Microrobots may become an effective tool in breaking down the dangerous blood vessel blockages that cause strokes. But these spherical robots technically aren’t autonomous machines. Instead, they’re tiny, magnetically guided beads packed with lifesaving medications, along with small amounts of a radioactive tracer to help doctors track their journey.
Current treatments for stroke patients often involve injectable drugs that dissolve a blockage in the blood vessel called a thrombus. Given the vast nature of the circulatory system, the procedure frequently requires a high dosage of medication to ensure that a proper amount reaches the target area. This makes the procedure itself inherently risky, with possible serious side effects including internal bleeding.
Robotics researchers at Switzerland’s ETH Zurich now believe that an alternative strategy may be on the way. As they detail in a study published in the journal Science, they’ve designed a soluble gel capsule imbued with just enough iron oxide nanoparticles to magnetize it.
“Because the vessels in the human brain are so small, there is a limit to how big the capsule can be. The technical challenge is to ensure that a capsule this small also has sufficient magnetic properties,” robotics researcher and study coauthor Fabian Landers said in a statement.
Landers and his team added nanoparticles of the element tantalum to allow for X-ray tracing. It’s taken years to find the right balance of components, but the researchers say they now have a magnetic microrobot capable of reliably navigating the human body’s roughly 360 arteries and veins.
“Magnetic fields and gradients are ideal for minimally invasive procedures because they penetrate deep into the body and–at least at the strengths and frequencies we use–have no detrimental effect on the body,” explained study coauthor microrobotist and study coauthor Bradley Nelson.
That said, the small delivery agents are only as good as they are effective. To test their invention, Landers and Nelson first used a catheter to inject the microrobot into artificial silicone models of both human and animal blood vessels. Based on an already available design, the specialized catheter includes an internal guidewire linked to a polymer gripper that opens to release the microrobot. However, it’s not as simple as slowly guiding the device at a single speed until it reaches its destination.
“The speed of blood flow in the human arterial system varies a lot depending on location. This makes navigating a microrobot very complex,” Nelson said.
This means that the guidance system relies on three separate strategies in order to navigate through every arterial region in a head. Using one rotating magnetic field, the team successfully and precisely guided the microrobot at speeds of up to 4 millimeters per second.
In another model, a shifting magnetic field gradient pulled the device along the stronger field, even against the blood flow’s current. In some cases, the microrobot reached a velocity of 20 centimeters per second.
“It’s remarkable how much blood flows through our vessels and at such high speed,” said Landers. “Our navigation system must be able to withstand all of that.”
After successful lab demonstrations, researchers moved on to clinical tests using pigs. In 95 percent of test scenarios, the microrobot delivered the thrombus medication to the correct destination. The procedure also showed promise in a sheep’s cerebrospinal fluid, indicating it could be used for many other medical applications.
“This complex anatomical environment has enormous potential for further therapeutic interventions, which is why we were so excited that the microrobot was able to find its way in this environment, too,” Landers said.
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