Modulating Polyalanine Motifs of Synthetic Spidroin for Controllable Preassembly and Strong Fiber Formation.

Spider dragline (major ampullate) silk is one of the toughest known fibers in nature and exhibits an excellent combination of high tensile strength and elasticity. Increasing evidence has indicated that preassembly plays a crucial role in facilitating the proper assembly of silk fibers by bridging the mesoscale gap between spidroin molecules and the final strong fibers. However, it remains challenging to control the preassembly of spidroins and investigate its influence on fiber structural and mechanical properties. In this study, we explored to bridge this gap by modulating the polyalanine (polyA) motifs in repetitive region of spidroins to tune their preassemblies in aqueous dope solutions. Three biomimetic silk proteins with varying numbers of alanine residues in polyA motif and comparable molecular weights were designed and biosynthesized, termed as N16C-5A, N15C-8A, and N13C-12A, respectively. It was found that all three proteins could form nanofibril assemblies in the concentrated aqueous dopes, but the size and structural stability of the fibrils were distinct from each other. The silk protein N15C-8A with 8 alanine residues in polyA motif allowed for the formation of stable nanofibril assemblies with a length of approximately 200 nm, which were not prone to disassemble or aggregate as that of N16C-5A and N13C-12A. More interestingly, the stable fibril assembly of N15C-8A enabled spinning of simultaneously strong (623.3 MPa) and tough (107.1 MJ m-3) synthetic fibers with fine molecular orientation and close interface packing of fibril bundles. This work highlights that modulation of polyA motifs is a feasible way to tune the morphology and stability of the spidroin preassemblies in dope solutions, thus controlling the structural and mechanical properties of the resulting fibers.

Functionalization and Reinforcement of Recombinant Spider Dragline Silk Fibers by Confined Nanoparticle Formation.

Spider dragline silk is a remarkable protein fiber that is mechanically superior to almost any other natural or synthetic material. As a sustainable supply of natural dragline silk is not feasible, recombinant production of silk fibers with native-like mechanical properties and non-native physiochemical functions is highly desirable for various applications. Here, we report a new strategy for simultaneous functionalization and reinforcement of recombinant spider silk fibers by confined nanoparticle formation. First, a mimic silk protein (N16C) of spider Trichonephila clavipes was recombinantly produced and wet-spun into fibers. Drawing the as-spun fibers in water led to post-drawn fibers more suitable for the templated synthesis of nanoparticles (NPs) with uniform distribution throughout the synthetic fibers. This was exemplified using a chemical precipitation reaction to generate copper sulfide nanoparticle-incorporated fibers. These fibers and the derived fabric displayed a significant photothermal effect as their temperatures could increase to over 40 °C from room temperature within 3 min under near-infrared laser irradiation or simulated sunlight. In addition, the tensile strength and toughness of the nanofunctionalized fibers were greatly enhanced, and the toughness of these synthetic fibers could reach 160.1 ± 21.4 MJ m-3, which even exceeds that of natural spider dragline silk (111.19 ± 30.54 MJ m-3). Furthermore, the confined synthesis of gold NPs via a redox reaction was shown to improve the ultraviolet-protective effect and tensile mechanical properties of synthetic silk fibers. These results suggest that our strategy may have great potential for creating functional and high-performance spider silk fibers and fabrics for wide applications.