Facile, Flexible, Cost-Saving, and Environment-Friendly Paper-Based

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Functional Nanostructured Materials (including low-D carbon)

A Facile, Flexible, Cost-Saving and Environment-Friendly Paper-Based Humidity Sensor for Multifunctional Applications Zaihua Duan, Yadong Jiang, Mingguo Yan, Si Wang, Zhen Yuan, Qiuni Zhao, Ping Sun, Guangzhong Xie, Xiaosong Du, and Huiling Tai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05709 • Publication Date (Web): 28 May 2019 Downloaded from http://pubs.acs.org on May 29, 2019

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ACS Applied Materials & Interfaces

A Facile, Flexible, Cost-Saving and Environment-Friendly Paper-Based Humidity Sensor for Multifunctional Applications

Zaihua Duan,† Yadong Jiang,† Mingguo Yan,‡ Si Wang,† Zhen Yuan,† Qiuni Zhao,† Ping Sun,§ Guangzhong Xie,† Xiaosong Du,† Huiling Tai*,†

†

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of

Optoelectronic Science and Engineering, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, P.R. China ‡

College of Science, Sichuan Agriculture University, Yaan 625014, P.R. China

§

College of Optoelectronic Engineering, Chengdu University of Information Technology,

Chengdu 610225, P.R. China

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ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Developing a facile, cost-saving and environment-friendly method for fabricating a multifunctional humidity sensor is of great significance to expand its practical applications. However, the most of humidity sensors involve complex fabrication process, resulting in their high cost and narrow application fields. Herein, a multifunctional paper-based humidity sensor with many advantages is proposed. This humidity sensor is fabricated using conventional printing paper and flexible conductive adhesive tape by a facile pasting method, in which the paper is used as both the humidity sensing material and the substrate of the sensor. Owing to the moderate hydrophilicity of the paper and the rational structure design of the paper-based humidity sensor, the sensor exhibits an excellent humidity sensing response of more than 103 as well good linearity (R2 = 0.9549) within the humidity range from 41.1% to 91.5% relative humidity. Furthermore, the paper-based humidity sensor is of good flexibility and compatibility, endowing it with multifunctional applications for breath rate, baby diaper wetting, noncontact switch, skin humidity and spatial localization monitoring. Although the resistance of the paper-based humidity sensor is relatively large, the humidity sensing response signals of the sensor can be conveniently processed by the designed signal processing system. The readily available starting materials and facile fabrication technique provide useful strategies for the development of multifunctional humidity sensors. KEYWORDS: paper electronics, humidity sensor, multifunction, flexibility, cost-saving, environment-friendly

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1. INTRODUCTION Well-known humidity sensor plays an important role in controlling the living systems and monitoring the industrial process.1−4 With the in-depth research, the novel applications of the humidity sensor have expanded to other fields including breath rate,4−6 noncontact interface localization,7,8 noncontact

switch,9,10 skin humidity11,12

and

baby diaper wetting

monitoring13,14. However, in order to obtain high performance (e.g., good flexibility, high sensitivity and fast response/recovery speeds) humidity sensors to meet these special applications mentioned above, the fabrication processes of these humidity sensors usually involve complex micro/nano structure materials synthesis route, advanced device assembly process and even poisonous chemical reagents.4−14 In general, in addition to requiring the humidity sensor to be of excellent humidity sensing properties, a humidity sensor applied to special occasions (e.g., breath rate, skin humidity and baby diaper wetting monitoring) should also require the following points: i) flexible for wearable demand; ii) nontoxic to be used in human body; iii) disposable to avoid the cross infection or contamination in patient breath rate and baby diaper wetting monitoring; iv) low cost and facile fabrication technology to meet the needs of the general public; v) degradable and non-polluting for environment. Therefore, it is significant to develop a humidity sensor bearing these essential rules to achieve special applications. Paper materials comprised of cellulose have been attracted remarkably increasing research and commercial interests for the flexible electronics (e.g., sensors,15−18 supercapacitors,19,20 transistors,21 and nanogenerators22−24) due to their superiorities including low price, natural abundance, light weight, mature manufacturing process, specific structural properties, 3



