Researchers at the University of Twente and Radboudumc in the Netherlands have guided a screw-shaped microrobot through real brain tissue using an external magnet. The experiments on sheep brains represent a step towards minimally invasive treatments for deep-seated brain conditions, such as blood clots, tumours and vascular abnormalities.
A key innovation is a model that enables the researchers to predict when the robot’s magnetic control becomes unstable. This allows them to determine in advance how fast a robot can move safely and in a controlled manner through different types of tissue.
Via blood vessels to the brain
Deep-seated abnormalities in the brain are often difficult to reach without invasive surgery. The researchers are therefore working on an alternative in which a small robot is guided to the desired location via the bloodstream. There, it could move through the vessel wall and then penetrate the brain tissue to reach a lesion directly.
The robot is powered by a magnet outside the body. The magnetic field causes the screw-shaped robot to rotate, enabling it to move forwards through soft tissue. However, there is a significant technical limitation: if the external magnet rotates too quickly, the robot can no longer keep up with the rotation. This tipping point is known as the step-out frequency. Above this threshold, the robot may slip, stop or move unpredictably. Precise control is essential, particularly for applications within the brain.
Model predicts threshold for safe control
The research team developed a model that can be used to calculate this critical threshold in advance. They utilised Buckingham Pi theory, which allows various physical variables to be combined into a limited number of dimensionless parameters. Among other factors, the robot’s dimensions, the magnetic strength and the stiffness of the tissue are taken into account.
The resistance of the tissue appears to have a major influence. In soft material, the robot was able to follow the magnet up to approximately 30 revolutions per second. In the stiffest material tested, that limit was less than one revolution per second. According to lead author Ewout Ligtenberg, after a single measurement in tissue of known stiffness, the model can predict how the same robot design will behave in other types of tissue. This reduces the need for separate experiments when developing and optimising new robots.
Tested in real brain tissue
The researchers first validated their model using four different robot designs in gelatine and subsequently in sheep brains. Without blood flow, the robot continued to rotate in synchronisation with the magnet up to approximately 1.8 revolutions per second. When blood was pumped through the vessels to better mimic the conditions in a living brain, that limit fell to less than 0.45 revolutions per second. The additional pressure increased the resistance of the tissue.
The tiny robot was able to burrow into the brain tissue at a rate of approximately 0.2 millimetres per second. When retracting, it reached 2.9 millimetres per second, as it was able to move back through the channel it had previously created. In a soft gel model of the brain, the same robot was guided to specific targets using real-time camera images. In some experiments, an accuracy of less than one millimetre was achieved.
Magnetically controlled microrobots
The research was carried out at the TechMed Centre and MESA+ at the University of Twente, in collaboration with Michiel Warlé’s team at Radboudumc. The robots’ biocompatible coating was developed using LipoCoat. International research and industrial partners were also involved. The results have been published in *Advanced Science*. Although the technology is still at the research stage, the predictive model developed provides a basis for the further development of magnetically controlled micro-robots that could eventually target deep-seated brain lesions without the need for open brain surgery.
Last year, Researchers at ETH Zurich developed a magnetically controlled microrobot designed to deliver medicines directly to specific locations in the body. The technology could support minimally invasive treatments for conditions such as stroke, infections and tumours while reducing the systemic side effects associated with conventional drug delivery.
The microrobot consists of a tiny capsule made from soluble gel containing magnetic iron oxide nanoparticles. External electromagnetic fields guide it through blood vessels, while tantalum nanoparticles make the robot visible using X-ray imaging. A specially developed navigation system enables the microrobot to roll along vessel walls, move against blood flow and navigate vascular branches at speeds of up to 4 millimetres per second.
References
Advanced Science (research)
