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Highly sensitive and skin-like pressure sensor based on asymmetric double-layered structures of reduced graphite oxide
Accepted Manuscript Title: Highly sensitive and skin-like pressure sensor based on asymmetric double-layered structures of reduced graphite oxide Authors: Yunsong Zhu, Junwen Li, Hongbing Cai, Yiming Wu, Huaiyi Ding, Nan Pan, Xiaoping Wang PII: DOI: Reference:
S0925-4005(17)31539-3 http://dx.doi.org/10.1016/j.snb.2017.08.116 SNB 22983
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
19-4-2017 29-7-2017 12-8-2017
Please cite this article as: Yunsong Zhu, Junwen Li, Hongbing Cai, Yiming Wu, Huaiyi Ding, Nan Pan, Xiaoping Wang, Highly sensitive and skin-like pressure sensor based on asymmetric double-layered structures of reduced graphite oxide, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.08.116 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 proof before it is published in its final 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.
Title page of SNB 22983 Highly sensitive and skin-like pressure sensor based on asymmetric double-layered structures of reduced graphite oxide Yunsong Zhu1, Junwen Li1, Hongbing Cai2, Yiming Wu1, Huaiyi Ding2, Nan Pan2,3, and Xiaoping Wang1,2,3,,a)
1
Department of physics, University of Science and Technology of China, Hefei 230026, China 2 Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China 3 Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China a) E-mail: [email protected] Graphical abstract
Highlights
A resistive pressure sensor based on asymmetric double-layered structures was fabricated by direct laser reduction of graphite oxide. The sensor shows high sensitivity (~2 kPa-1) , and the response frequency up to ~2kHz. The sensor can monitor the wrist pulse in real-time.
Abstract： In this work, a resistive pressure sensor based on asymmetric double-layered structures was fabricated readily by direct laser reduction of graphite oxide(GO) to graphene. The pressure sensor shows high sensitivity (~2 kPa-1), fast response time (~0.15 ms) and high cycle stability. Its response frequency to the acoustic vibration can be up to ~2 kHz. More importantly, the sensor can be pasted on the skin to monitor the wrist pulse in real-time. In addition, it can detect the finger touching and wrist bending. The high performance of this skin-like sensor, along with its facilely scaled up fabrication process, facilitates its great potential used in the wide applications such as pressure sensing, health monitor and even human-computer interaction.
Keywords: pressure sensing, asymmetric double-layered structure, reduced graphite oxide
1. Introduction
With the development of epidermal electronics, skin-like pressure sensors have drawn great attention and shown promising applications in human-computer interaction[1-4] and health monitor[5-8]. High sensitivity and fast response time hold both key elements of these applications, especially for non-invasive pulse wave detection. Various of skin-like pressure sensors had been demonstrated in the past, which are capacitive[3, 9, 10], triboelectric [11], and resistive[12-21]. Among these devices, resistive sensors appear to be more promising. However, piezoresistive materials in applications of skin-like pressure sensors need to be flexible, which limits the choice of suitable materials. Graphene has recently attracted much attention due to its excellent electrical and mechanical properties. Sensors based on graphene-wrapped foam[14] and fiber[12] have wide work range. However, the responses are too slow (about a few seconds) to be used in real-time monitor. Graphene woven fabrics strain sensor has an extremely
high gauge factor due to its special crisscross configuration[1], while the response is not enough fast. In order to overcome this shortcoming, the contact resistance type devices are proposed[16, 21-23]. The performance of this kind of sensors can be further improved through fabricating 3-D microstructures on the contact surface. The microstructures provide a plenty of voids that enable their surfaces to deform elastically, and the surface deformation stores and releases energy reversibly to minimize the viscoelasticity[24]. For example, by combining microstructured polydimethylsiloxane (PDMS) and graphene, high sensitivity and fast response time have be achieved[19], although the process for the fabrication 3-D microstructures is complex and high-cost. Currently, a easy and cheap way to obtain 3-D microstructures by laser reduction of graphite oxide is reported[25]. Moreover, the sensor based on laser-scribed graphene has also been demonstrated[21]. However, its symmetric double conductive layered structure makes it inconvenient to set up the initial resistance of the sensor, which is important to determine the sensitivity of the device.
Herein, we present a new type of epidermal resistive pressure-sensing device based on asymmetric double-layered structures, in which one of the key layers is interdigitated electrodes and the other is a conductive film. The initial resistance(R0) can be readily controlled by the geometric parameters of interdigitated electrodes. In this study, both the electrodes and conductive film are produced by laser reduction of graphite oxide. Due to the thermal expansion in the reduction process induced by the laser irradiation, an amount of protruded structuresare naturally formed on the surfaces of the electrodes and conductive film, leading to the fabricated sensor with high sensitivity. The sensitivity of this all graphene based device can reach as high as ~2 kPa-1 and its response time is as short as ~0.15 ms. Moreover, it is also demonstrated that this skin-like resistive pressure sensor not only can detect the finger touching and the wrist bending, but also can monitor the wrist pulse in real-time.
