Magnetite Nanostructured Porous Hollow Helical Microswimmers for Targeted Delivery

Bacteria-inspired magnetic helical micro-/nanoswimmers can be actuated and steered in a fuel-free manner using a low-strength rotating magnetic field, generating remotely controlled 3D locomotion with high precision in a variety of biofluidic environments. They are therefore envisioned for biomedical applications related to targeted diagnosis and therapy. In this article, a porous hollow microswimmer possessing an outer shell aggregated by mesoporous spindle-like magnetite nanoparticles (NPs) and a helical-shaped inner cavity is proposed. The fabrication is straightforward via a cost-effective mass-production process of biotemplated synthesis using helical microorganisms. Here, Spirulina-based fabrication is demonstrated as an example. The fabricated microswimmers are superparamagnetic and exhibit low cytotoxicity. They are also capable of performing structural disassembly to form individual NPs using ultrasound when needed. For the first time in the literature of helical microswimmers, a porous hollow architecture is successfully constructed, achieving an ultrahigh specific surface area for surface functionalization and enabling diffusion-based cargo loading/release. Furthermore, experimental and analytical results indicate better swimming performance of the microswimmers than the existing non-hollow helical micromachines of comparable sizes and dimensions. These characteristics of the as-proposed microswimmers suggest a novel microrobotic tool with high loading capacity for targeted delivery of therapeutic/imaging agents in vitro and in vivo.