favorable mechanical bendability, biocompatibility and nontoxicity over their counterparts (e.g., silicon wafer, polyethylene terephthalate (PET) and polyimide (PI)).25−30 Paper materials usually play the substrate role in paper-based electronic systems, and the electrical properties and mechanical/chemical stability of paper-based electronics are susceptible to environmental moisture due to the hydrophilic properties of the cellulose-rich paper, so researchers have to take many measures to avoid the influence of humidity on paper-based electronics.29−31 Inspired by the water molecules absorption characteristic of the paper, with converse thinking, George M. Whitesides and co-workers fabricated a humidity type paper-based electrical respiration sensor via a digitally printing craft robot in 2016, and this work paves the way for the development of the paper-based humidity sensors.6 Nevertheless, one of prominent limitations as mentioned in the conclusion of above work is that the electrodes of the paper-based humidity sensor are prone to cracking if the paper is folded,6 so the compatibility between the traditional rigid electrodes and flexible paper material needs to be considered.6,27 Although the conductive pencil trace is of good compatibility with paper and can be regarded as a good candidate for paper-based device electrodes, the electrical properties of the pencil trace is vulnerable to bending.15,29 In addition, there are still other problems to be solved in the practical applications for paper-based humidity sensors. Firstly, the fabricating technologies such as digital printing electrode technology involved in the above work are strict and expensive.6 Secondly, the advantages and multifunctional applications of the paper-based humidity sensors need to be further explored. Thus, developing a facile fabricating method and choosing a suitable flexible electrode material for improving the performance of the paper-based humidity sensors are challengeable and highly desired. 4


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In this work, a multifunctional humidity sensor based on paper material and flexible conductive adhesive tape electrodes is proposed. Via the rational materials selection and facile pasting method, as-fabricated paper-based humidity sensor exhibits excellent humidity sensing properties which the response is more than 103 within the humidity range from 41.1% to 91.5% relative humidity (RH), and the multifunctional applications (e.g., breath rate, baby diaper wetting, noncontact switch, skin humidity and spatial localization monitoring) of the paper-based humidity sensor were studied. Moreover, the signal processing system was designed for this paper-based humidity sensor. Given such features as flexible, cost-saving, eco-friendly, multifunctional, as well as facilely to fabricate, this paper-based humidity sensor will be of great advantages and potential in practical applications.

2. RESULTS AND DISCUSSION 2.1. Characterization. Figure 1a shows the fabrication process of the paper-based humidity sensor. It consists of two simple steps: Firstly, two polyester conductive adhesive tapes and two Al wires were pasted on the surface of the paper to form two electrodes (electrodes distance: 0.5 mm), and it is worth mentioning that the polyester conductive adhesive tape with excellent flexibility can ensure the good compatibility and stability between the electrodes and flexible paper material. Then, the PI adhesive tape with high insulation and temperature stability (260 C) was used to encapsulate the sensor for enhancing its mechanical bendability. Figure 1b−d displays the surface and cross-section scanning electron microscope (SEM) images of the sensor. As shown in Figure 1b, the gap distance between two electrodes is about 0.5 mm which is consistent well with the design parameter, and the polyester conductive adhesive tape is composed of unique fabric structure which 5



contributes to improve the mechanical strength of the electrodes. The paper is composed of a typical cellulose fibers network-like structure as shown in Figure 1c. It can be observed that the interfaces among the paper, electrodes and encapsulation layers are closely combined from the cross-section SEM image in Figure 1d, which are favorable to the conduction and stability of the sensor. The chemical compositions of the paper surface were characterized by X-ray photoelectron spectroscopy (XPS), and the survey spectrum of the paper in Figure S1 contains the expected elemental peaks including C and O (note that hydrogen cannot be detected by XPS). The binding energy of O 1s at 532.3 eV and 535.2 eV can be assigned to O–C and –OH/H2O species (Figure 1e), and the C 1s XPS spectrum can be ascribed to C–C/C=C, C–O, C=O and O–C=O species (Figure 1f).32−34 The XPS analysis results show that the paper contains rich hydrophilic oxygen-containing species which are contribute to the water molecules adsorption of the paper.35 In order to further evaluate the hydrophilicity of the paper, the dynamic contact angles of the paper within 20 min were recorded as shown in Figure 1g. At the beginning of 1 s, the droplet spreads on the surface of paper, forming a contact angle of 70.9. Then the contact angle decreases to 55.6 at 10 min and 42.1 at 20 min, respectively. In general, the water contact angle is smaller than 90°then the material surface is considered hydrophilic and the smaller the water contact angle is, the stronger hydrophilicity of the material is.3,36,37 Hence, the paper with a contact angle less than 90 is proved to be hydrophilic. In addition, it is noteworthy that the contact angle of the paper reaches to 42.1 from 70.9 taking a relatively long time of 20 min, indicating a moderate hydrophilicity of the paper.36,37 We know that the hydrophilicity of the humidity sensitive materials has great influence on the 6