2. Material and methods
2.1 Preparation of PDMS sheet The PDMS mixture of base and cross-linker (the weight ratio of base to cross linker was 10:1) was stirred at least for 10 min. Then, the PDMS mixture was poured in a clean disposable plastic petri dish, cured at room temperature, and peeled off from the petri dish to get the PDMS sheet. The thickness of the sheet is about 100 m.
2.2 Preparation of GO films GO was synthesized using a modified Hummer’s method[26]. About 20 ml GO water solution (2mg/mL) was decanted into the culture dish tiled with a PDMS sheet. After drying, a thin GO film was left on the PDMS substrate.
2.3 Device fabrication A home-made laser engraving machine was used for laser treatment. The machine uses a laser (rated power output = 200mW, wavelength = 650 nm) to reduce GO to the graphene. This process is under control of a computer. “Inkscape” software is used to draw device patterns and generate G-Codes. Then, “Universal-G-Code-Sender” software is used to send the G-codes to an Arduino board flashed with “Grbl v0.8” to control the two stepper motors.
2.4 Pressure response measurement To measure the responses of the sensor to static low pressures, a system containing an electromagnetism balance and a source-meter was designed, which makes it possible to detect low pressure accurately. A small plate on the device is adopted to loading the load. The salt is used as the load in our experiment because it can be varied readily little by little. To investigate the responses of the sensors to dynamic pressures, a system containing a computer-controlled stepping motor and a source-meter was
designed. Stepper motor driver a hanging heavy objects up and down to apply and release the pressures. Moreover, our sensor could be also applied to detect acoustic vibrations. To identify such a capability, we attached our sensor to a sponge and placed them close to a speaker.
2.5 Characterization The electrical testing was done using a source-meter (Keithley 2400), a DC power supply (Agilent E3617A) and an oscilloscope (Tektronix TDS2012). The weight was measured by an electromagnetic balance (Hang-ping FA1204B). Scanning electron microscopy (SEM) images were performed by Zeiss Sigma 300. Atomic force microscopy (AFM) images were carried out with Bruker Dimension Icon. Raman spectra were measured by LabRam HR 800. X-ray photoelectron spectroscopy (XPS) spectra were taken by Thermo Scientific ESCALAB 250Xi.
3. Results and discussions
The process for the fabrication of graphene-based pressure sensors is schematically illustrated in Fig. 1. Firstly, the GO water solution was decanted into the culture dish tiled with a PDMS sheet(Fig. 1a). After drying, a thin insulator GO film was left on the substrate(Fig. 1b). Subsequently, a home-made laser engraving machine was employed to direct write the interdigitated electrodes and conductive film(Fig. 1c), in which process the GO was reduced to laser-scribed graphene (LSG) by the irradiation with an laser(wavelength is 650 nm, and rated power output is 200 mW)[25]. Both of the interdigitated electrodes and the conductive film were then washed by water to remove unexposed GO. Eventually, they were assembled face-to-face to form the sensor (Fig. 1d to 1e) and the interdigitated electrodes were wired out (Fig. 1f). The photography of the sensor attached on the skin is shown in Fig. 1g. The detailed
layout and fabrication of the sensor is shown in the Figures S1 and S2a in the Supporting Information. The SEM observations of the conductive layer consisted of crisscross LSG grids and the interdigitated electrodes are shown in Supplementary Figure S2b and S2c, respectively.
Before the laser scribing, the surface of GO film is quite flat. However, the protruded structures can be formed on the surface of laser-scribed graphene film (Fig 1.h). The height of the protruded structures is estimated by AFM to be about 5 μm (Supplementary Figure S3). The GO and LSG films are further characterized by the XPS and Raman spectroscopy, and the results are shown in Supplementary Figure S4 and Figure S5, respectively. It is found clearly that the GO film can be readily reduced through the laser scribing.
The sensing principle is attributed to the variation of the pressure-dependent contact resistance between the conductive film and the interdigitated electrodes. It can be considered that, as two layers are assembled face to face under unloading condition, some protruded structures between two layers contact, while some others are not (as shown in the left of Supplementary Figure S6a). This results in the initial resistance of the device. On applying an external pressure, a compressively elastic deformation of protrusions occurs, and more protruded structures can contact each other, besides that the contacted areas increase (see right of Supplementary Figure S6a). This leads to enhance the conductive pathways and decrease the resistance of the sensor. After the external force removed, the deformation of protrusions disappears, resulting in recovering of the sensor resistance. SEM observation of one protruded structure in the sensor before and after applied pressure are shown in Supplementary Figure S6b and Figure S6c, respectively.
