The role of charged interfaces in reversible and irreversible deformation of biological tissue

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.

Compared to man-made materials biological materials often show superior mechanical properties. These materials often are light, show a high stiffness combined with an elevated toughness, they possess self-healing mechanisms and they naturally are "green". Thus, biological materials are a natural source of inspiration for the materials scientist in designing new materials with desired and tailored (mechanical) properties. To ensure a successful transfer of biological principles into technological applications, a thorough understanding - also from a fundamental point of view - of the first is a necessary prerequisite. This understanding is the starting point of the FWF funded project "The role of charged interfaces in reversible and irreversible deformation of biological tissue". In this project one specific strategy of nature to build stiff and tough materials was investigated in detail, namely the strategy of using so called sacrificial bonds (SBs). In polymeric structures SBs are additional cross-links that provide bonds weaker than the covalent backbone of the structure. Thus, upon loading it is first the cross-link that opens "sacrificially" allowing the part of the polymer that was shielded by the SB from the external load to stretch, i.e. hidden length is revealed. This mechanism provides a very efficient energy dissipation mechanism. Furthermore, often these bonds are also reversible. Thus, whenever the load is released after some time the SBs may reform and the original mechanical properties of the structure may be re-gained. This kind of self-healing mechanism involves no living cells and is completely passive. SBs are found in a variety of biological materials, like bone, wood, silk or the mussel byssus.In the FWF project a simple, computational model was built to understand the influence of reversible cross-links on the mechanical properties of polymeric materials. One or many polymeric chains grafted between two plates were investigated. Some of the monomers were defined as sticky, i.e. they could form an additional cross-link. The results showed that entropy and temperature play an important role in the stability of these SBs. This finding is surprising because the SBs investigated in the project were motivated by metal coordination bonds, found e.g. in the mussel byssus, that have a strength close to covalent bonds, i.e. their binding energy is much larger than the thermal energy. Furthermore, it could be shown that the topology of the SBs has a large influence on the mechanical behavior of the structure. In cyclic loading experiments the energy dissipation of the system was investigated and similar load-displacement curves as found in experiments could be reproduced. For systems consisting of more than one chain the most surprising result obtained is that the presence of weak reversible cross-links may decrease the strength of the system.