A flexible 4 × 4 sensor array with 16 micro-scale capacitive units has been demonstrated based on flexible piezoelectric poly(vinylidene fluoride) (PVDF) film. The piezoelectricity and surface morphology of the PVDF were examined by optical imaging and piezoresponse force microscopy (PFM). The PFM shows phase contrast, indicating clear interface between the PVDF and electrode. The electro-mechanical properties show that the sensor exhibits excellent output response and an ultra-high signal-to-noise ratio. The output voltage and the applied pressure possess linear relationship with a slope of 12 mV/kPa. The hold-and-release output characteristics recover in less than 2.5 μs, demonstrating outstanding electro- mechanical response. Additionally, signal interference between the adjacent arrays has been investigated via theoretical simulation. The results show the interference reduces with decreasing pressure at a rate of 0.028 mV/kPa, highly scalable with electrode size and becoming insignificant for pressure level under 178 kPa. Keywords: Piezoelectricity, PVDF film, Tactile pressure, Flexible sensor Background bio-medical field [9, 10]. Sharma et al. designed a pres- Poly(vinylidene fluoride) (PVDF) is a chemically stable sure sensor for smart catheter with PVDF film; it could piezoelectric polymer material that has many applica- be integrated onto a catheter for real-time pressure tions in different fields for its pyroelectric, piezoelectric, measurement . Bark et al. developed a pulse wave and ferroelectric properties [1, 2]. Especially, owing to sensor system to non-intrusively measure heart pulse the outstanding mechanical properties (the Young’s wave signals from driver’s palms based on PVDF; results modulus 2500 MPa and strength at break point ~ show that the sensor system can provide clear pulse 50 MPa), the pressure sensor based on PVDF shows a wave signals for heart rate variability analysis, which can good mechanical property such as flexibility and antifa- be used to detect driver’s vigilant state to avoid traffic tigue [3, 4]. Compared with the commonly used pressure accidents . Lee et al. fabricated a sensor with PVDF sensors based on ferroelectric PZT family materials, the and ZnO nanostructures and it could detect the changes PVDF-based pressure sensor is nontoxic and biocompat- in pressure and temperatures for artificial skin . The ible [5, 6]. Most importantly, the PVDF-based sensor sensor, however, only detects pressure at a single point was more soft and tough than PZT-based sensor due to with large dimension. the high flexibility coefficient of PVDF film, which could Real-world applications, such as patched biosensor for be made the required shapes for complex strain sensing detecting the human body pressure, demand multipoint [7, 8]. Accordingly, the PVDF-based pressure sensor is sensing, structurally flexibility, and ultra-high sensitivity thought to be one of the potential flexible bio-sensor for [14–16]. In this reported work, a 4 × 4 flexible sensor pressure characterization in the rapid development of array based on piezoelectric PVDF film is demonstrated, showing ultra-high sensitivity of 12 mV/kPa and fast output response of 2.5 μs. The magnitude and spatial * Correspondence: firstname.lastname@example.org distribution of the pressure applied on a human finger State Key Laboratory of Electronic Thin Films and Integrated Devices, are characterized. University of Electronic Science and Technology of China, Chengdu 610054, China Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Lu et al. Nanoscale Research Letters (2018) 13:83 Page 2 of 6 Design and Experimental data acquisition with four differential analog signals was Design and Fabrication of the Sensor Array set with differential model. The output voltage signal The proposed sensor array has a sandwich structure based from the proposed sensor was obtained by changing the on a PVDF thin film with the thickness of about 50 μm connection between sensor array and the DAQ. (Jinzhou Kexin Inc., China). The aluminum electrode arrays with the thickness of 20 μmwerecovered on both Results and Discussion sides of the PVDF film. Figure 1a shows a schematic Figure 2a shows the surface morphology of the sensor design of the sensor. The sensor has 16 micro-capacitor after etching of Al, checked by an optical microscope. The units; every 4 units share one connecting wire to minimize fairly bright and dark contrast suggests a clear interface the amount of the electrode wires. between PVDF and the etched Al electrodes. Figure 2b, c To fabricate the sensor array, a slide glass covered with shows the surface morphology and phase signal of the polydimethylsiloxane (PDMS) was prepared as a stiff sub- PVDF film of the pressure sensor. It is indicated that the strate. The PVDF thin film covered by Al on both sides surface of PVDF is smooth with a tissue structure. The was loaded on the substrate. Then, the photoresist was phase image of PFM measure in Fig. 2c shows a strong spin-coated on the surface of the film with a speed of response of the piezoelectric domain which is consistent 3000 rpm for 40 s. After photolithography and wet etching with the surface structure seen in Fig. 2b. These results of Al by a mask aligner system (ABM, Inc., USA), the 16 suggest that the as-prepared sensor based on the PVDF capacitor units with 4 × 4 square structure were prepared. film exhibits a good piezoelectricity. After that, the flexible sensor on the PDMS