MIT engineers have designed tiny robots that can help drug-delivery nanoparticles push their way out of the bloodstream and into a tumor or another disease site. The magnetic microrobots, inspired by bacterial propulsion, could overcome one of the biggest obstacles to delivering drugs with nanoparticles: getting the particles to exit blood vessels and accumulate in the right place.
"Our idea was to see if you can use magnetism to create fluid forces that push nanoparticles into the tissue," explains Simone Schuerle, a former MIT postdoc and lead author of the paper, which was published last week in Science Advances.
The robots that Schuerle, now an assistant professor at ETH Zurich, used in this study are 35 hundredths of a millimeter long, similar in size to a single cell, and can be controlled by applying an external magnetic field. This bioinspired robot, which the researchers call an "artificial bacterial flagellum," consists of a tiny helix that resembles the flagella that many bacteria use to propel themselves. These robots are 3-D-printed with a high-resolution 3-D printer and then coated with nickel, which makes them magnetic.
To test a single robot's ability to control nearby nanoparticles, the researchers created a microfluidic system that mimics the blood vessels that surround tumors. The channel in their system, between 50 and 200 microns wide, is lined with a gel that has holes to simulate the broken blood vessels seen near tumors.
Using external magnets, the researchers applied magnetic fields to the robot, which makes the helix rotate and swim through the channel. Because fluid flows through the channel in the opposite direction, the robot remains stationary and creates a convection current, which pushes 200-nanometer polystyrene particles into the model tissue. These particles penetrated twice as far into the tissue as nanoparticles delivered without the aid of the magnetic robot.
The researchers also developed a variant of this approach that relies on swarms of naturally magnetotactic bacteria instead of microrobots. For this study, the researchers used Magnetospirillum magneticum, which naturally produces chains of iron oxide. These magnetic particles, known as magnetosomes, help bacteria orient themselves and find their preferred environments.
The researchers discovered that when they put these bacteria into the microfluidic system and applied rotating magnetic fields in certain orientations, the bacteria began to rotate in synchrony and move in the same direction, pulling along any nanoparticles that were nearby. In this case, the researchers found that nanoparticles were pushed into the model tissue three times faster than when the nanoparticles were delivered without any magnetic assistance.