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Using a single rotating magnetic field, RBC biohybrid micromotors can be controlled to achieve propulsionviaswimming and rolling modes. The propulsion mechanism, directional control, and behavior in different fluids is investigated. 

Here, we use magnetically driven self-assembled achiral swimmers made of two to four superparamagnetic micro-particles to provide insight into how swimming kinematics develop in complex, shear-thinning fluids. Two model shear-thinning polymer fluids are explored, where measurements of swimming dynamics reveal contrasting propulsion kinematics in shear-thinning fluids vs a Newtonian fluid. When comparing the velocity of achiral swimmers in polymer fluids to their dynamics in water, we observe kinematics dependent on (1) no shear-thinning, (2) shear-thinning with negligible elasticity, and (3) shear-thinning with elasticity. At the step-out frequency, the fluidic environment's viscoelastic properties allow swimmers to propel faster than their Newtonian swimming speed, although their swimming gait remains similar. Micro-particle image velocimetry is also implemented to provide insight into how shear-thinning viscosity fluids with elasticity can modify the flow fields of the self-assembled magnetic swimmers. Our findings reveal that flow asymmetry can be created for symmetric swimmers through either the confinement effect or the Weissenberg effect. For pseudo-chiral swimmers in shear-thinning fluids, only three bead swimmers show swimming enhancement, while four bead swimmers always have a decreased step-out frequency velocity compared to their dynamics in water. 

Abstract We report the effects of polymer size, concentration, and polymer fluid viscoelasticity on the propulsion kinematics of achiral microswimmers. Magnetically driven swimmer's step‐out frequency, orientation angle, and propulsion efficiency are shown to be dependent on fluid microstructure, viscosity, and viscoelasticity. Additionally, by exploring the swimming dynamics of two geometrically distinct achiral structures, we observe differences in propulsion efficiencies of swimmers. Results indicate that larger four‐bead swimmers are more efficiently propelled in fluids with significant elasticity in contrast to smaller 3‐bead swimmers, which are able to use shear thinning behavior for efficient propulsion. Insights gained from these investigations will assist the development of future microswimmer designs and control strategies targeting applications in complex fluids.