Shape anisotropy-governed locomotion of surface microrollers on vessel-like microtopographies against physiological flows

Abstract

Significance Controlled microrobotic navigation in blood vessels holds significant potential to revolutionize targeted drug delivery. Navigation on the surface of the blood vessels is advantageous because of decreased flow velocities; however, surface microtopography of blood vessels, in the size scale of the microrobots, is a major hurdle for robust locomotion of surface-rolling microrobots against the blood flow. Here, we show the effect of the body-shape anisotropy of the microrollers on their locomotion capability over vessel-like microtopographies. We demonstrate by experiments and simulations that the microrollers with slender bodies are more robust to locomote on biological microtopographies due to their favorable hydrodynamic interactions. Thus, such anisotropically shaped microrollers would be more viable and robust in future in vivo medical applications. Surface microrollers are promising microrobotic systems for controlled navigation in the circulatory system thanks to their fast speeds and decreased flow velocities at the vessel walls. While surface propulsion on the vessel walls helps minimize the effect of strong fluidic forces, three-dimensional (3D) surface microtopography, comparable to the size scale of a microrobot, due to cellular morphology and organization emerges as a major challenge. Here, we show that microroller shape anisotropy determines the surface locomotion capability of microrollers on vessel-like 3D surface microtopographies against physiological flow conditions. The isotropic (single, 8.5 µm diameter spherical particle) and anisotropic (doublet, two 4 µm diameter spherical particle chain) magnetic microrollers generated similar translational velocities on flat surfaces, whereas the isotropic microrollers failed to translate on most of the 3D-printed vessel-like microtopographies. The computational fluid dynamics analyses revealed larger flow fields generated around isotropic microrollers causing larger resistive forces near the microtopographies, in comparison to anisotropic microrollers, and impairing their translation. The superior surface-rolling capability of the anisotropic doublet microrollers on microtopographical surfaces against the fluid flow was further validated in a vessel-on-a-chip system mimicking microvasculature. The findings reported here establish the design principles of surface microrollers for robust locomotion on vessel walls against physiological flows.