Deproteinized young bone reveals a continuous mineral phase and its contribution to mechanical properties with age

Bone is composed of mineral (apatite crystals) phase, organic (proteins, mainly collagen, and other organics) phase, and water at the nanoscale. Mineral contributes to elastic modulus and strength while organics and water give bone its toughness. Changes in the mineral composition and arrangement with age during development, and the resulting mechanical properties, are of high scientific and clinical interests. The mineral phase can be studied by removing the organic phase from bone using sodium hypochlorite (NaClO). This process is called deproteinization (DP). In this paper, four age groups (3-week, 4-week, 16-week, and 24-week) were compared in both untreated and DP bone states. We find that the deproteinized bone samples from all these age groups are self-standing. Thus, the mineral phase forms a continuous structure, even in the 3-week bone. Micro-computed tomography imaging displays a significant porosity decrease with the increase in age. Raman spectroscopy reveals increases in both the carbonate-to-phosphate ratio and the mineral crystallinity with age. Compression test results show that the elastic modulus and strength of the mineral phase alone are positively related to age. The elastic modulus of DP bone is less affected by age than that of the untreated bone. Overall, the mechanical properties of the mineral phase increase with age due to a decrease in porosity and an increase in the mineral content.