For decades, engineers have been trying to build medical robots that can deliver drugs or do surgery inside the human body—a somewhat less fantastic version of the 1966 sci-fi film Fantastic Voyage. Now, scientists have manipulated spirulina, a microscopic plant and food supplement, to travel through people in response to magnetic signals. The biohybrid robot could one day carry drugs to specific parts of the body, minimizing side effects. What’s more, the robot—and its magnetic coat—appear to kill cancer cells.
Spirulina, an alga, looks like a tiny coiled spring at the microscopic level. Researchers had been trying, and succeeding to various degrees, to build bots out of rods, tubes, spheres, and even cages no bigger than a cell. Outfitting these tiny devices with an ample power supply has been quite a challenge, as most potential fuels are toxic to humans. Another problem is steering such a microrobot through the body’s maze of proteins and other molecules, which requires both a way to control its movements and to see where it is.
So Li Zhang, a materials scientist at the Chinese University of Hong Kong in Shatin, turned to magnetism—and living organisms. Magnetic fields created outside the body can penetrate living tissue without harm, allowing researchers to move magnetized objects around inside. For maximum mobility, a helical body propelled by twirling works best. Enter spirulina. “It’s surprising that you can find in nature such a convenient structure and that it can behave so nicely,” says Peer Fischer, a physical chemist at the Max Planck Institute for Intelligent Systems in Stuttgart, Germany, who was not involved in the study.
Several years ago, Zhang and his colleagues used the alga as inspiration for a synthetic microbot, which worked to some degree. This time, the scientists decided to use the alga itself. They needed a way to track the robot in the body, and the alga produces a fluorescent glow. The researchers wondered whether they could follow the robot's course near the body surface by detecting this fluorescence, and then use a commonly used medical imaging technology called nuclear magnetic resonance (NMR) to track it in deeper parts of the body. NMR works by detecting magnetic particles given to a patient before the imaging takes place.
They developed a one-step method to magnetize the alga, coating millions of spirulina with iron oxide nanoparticles. A longer dip time allows for more control, but a shorter dip time allows researchers to detect the fluorescence more readily. When the bot is too deep for that technique to work, NMR can still follow the robot’s course because of the coating, the researchers report today in Science Robotics. Using NMR, they observed the microrobots swarm in a rat’s stomach as directed by the magnetic field.
“It’s a step forward that you can track these swimmers in the body,” says Joseph Wang, a nanoengineer at the University of California, San Diego, who is developing a different sort of medical microbot. “And it’s biocompatible and low cost.”
That biocompatibility is an important feature. The microrobot degrades in hours or days, depending on how thick the coating is; yet it doesn’t damage most cells. The one exception was cancer cells, some 90% of which were destroyed after tumor cells growing in a lab dish were exposed to the spirulina for 48 hours. Further tests indicated that spirulina produces a compound that’s toxic just to cancer cells. “The [cancer-killing] behavior seems to be an interesting, unexpected feature,” Fischer says.
But there’s still a long way to go for the half-dozen teams around the world now developing such microrobots. Zhang’s team, for example, still needs to show that its microbot can carry cargo—such as drugs attached to or within the spiral—and deliver those drugs more effectively than just taking a pill or getting an injection. “It’s still not ready for a doctor to use,” Wang says, but he thinks it might be ready in another 10 years. “Everyone wants to realize this fantastic voyage.”