A team of researchers at ETH Zurich in Switzerland developed and tested the method, which combines ultrasound and a rotating magnetic field.
Researchers around the world are researching and developing microvehicles to enable physicians to take biopsies, insert stents and deliver drugs to areas that are difficult to reach. In most cases, they are powered and controlled by acoustic and magnetic fields, or by using light.
Until now, propelling microvehicles against a fluid flow has been a major challenge, but it is necessary for the devices to navigate against the direction of blood flow.
In their lab experiment, the research team headed by ETH professors Daniel Ahmed and Bradley Nelson used magnetic beads made of iron oxide and a polymer with a diameter of 3 micrometres. A magnetic field induced the particles to cluster into a ‘swarm’ with a diameter of between 15 and 40 micrometres. The scientists studied the behaviour of the swarm in a thin glass tube with liquid flowing through it. The glass tubes had a diameter of 150 to 300 micrometres, a similar size to the blood vessels in a tumour.
To propel the swarm against the flow in the tube, the researchers applied the same trick canoeists use on a river – hugging the riverbank to paddle upstream, because the friction of the bank makes the current slower than in the middle of the river.
Using ultrasound at a specific frequency, the scientists first guided the cluster of microbeads close to the wall of the tube. They then switched to a rotating magnetic field to propel it against the flow. A YouTube video shows the particles travelling with the flow before reversing direction.
The researchers plan to investigate how the microvehicles respond in the blood vessels of animals. “As both ultrasound waves and magnetic fields penetrate body tissue, our method is ideal for controlling microvehicles inside the body,” said Prof Ahmed.
Microsurgery, such as unclogging blocked blood vessels, is a potential future application for the method. They could also one day be used to deliver cancer drugs to tumours via the blood vessels, releasing them directly into tumour tissue. Another potential application is transferring drugs from blood vessels into the tissues of the brain.
The research was published in Nature Machine Intelligence.
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