A composites theory predicts the dependence of stiffness of cartilage culture tissues on collagen volume fraction

The tensile stiffness of tissue grown from chondrocyte culture was both mea sured experimentally and predicted using a composites model theory relating tissue microstructure to macroscopic material stiffness. The tissue was al tered by several treatment protocols to provide a wide range of collagen fi bril volume fraction (0.015-0.15), The rate of change of tissue modulus wit h change in collegen volume fraction predicted by the theory was within 14% of the slope of the linear fit through the experimental data, without the use of fitting parameters for the theoretical value of the slope. Use of th e model to simulate cytokine mediated tissue digestion suggests that the ac tion of IL-1 beta and retinoic acid is mainly removal of proteoglycans and some removal of collagen. The model also indicates that the matrix and coll agen remaining in the tissue has the same elastic properties as the untreat ed tissue, and is not damaged due to the alteration. Young's modulus of the collagen fibrils is predicted to be 120 MPa, a value in the range of previ ous studies. This value is dependent mainly on the matrix modulus and colla gen fibril volume fraction and not on Poisson's ratio of either matrix or f ibril. Poisson's ratio of the tissue depends primarily on the Poisson's rat io of the matrix. (C) 1999 Elsevier Science Ltd. All rights reserved.