Stimuli-responsive hydrogel microfibers with controlled anisotropic shrinkage and cross-sectional geometries.

Stimuli-responsive microfibers are fabricated by extruding mixed solutions of poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAM-AAc) and sodium alginate (Na-alginate) using a microfluidic spinning system. The fabricated microfibers shrink and swell with temperature and/or pH. By controlling the extruded laminar flow, microfibers capable of anisotropic shrinkage are fabricated. Cross-sectional microscale geometries of microfibers, including double layering and hollowness, are successfully controlled by patterning the laminar flow during microfiber formation, resulting in hydrogels capable of folding/unfolding motions and fluid pumping. In addition, macroscopic 3D-bundle structures are assembled with these microfibers. We believe that our microfibers can be applied to various applications such as soft actuators, soft robots, and micropumps.