The response of the sensor to static pressures was first tested. To measure the responses to low pressures, a system containing an electromagnetic balance and a source-meter was designed. A small plate between the device and load determined the sensitive area to be 300 mm2. Fig. 2a is the current–voltage (I-V) curve of the device under different pressures. As seen, all curves show good linearity in the voltage between 0-10V, indicative of ohmic contacts of the device. Therefore, the resistance (or the sensitivity) of the device is independent of the working voltage. Fig. 2b presents the response of the sensor resistance to various pressures between 0~2300 Pa. Clearly, the resistance drastically decreases with increasing pressure. Fig. 2c shows the variation of the relative resistance((R0-R)/R0) of the device with the pressure, where R and R0 denote the resistance with and without applied pressure, respectively. Therefore, the sensitivity of the device, defined as S=δ(△R/R0)/δP[14], can be fitted and calculated from the trace in Fig. 2c to be ~2 kPa-1 in the low pressure range (<200 Pa). However, it drops to about 0.02 kPa-1 at pressures >500 Pa, due to that the contact area between the electrodes becomes saturated under the large compression force.
Note that, as demonstrated in Table I, even our device is composed by the reduced graphite oxide, its sensitivity is still higher than those of sensors constructed by either graphene-based material or graphite-based material[15, 16, 20, 21], and close to the microstructure enhanced graphene sensor [19]. This is attributed to the uniquely asymmetric double-layered structure of our device, because that the initial resistance of the sensor can be tuned readily through length, width and spacing of the interdigitated electrodes.
In order to test the cycle stability of the sensor, a home-made system containing a computer controlled stepping motor, a hanging weight and a source-meter was
designed. Fig.2d demonstrates a typical result. As seen, the output current of the sensor was stably maintained over 500 load-unload cycles under a pressure of 700 Pa, indicating the high durability of the sensor. It is worth mentioning that the supplied voltage on the sensor here is 5 V and the power consumption is less than 250 W.
The response of the sensor to the pressure pulse was measured using an oscilloscope. Experimental setup is illustrated in Fig. 3a. Fig. 3b is a representative result, and the response time can be estimated as short as 0.15 ms, while the relaxation time is 0.3 ms. These results are two or three orders of magnitude faster than those of previous reports[3, 13, 21]. We attribute the quick response and relaxation behavior of our device to its unique structure. As mentioned in the preceding paragraph, the protruded structures on the electrodes and the conductive film enable their surfaces to produce the elastic deformation and minimize the viscoelasticity.
We also investigate the dynamic response performance of the sensor to the acoustic vibration. As seen in Fig. 3c, the response shows high-fidelity to sine wave vibrations of 500 Hz, and becomes weak distortion as the wave frequency is up to 2 kHz. The behavior is consistent with the result demonstrated in Fig. 3b, in which the sum of the response and relaxation time is 0.45 ms, corresponding to the limitation of responding frequency up to ~2.2 kHz.
Finally, we demonstrate the response performance of the skin-like sensor to the sphygmus, finger touching and wrist bending. To this end, the sensor is first pasted on the wrist (Fig.4a) and the response to the sphygmus is monitored and recorded. As shown in Fig.4b, the heartbeat is counted to be about 75 per minute when the tester sits quietly. More importantly, the detailed health information can be evaluated in the
recorded radial artery pulse waves. As seen, the repeated beatings from the sum of the incident wave and reflected wave from the hand (P1), and the reflected wave from the lower body minus end-diastolic pressure (P2) can be well distinguished. Additionally, ΔTDVP, the time difference between P1 and P2 and representing a measure of arterial stiffness[27, 28], is found to be 0.25±0.02s. The value is compatible with a healthy man in late twenties, suggesting reliable of the measurement. In the same manner, the responses of the sensor to finger touching and wrist bending are also demonstrated in Fig. 4c. The results further verify the sensor possesses high sensitive and skin-like features.
4. Conclusions
In summary, we fabricate a resistive pressure sensor based on asymmetric double-layered structures by direct laser reduction of graphite oxide (GO) to graphene. The fabrication process is scalable and low-cost. This skin-like resistive pressure sensor demonstrates high sensitivity (~2 kPa-1) and very short response times (~0.15 ms). Moreover, the sensor can detect finger touching and monitor blood pulses in real-time. This all-carbon-based pressure sensor has many potential applications such as pressure sensing, health monitor and even human-computer interaction in the future.
