Journal of Materials Science & Technology, 2020, 45(0): 241-247 DOI: 10.1016/j.jmst.2019.11.014

Research Article

Facilely prepared layer-by-layer graphene membrane-based pressure sensor with high sensitivity and stability for smart wearable devices

Liu Taoa,b, Zhu Caizhenb, Wu Weia,c, Liao Kai-Ningd, Gong Xianjinga, Sun Qijuna, Li Robert K.Y.,a,*

Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

Institute of Low-dimensional Materials Genome Initiative, College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, 518060, China

National Engineering Research Center of Novel Equipment for Polymer Processing, Key Laboratory of Polymer Processing Engineering of Ministry of Education, School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou, 510640, China

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, China

Corresponding authors: * E-mail address:aprkyl@cityu.edu.hk(R.K.Y. Li).

Received: 2019-09-21   Accepted: 2019-11-2   Online: 2020-05-15

Abstract

With the prosperous development of artificial intelligence, medical diagnosis and electronic skins, wearable electronic devices have drawn much attention in our daily life. Flexible pressure sensors based on carbon materials with ultrahigh sensitivity, especially in a large pressure range regime are highly required in wearable applications. In this work, graphene membrane with a layer-by-layer structure has been successfully fabricated via a facile self-assembly and air-drying (SAAD) method. In the SAAD process, air-drying the self-assembled graphene hydrogels contributes to the uniform and compact layer structure in the obtained membranes. Owing to the excellent mechanical and electrical properties of graphene, the pressure sensor constructed by several layers of membranes exhibits high sensitivity (52.36 kPa-1) and repeatability (short response and recovery time) in the loading pressure range of 0-50 kPa. Compared with most reported graphene-related pressure sensors, our device shows better sensitivity and wider applied pressure range. What’s more, we demonstrate it shows desired results in wearable applications for pulse monitoring, breathing detection as well as different intense motion recording such as walk, run and squat. It’s hoped that the facilely prepared layer-by-layer graphene membrane-based pressure sensors will have more potential to be used for smart wearable devices in the future.

Keywords: Graphene membrane ; Self-assembly ; High sensitivity ; Wearable devices ; Human motions

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Cite this article

Liu Tao, Zhu Caizhen, Wu Wei, Liao Kai-Ning, Gong Xianjing, Sun Qijun, Li Robert K.Y.. Facilely prepared layer-by-layer graphene membrane-based pressure sensor with high sensitivity and stability for smart wearable devices. Journal of Materials Science & Technology[J], 2020, 45(0): 241-247 DOI:10.1016/j.jmst.2019.11.014

1. Introduction

With the recent prosperous development of medical diagnosis, artificial intelligence (AI) and electronic skins [[1], [2], [3], [4], [5]], wearable electronic devices have drawn much considerable attention in our daily life [[6], [7], [8], [9]]. Among these different kinds of wearable devices, flexible pressure sensors based on the change of electrical resistance have received extensive attention due to some special properties such as fast response and high sensitivity, light weight, easy processing, and low cost [[10], [11], [12]]. In actual pressure sensor applications, the contact area will change by motion induced pressure change, which can come from touching, pressing, heartbeat, or respiration induced pulse, further resulting in the change of electrical resistance [13,14]. In order to detect the pressure induced from the external forces accurately, flexible pressure sensors that can be processed easily, with high sensitivity (especially with a wide pressure window ranged from 0-20 kPa) are highly desirable [15,16].

