Hydrogels are widely used in tissue engineering,[1] drug delivery,[2] and wound dressings.[3] Recently, efforts have been made to integrate photonic functions into hydrogel materials for novel biomedical applications, including functional optical fibers for light delivery and monitoring of blood oxygenation levels in biological tissues,[4] cell-containing hydrogel implants for toxicity sensing and optogenetic therapy,[5] and nanoparticle-embedded hydrogels for detection of biochemical analytes.[6] However, due to the weak and brittle nature of common synthetic hydrogels,[7] these hydrogel photonic devices are relatively fragile against external stress and strain. The low mechanical strength poses a practical challenge for these hydrogels’ applications in wearable or implantable devices because the body motion and tissue movement may degrade or damage the structure and function.[8] A number of recipes for polymer gels to obtain mechanical toughness, resilience, and elasticity, as well as flexibility have been demonstrated.[9, 10] One of the most promising approaches is to form a hybrid of ionic and covalent polymer networks where the covalently crosslinked long-chain polymers give high stretchability of the hydrogel and the reconfigurable ionically crosslinked polymers enhance the hydrogel’s toughness by dissipating mechanical energy under deformation.[11] The hybrid network of ionically crosslinked alginate and covalently crosslinked polyacrylamide has shown remarkable stretching capability, high fracture energy of~ 9,000 J m− 2,[11] and biocompatibility.[7, 12] Despite recent enhancements of hydrogels’ mechanical properties, highly stretchable and robust hydrogel photonic devices have not been achieved yet.

Here, we report the design and fabrication of highly stretchable and tough optical fibers made of optically-optimized alginate-polyacrylamide hydrogel materials in a core/clad stepindex structure (Figure S1). The fibers can be elongated to an axial strain of 700% and then relaxed over multiple cycles. Functional molecules, such as organic dyes, may also be incorporated into the porous matrix of hydrogels by solution doping. Harnessing these unique properties, we have devised a novel, simple technique for measuring axial strain and accomplished distributed strain sensing based on dye-doped hydrogel fibers with a large dynamic strain range.