Acknowledgements
We acknowledge the financial supports from MOST of China (2016YFA0200602), National Natural Science Foundation of China (21421063, 11374274, 11404314, 11474260, 11504364).
References
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Yunsong Zhu is currently a Ph.D. student in the Department of physics at the University of Science and Technology of China. His research focuses on epidermal electronics. Junwen Li is currently a Ph.D. student in the Department of physics at the University of Science and Technology of China. His research interest lies in photocatalytic hydrogen production. Hongbing Cai is a postdoctoral fellow of the Hefei National Laboratory for Physical Sciences at the Microscale at the University of Science and Technology of China. His research focuses on micro- and nanomachining processes. Yiming Wu is currently a Ph.D. student in the Department of physics at the University of Science and Technology of China. His current research involves nanomaterials and nanodevices. Huaiyi Ding is a postdoctoral fellow of the Hefei National Laboratory for Physical Sciences at the Microscale at the University of Science and Technology of China. His research is focused in the areas of optical metamaterials and nanolasers Nan Pan received B.S. and Ph.D. degrees in condensed matter physics from University of Science and Technology of China (USTC) in 2002 and 2008, respectively. He is a professor in Hefei National Laboratory for Physical Sciences at the Microscale (HFNL) as well as in Department of Chemical Physics, USTC. His research interests include probing and tailoring the properties of low-dimensional nanosystems, designing hybrid-/hetero- materials for multi-purpose applications especially in nanoscale optoelectronics, photonics and photovoltaics. Xiaoping Wang is a full professor of Physics in University of Science and Technology of China (USTC) and the vice director of Hefei National Laboratory for Physical Sciences at the Microscale (HFNL), USTC. He received his B.S. in 1987, M.S. in 1990, and Ph.D. in 2001 from USTC. His current research focuses on the nanostructures and nanodevices.
Figure 1. Skin-like graphene pressure sensor based on asymmetric double-layered structures. (a-f) Schematic illustration of the fabrication process. (g) Photograph of a sensor attached on the skin. (scale bar, 0.5 cm). (h) Top view SEM image of the laser-scribed graphene surface. (scale bar, 100 m).
Figure 2. Characterization of the pressure response. (a) Current–voltage curve under various pressures. (b) Resistance response of sensor to various pressures. (c) The variation of relative resistance change with the pressure. The dotted lines are fitting results. (d) Stability of pressure responseto the 500-cycle of loading/unloading under the pressure of 700 Pa.
Figure 3. Dynamic response of the sensor. (a) Schematic illustration of the experimental setup. (b) Response to a pulse pressure. (c) Dynamic responses to sine wave sound vibrating at different frequencies.
Figure 4. Applications of the skin-like pressure sensor. (a) Photograph of a sensor on the wrist (scale bar, 1cm). (b) Real-time responses of the sensor tosphygmus. (c) Detection of wrist bending and finger touching.
Table I. Comparison of the pressure sensing for previous and present work Sensitivity
Response/relaxation
Sizes
Potential
(Pa)
time (ms)
(cm2)
applications
0.31
0-3000
5/5
4
Ag NWs-embedded PDMS
3.8
45-500
150/150
0.04
SBS/Ag NPs conductive fiber
0.21
0-2000
40/10
0.0016
CNT microyarns/PDMS
0.05
0-100
63/-
6.3
HMI, ES
[10]
CVD graphene/PDMS
0.24
0-250
1500/1500
1.7
HM, ES
[16]
CNT/graphene hybridfoam
0.19
0-2500
-/-
1
ES
[17]
Ag NWs/Wrinkle PU fiber
0.12
0-600
35/15
3
WE, ES
[18]
SSNPs/ PU
2.46
0-1000
30/30
-
ES, HMI
[13]
rGO/microstructured PDMS
5.5
0-100
0.2/-
2
ES, HMI
[19]
graphite-on-paper
0.610-4
0-2105
-/-
4
ES
[20]
graphite NPL/CNT/PDMS
0.6
0-3.2105
-/-
1
HMI, WE
[15]
LSG
0.96
0-50000
0.4/212
1
ES, HMI
[21]
LSG/PDMS
2
0-200
0.15/0.3
1.6
Materials
(kPa-1)
Range
Ref.
Ag NWs/NPs /micropattern PDMS
ES, HMI
[11]
WE, ES
[3]
WE,HMI
[9]
our HMI,HM
work
Note: Ag, siver; NPs, nanopaticles; CNT, carbon nanotube; NWs, nanowires; NPL, nanoplatelet; SSNPs, sea-urchin shaped metal nanoparticles; rGO, reduced graphene oxide; LSG, laser-scribed graphene; PDMS, polydimethylsiloxane; SBS, styrene-block-butadienstyrene; PU, polyurethane; WE, wearable electronics; HMI, human-machine interfaces; HM: health monitor; ES, electronic skin.