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performance of humidity sensors, if the hydrophilicity of humidity sensitive material is too strong, it is not conducive to desorption of the water molecules, resulting in a slow recovery speed of the humidity sensor, on the contrary, if the hydrophilicity is too weak, it is not conducive to adsorption of the water molecules, resulting in the small response value and slow response speed of the humidity sensor. Herein, we can predict that the paper with a moderate hydrophilicity as a humidity sensitive material not only can adsorb water molecules, but also has a good desorption rate for water molecules.

Figure 1. (a) Fabrication process of the paper-based humidity sensor. (b) Surface SEM image of the humidity sensor removed encapsulated PI adhesive tape. (c) Surface SEM image of the paper. (d) Cross-section SEM image of the humidity sensor. XPS spectrums of (e) O 1s and (f) C 1s. (g) Dynamic contact angles of the paper. 7



2.2. Humidity Sensing Performance. Before investigating the various applications of the paper-based humidity sensor, the conventional humidity sensing properties of the sensor were measured by a homemade humidity sensor measuring equipment with the semiconductor characterization system (Figure S2). In order to determine the appropriate bias voltage of the sensor, the output currents of the sensor under different bias voltages at low RH of 7.2% were measured. As can be seen from Figure S3, the humidity sensor is of acceptable signal-to-noise ratio and relatively low power consumption at the bias voltage of 10 V, so the 10 V is used as the bias voltage in the following tests. Figure 2 shows the humidity sensing properties of the humidity sensor. In the linear coordinate system of Figure 2a,b (blue dotted line), the current of the humidity sensor has no obvious changes at the low RH range (7.2%–72.0% RH) while increases dramatically above 72.0% RH, indicating that the humidity sensor is of nonlinear response within the whole humidity range (7.2%–91.5% RH) and a high RH switching point at 72.0% RH (ON/OFF ratio (I91.5% RH/I79.3% RH)  58). Although the paper-based humidity sensor has no obvious response to the RH below 72.0%, it can still satisfy the detection requirements of the high humidity objects such as breath rate and baby diaper wetting detection which their trigger RH (nearly 100%) is higher than 72.0% RH.6,13,14 Meanwhile, the high RH switching point is conducive to avoiding the environmental humidity interference when the sensor is used for the certain special occasions with high RH and so the following application testing results of the humidity sensor are exhibited in the linear coordinate system. In addition, the linear response range of the humidity sensor can be enlarged through proper data processing.7 The semi logarithmic coordinate system of the inset in Figure 2b shows the humidity sensor is of good linear response (R2 = 0.9549) at the humidity range of 41.1% to 8


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91.5% RH and the response is more than 103 (I91.5% RH/I41.1% RH = 1647), which shows a better performance in terms of response values compared with other paper-based humidity sensors in the reports (Table S2),17,38-42 and makes it also suitable for the accurate humidity detection of some special humidity environments above 41.1% RH.

Figure 2. Humidity sensing properties of the humidity sensor. (a) Dynamic response characteristic curves of the humidity sensor at different RH. (b) Current versus RH curves and the inset shows the linear fitting curve of current versus RH. (c) Response and recovery 9



