Rapid and Flexible Humidity Sensor Based on Laser-Induced Graphene for Monitoring Human Respiration

Respiration is an important physiological parameter used to assess human health and metabolic activity. Herein, we propose a laser-induced graphene (LIG)-based humidity sensor for respiratory monitoring. This sensor is fabricated through a combination of laser irradiation and intense pulsed light (IPL) sintering techniques. Initially, an ink containing copper nanoparticles (CuNPs) and graphene nanoplatelets (GnPs) is coated onto a polyimide (PI) substrate. The LIG is formed on the PI film using laser irradiation. To establish a reliable electrical connection between the LIG and the copper electrode, the ink undergoes rapid IPL sintering, resulting in an IPL-sintered copper electrode. This technique not only optimizes the fabrication process but also obviates the need for traditional approaches, such as copper wire bonding, electrode patterning, or the application of conductive paint on the LIG sensor. The humidity-sensing capabilities of the sensor are assessed under various relative humidity (RH) conditions. The sensor's response escalates from roughly 15 to 92% as RH levels increase from 13 to 67%. The sensor showed minimal response to various potential interfering gases like ammonia, ethanol, carbon monoxide, sulfur dioxide, and nitrogen dioxide (with responses of 0.4, 1.87, 0.102, 0.12, and 0.29%, respectively), confirming its high selectivity for RH (91.2%). Additionally, the sensor demonstrates exemplary reproducibility, as evidenced by its consistent responses (approximately 47.65, 49.13, 48.65, 49.09, and 49.39) over five cycles at 40% RH. The LIG sensor is used to monitor a wide range of respiratory patterns, including normal, slow, fast, and apnea events. The sensor effectiveness is proven through the consistent detection of human breathing patterns over 30 min, demonstrating its stability and reliability for extended use in continuous respiratory monitoring. These findings highlight the significant potential of LIG sensors as advanced precision devices in clinical respiratory monitoring with potential integration into modern medical practices.