Mucus, often overlooked in its simplicity, plays a crucial role in maintaining human health. This sticky substance acts as a protective barrier, trapping dust particles, bacteria, and other unwanted intruders before they can reach the depths of the lungs. But how does the body effectively clear this mucus?
According to Meike, unraveling this seemingly straightforward question is far more intricate than one might assume. "Biology is incredibly complex," she explains. "Considering all the factors involved, it’s virtually impossible to fully explain these biological phenomena using traditional methods." To tackle this challenge, Meike turned to computer simulations, simplified models of reality, to elucidate the underlying mechanisms at play.
Cleaning
The airways of all’animals are lined with ciliated cells, equipped with tiny hair-like structures known as cilia. These cilia move rhythmically, propelling a thin layer of fluid along the airway surfaces. In large mammals, including humans, this fluid layer contains mucus strands that capture and remove harmful particles. Mucus strands are produced in small glands and initially move with the fluid flow. However, at a certain point, they take an unexpected turn, rotating 90 degrees. "This is quite clever," Meike illustrates. "When you clean something, you don’t move the broom in the same direction as the dirt; instead, you sweep perpendicularly."Rules in physics
"From a physics standpoint, these strands shouldn’t be able to make this abrupt turn," Meike explains, pointing to her dissertation filled with complex physical formulas. "According to the laws of physics, an elongated object in a flow should continue moving in a straight line. Imagine holding a string in flowing water," she continues. "The string will follow the direction of the current, just like in the wind."Based on these principles, the behavior of mucus strands seems to defy the laws of physics. However, Meike contends that this doesn’t imply a flaw in physics but rather suggests the presence of additional factors at play. Through her computer simulations, Meike explored the influence of tiny sticky particles in the lungs on the long, rotating mucus strands. Her findings revealed that these smaller mucus droplets could adhere to the longer strands, influencing their rotation, shape, and speed.
Mucus exhibits properties of both a liquid and a solid, making it a viscoelastic substance. This means it exhibits both viscosity (resistance to flow) and elasticity (the ability to return to its original shape after deformation). Interestingly, humans cannot swim in viscoelastic materials, as our limbs cannot effectively push through the material. However, some tiny organisms, known as microswimmers, have mastered this challenge. They employ unique swimming techniques, such as whip-like tail movements (sperm cells), rotational motion, or cilia-driven propulsion (bacteria). In a side project alongside her main research, Meike investigated the movement of these micro-swimmers in viscoelastic materials like mucus using simulations. In such environments, the model micro-swimmers exhibited significantly more directional changes compared to non-viscoelastic materials. This, Meike discovered, was attributed to local interactions between the micro-swimmer and its surroundings.
Simulating mucus
While direct observation of mucus transport within human and animal lungs remains challenging, simulations offer a valuable tool for understanding these processes. "Simulations help us think critically about a system and test various hypotheses," Meike explains. Additionally, they bridge the gap between theory and practice: "When theoretical analysis reaches its limits, we turn to simulations. Similarly, simulations can enhance our understanding of experimental results."I prefer to apply physics to the real world
Physicist Meike Bos
