Biological microbots made from bubbles have been tracked moving inside the brain of a living mouse as they were steered by ultrasound.
鈥淲hile the mouse is under the microscope, we can see the small particles moving in the blood,鈥� says at ETH Zurich in Switzerland.
Many people hope that microbots 鈥� which are often made from simple biological compounds or tissues when used for medicine, rather than metal and microchips 鈥� can one day deliver drugs to specific cells in the human body. However, the technology has obstacles to overcome before it can be used clinically.
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One of the big challenges is that these small bots may need to travel against the flow of blood, which could carry away small objects. Another issue is how you track the location of a microbot once it is inside the body.
Now, Ahmed and his colleagues have come up with a way to address both of these problems by adapting microbubbles, which are bubbles less than 1 micrometre wide with a shell made from molecules called lipids.
For ultrasound scans, such biodegradable microbubbles are sometimes injected into people鈥檚 blood because they focus ultrasound waves and increase the quality of the image produced.
Microbubbles either move away from or towards ultrasound waves depending on the frequency, so Ahmed and his colleagues decided to try to use ultrasound to direct the bubbles where they wanted them to go and watch their progress.
First, they tagged microbubbles with fluorescent markers so their path could be followed using a microscope once they were injected into a mouse.
They then attached a transducer that could produce various frequencies of ultrasound to the head of a mouse and injected microbubbles into the animal鈥檚 bloodstream.
Ahmed and his colleagues found that they could use the ultrasound waves to make the microbubbles self-assemble into a single entity and were able to create microbots ranging in size from six to 30 bubbles.
The researchers then used ultrasound to move the microbots through blood vessels in the mouse鈥檚 brain as they watched.
鈥淭his is the first time that we鈥檝e been able to show in real time the navigation of microbots in the brain,鈥� says Ahmed. 鈥淲ithout real-time tracking 鈥� it鈥檚 super difficult to know what you are doing inside there.鈥�
They also found they could move the microbubbles towards the wall of a blood vessel, which is important because blood flow isn鈥檛 as strong at the walls, says Ahmed, and so the microbubbles experience less drag when moving against the direction of the blood.
The technique allowed them to achieve speeds of up to 1.5 micrometres per second when moving against the flow of blood 鈥� which had a speed ranging from 2 to 10 millimetres per second.
So far, the researchers have only been able to move the bubble to the upper regions of the brain, called the cortex. Navigating deeper would be hard because it becomes trickier to direct a bubble via more intricate blood vessels, says Ahmed.
at University College London says the engineering work involved in this study is impressive. 鈥淚 think any therapeutic application of this technology, however, has a very long way to go,鈥� he says.
The microbots could potentially be used to unblock blood clots that result in strokes, says Ahmed, but that would require more accurate control of the bubbles. 鈥淭o take it to the next level, we have to control microbots in any way we want,鈥� he says.
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