Carbonate substitution significantly affects the structure and mechanics of carbonated apatites

Bone mineral comprises nanoparticles of carbonate-substituted bioapatite similar to hydroxylapatite. Yet mechanical values of macroscopic-sized geological hydroxylapatite are often used to model bone properties due to a lack of experimental data for bioapatite. Here, we investigated the effects of carbonate substitution and hydration on biomimetic apatite response to load using in situ hydrostatic pressure loading and synchrotron x-ray diffraction. We find that increasing carbonate levels reduced the bulk modulus and elastic strain ratio. Elastic constants, determined using computational optimization techniques, revealed that compliance values and elastic moduli decreased with increasing carbonate content, likely a result of decreased bond strength due to CO32- substitution and Ca2+ loss. Hydration environment had no clear effects on the elastic properties likely due to dissolution and reprecipitation processes modifying the crystal structure organization. These results reinforce the need to consider carbonate composition when selecting mechanical properties and provide robust data for carbonate-substituted apatite stiffness. Statement of significance Over half of bones and teeth are composed of nano-scale carbonated-apatite mineral crystals providing them with strength and stiffness. Accurate mechanical property data for these nanocrystals is necessary to understand bone mechanics and develop new and improved orthopedic products and biomechanical models. However, this experimental mechanical data is lacking due to the crystals' small size. In this study, we apply X-ray techniques with in situ hydrostatic loading to measure the mechanics of biomimetic bone mineral nanocrystals with varying compositions and environments. These are some of the first experimental results showing a decrease in apatite elastic properties with increasing carbonate content. This knowledge will allow for significantly improved modeling of bone mechanics and more accurate biomaterial development.(C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

Increasing carbonate levels decrease the elastic properties of biomimetic apatite, while hydration environment has no significant effect. The study emphasizes the importance of considering carbonate composition in selecting mechanical properties and provides valuable data for carbonate-substituted apatite stiffness.