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All rGO-on-PVDF-nanofibers based self-powered electronic skins
Author’s Accepted Manuscript All rGO-on-PVDF-nanofibers based self-powered electronic skins Yuanfei Ai, Zheng Lou, Shuai Chen, Di Chen, Zhiming M. Wang, Kai Jiang, Guozhen Shen www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(17)30179-9 http://dx.doi.org/10.1016/j.nanoen.2017.03.039 NANOEN1867
To appear in: Nano Energy Received date: 19 February 2017 Revised date: 17 March 2017 Accepted date: 21 March 2017 Cite this article as: Yuanfei Ai, Zheng Lou, Shuai Chen, Di Chen, Zhiming M. Wang, Kai Jiang and Guozhen Shen, All rGO-on-PVDF-nanofibers based selfpowered electronic skins, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.03.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
All rGO-on-PVDF-nanofibers based self-powered electronic skins Yuanfei Aia,b, Zheng Lou a,*, Shuai Chen a,c, Di Chenc, Zhiming M. Wangb, Kai Jiangd,*, Guozhen Shena,e,* a
State Key Laboratory of Superlattices and Microstructures, Institution of Semiconductors,
Chinese Academy of Science, Beijing, 100083, China b
Institute of Fundamental and Frontier Sciences, University of Electronic Science and
Technology of China, Chengdu, 610054, China c
School of Mathematics and Physics, University of Science and Technology Beijing, Beijing
100083, China d
Institute & Hospital of Hepatobiliary Surgery, Key Laboratory of Digital Hepatobiliary
Surgery of Chinese PLA, Chinese PLA Medical School, Chinese PLA General Hospital, Beijing 100853, China e
College of Materials Science and Opto-electronic Technology, University of Chinese Academy
of Sciences, Beijing 100029, China
[email protected] (Z. Lou)
[email protected] (K. Jiang)
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[email protected] (G. Shen).
*
Corresponding authors.
ABSTRACT
With the rapid popularization of electronic products and portable devices, multifunctional integrated devices which can integrate many functions in one device and miniaturize their volume, are constantly evolving in recent years. Herein, four kinds of planar devices namely micro-supercapacitors, pressure sensor, photodetector and gas sensor were modularly manufactured, all with the reduced graphene oxide (rGO) encapsulated poly(vinylidene fluoridetrifluoroethylene) [P(VDF-TrFE)] (PVDF) nanofibers (NFs) as the functional materials. They were integrated into a single pixel to form a self-powered multifunctional electronic skin system, where the micro-supercapacitors could drive the three sensors to detect the change of the environment conditions and physiological signs of health, like the functions of human skins and sense organs. The technology is quite simple and efficient, which can in principle be scaled-up to fabricate more compact and higher performance e-skins for applications in wearable electronics or bionics field.
KEYWORDS: electronic skins; graphene; sensor; microsupercapacitor; photodetector; wearable electronics
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1. Introduction As bionics science and robotics science developing, electronic skins, which can mimic human skins and organs to sense physical environment, monitor human activity and personal healthy, are drawing extensive attentions and boosting rapidly in recent years [1-10]. As is well-known, the human skin is a complicated large-scale multifunctional integrated system, that can realize many functions together such as sensing the touching, pinching, stretching and temperature, creating signals by self and transmitting signals, protecting the body. To mimic the comprehensive properties of human skins, the artificial electronic skins (E-skins) are required to integrate diverse sensing modules that can simultaneously differentiate among various physical stimuli including strain, twist, temperature, light, humidity and the environmental gases [11-21]. Besides, the power units are also required to be integrated into the multifunctional E-skins to form into self-powered systems, which are especially favorable for next-generation multifunctional e-skins [22-29]. Previous studies of multifunctional E-skins or self-powered E-skins have mainly focused on developing the desired E-skin systems by integrating various sensors made of different sensing materials. For example, Javey et al have reported a flexible skin-conforming platform which merged five different sensors to measure the sweat metabolites, electrolytes and skin temperature, using glucose and lactate oxidase, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), carbon nanotubes and Cr/Au metal microwires as the sensing materials, respectively [30]. Gao and his group presented a flexible self-powered system containing a micro-supercapacitor (MSC) with the MnO2-PPy and V2O5-PANI composites as positive and
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negative electrodes, respevtively, and a photodetector with the perovskite nanowires as the photo sensing materials [31]. However, in some cases, practical E-skins require low-cost and facile fabrication processes by developing rational device architecture designs using simple sensing materials [23]. Reduced graphene oxide (rGO) is a graphene derivative, which can be mass produced using simple solution process by exfoliating from graphite. The surface groups on rGO make it very sensitive to environmental conditions, such as pressure, light, chemicals, temperatures etc. rGO is also an excellent candidate for energy storage applications [32-42]. With these attractive features, all rGO based multifunctional self-powered E-skins may be fabricated for applications to robotics, health monitoring, and medical implant services. In this work, by the adoption of rGO-on-PVDF-nanofibers as both the sensing and energy storage materials, we developed a multifunctional self-powered E-skin, where three types of sensors (pressure sensor, photodetector and gas sensor) and three on-chip micro-supercapacitors (MSCs) in parallel are integrated into a single pixel. In this integrated system, the MSCs could get recharged through the I/O pins to storage energy beforehand, and then provide a relatively stable and durable driving voltage (0.4 V) to power the other sensors to detect the environmental conditions and physiological signs of health. The multifunctional self-powered devices developed here not only avoided the use of different materials separately, but also enabled sensors and power unit integration with a very simple and efficient way. 2. Experimental section The structure and fabrication of the mulctifunctional self-powered E-skins were depicted in Figure 1. rGO-on-PVDF-NFs used in this work were prepared according to our previous work [42]. Briefly, as shown in Figure 1a, the PVDF nanofibers were fabricated by the electrospinning
