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Molecular mechanics of mineralized collagen fibrils in bone.

Nair AK, Gautieri A, Chang SW, Buehler MJ - Nat Commun (2013)

Bottom Line: Here we perform full-atomistic calculations of the three-dimensional molecular structure of a mineralized collagen protein matrix to try to better understand its mechanical characteristics under tensile loading at various mineral densities.We find that as the mineral density increases, the tensile modulus of the network increases monotonically and well beyond that of pure collagen fibrils.These findings reveal the mechanism by which bone is able to achieve superior energy dissipation and fracture resistance characteristics beyond its individual constituents.

View Article: PubMed Central - PubMed

Affiliation: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.

ABSTRACT
Bone is a natural composite of collagen protein and the mineral hydroxyapatite. The structure of bone is known to be important to its load-bearing characteristics, but relatively little is known about this structure or the mechanism that govern deformation at the molecular scale. Here we perform full-atomistic calculations of the three-dimensional molecular structure of a mineralized collagen protein matrix to try to better understand its mechanical characteristics under tensile loading at various mineral densities. We find that as the mineral density increases, the tensile modulus of the network increases monotonically and well beyond that of pure collagen fibrils. Our results suggest that the mineral crystals within this network bears up to four times the stress of the collagen fibrils, whereas the collagen is predominantly responsible for the material's deformation response. These findings reveal the mechanism by which bone is able to achieve superior energy dissipation and fracture resistance characteristics beyond its individual constituents.

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Mechanical properties of collagen fibrils at different mineralization stages.(a) Fibril unit cell with mineral content used to perform tensile test by measuring stress versus strain. (b) Stress–strain plots for non-mineralized collagen fibril (0%), 20% mineral density and 40% mineral-density cases. (c) Modulus versus strain for 0, 20 and 40% mineral density showing an increase in modulus as the mineral content increases. The error bars in b are computed from the maximum and minimum values of the periodic box length along the x direction at equilibrium.
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f3: Mechanical properties of collagen fibrils at different mineralization stages.(a) Fibril unit cell with mineral content used to perform tensile test by measuring stress versus strain. (b) Stress–strain plots for non-mineralized collagen fibril (0%), 20% mineral density and 40% mineral-density cases. (c) Modulus versus strain for 0, 20 and 40% mineral density showing an increase in modulus as the mineral content increases. The error bars in b are computed from the maximum and minimum values of the periodic box length along the x direction at equilibrium.

Mentions: We carry out tensile tests on non-mineralized (0%), 20 and 40% mineral density samples (Fig. 3a). As observed from Fig. 3b, as the mineral content increases, the stress–strain behaviour of the mineralized collagen microfibril also changes compared with pure collagen fibrils, with the mineralized cases showing an increasingly higher modulus as higher mineral densities are reached. To quantify the variation of the modulus for different strain levels, we plot the modulus as a function of strain as shown in Fig. 3c. The 0% case has an initial modulus of 0.5 GPa at a load less than 20 MPa, and increases to 1.1 GPa as the stress increases to 100 MPa. For larger deformation, the modulus approaches 2 GPa. These moduli are well within the range of values reported for collagen fibrils under tensile loading using both experiment and simulation2930. For the 20% mineral-density case, the initial modulus is 1.3 GPa (stress less than 20 MPa) and increases to 2.7 GPa at 100 MPa. This shows that even a relatively small mineral content severely alters the stress–strain behaviour of collagen microfibril model and increases its modulus by ~150%. Similarly, the 40% mineral-density case has an initial modulus of 1.5 GPa, approaching 2.8 GPa. For stresses at 100 MPa, the 20 and 40% mineral-density cases have very similar tangent moduli. However, as shown in Fig. 3c, as the strain increases beyond 10%, the modulus for the 40% mineral-density case also increases, indicating that at higher strains the mineral content provides additional stiffness to the collagen–HAP composite. The moduli identified here for the cases with 20 and 40% mineral content is consistent with a recent experimental study31, which showed that mineralized collagen fibrils from antler had a modulus of 2.38±0.37 GPa for strain less than 4%.


