Fabrication Technologies for Soft, Multimaterial Optical Fibers for In Vivo Diagnostics and Phototherapy, With a Focus on Extrusion Printing

ABSTRACT Multimaterial optical fibers provide a versatile platform for integrating diverse functionalities—such as waveguiding, side emission, sensing, and actuation—into a single filament. Although traditional multimaterial fibers have primarily been fabricated from rigid materials such as silica and thermoplastics for optoelectronic applications, recent developments have shifted the focus toward soft systems composed of elastomers, hydrogels, and their composites. Owing to their mechanical compliance and biocompatibility, these soft fibers are particularly well suited for wearable, implantable, and tissue‐integrated devices used in diagnostics and phototherapy. This review provides a comprehensive overview of the rapidly developing field of soft multimaterial optical fibers, highlighting key material combinations and fabrication strategies that enable multifunctional performance. Particular emphasis is placed on extrusion‐based multimaterial printing—including coaxial and segmented extrusion—which has significantly expanded the architectural and functional design space of soft optical fibers. Remaining challenges, including material compatibility, interfacial and surface quality, and printing resolution, are critically discussed. Finally, the review outlines emerging opportunities for advancing these fabrication approaches toward practical and clinically relevant biomedical applications.