S0925-4005(17)31539-3 http://dx.doi.org/10.1016/j.snb.2017.08.116 SNB 22983
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
19-4-2017 29-7-2017 12-8-2017
Please cite this article as: Yunsong Zhu, Junwen Li, Hongbing Cai, Yiming Wu, Huaiyi Ding, Nan Pan, Xiaoping Wang, Highly sensitive and skin-like pressure sensor based on asymmetric double-layered structures of reduced graphite oxide, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.08.116 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 proof before it is published in its final 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.
Title page of SNB 22983 Highly sensitive and skin-like pressure sensor based on asymmetric double-layered structures of reduced graphite oxide Yunsong Zhu1, Junwen Li1, Hongbing Cai2, Yiming Wu1, Huaiyi Ding2, Nan Pan2,3, and Xiaoping Wang1,2,3,,a)
1
Department of physics, University of Science and Technology of China, Hefei 230026, China 2 Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China 3 Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China a) E-mail: [email protected] Graphical abstract
Highlights
A resistive pressure sensor based on asymmetric double-layered structures was fabricated by direct laser reduction of graphite oxide. The sensor shows high sensitivity (~2 kPa-1) , and the response frequency up to ~2kHz. The sensor can monitor the wrist pulse in real-time.
Abstract： In this work, a resistive pressure sensor based on asymmetric double-layered structures was fabricated readily by direct laser reduction of graphite oxide(GO) to graphene. The pressure sensor shows high sensitivity (~2 kPa-1), fast response time (~0.15 ms) and high cycle stability. Its response frequency to the acoustic vibration can be up to ~2 kHz. More importantly, the sensor can be pasted on the skin to monitor the wrist pulse in real-time. In addition, it can detect the finger touching and wrist bending. The high performance of this skin-like sensor, along with its facilely scaled up fabrication process, facilitates its great potential used in the wide applications such as pressure sensing, health monitor and even human-computer interaction.
Keywords: pressure sensing, asymmetric double-layered structure, reduced graphite oxide
1. Introduction
With the development of epidermal electronics, skin-like pressure sensors have drawn great attention and shown promising applications in human-computer interaction[1-4] and health monitor[5-8]. High sensitivity and fast response time hold both key elements of these applications, especially for non-invasive pulse wave detection. Various of skin-like pressure sensors had been demonstrated in the past, which are capacitive[3, 9, 10], triboelectric [11], and resistive[12-21]. Among these devices, resistive sensors appear to be more promising. However, piezoresistive materials in applications of skin-like pressure sensors need to be flexible, which limits the choice of suitable materials. Graphene has recently attracted much attention due to its excellent electrical and mechanical properties. Sensors based on graphene-wrapped foam[14] and fiber[12] have wide work range. However, the responses are too slow (about a few seconds) to be used in real-time monitor. Graphene woven fabrics strain sensor has an extremely
high gauge factor due to its special crisscross configuration[1], while the response is not enough fast. In order to overcome this shortcoming, the contact resistance type devices are proposed[16, 21-23]. The performance of this kind of sensors can be further improved through fabricating 3-D microstructures on the contact surface. The microstructures provide a plenty of voids that enable their surfaces to deform elastically, and the surface deformation stores and releases energy reversibly to minimize the viscoelasticity[24]. For example, by combining microstructured polydimethylsiloxane (PDMS) and graphene, high sensitivity and fast response time have be achieved[19], although the process for the fabrication 3-D microstructures is complex and high-cost. Currently, a easy and cheap way to obtain 3-D microstructures by laser reduction of graphite oxide is reported[25]. Moreover, the sensor based on laser-scribed graphene has also been demonstrated[21]. However, its symmetric double conductive layered structure makes it inconvenient to set up the initial resistance of the sensor, which is important to determine the sensitivity of the device.
Herein, we present a new type of epidermal resistive pressure-sensing device based on asymmetric double-layered structures, in which one of the key layers is interdigitated electrodes and the other is a conductive film. The initial resistance(R0) can be readily controlled by the geometric parameters of interdigitated electrodes. In this study, both the electrodes and conductive film are produced by laser reduction of graphite oxide. Due to the thermal expansion in the reduction process induced by the laser irradiation, an amount of protruded structuresare naturally formed on the surfaces of the electrodes and conductive film, leading to the fabricated sensor with high sensitivity. The sensitivity of this all graphene based device can reach as high as ~2 kPa-1 and its response time is as short as ~0.15 ms. Moreover, it is also demonstrated that this skin-like resistive pressure sensor not only can detect the finger touching and the wrist bending, but also can monitor the wrist pulse in real-time.
