Investigations conducted within the framework of a program of cooperation between the Institut für Angewandte und Experimentelle Mechanik (Institute of Applied and Experimental Mechanics) at the university of Stuttgart and the Massachusetts Institute of Technology (MIT) hold the promise of great progress in the development of new multi-functional materials and in our understanding of illnesses such as Alzheimer's disease.
For the first time, scientists have succeeded in using atomistic calculations to explain fundamental fracturing mechanics of biological materials. Building on this, they have developed a theory which makes it possible to predict the strength and robustness of biological nano-structures. The renowned American journal, "Proceedings of the National Academy of Sciences", will report on this work as the front-page topic of issue 42 of the journal, which is to appear on October 16.*)
Why is spider silk stronger than steel? What makes bones so firm and yet flexible? How is it possible that cells can be reversibly stretched to many times their original length? And what molecular mechanisms cause proteins to malfunction mechanically, a circumstance which plays a central role in illnesses such as Alzheimer's, premature ageing and degenerative muscular diseases? All these phenomena are caused by intelligent, multi-functional biological nano-structures.
The goal of the scientists is to decode the properties of these structures in order to create new materials that can benefit human beings or to find new ways of curing genetically inherited illnesses. Headed by project manager, Prof. Markus Buehler from MIT and including Theodor Ackbarow, an exchange student from Stuttgart university currently working at MIT, the group is pursuing an approach which involves investigating mechanical behaviour on an atomic level by means of simulations on high-performance computers. This enables them to draw conclusions on the macroscopic level.
A breakthrough has now been achieved in this area of research. Deformation mechanisms of protein materials in the cyto-skeleton of the cell and in amyloid fibrils such as occur in the case of Alzheimer's disease have been explained on an atomic level for the very first time. "What is special about biological protein materials is that they are usually composed of very 'soft' hydrogen bonds", explained Ackbarow. Nevertheless, biological materials can be as strong as glass or steel. The research results show that the existence of hierarchical material structures from nano to macro is the key to achieving this unusual property.
The hierarchical structures make it possible to combine apparently contradictory material characteristics such as strength and robustness or self-healing and self-adaptation and, at the same time, strengthen the weak chemical bonds. As a result, robust materials which continuously adapt themselves to their environment can be created in spite of their having weak chemical bonds. "We have been able to verify how hierarchies can be used as an additional design variable in biological materials in order to resolve the conflict between robustness and strength such as occurs in synthetic materials", said Prof. Buehler. "This opens up new methods of material synthesis and will also help us to understand a large variety of illnesses."
Prediction of the mechanical properties of protein structures
The researchers observed that, due to the hierarchical structure, different deformation and fracture mechanisms become active, depending on the actual speed of deformation. If, for example, a cell is actively deformed, mechanisms are activated which ensure that the tissue remains soft and thus enable deformation with minimal application of force. If, in contrast, the tissue is subjected to sudden shock-like stress, other fracture mechanisms are activated which lead to local reinforcement of the material. Proceeding from this knowledge, the researchers were able to develop a strength model which makes it possible to predict the mechanical properties of protein structures solely on the basis of the properties possessed by the chemical compounds and the geometry of the molecules. This is the first step in the development of biological materials which not only adapt themselves to mechanical stress or have an ability to regenerate themselves but can also be manufactured at moderate temperatures.
The results were achieved in the framework of collaboration between the Institut für Angewandte und Experimentelle Mechanik (headed by Prof. Lothar Gaul) of the university of Stuttgart and the MIT Laboratory for Atomistic and Molecular Modeling of Prof. Markus Buehler, who himself studied and obtained his doctorate at Stuttgart university. Dipl.-Ing. Theodor Ackbarow, who studied technology management at Stuttgart university until July 2007, made some important contributions to the planning, performance and evaluation of the virtual experiments as well as to development of the strength theory.
*)Theodor Ackbarow, Xuefeng Chen, Sinan Keten, Markus J. Buehler: "Hierarchies, multiple energy barriers and robustness govern the fracture mechanics of alpha-helical and beta-sheet protein domains", Proc . Nat'l Academy of Sciences USA, Vol. 104 (42), pp. 16410-16415, 2007.
Source: idw
