Micromechanics of the connective tissue matrix of the lung

The number of macromolecules thought to play an important role in tissue rheology is small, including collagen, elastin and proteoglycans. Yet different tissues with similar constituents exhibit different rheological behavior. We hypothesized that the rheological behavior of the lung parenchyma arises from 1) the structural complexity of the collagen network within the alveolar wall and 2) the mechanical interaction between collagen and proteoglycans. In disease, active cellular remodeling alters the complexity of tissue architecture which changes its fractal dimension. We have developed a novel technique to visualize macromolecular complexes in the alveolar walls of a slice of intact lung tissue during macroscopic deformation. Collagen or elastin from normal and elastase-treated rat lungs were labeled and the rheology of the samples were assessed while the deformation of the macromolecules were imaged. The data suggest that 1) folding and relaxation of collagen may be important in the hysteretic behavior of the normal lung; 2) increased macroscopic hysteresis in emphysematous lungs is related to increased hysteresis during folding of alveolar walls; 3) proteoglycans stabilize the folding of the alveolar walls; and 4) the macroscopic power law stress relaxation may be related to the fractal organization of collagen within the alveolar wall.