curves for three cycles. (d) Amplified response and recovery curve in the linear coordinate system. (e) Response and recovery characteristics of the sensor between 72.0% and 91.5% RH at the RH switching times of 3, 6 and 9 s. (f) Response curves of three humidity sensors with different electrodes distance of 0.5, 1 and 2 mm. Figure 2c shows the continuous response and recovery curves of the humidity sensor in semi logarithmic coordinate system between 7.2% RH and 91.5% RH, indicating a good response and recovery characteristic of the paper-based humidity sensor. In order to present the response and recovery times clearly, an amplified recovery and response curve in the linear coordinate system is shown in Figure 2d. As can be seen, the response and recovery times of the humidity sensor are calculated to be about 472 s and 19 s, respectively. In terms of response and recovery speeds, the relatively slow response and recovery speeds of the humidity sensor in the whole humidity of 7.2% to 91.5% RH seem to be difficult for some special applications such as breath rate monitoring, which the breath rate of a healthy adult at rest is about 3–6 s per breath interval.43,44 In fact, the response and recovery times of 7.2% to 91.5% RH cannot fully reflect the response/recovery speeds in practical applications because the reference humidity (~70% RH) in the respiratory rate test is much higher than 7.2% RH. In addition, the humidity sensor only needs to make a transient response to expiration and it is not necessary for humidity sensor to completely adsorb and desorb the water molecules of the breath. In order to prove that the paper-based humidity sensor is of potential application for breath monitoring, the response and recovery characteristics of the sensor between 72.0% RH and 91.5% RH were measured at the short RH switching times of 3, 6 and 9 s, respectively. Notably, 72.0% RH here as a reference humidity is use to simulate the high humidity field of 10


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about 70.0% RH within 2 cm around the mouth cavity. As shown in Figure 2e, we can see that the longer of the humidity switching intervals is, the greater of the current change is, which can be easily understood by the relatively slow adsorption and desorption speeds of water molecules. In addition, we notice that the response and recovery curves move downward as the switching intervals become longer. As shown in Figure 2d, the response and recovery times of the humidity sensor are about 472 s and 19 s, respectively. That is to say, the adsorption rate of water molecules is far less than the desorption rate. During the same switching intervals (3, 6, 9 s) of adsorption and desorption, it means that desorption will be more dominant, so the absolute value of the current will be lower with the increase of switching intervals. In sum, the paper-based humidity sensor presents obvious response during quickly switching process of RH, endowing it with potential application for breath rate monitoring. The electrodes gap distance as an important parameter affects the humidity sensing properties of the paper-based humidity sensor, so the response curves of three humidity sensors with different electrodes gap distances (0.5, 1 and 2 mm) are tested as shown in Figure 2f. With the increase of the electrodes distance, the response value of the humidity sensor gradually decreases and the analogous phenomenon occurs on the previous report.17 It has been confirmed that the adsorbed water molecules are discontinuous when the gap between the two electrodes is too large, which will result in a discontinuous carriers’ transport channel even at high RH.45 Considering the humidity sensing performance of those three sensors and the accuracy of manual fabrication process that the electrodes distance of 0.5 mm can be facilely realized manually, this paper mainly studies the humidity sensing properties 11



and applications of the paper-based humidity sensor with the electrodes distance of 0.5 mm. The anti-mechanical bendability is an important parameter for wearable electronics and the bending influence on paper-based humidity sensor is studied as shown in Figure S4. It is noted that the electrodes length of the sensor is extended for expediently bending test and the details of bending test system refers to our previous work.46 It can be seen that the real-time current of the sensor has no obvious change during the bending process (Figure S4a). After different bending times of 0−200 (bending angle: 60), the response and recovery times in linear coordinate system (Figure S4b) are still about 472 s and 19 s, and response values in semi logarithmic coordinate system (Figure S4c) are still more than 103, which mean that the paper-based humidity sensor is of favorable anti-mechanical bendability. In addition, the SEM images of the humidity sensor before bending at flat state and after bending 100 times at bending state reveal that the effects of bending on the gaps among the cellulose fibers can be neglected (Figure S5). The wire-like geometry of cellulose fibers is of great resilience to stress and the deformation process hardly causes the crack formation laterally for its tiny size in diameter,47 so the bending has little influence on the performance of the paper-based humidity sensor. The humidity sensing characteristics of the paper-based humidity sensor depend on the adsorption and desorption of water molecules on the paper. From the chemical structure of the cellulose in paper (Figure 3a), it can be seen that the cellulose contains abundant hydrophilic –OH groups,31,48 which is consistent with the results of XPS analysis (Figure 1e). As shown in Figure 3b, at the first stage of adsorption, the water molecules are adsorbed on –OH groups via hydrogen bond and no proton can be conducted in this stage due to the restriction from the 12


