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Research Papers

Ab Initio Investigation of Strain Dependent Atomistic Interactions at Two Tropocollagen-Hydroxyapatite Interfaces

[+] Author and Article Information
Vikas Tomar

e-mail: tomar@purdue.edu
School of Aeronautics and Astronautics,
Purdue University,
West Lafayette, IN 47907

1Corresponding author.

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received July 30, 2012; final manuscript received January 29, 2013; published online March 28, 2013. Assoc. Editor: Xi Chen.

J. Eng. Mater. Technol 135(2), 021015 (Mar 28, 2013) (8 pages) Paper No: MATS-12-1185; doi: 10.1115/1.4023782 History: Received July 30, 2012; Revised January 29, 2013

Tropocollagen (TC) and hydroxyapatite (HAP) interfaces are one of the main load bearing entities in bone family of materials. Atomistic interactions in such interfaces occur in a variety of chemical environments under a range of biomechanical loading conditions. It is challenging to investigate such interactions using traditional analytical or using classical molecular simulation approaches owing to their limitations in predicting bond strength change as a function of change in chemical environment. In the present work, 3D ab initio molecular dynamics simulations are used to understand such atomistic interactions by analyzing tensile strain dependent deformation mechanism and strength of two structurally distinct idealized TC-HAP interfaces in hydrated as well as unhydrated environments. Analyses suggest that the presence of water molecules leads to modification of H-bond density at the interfaces that also depends upon the level of strain. TC molecules become stiffer in the presence of water due to the presence of H-bonds. Bond forming-and-breaking cycle change as a function of H-bond density lies at the heart of TC-HAP interfacial shear deformation. Consequently, interfaces with TC molecule placed flat on the HAP crystal surface experience significantly higher shear stress during deformation in comparison to the interfaces with TC molecule placed with their axes perpendicular to the HAP surface.

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Figures

Grahic Jump Location
Fig. 1

A representation of the hierarchical structure in bone, (a) microstructure of trabecular bone resembling the honeycomb structure with strut like structures, (b) one such strut called trabecula, having aligned mineralized collagen fibers, (c) fracture surface of human bone showing mineralized collagen fibrils, and (d) a schematic of staggered and layered assembly of tropocollagen molecules (wavy structures) and hydroxyapatite platelets to form a mineralized collagen fibril. Solid and dotted ellipses show two possible types of interactions between TC and HAP surface, one with TC parallel to HAP [001] surface (Transverse Interface Model) and other with TC perpendicular to HAP surface (L). Image in (a) is borrowed from [29] and (b, c) from [12], respectively.

Grahic Jump Location
Fig. 2

Snapshots showing TC-HAP interfaces, (a) with longitudinal interface (i.e., TC longitudinal axis perpendicular to HAP [001] surface), and (b) transverse interface (i.e., TC longitudinal axis parallel to HAP [001] surface). In both, transverse and longitudinal cases, supercells with water and without water are shown. The direction of loading (x direction) applied to all supercell is also shown in the transverse hydrated case.

Grahic Jump Location
Fig. 3

Snapshot of longitudinal interface model showing TC and HAP constituents in detailed atomistic representations

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Fig. 4

Plots showing von-Mises stress, virial stress, and maximum shear stress in the case of (a) transverse unhydrated interface, (b) transverse hydrated interface, (c) longitudinal unhydrated interface, and (d) longitudinal hydrated interface. Corresponding snapshots of all interfaces at 50% strain value are depicted on top of each plot.

Grahic Jump Location
Fig. 5

Plots showing number of hydrogen bonds associated with TC molecule and HAP surface as a function of strain in the case of (a) transverse interface and (b) longitudinal interface for hydrated and unhydrated cases. The shown hydrogen bonds do no include hydrogen bond contribution from water-water interaction. On an average, transverse interface involves more hydrogen bonds compared to longitudinal interface.

Grahic Jump Location
Fig. 6

Plots showing a comparison of atomic level forces acting on TC molecule as a function of deformation strain value and with or without presence of water for (a) transverse and (b) longitudinal interfaces. In plot (a) dependent axis is a broken axis and is plotted in this manner for the sake of visual clarity in variation of forces with respect to strain values.

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