Hollow spherical shells made of various oxides have long been used as filler materials to prepare lightweight materials with high specific strength. We present the fabrication of bubble-filled silica microfibers with controlled size and structure by using multiphase flow in a coaxial glass capillary device. The multiphase flow is controlled to generate a jetting gas-in-water-in-oil core-sheath stream, which is converted to flexible nanoparticle-polymer composite fibers filled with bubbles. The production rate of these bubble fibers can reach up to 2000 cm/min. These flexible fibers can be manipulated to desired shapes or architectures and subsequently be sintered via thermal treatment. We demonstrate the fabrication of bubble fiber-filled light weight composite, which potentially has a significant advantage over conventional light-weight composites made with spherical hollow particles. (C) 2015 Elsevier B.V. All rights reserved.