substrate was A typical result of the output signal is shown in Fig. 3a picked up from the slide glass. The electrodes of each cap- when a constant pressure of 98.1 kPa was applied on one acitor were connected with the conductive wires through of the squared electrode of the sensor . The x-axis and silver glue. In order to obtain good bio-compatibility, the y-axis show the time and the output voltage of the squared sensor was packaged by being covered with PDMS on the electrode of the sensor, respectively. The output voltage top and heated for 12 h at 60 °C. Figure 1b displays a was converted from charge (Q) generated by the PVDF photograph of the bent pressure sensor, illuminating that film of the sensor. Based on the piezoelectricity equation the sensor is flexible. (where d is a piezoelectric constant when the direction of polarization is the same with the direction of electric Piezoelectric Property of the Sensor Array Based on the field and F means pressure is applied on the z-direction PVDF Film with the same direction of d ), a relation between output Piezoresponse force microscopy (PFM) study (Seiko, Inc., voltage and pressure could be established. The raw data Japan) was carried out to characterize the surface morph- were obtained by applying a band block of 49–51 Hz. The ology and piezoelectric properties of the PVDF film of the arrow line of this figure indicates the signals of about proposed sensor under an AC bias voltage of 2 V with a 123.1 mV which was generated by the pressure applied on scanning area size of 2 × 2 μm . the sensor. The output voltage of the sensor by the pres- sure is shown clearly in the signal with low noise and high Calibration for the Sensor Array signal-to-noise ratio. In order to confirm the synchronal To calibrate the sensor, various pressures were applied property of the sensor array, an equal pressure of on the proposed sensor in an electro-mechanical experi- 113.2 kPa was applied on four units of the sensor, simul- mental platform connecting to a data acquisition (DAQ- taneously. The output voltage signals induced by the pres- USB6008) equipment from National Instruments. The sure were showed in Fig. 3b. The nearly same output Fig. 1 a Schematic diagram of the sensor array. b Physical picture of the ultimate device Lu et al. Nanoscale Research Letters (2018) 13:83 Page 3 of 6 Fig. 2 a Surface morphology of the proposed sensor after etching technology. b Surface morphology and c phase PFM images of PVDF film of the sensor value of about 190 mV was obtained from the four units output response curves were shown in Fig. 4b.The sensor of the sensor at the same time, which suggests that the shows a stable characteristic of electro-mechanical response sensor array exhibited a high stability and synchronal with theresponsetimeofabout2ms underdifferent pres- property by applying multipoint pressure. For calibrating sures, and the output voltages of the sensor under different the sensor array, different pressures in the range of 60– pressures are consistent with the linear calibration curve 150 kPa were applied on the sensor array; the output volt- obtained above. age vs. the applied pressure were obtained and plotted as Next, application of pressure on selective point is studied. the calibration curve shown in Fig. 3c, which exhibits a Signal interference is shown between the adjacent arrays, linear relationship. The slope of the linear curve is about when pressure was applied on the electrode of one of the 2.9 mV/kPa, and there is an offset of − 159.2 mV in the arrays. The simulation of signal interference was conducted calibration curve. via COMSOL Multiphysics on arrays. Each electrode area The hold-and-release output response of one squared is 1.4 mm . The geometry of the structure is shown in electrode of the sensor was obtained by applying an im- Fig. 5a. The additional strain, when pressure was applied on pulse pressure with various frequencies. The plotted curve electrode A, is seen in Fig. 5b, indicating the strain in Fig. 4a shows the typical response of the sensor by apply- increases with the distance away from electrode A. The ingthe impulsepressureofabout 75.1 kPawithafrequency interference in potential difference with a pressure level of of 90 Hz. The positive output voltage corresponds to the 20~80 kPa was studied, shown in Fig. 5c. The potential dif- compression of the electrode square of the sensor array, ference and pressure exhibit a linear relationship with a −4 and the negative output voltage corresponds to the relax- slope of 0.028 mV/kPa and an intercept of 5 × 10 mV, im- ation. As seen in the inset of Fig. 4a, the similar hold-and- plying very-low-level interference. A pressure under release output response has also been observed in the bare 178 kPa would generate signal interference less than 5 mV piezoelectric PVDF film . Theresponsetime ofoutput which is negligible [16, 17]. In addition, the dependence of voltage of the sensor is less than 2 ms, which suggests the interference on array electrode size has been investigated. sensor exhibits a good electro-mechanical response prop- Figure 5d shows the result with electrode sizes of 1.2, 1.0, erty. The impulse pressures within the range of 60–150 kPa and 0.8 mm . It shows that a linear relationship between were applied on the sensor array. The hold-and-release interference potential difference and pressure (in the range Fig. 3 Filtered output voltages for a an electrode square and b four