DYNAMIC MODULI OF RABBIT LUNG-TISSUE AND PIGEON LIGAMENTUM PROPATAGIALE UNDERGOING UNIAXIAL CYCLIC LOADING

In fibrous connective tissue networks, mechanical loads may be transfe rred from one fiber to the next by friction between slipping fibers (J . Appl. Physiol. 74: 665-681, 1993). Here we tested that hypothesis; i t predicts that elastance of fibrous networks increases with increasin g frequency, decreases with increasing strain amplitude (Delta epsilon ), and decreases with tissue swelling by solvent. Similarly, it predic ts that hysteresivity (eta) decreases with increasing frequency, incre ases with increasing Delta epsilon, decreases with tissue swelling, an d, importantly, exceeds that of isolated fibrous constituents of the m atrix. Elastance and eta of two structurally dissimilar connective tis sues were measured, the rabbit lung parenchymal strip (a loose collage nous tissue) and the pigeon ligamentum propatagiale (an elastin-rich t issue). Experiments covered the frequency range 0.03125-3.125 Hz. Elas tance of lung parenchyma was substantially lower than that of propatag ial ligament, increased linearly with the logarithm of frequency, and decreased with Delta epsilon; that of ligamentum propatagiale was inse nsitive to both frequency and Delta epsilon. eta of lung parenchyma de creased moderately with increasing frequency and assumed values of sim ilar to 0.1, but eta of ligamentum propatagiale was frequency and Delt a epsilon invariant and assumed values an order of magnitude smaller. These tissues also showed disparate mechanical responses when exposed to hypertonic bath solutions. Although there were some quantitative di fferences between predictions and experimental observations, the dynam ic behavior of lung parenchyma was generally consistent with that of a network in which load is transferred from one fiber to the next by th e agency of friction acting at slipping interface surfaces.