A dynamic compression–relaxation model for lung surfactants

Lung surfactants are mixtures of phospholipids and proteins that coat the alveolar surface. Upon compression (exhalation), these films reduce the surface tension of the alveolar fluid to a low value (<5 mJ/m2) to avoid alveoli collapse. In this work, the mechanical properties of these insoluble surfactant films are studied using a model for the response of surface tension to dynamic (cyclic) changes in surface area. This model introduces the likely effect that compression and relaxation occur simultaneously. The model treats the compression/expansion effects with the conventional definition of dilatational elasticity of insoluble surfactants, and treats the relaxation/adsorption of the film as first order processes driven by the excess surface energy of the film. The model was used to fit the dynamic surface tension (obtained using axisymmetric drop shape analysis—ADSA) of drops of lung surfactant preparations subject to compression/expansion cycles in a constrained sessile drop (CSD) device. As a result of this fit, the elasticity of the surfactant film and the adsorption/relaxation constants for the surfactant preparation are determined.