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technology, which were then suspended in a mixed aqueous solution with graphene oxide suspension. Hydrazine (35 wt%) was then dropped into the mixed solution to reduce the GO to graphene. Finally, the graphene-encapsulated PVDF NFs were obtained. 500 μm thick PDMS substrate was fabricated by stirring the mixture with base and cross-linker at 30 min, followed with spin-coating onto the silicon substrate with solidifying for 2 h and peeling off from the silicon substrate. To fabricate the planar MSCs, two pieces of rGO-on-PVDF-NFs films transferred on PDMS substrates were used as the electrodes, one piece of cellulose paper acted as the separator and PVA–KOH gel were prepared as the electrolyte according to our previous work [43,44]. The rGO-on-PVDF-NFs based pressure sensor, photodetector and gas sensor are of similar device structure and are prepared by depositing 80 nm Ni films on two ends of the nanofiber films as the electrodes. The distance between adjacent electrodes was set to be 0.5cm for the pressure sensor, 50 μm for the photodetector and 0.5 cm for the gas sensor, respectively. All the above four types of devices are integrated onto a single PDMS substrate and fabricated in sequence. Thermally evaporated Ni and Ag tapes electrodes were used as electrical interconnections between the devices and the circuit diagram was depicted in Figure S2. The supercapacitors unit could get recharged by external power source through I/O pins. The whole system was packed with another thin layer of PDMS with only the sensing materials exposed to air (Figure S1). The electrochemical and electronic measurements were performed in air. The electrochemical performance were measured with an electrochemical workstation (Chenhua, CHI 760E). The force gauge (Shandu SH-500B) was used to apply the static pressure. The photoresponse and the output electric signals were collected and analyzed by the Keithley 4200 semiconductor characterization system and electrochemical workstation (Chenhua, CHI 760E), respectively. 5
Figure 1. (a) Schematic illustration of the synthesis of rGO-on-PVDF-NFs; (b) E-skin integrated with multisensors and micro-supercapacitors with the as-synthesized rGO-on-PVDF-NFs as the functional materials. 3. Results and discussion The electrochemical performance of the MSCs in a single E-skin pixel was characterized. The electrochemical properties of the rGO-on-PVDF-NFs were first measured in a three-electrode electrochemical cell with 2 M KOH electrolyte and the results were shown in Figure S3. From the results, the specific capacitances of the hybrid film was calculated to be 595.4 F/g at the current densities of 5 A/g. All-solid-state MSCs were then fabricated with the rGO-on-PVDFNFs as the electrodes and Figure 2a shows the device structure of a single MSC, where two pieces of rGO-on-PVDF-NFs films were separated by a piece of cellulose paper. Figure 2b 6
showed the CV curves of a single MSC measured at a potential window of 0~1 V. From the curves, we can see that, as the scan rate increased from 10 to 1000 mV/s, closed areas of the CV curves were augmentative while their shapes remained quasi-rectangular. Figure 2c presented the galvanostatic charging-discharging curves of the MSC in the voltage window of 0~1 V. The capacitances evaluated from the curves were 13.46, 10.69, 9.32, 6.80, 4.98, 3.86 and 2.77 mF, corresponding to the current of 0.5, 0.8, 1.0, 2.0, 3.0, 4.0 and 5.0 mA, respectively, as shown in Figure 2d. The charging-discharging cycle stability was also investigated. Figure 2e shows its capacitances stability at the current of 3 mA for 3000 cycles. The initial specific capacitance was 9.91 mF and it remained 9.62 mF (97.1% of the initial capacitance) after 3000 cycles, demonstrating its good cycling performance. The volume energy density and power density of our MSC were calculated and shown in Figure 2f. The device exhibited an energy density of 0.071 mWh/cm3 at a power density of 0.5 mW/ cm3. Specially, even at a high power density of 5.03 mW/ cm3, the device still had an attractive energy density of 0.005 mWh/ cm3.
Figure 2. Micro-supercapacitors with the rGO-on-PVDF-NFs as the electrodes: (a) Device structures; (b) CV curves of the MSCs; (c) galvanostatic CD curves of the MSCs; (d) capacitances of the MSCs at varied galvanostatic CD current densities; (e) cycling performance of the MSCs at the current density of 3 mA; (f) energy and powder densities of the MSCs.
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To be used as the power unit for our self-powered E-skin system, four MSCs in parallel were designed and fabricated on the E-skin matrix as shown in Figure 3a. Figures 3b and 3c compared the CV and galvanostatic CD curves of a single MSC and four MSCs in parallel at the scan rate of 100 mV/s and the current of 5 mA, respectively. The results indicated the superior potential of the MSCs to be patterned and integrated as series combinations to extend the capacity and working current for practical applications. The flexibility and stability of the MSCs on the E-skin matrix were then studied by bending the devices under different states, as demonstrated in Figure 3d. The capacitance exhibited no attenuation during 400 cycles, indicating its excellent mechanical and electrochemical stability. We also attached the MSCs on E-skin matrix to human body to check its flexibility. As shown in Figure 3e, repeated bending-restoring the device did give obvious capacitance degradation, further confirming its excellent mechanical stability.
Figure 3. Four MSCs in parallel on the multifunctional E-skins: (a) Device position on the substrate; (b) CV and (c) galvanostatic CD curves of one MSC and four MSCs in parallel at the scan rate of 100 mV/s and the current of 5 mA, respectively; (d) Flexibility performance of the four MSCs in parallel; (e) the capacitance stability of the MSCs during repeatedly bendingrestoring cycles at galvanostatic current of 10 mA.
