Effect of calcium ions on the nanostiffness of articular cartilage

Calcium is one of the major elements in human body, and critical in the biological and physiological functions. Soft tissue hardening due to calcification has been reported in various diseases. However, the quantitative relationship between the mechanical properties of soft tissues and calcification has not yet been fully understood. In this study, using human articular cartilage as prototype material, the relationship between the ex vivo calcium concentration and the nanostiffness of soft tissues was investigated by indentation-type atomic force microscopy. It is found that the nanostiffness of tissue shows a significant dependence on concentration of calcium ions, when it is higher than the normal serum calcium concentration. Structural observation of collagen fibrils indicated that the increase of nanostiffness should be the result of the increase of crosslinking with the introduction of calcium among collagen molecules.