All‐Aqueous Pullulan Fibers Enabling Visible‐to‐Near‐Infrared Waveguiding with Mechanical and Thermal Resilience

Polymer optical fibers (POFs) are widely used in short‐distance communication and sensing, but current fibers rely on petroleum‐based polymers such as polymethyl methacrylate (PMMA), which soften near 100 °C and lack renewability. Cellulose‐derived fibers have been proposed as sustainable alternatives, yet progress is hindered by solvent‐intensive fabrication and intrinsic limits including narrow transmission and high optical loss. An all‐aqueous borax gel‐spinning method is presented using the water‐soluble polysaccharide pullulan, eliminating organic solvents and thermal processing. The process yields transparent amorphous fibers with tensile strength of ≈200 MPa, exceeding that of PMMA and comparable to regenerated cellulose. These fibers retain their elastic modulus up to 200 °C, an unusual property for transparent amorphous polymers. The water‐only fabrication suppresses chromophore formation, preserving optical clarity. Pullulan's amorphous, non‐aromatic structure allows low‐loss waveguiding across the visible to near‐infrared (NIR, up to 1300 nm) spectrum, enabling efficient blue‐light transmission for the first time in polysaccharide fibers to the knowledge. Initial measurements show 1.15 dB cm⁻¹ attenuation at 405 nm and transmission from 350 to 1300 nm. In addition, the intrinsic hygroscopicity of pullulan enables direct humidity sensing without additives. Together, these results identify pullulan–borax fibers as a sustainable, multifunctional platform for next‐generation green photonics.