Role of hydroxyapatite crystal shape in nanoscale mechanical behavior of model tropocollagen–hydroxyapatite hard biomaterials

Shape of mineral (e.g. hydroxyapatite (HAP) or aragonite) crystals can be a strong determinant of the nanoscale strength of hard biological materials such as bone, dentin, and nacre. This work presents an understanding of the effect of HAP shape variation on nanoscale strength of model TC-HAP biomaterials. For this purpose, 3-dimensional molecular dynamics analyses of direction dependent tensile deformation in two structurally distinct TC-HAP cells with HAP crystals in needle shaped configuration and plate shaped configuration are performed. Analyses point out that the peak interfacial strength for failure is the highest for supercells with plate shaped HAP crystals. In addition, the plate shaped HAP crystals result in the localization of peak stress over a larger length scale indicating higher fracture strength. Peak strength during transverse loading is always found to be lower than that during the longitudinal loading. However, interfacial strength shows a reverse trend. Overall, analyses point out that HAP crystal shape along with the optimal direction of applied loading with respect to the TC-HAP orientation strongly influence biomaterial strength at the nanoscale.