The Structural and Physical Components Affecting the Mechanics of Composite Mussel Byssus Fibers

Materials found in nature exhibit remarkable properties allowing natural living systems to survive. An outstanding example of this is the byssus of marine mussels. Mussels utilize byssal threads in the byssus to anchor themselves onto a variety of surfaces and endure the harsh intertidal environment. Byssal threads display a composite microstructure as well as intricate macro-scale architectures. This dissertation presents four studies that address questions regarding byssal thread geometry, physical parameters affecting adhesion, and the relationship between the thread microstructure and mechanical properties. Mussels utilize a mushroom-shaped geometry for their byssal threads: the threads consist of a distal thread (stalk) terminating in a plaque (mushroom-tip). Previous studies on adhesion associated with the mushroom-shaped geometry have focused on the effects of geometrical parameters such as tip thickness and the ratio of the stalk to tip radius. Mussels deposit byssal threads radially, which are loaded at various angles during wave motion. This introduces a more complex geometry than previously studied in regard to adhesion and detachment. Due to these differences, we focused on the effects of casting angle and loading angle on adhesive strength utilizing synthetic mimics. We find that the optimal configuration for adhesive strength is when the loading angle and casting angle are equivalent. Evidence suggests that suction may play a role in the adhesive strength of mushroom shaped structures. Using byssal threads as inspiration, we utilized synthetic mimics to study the effects of suction at the macroscopic scale. To determine the critical stress necessary for defect propagation and detachment a fracture mechanics-based model is introduced, and compared with experimental results. The findings indicate that there is a greater increase in adhesive strength due to suction at the macro-scale, which is length-scale dependent. Lastly, we assess the relationship between the thread microstructure and mechanical properties. Different protein domains in the collagenous core were targeted with chemical treatments and stress relaxation measurements were conducted to determine which energy dissipative mechanisms are present during the relaxation process. This complements previous studies which largely focused on elastic properties, by concentrating on the viscoelastic properties of the threads. Results show that the silk-like domains are largely responsible for energy dissipation via protein unfolding and/or rearrangement during the relaxation process. Under cyclic loading, distal threads exhibit a stress-strain behavior reminiscent of shapememory and superelastic effects observed in some metal alloys. Previous studies have revealed that distal threads undergo phase transitions in their microstructure as they are loaded. A hyperelastic Neo-Hookean-based model is introduced that incorporates the mechanical properties from two distinct phases in the microstructure to address the contributions from the collagen core. In addition, a Mullins-based model is used to fit the composite cyclic data and provide insight into the mechanical response of the composite thread.