Physical signals and solute transport in cartilage under dynamic unconfined compression: finite element analysis

A specialized model for charged hydrated soft tissue containing uncharged solutes (such as glucose and uncharged growth factor) was presented based on the more general, mechanoelectrochemical mixture theory (Gu et al., J. Biomech. Eng. 120:169-180, 1998: Lai et al., J. Biomech. Eng. 113:245-258, 1991). This model was applied to analyze the mechanical, chemical and electrical signals within the cartilage sample under dynamic unconfined compression (5% dynamic strain) using a finite element method. The effects of the permeable loading platen, loading frequency, and fixed charged density on the physical signals and the transport of fluid, ions, and uncharged solutes were investigated. Numerical analyses show that a permeable platen will increase the rate of dynamic fluxes of fluid, ion, and uncharged solute in the region near the permeable platen, but not the fluid pressure and electrical potential in the central region of the tissue at 0.1 Hz. The increase in fixed charge density (FCD) will decrease the dynamic fluxes of fluid, ion, and uncharged solute, but increase the fluid pressure and electrical potential within the tissue. For both permeable and impermeable loading platen cases, the electrical current density within the tissue is close to zero (approximately 10 microA/m2) except at the small region near a corner of the sample. On the radial edge of the sample, the dynamic solute flux for the large neutral solute is different from that for small neutral solute (glucose). This study is important for understanding mechanobiology of cartilage and for designing a bioreactor to be used in cartilage tissue engineering.