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The excellent electrochemical performance and mechanical stability and flexibility of the MSCs make them good candidates to be used as driven power for sensors in the E-skin system. Figure S4 shows the I-T curves of the three types of sensors when driven by the MSCs in a static state. The static current of each sensor could keep in a relatively stable range, thanks to the high capacitance of supercapacitors and high resistance of the sensors. The E-skin matrix with pressure sensor driven by the integrated MSCs was then attached to different parts of a human body to detect the wrist pulse, neck pulse, voice and swallowing. Figure 4a shows the device attached to hand. When a certain pressure was applied to the pressure sensor, the little increase in pressure caused the increase of conductivity because of the increase in the contacting sites of elastic rGO-on-PVDF-NFs [42]. From Figure 4b, we can see that when the applied pressure increased/decreased every 5 kPa, the current of the sensor driven by the MSCs increased/decreased about 0.13 μA. With a very high sensitivity, the rGO-on-PVDF-NFs based pressure can even be used to detect very tiny things, like mung bean as demonstrated in Figure 4c. And the device exhibits a fast response and recovery time of 0.4 s and 0.3 s, respectively. Due to the excellent performance of the pressure sensors, the as-fabricated E-skin can monitor the human physiological signals. Here, the flexible integrated self-powered system is wearable and can monitor the pulse in real time (Figure 4d-f). According to the measurement results of the volunteer (male, 26 years old, 176 cm height, 62 kg weight), a typical radial artery pulse waveform with two clearly distinguishable peaks in one cycle pulse and 75 beats per minute has been shown in Figure 4e. From Figure 4e, we can easily derive two parameters that are most commonly used for health monitoring, namely the radial augmentation index AIr=P 2/P1 [42], which is calculated to be about 0.71. It is a characteristic value for a healthy adult male [45].
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Moreover, our sensor was able to resolve small features in the diastolic tail of the pulse pressure wave that the majority of the sensors used in arterial tonometry do not detect, and hence could potentially be used for more accurate diagnostics. Figure 4f and Figure S5a show that the heart rates of neck pulse and the chest were also 75 beats per minute which means that the E-skin can accurately monitor the human heart rate which can provide many useful information for the early diagnosis of cardiovascular diseases. In addition to the heart rate, the monitoring of respiratory rate is also important as an early warning system for sudden infant death syndrome. As shown in Figure S5b is the potential use of our E-skin to monitor human breathing with wearable device attached to the abdomen. The result shows that periodic respiration produces a reliable and stable resistance change.
Figure 4. Pressure sensor on the multifunctional E-skins: (a) Device attached on hand; (b) The output current signal based on the successive and additive pressure; (c) Transient response to the loading and removal of a mung bean (35 mg); (d) The integrated film device attached to wrist and neck; (e) Measurement of the device on wrist; (f) Measurement of the device on neck; (g) The integrated film device attached on throat (h) Responsive curves when wearer spoke “CAS”; (i) Responsive curves when swallowing.
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E-skin attached onto the subject’s throat can also monitor pressure difference of the muscle movement during speech (Figure 4g). As shown in Figure 4h and Figure S6, each of the repeatedly pronouncing of the word of “CAS”, “Nihao” and “OK” has its proper current-time curves and distinct patterns, which mainly due to the complex muscle movements during speech. Such an accurately specific phonation recognition provides a great potential application value in speech rehabilitation training. Figure 4d shows the reproducible sensing of the swallowing with the similar I-t curves over two repeated measurements. These results indicated that the selfpowered E-skin can provide a facile and efficient method for monitoring multiple human physiological parameters in real-time.
Figure 5. Photodetector on the multifunctional E-skins: (a) Sensing mechanism of the photodetector powered by integrated MSCs; (b) Light detection of the device with exposure of 20, 40 and 60 mW/cm2 white light; (c) The current on/off ratios of the device upon different wavelength light; (d) Sensing of the device under different bending state exposted to 20 mW/cm2 white light.
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As is well known, upon the absorption of light, the electron–hole pairs generated in graphene in the thin channel would normally recombine in tens of picoseconds, depending on the quality and carrier concentration of the graphene [23,46-48]. A rGO-on-PVDF-NFs based photodetector was then integrated into the E-skin matrix to detect white light. Figure 5a shows the device structure and sensing mechanism of the photodetector driven by the integrated MSCs. The photoresponse switching behavior of the photodetector to white lights with different light intensities is illustrated in Figure 5b. The response times defined as the peak value from 10% to 90% of the photocurrent were about 30 s under different intensities. It is also shown that the photocurrent upon turning on/off the light irradiation during eight cycles has no obvious change, indicating the excellent reproducibility of the photodetector and good voltage stabilization of the MSC. Figure 5c is the responsivity of the device upon different wavelength light with 1 μW/cm 2 intensity. The responsivity can be defined as R=ΔI/I0, where ΔI is the difference between the photocurrent and dark current, I0 is the dark current. It has been confirmed that as the wavelength decreased, the responsivity to light illumination increases bit by bit. When 233 nm UV light illuminated the wearable device, the responsivity reaches the highest value. The sensing stability under different bending states is an important factor to measure the quality of a wearable E-skin. As shown in Figure 5d, the photoresponse of the photodetector driven by the MSC under different curvatures have been measured. The result shows that the photocurrent under different curvatures exhibits a great stable without a clear change, revealing good electrical stability and mechanical flexibility of the E-skin. And it can also be confirmed that the MSC can provide a stable working voltage for the wearable device, which indicated the stability and feasibility of the self-powered E-skin system.
