Self-Propelled Microbots Navigate Blood Vessels

With the advent of nanotechnology, researchers are getting closer to creating a microscopic device that could swim through the bloodstream and directly pinpoint the site of disease, such as a tumor, which could potentially provide new treatment options. To reach to a tumor, however, such a device would have to be small and agile enough to navigate through a maze of tiny blood vessels, some much thinner than a human hair.

Researchers from the École Polytechnique de Montréal (QC, Canada), led by professor of computer engineering Dr. Sylvain Martel, have joined live, swimming bacteria to microscopic beads to develop a self-propelling device, called a nanobot. While other scientists have previously attached bacteria to microscopic particles to exploit their natural propelling motion, Dr. Martel's team is the first to demonstrate that such hybrids can be navigated through the body utilizing magnetic resonance imaging (MRI) technology.

To do this, the investigators used bacteria that naturally contain magnetic particles. In a natural environment, these particles help the bacteria navigate toward deeper water, away from oxygen. However, by altering the neighboring magnetic field using an extended set-up attached to an MRI machine, the researchers were able to induce the bacteria to move themselves in any direction they wanted.

The bacteria move using tiny corkscrew-like tails called flagella, and these specific bacteria are faster and stronger than most, according to Dr. Martel. Moreover, they are only two microns in diameter--small enough to fit through the smallest blood vessels in the human body. The team treated the polymer beads approximately 150 nanometers in size with antibodies so that the bacteria would attach to them. Eventually, the researchers plan to modify the beads so that they also carry cancer-killing drugs.

In 2007, Dr. Martel and his group published a study in the journal Applied Physics Letters describing how they utilized an MRI machine to maneuver a 1.5-mm magnetic bead with a bacteria propeller through the carotid artery of a living pig at 10-cm per second. The investigators latest study, presented at the IEEE 2008 Biorobotics Conference, held in Scottsdale, AZ, USA, October 2008, revealed that they could track and steer microbeads and bacteria or bacteria alone through a facsimile of human blood vessels using the same approach. The investigators have performed similar experiments in rats and rabbits, according to Dr. Martel.

The bacterial microbots, however, would not be able to work in larger blood vessels in on their own. The current would be too strong for them to swim against it. Therefore, the researchers envisage using a larger, magnetically maneuverable micro-vehicle to carry the microbots close to a tumor. According to Dr. Martel, the vehicle would have to be composed of a polymer or something similar to release the bacteria while the vehicle remains there and dissolves

Dr. Martel's vehicle contains magnetic nanoparticles and it can be moved at approximately 200 microns per second. He reported that he and his team corrected the micro-vehicle's path approximately 30 times a second. While they have developed the micro-vehicle and bacterial microbots independently, they are now working to combine the two technologies. They are positive that they will soon succeed in that endeavor.

Contrariwise, however, other investigators have suggested 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 Dr. Martel stated that he is very confident that this will not be a problem. Because the immune system has not encountered these bacteria before, he noted, it would not have time to destroy the microbots before they reach their target.

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École Polytechnique de Montréal