2. Material and methods
2.1 Preparation of PDMS sheet The PDMS mixture of base and cross-linker (the weight ratio of base to cross linker was 10:1) was stirred at least for 10 min. Then, the PDMS mixture was poured in a clean disposable plastic petri dish, cured at room temperature, and peeled off from the petri dish to get the PDMS sheet. The thickness of the sheet is about 100 m.
2.2 Preparation of GO films GO was synthesized using a modified Hummer’s method[26]. About 20 ml GO water solution (2mg/mL) was decanted into the culture dish tiled with a PDMS sheet. After drying, a thin GO film was left on the PDMS substrate.
2.3 Device fabrication A home-made laser engraving machine was used for laser treatment. The machine uses a laser (rated power output = 200mW, wavelength = 650 nm) to reduce GO to the graphene. This process is under control of a computer. “Inkscape” software is used to draw device patterns and generate G-Codes. Then, “Universal-G-Code-Sender” software is used to send the G-codes to an Arduino board flashed with “Grbl v0.8” to control the two stepper motors.
2.4 Pressure response measurement To measure the responses of the sensor to static low pressures, a system containing an electromagnetism balance and a source-meter was designed, which makes it possible to detect low pressure accurately. A small plate on the device is adopted to loading the load. The salt is used as the load in our experiment because it can be varied readily little by little. To investigate the responses of the sensors to dynamic pressures, a system containing a computer-controlled stepping motor and a source-meter was
designed. Stepper motor driver a hanging heavy objects up and down to apply and release the pressures. Moreover, our sensor could be also applied to detect acoustic vibrations. To identify such a capability, we attached our sensor to a sponge and placed them close to a speaker.
2.5 Characterization The electrical testing was done using a source-meter (Keithley 2400), a DC power supply (Agilent E3617A) and an oscilloscope (Tektronix TDS2012). The weight was measured by an electromagnetic balance (Hang-ping FA1204B). Scanning electron microscopy (SEM) images were performed by Zeiss Sigma 300. Atomic force microscopy (AFM) images were carried out with Bruker Dimension Icon. Raman spectra were measured by LabRam HR 800. X-ray photoelectron spectroscopy (XPS) spectra were taken by Thermo Scientific ESCALAB 250Xi.
3. Results and discussions
The process for the fabrication of graphene-based pressure sensors is schematically illustrated in Fig. 1. Firstly, the GO water solution was decanted into the culture dish tiled with a PDMS sheet(Fig. 1a). After drying, a thin insulator GO film was left on the substrate(Fig. 1b). Subsequently, a home-made laser engraving machine was employed to direct write the interdigitated electrodes and conductive film(Fig. 1c), in which process the GO was reduced to laser-scribed graphene (LSG) by the irradiation with an laser(wavelength is 650 nm, and rated power output is 200 mW)[25]. Both of the interdigitated electrodes and the conductive film were then washed by water to remove unexposed GO. Eventually, they were assembled face-to-face to form the sensor (Fig. 1d to 1e) and the interdigitated electrodes were wired out (Fig. 1f). The photography of the sensor attached on the skin is shown in Fig. 1g. The detailed
layout and fabrication of the sensor is shown in the Figures S1 and S2a in the Supporting Information. The SEM observations of the conductive layer consisted of crisscross LSG grids and the interdigitated electrodes are shown in Supplementary Figure S2b and S2c, respectively.
Before the laser scribing, the surface of GO film is quite flat. However, the protruded structures can be formed on the surface of laser-scribed graphene film (Fig 1.h). The height of the protruded structures is estimated by AFM to be about 5 μm (Supplementary Figure S3). The GO and LSG films are further characterized by the XPS and Raman spectroscopy, and the results are shown in Supplementary Figure S4 and Figure S5, respectively. It is found clearly that the GO film can be readily reduced through the laser scribing.
The sensing principle is attributed to the variation of the pressure-dependent contact resistance between the conductive film and the interdigitated electrodes. It can be considered that, as two layers are assembled face to face under unloading condition, some protruded structures between two layers contact, while some others are not (as shown in the left of Supplementary Figure S6a). This results in the initial resistance of the device. On applying an external pressure, a compressively elastic deformation of protrusions occurs, and more protruded structures can contact each other, besides that the contacted areas increase (see right of Supplementary Figure S6a). This leads to enhance the conductive pathways and decrease the resistance of the sensor. After the external force removed, the deformation of protrusions disappears, resulting in recovering of the sensor resistance. SEM observation of one protruded structure in the sensor before and after applied pressure are shown in Supplementary Figure S6b and Figure S6c, respectively.