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Figure 6. Gas sensor on the multifunctional E-skins: (a) Responses of the MSCs-powered gas sensor to acetone with different concentrations (100 ppm, 200 ppm, 500 ppm and 1000 ppm); (b) The responsivity of the sensor under different acetone concentrations; (c) The response and recovery time of the sensor; (d) Sensing stability of the device under different bending state in 500 ppm acetone. Our rGO-on-PVDF-NFs could also be used as sensing materials to successfully detect the volatile organic compounds (VOCs) at room temperature using the energy supplied by integrated MSC array. The schematically structure of the rGO-on-PVDF-NFs based gas sensor was shown in the middle of Figure 6. Briefly, the rGO-on-PVDF-NFs were coated on the PDMS films with two electrodes at the end of each side. Figure 6a shows the change of conductance in sensing layer by introducing various concentration acetone gases, which make rGO-on-PVDF-NFs based gas sensor achieve a high response detection of 0.12-0.31 for 100-1000 ppm acetone. The responsivity was defined as S=ΔI/I0, where ΔI is the difference current between in the air and in
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the target gas, I0 is the current in the air. Figure 6b displays the measurement response as a function of the target acetone gas concentration. We can see clearly that the response increase quickly with an increase in acetone concentration. Response and recovery speed is also a significant parameter which is very useful for real sensing application. As shown in Figure 6c, the rGO-on-PVDF-NFs based gas sensor displays a rapid response and recovery times of 5.5 s and 30 s, respectively, at room temperature. The definition of the response and recovery time was a period of time taken by the 90% current change. The rGO-on-PVDF-NFs based gas sensor also demonstrated good performance for other toxic organic gases such as toluene and formaldehyde (Figure S7). Figure 6d displays the acetone sensing while the entire self-powered E-skin system is under different bending curvatures. The current change due to the acetone gas remains the same response of 0.25 under different conditions, indicating the excellent mechanical stability of the integrated E-skin. 4. Conclusions In conclusion, we demonstrate a flexible self-powered multifunctional E-skin integrated with three types of sensors to monitor environmental signals and four MSCs to power the entire system. Novelty, all of the sensors and MSCs were made from the same rGO-on-PVDF-NFs. The rGO-on-PVDF-NFs based MSCs could get repeatedly recharged and drive the multisensors with the bias voltage of about 0.4 V for a long time. The fabricated integrated system could monitor biosignals by wearing the human body, such as a neck pulse, saliva swallowing, voice, and body movement by the pressure sensing part, detect the brightness of light by the photodetector and the concentration of VOCs by the gas sensing part. More importantly, the entire E-skin system exhibited a great mechanical flexibility under different bending curvatures.
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Our work clearly demonstrates the great potential of the as-designed integrated system for application to next-generation wearable electronics and artificial intelligence applications. Acknowledgements This work was supported by the National Science Fund for Distinguished Young Scholars (61625404), the National Natural Science Foundation of China (61504136, 51672308), Beijing Natural Science Foundation (4162062) and the Key Research Program of Frontier Sciences, CAS (QYZDY-SSW-JWC004). Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/xxxx.
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Yuanfei Ai is currently a Ph.D. candidate at University of Electronic Science and Technology of China (UESTC). His current scientific interests focus on the design and synthesis of three dimensional nanostructures, and investigation of their fundamental properties and potential applications in flexible energy storage devices and integrated devices.
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Zheng Lou received his Ph.D. degree from Jilin University in 2014. He joined the Institute of Semiconductors, Chinese Academy of Sciences as an Assistant Professor in 2014. His current research focuses on flexible electronics, including pressure sensors, electronic-skin, transistors and photo-detectors.
Shuai Chen received his M.S. degree from Qingdao University in 2015. He is a Ph.D. candidate at School of Mathematics and Physics, University of Science and Technology Beijing from 2015. His research interest focuses on flexible sensors.
Di Chen is a professor at University of Science and Technology Beijing. She received her PhD degree from the University of Science and Technology of China in 2005. Her current research interest is the advanced technology for designing nanostructure for sustainable energy applications, including energy storage and photocatalysts.
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Zhiming M. Wang received his B. S. degree (1992) in physics from Qingdao University, B. S. degree (1995) in physics from Peking University and Ph. D degree (1998) in Physics from Institute of Semiconductors, Chinese Academy of Sciences. He joined University of Electronic Science and Technology of China (UESTC) as “Professor of National 1000-Talents Program” in 2011. His current research focused on compound semiconductors, epitaxial crystal growth, molecular beam epitaxy (MBE) and quantum dots technology.
Kai Jiang received his MB/BS degree from Second Military Medical College, Shanghai, China in 1991, and MD/PhD degrees from Chinese PLA Postgraduate Medical College, Beijing, China in 1998. He further studied in the Queen Mary Hospital of University of Hong Kong in 2002, and Universit at de Barcelona, Span in 2008. He has been with the Department of Hepatobiliary Surgery, Chinese PLA General Hospital since 1991, where he is currently a Professor of surgery and vice Dean of the Department. His current research interests focus on surgical operation and applications of nanotechnology in clinical medicine.
Guozhen Shen received his B. S. degree (1999) in Chemistry from Anhui Normal University and Ph. D degree (2003) in Chemistry from University of Science and technology of China. He joined the Institute of Semiconductors, Chinese Academy of Sciences as a Professor in 2013. His 19
current research focused on flexible electronics and printable electronics, including transistors, photodetectors, sensors and flexible energy storage and conversion devices.
Highlights
A multifunctional self-powered E-skin was fabricated. rGO-on-PVDF-nanofibers act as “4-in-1” material for the E-skin. Micro-supercapacitors, pressure sensors, photodetectors and gas sensors are integrated into a single pixel.
