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 30–70% 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 non-smokers 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.