Dynamic compression of soft tissues, such as articular cartilage, affects tissue mechanical properties and metabolic activity. The effect is attributed, in part, to the water and solutes movement in the extracellular matrix, which alters the mechanical (e.g. fluid shear stress) and chemical (e.g. growth factors, cytokines and hormones) microenvironment for cells in the soft tissue (Guilak et al. 1997). Experimental studies on cartilages suggest that external dynamic load enhances solute transport to inner regions of specimens, particularly for large sized molecules (O’Hara et al. 1990). Few theoretical studies, however, have been carried out to clarify the underlying mechanism and to quantify the enhancement. This study aims to highlight and to quantify the contribution of dynamic loads on solute transport to regions of tissue where cells would, otherwise, remain inactive due to the low solute concentration there.

In our analysis, poroelastic theory was used for the deformation of the solid matrix and the movement of water within (Wang & Parker, 1995). The solid phase represented the matrix of collagens and proteoglycans, and the liquid phase the interstitial fluid. A simplified two-dimensional model was used, that consisted of a deformable matrix embedded with cells immersed in solution in a rigid impermeable well. The top surface of the matrix was in direct contact with the solution with known solute concentration. Solute diffused into the matrix and was consumed by cells. Mechanical cyclic loads were applied in the central region of the top surface of the matrix, causing its deformation and extracellular fluid movement. Resulting cell density in the matrix varied with the time and the location. Solute diffusion coupled with the movement of the extracellular fluid contributed to the solute transport in the matrix.

Governing equations for the matrix deformation, interstitial fluid movement and solute transport were solved numerically. Comparisons on solute transport were made between different loading frequencies and amplitudes. Different sized molecules were also considered in our study. Results from the model confirmed experimental findings that cyclic loads facilitated solute transport in soft tissues and the effect was more significant for large sized molecules. Furthermore, we found that higher loading frequency and bigger loading amplitude introduced better improvement to solute transport. Quantitative analysis of solute concentration distribution in the tissue made it possible to predict regions where cells were activated by the improved solute supply. Activation of cells occurred often without significant elevation of solute concentration in the tissue. The fact that more cells in tissues became metabolically active under dynamic loads exemplified most directly their effects on solute transport in soft tissues.

In conclusion, dynamic loads on extracellular matrix promote solute transport to inner regions of the tissue. Poroelastic theory makes it possible to predict the enhancement quantitatively for different sized solutes under different loading conditions.