A flexible calligraphy-integrated in situ humidity sensor

Measurement 147 (2019) 106853

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A flexible calligraphy-integrated in situ humidity sensor Yalei Zhang, Yue Cui ⇑ College of Engineering, Peking University, Beijing 100871, PR China

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Article history: Received 30 May 2019 Received in revised form 17 July 2019 Accepted 22 July 2019 Available online 25 July 2019 Keywords: Sensor Humidity PEDOT:PSS Printed Flexible Calligraphy

a b s t r a c t The development of a humidity sensor integrated with calligraphy art could open up exciting opportunities for the development of ‘‘smart art”, which can be applied in arts, architecture, and environmental monitoring. PEDOT:PSS has shown excellent electrical and sensing properties as a conducting polymer. Calligraphy has a long history in both eastern and western countries, and is still widely used as an art form. Here, we show, for the first time, the development of an ‘‘invisible” and flexible humidity sensor by integrating calligraphy and a PEDOT:PSS sensing electrode. The sensor is embedded onto calligraphy text and can detect surrounding environmental humidity without being visible. The sensor exhibits rapid and highly sensitive detection of humidity with a detection range of 10–90% relative humidity with a measuring time of about 30 s. We anticipate that these results could provide significant opportunities for both fundamental studies as well as practical applications of flexible sensors. Ó 2019 Published by Elsevier Ltd.

1. Introduction Humidity detection is very important in a variety of areas, such as the construction of electronic and optical devices [1–3], healthcare conditions [4], environmental monitoring, and plant growth [5]. Many types of humidity sensors have been developed on silicon chips [6,7], plastic substrates [8], photopapers [9], etc. However, these sensors may tend to occupy extra space and therefore be clearly visible. Thus, it is highly desired to develop an ‘‘invisible” humidity sensor that occupies almost no space. Such a sensor will also be lightweight. Art has developed throughout human history as an important part of our lives. Calligraphy is a typical art form that has been widely used in both western and eastern countries [10]. Integrating art and sensors to develop ‘‘smart art” could provide significant opportunities for re-shaping peoples’ living styles. For example, a humidity sensor integrated into a calligraphy could provide an in situ method for detecting the humidity on the calligraphy directly. Such an integrated in situ sensor has many unique advantages, such as light weight, small size, and ‘‘invisibility”. Furhter, since there is no distance between the sensor and the object, it could provide the most accurate information for the environment of the piece of art. Electronic humidity sensors have attracted considerable attention as miniaturized devices that can produce rapid and sensitive measurements [11,12]. Various electronic humidity sensors have ⇑ Corresponding author. E-mail address: [email protected] (Y. Cui). https://doi.org/10.1016/j.measurement.2019.106853 0263-2241/Ó 2019 Published by Elsevier Ltd.

been developed, such as Pt thin film [13], graphene oxide [14], ZnO nanostructures [15], and GaN nanowires [14,16]. PEDOT:PSS is a cost-effective and easily processed conducting polymer that has been widely used in a variety of sensors [17–19], such as solar cells [20,21] and thermoelectric devices [22]. Recently, PEDOT:PSS has been incorporated into the sensing electrode for detecting humidity as well [23–25]. However, until now, there has been no report for embedding PEDOT:PSS into an artwork to construct an ‘‘invisible” and flexible humidity sensor. In this work, we demonstrate, for the first time, a calligraphyintegrated humidity sensor by using PEDOT:PSS as the sensing electrode. Our sensor is constructed by printing the PEDOT:PSS ink directly on a calligraphy text. With a change in humidity, the water molecules interact with the PEDOT:PSS electrode, thus changing the resistance of the electrode, which generates a sensing signal. 2. Material and methods 2.1. Apparatus A digital multimeter was purchased from Keysight Technologies, Inc. (Delaware, USA). A hot plate was purchased from YiHeng, Inc. (Shanghai, China). An optical microscope was purchased from Cewei, Inc. (Shanghai, China). A computer installed with humidity data logging software and digital resistance logging software was obtained from Hewlett-Packard (California, USA). A commercial humidity sensor was purchased from Mianyang Pingyang Technology Co., Ltd. (Wenzhou, China).

