Study of chemically synthetic spider silk inspired materials

Abstract

Spider silks are renowned for their remarkable mechanical properties. It is the unique hierarchical structure of the silks makes them stand out for high strength and extra-high toughness among all materials. The hierarchical structure is from the self-assembly of macromolecules with various amino acid sequenced proteins secreted by different glands during natural spinning, including β-sheet crystals, α-helix and random coils in the amorphous region in a spider silk. However, due to the difficulty in culturing spiders in large scale like silkwarm, artificial spider silks have always been the dream of scientists and many important industries such as defense and space. In recent decades, with the bloom of transgenetic technology, gene engineering has been widely used in preparation and study of artificial spider silk protein materials. However, since the recombinant/regenerated proteins can hardly form a hierarchical structure as effectively as natural ones, even if the same protein can be synthesized by genetic engineering, the mechanical properties of the resultingt artificial spider silks are far from satisfactory. In addition, due to the high-cost and low-productivity, it is extremely difficult to achieve mass production. On the other hand, the problems of existing chemical synthesis methods are: (1) synthesis process is tedious, costly and inefficient; (2) the molecular weight of the obtained copolypeptides is very low; (3) the mechanical properties of the subsequent polymers are unsatisfactory. Therefore, inspired by the formation of β-sheet crystals in the process of spider silk formation, combining with the innovative molecular design, a novel synthetic strategy was explored and developed in this research project. In this thesis, to mimic the α-helix and β-sheet in natural spidroins, PBLG with certain degree of polymerization was prepared via ROP of BLG NCAs. The thermo-responsive shape memory behavior of PBLG was studied for the first time. Then polyurethane/ureas (PUUs) containing PBLG copolymer with different degree of polymerization (3 and 7) were synthesized through a facile polyurethane synthesis route. The effect of PBLG's DP on the structure and performance was elucidated. In addition, by varying the peptide proportion in PUUs, we clarified the influence of peptides on mechanical properties and found a relatively suitable weight ratio that the PUU film can express the most excellent mechanical properties among all samples. Next, a dry spinning system was developed to fabricate the PUU fibers. During the spinning process, the secondary structure change accompanying the solid fiber formation was studied. Most importantly, one of the fibers exhibited a super toughness that is comparable to the most tough spider silk in the world, the aciniform silk of Argiope trifasciata, which is achieved by introducing β-sheet crystals and α-helical peptides simultaneously in a pseudoprotein polymer. Structure analysis reveals that like native spider silks, α-helix, β-sheet and random coils were all composed in the polypeptides. The work done in this project would directly provide an effective and facile fabrication method for the biomimetic spider silk materials and establish a research model for understanding the composition-structure-property relationship of the spider silk with a hierarchical structure. More importantly, this work could lay a solid foundation for the exploration and preparation of extraordinarily high-performance materials, particularly fibers of protein origins.