Robot drills through real brain tissue for the first time

Researchers from the University of Twente and Radboudumc (HealthTech Nexus) have steered a tiny screw-shaped robot through real brain tissue, controlled only by a magnet outside the body. The test used sheep brain tissue in the lab. The team built a model that predicts exactly when such a robot loses control, a step towards treating stroke and tumours deep in the brain without open surgery.

Deep-seated brain lesions, blood clots after a stroke, tumours, and vascular malformations are often hard to reach without cutting open the skull. The research team envisions a different route: a robot that travels through the bloodstream, drills through the artery wall at the right spot, and continues into the brain tissue to reach the lesion directly.

Predicting when the robot loses control

An external magnet spins the robot, which drills forward like a screw. Spin the magnet faster, and the robot goes faster too, up to a point. Beyond that point, called the step-out frequency, the robot can no longer keep pace. It slips, stalls, or moves erratically. The researchers can now predict that moment with a new model.

The researchers built their model using the Buckingham Pi theorem, a method that reduces a complex physical problem to a handful of dimensionless numbers. It combines the robot’s size, its magnetic strength and the tissue’s stiffness into a single equation. That tipping point depends on how stiff the surrounding tissue is. In soft tissue, the robot kept pace with the magnet up to roughly 30 rotations per second. In the stiffest tissue tested, that dropped to less than one rotation per second.

“Push a magnetic robot too fast and it simply stops listening to the magnet,” says Ewout Ligtenberg, the study’s first author. “We can now predict exactly when that happens, for any tissue, from a single test. That takes out a lot of guesswork when designing robots for the brain.

One measurement and the model predicts the rest

Once the model is calibrated with one measurement in a tissue of known stiffness, it predicts how that same robot design will behave in any other tissue, without further testing. The team validated the model on four robot designs in gelatine, then in real sheep brain tissue. Without blood flow, the robot stayed in sync with the magnet up to about 1.8 rotations per second. Once blood was pumped through the vessels, mimicking a living brain, that threshold dropped below 0.45 rotations per second. The pressurised tissue pushed back harder.

The robot drilled into brain tissue at 0.2 millimetres per second. Reversed, it retraced its own channel back out at 2.9 millimetres per second, considerably faster, since it no longer had to drill a new path. Using real-time camera tracking, the same robot design was also steered towards specific targets inside a soft gel model of the brain, reaching them with an accuracy of less than a millimetre in some trials.

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