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, most humidity sensors involve a 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 as good linearity (R 2 = 0.9549) within the humidity range from 41.1 to 91.5% relative humidity. Furthermore, the paper-based humidity sensor has 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.

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For a long time, the daily paper and carbon ink have been used for writing and painting. With the development of the electronic technology, they are expected to play new roles in electronic devices. Herein, combining the unique characteristics of the paper (rough surface, hydrophilicity) and carbon ink (conductivity), this work rationally proposed two low-cost, eco-friendly, flexible, multifunctional pressure and humidity sensors. The results show that as-fabricated paper-based (PB) pressure sensor has a good sensitivity of 0.614 kPa−1 in the pressure range of 0–6 kPa. The PB humidity sensor has a large response of ∼2120 (current ratio at 91.5% relative humidity (RH) and 0% RH). The PB pressure sensor is proven to be useful for multiple pressure related contact detections, and the PB humidity sensor can be used for many humidity related non-contact detections. Interestingly, combining the different detecting modes of pressure and humidity sensors, some same detecting functions (e.g. switch, respiratory frequency and speech recognition) are realized from contact to non-contact using PB pressure and humidity sensors, which greatly enhance the wearable comfort. Compared with previous reports, this work demonstrates a much simpler approach without expensive raw materials, toxic reagents and high temperature treatment to achieve outstanding sensing performances of the multifunctional pressure and humidity sensors.

Cost and fabrication technique, as two important factors affecting the development and practical applications of electronic devices, have attracted extensive attention of researchers. In order to overcome the shortcomings of high-cost raw materials and complex fabrication process, in the present work, a low-cost (the cost of per strain sensor is less than one cent ($0.01)) and environmentfriendly strain sensor based on common daily goods of carbon ink and elastic core-spun yarn is fabricated by a facile solution processing method. The results show that the as-fabricated strain sensor possesses acceptable sensitivity (a gauge factor of 6.1 under the strain range of 0.5−20%) and rapid response and recovery speed (response/recovery times under a strain of 10%: 110/80 ms). Moreover, the facile solution processing method and tailorable length of the elastic core-spun yarn allow it to be fabricated into strain sensors with different specifications for meeting the needs of various application situations. Based on its many advantages, the strain sensor is successfully used for monitoring multiple human motions. This facile approach can be used for developing low-cost strain sensors and also provides inspiration for developing new applications of the daily goods such as carbon ink in electronic field.