Microscopic airway reopening through cascades of plugs ruptures

The inner surface of lung airways is covered by a thin layer of mucus whose thickness is usually about 2 or 3 % of the total radius of the duct. However certain diseases like asthma, chronic bronchitis or allergies can induce a hypersecretion of mucus, leading to the formation of liquid plugs which occlude the airways. These plugs can considerably alter the distribution of air during the breathing cycle. It is therefore fundamental to understand the propagation of air in the presence of such plugs and in particular airway reopening. Some studies have been performed on real lungs but there was no visualization of the airways, and only information at the entrance was reported. The purpose of this experimental work is to create a synthetic network, reproducing only the main features of the lung airways, to visualize and understand the physics of airway reopening. The human lung is made of about 24 generations with diameters ranging from about 2 cm for the trachea to 100 μm for the smallest ones. As a consequence, the physics is very different for the first and the last generations. The present work focuses on the last micrometric generations for which inertia and gravity can be neglected (small Reynolds and Bond numbers). For this purpose a binary network made of PDMS was designed and fabricated. It is composed of 6 generations with a width of 700 μm for the first generation and a width ratio of 0.8 between the branches of successive generations. A random initial distribution of plugs is inserted inside this network by using syringe pumps and finally some air is introduced inside the airways. The reopening of the network takes place through a series of cascades of plugs ruptures. A single cascade can be explained by a simple model, based on the flow resistance of the plugs and the liquid deposited on the walls. The correlation between successive cascades is extracted from a careful analysis of the data. This study improves considerably our understanding of cascades of plug ruptures, which might be valuable to enhance the treatment of such diseases.