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two adjacent hydrogen bonds in cellulose.49 As the water molecules continue to adsorb on the surface of paper, the second water molecules layer is physically adsorbed on the first water molecules layer and it should be noted that this layer is less ordered than the first layer due to the existence of one hydrogen bond locally.49 With more water molecules layers condensing on the surface of paper, the ordering of the water molecules may gradually disappear and protons will have more and more freedom to move inside the condensed water through the Grotthuss mechanism.49 At low RH condition (7.2%−41.1% RH), only a small amount of water molecules are adsorbed on the surface of the paper to form the one water molecules layer or few water molecules layers, resulting in the discontinuous proton conduction and poor humidity sensing response in low RH.49 With the increasing of RH (41.1%−91.5% RH), on the one hand, the proton conduction becomes easier. On the other hand, H3O+ and OH− ions can be easily produced and form good ionic conductivity in bulk liquid water (H2O → H+ + OH−, H2O + H+ → H3O+, H2O + H2O  H3O+ + OH−, Figure 3c), resulting in a large response of the paper-based humidity sensor at high RH.49−51

Figure 3. (a) Chemical structure of the cellulose. (b) Water molecules adsorption process and protons hopping conduction. (c) Schematic diagram of ionic conductive species on the surface of the cellulose fibers. 13



2.3. Demonstration of Multifunctional Applications. Benefiting from the fast response and recovery characteristics of the flexible paper-based humidity sensor in the short interval periods, we investigated its potential application for human breath rate monitoring and the humidity sensor was fixed on a mask for conveniently measuring the breath rate. We know that the human breath is usually through nose, but we also use mouth breathing in some special cases such as strenuous exercise and nasal congestion (colds and nasitis). In previous reports, some humidity sensors had been reported for monitoring the nasal breathing,5,52 while others were used to monitoring mouth breathing.13,53−55 The RH of the breath exhaled from the mouth is usually higher than the nose’s due to the saliva in the mouth (Figure S6), so it is necessary to test the response of the humidity sensor for mouth and nose breathing separately. As can be seen in Figure 4a, the current change produced by mouth breathing is larger than that of nose breathing. The response of the humidity sensor at different mouth breathing rates is also tested as shown in Figure 4b, proving that the humidity sensor can meet the needs of normal breathing rates.43,44 In order to further verify that the paper-based humidity sensor is of excellent flexibility and can resist the bending influence during breathing rate monitoring, the humidity sensor is continuously bent during the mouth breathing rate test as shown in Figure 4c and Movie S1. There is no significant influence on the breathing response signal after the continuous bending the sensor, indicating that the paper-based humidity sensor is of good anti-bending property. In addition, the paper-based humidity sensors based on Cu and Al foil adhesive tape electrodes were fabricated for comparing with the humidity sensor of polyester electrodes (Figure S7a). Although the paper-based humidity sensors with Cu and Al electrodes are of the same good humidity sensing properties for breathing monitoring as the 14


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polyester electrodes humidity sensor (Figure S7b), the rigid electrodes of Cu and Al cannot resist the continuous bending and twisting (Figure S7c) and the paper-based humidity sensor with polyester electrodes can still detect the mouth breathing rate after continuous bending and twisting (Figure S7d). In order to test the potential application of the paper-based humidity sensor in baby diaper wetting monitoring, the sensor was attached on the surface of baby diaper and the urine here was simulated with 100 mL water. The real-time current curve of the humidity sensor in baby diaper wetting process is shown in Figure 4d and Movie S2, indicating that the humidity sensor is of good monitoring performance for baby diaper wetting. At the same time, the influence of water quantity on the response of the sensor is studied. As can be seen from the Figure S8, when the volume of water is less than 10 mL, the current of the sensor does not change significantly and the current of the sensor increases significantly when the volume of water is larger than 50 mL. This information is helpful for setting alarm threshold in the practical circuit system. In addition, there is no obvious response for the polyester conductive adhesive tapes which are directly attached on the surface of the baby diaper (Figure S9), further illustrating that the response is produced by the paper-based humidity sensor. The changes of wind and temperature also are involved during the breathing rate and baby diaper wetting monitoring, so the responses of the humidity sensor to temperature and wind are tested as shown in Figure S10, and the results show that the humidity sensor has no noticeable responses to the changes of temperature and wind. Moreover, the consistent performance of the batch fabricated sensors is very important for simplifying instrument calibration and replacing faulty sensors, especially for the disposable humidity sensors that can be used in 15



