Mechanical Testing of Engineered Spider Silk Filaments Provides Insights into Molecular Features on a Mesoscale

Spider dragline silk shows the highest toughness in comparison to all other known natural or man-made fibers. Despite a broad experimental foundation concerning the macroscopic silk thread properties as well as a thorough simulation-based molecular understanding, the impact of the mesoscale building blocks, namely nano-/submicrometer-sized filaments, on the mechanical properties of the threads remains the missing link. Here, we illustrate the function of these mesoscaled building blocks using electrospun fibers made of a recombinant spider silk protein and show the impact of β-sheet content and fiber hydration on their mechanical performance. Specifically elucidating the interplay between β-sheet-cross-linking (fiber strength) and structural water (fiber extensibility), the results bridge the gap between the molecular and the macroscopic view on the mechanics of spider silk. It is demonstrated that the extensibility of the here used single (MaSp2-like) protein system is in good accordance with the simulated extensibilities published by other groups. Furthermore, sufficient hydration of the fibers is shown to be a prerequisite to obtain a toughness in the range of that of natural dragline silk. Preliminary studies on electrospun fibers of the MaSp2-based recombinant spider silk proteins used in this work have indicated their basic applicability in the technical field of filter systems as well as in regenerative medicine. The presented work provides a fundamental understanding of the mechanical performance of such fibers under different wetting conditions, a prerequisite to further specify their potential for such applications.