Optimization of Tail Geometry for the Propulsion of Soft Microrobots

Recent advances in smart materials and microfabrication techniques lead to the development of microrobots for on-demand and targeted therapy. Self-folded hydrogel tubes are particularly promising vehicles as they provide relatively large surface area-to-volume ratio and cargo space for therapeutic agents. In this paper, we decorate these microstructures with an artificially approximated bacterial flagellum to enable efficient swimming in fluidic environments. Flexibility enhances overall motility of the soft microrobot through synergistic propulsion generated by the tubular body and the flagellum, a feature that has not been observed in conventional microrobots manufactured from rigid materials. While the flagellum is applying forward thrust, a precession is induced on the body due to wobbling of the tail that can provide extra speed depending on the tail design. A simple model based on resistive force theory explains the direction-dependent changes in swimming motility and the role of tail geometry.