patient breathing rate and baby diaper wetting monitoring. In order to illustrate the batch consistency of the paper-based humidity sensor, the humidity sensing performance of three humidity sensors are tested as shown in Figure S11. As can be seen, the responses of three humidity sensors for mouth breathing rate (Figure S11a) and baby diaper wetting monitoring (Figure S11b) are of good consistency. The facile fabrication method and mature commercial materials endow the paper-based humidity sensor with good consistency. Moreover, to avoid the e-waste and dissemination of bacteria after using by patients, the degradable characteristic of the sensors is significant.56 To illustrate this, as-fabricated paper-based humidity sensor was directly disposed by a simple and cost-saving combustion method and the results show that the sensor can be burnt out within about 3 s and produces little ashes indicating its easy disposal (Figure S12).

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Figure 4. Applications of the humidity sensor. (a) Breathing response curves from mouth and nose. (b) Response curve at different mouth breathing rates. (c) Mouth breathing response curve during continuously bending the humidity sensor. (d) Real-time current curve of the humidity sensor during the baby diaper wetting process. To study the potential applications of the humidity sensor in vertical space localization and noncontact switch, a wetted filter paper as a RH source was placed on the plastic pipes with different heights to simulate the vertical spatial humidity distribution. Figure 5a shows the real-time current change curve versus different heights of plastic pipe. It can be seen that the current of the humidity sensor increases with the lowering the height of the plastic pipes and the current change of the humidity sensor gradually decreases when the plastic pipe height is higher than 10 mm (Figure 5b), indicating that the humidity sensor is of potential application in near distance localization of the humidity objects. Motivated by its ability for vertical spatial humidity detection, the humidity sensor also can be applied to the moisture detecting around fingertips and noncontact switch. As shown in Figure 5c and Movie S3, the current of the humidity sensor increases when the finger is 5 mm away from the humidity sensor and decreases when the finger is moved away, which demonstrates that the humidity sensor is of good noncontact switching characteristic and has potential application in future untouched switches to decrease the risk of bacterial transmission.7 The water content of human skin is a crucial parameter for evaluating human health and skin conditions.9 However, the human skin contains no specific receptors for humidity sensing and is instead able to sense the humidity changes via mechanoreceptors and thermoreceptors.11 Considering the good flexibility and human compatibility of the 17



paper-based humidity sensor, the sensor was fixed on a flexible hollowed PET film (thickness: 75 m) for skin humidity monitoring as shown in Figure 5d. The current of the humidity sensor is about 37 nA for dry skin (Figure 5e). After spraying moisturizing water on the skin, the current of the humidity sensor increases first, then decreases gradually and maintains at ~110 nA for a period of time (Figure 5f). It should be noted that it takes some time (about 5 s) to fix the sensor on skin. During this period, the sensor will adsorb water molecules from the moisturizing water on the skin. After fixing the sensor, the system starts to acquire current data, so the recorded time of current increase is very short. The above results indicate that the humidity sensor can be used to monitor the skin humidity and also has potential application for moisturizing properties evaluating of the moisturizing products.

Figure 5. (a) Dynamic response curve at different heights of plastic pipe. (b) Current versus heights curve. (c) Noncontact response curves for five cycles with the height of 5 mm. (d) Schematic diagram of the humidity sensor fixed on the flexible PET film (thickness: 75 m)

