Airway Closure: Elastoviscoplastic and Kinematic Hardening Effects in a Mucus Model

Airway closure is investigated numerically by considering the elastoviscoplastic (EVP) properties of airway mucus. Isotropic kinematic hardening is integrated into the Saramito-Herschel-Bulkley model to model the liquid film. The rheological model is fitted to experimental mucus data under healthy, asthma, chronic obstructive pulmonary disease (COPD) and cystic fibrosis (CF) conditions. A range of undisturbed initial liquid film thickness and mean surface tensions is considered for the simulations. Airway wall stresses and yielding of the liquid film is examined. In our results, we find that yielding starts immediately in the kinematic hardening induced model whereas it starts near the closure in the Saramito-HB model. Comparing the wall stresses between the two models, kinematic hardening is observed to significantly affect the closure time. The individual effects of the mucus rheological parameters on the wall stresses are examined. Elastic modulus is found to influence the closure time and wall stress excursions and gradients. The polymeric viscosity specifically affects the relaxation of stresses after the closure. The yield stress is found to also have a important effect on the closure time. A sensitivity analysis is performed as well, in order to account for possible uncertainties in determining the model parameters.