MRI-powered ‘millirobots’ could swim around the body, drive needles, puncture tissues

(noppasit TH/Shutterstock)
(noppasit TH/Shutterstock)

MRI is a staple of surgical imaging, but it has the potential to do much more than take pictures. In 2011, bioengineer Pierre Dupont, PhD, and colleagues demonstrated that an MRI machine’s magnetic field could power a motor strong enough to control a robotic instrument, in this case driving a needle into an organ to do a biopsy.

But Dupont, head of the Pediatric Cardiac Bioengineering Lab at Boston Children’s Hospital, wants to go further. “We had this idea, admittedly fanciful: What if you could swim robots through the body?” he says. “If you could inject something systemically and steer it to just hit your target, that would be a cool application.”

Under an MRI magnetic field, teams of magnetized robots could, say, break open a cyst, unclog an artery or puncture a membrane to unblock fluid flow in the brain. Or swim to a part of the body targeted by a drug, take a small sample to verify drug concentrations and swim back out. An advantage of an MRI system is that you’d be able to image the robots and track them.

To test the idea, Dupont’s team first created a two-part magnetic “millirobot” for drug delivery. The first part is a hydrogel loaded with a drug and a magnetized element. The two parts can swim to a desired location in the body for drug delivery and then dock with each other through magnetic attraction. Their collision compresses the hydrogel, causing release of the drug.

But could the robots be powered enough to do more physically demanding surgical tasks? Just swimming a robot through body fluids under MRI power requires a certain amount of force to overcome the fluid friction, leaving not enough force to perform the actual surgery.

“I asked one of my postdocs, Aaron Becker, to figure out how we could produce a force large enough to penetrate tissue,” says Dupont.

A few weeks later, he came across Becker and a second postdoc, Ouajdi Felfoul, in their darkened office wearing safety goggles and smashing light bulbs. They had created something called a Gauss gun. It consisted of a course of magnetic ball bearings that go faster and faster the further down the course they go:

Inside an MRI scanner, ball bearings turn into magnets. To keep them all from sticking together, Becker added spacers in between. When magnetic attraction pulls in the first ball, the force of the collision launches a second ball toward a second magnet, whose attractive pull makes the ball pick up speed. When that ball slams into the magnet, it launches a third ball that goes even faster, pulled by a third magnet. This video from the Discovery Channel breaks it down:

And here’s Becker and Felfoul’s experiment inside a clinical MRI scanner—presented in May at the IEEE International Conference on Robotics and Automation—with millirobots guided through simulated body fluids to create a Gauss gun:

There’s still much work to be done before swimming robots could be employed in minimally invasive surgery. For a start, Dupont would like to be able to shrink the robots to nanometer size and enable them to surf in flowing blood, steering through branching blood vessels.

In the meantime, Dupont is discussing possible applications with clinicians throughout Boston Children’s Hospital. The technology, developed with National Science Foundation funding, still needs a killer app—a sure-fire solution to an unmet surgical need that would justify the substantial costs of mounting a clinical trial. Essentially, Dupont is telling the surgeons, “I’ve created this really crazy hammer—any of you guys have a nail I can hit?”

Read more about Dupont’s millirobots in IEEE Spectrum.