The effect of tensile and compressive loading on the hierarchical strength of idealized tropocollagen-hydroxyapatite biomaterials as a function of the chemical environment.

Hard biomaterials such as bone, dentin and nacre have primarily a polypeptide phase (e.g. tropocollagen (TC)) and a mineral phase (e.g. hydroxyapatite (HAP) or aragonite) arranged in a staggered manner. It has been observed that the mechanical behaviour of such materials changes with the chemical environment and the direction of applied loading. In the presented investigation, explicit three-dimensional molecular dynamics (MD) simulations based analyses are performed on idealized TC-HAP composite biomaterial systems to understand the effects of tensile and compressive loadings in three different chemical environments: (1) unsolvated, (2) solvated with water and (3) calcinated and solvated with water. The MD analyses are performed on two interfacial supercells corresponding to the lowest structural level (level n) of TC-HAP interactions and on two other supercells with HAP supercells arranged in a staggered manner (level n+1) in a TC matrix. The supercells at level n+1 are formed by arranging level n interfacial supercells in a staggered manner. Analyses show that at level n, the presence of water molecules results in greater stability of TC molecules and TC-HAP interfaces during mechanical deformation. In addition, water also acts as a lubricant between adjacent TC molecules. Under the application of shear stress dominated loading, water molecules act to strengthen the TC-HAP interfacial strength in a manner similar to the action of glue. An overall effect of the observed mechanisms is that, in a staggered arrangement, tensile strength increases in the presence of water and calcinated water environments. On the other hand, corresponding compressive strength decreases under similar circumstances. Fundamentally, supercells with primarily normal load transfer at the TC-HAP interfaces are stronger in tensile shear loading. On the other hand, supercells with primarily tangential or shear load transfer at the TC-HAP interfaces are stronger in compressive shear loading. A combination of changes in chemical environment from vacuum to calcinated water and changes in interfacial configurations in a staggered arrangement could be chosen to make the TC-HAP material stronger under applied deformation.