Abstract: Inspired by bacterial flagella in nature, magnetic helical microswimmer is an ideal model to perform complex task in a low Reynolds number environments. Shape Memory Polymers (SMPs) with desirable properties are considered as one of the most preferred options for the development of small-scale robots. However, fabricating and programming strategies are still challenging. Here, we report an approach to fabricate helical microswimmers based on thermoplastic SMP (polylactic acid). Melt-spun polylactic acid fibers containing magnetic particles were enwound to form helical microstructures. Their shape recovery behaviors were programmed by annealing and pre-deformation. Three forms of helical microswimmers (constant-helix-angle conical helix, constant-pitch conical helix, and straight helix) with controlled morphological parameters were tailored. The obtained microswimmers showed 3D locomotion capability under rotating magnetic fields. The maximum swimming velocity of microswimmers was nearly six body lengths per second, and the near-wall swimming of conical helixes along their sharp end exhibited a smaller drift. Moreover, we demonstrated programmed shape-switching processes (spring-like contraction and elongation, coiling and uncoiling) and self-repairing of the microswimmers. As demonstrations of potential applications, tasks of mobile microstent, cargo delivery, and minimally invasive injection were carried out. The multifunctional shape-memory microswimmers have immense potential in a variety of applications.

Key words: helical microrobot, magnetic actuation, Shape Memory Polymer (SMP), polylactic acid, magnetic particles