At some point, doctors will be able to use micro-vehicles small enough to navigate our blood vessels to take biopsies, insert stents, and deliver drugs precisely to hard-to-reach places in the body. Scientists around the world are currently researching and developing suitable micro-vehicles. In most cases they are fed and controlled by acoustic and magnetic fields or by using light. Until now, however, it has been a major challenge to propel micro-vehicles against a flow of liquid. This would be necessary so that the micromachines in blood vessels can navigate against the direction of blood flow. Researchers at ETH Zurich have now developed micro-vehicles that can be manipulated by an external field and swim against the current.
In their laboratory experiment, the research team headed by Daniel Ahmed and Bradley Nelson, professors at the Institute for Mechanical and Process Engineering at ETH Zurich, used magnetic beads made of iron oxide and a polymer with a diameter of 3 micrometers. A magnetic field causes these particles to collect in a swarm with a diameter between 15 and 40 micrometers. The scientists studied the behavior of this swarm in a thin glass tube through which liquid flows. The glass tubes were 150 to 300 micrometers in diameter, a size similar to the blood vessels in a tumor.
To drive the micro-swarm against the current in the tube, the ETH researchers used the same trick that canoeists use on a river: They hug the river bank to paddle upstream because the friction of the bank makes the current there slower than in the river Middle of the river.
Using ultrasound at a certain frequency, the scientists first guided the microsphere cluster close to the tube wall. Then the researchers switched to a rotating magnetic field to propel the swarm against the current.
As a next step, the researchers want to investigate how the micro-vehicles react in the blood vessels of animals. "As both ultrasonic waves and magnetic fields penetrate the body tissue, our method is ideal for controlling micro-vehicles in the body," says ETH professor Ahmed.
Microsurgery, such as clogging clogged blood vessels, is one of the future uses that researchers hope this method will find use. In addition, the micro-vehicles could one day be used to deliver cancer drugs to tumors through the blood vessels and release them directly into the tumor tissue. Another area of application is the transfer of drugs from blood vessels to the tissues of the brain.