Exploring silk protein assembly mechanisms for high-performance materials

Silk proteins present remarkable potential as the building blocks for high-performance materials. Due to their unique protein structure, they exhibit a variety of properties, such as a combination very high tensile strength and elasticity, biodegradability, biocompatibility, and ability to be spun in natural conditions — qualities uncommon in synthetic fibres. While challenging to acquire through natural means, the application of gene engineering allows to produce substantial quantities of recombinant silk proteins. However, the assembly pathways leading to the formation of functional materials remain largely unknown, with the evidence suggesting that crucial insights lie within the intricate spinning mechanism of silk-producing animals. Publication 1 delves into an investigation of the native spinning system of Bombyx mori silkworm. Amino acid mapping throughout the middle silk gland revealed a gradual change in various amino acids, attributed to the increase in different sericin proteins. This change correlated with the extensional behaviour of the silk dope, which was further enhanced by a straightforward pH modification inspired by the native spinning system. Publication 2 introduces a more sophisticated silk pulling device designed for controlled pulling and tensile testing, with a focus in automation. The device was employed to pull fibers from regenerated silk solution. Although immune to the pH induced dimerization due to irreversible changes by the regeneration, the fibroin fibers displayed a considerable median tensile strength of 147 MPa. In addition, a recombinant silk protein was observed to preassemble at an air-water interface, allowing pulling of thin fibers from droplets of silk. Liquid-liquid phase separation was found to be a significant factor in creating stronger fibers, demonstrating the necessity of controlling this intricate aspect of molecular assembly. Not limited to fibers, silk proteins find application as building blocks for films. Publication 3 details the development of a simple deposition method to create highly hydrophobic silk films. These films, characterized by the arrangement of intrinsically hydrophobic regions of the silk protein, formed in the presence of a specific salt concentration and high relative humidity during drying. Notably, the phenomenon was not strictly dependent on the silk protein sequence or the type of salt, although certain variations displayed superior performance. The films were altered by wetting, disrupting the interaction between the silk and salt. Although the lack of durability limits applicability of the silk films, this work demonstrates the versatile nature of the amphiphilic silk proteins.