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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 25034−25042
All Paper-Based Flexible and Wearable Piezoresistive Pressure Sensor Lei Gao,† Chengxian Zhu,† Lin Li,† Chengwu Zhang,† Jinhua Liu,*,† Hai-Dong Yu,*,† and Wei Huang*,†,‡ †
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Institute of Advanced Materials (IAM) & Key Laboratory of Flexible Electronics (KLOFE), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P. R. China ‡ Xi’an Institute of Flexible Electronics, Northwestern Polytechnical University, 127 West Youyi Road, Xi’an 710072, P. R. China S Supporting Information *
ABSTRACT: Flexible and wearable pressure sensors are of paramount importance for the development of personalized medicine and electronic skin. However, the preparation of easily disposable pressure sensors is still facing pressing challenges. Herein, we have developed an all paper-based piezoresistive (APBP) pressure sensor through a facile, costeffective, and environmentally friendly method. This pressure sensor was based on a tissue paper coated with silver nanowires (AgNWs) as a sensing material, a nanocellulose paper (NCP) as a bottom substrate for printing electrodes, and NCP as a top encapsulating layer. The APBP pressure sensor showed a high sensitivity of 1.5 kPa−1 in the range of 0.03−30.2 kPa and retained excellent performance in the bending state. Furthermore, the APBP sensor has been mounted on the human skin to monitor physiological signals (such as arterial heart pulse and pronunciation from throat) and successfully applied as a soft electronic skin to respond to the external pressure. Due to the use of the common tissue paper, NCP, AgNWs, and conductive nanosilver ink only, the pressure sensor has advantages of low cost, facile craft, and fast preparation and can be disposed off easily by incineration. We believe that the developed sensor will propel the advancement of easily disposable pressure sensors and green paper-based flexible electronic devices. KEYWORDS: pressure sensors, paper-based electronic devices, nanocellulose paper, wearable, piezoresistive
1. INTRODUCTION Pressure sensors could be attached to the skin or on clothes to monitor physiological signals or external pressure under the continuous working condition without interrupting or limiting humans’ normal activities.1,2 They are of critical importance for plentiful applications such as flexible electronic skins,3−5 breath monitoring,6 heartbeat supervising,7,8 and acquisition of other physiological signals.9,10 Over the past few years, a majority of pressure sensors have been built by using piezocapacitive,11 piezoelectric,12−14 triboelectric,15,16 and piezoresistive effects.17 Among them, piezoresistive pressure sensors are promising to achieve the goal of supervising vital human signals and realizing real-time personalized medicine due to their advantages of facile craft, low power consumption, and high sensitivity in the low-pressure range.18,19 However, most pressure sensors usually use a nondegradable artificial polymer for the encapsulation of devices and for the preparation of the sensing material and bottom substrates.20−22 Given the pressing issue of electronic waste caused by inapposite discard of obsolete devices,23 green pressure sensors that can be easily disposed off are in urgent demand.24,25 © 2019 American Chemical Society
As a natural biodegradable material, paper has been used in everyday life for more than 2000 years due to its excellent attributes including flexible, low-cost, abundant, biocompatible, and environmentally friendly properties.26 A new surge of interest in flexible electronic devices based on micro-/ nanostructured paper has arisen recently.27−29 The microstructured paper with a porous and rough structure can be used as an active material for flexible electronic devices after a special treatment.30−33 A highly transparent nanostructured paper with a smooth surface, also called nanocellulose paper (NCP), has been used as substrates for flexible electronic devices in recent years34−36 because it can solve the problem that common paper with high surface roughness will decline conductivity of electrodes.37,38 Although several paper-based pressure sensors have been reported, these pressure sensors with a paper-based sensing material need a petroleum-based polymer as an encapsulating layer and the fabrication of Received: April 29, 2019 Accepted: June 25, 2019 Published: June 25, 2019 25034
DOI: 10.1021/acsami.9b07465 ACS Appl. Mater. Interfaces 2019, 11, 25034−25042
Research Article
ACS Applied Materials & Interfaces
