Effect of Heating on Avian (Cortical and Medullary) Bone Chemistry, Mineralogy and Structural Organization

The study of bone changes induced by heating is highly relevant for forensic and archeological analyses as well as for the production of bone-derived materials with novel properties and applications. In the present study, we study in detail how different types of avian bone (cortical, medullary) transform during thermal treatments (up to 800 °C) using different analytical techniques such as thermogravimetry (TGA-DSC), electron microscopy, X-ray diffraction and infrared spectroscopy. We show that bone transformation following thermal treatments is strongly influenced by bone architecture, the composition of the organic matrix, and the integration of the mineral with the organic fractions. For instance, in avian cortical bone, the apatite nanocrystals are integrated within collagen fibrils and coated with phosphorylated proteins. During heating, the collagen losses structural order and denatures (at around 200 °C), losing all structural integrity at 300 °C. In the bone mineral fraction, there is a gradual conversion of phosphate, in poorly crystalline/amorphous environments, into apatite (up to 400 °C). However, it is not until all organics are completely lost at around 600 °C that recrystallization sets in with a rapid increase in the size of apatite crystals. Also, during recrystallization, foreign ions (Mg2+, Na+) are expelled from the apatite lattice to the crystal surface, and the degree of preferential orientation of the apatite crystals increases as larger, well-oriented apatite crystals grow epitaxially at the expense of smaller, randomly oriented crystals. However, the scenario is different for the medullary bone. In this case, with an organic matrix rich in noncollagen proteins and proteoglycans, the recrystallization sets in at much lower temperatures (around 400 °C compared to 600 °C in cortical bone). Thus, the association of mineral and organic components controls recrystallization, particularly in the case of apatite nanocrystals within collagen fibrils in cortical bone. Also, the calcination process creates additional microporosity in both types of bone, increasing the bone mineral surface area and reactivity. The information obtained in this study provides a better understanding of the dynamics of bone transformation during alteration in natural processes (e.g., diagenesis, burning) and how bone mineral characteristics can be modified for specific applications (e.g., bone grafts, waste removal, or chromatography).