Graphical Abstract
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PII: DOI: Reference:
S2211-2855(17)30179-9 http://dx.doi.org/10.1016/j.nanoen.2017.03.039 NANOEN1867
To appear in: Nano Energy Received date: 19 February 2017 Revised date: 17 March 2017 Accepted date: 21 March 2017 Cite this article as: Yuanfei Ai, Zheng Lou, Shuai Chen, Di Chen, Zhiming M. Wang, Kai Jiang and Guozhen Shen, All rGO-on-PVDF-nanofibers based selfpowered electronic skins, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.03.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
All rGO-on-PVDF-nanofibers based self-powered electronic skins Yuanfei Aia,b, Zheng Lou a,*, Shuai Chen a,c, Di Chenc, Zhiming M. Wangb, Kai Jiangd,*, Guozhen Shena,e,* a
State Key Laboratory of Superlattices and Microstructures, Institution of Semiconductors,
Chinese Academy of Science, Beijing, 100083, China b
Institute of Fundamental and Frontier Sciences, University of Electronic Science and
Technology of China, Chengdu, 610054, China c
School of Mathematics and Physics, University of Science and Technology Beijing, Beijing
100083, China d
Institute & Hospital of Hepatobiliary Surgery, Key Laboratory of Digital Hepatobiliary
Surgery of Chinese PLA, Chinese PLA Medical School, Chinese PLA General Hospital, Beijing 100853, China e
College of Materials Science and Opto-electronic Technology, University of Chinese Academy
of Sciences, Beijing 100029, China
[email protected] (Z. Lou)
[email protected] (K. Jiang)
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[email protected] (G. Shen).
*
Corresponding authors.
ABSTRACT
With the rapid popularization of electronic products and portable devices, multifunctional integrated devices which can integrate many functions in one device and miniaturize their volume, are constantly evolving in recent years. Herein, four kinds of planar devices namely micro-supercapacitors, pressure sensor, photodetector and gas sensor were modularly manufactured, all with the reduced graphene oxide (rGO) encapsulated poly(vinylidene fluoridetrifluoroethylene) [P(VDF-TrFE)] (PVDF) nanofibers (NFs) as the functional materials. They were integrated into a single pixel to form a self-powered multifunctional electronic skin system, where the micro-supercapacitors could drive the three sensors to detect the change of the environment conditions and physiological signs of health, like the functions of human skins and sense organs. The technology is quite simple and efficient, which can in principle be scaled-up to fabricate more compact and higher performance e-skins for applications in wearable electronics or bionics field.
KEYWORDS: electronic skins; graphene; sensor; microsupercapacitor; photodetector; wearable electronics
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1. Introduction As bionics science and robotics science developing, electronic skins, which can mimic human skins and organs to sense physical environment, monitor human activity and personal healthy, are drawing extensive attentions and boosting rapidly in recent years [1-10]. As is well-known, the human skin is a complicated large-scale multifunctional integrated system, that can realize many functions together such as sensing the touching, pinching, stretching and temperature, creating signals by self and transmitting signals, protecting the body. To mimic the comprehensive properties of human skins, the artificial electronic skins (E-skins) are required to integrate diverse sensing modules that can simultaneously differentiate among various physical stimuli including strain, twist, temperature, light, humidity and the environmental gases [11-21]. Besides, the power units are also required to be integrated into the multifunctional E-skins to form into self-powered systems, which are especially favorable for next-generation multifunctional e-skins [22-29]. Previous studies of multifunctional E-skins or self-powered E-skins have mainly focused on developing the desired E-skin systems by integrating various sensors made of different sensing materials. For example, Javey et al have reported a flexible skin-conforming platform which merged five different sensors to measure the sweat metabolites, electrolytes and skin temperature, using glucose and lactate oxidase, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), carbon nanotubes and Cr/Au metal microwires as the sensing materials, respectively [30]. Gao and his group presented a flexible self-powered system containing a micro-supercapacitor (MSC) with the MnO2-PPy and V2O5-PANI composites as positive and
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negative electrodes, respevtively, and a photodetector with the perovskite nanowires as the photo sensing materials [31]. However, in some cases, practical E-skins require low-cost and facile fabrication processes by developing rational device architecture designs using simple sensing materials [23]. Reduced graphene oxide (rGO) is a graphene derivative, which can be mass produced using simple solution process by exfoliating from graphite. The surface groups on rGO make it very sensitive to environmental conditions, such as pressure, light, chemicals, temperatures etc. rGO is also an excellent candidate for energy storage applications [32-42]. With these attractive features, all rGO based multifunctional self-powered E-skins may be fabricated for applications to robotics, health monitoring, and medical implant services. In this work, by the adoption of rGO-on-PVDF-nanofibers as both the sensing and energy storage materials, we developed a multifunctional self-powered E-skin, where three types of sensors (pressure sensor, photodetector and gas sensor) and three on-chip micro-supercapacitors (MSCs) in parallel are integrated into a single pixel. In this integrated system, the MSCs could get recharged through the I/O pins to storage energy beforehand, and then provide a relatively stable and durable driving voltage (0.4 V) to power the other sensors to detect the environmental conditions and physiological signs of health. The multifunctional self-powered devices developed here not only avoided the use of different materials separately, but also enabled sensors and power unit integration with a very simple and efficient way. 2. Experimental section The structure and fabrication of the mulctifunctional self-powered E-skins were depicted in Figure 1. rGO-on-PVDF-NFs used in this work were prepared according to our previous work [42]. Briefly, as shown in Figure 1a, the PVDF nanofibers were fabricated by the electrospinning