electrode squares of the sensor array. c Liner calibration curve fit of the proposed sensor Lu et al. Nanoscale Research Letters (2018) 13:83 Page 4 of 6 Fig. 4 The hold-and-release output response from the pressures of a 75.1 kPa, b 58.2 kPa, c 67.8 kPa, d 81.9 kPa, e 98.1 kPa, and f 153.6 kPa; the inset shows the hold-and-release output response obtained from bare PVDF film of 20~60 kPa) can be still observed in the smallest elec- and distribution of the finger. Figure 6 shows a snap of the trode. The fitting slopes for interface voltage are 0.01748, pressure distribution of the thumb finger characterized by 0.01181, and 0.00574 mV/kPa, respectively, for the three the sensor during the three movements of the finger, structures with the noted observation of reduced interfer- respectively. In Fig. 6a, it could be clearly seen that the ence potential in smaller electrode size. pressure of 76 kPa was focused in the center of the thumb For a simple practical application, the sensor was applied finger during the shiatsu movement, which are quite differ- to measurethe pressurestateand distribution of thefinger ent with the kneading and the rub seen in Fig. 6b, c, of human hand. As we all have known, the complex finger respectively. Figure 6b shows the pressure from the front of movement consists of some basic skills, such as shiatsu, the thumb finger is higher than the other parts of the finger kneading, rub, friction, and so on . In our experiments, during the kneading movement, while the pressure of the three most commonly used movements including shiatsu, thumb finger is fairly uniform (about 68 kPa) during the kneading, and rub were selected to test the pressure state rub movement as shown in Fig. 6c. The observed pressure Fig. 5 a Physical dimensions used for theoretical simulation. b Displacement and c liner curve-fitting between interference voltage and applied pressure with an array size of 1.4 mm. d Obtained results using array sizes of 0.8, 1.0, and 1.2 mm, respectively Lu et al. Nanoscale Research Letters (2018) 13:83 Page 5 of 6 Fig. 6 The pressure state and distribution of the thumb finger movement characterized by the proposed sensor: a the shiatsu, b the kneading, and c the rub distribution in the finger is somewhat similar with the Competing interests The authors declare that they have no competing interests. previous reports in clinical observation [17, 20]. According to our measurements, the strain sensor based on flexible ferroelectric PVDF film prove to be sensitive for Publisher’sNote characterize the complex finger movement. It is expected Springer Nature remains neutral with regard to jurisdictional claims in to explore the skill of the human finger more precisely by published maps and institutional affiliations. using the proposed sensor, and it would also be helpful to Author details develop the robot to replace human fingers in the future. 1 State Key Laboratory of Electronic Thin Films and Integrated Devices, In conclusion, a 4 × 4 sensor array with 16 capacitor University of Electronic Science and Technology of China, Chengdu 610054, China. State Key Laboratory of Luminescent Materials and Devices, South units based on the piezoelectric PVDF thin film has been China University of Technology, Guangzhou 510006, China. AML, fabricated and packaged with PDMS. The sensor array Department of Engineering Mechanics, Tsinghua University, Beijing 100084, exhibits flexible and high sensitive properties. The hold- China. College of Nanoscale Science and Engineering (CNSE), State University of New York, Albany, NY 12203, USA. and-release output response of the sensor was obtained by applying impulse pressures with various frequencies, Received: 1 February 2018 Accepted: 2 March 2018 which indicated the sensor array could generate 20– 300 mV voltage signals within 2 ms when applying a pres- sure in the range of 60–150 kPa. The obviously different References pressure distributions in the finger during the finger 1. Harris GR, Preston RC, DeReggi AS (2000) The impact of piezoelectric PVDF on medical ultrasound exposure measurements, standards, and regulations. movement of human hand have been observed by using IEEE Trans Ultrason Ferroelectr Freq Control 47(6):1321–1335 the proposed sensor, which is expected to explore the skill 2. 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ZYGX2016J055 J Appl Polym Sci 132(13) and ZYGX2016KYQD131), and the Technology Innovative Research Fund of 6. Shu FF (2007) Application of PVDF piezoelectric-film sensor to plantar Sichuan Province of China (Grant No. 2015TD0005). This work was also pressure measurement. In 6th China International Silk Conference/2nd supported by the National Natural Science Foundation of China (Grant No. International Textile Forum, pp 322–326 61474016). 7. Liu X, Liu S, Han MG, Zhao L, Deng H, Li J, Zhu Y, Krusin-Elbaum L, O’Brien S (2013) Magnetoelectricity in CoFe O nanocrystal-P (VDF-HFP) thin films. 2 4 Availability of data and materials Nanoscale Res Lett 8(1):374 The datasets generated during and/or analyzed during the current study are 8. Jo SH, Lee SG, Lee YH (2012) Ferroelectric properties of PZT/BFO multilayer available from the corresponding authors on reasonable request. thin films prepared using the sol-gel method. Nanoscale Res Lett 7(1):54 9. 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Nanoscale Research Letters – Springer Journals
Published: Mar 14, 2018
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