pH-dependent surface chemistry and thickness affect the mechanics of hydroxyapatite nanocrystals

Hydroxyapatite (HAP) constitutes almost the entire mineral component of bone and is therefore one of its fundamental building blocks. In the present work, we perform mechanical tests of uniaxial compression and tension on a single HAP nanocrystal, through Molecular Dynamics simulations. We focus on hydroxyapatite nanocrystals with surface compounds arising from protonation and chemical modification under different external pH conditions and investigate how crystal thickness and pH variations influence the mechanical properties of a single HAP crystal. We show that the presence of surface compounds on HAP single crystals modifies the mechanical properties of the crystal itself. We also show that crystal thickness could play a role in determining crack paths and failure modes in tensile tests, although it is not the only contributing factor. Our work suggests a path for more in-depth analysis on the mechanical properties of hydroxyapatite single crystals that should take into account chemical modifications arising from different pH values present in different biological contexts, along with nanoscale fracture theories.