Flexible Gelatin-Modified Graphene Humidity Sensors for Breath and Food Monitoring

Advances in biomaterials and carbon nanostructures are enabling next-generation relative humidity (RH) sensors with broad applications. Here we report a gelatin-modified graphene ink (Ge-GTr), an eco-friendly formulation that delivers high-performance RH sensing when printed by aerosol jet. Devices fabricated with optimized 1.0Ge-GTr ink exhibited reliable operation across 10%-90%RH at room temperature, with two distinct response regimes: 0.58%RH-1 sensitivity at ≤60%RH (charge transfer/ dielectric screening) and 2.5%RH-1 at ≥60%RH (gelatin micelle swelling). The sensors achieved a peak response of 102% at 90%RH, resolution of ±2%RH, response time of ∼75 s, and only 3% hysteresis between 30%RH and 70%RH. They also showed repeatable cycling behavior, robust temperature stability, and excellent mechanical flexibility. Demonstrations in breath analysis and food packaging confirmed their practical utility: The sensors distinguished breathing patterns of smokers and nonsmokers and detected spoilage-related RH in meat packages, while maintaining high selectivity to RH over interfering gases. These results highlight Ge-GTr as a sustainable route to flexible, printed RH sensors for health monitoring and smart packaging applications.