Various material systems such as polymer/nanoparticle composites [[17], [18], [19]], carbon films and carbon sponges [[20], [21], [22], [23]], conductive hydrogels [[24], [25], [26]] have been investigated for their piezoresistive behaviour. Gong et al. [17] fabricated flexible pressure sensors with ultrahigh sensitivity by sandwiching gold nanowires-implanted tissue paper sheets between two thin films of polydimethylsiloxane (PDMS) to monitor blood pulses in real time and detect tiny vibration forces from voice. Jian et al. [20] reported a facile bioinspired approach to develop high-performance pressure sensors based on highly conductive aligned carbon nanotubes/graphene hybrid and microstructured-PDMS films. The obtained sensors demonstrated ultrahigh sensitivity of 19.8 kPa-1 (<0.3 kPa), low operating voltage of 0.03 V, short response time of 16.7 ms, along with reasonable stability for more than 35,000 cycles of loading-unloading. Li et al. [23] prepared a highly stretchable and sensitive strain sensor based on three-dimensional graphene foam/PDMS composites and it could be stretched to 30 % of strain and the gauge factor reached 98.66 under 5 % of applied strain. Lubineau’s group [26] reported a flexible pressure sensor by embedding single-walled carbon nanotubes/alginate hydrogel spheres between conductive electrodes and it displayed high sensitivity of 0.176 ΔR/R0 /kPa as well as a low detectable limit of 10 Pa.

Graphene, emerging as a new two-dimensional (2D) nanomaterial in the last decade, has attracted more and more interest in soft electronics [[27], [28], [29]]. The 2D hexagonal honeycomb structure arranged by sp2-bonded carbon atoms enable graphene sensitive to mechanical force since its band gap can be easily tuned by an external stimulus [30,31]. Tao and Zhang fabricated graphene-paper by mixing tissue paper with GO and the paper-based pressure sensor showed satisfactory performance in the pressure range of 0-20 kPa, which was very suitable for smart wearable applications [14]. Sheng’s group [32] reported a blade coating-thermal treatment strategy to fabricate highly sensitive pressure sensors based on honeycomb-like graphene film decorated with bubbles and it became a promising candidate for future low-cost artificial skins. As far as reported, the sensitivity and stability of the graphene-based pressure sensor cannot completely satisfy the requirements for wider applications. It is still of great challenge to fabricate graphene-based pressure sensors with ultrahigh sensitivity for a wider pressure range.

In this work, we demonstrate graphene membranes (GMs) with a layer-by-layer structure can be successfully fabricated via a facile self-assembly and air-drying (SAAD) method. Due to the excellent electrical properties of graphene films, the GM-based pressure sensor exhibited excellent sensitivity (52.36 kPa-1) and repeatability in the pressure range of 0-50 kPa. In addition, the pressure sensor shows excellent performance in wearable applications to detect human motions such as pulse, breathing as well as several types of intense motion. As far as we can concern, compared with most graphene-related pressure sensors, our device shows better sensitivity and stability, which will make a difference in the applied fields. We hope the facilely prepared layer-by-layer graphene membrane-based pressure sensor will have huge potential in smart soft devices for future applications.

2. Experimental

2.1. Materials

Natural graphite flakes (average diameter of 13.0 μm) were purchased from Qingdao Huadong Graphite Factory (Shandong, China). Concentrated sulfuric acid (H2SO4, 95-98 %), sodium nitrate (NaNO3, 99 %), potassium permanganate (KMnO4, 99.5 %), concentrated hydrochloric acid (HCl, 36-38 %) and ascorbic acid (99 %) were from Beijing Chemical Factory (Beijing, China) and used as received.

2.2. Preparation of graphene membranes (GMs) by self-assembly and air-drying (SAAD) method

Graphite oxide (GO) was synthesized by oxidizing natural graphite flakes according to the previous modified Hummers method [33]. 5 ml GO suspension with a concentration of 6 mg mL-1 was mixed with 30 mg ascorbic acid under ultrasonication. Then the suspension was transferred into a plastic petri dish and heated at 70 °C for 3 h in an oven to obtain a graphene hydrogel. Finally, the thin hydrogel was air-dried to reduced graphene oxide membrane (rGOM) and the GM was obtained by annealing at 1000 °C for 1 h under Argon atmosphere.

