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

与合成材料相比，生物材料具有更好的韧性，这被认为取决于底层的纳米级结构。丝素就是这样一种基于蛋白质的纳米结构生物材料。此前，在DFG SachbeiHilfe的第一个资助期，我们已经解决了贝塔-折叠蛋白质晶体和周围丝素中无序的蛋白质链之间的相互作用。为了研究这些体系，我们使用了纳米尺度的分子动力学(MD)模拟来获得参数，这些参数被输入到用有限元分析(FEA)求解的更大的材料尺度模型中。在以前的研究中，所建立的纤维模型在力学特性方面与实验数据显示出很好的一致性。更重要的是，我们观察到了一个意想不到的现象，即拉伸导致沿纤维轴的晶区有序性增加，这一点现在--令人兴奋--得到了我们合作者的小角中子散射实验的证实。在下一个资助期，我们建议沿着这些线提高我们对丝绸力学的理解。我们将系统地评估丝绸机械性能的决定因素，包括强度、韧性、摩擦和自序性。为此，我们将采用并进一步改进我们在第一个资助期开发的多尺度自下而上模拟方法。重要的是，作为这项提议的主要新奇之处，我们将不再假定MD和FEA的特定结构前置，而是将后退一步，彻底解决在剪切流下发生的结构形成问题，同样遵循新的多尺度自下而上的方法。我们期待这项工作计划代表着在理解和重新设计纳米生物材料的组装、结构和力学之间的复杂耦合方面的飞跃。

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.