Progress and future of relative humidity sensors: a review from materials perspective

Measurement of relative humidity demands sensors with different requirements and specifications. There has been a lot of work done on the development of humidity sensors with more than 100,000 research articles already published. The huge research magnitude suggests huge potential in this field but the question arises as to why such a seemingly simple task requires so much research efforts? Which parameters are exactly targeted? What are the difficulties faced? And, what are the different approaches adopted to achieve the requirements? The research on humidity sensors usually targets improvement of various performance parameters while keeping the cost low and the fabrication process easy. As many parameters are interdependent, the criteria to prioritize some parameters over others based on the target application are also considered. A major portion of research work in this field is based on the development of novel materials to achieve these targets, which are set according to desired applications. This focus review discusses the most widely employed and the best performing material categories including polymers, composites, 2D materials and carbon-based materials for the development of humidity sensors. The material properties, selection process, sensing mechanism, and the pros and cons of various categories for different target applications are discussed. A comprehensive overview of commercial humidity sensors is also presented. The aim of this review is to provide clear guidelines for researchers working in this field, so as to provide a new path which needs to be adopted and to discourage redundant research related to humidity sensors development.