Mucociliary Transport in Healthy and Diseased Environments

Mucociliary clearance in the lung is the primary defense mechanism that protects the airways from inhaled toxicants and infectious agents. The system consists of a viscoelastic mucus layer on top of a nearly-viscous periciliary layer surrounding the motile cilia. In healthy environments, the thickness of the periciliary layer is comparable to the cilia length. Perturbations to this system, whether due to a genetic disorder or acquired causes, are directly linked to infection and disease. For example, depletion of the periciliary layer is typically observed in diseases such as chronic obstructive pulmonary disease and cystic fibrosis. Clinical evidence connects the periciliary layer depletion to reduced rates of mucus clearance. In this work, we develop a novel computational model to study mucociliary transport in a microfluidic channel. We systematically vary the viscoelastic properties and thickness of the mucus layer to emulate healthy and diseased environments. We assess cilia performance in terms of three metrics: flow transport, internal power expended by the cilia, and transport efficiency. We find that, compared to a control case with no mucus, a healthy mucus layer enhances cilia performance in all three metrics. That is to say, a healthy mucus layer not only improves flow transport, resulting in better clearance of harmful substances, but it does so at an energetic advantage to the cilia. Further, stiffer mucus enhances transport efficiency. In contrast, in diseased environments where the periciliary layer is depleted, mucus hinders transport, and stiffer mucus leads to a substantial decrease in transport efficiency. This decrease in efficiency is accompanied by an increase in the