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and the photograph of the flexible skin moisture sensor. Real-time current curves of (e) dry skin and (f) after spraying moisturizing water. Sensor array is of significance for observing the responses toward the stimulus as well reflecting the position of the stimulus. By taking the merits of the paper-based humidity sensor, an in-plane 5 × 5 integrated humidity sensor array was facilely integrated for RH intensity and location detection as shown in Figure 6a. For presenting the RH distributions of specific locations expediently, five polyvinyl chloride (PVC, thickness: 3 mm) mask plates with different letters “U”, “E”, “S”, “T” and “C” (the abbreviations of “University of Electronic Science and Technology of China”) are prepared and Figure 6b shows the one of mask plates of the letter “U”. A hand with sweat is positioned over the humidity sensor array to form a large and non-uniform humidity field as shown in Figure 6c. Compared with the covered part in the mask plate, the hollow part is easier to absorb water molecules due to directly exposing the humidity source. Hence, the shape change of the mask plates will affect the current flowing through the sensors in array.7 It can be seen that the different RH distributions (Figure 6d−h) are revealed through the current mapping of the humidity sensor array (Figure 6i−m). The clearly distinguishable current change for each letters shape indicates that the humidity sensor array could be used for noncontact spatial localization of the moist objects.7,8,10

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Figure 6. Schematic diagrams of (a) 5 × 5 integrated humidity sensor array and (b) the mask plate shape of the letter “U”. (c) Photograph of a hand with sweat positioned over the humidity sensor array and (d−h) five different shape mask plates positioned over the humidity sensor array. (i−m) Current mapping of RH distributions. 2.4. Design of Signal Processing System. The paper is of a relatively large resistance in dry air and its resistance will decrease drastically after adsorbing the water molecules, which is conducive to the humidity sensing response of the paper-based humidity sensor to some extent, but it also makes it hard to design the signal processing system. In consideration of the relatively large resistance of the paper-based humidity sensor and the development trend of the flexible wearable electronics, it is necessary to design an integrative and flexible signal processing system for the humidity sensor. Flexible circuit board (flexible substrate material: PI) was made by the circuit company (Shenzhen EDADOC Technology Co., Ltd., China) according to the circuit diagrams we designed. Figure 7a shows the photograph of the signal processing system, which is mainly composed of flexible circuit board, humidity sensor, 20


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signal acquisition circuit, analog-digital converter (ADC), microcontroller unit (MCU) and bluetooth module. The schematic diagram of the data acquisition circuit for the large resistance is shown in Figure S13, the voltage across the sensor (R3) varies with its resistance, so the humidity sensor’s signal is obtained by measuring the voltage across the sensor. Figure S14 shows the schematic diagrams of the bluetooth module, MCU and ADC. Figure 7b shows the good flexibility of signal processing system under the bending state, which is conducive to good compatibility between the signal processing system and the human body. Taking the mouth breathing rate detection as an example, the whole signal processing system including the lithium battery is embedded in a mask by bending as shown in Figure 7c, and the voltage value across the paper-based humidity sensor can be transmitted to the laptop terminal in real time through bluetooth module. Figure 7d shows the voltage curve of the mouth breathing response, indicating that the as-designed signal processing system is useful. It is worth noting that this flexible signal processing system fills the shortage of other humidity sensors lacking processing circuits.4,5,7−9 Meanwhile, the working voltage of the signal processing system is 5 V and the size of the flexible circuit board is ~2 × 5 cm2, which is of lower working voltage and smaller size than previous report of the George M. Whitesides’ group (25 V, ~7 × 10 cm2).6 Moreover, the good portability and scalability of the signal processing system may also enable it to be applied to other sensors.

21



Figure 7. Photographs of the signal processing system under (a) flat and (b) bending states. (c) Photograph of the breathing monitoring system wore on the face. (d) Voltage curve of the mouth breathing response.

3. CONCLUSIONS In summary, we presented a flexible, cost-saving, environment-friendly and multifunctional paper-based humidity sensor via a facile method. The humidity sensor is of an excellent humidity sensing response of more than 103 within the humidity range from 41.1% to 91.5% RH. The main advantages of the obtained paper-based humidity sensor are as 22


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follows: i) The raw materials (paper, conductive adhesive tape) involved are very cheap and the pasting fabrication method is very simple; ii) One of the known limitations of poor flexibility for paper-based humidity sensors has been solved using conductive adhesive tape,6 which is also expected to be used in other flexible paper-based electronics; iii) In addition to the application of breathing rate detection, the other applications of the paper-based humidity sensor in baby diaper wetting, noncontact switch, skin humidity and spatial localization monitoring are also explored. iv) The signal processing system for the paper-based humidity sensor is optimized in terms of integration, flexibility and power consumption. Although the fabrication technics and daily available materials in this work simplify the fabrication process, reduce the cost and expand the application fields of the paper-based humidity sensors, two improvements remain for paper-based humidity sensor: i) For the daily environmental RH detection and special breathing rate monitoring with low RH, the linear response range of the paper-based humidity sensor needs to be expanded, especially for the detection of low RH; ii) Without sacrificing the response value, the resistance of the paper-based humidity sensor should be properly reduced to further reduce the power consumption and design costs of the signal processing system.

