Tough Gel-Fibers as Strain Sensors Based on Strain–Optics Conversion Induced by Anisotropic Structural Evolution

The advocacy of smart living results in a high demand for wearable and flexible sensors to monitor human motions. Among these, sensors based on strain–optics conversion are attractive due to their inherent electrical safety and electromagnetic immunity in comparison to strain–electricity conversion sensors. Particularly, hydrogel-based optical fiber sensors are biocompatible, flexible, and stretchable and thus are potentially applicable to health monitoring, human–machine intelligence, and soft robots. Nonetheless, hydrogel-based optical fibers still demonstrate challenges such as limited stretch ratios from chemical cross-linking networks and insufficient light transmittance from dehydration or nucleation of water. Herein, flexible and stretchable strain sensors based on glycerol-introducing nanocomposite hydrogel fibers (GN-Fibers) were achieved via dynamic stretching of a reactive pregel from monomer/nanoparticle hybrid precursors in a glycerol–water cosolvent. The resultant GN-Fibers evolved with anisotropic microstructures, displaying excellent tensile strength (9.76 MPa), high elastic modulus (32.63 MPa), low light propagation attenuation (0.26 dB cm–1), and broad strain range. Owing to the use of glycerol–water, such GN-Fibers also exhibited long-term moisture-retaining and antifreezing properties. In addition, GN-Fibers functioned well as sensors based on strain–optics conversion to monitor stretching and compressing behaviors. It is believed that such an optical fiber based strain sensor is a gateway to fabrication of next-generation wearable and flexible devices for health monitoring or artificial intelligence.