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2.2. Chemicals Xuan paper was purchased from Nanchang Merlin John Optical Media Co., Ltd. (Nanchang, China). PEDOT:PSS (ratio: 1:2.5, conductivity: 500 s/cm) was from Ouyi Inc. (Shanghai, China). Swabs were obtained from Suzhou Honest Good Electronics Co., Ltd. (Jiangsu, China). Copper papers were purchased from Hangzhou Building Materials Trade Co., Ltd. (Hangzhou, China). Silver paste was from Nanjing Xylite, Inc. (Nanjing, Jiangsu, China). Tweezers were from Prokits Industries Co., Ltd. (Taiwan, China). A plastic case was from Guangzhou Heng Jia Food Equipment Co., Ltd. (Guangzhou, China). A hose was obtained from Shenzhen Flying Rubber Co., Ltd. (Shenzhen, China). 2.3. Preparation of the sensor A Chinese calligraphy text was written using a brush and black ink on Xuan paper, which is Chinese art paper. The PEDOT:PSS electrode was printed on the calligraphy text using a swab, followed by drying at 160 °C for 10 min. After cooling down to room temperature, the PEDOT:PSS paper was cut into a long strip of size 5 mm  40 mm. In order to prepare the electrode, the silver paste was added to both ends of the PEDOT:PSS strip, and copper paper was then connected to the silver paste, followed by baking at 100 °C for 30 min. The sensing sample was thus obtained. 2.4. Sensing measurement The performance of the calligraphy-integrated PEDOT:PSS humidity sensor was measured by a Keysight digital multimeter at room temperature (21 °C). To evaluate the performance of the developed sensor, it was compared with a commercial humidity sensor. Both of the humidity sensors were placed in a sealed plastic box, in order to reduce the influence of other external factors. The humidity in the box was changed by a humidifier connected to the box by a hose. The resistance of the PEDOT:PSS was changed by humidity, and the resistance change was measured by a digital multimeter. The resistances of the calligraphyintegrated PEDOT:PSS electrode were in accordance with the values shown on the commercial humidity sensor. A calibration curve was plotted between the resistance of the calligraphy-integrated PEDOT:PSS sensor and the humidity.

Fig. 1. The sensor diagram and the experimental apparatus. (a) A diagram of the sensor configuration and dimension. (b) A camera image of the experimental apparatus. Images of a calligraphy-integrated humidity sensor. (a) A camera image of a calligraphy written Peking University in Chinese characters. A PEDOT:PSS electrode was printed area within the area marked with a blue box. (b) An optical image of the calligraphy without the PEDOT:PSS electrode. (c) An optical image of the calligraphy with the PEDOT:PSS electrode on it. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3. Results and discussions Fig. 1 shows the schematic diagram of the sensor and the experimental apparatus. As shown in Fig. 1a, the sensing device is built on a calligraphy artwork. In the device, the sensing electrode is made of PEDOT:PSS, and it connects to the silver paste electrodes that further connect to the copper paper-based electrodes. The PEDOT:PSS electrode has a dimension of 5 mm by 40 mm, the sliver paste electrode has a dimension of 5 mm by 10 mm, and the copper paper-based electrode has a dimension of 5 mm by 50 mm. Fig. 1b shows a camera image of the experimental apparatus. Both of a PEDOT:PSS sensor and a detection probe from a commercial sensor are put in a box for measuring the humidity in the box that can be changed by a humidifier. A multimeter measures the resistance of the PEDOT:PSS electrode during sensing, which is correlated to the humidity level and compared with that from a commercial sensor. Fig. 2 shows the camera image and optical image of the calligraphy-integrated humidity sensor. As shown in Fig. 1a, Chinese calligraphy text of ‘‘Peking University” was written on a Xuan paper, and the color of the text is black owing to the ink color. The Xuan paper is soft and flexible, and can function as an excellent

Fig. 2. Images of a calligraphy-integrated humidity sensor. (a) A camera image of a calligraphy written Peking University in Chinese characters. A PEDOT:PSS electrode was printed area within the area marked with a blue box. (b) An optical image of the calligraphy without the PEDOT:PSS electrode. (c) An optical image of the calligraphy with the PEDOT:PSS electrode on it. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

substrate to be incorporated with sensors. The PEDOT:PSS electrode was printed onto part of the calligraphy text, the written area within the blue box. Fig. 1(b) shows the camera image of the calligraphy-integrated PEDOT:PSS sensor. As can be seen, there is no large difference between the two images. Thus, the PEDOT: PSS calligraphy-integrated humidity sensor does not change the