The response of the sensor to static pressures was first tested. To measure the responses to low pressures, a system containing an electromagnetic balance and a source-meter was designed. A small plate between the device and load determined the sensitive area to be 300 mm2. Fig. 2a is the current–voltage (I-V) curve of the device under different pressures. As seen, all curves show good linearity in the voltage between 0-10V, indicative of ohmic contacts of the device. Therefore, the resistance (or the sensitivity) of the device is independent of the working voltage. Fig. 2b presents the response of the sensor resistance to various pressures between 0~2300 Pa. Clearly, the resistance drastically decreases with increasing pressure. Fig. 2c shows the variation of the relative resistance((R0-R)/R0) of the device with the pressure, where R and R0 denote the resistance with and without applied pressure, respectively. Therefore, the sensitivity of the device, defined as S=δ(△R/R0)/δP[14], can be fitted and calculated from the trace in Fig. 2c to be ~2 kPa-1 in the low pressure range (<200 Pa). However, it drops to about 0.02 kPa-1 at pressures >500 Pa, due to that the contact area between the electrodes becomes saturated under the large compression force.
Note that, as demonstrated in Table I, even our device is composed by the reduced graphite oxide, its sensitivity is still higher than those of sensors constructed by either graphene-based material or graphite-based material[15, 16, 20, 21], and close to the microstructure enhanced graphene sensor [19]. This is attributed to the uniquely asymmetric double-layered structure of our device, because that the initial resistance of the sensor can be tuned readily through length, width and spacing of the interdigitated electrodes.
In order to test the cycle stability of the sensor, a home-made system containing a computer controlled stepping motor, a hanging weight and a source-meter was
designed. Fig.2d demonstrates a typical result. As seen, the output current of the sensor was stably maintained over 500 load-unload cycles under a pressure of 700 Pa, indicating the high durability of the sensor. It is worth mentioning that the supplied voltage on the sensor here is 5 V and the power consumption is less than 250 W.
The response of the sensor to the pressure pulse was measured using an oscilloscope. Experimental setup is illustrated in Fig. 3a. Fig. 3b is a representative result, and the response time can be estimated as short as 0.15 ms, while the relaxation time is 0.3 ms. These results are two or three orders of magnitude faster than those of previous reports[3, 13, 21]. We attribute the quick response and relaxation behavior of our device to its unique structure. As mentioned in the preceding paragraph, the protruded structures on the electrodes and the conductive film enable their surfaces to produce the elastic deformation and minimize the viscoelasticity.
We also investigate the dynamic response performance of the sensor to the acoustic vibration. As seen in Fig. 3c, the response shows high-fidelity to sine wave vibrations of 500 Hz, and becomes weak distortion as the wave frequency is up to 2 kHz. The behavior is consistent with the result demonstrated in Fig. 3b, in which the sum of the response and relaxation time is 0.45 ms, corresponding to the limitation of responding frequency up to ~2.2 kHz.
Finally, we demonstrate the response performance of the skin-like sensor to the sphygmus, finger touching and wrist bending. To this end, the sensor is first pasted on the wrist (Fig.4a) and the response to the sphygmus is monitored and recorded. As shown in Fig.4b, the heartbeat is counted to be about 75 per minute when the tester sits quietly. More importantly, the detailed health information can be evaluated in the
recorded radial artery pulse waves. As seen, the repeated beatings from the sum of the incident wave and reflected wave from the hand (P1), and the reflected wave from the lower body minus end-diastolic pressure (P2) can be well distinguished. Additionally, ΔTDVP, the time difference between P1 and P2 and representing a measure of arterial stiffness[27, 28], is found to be 0.25±0.02s. The value is compatible with a healthy man in late twenties, suggesting reliable of the measurement. In the same manner, the responses of the sensor to finger touching and wrist bending are also demonstrated in Fig. 4c. The results further verify the sensor possesses high sensitive and skin-like features.
4. Conclusions
In summary, we fabricate a resistive pressure sensor based on asymmetric double-layered structures by direct laser reduction of graphite oxide (GO) to graphene. The fabrication process is scalable and low-cost. This skin-like resistive pressure sensor demonstrates high sensitivity (~2 kPa-1) and very short response times (~0.15 ms). Moreover, the sensor can detect finger touching and monitor blood pulses in real-time. This all-carbon-based pressure sensor has many potential applications such as pressure sensing, health monitor and even human-computer interaction in the future.
Acknowledgements
We acknowledge the financial supports from MOST of China (2016YFA0200602), National Natural Science Foundation of China (21421063, 11374274, 11404314, 11474260, 11504364).
