Spider silk protein has a reputation of being among the strongest and toughest of materials, pound for pound. Stronger than some steel alloys and tougher than Kevlar. It could have applications ranging from superthin sutures for sutures for surgery and bulletproof materials. Unfortunately, unlike silk worms, spiders are notoriously difficult to farm because of their territorial and cannibalistic nature and so spider silk has always evaded mass production. Materials scientists have thus spent many years searching for synthetic alternatives taking inspiration from our arachnid friends.
Now, Fuzhong Zhang and colleagues at Washington University St. Louis, Missouri, have engineered bacteria to produce a biosynthetic spider silk that is comparable in physical characteristics with its natural counterparts. The engineered protein is twice as large as any of this type of protein synthesized before. Its protein chains are 556 kilodaltons and contain 192 repeat motifs of the Nephila clavipes dragline protein spidroin. Previously, the largest biosynthetic spider silk protein was just 285 kDa. Even natural dragline silk is usually only 370 kDa, although there are heavier outliers. The protein's tensile strength and toughness positively correlates with its molecular weight as is the case with natural silk. [Bowen, C.H., et al., Biomacromol. (2018) DOI: 10.1021/acs.biomac.8b00980]
"People already knew about this correlation, but only with smaller-sized proteins," explains Zhang, "We found that even at this large size, there is still a very good correlation." The key to the team's success was to use repeated motifs from spider silk to engineer their synthetic version so that they could make it as big as possible. Of course, there is a limit at which point the bacteria can no longer cope with the length of the protein and their enzymes cleave it into smaller chunks. The team circumvented this well known problem by adding an extra DNA sequence that promotes a chemical reaction between the proteins formed so that they fuse.
With this material in hand, the team spun their synthetic silk proteins into fibers for mechanical tests. They measured tensile strength at 1.03 ± 0.11 gigapascals),modulus at 13.7 ± 3.0 GPa, extensibility of 18 ± 6%, and toughness 114 ± 51 megajoules per cubic meter.
"We will continue to work on making the process more scalable and economical by making it easier to handle, reducing the amount of chemicals needed, and increasing the robustness and efficiency," Zhang adds. The team now hopes to explore the limits of their new approach, hoping that they can add yet more mass to their biosynthetic silk and perhaps emulate the properties of spider silk and perhaps one day make a material that can outperform it.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase. His popular science book Deceived Wisdom is now available.