Fiber is one of the most fundamental material forms seen in human life. Befitting from their long and bendable shape, fibers with different specialties and different dimensions are used in a multitude of applications, ranging from fabrics [1] to telecommunications [2], from generating laser [3] to sensing and actuating, etc. [4–6]. In recent years, major breakthroughs were made, demonstrating that fibers have novel optical [7–9], electronic [10], acoustic [11, 12] and cell interfacing [13, 14] properties that enable new functionalities. Functional fibers and related application research are at the crossroads of many disciplines, including optics, materials science, device physics, nanotechnology, and fluid dynamics.