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Scientists at Washington University in St Louis have used microbes to make synthetic muscles. They used a synthetic chemistry approach to polymerise proteins inside engineered microbes. This enabled the microbes to produce the high molecular weight muscle protein, titin – one of the three major proteins of muscle tissue – which was then spun into fibres.
The research, carried out at the McKelvey School of Engineering, is published in the journal Nature Communications.

“Its production can be cheap and scalable. It may enable many applications that people had previously thought about, but with natural muscle fibres,” said Fuzhong Zhang, professor in the Department of Energy, Environmental and Chemical Engineering. Now, these applications may come to fruition without the need for actual animal tissues.

The synthetic muscle protein has been produced in Zhang’s lab. Critical to its mechanical properties is the large molecular size of titin. “It’s the largest known protein in nature,” said Cameron Sargent, a PhD student in the Division of Biological and Biomedical Sciences and a first author on the paper along with Christopher Bowen, a recent PhD graduate of the Department of Energy, Environmental and Chemical Engineering.

Muscle fibres have been of interest for a long time, Zhang said. Researchers have been trying to design materials with similar properties to muscles for various applications, such as in soft robotics.

“We wondered, ‘Why don’t we just directly make synthetic muscles?’” he said. “But we’re not going to harvest them from animals — we’ll use microbes to do it.”

To circumvent some of the issues that typically prevent bacteria from producing large proteins, the research team engineered bacteria to piece together smaller segments of the protein into ultra-high molecular weight polymers around two megadaltons in size — about 50 times the size of an average bacterial protein. They then used a wet-spinning process to convert the proteins into fibres that were around 10 microns in diameter, or a tenth the thickness of human hair.

Working with collaborators Young Shin Jun, professor in the Department of Energy, Environmental and Chemical Engineering, and Sinan Keten, professor in the Department of Mechanical Engineering at Northwestern University, the group then analysed the structure of these fibres to identify the molecular mechanisms behind their exceptional toughness, strength and damping capacity, or the ability to dissipate mechanical energy as heat.

Besides use in fancy clothes or protective armour – the fibres are tougher than Kevlar, the material used in bulletproof vests – Sargent pointed out that this material has many potential biomedical applications as well. Because it’s nearly identical to the proteins found in muscle tissue, this synthetic material is presumably biocompatible and could therefore be a great material for sutures and tissue engineering.

“The beauty of the system is that it’s really a platform that can be applied anywhere,” Sargent said. “We can take proteins from different natural contexts, then put them into this platform for polymerisation and create larger, longer proteins for various material applications with a greater sustainability.”

The research was supported by the Office of Naval Research and an Early Career Faculty grant from NASA’s Space Technology Research Grants Programme.

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