drying procedure, the sides of the hydrogel sheet were attached to the PMMA by adhesive tapes to prevent shrinkage. 2.3. Conductive Tracks and Interdigitated Electrodes. The ink of a commercial refill pen (0.5 mm line width) was removed thoroughly, and then, the pen was applied with ultrasound in ethyl alcohol for 2 h to wash off the pristine internal oil and carbon ink. Afterward, the clear refill pen was blow-dried by an air blower and the conductive nanosilver ink was injected into its interior to form a conductive ink pen for further direct writing. Here, we used glass slides as spacers and exploited the ruler to facilitate the writing. The tracks can be easily written on different substrates including PET, printing paper, and NCP under the normal writing speed. Afterward, the designed pattern was written and the substrate was put instantly into a drying oven to be annealed at 140 °C for 10 min to form final conductive tracks. After annealing, the configuration of NCP will not change (Figure S1). The interdigitated electrodes were made on the nanocellulose paper by the same method and under the same conditions. The width of conductive tracks was 0.5 mm, and the interval width between adjacent finger electrodes was 1.5 mm. With the facile direct writing method, 10 finger electrodes were made in each interdigitated electrode. Then, conductive copper foil tapes were fixed at the end of the interdigitated electrode. 2.4. Fabrication of the Sensing Material and Pressure Sensor. The original tissue paper was tailored into 1 cm × 1 cm square pieces and then immersed in 2.5 mg/mL silver nanowire aqueous dispersion for 20 s. Wet tissue paper was put on a plastic Petri dish and was flattened by tweezers to prevent from deformation during drying. These wet tissue papers were annealed in a drying oven at 70 °C for 5 min to evaporate water solution. After drying, the color of these tissue papers turned from white to light gray and then these AgNWs/tissue papers were applied as a sensing material for the pressure sensor. Adhesive NCP was prepared by coating oil-based glue on the surface of pure NCP. AgNWs/tissue paper was put on the center of interdigitated electrodes and then encapsulated by adhesive NCP under appropriate pressure to form a final pressure sensor for further testing. To facilitate the measurement of devices, copper foil tape was connected to the end of interdigitated electrodes. 2.5. Characterization. Digital photographs of conductive tracks and devices were obtained by a Canon camera. The features and folding ability of conductive silver tracks were observed by an optical microscope. Surface morphology analysis of different substrates with silver tracks was performed on a scanning electron microscope (SEM; JSM-7800F, JEOL Corp.). The electrical resistance of silver tracks on substrates and AgNWs/tissue paper was measured by a four-point probe instrument (ST2253, Suzhou Jingge Electronic Corp.). For the device characterization, the I−t data was collected by an electrochemical workstation that was connected to a computer. The quite time was 0.02 s, and the working voltage was 0.1 V. To distribute the external pressure uniformly, the 18 mm × 18 mm thin cover glass was first put on top of the pressure sensor before testing. The adhesive NCP encapsulating layer and cover glass can apply small pressure to the sensor, which caused the original based current (I0) to be 6 × 10−7 A at the fixed working voltage of 0.1 V. The response to the external pressure was tested by applying a sequence of metal weights on the top of the sensor. The objects (1, 5, 10, 20, 50, 100, 200, 300, 500, 700, and 1000 g) can produce corresponding pressure (30, 151, 302, 605, 1512, 3025, 6049, 9074, 15 123, 21 172, and 30 247 Pa) on the cover glass surface. The flattened sensor was touched by a finger under different forces to show the response capacity of touch. To monitor the adult human pulse and pronunciation, the pressure sensor combined with a band-aid was attached to the wrist and throat, respectively. Based on the pressure sensor, 3 × 3 sensor arrays were assembled to form a soft electronic skin for monitoring the external touch pressure.
interdigitated electrodes and sensing materials needs expensive instruments and complex craft, which hinder the development of an easily disposable, low-cost, and green paper-based pressure sensor.39 In this work, we have developed an all paper-based piezoresistive (APBP) pressure sensor with low-cost, facile craft, and fast preparation advantages. The APBP pressure sensor was constructed by the assembly of three components: NCP with a smooth surface was used as a flexible substrate on which silver interdigitated electrodes were printed by a direct writing method; a tissue paper with a porous structure and a rough surface was used as a sensing material with coating of silver nanowires (AgNWs) by a dip-drying method; and adhesive NCP was used as the top encapsulating layer to replace the conventional petroleum-based polymer. The obtained pressure sensor exhibited excellent performance including a high sensitivity of 1.5 kPa−1 (0.03−30.2 kPa), an ultralow energy consumption (∼10−8 W), a low working voltage (0.1 V), and a rapid response time (90 ms). The pressure sensor was successfully applied to monitor human physiological signals including finger touch, arterial heart pulse, and pronunciation and used to prepare a low-cost soft electronic skin to respond to the external pressure. Also, the APBP pressure sensor can be disposed off easily by incineration. We believe that the proposed APBP pressure sensor should find profound applications in healthcare and flexible electronic skin. This work will pave the way to build low-cost and easily disposable paper-based pressure sensors and will impel the development of green flexible electronic devices.
2. EXPERIMENTAL SECTION 2.1. Materials. Cotton linter pulps (degree of polymerization
