Distributed and multimodal strain sensing performance of flexible hydrogel functional optical fibers

Traditional silica-based optical fiber strain sensors suffer from a limited range of strain detection, mismatched mechanical performance, and complicated sensing structures that hinder their practical applications involving complex strain patterns and large strain measurement. To solve these issues, the development of novel flexible optical fiber strain sensors with distributed and multimodal strain sensing performance is critical but challenging. In this work, a flexible hydrogel optical fiber with a step-index core/cladding structure was successfully designed and fabricated using a simple thermal polymerization method involving calcium alginate and polyacrylamide. The resulting flexible fiber exhibited a large stretch deformation capacity and good optical transmission property as well as an effective and reversible strain response. Significantly, a flexible hydrogel functional fiber was further constructed by doping quantum dots with different fluorescent emission wavelengths in different positions. By exploiting a combination of emission wavelength encoding and transmission loss, the assembled flexible hydrogel functional optical fibers could distinguish and recognize the locations, magnitudes, and modes (stretch or bend) of mechanical deformation through a single wavelength of visible light (365 nm) excitation. An ultra-large dynamic range (0-120%) of axial strain and bending deformation (10°-80°) could be effectively detected. This work offers a versatile solution to develop promising stretchable and distributed fiber strain sensors. Flexible fiber enables large-scale, multi-mode, and distributed strain sensing and provides a versatile solution for wearable and implantable strain sensors.