References
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Yunsong Zhu is currently a Ph.D. student in the Department of physics at the University of Science and Technology of China. His research focuses on epidermal electronics. Junwen Li is currently a Ph.D. student in the Department of physics at the University of Science and Technology of China. His research interest lies in photocatalytic hydrogen production. Hongbing Cai is a postdoctoral fellow of the Hefei National Laboratory for Physical Sciences at the Microscale at the University of Science and Technology of China. His research focuses on micro- and nanomachining processes. Yiming Wu is currently a Ph.D. student in the Department of physics at the University of Science and Technology of China. His current research involves nanomaterials and nanodevices. Huaiyi Ding is a postdoctoral fellow of the Hefei National Laboratory for Physical Sciences at the Microscale at the University of Science and Technology of China. His research is focused in the areas of optical metamaterials and nanolasers Nan Pan received B.S. and Ph.D. degrees in condensed matter physics from University of Science and Technology of China (USTC) in 2002 and 2008, respectively. He is a professor in Hefei National Laboratory for Physical Sciences at the Microscale (HFNL) as well as in Department of Chemical Physics, USTC. His research interests include probing and tailoring the properties of low-dimensional nanosystems, designing hybrid-/hetero- materials for multi-purpose applications especially in nanoscale optoelectronics, photonics and photovoltaics. Xiaoping Wang is a full professor of Physics in University of Science and Technology of China (USTC) and the vice director of Hefei National Laboratory for Physical Sciences at the Microscale (HFNL), USTC. He received his B.S. in 1987, M.S. in 1990, and Ph.D. in 2001 from USTC. His current research focuses on the nanostructures and nanodevices.
Figure 1. Skin-like graphene pressure sensor based on asymmetric double-layered structures. (a-f) Schematic illustration of the fabrication process. (g) Photograph of a sensor attached on the skin. (scale bar, 0.5 cm). (h) Top view SEM image of the laser-scribed graphene surface. (scale bar, 100 m).
Figure 2. Characterization of the pressure response. (a) Current–voltage curve under various pressures. (b) Resistance response of sensor to various pressures. (c) The variation of relative resistance change with the pressure. The dotted lines are fitting results. (d) Stability of pressure responseto the 500-cycle of loading/unloading under the pressure of 700 Pa.
Figure 3. Dynamic response of the sensor. (a) Schematic illustration of the experimental setup. (b) Response to a pulse pressure. (c) Dynamic responses to sine wave sound vibrating at different frequencies.
Figure 4. Applications of the skin-like pressure sensor. (a) Photograph of a sensor on the wrist (scale bar, 1cm). (b) Real-time responses of the sensor tosphygmus. (c) Detection of wrist bending and finger touching.
Table I. Comparison of the pressure sensing for previous and present work Sensitivity
Response/relaxation
Sizes
Potential
(Pa)
time (ms)
(cm2)
applications
0.31
0-3000
5/5
4
Ag NWs-embedded PDMS
3.8
45-500
150/150
0.04
SBS/Ag NPs conductive fiber
0.21
0-2000
40/10
0.0016
CNT microyarns/PDMS
0.05
0-100
63/-
6.3
HMI, ES
[10]
CVD graphene/PDMS
0.24
0-250
1500/1500
1.7
HM, ES
[16]
CNT/graphene hybridfoam
0.19
0-2500
-/-
1
ES
[17]
Ag NWs/Wrinkle PU fiber
0.12
0-600
35/15
3
WE, ES
[18]
SSNPs/ PU
2.46
0-1000
30/30
-
ES, HMI
[13]
rGO/microstructured PDMS
5.5
0-100
0.2/-
2
ES, HMI
[19]
graphite-on-paper
0.610-4
0-2105
-/-
4
ES
[20]
graphite NPL/CNT/PDMS
0.6
0-3.2105
-/-
1
HMI, WE
[15]
LSG
0.96
0-50000
0.4/212
1
ES, HMI
[21]
LSG/PDMS
2
0-200
0.15/0.3
1.6
Materials
(kPa-1)
Range
Ref.
Ag NWs/NPs /micropattern PDMS
ES, HMI
[11]
WE, ES
[3]
WE,HMI
[9]
our HMI,HM
work
Note: Ag, siver; NPs, nanopaticles; CNT, carbon nanotube; NWs, nanowires; NPL, nanoplatelet; SSNPs, sea-urchin shaped metal nanoparticles; rGO, reduced graphene oxide; LSG, laser-scribed graphene; PDMS, polydimethylsiloxane; SBS, styrene-block-butadienstyrene; PU, polyurethane; WE, wearable electronics; HMI, human-machine interfaces; HM: health monitor; ES, electronic skin.