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technology, which were then suspended in a mixed aqueous solution with graphene oxide suspension. Hydrazine (35 wt%) was then dropped into the mixed solution to reduce the GO to graphene. Finally, the graphene-encapsulated PVDF NFs were obtained. 500 μm thick PDMS substrate was fabricated by stirring the mixture with base and cross-linker at 30 min, followed with spin-coating onto the silicon substrate with solidifying for 2 h and peeling off from the silicon substrate. To fabricate the planar MSCs, two pieces of rGO-on-PVDF-NFs films transferred on PDMS substrates were used as the electrodes, one piece of cellulose paper acted as the separator and PVA–KOH gel were prepared as the electrolyte according to our previous work [43,44]. The rGO-on-PVDF-NFs based pressure sensor, photodetector and gas sensor are of similar device structure and are prepared by depositing 80 nm Ni films on two ends of the nanofiber films as the electrodes. The distance between adjacent electrodes was set to be 0.5cm for the pressure sensor, 50 μm for the photodetector and 0.5 cm for the gas sensor, respectively. All the above four types of devices are integrated onto a single PDMS substrate and fabricated in sequence. Thermally evaporated Ni and Ag tapes electrodes were used as electrical interconnections between the devices and the circuit diagram was depicted in Figure S2. The supercapacitors unit could get recharged by external power source through I/O pins. The whole system was packed with another thin layer of PDMS with only the sensing materials exposed to air (Figure S1). The electrochemical and electronic measurements were performed in air. The electrochemical performance were measured with an electrochemical workstation (Chenhua, CHI 760E). The force gauge (Shandu SH-500B) was used to apply the static pressure. The photoresponse and the output electric signals were collected and analyzed by the Keithley 4200 semiconductor characterization system and electrochemical workstation (Chenhua, CHI 760E), respectively. 5
Figure 1. (a) Schematic illustration of the synthesis of rGO-on-PVDF-NFs; (b) E-skin integrated with multisensors and micro-supercapacitors with the as-synthesized rGO-on-PVDF-NFs as the functional materials. 3. Results and discussion The electrochemical performance of the MSCs in a single E-skin pixel was characterized. The electrochemical properties of the rGO-on-PVDF-NFs were first measured in a three-electrode electrochemical cell with 2 M KOH electrolyte and the results were shown in Figure S3. From the results, the specific capacitances of the hybrid film was calculated to be 595.4 F/g at the current densities of 5 A/g. All-solid-state MSCs were then fabricated with the rGO-on-PVDFNFs as the electrodes and Figure 2a shows the device structure of a single MSC, where two pieces of rGO-on-PVDF-NFs films were separated by a piece of cellulose paper. Figure 2b 6
showed the CV curves of a single MSC measured at a potential window of 0~1 V. From the curves, we can see that, as the scan rate increased from 10 to 1000 mV/s, closed areas of the CV curves were augmentative while their shapes remained quasi-rectangular. Figure 2c presented the galvanostatic charging-discharging curves of the MSC in the voltage window of 0~1 V. The capacitances evaluated from the curves were 13.46, 10.69, 9.32, 6.80, 4.98, 3.86 and 2.77 mF, corresponding to the current of 0.5, 0.8, 1.0, 2.0, 3.0, 4.0 and 5.0 mA, respectively, as shown in Figure 2d. The charging-discharging cycle stability was also investigated. Figure 2e shows its capacitances stability at the current of 3 mA for 3000 cycles. The initial specific capacitance was 9.91 mF and it remained 9.62 mF (97.1% of the initial capacitance) after 3000 cycles, demonstrating its good cycling performance. The volume energy density and power density of our MSC were calculated and shown in Figure 2f. The device exhibited an energy density of 0.071 mWh/cm3 at a power density of 0.5 mW/ cm3. Specially, even at a high power density of 5.03 mW/ cm3, the device still had an attractive energy density of 0.005 mWh/ cm3.
Figure 2. Micro-supercapacitors with the rGO-on-PVDF-NFs as the electrodes: (a) Device structures; (b) CV curves of the MSCs; (c) galvanostatic CD curves of the MSCs; (d) capacitances of the MSCs at varied galvanostatic CD current densities; (e) cycling performance of the MSCs at the current density of 3 mA; (f) energy and powder densities of the MSCs.
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To be used as the power unit for our self-powered E-skin system, four MSCs in parallel were designed and fabricated on the E-skin matrix as shown in Figure 3a. Figures 3b and 3c compared the CV and galvanostatic CD curves of a single MSC and four MSCs in parallel at the scan rate of 100 mV/s and the current of 5 mA, respectively. The results indicated the superior potential of the MSCs to be patterned and integrated as series combinations to extend the capacity and working current for practical applications. The flexibility and stability of the MSCs on the E-skin matrix were then studied by bending the devices under different states, as demonstrated in Figure 3d. The capacitance exhibited no attenuation during 400 cycles, indicating its excellent mechanical and electrochemical stability. We also attached the MSCs on E-skin matrix to human body to check its flexibility. As shown in Figure 3e, repeated bending-restoring the device did give obvious capacitance degradation, further confirming its excellent mechanical stability.
Figure 3. Four MSCs in parallel on the multifunctional E-skins: (a) Device position on the substrate; (b) CV and (c) galvanostatic CD curves of one MSC and four MSCs in parallel at the scan rate of 100 mV/s and the current of 5 mA, respectively; (d) Flexibility performance of the four MSCs in parallel; (e) the capacitance stability of the MSCs during repeatedly bendingrestoring cycles at galvanostatic current of 10 mA.
