Artificial Microtubules from Microrobot Swarm Unraveling for Directed Cargo Transport under Confinement

Cargo transport on artificial microtubules (AMTs) has been extensively explored in various biomedical applications, including drug delivery and cancer therapy. However, the transport velocity and directional controllability are still poor compared to their natural counterparts, especially under confinement. Here, we show that confinement can induce the spontaneous formation of AMTs from structural transformations of microrobot swarms, and that these AMTs are particularly adept at directionally controlled cargo transport. It is shown that a magnetically driven microrobot cluster, when locally pinned, can be unraveled along the boundary into a strand that can function as a microtubule thanks to collective tangential flows. We demonstrate with both experiments and simulations that the local pinning required for AMT formation can come from a single pinned particle, or from corners and necks ubiquitous in microchannels and cavities. By modulating the driving magnetic field, the microtubule formation process can be precisely manipulated following periodic perturbations and re-establishments of local order, thereby simultaneously controlling the velocity and direction of cargo transport. The general applicability of this method has also been tested in biological environments including flowing blood, as well as in an in vitro delivery model under confinement.