Humidity-compatible chemiresistive hydrogel sensor for real-time breath CO2 monitoring

Abstract

Real-time monitoring of exhaled carbon dioxide (CO₂) is essential for assessing metabolic status and tracking disease progression. However, the high moisture content of exhaled breath poses a significant challenge to the accuracy and stability of CO₂ detection. Here, we present a flexible chemiresistive CO₂ sensor based on a dimethylamine-functionalized hydrogel, in which water molecules critically facilitate the CO₂-induced reversible protonation of tertiary amine groups, thereby modulating the hydrogel's ionic conductivity for real-time breath analysis. As a result, the hydrogel synthesized from N-[3-(dimethylamino) propyl] methacrylamide (DMAPMA) exhibits a high CO₂ response of 37.3 % at 10,000 ppm and a low detection limit of 100 ppm. To further enhance mechanical durability under respiratory-induced stress, N,N-Dimethylacrylamide (DMAA) was incorporated into the hydrogel, increasing the tensile strain limit from 34.2 % to 51.0 % without compromising CO₂ sensing performance. In addition, we demonstrate the integration of the hydrogel sensor into a wireless smart mask system, enabling multimodal monitoring of respiratory CO₂ patterns, physical activity, and postprandial metabolic changes. This platform offers a scalable, low-power solution for wearable CO₂ sensing in personalized health monitoring.