Molecular mechanics of mineralized collagen fibrils in bone.

Nair AK, Gautieri A, Chang SW, Buehler MJ - Nat Commun (2013)

Mechanical properties of collagen fibrils at different mineralization stages.(a) Fibril unit cell with mineral content used to perform tensile test by measuring stress versus strain. (b) Stress–strain plots for non-mineralized collagen fibril (0%), 20% mineral density and 40% mineral-density cases. (c) Modulus versus strain for 0, 20 and 40% mineral density showing an increase in modulus as the mineral content increases. The error bars in b are computed from the maximum and minimum values of the periodic box length along the x direction at equilibrium.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3644085&req=5

f3: Mechanical properties of collagen fibrils at different mineralization stages.(a) Fibril unit cell with mineral content used to perform tensile test by measuring stress versus strain. (b) Stress–strain plots for non-mineralized collagen fibril (0%), 20% mineral density and 40% mineral-density cases. (c) Modulus versus strain for 0, 20 and 40% mineral density showing an increase in modulus as the mineral content increases. The error bars in b are computed from the maximum and minimum values of the periodic box length along the x direction at equilibrium.
Mentions: We carry out tensile tests on non-mineralized (0%), 20 and 40% mineral density samples (Fig. 3a). As observed from Fig. 3b, as the mineral content increases, the stress–strain behaviour of the mineralized collagen microfibril also changes compared with pure collagen fibrils, with the mineralized cases showing an increasingly higher modulus as higher mineral densities are reached. To quantify the variation of the modulus for different strain levels, we plot the modulus as a function of strain as shown in Fig. 3c. The 0% case has an initial modulus of 0.5 GPa at a load less than 20 MPa, and increases to 1.1 GPa as the stress increases to 100 MPa. For larger deformation, the modulus approaches 2 GPa. These moduli are well within the range of values reported for collagen fibrils under tensile loading using both experiment and simulation2930. For the 20% mineral-density case, the initial modulus is 1.3 GPa (stress less than 20 MPa) and increases to 2.7 GPa at 100 MPa. This shows that even a relatively small mineral content severely alters the stress–strain behaviour of collagen microfibril model and increases its modulus by ~150%. Similarly, the 40% mineral-density case has an initial modulus of 1.5 GPa, approaching 2.8 GPa. For stresses at 100 MPa, the 20 and 40% mineral-density cases have very similar tangent moduli. However, as shown in Fig. 3c, as the strain increases beyond 10%, the modulus for the 40% mineral-density case also increases, indicating that at higher strains the mineral content provides additional stiffness to the collagen–HAP composite. The moduli identified here for the cases with 20 and 40% mineral content is consistent with a recent experimental study31, which showed that mineralized collagen fibrils from antler had a modulus of 2.38±0.37 GPa for strain less than 4%.

Bottom Line: Here we perform full-atomistic calculations of the three-dimensional molecular structure of a mineralized collagen protein matrix to try to better understand its mechanical characteristics under tensile loading at various mineral densities.We find that as the mineral density increases, the tensile modulus of the network increases monotonically and well beyond that of pure collagen fibrils.These findings reveal the mechanism by which bone is able to achieve superior energy dissipation and fracture resistance characteristics beyond its individual constituents.

View Article: PubMed Central - PubMed

Affiliation: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.

ABSTRACT
Bone is a natural composite of collagen protein and the mineral hydroxyapatite. The structure of bone is known to be important to its load-bearing characteristics, but relatively little is known about this structure or the mechanism that govern deformation at the molecular scale. Here we perform full-atomistic calculations of the three-dimensional molecular structure of a mineralized collagen protein matrix to try to better understand its mechanical characteristics under tensile loading at various mineral densities. We find that as the mineral density increases, the tensile modulus of the network increases monotonically and well beyond that of pure collagen fibrils. Our results suggest that the mineral crystals within this network bears up to four times the stress of the collagen fibrils, whereas the collagen is predominantly responsible for the material's deformation response. These findings reveal the mechanism by which bone is able to achieve superior energy dissipation and fracture resistance characteristics beyond its individual constituents.

Show MeSH
Related in: MedlinePlus