Flexible, Highly Sensitive, and Wearable Pressure and Strain Sensors

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Flexible, Highly Sensitive, and Wearable Pressure and Strain Sensors with Graphene Porous Network Structure Yu Pang,† He Tian,‡ Luqi Tao,† Yuxing Li,† Xuefeng Wang,† Ningqin Deng,† Yi Yang,† and Tian-Ling Ren*,† †

Institute of Microelectronics, Tsinghua University, 100084 Beijing, China Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089, United States

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S Supporting Information *

ABSTRACT: A mechanical sensor with graphene porous network (GPN) combined with polydimethylsiloxane (PDMS) is demonstrated by the first time. Using the nickel foam as template and chemically etching method, the GPN can be created in the PDMS-nickel foam coated with graphene, which can achieve both pressure and strain sensing properties. Because of the pores in the GPN, the composite as pressure and strain sensor exhibit wide pressure sensing range and highest sensitivity among the graphene foam-based sensors, respectively. In addition, it shows potential applications in monitoring or even recognize the walking states, finger bending degree, and wrist blood pressure. KEYWORDS: graphene network, porous structure, piezoresistivity, pressure sensor, strain sensor ressure and strain sensors with flexible, stretchable, and wearable characteristics have shown great potential applications in physiological activities, artificial e-skins, and health care monitoring/diagnosis.1−3 There are three main transduction mechanisms, capacitance, piezoelectricity, and piezoresistivity, used for converting applied force into different electrical signals, in which the piezoresistive sensor displays the superiorities of simple device structure, large measurement range, and high sensitivity.2,4,5 Moreover, most of the piezoresistive sensors exhibit only resistance variation responding to the pressure or strain stimulation,6−10 and few of them can be used as both pressure and strain sensors.11 However, sensors with multifunctional properties are essential to broadening their applications. For this type of piezoresistive sensors, generally, the wearable pressure and strain sensors are composed of conductive materials and flexible polymer substrates. PDMS, an intrinsic elastic and extensible material with high transparency, responds readily to tensile, torsion, and compression and has been widely used as a flexible substrate for various sensors.6,11,12 For the conductive materials, different kinds of carbon materials, such as carbon blacks,13 carbon nanotubes,14 and graphene,15−17 are the most promising active materials employed in the pressure and strain sensors. Because of the remarkable conductivity, extraordinary

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elasticity, and stiffness properties of the graphene it exhibits capacity of fabricating high-performance sensors with high sensitivity, fast response time, and good linearity between applied force and resistance variation.16−19 Recently, the graphene foams prepared by reduced graphene oxide (rGO) or chemical vapor deposition (CVD) have drawn great attention in fabricating stretchable and flexible pressure and/or strain sensors.11,15,17−19 Nevertheless, all the porous structure can not be effectively left in the sensors after the polymers infiltrated into the as-prepared graphene foam,15,17,18 whereas the pores can achieve enhanced sensing performance.4,20 Consequently, it is critical to establish a route to keep a graphene structure with pores for the sensor utilization. Herein, we employ the chemical solution to etch the nickel foam template coated with multilayer graphene, and the GPN can remain in the PDMS substrate. To best of our knowledge, this work is the first demonstration of in situ GPN prepared in the polymer and used for the pressure and strain application. Importantly, the porous structure can induce more effect on the Received: July 5, 2016 Accepted: September 29, 2016

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DOI: 10.1021/acsami.6b08172 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic process for fabricating pressure and strain sensors with the GPN structure; (b) XRD pattern and (c) Raman spectrum of nickel foam coated with graphene by CVD; (d) photograph of a bent GPN-PDMS composite; (e) SEM image of the nickel foam coated with graphene and (f) magnified SEM image of one branch clearly showing graphene layers covered on the nickel foam; (g) SEM image of the GPN-PDMS composite and (h) magnified SEM image of a typical connected network with three branches.

