Toward Environmentally Friendly Hydrogel‐Based Flexible Intelligent Sensor Systems

The next‐generation technologies including digital health, precision agriculture, human–machine interfaces (HMIs), Internet of Things always call for intelligent systems that can take decision like human brain by acquiring and analysing inputs from the real world. Mechanically flexible sensors, which conform to the dynamic, nonplanar surfaces, have attracted considerable attention for continuously acquiring signals from the physical world. Combining such sensors with a machine learning (ML) algorithm that acts as a brain by processing and analysing the sensors' signals makes the framework of an intelligent system. Recently, hydrogels have emerged as excellent materials for making inherently soft, stretchable, biocompatible, and environmentally friendly flexible sensors. Converging such hydrogel‐based flexible sensors with suitable ML algorithms opens up opportunities for environmental‐friendly flexible intelligent sensor systems and also meets the global demands of sustainable develeopment. Tremendous efforts have been put in this field over the last decade to realize different types of hydrogels, their sensor applications and to study their ability to interface with practical systems like electronic circuitries and biological bodies. This review surveys the recent works on hydrogel‐based flexible sensor systems and comprehensively discusses about materials, sensors, applications, and their data processing mainly focusing on ML. The integration of these flexible sensing devices with conventional rigid electronics and biological systems is also discussed. Finally, the discussion and comments on the future prospects of this field are concluded.