When treating an ischemic stroke - where a clot is blocking the flow of oxygen to the brain - every minute counts. The more quickly doctors can remove the clot and restore blood flow, the more brain cells will survive, and the more likely patients are to have a good outcome. But current technologies only successfully remove clots on the first try about 50% of the time, and in about 15% of cases, they fail completely.
Researchers at Stanford Engineering have developed a new technique called the milli-spinner thrombectomy that could significantly improve success rates in treating strokes, as well as heart attacks, pulmonary embolisms, and other clot-related diseases. In a paper published June 4 in Nature, the researchers used both flow models and animal studies to show that the milli-spinner significantly outperforms available treatments and offers a new approach for fast, easy, and complete clot removal.
"For most cases, we're more than doubling the efficacy of current technology, and for the toughest clots - which we're only removing about 11% of the time with current devices - we're getting the artery open on the first try 90% of the time," said co-author Jeremy Heit, chief of Neuroimaging and Neurointervention at Stanford and an associate professor of radiology. "It's unbelievable. This is a sea-change technology that will drastically improve our ability to help people."
Taking advantage of tangles
Blood clots are held together by tangles of fibrin, a tough, thread-like protein that traps red blood cells and other material to form a sticky clump. Typically, doctors try to remove them by inserting a catheter into the artery and either vacuuming up the clot or snaring it with wire mesh. But these methods don't always work and can snap the fibrin threads, causing pieces of the clot to break off and get lodged in new, harder to reach places.
"With existing technology, there's no way to reduce the size of the clot. They rely on deforming and rupturing the clot to remove it," said Renee Zhao, an assistant professor of mechanical engineering and senior author on the paper. "What's unique about the milli-spinner is that it applies compression and shear forces to shrink the entire clot, dramatically reducing the volume without causing rupture."
The milli-spinner, which also reaches the clot through a catheter, consists of a long, hollow tube that can rotate rapidly, with a series of fins and slits that help create a localized suction near the clot. This applies two forces - compression and shear - to roll the fibrin threads into a tight ball without breaking them.
Close-up of the milli-spinner, which consists of a long, hollow tube that can rotate rapidly, with a series of fins and slits near the clot that help create a localized suction. Through its innovative design, the milli-spinner can shrink blood clots without rupturing them. | Andrew Brodhead
Imagine a loose ball of cotton fibers (or a handful of long hair pulled from a hairbrush, if you'd prefer). If you press it between your palms (compression) and rub your hands together in a circle (shear), the fibers will become increasingly tangled into a smaller, denser ball. The milli-spinner is able to do this same thing to the fibrin threads in a clot, using suction to compress the clot against the end of the tube and rapidly spinning to create the necessary shear.
Zhao and her colleagues showed that the milli-spinner could reduce a clot to as little as 5% of its original volume. The process shakes free the red blood cells, which move normally through the body once they aren't trapped in fibrin, and the now-tiny fibrin ball is sucked into the milli-spinner and out of the body.
"It works so well, for a wide range of clot compositions and sizes," Zhao said. "Even for tough, fibrin-rich clots, which are impossible to treat with current technologies, our milli-spinner can treat them using this simple yet powerful mechanics concept to densify the fibrin network and shrink the clot."
A surprising success
The milli-spinner design is an extension of Zhao's work on millirobots - tiny, origami-based robots built to swim through the body to dispense medicine or assist with diagnostics. The spinning hollow structure with fins and slits was intended as a propulsion mechanism, but when the researchers realized that it was also creating localized suction, they decided to see if it could have other uses as well.
"At first, we simply wondered whether this suction could help remove a blood clot," Zhao said. "But when we tested the spinner on a clot, we observed a striking clot color change, from red to white, along with a dramatic reduction in volume. Honestly, it felt like magic. We didn't fully understand the mechanism at the time."
Intrigued by this unexpected and unprecedented clot response, the researchers set out to uncover the underlying mechanism and then went through hundreds of design iterations to make the milli-spinner as efficient and effective as possible. But they haven't forgotten about its propulsion possibilities. Zhao and her colleagues are also working on an untethered version of the milli-spinner that could swim freely through blood vessels to target and treat clots.
While they have focused on treating blood clots first, there are many other potential uses for the milli-spinner, Zhao said. She and her team are already working on using the milli-spinner's localized suction to capture and remove kidney stone fragments.
"We're exploring other biomedical applications for the milli-spinner design, and even possibilities beyond medicine," Zhao said. "There are some very exciting opportunities ahead."
Knowing the difference it could make for stroke patients and those with other blood clot-related diseases, Zhao, Heit, and their colleagues are hoping to get the milli-spinner thrombectomy approved for patient use as soon as possible. They have started a new company that licenses the technology from Stanford in order to develop and bring it to market, with clinical trials planned for the near future.
"What makes this technology truly exciting is its unique mechanism to actively reshape and compact clots, rather than just extracting them," Zhao said. "We're working to bring this into clinical settings, where it could significantly boost the success rate of thrombectomy procedures and save patients' lives."
