At the micron scale, objects have difficulty swimming because the viscosity of water prevents objects from relying on momentum to maintain forward motion. Now, researchers at the Georgia Institute of Technology (Atlanta) have used complex computational models to design swimming microrobots that could overcome these challenges. Ultimately, the researchers' work could contribute to the development of robots capable of serving as drug-delivery vehicles.

Designed by Alexander Alexeev, an assistant professor at Georgia Tech's George W. Woodruff School of Mechanical Engineering, and collaborators Hassan Masoud and Benjamin Bingham, the robots consist of a responsive gel body about 10 µm long with two propulsive flaps attached to opposite sides. A steering flap sensitive to specific stimuli would be located at the front of the swimmer.

The responsive gel body would undergo periodic expansions and contractions triggered by oscillatory chemical reactions, oscillating magnetic or electric fields, or cycles of temperature changes. These expansions and contractions would create a beating motion in the rigid propulsive flaps attached to each side of the microswimmer. Combined with the movement of the gel body, the beating motion would move the microswimmer forward. The trajectory of the microswimmer would be controlled by a flexible steering flap on its front. The flap would be made of a material that deforms based on changes in light intensity, temperature, or magnetic field.

“You can’t swim at the small scale in the same way you swim at the large scale,” Alexeev comments. “There is no inertia, which is how you keep moving at the large scale. What happens at the small scale is counterintuitive to what you expect at the large scale.”

The computational fluid modeling the researchers used allowed them to study a range of parameters in materials, oscillation rates, and flexibility. Their studies, Alexeev remarks, will provide experimentalists with a starting point for actually building prototypes of the flexible gel robots.

While the microswimmers will achieve a top speed on the order of a few micrometers per second, according to Alexeev, that speed should be enough to accomplish their mission. “If your body is micrometers in size, that kind of speed is really not too bad,” he adds. “The swimming speed will be rather slow, but at that size scale, you don’t really need to go very fast since you only need to go short distances.”