4. EXPERIMENTAL SECTION 4.1. Fabrication of the Paper-based Humidity Sensor. Two polyester conductive adhesive tapes (width: ∼5 mm, thickness: ∼130 m, conductive components: electroplated Cu-Ni alloy, surface resistivity ≤0.025  sq-1, adhesive force ≥1.3 kg sq-1, Shenzhen Huijia Adhesive Products Co., Ltd., China) and two Al wires were firstly pasted on the surface of the conventional A4 printing paper (80 g m-2, thickness: ∼107 m, Asia Symbol (Guangdong) 23



Paper Co., Ltd., China) to form two electrodes, which the gap distances between two electrodes of three kinds of humidity sensors were about 0.5, 1 and 2 mm, respectively. Then, the PI adhesive tape (width: ∼5 mm, thickness: ∼55 m, Shenzhen Huijia Adhesive Products Co., Ltd., China) were pasted on the surface of the sensor and only exposed the gap between two electrodes. 4.2. Characterization and Measurement of the Paper-based Humidity Sensor. The surface and the cross-section morphologies of the sensor were investigated by using a SEM (JSM-6390A, JEOL, Japan) at 15 kV. The chemical compositions of the paper surface were characterized by XPS (ESCALAB 250Xi, Thermo Scientific, USA). The dynamic contact angles of the paper were recorded by using an optical contact angle meter (OCA15EC, Dataphysics, Germany). The humidity sensing properties of the paper-based humidity sensors were measured by the homemade humidity sensor measuring equipment with a semiconductor characterization system (Keithley 4200-SCS) and the RH generation system to measure humidity sensing characteristics is shown elsewhere (Figure S2).57 The other application tests of the paper-based humidity sensors were performed in an open air environment of laboratory. The current signals of humidity sensor array in Figure 6 were acquired by testing each sensor unit one by one. The temperature and RH of the laboratory were controlled at 25±1 C and 60±5% RH by an air conditioner, and the real-time RH and temperature were recorded by a high-accuracy hygrometer (CEM, DT-625, Shenzhen Everbest Machinery Industry Co. Ltd., China). It is noteworthy that the fluctuation of laboratory humidity (60±5% RH) has some effects on the reference current of the paper-based humidity sensor in practical application tests. The response of the humidity sensor in this 24


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study is defined as the ratio of the current at high RH (IH) to that at low RH (IL), namely, response = IH/IL. The time taken by the humidity sensor to achieve 90% of the current change from 7.2% to 91.5% RH is defined as the response time (res) in the case of adsorption or the recovery time (rec) in the case of desorption from 91.5% to 7.2% RH. ◼

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Detailed XPS, schematic diagram of the humidity sensor measuring system, sensing characteristics of the humidity sensors, schematic diagram of the data acquisition circuit (PDF). Movie S1: Mouth breathing rate response during bending the humidity sensor (AVI). Movie S2: Real-time current curve of the humidity sensor during the baby diaper wetting process (AVI). Movie S3: Noncontact switching characteristic of the humidity sensor (AVI). ◼

AUTHOR INFORMATION

Corresponding author *E-mail: [email protected] (H. T.). ORCID Huiling Tai: 0000-0001-5966-3843 Notes The authors declare no competing financial interest. ◼

ACKNOWLEDGMENTS 25



This work is supported by the National Science Funds for Excellent Young Scholars of China (Grant No. 61822106), National Science Funds for Creative Research Groups of China (Grant No. 61421002) and Natural Science Foundation of China (Grant No. 61671115). ◼

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Facile, Flexible, Cost-Saving, and Environment-Friendly Paper-Based

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