Researchers have developed a bubble microrobot that uses ultrasound to guide it through tiny, complex brain blood vessels. The "microvehicles" have been successfully tested on mice and could become a means of precisely delivering drugs to treat conditions such as brain cancer and stroke.

There are more than 404 miles (650 kilometers) of blood vessels in our brains. Advances in nanotechnology have enabled the development of tiny robots that can navigate previously inaccessible areas through tiny, intricate pathways, provide precise drug delivery, and perform minimally invasive surgeries.

Given the complexity of vascular networks and the blood flow pressures encountered, a method of guiding microrobots is needed. Using magnetic fields to guide microrobots through blood vessels in the brain allows for precise manipulation, but because the microrobots must be magnetic, this limits their biodegradability.

Now, researchers at ETH Zurich, the University of Zurich and the University Hospital Zurich have collaborated to develop microcarriers - gas-filled microbubbles coated with lipids - that can use ultrasound to navigate the narrow and complex blood vessels of the mouse brain.

Daniel Ahmed, one of the corresponding authors of the study, said: "In addition to being widely used in the medical field, ultrasound is also safe and can penetrate deep into the human body."

These small, smooth, gas-filled microbubbles are between 1.1 and 1.4 microns in diameter and are made from a fluorescent contrast agent currently used in ultrasound imaging. Over time, they dissolve in the body and their lipid shells are made of the same material as biological cell membranes.

Acoustic microrobot navigation combined with real-time optical imaging Research by DelCampo Fonseca et al. found that the microrobot can dissolve in the body for a long time, and its lipid shell is made of the same material as biological cell membranes.

The researchers injected microbubbles into mice and allowed them to circulate in the animals' blood. The microscope enables real-time imaging of the robot. The researchers mounted up to four ultrasound sensors on the outside of mice's heads and found that the microrobots responded to sound waves by self-assembling into swarms and navigating along brain blood vessels.

The robots are guided by adjusting the output of each sensor at speeds up to 1.5 microns/second and successfully move in reverse directions with blood flow velocities up to 10 mm/second. The results show that the acoustic micromanipulator can work under physiological conditions in vivo. The researchers analyzed brain tissue after ultrasound driving and found that the microrobot neither damaged the inner walls of blood vessels nor caused nerve cell death.

Creating microbubbles from a substance that is already in use has its advantages. "Because these bubbles, or vesicles, are already approved for human use, our technology is likely to be approved for human treatment more quickly than other types of microcarriers currently in development," Ahmed said.

Now that they have demonstrated that their microrobot can navigate the brain blood vessels of mice, the next step for the researchers is to attach drug molecules to the outside of the microbubble shell. If successful, the ultrasound-activated microcarriers could potentially be used to treat cancer, stroke and mental illness.

The research was published in the journal Nature Communications.