Wearable fiber-optic sensors: Recent advances and future opportunities

The past years have witnessed the fast development of wearable electronic sensors, which have broad applications in biomechanics, personal healthcare, electronic skin, and human-machine interface. Nevertheless, conventional electronic sensors suffer from susceptibility to electromagnetic interference, electrochemical corrosion, and electrical safety issues. Compared with electronic devices, optical sensor devices have the advantages of anti-electromagnetic interference and electrical safety, and optical signals have the featured properties of large transmission bandwidth, multiplexing, flexible modulation, and demodulation methods, which show great potential in multi-degree-of-freedom sensing. The research of wearable optical fiber sensors is emerging recently, which can be complementary to electric sensors. This work focuses on the recent advances in wearable fiber-optic sensors, including silica fiber and polymer fiber devices. In the silica fiber section, the sensing mechanism, fabrications, and wearable sensing applications of fiber Bragg gratings and microfiber structures are mainly focused. In particular, microfibers with diameters of tens of nanometers to several micrometers, show many different intriguing properties compared with traditional bulk fibers, such as strong light field confinement, strong evanescent fields, high nonlinearity, and mechanical strength, which are promising for high-dense integration. In the polymer fiber section, the fundamental sensing mechanisms and fiber-optic sensing applications are discussed. Compared with rigid silica fibers, polymer optical fibers have similar mechanical modulus to human tissues, better biocompatibility and stretchability, and they are also easy to process and mix with other functional organic materials, and some polymer materials are also degradable or have the self-healing capability. In addition, the hydrogel fibers for in vivo chemical sensing are highlighted. In the final section, it summarizes the pros and cons of typical wearable sensors, and the challenges and future opportunities of this field are discussed.