542, 54-62. (39) Chen, M.; Li, Z.; Li, W.; Shan, C.; Li, W.; Li, K.; Gu, G.; Feng, Y.; Zhong, G.; Wei, L.; Yang, C. Large-scale Synthesis of Single-crystalline Self-standing SnSe2 Nanoplate Arrays for Wearable Gas Sensors. Nanotechnology 2018, 29, 455501. (40) Chung, K. M.; Warmwangi, D.; Woda, M.; Wutting, M.; Bensch, W. Investigation of SnSe, SnSe2, and Sn2Se3 Alloys for Phase Change Memory Application. J. App. Phys., 2008, 103, 083523.

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(41) Zhou, X.; Gan, L.; Tian, W.; Zhang, Q.; Jin, S.; Li, H.; Bando, Y.; Golberg, D.; Zhai, T. Ultrathin SnSe2 Flakes Grown by Chemical Vapour Deposition for High Performance Photodetectors. Adv. Mater. 2015, 27, 8035-8041.

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Figure 1 PDMS/Au/Au/SnSe2 pressure sensors. (a) Schematic illustration of the fabrication process of the PDMS/Au/Au/SnSe2 pressure sensors. (b-d) Top-view SEM images of the flower-shaped SnSe2 nanoplates. (e-g) Top and cross-sectional-view SEM images and height profile of the micropatterned PDMS. (h-j) Optical micrograph of the SnSe2/Au interdigital electrodes. 20



Figure 2 Spectroscopic characteristics of SnSe2 nanoplates on glass substrate. (ab) XRD pattern and Raman spectra of SnSe2 nanoplates on glass substrate. (c-d) SEM image of the obtained SnSe2 nanoplates and the corresponding Sn and Se elemental mapping. The inset of d is the EDS spectrum of the SnSe2 nanoplates. The elemental ratio Sn: Se is 32.18: 67.82, which is close to 1:2.

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Figure 3 Pressure-sensing properties of the PDMS/Au/Au/SnSe2 pressure sensors. (a) Schematic illustration of the experimental setup. (b) The comparison of pressure sensitivities of different sensor structures: PDMS/Au/Au (red), PDMS/Au/Au/SnSe2 (blue). (c) Working mechanism of the PDMS/Au/Au (“surface contact” to “surface saturated”) and PDMS/Au/Au/SnSe2 (“point contact” to “point saturated” to “surface contact” to “surface saturated”) pressure sensors: schematic illustrations of conduction path models under different loading state. (d) AFM images of the Au film (top) and SnSe2 film (down), the peak-to-peak roughness are 4.5 nm and 515 nm for Au film and SnSe2 film, respectively.

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Figure 4 Pressure-sensing properties of the PDMS/Au/Au/SnSe2 pressure sensors. (a) Cycle testing of the PDMS/Au/Au/SnSe2 pressure sensors at 25 Pa. (b) Timeresolved current change response of the PDMS/Au/Au/SnSe2 pressure sensor under repeated pressures ranging from 40 Pa to 12 kPa. (c) I-V curves of the PDMS/Au/Au/SnSe2 pressure sensor with different applied pressures. (d) Response/ recovery time of the PDMS/Au/Au/SnSe2 pressure sensor under the pressure of 20 Pa. (e) Response of the PDMS/Au/Au/SnSe2 pressure sensor to a fragment of glutinous rice.

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Figure 5 PDMS/Au/Au/SnSe2 pressure sensor applications for the detection of both the intensity and distribution of the mechanical load and human physiology monitoring. (a-c-e) Current mapping for the pressure of a right-angle-type hexagon wrench by 3 × 3 PDMS/Au/Au/SnSe2 pressure sensor arrays, the whole sample dimension (glass/SnSe2) is 10 cm×10 cm. (b-d-f) Current mapping for the pressure of some complex objects (screw, nut, prill, et. al.) by 4×4 PDMS/Au/Au/SnSe2 pressure sensor arrays, the whole sample dimension is 5 cm×5 cm (glass/SnSe2). (g-h), Realtime monitoring of pulse waves. “P”, “T” and “D” represents P-wave (percussion), Twave (tidal) and D-wave (diastolic) peaks, respectively. 24



Graphic for manuscript

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Flexible and High Performance Piezoresistive Pressure Sensors

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