Mucus on the human airway surface normally provides a fluid barrier to trap and remove inhaled hazardous particulates such as viruses and bacteria, a physiological function known as mucus clearance. This function, however, can fail if the mucus has abnormal rheological properties, as in the case of certain lung diseases such as asthma. Despite its importance, airway mucus rheology has not been well studied so far, largely because of its complex nature and limited availability. Therefore, in this study, we prepared mucin-based protein solutions as simulated normal and asthmatic airway mucus (NM and AM, respectively) and subsequently studied them in both linear and nonlinear rheological conditions using either conventional steady-state or large amplitude oscillatory shear experiments together with nonlinear multi-mode Giesekus model analysis. We also examined the microscopic structure of the simulated airway mucus by optical or atomic force microscopy. We found that both NM and AM exhibited typical nonlinear rheological behaviors of protein solutions. However, as compared to NM, AM was much more solid-like, and the viscosity, yield stress, and dynamic modulus were more than ten times that of NM. These differences in macroscopic rheological behaviors between NM and AM could be attributed to their different microstructures. Taken together, this study provides evidence that airway mucus may dramatically change its rheological behaviors with changing chemical composition and microstructure as occurring in diseased conditions such as AM. Thus, the presented rheological assessment and modeling analysis, together with the microscopic characterization of simulated airway mucus, may have important values for better understanding the critical roles of mucus rheology in the determination of the mucus clearance function in health and disease as well as the development of pulmonary drug delivery systems.
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