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appearance of the calligraphy text, and it can be therefore be considered invisible in a sense. Fig. 3 shows the influence of the size of the sensor on the sensing performance. Different lengths of the sensing electrodes ranging from 2 cm to 6 cm were studied with a constant width of 0.5 cm, and different widths of the sensor ranging from 0.3 cm to 1.5 cm were studied at a constant length of 4 cm. As can be seen from Fig. 2a, the signal increases as the length of the sensor goes up. The signal is set as 100% when its length is 4 cm, the signal is 41.34%, 90.06%, 126.35%, 174.14% when its length is 2 cm, 3 cm, 5 cm, and 6 cm respectively. As can be seen from Fig. 2b, the signal will decrease as the width of the sensor goes up. The signal is set as 100% when its width is 0.5 cm, the signal is 124.03%, 87.47%, 35.93%, 33.96% when its width is 0.3 cm, 0.8 cm, 1 cm, 1.5 cm respectively. These results demonstrate the size of the sensor could affect the sensing response, and a sensing electrode with a width of 0.5 cm and a height of 4 cm is sufficient enough for generating an excellent sensing signal. Therefore, a dimension of 0.5 cm by 4 cm is chosen for other studies. Further, more external factors could potentially affect the sensing signal as well, such as temperature, air flow rate, etc. In this work, we tried to keep other factors to

Fig. 3. The effect of the size of the sensor on the sensing performance. (a) The influence on the length on the sensor. Width is 0.5 cm. (b) The influence on the width on the sensor while the length is 4 cm. All signals are calculated by the following formula: resistance change per humidity = (R2–R1)/(H2–H1). Relative sensing signal (%) was calculated by normalizing the sensing response to that with a sensor dimension of 0.5 cm by 4 cm.

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be constant during the measurement of humidity. For future work, a sensor array is desired for the measurement, since it could possess multiple sensors which can generate a more accurate result by excluding the influence from other factors. Fig. 4 shows the characterization of the humidity sensor. The humidity was controlled by a humidifier in a step-wise manner. Since the humidity was changed over time, a humidity versus time curve for the commercial humidity sensor is shown in

Fig. 4. Characterization of a PEDOT:PSS humidity sensor. (a) A humidity versus time curve by a commercial humidity sensor. (b) A resistance versus time curve with the calligraphy-integrated PEDOT:PSS humidity sensor. (c) Calibration curve of the PEDOT:PSS sensor for the detection of humidity.

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Fig. 4a. The resistance of the calligraphy-integrated PEDOT:PSS sensor changed with respect to the change in humidity, and the resistance vs. time response curve of the PEDOT:PSS humidity sensor is shown in Fig. 4b. As can be seen from Fig. 4, the response of the PEDOT:PSS humidity sensor to the change in humidity is obvious and rapid. When the humidity increased, the resistance of the sensor also increased, and returned to stability only after about 30 s. Further, the PEDOT:PSS sensor could be used several times for detecting humidity with little change in the sensing property. Fig. 4c shows the calibration curve for the humidity sensors. The Y axis represents resistance, and the X axis represents the humidity shown by the commercial humidity sensor. As can be seen, the change in the resistance for the PEDOT:PSS humidity sensor is consistent with the humidity change from the commercial sensor. The resistance increases with the increase in humidity, and the change in resistance is directly proportional to the humidity shown on the commercial sensor, e.g., the resistance of PEDOT:PSS sensor is 374 kX when the humidity is 10%, and it increased by 1.747 times when the humidity is 90%. The results show that the sensor has a wide detection range for humidity from 10% to 90% with a slope

Fig. 6. The effect of bending numbers on the PEDOT:PSS humidity sensor. (a) The effect of bending numbers on the resistance of the sensing electrode. Relative resistance (%) was calculated by normalizing the absolute resistance with a given bending number to that without any bending. (b) The effect of bending numbers on the sensing signal of the sensor. Relative sensing slope was calculated by normalizing the slope of the calibration curve with a given bending numbers to that without any bending. The humidity is 17%. The bending angle is 180°.

