Flexible sensors for food monitoring. Part I: Principle

Monitoring and maintaining food quality, safety, and authenticity are the most important concerns in the food industry. The cutting-edge flexible sensors for food monitoring precisely meet the needs of acquiring information on multiple parameters in small space and more reasonable layout, providing data on mechanical deformations, and conformably attaching to arbitrarily curved surfaces. Flexible sensing materials with a large specific surface area, high carrier mobility and carrier density, dense active sites, outstanding tunability, and processability, such as two-dimensional carbon nanomaterials, conductive polymers, and nanohybrid materials, have further improved the sensitivity, stability, and selectivity of flexible sensors. This article attempts to critically review state-of-the-art developments with respect to materials, fabrication techniques, and sensing mechanisms of devices, as well as the applications of the electrically-transduced flexible sensors. In addition, this review elaborates on the transduction mechanisms of several typical transducers, with a focus on the physics behind, including the modulation of doping level, Schottky barrier, and interfacial layer that typically lead to changes in conductivity, work function, and permittivity. We also highlight the benefits, technical challenges with corresponding solutions of current flexible sensors, and discuss potential strategies to overcome limitations in energy consumption, quantify the trade-offs in maintaining quality and marketability, optimize wireless communication, and explore new sensing patterns.