Carbon nanofibers and their composites for wearable piezoresistive physical sensors

In the last decades, flexible piezoresistive sensors have attracted tremendous attention owing to their wide-range applications in electronic skin, health monitoring, soft robots and human-machine interfaces. Thanks to the flexibility, porosity, excellent electrical and mechanical properties, carbon nanofibers (CNFs) exhibit promising potential in wearable sensors and electronics. While carbon-based materials have been extensively studied and reported as promising candidates for piezoresistive sensing materials, CNF-based sensors, particularly for temperature and humidity sensors, are rarely reviewed. This comprehensive review summarizes recent advancements in the design and preparation strategies for fabricating CNFs and their composites for wearable piezoresistive physical sensors. It highlights the fabrication methods including chemical vapor decomposition (CVD), electrospinning and template synthesis. Various structures of CNFs, including random, aligned, core/sheath, hollow, porous and doping CNFs, with corresponding fabricating technology, are summarized and compared. In addition, the sensing performance and working mechanisms of CNFs in wearable piezoresistive sensors, such as strain, pressure, temperature and humidity, are emphasized and discussed. Finally, the challenges and potential opportunities of CNF-based piezoresistive sensors are revealed to offer new insights for future development.