The ability to propel against flows, that is, to perform positive rheotaxis, can provide exciting opportunities for applications in targeted therapeutics and non-invasive surgery. So far no biocompatible technologies exist for navigating microparticles upstream when they are in a background fluid flow. Inspired by many naturally occurring microswimmers—such as bacteria, spermatozoa and plankton—that utilize the no-slip boundary conditions of the wall to exhibit upstream propulsion, here we report on the design and characterization of self-assembled microswarms that can execute upstream motility in a combination of external acoustic and magnetic fields. Both acoustic and magnetic fields are safe to humans, non-invasive, can penetrate deeply into the human body and are well-developed in clinical settings. The combination of both fields can overcome the limitations encountered by single actuation methods. The design criteria of the acoustically induced reaction force of the microswarms, which is needed to perform rolling-type motion, are discussed. We show quantitative agreement between experimental data and our model that captures the rolling behaviour. The upstream capability provides a design strategy for delivering small drug molecules to hard-to-reach sites and represents a fundamental step towards the realization of micro- and nanosystem navigation against the blood flow.
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