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The excellent electrochemical performance and mechanical stability and flexibility of the MSCs make them good candidates to be used as driven power for sensors in the E-skin system. Figure S4 shows the I-T curves of the three types of sensors when driven by the MSCs in a static state. The static current of each sensor could keep in a relatively stable range, thanks to the high capacitance of supercapacitors and high resistance of the sensors. The E-skin matrix with pressure sensor driven by the integrated MSCs was then attached to different parts of a human body to detect the wrist pulse, neck pulse, voice and swallowing. Figure 4a shows the device attached to hand. When a certain pressure was applied to the pressure sensor, the little increase in pressure caused the increase of conductivity because of the increase in the contacting sites of elastic rGO-on-PVDF-NFs [42]. From Figure 4b, we can see that when the applied pressure increased/decreased every 5 kPa, the current of the sensor driven by the MSCs increased/decreased about 0.13 μA. With a very high sensitivity, the rGO-on-PVDF-NFs based pressure can even be used to detect very tiny things, like mung bean as demonstrated in Figure 4c. And the device exhibits a fast response and recovery time of 0.4 s and 0.3 s, respectively. Due to the excellent performance of the pressure sensors, the as-fabricated E-skin can monitor the human physiological signals. Here, the flexible integrated self-powered system is wearable and can monitor the pulse in real time (Figure 4d-f). According to the measurement results of the volunteer (male, 26 years old, 176 cm height, 62 kg weight), a typical radial artery pulse waveform with two clearly distinguishable peaks in one cycle pulse and 75 beats per minute has been shown in Figure 4e. From Figure 4e, we can easily derive two parameters that are most commonly used for health monitoring, namely the radial augmentation index AIr=P 2/P1 [42], which is calculated to be about 0.71. It is a characteristic value for a healthy adult male [45].
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Moreover, our sensor was able to resolve small features in the diastolic tail of the pulse pressure wave that the majority of the sensors used in arterial tonometry do not detect, and hence could potentially be used for more accurate diagnostics. Figure 4f and Figure S5a show that the heart rates of neck pulse and the chest were also 75 beats per minute which means that the E-skin can accurately monitor the human heart rate which can provide many useful information for the early diagnosis of cardiovascular diseases. In addition to the heart rate, the monitoring of respiratory rate is also important as an early warning system for sudden infant death syndrome. As shown in Figure S5b is the potential use of our E-skin to monitor human breathing with wearable device attached to the abdomen. The result shows that periodic respiration produces a reliable and stable resistance change.
Figure 4. Pressure sensor on the multifunctional E-skins: (a) Device attached on hand; (b) The output current signal based on the successive and additive pressure; (c) Transient response to the loading and removal of a mung bean (35 mg); (d) The integrated film device attached to wrist and neck; (e) Measurement of the device on wrist; (f) Measurement of the device on neck; (g) The integrated film device attached on throat (h) Responsive curves when wearer spoke “CAS”; (i) Responsive curves when swallowing.
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E-skin attached onto the subject’s throat can also monitor pressure difference of the muscle movement during speech (Figure 4g). As shown in Figure 4h and Figure S6, each of the repeatedly pronouncing of the word of “CAS”, “Nihao” and “OK” has its proper current-time curves and distinct patterns, which mainly due to the complex muscle movements during speech. Such an accurately specific phonation recognition provides a great potential application value in speech rehabilitation training. Figure 4d shows the reproducible sensing of the swallowing with the similar I-t curves over two repeated measurements. These results indicated that the selfpowered E-skin can provide a facile and efficient method for monitoring multiple human physiological parameters in real-time.
Figure 5. Photodetector on the multifunctional E-skins: (a) Sensing mechanism of the photodetector powered by integrated MSCs; (b) Light detection of the device with exposure of 20, 40 and 60 mW/cm2 white light; (c) The current on/off ratios of the device upon different wavelength light; (d) Sensing of the device under different bending state exposted to 20 mW/cm2 white light.
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As is well known, upon the absorption of light, the electron–hole pairs generated in graphene in the thin channel would normally recombine in tens of picoseconds, depending on the quality and carrier concentration of the graphene [23,46-48]. A rGO-on-PVDF-NFs based photodetector was then integrated into the E-skin matrix to detect white light. Figure 5a shows the device structure and sensing mechanism of the photodetector driven by the integrated MSCs. The photoresponse switching behavior of the photodetector to white lights with different light intensities is illustrated in Figure 5b. The response times defined as the peak value from 10% to 90% of the photocurrent were about 30 s under different intensities. It is also shown that the photocurrent upon turning on/off the light irradiation during eight cycles has no obvious change, indicating the excellent reproducibility of the photodetector and good voltage stabilization of the MSC. Figure 5c is the responsivity of the device upon different wavelength light with 1 μW/cm 2 intensity. The responsivity can be defined as R=ΔI/I0, where ΔI is the difference between the photocurrent and dark current, I0 is the dark current. It has been confirmed that as the wavelength decreased, the responsivity to light illumination increases bit by bit. When 233 nm UV light illuminated the wearable device, the responsivity reaches the highest value. The sensing stability under different bending states is an important factor to measure the quality of a wearable E-skin. As shown in Figure 5d, the photoresponse of the photodetector driven by the MSC under different curvatures have been measured. The result shows that the photocurrent under different curvatures exhibits a great stable without a clear change, revealing good electrical stability and mechanical flexibility of the E-skin. And it can also be confirmed that the MSC can provide a stable working voltage for the wearable device, which indicated the stability and feasibility of the self-powered E-skin system.
