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.

推动逆流的能力，即执行正流趋性，可以为靶向治疗和非侵入性手术的应用提供令人兴奋的机会。到目前为止，尚不存在用于在背景流体流中引导微粒向上游流动的生物相容性技术。受到许多自然存在的微型游泳者（例如细菌、精子和浮游生物）的启发，这些微型游泳者利用壁的无滑移边界条件来表现出上游推进力，在这里我们报告了自组装微型群的设计和表征，这些微型群可以在水中执行上游运动外部声场和磁场的组合。声场和磁场对人体都是安全的，非侵入性的，可以深入人体，并且在临床环境中得到了很好的发展。两个领域的结合可以克服单一驱动方法遇到的限制。讨论了执行滚动型运动所需的微群声致反作用力的设计标准。我们展示了实验数据和捕获滚动行为的模型之间的定量一致性。上游能力提供了一种将小药物分子输送到难以到达的位点的设计策略，代表着实现针对血流的微米和纳米系统导航的基本一步。