The scanning electron microscopy (SEM) image of the nickel foam coated with graphene (Figure 1e) show that the nickel skeleton owns a width of 20−70 μm and large pore sizes of 100− 350 μm. The magnified SEM image in Figure 1f clearly shows that the layer-by-layer graphene has been deposited on the nonplanar nickel surface, and the thickness of graphene layer is about 1 μm. After immersed into the PDMS solution and cured, no obvious damage on the coated graphene can be observed. It can be seen from Figure 1g that the three-dimensional GPN has been left in the PDMS substrate after chemically etching. Figure 1h shows a typically interconnected network structure with three graphene branches, and three parts including PDMS, graphene and pore space contained in the composite. The PDMS provides the flexible substrate with high elasticity, the graphene as the conductive material for the resistance variation, and the pore offer space for graphene connecting or not. Therefore, the GPN structure in the PDMS substrate can be easily achieved by this approach. The relative resistance variation versus pressure of GPNPDMS composite is shown in Figure 2a. Interestingly, the pressure sensor exhibits a decreased resistance with applied force increasing (see Figure S4), which is contrary to the graphene foam pressure sensor reported by Samad.17 It can be seen that the pressure sensor has two linear ranges, approximate 90% and 7% resistance decrease for the pressure ranges of 0−1000 and 1200− 1800 kPa, respectively. Although the GPN-PDMS composite as pressure sensor owns low sensitivity ((R − R0)/R0/pressure, where R and R0 represent resistance at loading and unloading, respectively) in the low pressure range, about 0.09 kPa−1, it is still a very high sensitivity compared to the sensitivity of 2.3 × 10−4 and 6.7 × 10−4 kPa−1 of reported pressure sensors with a large measurement range.11,21 The device exhibits an immediate response to external loading and unloading with the rising and dropping time of ∼100 and 80 ms, respectively, see Figure S5.

conductive path of graphene network, leading to a high sensitivity of 0.09 kPa−1 up to 1000 kPa for the pressure sensor and the highest sensitivity of 25.6 among the reported strain sensors with graphene foam. In addition, the sensors exhibit practical application in monitoring and recognizing human physiological activities. The fabrication process of GPN-PDMS sensor is shown in Figure 1a. First, the multilayer graphene were grown on the nickel foam template by the CVD method using the methane as carbon source, see the details in the Supporting Information. A prepolymer of PDMS and cross-linker was well-mixed, and then the graphene coated nickel foam was immersed into the prepolymer. Noted that the solution must submerge all the foam surface to ensure pores infiltrated with PDMS completely. To remove the air bubbles in the solution, it was put inside the vacuum desiccator for 1 h. The sample was cured in a hot plate at 100 °C. Because of the rigid mechanical properties of metal nickel, it can not achieve a flexible, compressible, and stretchable piezoresistive sensor. Thus, we used the hydrochloric acid to remove the nickel skeleton, and no residual nickel left in the composite (see Figure S1). Before the chemical etching, it is noted that the redundant PDMS need to be over cut for good contact between the nickel foam and acid solution. The XRD pattern of nickel foam coated with graphene in Figure 1b shows three strong nickel characteristic peaks and a weak graphene peak, indicating that a small amount of graphene has grown on the nickel foam. Moreover, the Raman spectrum exhibits two peaks at 1581 and 2700 cm−1, corresponding to the characteristic peaks of G and 2D of graphene, respectively. However, the defect-related D peak is not detected, which may be strongly suppressed by the high quality of graphene grown on the nickel foam.15 After removing the nickel skeleton, the composite displays good flexibility and can undertake bending, torsion and stretch, see the Figure 1d and Figure S2. B

DOI: 10.1021/acsami.6b08172 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 2. Relative resistance variation of the GPN-PDMS composite with (a) applied static compressive loading and (b) stretchable loading, the insets show the working mechanism response to the applied pressure and strain; the relative change on resistance of the GPN-PDMS composite with applied (c) press and (d) stretching cycles.

Figure 3. Signal variations of relative resistance corresponding to different (a) walking and (b) bending states, and (c) wrist blood pressure.

To investigate its promising practical applications in physiological activities, we have used the sensor to recognize the walking state and finger motion, and to monitor the blood pressure. So far, the wearable strain sensors on knee rather the pressure sensor under the foot has been widely applied to monitor the walking state because of low measuring range of the reported pressure sensors.2,7,8 As shown in Figure 3a, a pressure sensor was put on the heel and fixed by the sock. The resistance shows a decreased resistance response to the human walking (with a weight of 60 kg). The relative changes in resistance exhibit clearly difference signals for the walking and jogging, which can be recognized by the signal sharpness, signal frequency, and peak intensity. Interestingly, it has a very sharp peak to the impact of jogging, and the resistance display about 90% decline. The results indicate that the GPN structure as pressure sensor exhibits high robustness and rapid response time. Figure 3b shows the resistance variation corresponding to finger-bending motion. It can be seen that at the slow bending speed the resistances have about 12 and 25% increase for 30 and 90° bending of thumb finger, respectively. Particularly, an