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Figure 6. Gas sensor on the multifunctional E-skins: (a) Responses of the MSCs-powered gas sensor to acetone with different concentrations (100 ppm, 200 ppm, 500 ppm and 1000 ppm); (b) The responsivity of the sensor under different acetone concentrations; (c) The response and recovery time of the sensor; (d) Sensing stability of the device under different bending state in 500 ppm acetone. Our rGO-on-PVDF-NFs could also be used as sensing materials to successfully detect the volatile organic compounds (VOCs) at room temperature using the energy supplied by integrated MSC array. The schematically structure of the rGO-on-PVDF-NFs based gas sensor was shown in the middle of Figure 6. Briefly, the rGO-on-PVDF-NFs were coated on the PDMS films with two electrodes at the end of each side. Figure 6a shows the change of conductance in sensing layer by introducing various concentration acetone gases, which make rGO-on-PVDF-NFs based gas sensor achieve a high response detection of 0.12-0.31 for 100-1000 ppm acetone. The responsivity was defined as S=ΔI/I0, where ΔI is the difference current between in the air and in
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the target gas, I0 is the current in the air. Figure 6b displays the measurement response as a function of the target acetone gas concentration. We can see clearly that the response increase quickly with an increase in acetone concentration. Response and recovery speed is also a significant parameter which is very useful for real sensing application. As shown in Figure 6c, the rGO-on-PVDF-NFs based gas sensor displays a rapid response and recovery times of 5.5 s and 30 s, respectively, at room temperature. The definition of the response and recovery time was a period of time taken by the 90% current change. The rGO-on-PVDF-NFs based gas sensor also demonstrated good performance for other toxic organic gases such as toluene and formaldehyde (Figure S7). Figure 6d displays the acetone sensing while the entire self-powered E-skin system is under different bending curvatures. The current change due to the acetone gas remains the same response of 0.25 under different conditions, indicating the excellent mechanical stability of the integrated E-skin. 4. Conclusions In conclusion, we demonstrate a flexible self-powered multifunctional E-skin integrated with three types of sensors to monitor environmental signals and four MSCs to power the entire system. Novelty, all of the sensors and MSCs were made from the same rGO-on-PVDF-NFs. The rGO-on-PVDF-NFs based MSCs could get repeatedly recharged and drive the multisensors with the bias voltage of about 0.4 V for a long time. The fabricated integrated system could monitor biosignals by wearing the human body, such as a neck pulse, saliva swallowing, voice, and body movement by the pressure sensing part, detect the brightness of light by the photodetector and the concentration of VOCs by the gas sensing part. More importantly, the entire E-skin system exhibited a great mechanical flexibility under different bending curvatures.
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Our work clearly demonstrates the great potential of the as-designed integrated system for application to next-generation wearable electronics and artificial intelligence applications. Acknowledgements This work was supported by the National Science Fund for Distinguished Young Scholars (61625404), the National Natural Science Foundation of China (61504136, 51672308), Beijing Natural Science Foundation (4162062) and the Key Research Program of Frontier Sciences, CAS (QYZDY-SSW-JWC004). Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/xxxx.
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Yuanfei Ai is currently a Ph.D. candidate at University of Electronic Science and Technology of China (UESTC). His current scientific interests focus on the design and synthesis of three dimensional nanostructures, and investigation of their fundamental properties and potential applications in flexible energy storage devices and integrated devices.
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Zheng Lou received his Ph.D. degree from Jilin University in 2014. He joined the Institute of Semiconductors, Chinese Academy of Sciences as an Assistant Professor in 2014. His current research focuses on flexible electronics, including pressure sensors, electronic-skin, transistors and photo-detectors.
Shuai Chen received his M.S. degree from Qingdao University in 2015. He is a Ph.D. candidate at School of Mathematics and Physics, University of Science and Technology Beijing from 2015. His research interest focuses on flexible sensors.
Di Chen is a professor at University of Science and Technology Beijing. She received her PhD degree from the University of Science and Technology of China in 2005. Her current research interest is the advanced technology for designing nanostructure for sustainable energy applications, including energy storage and photocatalysts.
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Zhiming M. Wang received his B. S. degree (1992) in physics from Qingdao University, B. S. degree (1995) in physics from Peking University and Ph. D degree (1998) in Physics from Institute of Semiconductors, Chinese Academy of Sciences. He joined University of Electronic Science and Technology of China (UESTC) as “Professor of National 1000-Talents Program” in 2011. His current research focused on compound semiconductors, epitaxial crystal growth, molecular beam epitaxy (MBE) and quantum dots technology.
Kai Jiang received his MB/BS degree from Second Military Medical College, Shanghai, China in 1991, and MD/PhD degrees from Chinese PLA Postgraduate Medical College, Beijing, China in 1998. He further studied in the Queen Mary Hospital of University of Hong Kong in 2002, and Universit at de Barcelona, Span in 2008. He has been with the Department of Hepatobiliary Surgery, Chinese PLA General Hospital since 1991, where he is currently a Professor of surgery and vice Dean of the Department. His current research interests focus on surgical operation and applications of nanotechnology in clinical medicine.
Guozhen Shen received his B. S. degree (1999) in Chemistry from Anhui Normal University and Ph. D degree (2003) in Chemistry from University of Science and technology of China. He joined the Institute of Semiconductors, Chinese Academy of Sciences as a Professor in 2013. His 19
current research focused on flexible electronics and printable electronics, including transistors, photodetectors, sensors and flexible energy storage and conversion devices.
Highlights
A multifunctional self-powered E-skin was fabricated. rGO-on-PVDF-nanofibers act as “4-in-1” material for the E-skin. Micro-supercapacitors, pressure sensors, photodetectors and gas sensors are integrated into a single pixel.
Graphical Abstract
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