2.3. Characterization

GO, rGOM and GM were characterized by X-ray photoelectron spectroscopy (XPS) under a Physical Electronics PHI 5802 with a monochromatic Al Kα X-ray source (1486.6 eV) and transform infrared spectroscopy (FT-IR) under a PerkinElmer 100 spectrometer Fourier. Morphologies and microstructures of GM were observed by field emission scanning electron microscopy (FE-SEM, FEI Quanta 450) at 20 kV. X-ray diffraction (XRD) measurements were carried out using a Bruker AXS D2 diffractometer with Cu Kα radiation (λ = 1.54 Å) at a generator voltage of 30 kV and a generator current of 10 mA. The scanning range of diffraction angle (2θ) was from 5° to 60° with a fixed interval of 0.02°. The electrical conductivity of the rGOM and GM were measured by using a Hall Effect Measurement System (HMS-5300).

2.4. Fabrication of GM-based pressure sensors

Different number of layers of GMs and rGOMs were respectively stacked together to form the n-layer GM pressure sensor. And then the bottom and top side of the stacked membranes were stick to thin copper foil by silver paste. The loading of the pressure stress was performed with a force machine (Force Gauge Model M5-012) and the electrical curves of the prepared pressure sensors were carried out by using a digital source-meter (Agilent 4155C Semiconductor Parameter Analyzer).

3. Results and discussion

3.1. Fabrication and morphology of GMs

GO has been used widely as the graphene derivatives for the preparation of graphene films due to its affinity to water. There are different methods to fabricate GO films such as spin coating [[34], [35], [36]], rolling [37], casting [38,39], vacuum filtration [40,41], spray coating [42], etc. From the viewpoint of colloid chemistry, the formation of the GO films is induced by water evaporation at the interface of the air and GO sheets [43]. As is known, in the fluid-like GO dispersion, larger sheets prefer to sediment due to gravity [44] while smaller GO nanosheets with higher kinetic energy tend to move upward (Brownian motion). Finally, water evaporation results in a compact GO film with asymmetrical structures, which will influence the properties of the prepared graphene film to some degree.

In comparison, a facile self-assembly and air-drying (SAAD) method has been introduced to fabricate GO films by the air drying of rGO hydrogels. As is illustrated in Fig. 1(a), the suspension of GO-ascorbic acid is ultrasonicated for 30 min and then transferred to plastic petri dish in an oven at 70.0 °C. Later the suspension colour turns from brown to black but still homogeneous, indicating the appearance of chemical reduction. The rGO sheets then become hydrophobic owing to the removal of oxygenous groups from the carbon structure. The interaction of π-π stacking and hydrophobic effect promote partial interlocking and overlapping of rGO nanosheets with each other in three-dimensional space to bring about abundant physical crosslink sites to form hydrogel framework [45]. Another hour later the petri dish is moved out of the oven and the rGO membrane will be obtained after air-drying for one day. As explained above, air drying of hydrogel contributes to the uniform and compact layer structure in the rGO membrane, which can be demonstrated by the SEM image in Fig. 1(b). The thickness of the membrane is about 10 μm and it depends on the concentration of the GO dispersion.

Fig. 1.

Fig. 1.   (a) The schematic of fabricating GM. SEM of the cross-section of the layer-by-layer structure of rGOM (b) and GM (c and d).


Annealing is an effective method to remove most oxygen-containing groups of the GO nanosheets. Here graphene membranes are obtained under heat treatment at 1000 °C under the atmosphere of Argon. During the annealing process, the removal of functional groups and release of gas contributes to the expanded layer-by-layer structure of graphene films on the base of the uniform structures. As is shown in Fig. 1(c) and (d), the layers of the membranes are exfoliated by the gas produced in the reduction process and the thickness of the membranes increases to around 100 μm.

