Paper, as a flexible, low-cost, lightweight, tailorable, environmental-friendly, degradable, and renewable material, is emerging in electronic devices Especially, many kinds of paper-based (PB) sensors have been reported for wearable applications in recent years Among them, the PB gas, humidity, and strain sensors are widely studied for monitoring gas, humidity, and strain from the human body and the environment However, gas, humidity, and strain often coexist and interact, and the paper itself is hydrophilic and flexible, resulting in that it is still challenging to develop high-performance PB sensors specialized for gas, humidity, and strain detections Therefore, it is necessary to summarize and discuss them systematically In this review, we focus on summarizing the state-of-art studies of the PB gas, humidity, and strain sensors Specifically, the fabrications (electrodes and sensing materials) and applications of PB gas, humidity, and strain sensors are summarized and discussed The current challenges and the potential trends of PB sensors for gas, humidity, and strain detections are also outlined This review not only can help readers to understand the development status of the PB gas, humidity, and strain sensors but also is helpful for readers to find out and solve the problems in this field through comparative reading

Paper-Based Sensors for Gas, Humidity, and Strain Detections: A Review.

Breath analysis is an attractive strategy that holds tremendous potential to achieve non-implantable physical health management enabled by flexible humidity sensors for breathing behaviors (e.g., continuity, frequency) related to disease monitoring and chemiresistive gas sensors related early disease diagnosis. Compared to other techniques for breath component detection, non-invasive breathing diagnostics based on chemical sensors can offer several advantages like miniaturization, low power consumption, simple structure and cost-saving, which is helpful to enhance the portability of practical tests. Although extensive research has been carried out over the past two decades to improve sensing performances of breath gas sensors, many problems need to be further addressed when it comes to clinical disease diagnosis. Developing integrated gas sensor arrays have become one of the efficient solutions to improve detection accuracy. To get rid of external power supply, various novel sensors combining with self-powered technology are designed to exhibit a desirable development prospect in breath analysis. Thus, this review aims to summarize the latest research advances on wearable humidity-enabled breathing behaviors monitoring and typical biomarker gases-based disease screening, and also provides the prospects of future development from individual sensors to integrated devices and self-powered health monitoring systems.

Evolution of breath analysis based on humidity and gas sensors: Potential and challenges

Stretchable respiration sensors: Advanced designs and multifunctional platforms for wearable physiological monitoring.

For a long time, humidity sensors have been mainly used in detecting humidity of ambient conditions, such as industry, agriculture, and smart home. With the development of wearable electronic systems, humidity sensors have attracted great attention in human body-related (HBR) humidity detection (such as respiratory behavior, speech recognition, skin moisture, non-contact switch, and diaper monitoring). Therefore, significant efforts have been recently devoted to the development of HBR humidity sensors. Although remarkable achievements have been made in this field, many challenges still exist. Herein, we review and analyze the recent advances in HBR humidity sensors. Initially, we introduced the basic concept. Then, the various applications of humidity sensors for HBR humidity detection are systematically summarized and discussed. Finally, combined with the research progress of the HBR humidity sensor, the challenges in this field are prospected. We expect that this critical review will provide great insight and direct the research prospectus of the HBR humidity sensors.

Recent advances in humidity sensors for human body related humidity detection

Benefiting from the development of nanomaterials synthesis technology, the performance of many electronic devices, including humidity sensors, has been improved greatly. However, the synthesis of nanomaterials usually involves complex processes, expensive raw materials and even toxic reagents. Herein, the natural nanomaterials of halloysite nanotubes (HNTs) are deliberately selected for the fabrication of high-performance humidity sensor. Characterization results show that the HNTs have good hydrophilicity, hollow tubular nanostructure and large specific surface area, which contribute to humidity sensing performance of the humidity sensor. Further, the humidity sensor based on HNTs is fabricated and its humidity sensing properties are tested at the room temperature (25 °C). The results show that the impedance variation of the HNTs humidity sensor is five orders of magnitude within the humidity range from 0% to 91.5% relative humidity (RH) at the optimum working frequency (100 Hz), and its response time is only about 0.7 s. Notably, the HNTs humidity sensor exhibits wide humidity detection range of 0–91.5% RH, very low RH (7.2%) response characteristic and good linear responses at 0–28.8% and 28.8–91.5% RH. This work provides a simple, low-cost and environmental-friendly strategy for fabricating high-performance humidity sensor by exploring the natural nanomaterials like HNTs.

Halloysite nanotubes: Natural, environmental-friendly and low-cost nanomaterials for high-performance humidity sensor

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 ( R2 = 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.

Mingguo Yan

Papers

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