Mucus in a human lung has non-Newtonian properties such as shear-thinning, yield stress, and viscoelasticity. Rheological changes of mucus may greatly affect its ability to function as a lubricant, selective barrier, and the body’s first line of defense against infection (Lai 2009). Patients with COPD, asthma, or cystic fibrosis often have mucus hyper-secretion. Mucus plugging can cause total airway obstruction (Kant 2007). If mucus is too thick, for example in severe bronchitis or cystic fibrosis where the sputum viscosity can be more than 105 times that of water, patients experience great difficulty in mucus clearance, resulting in bacterial overgrowth (Lai 2009). The understanding of the mechanisms involved in mucus clearance is far from complete. For instance, the relation is not known between the non-Newtonian properties of the mucus layer and ciliary function in thickened mucus (Button 2008).

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