The artificial silk - dubbed polymeric amyloid fibre - was produced by bacteria that were genetically engineered in the lab of Fuzhong Zhang, a professor in the Department of Energy, Environmental & Chemical Engineering in the McKelvey School of Engineering at Washington University in St. Louis. The research has been published in ACS Nano.
In 2018 Zhang’s lab engineered bacteria that produced a recombinant spider silk with performance comparable with its natural counterparts.
"After our previous work, I wondered if we could create something better than spider silk using our synthetic biology platform," Zhang said in a statement.
The research team, which includes first author Jingyao Li, a PhD student in Zhang's lab, modified the amino acid sequence of spider silk proteins to introduce new properties, while keeping some of the attractive features of arachnid silk.
A problem associated with recombinant spider silk fibre -- without significant modification from natural spider silk sequence -- is the need to create β-nanocrystals, a main component of natural spider silk, which contributes to its strength.
"Spiders have figured out how to spin fibres with a desirable amount of nanocrystals," Zhang said. "But when humans use artificial spinning processes, the amount of nanocrystals in a synthetic silk fibre is often lower than its natural counterpart."
The team redesigned the silk sequence by introducing amyloid sequences that have high tendency to form β-nanocrystals. They created different polymeric amyloid proteins using three amyloid sequences as representatives. The resulting proteins had less repetitive amino acid sequences than spider silk, making them easier to be produced by engineered bacteria. The bacteria produced a hybrid polymeric amyloid protein with 128 repeating units. Recombinant expression of spider silk protein with similar repeating units has proven to be difficult.
The longer the protein, the stronger and tougher the resulting fibre. The 128-repeat proteins resulted in a fibre with gigapascal strength, which is stronger than common steel. The fibres’ toughness is higher than Kevlar and all previous recombinant silk fibres. Its strength and toughness are even higher than some reported natural spider silk fibres.
In collaboration with Young- Shin Jun, professor in the Department of Energy, Environmental & Chemical Engineering, and her PhD student Yaguang Zhu, the team confirmed that the high mechanical properties of the polymeric amyloid fibres come from the enhanced amount of β-nanocrystals.
"This demonstrates that we can engineer biology to produce materials that beat the best material in nature," Zhang said.
This work explored three of thousands of different amyloid sequences that could potentially enhance the properties of natural spider silk. "There seem to be unlimited possibilities in engineering high-performance materials using our platform," Li said. "It's likely that you can use other sequences, put them into our design and also get a performance-enhanced fibre."
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