Plot and go: Researchers plot a trajectory for their bacteria-powered microrobots, which are guided using an MRI machine. Credit: The NanoRobotics Laboratory, École Polytechnique de Montréal (EPM)
The 1966 science-fiction movie Fantastic Voyage famously imagined using a tiny ship to combat disease inside the body. With the advent of nanotechnology, researchers are inching closer to creating something almost as fantastic. A microscopic device that could swim through the bloodstream and directly target the site of disease, such as a tumor, could offer radical new treatments. To get to a tumor, however, such a device would have to be small and agile enough to navigate through a labyrinth of tiny blood vessels, some far thinner than a human hair.
Researchers at the École Polytechnique de Montréal, in Canada, led by professor of computer engineering Sylvain Martel, have coupled live, swimming bacteria to microscopic beads to develop a self-propelling device, dubbed a nanobot. While other scientists have previously attached bacteria to microscopic particles to take advantage of their natural propelling motion, Martel's team is the first to show that such hybrids can be steered through the body using magnetic resonance imaging (MRI).
To do this, Martel used bacteria that naturally contain magnetic particles. In nature, these particles help the bacteria navigate toward deeper water, away from oxygen. "Those nanoparticles form a chain a bit like a magnetic compass needle," says Martel. But by changing the surrounding magnetic field using an extended set-up coupled to an MRI machine, Martel and his colleagues were able to make the bacteria propel themselves in any direction they wanted.
The bacteria swim using tiny corkscrewlike tails, or flagella, and these particular bacteria are faster and stronger than most, says Martel. What's more, they are just two microns in diameter--small enough to fit through the smallest blood vessels in the human body. The team treated the polymer beads roughly 150 nanometers in size with antibodies so that the bacteria would attach to them. Ultimately, the researchers plan to modify the beads so that they also carry cancer-killing drugs.
"I think nature has provided an excellent solution to how to make small things swim," says Bradley Nelson, a professor at ETH Zurich, who has researched the use of artificial flagella. "What's interesting about Sylvain's work is that he's actually using nature to do it and not just learning from it."
Last year, Martel and his group published research in the journal Applied Physics Letters detailing how they used an MRI machine to maneuver a 1.5-millimeter magnetic bead with a bacteria propeller through the carotid artery of a living pig at 10 centimeters per second. The researchers' latest work, presented at the IEEE 2008 Biorobotics Conference last week, shows that they can track and steer microbeads and bacteria or bacteria alone through a replica of human blood vessels using the same approach. The group has carried out similar experiments in rats and rabbits, according to Martel.
The bacteria bots wouldn't be able to make it in larger blood vessels in on their own, however. The current would be too strong for them to swim against. So the researchers envisage using a larger, magnetically steerable microvehicle to carry the bots close to a tumor. "The vehicle will be a type of polymer, or possibly another type of material," says Martel. "We have a way to release the bacteria while the vehicle stays there and dissolves."
Martel's vehicle contains magnetic nanoparticles and can be moved at about 200 microns per second. He says that he and his team correct the microvehicle's course approximately 30 times a second. While they have developed the microvehicle and bacterial microbots independently, they are now working to combine the two technologies. "We think in two years we'll be able to do that," says Martel.
"This work is promising but, as with any transformative idea, there are a lot of challenges that need to be addressed," says Bahareh Behkam, an assistant professor at Virgina Polytechnic Institute, who has also used bacteria to propel microbeads. She suggests that it could be difficult to maintain normal blood flow and to retrieve the magnetic particles from the body after the procedure is complete.
Some researchers also question whether the body's immune system would attack the bacteria before they could reach a tumor, but Martel defends the approach. "We are very confident from our preliminary tests that this [scenario] will not be an issue," he says. Because the immune system has not encountered these bacteria before, he says, it would not have time to wipe out the microbots before they reach their target.
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