From assembly to mechanics: predictive scale bridging simulations of spider silk

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Biomaterials can feature superior toughness compared to synthetic materials, which is thought to hinge upon the underlying nano-scale structure. Silk is one such protein-based nano-structured biomaterials. We have previously, during the first funding period of this DFG Sachbeihilfe, addressed the interplay between the beta-sheet protein crystals and the surrounding disordered protein chains in silk. To study these systems, we have employed Molecular Dynamics (MD) simulations at the nano-scale to derive parameters, which were fed into larger material-scale models solved with finite element analysis (FEA). In these previous studies, the established fiber model showed very good agreement with experimental data with regard to its mechanical characteristics. More importantly, we observed an unexpected phenomenon, namely a stretch-induced increase in order of crystalline regions along the fiber axis, which now - excitingly - was confirmed by small angle neutron scattering experiments of our collaborators.In the next funding period, we propose to improve our understanding of silk mechanics along these lines. We will systematically assess the determinants of the mechanical properties of silk, including strength, toughness, friction, and self-ordering. To this end, we will employ and further refine our multi-scale bottom-up simulation approach as developed in our first funding period. Importantly, as the major novelty of this proposal, we will not anymore assume a certain structural preposition for the MD and FEA, but instead will go one step back and thoroughly address the question of structure formation as it occurs under shear flow, again following a new multiscale bottom-up approach. We expect the work program to represent a leap forward in understanding and redesigning the intricate coupling between assembly, structure, and mechanics of nanoscale biomaterials.