Tiny robotic tools, powered by magnetic fields, could enable minimally invasive brain surgery

A team of researchers at the University of Toronto and The Hospital for Sick Children (SickKids) have created a set of tiny robotic tools that could enable keyhole surgery in the brain.

In a paper published in Science Robotics, the team demonstrated the ability of the tools – only about three millimetres in diameter – to grip, pull and cut tissue.

The tools are powered by external magnetic fields rather than motors, enabling their extremely small size.

Current robotic surgical tools, widely used in surgeries that take place in the torso, are typically driven by cables connected to electric motors. But this approach starts to break down at smaller length scales, according to Eric Diller, associate professor in the Faculty of Applied Science & Engineering’s department of mechanical and industrial engineering.

“The smaller you get, the harder you have to pull on the cables,” he says. “And at a certain point, you start to get problems with friction that lead to less reliable operation.” 

Diller and his collaborators have been working for several years on an alternative approach. Instead of cables and pulleys, their robotic tools contain magnetically active materials that respond to external electromagnetic fields controlled by the surgical team. 

The system consists of two parts. The first comprises the tiny tools themselves – a gripper, a scalpel and a set of forceps. The second is a “coil table,” which is a surgical table with several electromagnetic coils embedded inside.

In this design, the patient would be positioned with their head on top of the embedded coils, with the robotic tools inserted into the brain by means of a small incision. 

By altering the amount of electricity flowing into the coils, the team can manipulate the magnetic fields, causing the tools to grip, pull or cut tissue as desired. 

To test the system, Diller and his team partnered with experts at the Wilfred and Joyce Posluns Centre for Image-Guided Innovation and Therapeutic Intervention (PCIGITI) at SickKids, including James Drake, former chief of pediatric neurosurgery and a professor of neurosurgery at U of T’s Temerty Faculty of Medicine, and Thomas Looi, PCIGITI’s project director and an assistant professor of otolaryngology-head and neck surgery at Temerty Medicine.

Together, they designed and built a life-sized model of a brain, made of silicone rubber, that simulates the geometry of a real brain. 

The team then used small pieces of tofu and bits of raspberries to simulate the mechanical properties of the brain tissue they would need to work with. 

“The tofu is best for simulating cuts with the scalpel, because it has a consistency very similar to that of the corpus collosum, which is the part of the brain we were targeting,” says Changyan He, an assistant professor at the University of Newcastle in Australia and a former U of T postdoctoral fellow co-supervised by Drake and Diller.

“The raspberries were used for the gripping tasks, to see if we could remove them in the way that a surgeon would remove diseased tissue.” 

The performance of these magnetically actuated tools was compared with that of standard tools handled by trained physicians.  

In the paper, the team reports that the cuts made with the magnetic scalpel were consistent and narrow, with an average width of 0.3 to 0.4 millimetres – more precise than cuts made using traditional hand tools, which ranged from 0.6 to 2.1 millimetres. 

As for the grippers, they were able to successfully pick up the target 76 per cent of the time.  

The team also tested the operation of the tools in animal models, where they found that they performed similarly well.  

“I think we were all a bit surprised at just how well they performed,” says He. “Our previous work was in very controlled environments, so we thought it might take a year or more of experimentation to get them to the point where they were comparable to human-operated tools.” 

Despite the encouraging results, Diller notes it may be a long time before these tools see the inside of an operating room. “There’s a lot we still need to figure out,” he says. “We want to make sure we can fit our field generation system comfortably into the operating room, and make it compatible with imaging systems like fluoroscopy, which makes use of X-rays.” 

Still, the team is excited about the potential of the technology.

“This really is a wild idea,” says Diller. 

“It’s a radically different approach to how to how to make and drive these kinds of tools, but it’s also one that can lead to capabilities that are far beyond what we can do today.”