Humans have always been fascinated by the idea of microrobots, the technology that could ultimately provide a slew of applications, including in healthcare, such as targeted drug delivery and microsurgical procedures. Designs of such robots are often inspired by natural microorganisms such as bacteria or algae.
Now, for the first time, a team of researchers at ETH Zurich has developed a microrobot design inspired by starfish larva, which use ciliary bands on their surface to swim and feed.
This yet-to-be-named microrobot measure is a rectangle and ten times smaller, only a quarter of a millimeter across. It swims through liquid by moving tiny surface hairs, or cilia, found on all kinds of microorganisms, including newborn starfish. The ultrasound-activated synthetic system mimics the natural arrangements of selfish ciliary bands and leverages nonlinear acoustics to replicate the larva’s motion and manipulation techniques.
Researchers used photolithography to construct a microrobot with appropriately inclined ciliary bands. They then applied ultrasound waves from an external source to make the cilia oscillate. The synthetic cilia beat back and forth more than ten thousand times per second, which is about a thousand times faster than those of a starfish larva. These beating cilia can be used to generate a vortex with a suction effect at the front and a vortex with a thrust effect at the rear, the combined effect “rocketing” the robot forward.
In laboratory experiments, the researchers succeeded in making the microrobot swim in a straight line through a liquid such as water. Adding tiny plastic beads to the water made it possible to visualize the vortices created by the microrobot. The researchers then arranged the ciliary bands so that a suction vortex was positioned next to a thrust vortex, imitating the feeding technique used by starfish larva. This arrangement enabled the robots to collect particles and send them out in a predetermined direction.
Researchers believe that such microswimmers could deliver drugs to diseased cells with pinpoint accuracy in the foreseeable future. “Our vision is to use ultrasound for propulsion, imaging, and drug delivery,” said Daniel Ahmed, who led the research. He believes one initial field of application could be the treatment of gastric tumors. Microrobots transport a drug specifically to the site of a stomach tumor and then deliver it there, which might make the drug’s uptake into tumor cells more efficient and reduce side effects.
Before this happens, however, a major challenge remains to be overcome: imaging. Researchers have plans to make the microrobots more visible by incorporating contrast agents such as those already used in medical imaging with ultrasound.
