  The ability to propel against flows, i.e., to perform positive rheotaxis, can
provide exciting opportunities for applications in targeted therapeutics and
non-invasive surgery. To date, 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 non-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 toward the realization of micro- and
nanosystem-navigation against the blood flow.
