Simulating the fluid dynamics of moving cells to map their location

As you read this sentence, trillions of cells are moving around in your body. From the red blood cells being pumped by your heart, to the immune cells racing across your lymphatic system, everything you need to live pulsates and flows in a turbulent dance of finely tuned biological machinery.

Because its physical properties are so unique, understanding the fluid dynamics of flowing biological cells like these has been an important topic of research. New insights can lead to the development of better microfluidic devices that study disease, and even improve the function of artificial hearts. However, live tracking and observing flowing cells as it moves across the body is still a challenge.

Now, utilizing numerical simulations, researchers from Japan have succeeded in recreating the fluid dynamics of flowing cells. In their paper, published in the Journal of Fluid Mechanics, the team created an in-silico cell model—a simulation of biological cells—by programming them as deformable “capsules,” and placed them in a simulated tube under a pulsating “flow,” mimicking how cells travel through a vessel.

They found that these capsules will move to a specific position in the tube depending on two factors: the deformation of the capsule and the pulsation frequency. Essentially, the system provides researchers the tool to identify where and how cells move through a vessel.

The fluid dynamics of a moving cell are quite unique. They will get pushed around through the body at regular intervals, and pass through tubes that can vary in size and composition under different flow conditions. Cells are also very flexible and will stretch and deform as they work through your body, something that also affects its fluid dynamics.

“To better understand cell behavior under unsteady flow, we constructed a numerical model that simulates the physics of cells in tubes under pulsating flows,” explains Associate Professor Naoki Takeishi from Kyushu University’s Faculty of Engineering, who led the study.

“This would allow us to figure out how cells statistically distribute in a system,” continues Takeishi. “In our experiment we simulated cells as deformable capsules. Because we were simulating capsule dynamics in a wide range of conditions, we required heavy computational resources.”

In their simulations, the team revealed that there exists a pulsation frequency at which the capsule stretches and shrinks, allowing it to move stably away from the tube’s center—where the flow is the fastest—toward areas with slower flow. Interestingly, even if the flow speed is increased, the pulse frequency remains the same. On the other hand, under slow flow conditions, capsules would tend to converge quickly to the center of the tube.

“Our results show that the behavior of flexible particles, like biological cells, in a flowing tube depends not only on the amount of deformation—that has already been known—but also on the pulsating frequency,” continues Takeishi. “Moreover, we can control the capsule position by adjusting that frequency.”

The team hopes their new findings can be utilized in research that requires precise cell and fluid manipulation, such as in cell alignment, sorting, and separation. These techniques are particularly relevant for isolating moving tumor cells in cancer patients.

“At present, there is no biological consensus on whether steady or unsteady blood flow is preferable in artificial hearts,” concludes Takeishi. “Our numerical results form a fundamental basis for further study, not only on the essential movement of cells in the body, but also in the development of artificial organs, particularly the heart and blood vessels.”