Tadpole-Like Flexible Microswimmers with the Head and Tail Both Magnetic

In analogy to eukaryotic cells that move by beating the flagella, magnetically powered micro/nanorobots with flexible filaments are capable of eluding the limitation of the scallop theorem to generate net displacement in a three-dimensional space, but they are limited by complicated fabrication and low speed. Here, we demonstrate a tadpole-like flexible microswimmer with a head and tail that are both magnetic by developing a magnetically assisted in situ polymerization method. The flexible microswimmer consists of a magnetic-bead head fixed to a nanochain bundle of magnetic nanoparticles (tail), and the tail length and stiffness can be adjusted simply by changing the duration and strength of the applied magnetic field during fabrication, respectively. For the microswimmer under an oscillating magnetic field, the magnetic head generates an undulatory motion, which can be further increased by the flexible magnetic tail. The magnetically induced undulation of the head and tail generates a traveling wave propagating through its flexible tail, resulting in efficient tadpole-like propulsion of the microswimmer. The flexible microswimmer runs at a maximum motion speed when the tail length is ∼5 times the diameter of the magnetic head, corresponding to ∼half the wavelength of the undulatory motion. The flexible microswimmers reported here are promising for active sensing and drug delivery, as the tails can be designed with various responsive hydrogels, and the results are expected to advance flexible micro/nanorobots.