Shashindra M. Pradhan

Platelet interlocks are the key to toughness and strength in nacre

Nacre, the inner layer of mollusk shells is a composite made of platelets of mineral aragonitic calcium carbonate with a few weight percent organic material sandwiched in between. The organic and nanostructural nuances are often suggested to be the reason for the extreme toughness of nacre. Here we report the presence of interlocks between platelets of nacre from red abalone. We also report and show, using three-dimensional finite element modeling, that interlocks are the key mechanism for the high toughness and strength of nacre. The observed rotation between platelet layers, which were earlier reported as defects of structure, are necessary for the formation of interlocks.

Directional dependence of hydroxyapatite-collagen interactions on mechanics of collagen

Bone is a biological nanocomposite composed primarily of collagen and hydroxyapatite. The collagen molecules self-assemble to from a structure known as a fibril that comprises of about 85-95% of the total bone protein. In a fibril, the molecular level interactions at the interface between molecular collagen and hydroxyapatite nanocrystals have a significant role on its mechanical response. In this study, we have used molecular dynamics and steered molecular dynamics to study directional dependence of deformation response of collagen with respect to the hydroxyapatite surface. We have also studied mechanical response of collagen in the proximity of (0001) and (101 0) surfaces of hydroxyapatite. Our simulations indicate that the mechanics of collagen pulled in different directions with respect to hydroxyapatite is significantly different. Similar results were obtained for collagen pulled in the proximity of different crystallographic surfaces of hydroxyapatite.

Structural hierarchy controls deformation behavior of collagen.

The structure of collagen, the most abundant protein in mammals, consists of a triple helix composed of three helical polypeptide chains. The deformation behavior of collagen is governed by molecular mechanisms that involve the interaction between different helical hierarchies found in collagen. Here, we report results of Steered Molecular Dynamics study of the full-length collagen molecule (~290 nm). The collagen molecule is extended at various pulling rates ranging from 0.00003/ps to 0.012/ps. These simulations reveal a new level of hierarchy exhibited by collagen: helicity of the triple chain. This level of hierarchy is apparent at the 290 nm length and cannot be observed in the 7-9 nm models often described to evaluate collagen mechanics. The deformation mechanisms in collagen are governed by all three levels of hierarchy, helicity of single chain (level-1), helical triple helix (level-2), and hereby described helicity of the triple chain (level-3). The mechanics resulting from the three levels is described by an interlocking gear analogy. In addition, remarkably, the full-length collagen does not show much unwinding of triple helix unlike that exhibited by short collagen models. Further, the full-length collagen does not show significant unwinding of the triple helix, unlike that exhibited by short collagen. Also reported is that the interchain hydrogen bond energy in the full-length collagen is significantly smaller than the overall interchain nonbonded interaction energies, suggesting that the nonbonded interactions have far more important role than hydrogen bonds in the mechanics of collagen. However, hydrogen bonding is essential for the triple helical conformation of the collagen. Hence, although mechanics of collagen is controlled by nonbonded interchain interaction energies, the confirmation of collagen is attributed to the interchain hydrogen bonding.

Multiscale Model of Collagen Fibril in Bone: Elastic Response

AbstractIn this paper, the development of a multiscale model of collagen fibril is described using a hierarchical approach by combining molecular dynamics simulations and FEM. Steered molecular dynamics was used to investigate the mechanical response of collagen under various conditions that mimic the organization of collagen molecules and mineral inside the collagen fibril. The steered molecular dynamics simulations showed that the elastic properties of collagen molecules are significantly higher in the proximity of mineral. The properties of collagen and interfaces at the molecular scale were carried over to the continuum model of collagen fibril. The model of collagen fibril (measuring approximately 1 μm in length and 50 nm in diameter) was constructed using FEM. Results showed that the deformation properties of collagen fibril are significantly influenced by interactions between collagen and mineral at the molecular scale, significantly affecting the elastic properties of fibril. Here, a model of fibr...

Evolution of Molecular Interactions in the Interlayer of Na-Montmorillonite Swelling Clay with Increasing Hydration

AbstractIn this work, the swelling behavior of sodium (Na)-montmorillonite clay with increasing amounts of hydration is studied using molecular dynamics. The molecular models of the dry clay and the hydrated clays, consisting of 2, 4, 6, 8, and 10 monolayers of water in the interlayer, are used in this study. This work captures the evolution of interaction energies in the interlayer of Na-montmorillonite swelling clay with increasing hydration and provides insight into swelling mechanisms. This work shows the important role of bound water and clay-Na interactions during swelling to stabilize clay structure during hydration. Changes to water molecule conformations in the interlayer during swelling are also reported. The results and insight provided by this work will help in modeling and predicting exfoliation and resulting particle breakdown in swelling clays, in addition to expounding the key role of interlayer interactions on swelling in smectite clays.

Mechanics of Collagen in the Human Bone: Role of Collagen-Hydroxyapatite Interactions

Here, we report results of our simulations studies on modeling the collagen-hydroxyapatite (HAP) interface in bone and influence of these interactions on mechanical behavior of collagen through molecular dynamics and steered molecular dynamics (SMD). Models of hexagonal HAP (10-10) and (0001) surface, and collagen with and without telopeptides were built to investigate the mechanical response of collagen in the proximity of mineral. The collagen molecule was pulled normal and parallel to the (0001) surface of hydroxyapatite. Water molecules were found have an important impact on deformation behavior of collagen in the proximity of HAP due to their large interaction energy with both collagen and HAP. Collagen appears stiffer at small displacement when pulled normal to HAP surface. At large displacement, collagen pulled parallel to HAP surface is stiffer. This difference in mechanical response of collagen pulled in parallel and perpendicular direction results from a difference in deformation mechanism of collagen. Further, the collagen molecule pulled in the proximity of HAP, parallel to surface, showed marked improvement in stiffness compared to absence of HAP. Furthermore, the deformation behavior of collagen not only depends on the presence or absence of HAP and direction of pulling, but also on the type of mineral surface in the proximity. The collagen pulled parallel to (10-10) and (0001) surfaces showed characteristically different type of load-displacement response. In addition, here we also report simulations on 300 nm length of collagen molecule indicating the role of length of model on the observed response in terms of both the magnitude of modulus obtained as well as the mechanisms of response of collagen to loading.