Study reveals unexpected movement of drug-delivery swimming robots

One day, tiny swimming robots may travel through the human body to deliver drugs. The medication would target only areas of need-chemotherapy drugs for a tumor, for example-avoiding healthy tissue and minimizing side effects. A research team led by Ebru Demir, an assistant professor of mechanical engineering and mechanics in Lehigh University's P.C. Rossin College of Engineering and Applied Science, with collaborators On Shun Pak (Santa Clara University) and Roberto Zenit (Brown University), is studying how tiny robots move through bodily fluids. They recently published a paper in the journal Applied Physics Letters detailing new foundational insights.

The challenge is that bodily fluids are difficult to navigate. Blood and mucus are non-Newtonian fluids, meaning their viscosity changes under shear stress-the frictional force generated when layers of fluid slide past one another or along a solid surface like a blood vessel wall. 

Testing locomotion in non-Newtonian fluids

"We found that the rheology, or properties of the fluid, affected the locomotion of the swimmers," says the paper's lead author, Amin Balazadeh Koucheh, a second-year PhD student in Demir's research group. "Specifically, we found that the swimmer reversed its direction of motion when we increased the actuation frequency in a non-Newtonian fluid. This phenomenon has been shown numerically by our research group, but this was the first time it's been shown experimentally." 

The team used two types of tiny swimmers, a sphere and a helix (modeled after the corkscrew shape of certain bacteria). They embedded a magnet within each, and tested them first in Newtonian fluid. (The viscosity of Newtonian fluids, such as water or alcohol, never changes, no matter how much the fluid is shaken or stirred.) They used millimeter-scale swimmers, but matched the physics of microswimming by increasing the viscosity of the test fluids.

By magnetically actuating the swimmers, the researchers caused them to rotate and move forward. As they increased the frequency of the rotation, the swimmers continued-as expected-to move forward at a higher rate of speed. 

At the same time, the way the swimmers pushed against the surrounding fluid near the wall also caused them to move sideways while moving forward, resulting in a diagonal trajectory. But when they placed the swimmers in a synthetic non-Newtonian fluid meant to simulate mucus or blood, the researchers observed an unexpected behavior: under the same magnetic actuation, the swimmers moved sideways in the opposite direction from what they did in a Newtonian fluid.

They start to slide backwards, in a sense. It's almost like changing the fluid changes where the finish line is for each swimmer."

Ben Ratnor MS'26, study co-first author 

Implications for targeted drug-delivery systems

The significance of such unexpected movement lies in the team's ultimate objective: successfully manipulating the swimmers. To do that, they need to first understand how the swimmers react to different environments. 

"Both types of swimmers showed this backward sliding motion," says Balazadeh Koucheh. "So we now understand that rheology alone is enough to affect this phenomenon rather than shape. It's a very nice finding since our aim is to control these swimmers inside shear-thinning fluids like mucus and blood." 

The next step for the research is to explore the effects of rheology on swimmers designed at the microscale-swimmers in this study were millimeter-sized-and with different shapes.

Although targeted drug delivery remains many years away, findings like this represent the incremental advances that will bring the concept from theory to practice.

"The same swimmer behaves completely differently depending on what it's swimming through and that's a powerful handle for control," says Demir. "We used to think of the fluid as just the medium. Now we're starting to see it as part of the machine."