Nanocellulose based hydrogel for flexible sensors: Current progress and future perspective

Hydrogels have found significant applications in flexible sensing due to their tunable modulus and excellent conformability to human skin. However, petroleum-based hydrogels (PPH), due to their origins from increasingly scarce petroleum resources and poor biocompatibility, still face limitations in widespread applications for detecting physiological signals in the human body. It is highly desirable to overcome these drawbacks and utilize new environmentally friendly materials. Hydrogels based on nanocellulose material (NCM) as fillers or substrates, are emerging as the most promising candidates for the next generation of flexible sensors, owing to their wide availability and excellent biocompatibility. This overview highlights the latest progress in utilizing nanocellulose-based hydrogel (NCH) to fabricate flexible sensors with diverse functionalities. We first provide a brief introduction of three types of NCM, including cellulose nanocrystal (CNC), bacterial cellulose (BC), and cellulose nanofibril (CNF). Following a discussion on the fundamental material characteristics essential for synthesizing flexible sensors, we outline the design principles and cross-linking strategies for hydrogels rooted in NCMs. Subsequently, we focus on the working principles and performance characteristics of flexible sensors for physical and chemical signals. Lastly, we summarize the challenges and prospects in the advancement of flexible sensors based on NCMs.