The design of advanced materials with coupled optical and mechanical properties is an important challenge in materials science; especially, the implementation of soft materials in optics has recently gained significant interest. Soft optical systems are particularly versatile in sensing, where large and repeated deformations require dynamically responsive materials. Here, stretchable step‐index optical fibers, which are capable of reversibly sustaining strains of up to 300% while guiding light, are demonstrated. A continuous and scalable melt‐flow process is used to coextrude two thermoplastic elastomers, thereby forming the fibers' high index core‐low index cladding structure. Deformation of the fibers through stretching, bending, and indentation induces detectable, predictable, reversible, and wavelength‐dependent changes in light transmission. Quantitative knowledge about the coupling of the fibers' mechanical and optical properties forms the basis for the design of fiber‐based sensors that are capable of reliably assessing extreme mechanical stimuli. The fibers utility in sensing scenarios is demonstrated in a knee brace for continuous knee motion tracking, a glove for control of a virtual hand model, and a tennis racket capable of locating ball impacts. Such devices can greatly improve quantitative assessment of human motion in rehabilitation, sports, and anywhere else where large deformations need to be monitored reliably.

No abstract