Flexible electronics are increasingly important in health management, offering the potential to revolutionize people's lives by enabling continuous monitoring of physiological signals and providing therapy without external power sources. Current flexible electronics often rely on separate power supplies, leading to complex circuit designs, increased system failure probability, and compromised comfort and aesthetics. The development of self-powered flexible electronics, including sensors and pacemakers, is therefore highly desirable. Self-powered sensors have been developed based on various mechanisms, such as triboelectric, piezoelectric, thermoelectric, and hydroelectric effects. However, the metal-air redox reaction, typically used in high-performance metal-air batteries, offers a unique advantage due to its ability to convert chemical energy into electrical energy without external stimulation and its potential for high stability. Previous research has shown that the output performance of metal-air batteries can be influenced by external stimuli, suggesting the possibility of developing a self-powered humidity sensor by leveraging the impact of humidity on the redox reaction. This approach offers the potential for a stable signal response without external energy input, unlike other self-powered sensor types.

The existing literature extensively covers the use of flexible electronics in health monitoring applications, including respiration monitoring for diagnosing and preventing respiratory diseases. However, a significant gap exists in the development of self-powered sensors for these applications, particularly those leveraging the metal-air redox reaction for energy harvesting. While several self-powered sensors have been reported using various energy harvesting mechanisms, these often lack the long-term stability and high sensitivity required for reliable health monitoring. The use of metal-air redox reactions in sensor development has been limited, primarily focusing on its application in batteries. This study aims to bridge this gap by exploring the potential of metal-air redox reactions in creating a highly sensitive and stable self-powered humidity sensor for health management applications.

The researchers developed a high-performance self-powered chemoelectric humidity (CEH) sensor using a novel design. The sensor's electrolyte layer was a rationally designed graphene oxide (GO)/silk fibroin (SF)/LiBr gel. The preparation involved mixing GO and natural silkworm SF in a LiBr aqueous solution. The SF, a natural biomacromolecule, contains abundant polar amino acids with oxygen-containing groups, enabling strong hydrogen bonds and π–π interactions with GO, leading to a uniform and stable ink. The ink's viscosity was adjustable by varying the GO content, allowing for flexible fabrication methods such as direct writing, screen printing, and extrusion printing. The resulting electrolyte layer was sandwiched between graphite paper (cathode) and aluminum foil (anode). The sensor's working mechanism is based on the metal-air redox reaction, where humidity affects the ion mobility within the electrolyte layer. The short-circuit current is measured to monitor humidity, and the parallel alignment of GO flakes in the SF gel helps maintain sensor lifetime. The sensor's performance was characterized by measuring the current change rate (CCR) under varying relative humidity (RH), response time, recovery time, and sensitivity across a wide RH range. The influence of temperature, pressure, and anode materials on sensor performance was also investigated. The long-term stability and mechanical stability of the sensor were evaluated through continuous operation and bending cycle tests. The applications of the CEH sensor were demonstrated through an integrated respiratory monitoring-diagnosing-treatment system and a non-contact human-machine interface.

The fabricated CEH sensor demonstrated excellent performance. It achieved a high sensitivity of 0.0933 µA/s/1% RH, a fast response time of 1.05 s, and a quick recovery time of 0.80 s within a wide RH range (11–84%). The sensor showed stable responses across a temperature range of 0–80 °C and was largely unaffected by changes in air pressure. Experiments using different anode materials (Cu foil, Cu wire, Cu mesh) showed consistent humidity responses, indicating the versatility of the sensor design. The sensor's anode metal (Al) consumption was low (approximately 0.18 mg/cm² in 12 h), making it cost-effective. While the open-circuit voltage decreased over time (from 1.24 to 0.51 V within 72 h of continuous operation), the sensor remained responsive to humidity changes even after 72 h of operation. It also demonstrated long-term storage stability (maintaining its humidity response after 180 days of dry storage) and excellent mechanical flexibility (sustaining performance after 300 bending cycles). Proof-of-concept applications demonstrated successful respiratory monitoring using an integrated system that transmits data wirelessly to a cloud server. A non-contact human-machine interface prototype using the sensor effectively prevented virus transmission, as shown by tests involving fluorescent phosphors simulating viruses.

The findings demonstrate the successful development of a highly sensitive, fast-responding, and stable self-powered humidity sensor based on the metal-air redox reaction. The sensor’s performance surpasses many previously reported flexible humidity sensors in terms of response and recovery times. The use of a biocompatible and flexible material like silk fibroin further enhances the sensor's potential for wearable and implantable applications. The integration of the sensor into functional systems like the respiratory monitoring and non-contact interface prototypes showcases its practical applicability in health management. The low cost and ease of regeneration further contribute to its practicality. The results contribute significantly to the field of self-powered sensors and offer new avenues for designing power-free devices for various applications. The unique combination of high sensitivity, fast response, stability, biocompatibility, and cost-effectiveness addresses critical limitations of existing humidity sensors, paving the way for advanced health monitoring and human-machine interaction technologies.

This research successfully demonstrated a high-performance, self-powered chemoelectric humidity sensor based on the metal-air redox reaction. The sensor's exceptional sensitivity, fast response time, and long-term stability, coupled with its biocompatibility and cost-effectiveness, make it highly suitable for various health management applications. The successful integration into a respiratory monitoring system and a non-contact human-machine interface highlights its potential for real-world use. Future research could explore the sensor’s integration into other wearable devices for broader health monitoring, investigate different anode materials for improved performance, and further optimize the sensor's design for even greater sensitivity and longevity.

While the sensor demonstrated excellent performance, a few limitations exist. The sensor's open-circuit voltage and responsiveness decrease gradually over extended periods of continuous use (72 h). Although it can be easily regenerated, this aspect limits its operational duration without replacement. Further research is needed to investigate the factors contributing to this voltage decrease and to find strategies to prolong the sensor's lifespan. Additionally, while the sensor's performance was tested under a controlled environment, further investigations are necessary to ensure reliable performance under diverse and unpredictable real-world conditions.