Advances in cellulose nanofiber hydrogels for self-powered wearable pressure sensors
The 21st century has seen rapid advancements in the field of flexible electronics field.Unlike traditional pressure sensors,flexible pressure sensors are lightweight,portable,and comfortable,making them a focal point of widespread interest.Despite these advantages,the issue of discontinuous energy supply during long-term monitoring of complex physiological signals in the human body remains unresolved.Flexible pressure sensors often require frequent charging or battery replacement,leading to signal detection interruptions and complicating the sensing process.Therefore,it is crucial to develop self-powered flexible pressure sensors that can harvest energy from environmental sources or the human body.Moreover,the flexibility of the pressure sensor and the degree of adhesion to the human skin have a great influence on the sensing performance.Hydrogels are three-dimensional network structures that are physically or chemically crosslinked,consisting mostly of water,with water content that can exceed 90％.The network density of the hydrogel network is controlled by crosslinking points,and it is a key factor affecting the mechanical properties of hydrogels.Therefore,hydrogels exhibit adjustable mechanical performance.During the compression process under external force,hydrogels undergo deformation through the sliding of polymer chains,which contributes to their flexibility.Hydrogels are ideal materials for flexible,wearable pressure sensing materials due to their adjustable modules and excellent compatibility with human skin.At the same time,cellulose nanofibers are natural biomass polymers,which are abundantly available on earth.They have good biodegradability and mechanical properties.Chemical modification of cellulose nanofiber can enhance the piezoelectric and triboelectric responses.Therefore,cellulose nanofiber-based hydrogels provide excellent sensing capabilities,mechanical flexibility,adhesion property,and biodegradability,making them ideal materials for developing flexible self-powered pressure sensors.This review summarizes recent development of flexible pressure sensors with self-powered capability based on cellulose nanofiber-based hydrogels.First,a brief introduction to the construction,microstructure and properties of cellulose nanofibers is provided,followed by an overview of the physical and chemical cross-linking strategies used in cellulose nanofiber-based hydrogels.Physical crosslinking includes hydrogen bonds,hydrophobic interactions,electrostatic interactions,and ionic interactions.Chemical crosslinking can be categorized into conventional chemical crosslinking and dynamic chemical crosslinking(such as borate ester bonds and disulfide bonds).Subsequently,flexible self-powered pressure sensors based on cellulose nanofiber hydrogels can be classified into piezoelectric and triboelectric types according to their energy supply mechanisms.Their working principles and practical applications in detecting human signals are discussed.Finally,the challenges and prospects for the development of flexible self-powered pressure sensors based on cellulose nanofibers are summarized.
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