Swelling-induced residual stress and the mechanism of curling in articular cartilage in vitro
The triphasic theory has been used to analyze the in vitro free-swelling behavior of two-dimensional strips of articular cartilage with an inhomogeneous distribution of negative fixed charge density. The results indicate that the model can predict the characteristic curling behavior of cartilage strips. The free swelling geometry demonstrated a dependence on aspect ratio of the strip length thickness, so that the curling or cartilage strips increased with decreasing cartilage thickness. It was concluded that a physiological variation in fixed charge density gives rise to a non-uniform distribution of residual stress and strain, and is thus responsible for the curling of excised samples in vitro. The mathematical solution presented in this study provides a means for calculating the residual strains, stresses and swelling pressures in the equilibrium free-swelling configuration and may prove useful in the development of an experimental protocol for measurement of the residual strains in vitro and in situ.