Mechanical Properties of Aging Soft Tissues

Abstract

Aging is a process that affects cells, the extracellular matrix (ECM), tissues, and organs. The lung is the entry of oxygen into the body and any dete-rioration in its ability to take up and distribute oxygen uniformly in the parenchyma compromises the cardiovascular system and hence contributes to the aging of the organism. In this chapter, we provide an overview of the biochemical, structural, and biomechanical properties of the aging lung parenchyma. We also discuss several measurement techniques that are suitable to assess the biomechanical properties of the lung. Following a review of general constitutive relations used in lung biomechanics, we derive a specific multiscale constitutive equation for the lung tissue strip that allows us to partition the contributions of collagen, elastin, their volume fraction, and their interaction with the proteoglycan matrix. This model provides a better understanding of how airspace enlargement, local stiffening of ECM fibers and macroscopic lung compliance are related to each other. These constitutive relations have important implications for lung function during aging. Specifically, there is an increase in ECM stiffness due to cross-linking of collagen which influences cellular behavior at the microscale. Despite ECM stiffening, at the scale of thousands of alveoli, parenchymal stiffness may be near normal and lung compliance may even increase in the elderly due to the enlargement alveoli enabling relatively normal gas exchange in the absence of exercise. Finally, we discuss possible new research directions that may help better understand and reduce the risk of pulmonary diseases of old age. 5.1 Introduction The remarkable increase in human life expectancy in the last 100 years prompted scientists to investigate the mechanisms of aging. Various molecular theories of cellular aging have been put forth since the middle of the last century [25, 48, 85].