Team Members

Priv. Doz. Dr. Markus Hartmann

Univ. Prof. Dr. Christoph Dellago

Dr. Stéphane Blouin

Dr. Eleftherios Paschalis

Varsha Margrette, MSc

Starting Date

1st July 2022

End Date

30th June 2026

Funding Agency

The Austrian Science Fund FWF (P 35715-N)

Project Description

In an aging society fracture of bone that leads to a loss of mobility and quality of life is one of the biggest challenges. Diabetes mellitus, a metabolic disease, characterized by the disability of the organism to control the sugar levels in the blood, has also detrimental effects on bone. The organic component of bone, the triple helical protein collagen provides the template upon which bone mineralization occurs and is also responsible for certain mechanical attributes of bone. These mechanical attributes are carefully tuned by enzymatically cross-linking different collagen molecules. These cross-links are placed at well-defined locations in the molecule. In contrast to enzymatic cross-links, non-enzymatic cross-links are detrimental to the mechanical performance of bone. These can occur anywhere in the molecule and accumulate with age. As non-enzymatic cross-links are formed via glycation, increased glucose levels in the blood stream, as found in diabetes, accelerate this natural aging process. A large amount of non-enzymatic cross-links leads to a brittle material that is prone to fracture.

In this project, we will investigate the change of collagen properties with aging and disease induced by a change in the cross-linking pattern. We will use computational as well as experimental methods. Spectroscopic methods allow to assess the number and type of cross-links. Scanning acoustic microscopy gives the possibility to measure the local mechanical properties of collagen. In parallel to the measurements, the molecular structure of different types of cross-links will be modeled in the computer. Their mechanical properties will be calculated by deforming these structures in silico. The knowledge of the mechanical behavior of single cross-links can then be used to deform larger structures, like a collagen fibril, consisting of many different cross-links. The number and type of cross-links known from experiment are an important input for the simulations. On the other hand, the simulations allow for the testing of different spatial configurations of cross-links; an information that is not accessible in the experiments. It is this combination of experimental and theoretical methods that make this project unique and that will allow for new findings that cannot be obtained otherwise.

Team Members

Priv. Doz. Dr. Markus Hartmann

Dr. Huzaifa Shabbir

Starting Date

1st September 2015

End Date

31st August 2020

Funding Agency

The Austrian Science Fund FWF (AP 27882-N27)

Publications

1. Huzaifa Shabbir and Markus A. Hartmann
   A high coordination of cross-links in fiber bundles prevents local strain concentrations
   Computational Materials Science 184, 109849 (2020)

2. Huzaifa Shabbir, Christoph Dellago and Markus A. Hartmann
   A high coordination of cross-links is beneficial for the strength of cross-linked fibers
   Biomimetics 4, 12 (2019)

3. Huzaifa Shabbir and Markus A. Hartmann
   Influence of reversible cross-link coordination on the mechanical behavior of a linear polymer chain
   New Journal of Physics 19, 093024 (2017)

Project Description

Polymeric structures are ubiquitous in natural as well as technological systems. The mechanical properties of these systems are strongly dependent on the degree and kind of crosslinking of the different polymeric chains. The vast majority of theoretical concepts investigating the effect of crosslinking deal with two-fold coordinated crosslinks, i.e. crosslinks formed between two monomers only. Nevertheless, a variety of materials is known that crosslink via so called tris-complexes, i.e. one crosslink is formed between three monomers. One example for this kind of material is the iron-dopa complex that – depending on pH – can exist in the mono-, bis- or tris-state. It is the aim of this project to close the aforementioned gap in the theoretical description and to investigate the effect of three-fold coordinated crosslinks on the mechanical properties of polymeric structures. It is expected that this effect is highly non-trivial. First, the force flow through a three-fold coordinated structure is completely different than through a bis-complex. Second, rupture of a tris-complex is always a two stage process. When the first monomer detaches, the tris-complex is transformed into a bis-complex. Subsequently, this bis-complex may either completely dissociate through a rupture of the remaining bond, or the tris-complex may be restored through the attachment of a different monomer. In the framework of this project a simple generic potential shall be used to describe reversible crosslinks of different coordination. The static and dynamic mechanical properties of the systems will be studied using Monte Carlo and molecular dynamics techniques. Two different geometries will be investigated: aligned fiber bundles and random fiber networks. Special emphasis in the analysis will be put on understanding the relation between backbone bending elasticity, sticky site density and polymer density on the one hand and the mechanical behavior of the system on the other hand. Computational loading tests will be performed to assess key mechanical parameters like stiffness, toughness or strength of the systems. The energy dissipation of these structures will be estimated by doing cyclic loading tests. It is the ultimate goal of this project to reveal the influence of coordination of crosslinks on the mechanical properties of crosslinked polymeric systems. This will lead to a better understanding of the functionality of biological structures as well as to the possibility to specifically tailor the mechanical properties of artificial systems.

Team Members

Priv. Doz. Dr. Markus A. Hartmann

Dr. S. Soran Nabavi

Starting Date

1st September 2011

End Date

30th June 2015

Funding Agency

The Austrian Science Fund FWF (P 22983-N20)

Publications

1. S. Soran Nabavi and Markus A. Hartmann
   Weak reversible cross-links may decrease the strength of aligned fiber bundles
   Soft Matter 12, 2047 (2016)

2. S. Soran Nabavi, Peter Fratzl and Markus A. Hartmann
   Energy dissipation and recovery in a simple model with reversible cross links
   Physical Review E 91, 032603 (2015)

3. S. Soran Nabavi, Matthew J. Harrington, Peter Fratzl and Markus A. Hartmann
   Influence of sacrificial bonds on the mechanical behavior of polymer chains
   Bioinspired, Biomimetic and Nanobiomaterials 3, 139 (2014)

4. S. Soran Nabavi, Matthew J. Harrington, Oskar Paris, Peter Fratzl and Markus A. Hartmann
   The role of topology and thermal backbone fluctuations on sacrificial bond efficacy in mechanical metalloproteins
   New Journal of Physics 16, 013003 (2014)

Project Description

Biological materials, such as bone or wood, show extraordinary mechanical properties. These materials combine a high stiffness with an elevated toughness. Crucial for the performance of biological materials is their hierarchical structuring over several length scales, as well as the coupling of stiff entities and the soft matrix on the nanoscale. Recent experiments give evidence that this coupling is realized by non-specific, reversible coulombic cross links. These so called sacrificial bonds provide the material with hidden length scales, considerably increasing the toughness of the composite. Sacrificial bonds have been observed in a variety of biological systems ranging from bone to sea shells and mussel fibers. Thus, coulombic cross linking seems to be a general strategy in nature to couple different phases and to provide the resulting composite material with superior mechanical properties. Within the framework of this project we want to build and test simple, physical model systems to microscopically describe the organic-inorganic interface in biological tissue and to understand its basic interactions. It is the spirit of the proposed project to start from simple models, simplifying the complicated situation as found in biological materials, which can be described as a multi-component system mainly consisting of proteins, ions, mineral and water. The developed models shall be refined step by step to account for the more complicated interactions. The models shall be investigated using computer simulation techniques, such as Monte Carlo or Molecular Dynamics. It is the goal of this project to better understand the complicated interactions at the organic-inorganic interface in biological materials. This shall: first, reveal the microscopic origin of diseases that are caused by pathological changes in the interactions of this interface, second, give the possibility to construct materials that are inspired by natures design principles and, third, give an input to larger scale, continuum models such as Finite Element calculations, that base on the description of the interface between the different constituents.