Figure 2.

The hierarchical structure of natural bone from macro to nanoscale. The schematic representation of the long bone structure is shown as an example. In the macroscopic view, bone cross-section can be divided into an external and an internal part. The bone external structure consists of compact cortical bone which is composed of densely packed cylindrical osteons. The internal cancellous bone has a porous trabecular structure. Osteons and trabeculae are made up of lamellae with different collagen fiber patterns. The collagen fibers (~1 µm) are composed of bundles of mineralized collagen fibrils (~100 nm), in which hydroxyapatite nanocrystals (nanoscale size) are deposited in the gaps between collagen molecules (tropocollagen triple helices). The figure was created using Servier Medical Art.