Magnetic resonance microscopy studies of cation diffusion in cartilage.

The diffusion of copper ions in bovine nasal cartilage (BNC), a dense connective tissue, was investigated to further the understanding of ion transport in charged biopolymer systems. Using an inversion-recovery null-point imaging technique, it was found that the diffusion rate of divalent copper ions into cartilage was significantly lower in normal BNC than in BNC in which the matrix fixed charges had been reduced by enzymatic digestion or acid neutralization. In normal cartilage, counterion diffusion was not well described by a simple Fickian process, likely owing to the high charge density of the constituent molecules. In contrast, in both digested and acid neutralized BNC, counterion diffusion appeared Fickian. Features of the ion transport process were modeled using a diffusion equation which included a linear sorption term to account for cation binding. The diffusion coefficient of copper in cartilage increased with decreasing matrix fixed charge and was constant for reservoir concentrations up to 30 mM. The activation energy for the diffusion of copper into BNC was determined to be 34.5 kJ/mol.