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P.J. Thurner, O.G. Andriotis, S. Desissaire, M.G. Jones, D.E. Davies:
"Experimental micro- and nanomechanics of collagen rich-tissues and individual collagen fibrils";
Vortrag: 90th Annual Meeting of the International Association of Applied Mathematics and Mechanics (GAMM 2019), Vienna; 18.02.2019 - 22.02.2019.

All tissues providing passive mechanical function are rich in collagens, which are thought to provide stiffness and toughness, whereas resilience is thought to be provided by elastin. Macroscopically, relationships between tissue-structure, composition and mechanical function have been explored but comparatively little has been done in this context on the micro- and nanometer length scale. Yet, perhaps the most important basic structural building block of collagen-rich tissues can be found between these scales: the collagen fibril. Fibrils are rope-like with diameters in the range of tens to hundreds of nanometers and they have extremely high aspect ratios with lengths reaching up to tens of millimeters. The capacity to measure mechanical properties at these levels is increasing continuously due to the development of approaches based on atomic force microscopy or MEMS devices. So far, investigations have shown interesting insights but a full and systematic view detailing structure-mechanical-function relationships for individual collagen fibrils and collagen-rich tissue micromechanics is still missing.
Insights into tissue mechanics at these levels are not only interesting from a basic science perspective, but are also important for understanding changes due to age and disease. That is, they may offer targets for diagnostics and treatment of pathologies. Furthermore, micro- and nanomechanics can be useful to elucidate mechanobiological effects as cells sense their environment at this level.
To elucidate this, examples will be presented demonstrating that tissue micromechanics and collagen fibril mechanics are dependent on the molecular structure of the tropocollagen molecule, chemical composition including chemical modifications (cross- links) as well as chemical environment, i.e. hydration. A specific focus is osmotic pressure and cross-linking. The effect of osmotic pressure has so far not been considered much on the level of individual collagen fibrils, but it exhibits significant influence on collagen mechanics. By employing poly-ethylene-glycol (PEG) as an osmotic pressure generating agent we found up to a six-fold increase in tensile stiffness of individual collagen fibrils. This elucidates the importance of hydration and noncovalent interactions for tissue mechanics. Classically, cross-linking of collagen is thought to be the major mechanism for tuning elasticity. We show that this effect is also present on the microscale. However, whether such cross-links have an effect on the tensile mechanics of individual collagen fibrils remains to be shown. It may well be that chemical cross-linking is more active in the extrafibrillar space, linking fibrils together, rather than tuning mechanics of individual fibrils. Further research is required to clarify this.