Scanning probe microscopy of intrafibrillar crystallites in calcified collagen

Vertebrate mineralized tissues are composite materials formed by the organized growth of carbonated apatite crystals within a matrix of collagen fibres. Calcified collagen from turkey tendon was investigated using scanning tunnelling microscopy (STM) and atomic force microscopy (AFM). Samples were treated with hydrogen peroxide to enhance the mineralized phase by removing part of the collagen matrix and the results compared with the untreated material. Plate-like crystalline entities with dimensions 35 nm × 5–8 nm by 1.5 nm were seen. These dimensions are consistent with previous reports using transmission electron microscopy (TEM) of calcified tendon and topographic imaging of tendon crystals. The resolution of the images obtained using STM is better than the previously reported pictures obtained using TEM or scanning electron microscopy (SEM). The value of 35 nm is the same as the gap region in the structure of the collagen fibrils. Stacking of plates and plate-aggregates are a dominant feature in the scanning images. These results support the concept of organized intra-fibril mineral crystals within the organic collagen matrix. Electron diffraction and X-ray diffraction were undertaken on the samples and the patterns recorded match those previously reported for carbonated apatite.