The working mechanism in the inset shows that more graphene conductive paths can be established when the external force applied, leading to a resistance decline. The resistance variation versus strain of GPN-PDMS composite (Figure 2b) also displays two linear regions within strain range of 0−18% and 22−40%, and the sensitivity ((R − R0)/R0/strain, where R and R0 represent resistance under stretch and 0% strain) are 2.6 and 8.5, respectively. Obviously, the sensitivity of GPN-PDMS composite as strain sensor is much higher than that of pressure sensor. This is due to the fact that it is easy to establish nonconductive path caused by large deformation in the length direction with respect to establish connected path in the height direction. To best of our knowledge, the sensitivity value of 8.5 is the highest one among the reported graphene foam based strain sensors.15,17 It can be seen from the inset of Figure 2b that the conductive network paths exhibit a sharply decrease after the force applied in the plane direction, which shows opposite working mechanism compared to the pressure sensor. The loading−unloading press and stretching cycles of the GPN-PDMS composite at 1666 kPa and 25% strain, respectively, are shown in Figure 2c, d. C

DOI: 10.1021/acsami.6b08172 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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approximate linear response between the increased resistance and bending degree can be achieved during the slow bending motion. Moreover, the sensor can also distinguish the repaid finger bending, which shows increased frequency for the resistance variation. It is noted that the resistance variation exhibits a decrease as a result of finger compression when it returns back to the horizon state. Compared with some other piezoresistive sensors,14,16,22,23 our sensor that uses GPN as conductive filler can not only distinguish bending frequency but also respond to bending degree, which makes it a potential artificial skin to human−machine interaction. We further investigated the blood pressure by fixing the sensor on the wrist, and the result shown in Figure 3c. The wrist pulses, about 78 beats per minute, can be counted via the relative resistance variation. In particular, some of the peaks display obvious three subtle peaks, representing radial artery pulse waveform (P1 and P3) and systolic augmentation shoulder (P2),6 which exhibits much higher resolution to wrist pulse than that of rGO graphene foam based sensors.17 In conclusion, we have reported a simple route to fabricate GPN structure retained in the PDMS composite as pressure and strain sensors. The pressure sensor displays a huge pressure sensing range up to 2000 kPa, whereas the strain sensor displays the highest sensitivity of 8.5 among the graphene foam based sensors. Moreover, they exhibit promising practical applications in monitoring or even recognizing the physiological activities, including walking state, finger bending, and wrist blood pressure.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08172. Detailed experimental section, characterization, photographs of PDMS-Ni foam composite, photographs of twisty and stretchable sensor, SEM images, demo of a LED, response time, and cyclic stress−strain curves (PDF)

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86(010)82316192. Author Contributions

Y. P. conceived the project and conducted the experiments. H. T., L.-Q. T., Y. -X. L. X.-F.W., N.-Q. D., and Y. Y. assisted in the experiments, analysis, and discussions of the results. Prof. T.-L.R. supervised the project. All authors contributed to the writing of the manuscript and gave approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation (61574083, 61434001), National Basic Research Program (2015CB352101), National Key Research and Development Program (2016YFA0200404), National Key Project of Science and Technology (2011ZX02403-002), and Special Fund for Agroscientific Research in the Public Interest of China (201303107). Also thankful for the support of the Independent Research Program (2014Z01006) of Tsinghua University, and Advanced Sensor and Integrated System Lab of Tsinghua University Graduate School at Shenzhen (ZDSYS20140509172959969). D

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ACS Applied Materials & Interfaces (20) Tang, Y.; Zhao, Z.; Hu, H.; Liu, Y.; Wang, X.; Zhou, S.; Qiu, J. Highly Stretchable and Unltrasensitive Strain Sensor Based on Reduced Graphene Oxide Microtubes-Elastomer Composite. ACS Appl. Mater. Interfaces 2015, 7, 27432−27439. (21) Graz, I.; Krause, M.; Gogonea, S.; Bauer, S.; Lacour, S.; Ploss, B.; Zirkl, M.; Stadlober, B.; Wagner, S. Flexible Active-Matrix Cells with Selectively Poled Bifunctional Polymer-Ceramic Nanocomposite for Pressure and Temperature Sensing Skin. J. Appl. Phys. 2009, 106, 034503. (22) Park, J.; Lee, Y.; Hong, J.; Ha, M.; Jung, Y. D.; Lim, H.; Kim, S. Y.; Ko, H. Giant Tunneling Piezoresistance of Composite Elastomers with Interlocked Microdome Arrays for Ultrasensitive and Multimodal Electronic Skins. ACS Nano 2014, 8, 4689−4697. (23) Gong, S.; Lai, D. T. H.; Su, B.; Si, K. J.; Ma, Z.; Yap, L. W.; Guo, P.; Cheng, W. Highly Stretchy Black Gold E-Skin Nanopatches as Highly Sensitive Wearable Biomedical Sensors. Adv. Electron. Mater. 2015, 1, 1400063.

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DOI: 10.1021/acsami.6b08172 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX