Rheological properties of lung parenchyma: relationship to structure of collagen and elastin fiber networks

The authors investigated the relationship between the structure of the connective tissue and the mechanical properties in parenchymal strips from guinea pig lungs. They assessed the storage and loss moduli using pseudorandom strain oscillations for frequencies from 0.07 to 2.4 Hz at mean static strains from 0 to 30%. To separately test how the components of the connective tissue matrix contribute to macroscopic tissue rheology, elastin or collagen fibers were labeled with fluorescent antibodies and manually stretched to the same macroscopic strains as the dynamic measurements under an inverted fluorescent microscope. About 10 images of both collagen and elastin fibers were taken at each strain level. The fibers form a continuous network of hexagons at low strains and ellipsoids at higher strains. The authors characterized deformation of the network by measuring the mean aspect ratio of the ellipsoids for both collagen and elastin (Ar,c and Ar,c respectively). Ar,e increased with increased strain in a linear fashion whereas Ar,c increased and then showed a plateau for higher strains. The heterogeneity of the fiber network decreased with increasing strain. There was a good correlation between Ar,c and the storage moduli. The correlation between Ar,c and the storage and loss moduli was only good for low strains, indicating the role collagen plays in limiting tissue deformation.