Magnetic Hammer Drives Tiny Medical Robot Through Brain Tissue

This millirobot has already probed a goat brain, and may someday maneuver through a human

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A tiny robot that jackhammers its way through the body sounds like the stuff of science fiction nightmares. But such a robot exists, and it could play an important role in the future of medicine.

A new study on the concept shows that millimeter-scale robots (known as millirobots) can penetrate lamb and goat brain tissue by responding to changes in the magnetic field generated by hospital medical scanners. That achievement could pave the way for fantastic voyages of biomedical discovery.

Many medical researchers have experimented with magnetic fields that push and pull tiny robots to move them around inside the human body. In this case, University of Houston researchers also created a “magnetic hammer” inside a bullet-shaped robot that would produce enough force to drive it into animal brains.

The robot contains a stainless-steel bead that is pulled back and forth inside the robot’s transparent acrylic body by directional changes in the magnetic field produced by a magnetic reso­nance imaging (MRI) scanner. When pulled in one direction, the bead compresses a mechanical spring at the back of the robot, which propels the bead forward when released to strike the robot’s front end, hammering it deeper into bodily tissue.

“The robot is the combination of the MRI system and this relatively simple component that could be mass produced,“ says Aaron Becker, an assistant professor in electrical and computer engineering at the University of Houston.

Such a robot could leverage standard MRI scanners in hospitals, which means physicians could simultaneously perform MRI imaging of patients and move millirobots around inside their bodies. The research by Becker and his colleagues was published in January in IEEE Robotics and Automation Letters.

Many researchers who experiment with miniature medical robots, which are not yet being used in any clinical applications, prefer to work with custom magnetic coils in order to exert more precise control over the orientation of the magnetic fields influencing the robot. By comparison, commercial MRI machines have a magnetic field that is fixed along a given orientation, which limits directional control.

To further complicate matters, MRI scanners have magnetic gradient field strength from 20 to 40 millitesla per meter—enough to maneuver a small robot through fluid-filled blood vessels, but not enough to help a robot punch through tissue.

The University of Houston researchers hit upon the magnetic hammer approach to make the most of the set gradient field strength of MRI scanners. While designing the robot, they also added other features to improve its penetrating power.

Julien Leclerc, a postdoctoral fellow in electrical and computer engineering at the university and a leader on the project, was thinking of hunting arrow tips when he put three titanium blades on the nose of the 50-millimeter-long, 7-⁠mm-diameter device. These blades let the robot push its way through lamb brain tissue—sourced from a local supermarket—based on the jackhammering action. That also means the millirobot leaves behind some minor wounds, similar to those inflicted by a Da Vinci robot’s probe tip intended for minimally invasive surgery.

The researchers also placed a single porcupine needle at the tip of the robot’s nose, which itself is made from 3D-printed thermoplastic. The needle’s backward-facing barbs helped anchor the robot in the tissue, so that it didn’t pull back each time the stainless-steel bead slid to its back end. (Future designs would use synthetic versions of the porcupine needle for sanitary reasons.)

Most previous studies have focused on moving tiny robots through human bodies rather than penetrating tissue, says Eric Diller, assistant professor of mechanical engineering at the University of Toronto. He described the magnetic hammer as “a clever mechanism that can effectively amplify the amount of force achievable from magnetically driven millirobots.”

“The approach here is ingenious to boost peak forces, but there could be limits in applying this force in a more general way,” Diller says. “Because it uses sliding parts, there could be difficulties in scaling the device smaller where friction and adhesive forces could jam the mechanism.”

It’s also not clear what specific medical tasks the magnetic hammer robot could best perform compared with less invasive medical approaches, or in place of more invasive procedures, according to Sylvain Martel, a professor at Polytechnique Montréal and director of the institution’s NanoRobotics Laboratory. “They have an exciting technology looking for an application,” Martel says.

The University of Houston team envisions the robot performing minor medical interventions such as piercing a cyst in the spinal canal or performing a biopsy under an MRI scanner. But they also floated the more ambitious idea of robots someday entering the spine of a human patient and traveling to the brain through cerebrospinal fluid. “Imagine the minimally invasive surgery that could be performed in the brain,” Leclerc says.

In any case, the team acknowledges that the millirobot with the magnetic hammer has a long way to go—perhaps pig bodies will be next—before it ends up in human trials. They’ve continued to tweak the robot’s design; one of Leclerc’s latest models sports retractable blades that could avoid doing further damage when pulling out of bodily tissue. Even at the smallest scales, biomedical robots must take special care.

This article appears in the February 2018 print magazine as “Magnetic Hammer Propels Tiny Medical Bot.”

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