Synergistic Water‐State Regulation and Network Densification Enable Low‐Loss Fine‐Core Hydrogel Flexible Optical Fibers for Wearable Soft Photonics

Abstract

ABSTRACT Biological soft photonics shows broad potential in wearable health monitoring. Hydrogel flexible optical fibers (HFOFs) possess significant application potential due to biocompatibility. However, while high water content and multi‐network architectures endow HFOFs with unique biological and mechanical advantages, they also introduce substantial optical losses, thereby limiting their application in meter‐scale flexible photonic systems. This study proposes a synergistic strategy combining water‐state regulation and network densification. By increasing the fraction of bound water, the absorption loss associated with free water is reduced, while network densification suppresses scattering loss arising from structural heterogeneity. Polyacrylamide hydrogel optical fibers are fabricated using a spider‐silk‐inspired draw‐spinning method. Combined continuous stretching and thermal treatment induce chain alignment, network densification, and the formation of hydrogen‐bonded nanoclusters. These structural changes create a confined hydration environment, resulting in a more compact hydrogel fiber network and increasing the bound‐water fraction to 70%. The fabricated HFOFs exhibit low optical loss (0.0745 ± 0.0012 dB/cm) and small diameter (120 µm), enabling a tenfold increase in light‐guiding distance. The low‐loss HFOFs are further integrated into skin‐conformal devices, successfully applied in silent speech recognition and braille recognition. This study proposes a general strategy for designing soft photonic materials with low loss and high compliance.