Bioinspired Stiffness-Controlled Protein Filaments Based on Understanding Hydrodynamics and Molecular Self-Assembly of Constitutive Proteins

Natural materials like collagen, microtubules, and muscle fibers feature ultralong, tough, and robust mechanical properties. This is induced by complex hierarchical structures formed by the self-assembly of stiffness-controlled filaments and fibrils. As a result, understanding dynamic mechanical properties and the self-assembly of filaments and fibrils has become the focus of studies aimed to better understand the hierarchical structures of natural fibers. Moreover, synthetic polymer-based filaments and fibrils with controlled stiffness have been developed for drug delivery and tissue engineering applications. Recently, protein-based filaments have received attention as synthetic counterparts for healthcare applications due to their biocompatibility and biodegradability. In addition, proteins have highly controllable monomers and diverse functionalities, and they can be genetically fused to yield versatile protein polymers that form exquisitely designed bioinspired filaments. It is reported that crosslinking self-assembled protein nanostructures allows for the preparation of the protein filaments, and modulating crosslinking density enables the control of the filament stiffness. However, methods to tailor the stiffness of protein filaments remains unclear due to the delicate processing required, including the complexity of the self-assembly processes involved with sophisticated molecular interactions.<br/>Herein, we report bioinspired stiffness-controlled protein filaments via the molecular self-assembly of designed protein polymers with constitutive coil-like and rod-like proteins. Using molecular dynamics simulations, biosynthesis, and characterization methods, we found that the hydrodynamics of the coil-like protein component showed high flexibility such that curvature was generated in the filament structures, while the rod-like protein component enhanced the stiffness. This finding leads to the investigation of the fundamental relationship between chain topology for designed protein polymers and their composition, structure, length, symmetry, as well as their interdependence. Our work provides insights into the scientific principles of protein filaments toward constructing biomimicking hierarchically structured protein fibers, which can open new avenues for healthcare applications in drug delivery, tissue engineering, and regenerative medicines.