Stronger than steel and harder than Kevlar, Microbially produced fibers

A new fiber, made by genetically engineered bacteria is stronger than steel and tougher than Kevlar.

Spider silk is claimed to be one of the world's strongest and toughest materials. Engineers at Washington University in St. Louis have created amyloid silk hybrid proteins using bacteria that have been genetically modified. The resulting strands are more durable and stronger than natural spider silk.

Their findings were reported in the ACS Nano journal.

To be precise, the artificial silk, named "polymeric amyloid" fiber, was created by bacteria that were genetically altered in the lab of Fuzhong Zhang, a professor in the McKelvey School of Engineering's Department of Energy, Environmental, and Chemical Engineering. 

Zhang has previously worked with spider silk. His lab developed bacteria in 2018 that produced a recombinant spider silk that outperformed its natural counterparts in all essential mechanical qualities.

"After our earlier work, I wondered whether we might use our synthetic biology platform to make something better than spider silk," Zhang added.

The research team, which included first author Jingyao Li, a PhD student in Zhang's lab, altered the amino acid sequence of spider silk proteins to add new qualities while retaining some of spider silk's appealing characteristics.

The requirement to manufacture -nanocrystals, a main component of natural spider silk that contributes to its strength, is an issue linked with recombinant spider silk fiber — without significant alteration from natural spider silk sequence. "Spiders have discovered how to spin threads with a desired amount of nanocrystals," Zhang explained. "However, when people utilize artificial spinning techniques, the amount of nanocrystals in synthetic silk fibers is frequently smaller than in natural silk fibers."

To address this issue, the researchers modified the silk sequence by using amyloid sequences with a high proclivity for forming -nanocrystals. They used three well-studied amyloid sequences as models to build other polymeric amyloid proteins. Because the proteins produced by modified bacteria contained less repeating amino acid sequences than spider silk, they were easier to make. The bacteria eventually created a 128-repeating-unit hybrid polymeric amyloid protein. It's been difficult to recombinantly manufacture spider silk protein with similar repeating units.

The stronger and tougher the resultant fiber is, the longer the protein. The 128-repeat proteins produced a fiber with gigapascal strength (the amount of force required to break a fiber of a particular diameter), which is stronger than conventional steel. The toughness of the fibers is higher than Kevlar and any prior recombinant silk fibers (a measure of how much energy is required to break a strand). It has even more strength and hardness than some natural spider silk fibers.

The researchers confirmed that the strong mechanical capabilities of polymeric amyloid fibers are due to an increased amount of -nanocrystals in collaboration with Young- Shin Jun, professor in the Department of Energy, Environmental, and Chemical Engineering, and her PhD student Yaguang Zhu.

In the Zhang lab, these new proteins and the accompanying fibers aren't the end of the tale when it comes to high-performance synthetic fibers. They have just just begun. "This shows that we can design biology to create materials that outperform nature's best," Zhang added.

This study looked at only three of the hundreds of possible amyloid sequences that could improve the characteristics of natural spider silk. "Using our technology, there appear to be limitless possibilities in creating high-performance materials," Li added. "It's likely that you'll be able to use other sequences, incorporate them into our design, and obtain a performance-enhanced fiber as well."