3.2. Reduction of the GMs during the annealing process

Fig. 2(a) shows the FT-IR spectrum of GO, rGOM and GM. The C-O band in alkoxy (at 1067 cm-1), C=O in carboxylate/carbonyl group (at 1727 cm-1) and the stretching vibration band of -OH (at 3421 cm-1) illustrate that there exist abundant oxygen-containing groups in GO. Then the peaks of the oxygenous groups are obviously weakened after the chemical reduction with ascorbic acid, while a broad peak at 1569 cm-1 is corresponded to the vibration band of C=C. In comparation with rGOM, the peaks in GM decrease more, which future certifies the reduction of graphene oxide to GM.

Fig. 2.

Fig. 2.   FT-IR spectrum (a), XRD curves (b) and XPS full scan spectra (c) of GM, rGOM and GO. C 1s XPS spectra of (d) GO and (e) rGOM and (f) GM.


XRD patterns of GO, rGOM and GM are displayed in Fig. 2(b). The typical diffraction peak of GO at 12.0° shifts to around 24.5° in the XRD pattern of GM, which reveals irregular stacking of rGO sheets and the appearance of graphite crystals in the obtained GM, suggesting the further reduction of GO. The chemical reduction is also demonstrated by comparing XPS spectra of GO, rGOM and GM (Fig. 2(c)). The two peaks at 286.0 and 533.0 eV are related to Carbon and Oxygen elements, respectively and the elemental ratio of C/O increases with the consecutive reduction from GO to GM, further indicating the removal of oxygen-containing groups in the annealing process. This is also evident when analyzing the peaks for oxygen-related groups in GO, rGOM and GM (Fig. 2(d)-(f)).

3.3. Flexibility and electric conductivity of the obtained membranes

The obtained GM has a high electric conductivity of 3.83 × 104 S/m compared with rGOM (354 S/m), which has a potential to be used in the process of pressure sensors due to its high electrical properties. Better conductivity endows the pressure with better sensitivity under a set voltage. And because of the layer-by-layer structures, the membrane has a good flexibility for applications in electronic device such as pressure sensors.

3.4. Resistance-pressure properties in the pressure sensor

One, five and ten layers of GMs and rGOMs were respectively stacked together to assemble the GM-based pressure sensor (n-GM and n-rGOM, n represents the number of layers) (Fig. 3(a) and (b)). Usually the sensitivity is calculated on the basis of the formula S=Δ(Rc)/ΔP, where Rc = ΔR/R is the change of measured electric resistance and P represents the pressure loaded on the sensor. Several (3-5) sensors with the same numbers of GM layers were fabricated and tested, and the U-I performances of the sensors show reasonable consistence (Fig. S1). The way of stacking GM layers ensures the reliable performance of the pressure sensors, resulting from the layer-by-layer structure of membranes.

Fig. 3.

Fig. 3.   (a) Schematic illustration displaying the structure of the n-GM pressure sensor. (b) Digital pictures of 5-GM pressure sensor formed by five layers of GMs. (The size of the pressure sensor is about 5 mm × 5 mm, and the thickness is about 2 mm.) (c) Response sensitivity comparison of 5-GM and 5-rGOM pressure sensor. (d) Resistance changes with pressure for GM sensor with 1, 5 and 10 layers of graphene membranes.


When comparing the sensitivity of 5-GM and 5-rGOM shown in Fig. 3(c), we can find that the 5-GM shows obvious higher response than the 5-rGOM in all range of pressure (0-50 kPa). On the one hand, higher electric conductivity of GM contributes to a shorter response time of the loader pressure. On the other side, in the SEM images show in Fig. 2(b) and (c), the rGOM is flat and dense, while GM has more holes and air gaps compared with the structures of rGOM, which attribute to higher sensitivity of GM-based pressure sensors. The layer-by-layer structure of GM makes the contact area of the multi-laminate membrane increase rapidly when under loading, which results in sharper decrease of the resistance. The comparation demonstrates the prepared GM can be a better pressure sensor alternative than unannealed rGOM.

