Human blood clots kill an estimated 100,000 to 300,000 Americans each year. Current treatments rely on medications that break down clots, which can be combined with a surgical procedure that mechanically alters the clot. However, clot-busting medications and surgery are both linked to unintended adverse events. This project designs and studies miniature magnetic swimmers as a minimally invasive alternative to these treatments. These devices are millimeter-scale objects that have a helical shape and contain a small permanent magnet. A rotating magnetic field is used to remotely control the swimmers. The field makes the swimmer rotate and the helical shape converts the rotational movement into a propulsive force (much like a boat's propeller). The swimmers can be steered by changing the orientation of the applied magnetic field. They can precisely navigate within liquids along pre-defined 3D paths and could potentially navigate within the bloodstream toward a blood clot. The rotational movement can be used to abrade the clot and restore an appropriate blood flow. This project introduces new biomedical paradigms and investigates the robust high-speed tetherless manipulation of magnetic swimmers in complex, time-varying environments. It requires multiple swimmer designs, innovative controllers, and integrated, dynamic sensing modalities. The project is divided into three synergetic objectives. First, a fluid-dynamics model for the swimmer must be created and validated experimentally. This model will be used to optimize the swimmer's geometry. Secondly, a robust swimmer controller will be built. The controller must be able to compensate for the variable blood flow present in the arteries. An ultrasound scanner will be used to track the position of the swimmer and obtain useful information about its environment. For example, the velocity of the swimmer can be used to infer the blood flow velocity. Finally, the feasibility of several surgical procedures will be studied via ex-vivo experiments. 3D navigation together with the removal of human blood clots will be performed inside a human heart phantom. Several swimmers end effectors will also be tested. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.