Fig. 5. The effect of bending angles on the PEDOT:PSS humidity sensor, bending while sensing. (a) The effect of bending angles on the resistance of the sensing electrode. Relative resistance (%) was calculated by normalizing the absolute resistance with a given bending angle to that without any bending. (b) The effect of bending angles on the sensing signal of the sensor. Relative sensing slope was calculated by normalizing the slope of the calibration curve with a given bending numbers to that without any bending. The humidity is 21%.

of 3.7125 and a R2 of 0.9208. The results demonstrate that the PEDOT:PSS sensor can detect humidity sensitively and rapidly. The accuracy of the sensor was studied as well by testing a variety of humidity values, including 14.5%, 36.5%, 50.1%, 61.2%, 71.7%, and 83.0% (from a commercial humidity sensor), and the developed sensor in this work shows relative accurate results with a small average error of 7.6%. The PEDOT:PSS and silver paste electrodes are ink-based electrodes, and these conductive electrodes have thin thicknesses with high flexibilities. The PEDOT:PSS electrode has a thickness of 0.02 mm, and the silver paste electrode has a thickness of 0.21 mm. These inks are painted into a flexible paper-based substrate, and the paper substrate has a thickness of 0.11 mm. Thus, the overall device is flexible and bendable. The flexible nature and capability of the sensor could enable the sensor to be used in various situations. Fig. 5 shows the effect of bending numbers on the PEDOT:PSS humidity sensor. The humidity in this study is 17%, and the bending angle is 180°. As shown in Fig. 5a, the resistance of the sensing electrode was not changed significantly with bending up

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60 min, 120 min, 240 min, 720 min, and 1440 min. Fig. 7a shows that the resistance of the sensor fluctuates between 10% during the measurement time. the initial resistance increases slightly (2%, compared to the initial data) after 10 min, and decreases 1% after 45 min, which is not a large difference compared to the initial resistance. Fig. 7b shows the change in the relative signal over time. The relative sensing signal (%) was calculated by normalizing the absolute resistance change per humidity with a given storage time to that obtained on the first measurement. The relative sensing signal fluctuated a little bit over 24 h, but generally the sensor showed excellent stabilities. For example, the sensing signal decreased by 2% after 5 min in air, increased by 7% after 30 min, and increased by 13% after 24 h. These results demonstrate the sensor has a relatively excellent stability over time, and can undergo repeated use for the detection of humidity. 4. Conclusion In this work, we have shown for the first time the development of a calligraphy-integrated flexible PEDOT:PSS humidity sensor. The sensing electrode is printed onto the calligraphy text, and owing to the integration of the sensing electrode and the calligraphy, the sensor is invisible and does not show a visible color compared with the calligraphy text color. Owing to the property of the calligraphy paper, the sensor is soft and flexible. The sensor shows a highly sensitive and rapid detection of humidity. The approach created here may provide new avenues for constructing a variety of artwork-based wearable sensors to detect other chemicals for a wide range of applications in healthcare, defense, and environmental monitoring. Declaration of Competing Interest

Fig. 7. The stability of a PEDOT:PSS humidity sensor stored under ambient condition. (a) The effect of storage time on the resistance of the sensing electrode. Relative resistance (%) was calculated by normalizing the absolute resistance with a given storage time to that the first time of measured. (b) The effect of storage time on the sensing signal of the sensor. All signals are calculated by the following formula: resistance change per humidity = (R2–R1)/(H2–H1), Relative signal (%) was calculated by normalizing the absolute resistance change with a given storage time to that obtained on the first measurement.

to 100 times. The resistance increased slightly for about 15% with bending up to 100 times. Fig. 5b shows the effect of bending numbers on the sensing ability. The slope of the calibration curve of the sensor was not affected significantly with bending up to 100 times. After bending up to 100 times, the slope of the calibration curve decreased slightly for about 8%. The results demonstrate that the sensing performance of the PEDOT:PSS humidity sensor was not significantly affected by the bending number, and the sensor can still show a relative excellent performance after bending for many times. Fig. 6 shows the effect of bending angle on the PEDOT:PSS humidity sensor. As can be seen from Fig. 6a, when the bending angle is 180°, the resistance increased by 39%. Fig. 6b shows the sensing slope for humidity detection with sensor bending at 0°, 30°, 60°, 90°, 120°, and 180°. It can be seen that the sensing slope decreased by 28% when the bending angle was 180° during sensing. Regarding the test error, the results demonstrate that bending angles only have a slight influence on the detection with the PEDOT:PSS humidity sensor. Fig. 7 shows the stability of PEDOT:PSS humidity sensor over time. The PEDOT:PSS sensor performance was measured initially, and then placed in air for 5 min, 10 min, 15 min, 30 min, 45 min,

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