Fig. 3(d) shows the sensitivity behaviours of three pressure sensors based on 1-GM, 5-GM and 10-GM in the pressure range of 0-50 kPa. Here pressure sensor with more numbers of GM will have a faster change in the contact area of the interlayers, which results that the sensitivity of the GM sensor increases with the number of GM layers. To be specific, in the range of 0-5 kPa, 10-GM and 5-GM have sensitivity of S3 = 52.36 kPa-1 and S2 = 32.14 kPa-1, which are much higher than that of 1-GM (S1 = 1.38 kPa-1). There exist more air gaps in the pressure sensor with more layers of membranes, and when the tiny pressure is applied to the sensor, the air gaps will decrease and the contact areas will increase largely, which further results in the high sensitivity of the pressure sensor. When compared with the other reported graphene-based pressure sensor, works by other pressure work, the sensitivity in this work is the highest as far as we know [20,23,26]. In the range of 5-20 kPa, the layer structure becomes compact with each other, which makes the change of the contact areas not as fast and results in the decreases of the sensitivity. When the pressure increases more than 20 kPa, the sensitivity will become much smaller but the value of δR still don't reach to more than 70 %, indicating that the long range of the pressure can be detected by the prepared sensor.

The response time is a very important factor for a pressure sensor and here we conduct a measurement of the response time of 5-GM sensor at different rate (0.2 mm/s, 1.0 mm/s and 2.0 mm/s, respectively) under a loading pressure of 1 kPa. As Fig. 4(a) shows, the response time reduces obviously with the increase of the moving rate of the pressure, which also demonstrates the fast sensitivity to pressure. At a rate of 0.2 mm/s, the response time and recovery time of the sensors are 100 and 80 ms, respectively. So, the GM-based pressure sensor is a realistic kind of sensors when the stress changes rapidly, and it is also possible to detect tiny pressure due to its high sensitivity. Stability under different pressure is another important factor to evaluate pressure sensors.

Fig. 4.

Fig. 4.   (a) Response time under different rate (0.2 mm/s, 1.0 mm/s and 2.0 mm/s) of a loading pressure of 1 kPa. (b) Response and Recovery time of 5-GM pressure sensor under a loading pressure of 1 kPa. (c) Test of repeatability characteristics of 5-GM in 6000 s under a pressure of 1.63 kPa. The inserted figure is the enlarged image of the characteristics from 3570-3590 s. (d) Response test of 5-GM at different pressure (300 Pa, 1 kPa, 10 kPa and 20 kPa, respectively).


Fig. 4(c) shows the stability testing results of 5-GM under a loading pressure of 1.63 kPa for 6000 s. The inserted enlarged view illustrates that the n-GM sensor has good repeatability under pressure for a long time, which certifies that the air gaps in the sensors can still reserve stably under repeated pressure, without any gradual degradation of the performance.

3.5. Comparations with other graphene-based pressure sensors

As far as concerned, some graphene-related pressure sensors with good sensitivity and stability have already been reported. As is shown in Table 1, we compared our work with the reported pressure sensors based on graphene. Compared with most other sensors, our pressure sensor exhibits higher sensitivity and wider applied pressure range. As to the loading-unloading cycles, we just show the cycles in 6000 s, and it still shows excellent stability. In addition, more wearable applications are demonstrated in detail as follows, which are not displayed in most other work.

Table 1   Comparations of the properties with graphene-related work.

WorkMaterialsSensitivityPressureStabilityApplications
[23]3DGF-PDMSgauge factor ~98.66-200 cyclesfinger bending
[26]SWCNT/alginate0.176 ΔR/R0 /kPa10 Pa3000 cycleswrist pulse, neck muscle
[20]CNT/graphene19.8 kPa-10.3 kPa3500 cyclesacoustic vibrations, bending, torsion
[14]tissue paper/GO17.2 kPa-10-20 kPa3000 sbreath, human motions
[32]graphene monoliths161 kPa-10-10 kPa-finger movement
[12]multilayer graphene nanoplatelets0.23 kPa-10-70 kPa-wearable health care systems
[13]graphene arrays5.53 kPa-10-1400 Pa250 sinformation transmission
[21]graphene foam---ultrasonic waves
This workgraphene membranes52.36 kPa-10-50 kPa6000 spulse, breath, jumping, walking, squatting

