To fabricate biocompatible composite films with tunable performance, both silk fibroin (SF, β-crystallite rich) and keratin (α-helix rich) materials are engineered at the mesoscale based on the molecular synergy. While SF materials display a hierarchical structure initiated from the β-crystallite molecular networks, keratin materials reveal the networks crossly linked by disulfide bonding. It is surprising to see that the β-crystallites of SF materials induce the conversion of the free unfolded molecular chains of keratin to β-folding (β-crystallites) in the SF/keratin composites. Furthermore, the α-helices from the keratin components in the SF/keratin composites can transit to β-sheets under stress, which gives rise to the strain-hardening, and a better flexibility and elasticity. It follows that the tensile and biodegradable performance of the SF/keratin composites can be programmed by adjusting the ratio of SF versus keratin in the composites. Raising the SF ratio in the composite films increases the density of β-crystallites in the networks, giving rise to the enhanced toughness and reduced biodegradation rate, but poor deformation recovery. On the other hand, increasing the ratio of keratin in the composite films increases the extensibility, strain-hardening effect. These make them excel bio-optical materials with controllable properties. macroscopic performance. Wool keratins are composed of the intermediate filament proteins and the matrix proteins. Intermediate filament proteins have an average molecular mass of 40-60 kDa while the matrix proteins are of 11-26 kDa. [15] Wool fibers are mainly composed of keratin intermediate filaments, which are organized as coiled coils with a diameter of 7 nm due to their hierarchical structures ( Figure 1B): each dimer (45 nm long) is formed by two individual right-handed α-helix chains to constitute protofibrils with disulfide bonds. [16] The dimers aggregate then into protofilaments (about 2 nm in diameter) end-to-end and side-by-side. These form protofibrils which eventually combine into helical intermediate filaments of wool fibers. [6] For the above structure, the disulfide cross-links are of great importance for stabilizing the multilevels of wool fibers. Consequently, the route of introducing the networks of keratins will promote the formation of disulfide bonds at heating. [17] According to the latest studies, [1] the performance of soft materials are directly correlated with the five structural factors of mesoscopic structures: (1) the topology of networks, (2) the correlation length among building units, (3) the orientation and (4) the interactions of structural units, and (5) the hierarchy of mesoscopic structure. [1] Based on this principle, the function and macroscopic performance of soft materials can be modified by the following two paths: [1,12,18] Path (1) is to modify the mesoscopic structure so as to acquire desired performance. This requires the knowledge on the correlation between the mesoscopic structure of soft materials and the performance, and ...
To design higher-strength natural scaffold materials, wool keratin (WK) rich in α-helix structures is used as a well-defined foreign substrate, which induces the formation of β-crystallites in silk fibroin (SF). Consequently, the macroscopic properties of silk materials (such as the rheological properties of SF hydrogels and the mechanical properties of stents) can be manipulated by governing the change in the hierarchical mesoscopic structure of silk materials. In this work, by monitoring the structure and morphology in the SF gel process, the mechanism of the effect of keratin on SF network formation was speculated, which was further used to design ultra-high-strength protein scaffolds. It has been confirmed that WK accelerates the gelation of SF by reducing the multistep nucleation barrier and increasing the primary nucleation sites, and then establishing a high-density SF domain network. The modulus of the protein composite scaffold prepared by this facile strategy can reach 11.55 MPa, and the MC-3T3 cells can grow well on the scaffold surface. The results suggest that freeze-dried biocompatible SF-based scaffolds are potential candidates for bone tissue engineering.
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