Dubey Devendra K, Tomar Vikas
Aerospace and Mechanical Engineering, University of Notre Dame, IN 46556, USA.
J Phys Condens Matter. 2009 May 20;21(20):205103. doi: 10.1088/0953-8984/21/20/205103. Epub 2009 Apr 8.
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.
诸如骨骼、牙本质和珍珠母等硬质生物材料主要具有以交错方式排列的多肽相（例如原胶原蛋白（TC））和矿物相（例如羟基磷灰石（HAP）或文石）。据观察，此类材料的力学行为会随化学环境和施加载荷的方向而变化。在本研究中，对理想化的TC - HAP复合生物材料系统进行了基于显式三维分子动力学（MD）模拟的分析，以了解在三种不同化学环境下拉伸和压缩载荷的影响：（1）未溶剂化，（2）用水溶剂化，（3）煅烧后用水溶剂化。MD分析是在对应于TC - HAP相互作用最低结构水平（n级）的两个界面超胞上进行的，以及在TC基质中以交错方式排列HAP超胞的另外两个超胞（n + 1级）上进行的。n + 1级的超胞是通过将n级界面超胞以交错方式排列形成的。分析表明，在n级时，水分子的存在会使TC分子和TC - HAP界面在机械变形过程中具有更高的稳定性。此外，水还在相邻的TC分子之间起到润滑剂的作用。在以剪切应力为主的载荷作用下，水分子的作用类似于胶水，可增强TC - HAP界面强度。观察到的这些机制的总体效果是，在交错排列中，在有水和煅烧水环境下拉伸强度会增加。另一方面，在类似情况下相应的压缩强度会降低。从根本上说，在TC - HAP界面主要进行法向载荷传递的超胞在拉伸剪切载荷下更强。另一方面，在TC - HAP界面主要进行切向或剪切载荷传递得超胞在压缩剪切载荷下更强。可以选择从真空到煅烧水的化学环境变化以及交错排列中界面构型的变化组合，以使TC - HAP材料在施加变形时更强。
