Stroke affects 12 million people globally each year, often causing death or lasting disability. Current treatment relies on systemic administration of clot-dissolving drugs, which circulate throughout the entire body. Because only a fraction reaches the blocked vessel, clinicians must administer high doses that significantly increase the risk of dangerous side effects, including internal bleeding. For decades, researchers have sought a way to deliver medication directly to the stroke-related thrombus without exposing the rest of the body to unnecessary drug levels. Now, a new magnetic microrobot system can navigate deep into the brain’s tiny vessels and deliver thrombolytic drugs directly to the blockage.

The work, by researchers at ETH Zurich (Zurich, Switzerland), represents a major leap toward minimally invasive, targeted therapy for stroke patients. The microrobot consists of a proprietary spherical capsule made of a dissolvable gel shell embedded with precision-engineered iron oxide nanoparticles for magnetic guidance and tantalum nanoparticles for X-ray visibility. Achieving strong magnetic responsiveness at such a small size was a key technical challenge, given the narrow diameter of brain vessels.

Inside the capsule, the researchers loaded active drugs—including a clot-dissolving agent, antibiotics, or anticancer medications—which can be released on demand. When exposed to a high-frequency magnetic field, the magnetic nanoparticles heat up, dissolving the gel shell and releasing the therapeutic payload exactly where it is needed. The team designed a two-step delivery system: first, the microrobot is inserted into blood vessels or cerebrospinal fluid using a catheter; second, an electromagnetic navigation platform steers it through branching arteries and high-velocity blood flow toward the target site. The catheter itself includes a flexible polymer gripper that releases the microrobot precisely within the vessel.

To maneuver through the brain’s complex vascular system, the researchers combined three navigation strategies. A rotating magnetic field allows the microrobot to roll along vessel walls; a magnetic field gradient can pull the capsule upstream even against rapid blood flow exceeding 20 centimeters per second; and an in-flow navigation method guides the microrobot through difficult junctions by shaping the magnetic gradient to direct blood flow into the correct branch. This integrated approach allowed precise movement through diverse anatomical pathways at speeds up to 4 millimeters per second.

Across more than 95% of experimental trials, the microrobot successfully reached its target and delivered the drug with high precision. Because magnetic fields penetrate deeply into the body without harmful effects at the strengths used, this method offers a safe, minimally invasive navigation strategy. To test their system under realistic conditions, the team developed highly accurate silicone vessel models based on patient anatomy, allowing extensive training and optimization without animal use. Following successful model experiments, researchers demonstrated the system’s feasibility in pigs, showing that all three navigation methods function under realistic blood flow conditions and that the microrobot remains visible throughout the procedure.

In their research published in Science, they also navigated the microrobot through the cerebrospinal fluid of a sheep, a complex anatomical environment that suggests broad potential for future neurological interventions. In addition to stroke treatment, the researchers envision applications in targeting localized infections or tumors. With clinical translation as their central goal, the team is now preparing for human trials. Their work reflects a commitment to delivering technologies that support surgeons in real time and offer patients safer, faster, and more effective therapeutic options.

“Doctors are already doing an incredible job in hospitals,” said researcher Fabian Landers, lead author of the paper. “What drives us is the knowledge that we have a technology that enables us to help patients faster and more effectively and to give them new hope through innovative therapies.”

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