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3.6. Detection of wearable motions

Considering its high sensitivity, large sense range of pressure, excellent repeatability and stability, the n-GM sensor should be well applied in many fields especially the wearable electronics. We mainly use it to detect pulse, respiration, human motions, etc. In the medical fields, pulse rate and breath are very important factor reflecting and evaluate the health state of a person, especially for patients suffering with some heart and pulmonary diseases. Firstly, as shown in Fig. 5(a), the n-GM sensor is tied to a person’s wrist with PI tape and it can show stable response to his pulse beat and the pulse rate is approximately 63 /min (Fig. 5(b)), nearly the same as we measure by ourselves. After exercise, the pulse rate increases to ~81 /min.

Fig. 5.

Fig. 5.   (a) Sensor application for detection of a person’s wrist pulse. (b) Measuring signal of the wrist pulse response before exercise. (c) Wrist pulse response of a person after exercise. (d) Detection curve of the breathing rate.


In addition, it can also be used to measure the breathing rate and it can be fixed to a transparent mask and they the person can wear to test it. It can sense the exhalant gas pressure and will decrease the resistance of the GM sensor. The curves in Fig. 5(d) shows the person’s respiration rate is almost 20 /min, which also conforms to the practical level. What’s more, finger bending has been detected by the GM pressure sensor with great sensitivity and stability (Fig. 6(a)), which further illustrates the GM pressure sensor can be applied as wearable electronics. We put the sensor on the shoe sole and it can be also be used to detect human walking gait. Fig. 6(b)-(d) show the response curves of the person’s different motions of running, walking and squatting. From the curves we can see that the response waveforms of different motions are distinguishable and stable, which illustrates the possibility of the commercial applications to detect human motions.

Fig. 6.

Fig. 6.   (a) The tested signal of real-time finger movement of bending and stretching. The inserted image shows a pulse sensor to monitor the movement. Response curves for the person’s movements of running (b), walking (c), squatting (d).


When comparing the curves in Fig. 4, Fig. 5, Fig. 6, the signals in Fig. 4 are smoother and the characteristic peaks are more obvious. The reason is that in practical applications, lots of factors have the chance to influence the stability and contribute to some cluttered noises, high or low peaks of the performance. This phenomenon also exists in most reported carbon-based pressure sensors [13,14,17,20,22,23]. Whatever, we can still easily find the obvious tendency from the curves, which can satisfy the requirements for the pressure applications. For higher accuracy, further refinements need to be developed when the pressure sensors based on GM are manufactured and used in the market.

4. Conclusion

In conclusion, graphene membranes with a layer-by-layer structure was prepared successfully by a facile self-assembly and air-drying method. Owing to the excellent electrical properties of graphene membranes, the membrane-based pressure sensor exhibits good sensitivity and repeatability in the loading pressure range of 0-50 kPa. As far as concerned, our device shows better sensitivity and wider applied pressure range, when compared with most graphene-related pressure sensors. What’s more, we demonstrate its applications for pulse monitoring, breathing detection as well as different intense motion recording such as walk, run and squat and it shows desired results in wearable devices for the detection of human motions. It’s hoped that the facilely prepared layer-by-layer graphene membrane-based pressure sensors will have great potentials in the modern fields of smart electronics for medical health, human motions and even artificial intelligence and robot technology development.

Acknowledgements

Financial support from the grant from the City University of Hong Kong (SRG 7004918), South China University of Technology (National Key Research and Development Program of China, No. 2016YFB0302000) and Shenzhen University (Ten Thousand People’s Scheme, Project No. 201,810,090,052) are gratefully acknowledged.

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