Biomimetic engineering of materials based on hagfish slime thread proteins
Date of Issue2016-06-23
School of Materials Science and Engineering
Biomimetic materials inspired from Nature have shown unique potential to create a new class of materials with a set of properties not found in synthetic materials. Silk, one classical example of biological protein-based materials, exhibits excellent mechanical performances for diverse applications. However, it suffers from drawbacks with regard to bioengineering processing. Another promising –but far less explored– biological fibrous material model is the hagfish slime thread, which in the native state can be as stiff and tough as silks. At the molecular level, hagfish slime threads (from the species Eptatretus stoutii, Es) are built from α-helical coiled-coil proteins (EsTKα and EsTKγ), which self-assemble into hetero-dimeric intermediate filaments (IFs), and these IFs are bundled intra-cellularly with a high degree of axial alignment into macroscopic threads. Upon draw-processing, native hagfish slime threads exhibit an irreversible structural transition from α-helical coiled-coil to aligned β-sheet (α to β transition). This strain-induced phase transformation endows native threads with remarkable mechanical properties that rival silks and approach Kevlar. This thesis describes the methods to create artificial EsTK-based fibers by using self-assembled coiled-coil nano-filaments produced by recombinant (rec) protein expression as building blocks. (rec)EsTKα and EsTKγ are expressed in an E. coli system, and subsequently purified by microfluidization and size exclusion chromatography (SEC). Using a self-assembly process based on step-wise removal of urea, (rec)EsTK-based filamentous networks are exploited. Both circular dichroism (CD) measurement and transmission electron microscopy (TEM) images confirm the helical coiled-coil filaments formation, which provides the basis for the production of larger scale materials fabrication. Notably, (rec)EsTK-based filamentous networks undergo α to β transition under oscillatory shearing, which mimics the native hagfish threads mechanical strengthening process. Using the self-assembled coiled-coil filaments as building blocks, artificial (rec)EsTK-based fibers are then developed. These artificial fibers experience mechanical performance enhancement during draw-processing, which is attributed to high content of aligned β-sheet structure formation, as evidenced by synchrotron wide-angle X-ray scattering (WAXS) and polarized Raman spectroscopy. The fibers are believed to exhibit a multi-step conformational transition, from coiled-coils to cross β-sheets during fiber picking-up formation, followed by cross β-sheets to elongated parallel β-sheets during draw-processing. Moreover, chemical cross-linking between lysine residues further enhances the stiffness and ultimate tensile strength (UTS), which approaches that of silks. Microfluidic devices are also utilized to create (rec)EsTK-based fibers. Using pressure as the driving force, the majority of self-assembled nano-filaments could be stretched, elongated, and aligned parallel to the direction of fluid flow. Although the resulting fibers are shorter and more brittle than manually picking up from the solution, microfluidics provide an elegant method for the alignment and bundling of (rec)EsTK-based filaments and will be further explored. Generally speaking, this thesis provides the foundation to produce protein-based artificial fibers by a ‘bottom-up’ approach from recombinant protein production, using self-assembled coiled-coil filaments as the fundamental building blocks. The tough and tunable mechanical responses of these protein-based fibrous materials could lead to a variety of engineering and biomedical applications and could notably rival artificial silk.