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Scientists Engineer Artificial Silk Using Bacteria

Microscopic image of 285-kD recombinant spider silk fiber

Finer than human hair, five times stronger by weight than steel, and biocompatible, spider dragline silk is suitable for a variety of biomedical applications such as artificial ligaments and surgical thread. However, because natural dragline silk cannot be easily obtained by farming spiders, scientists are attempting to mass produce artificial silk while maintaining the natural material's properties.

Attempting to do just that are Sang Yup Lee from the Korea Advanced Institute of Science and Technology (KAIST; Daejeon, South Korea), Young Hwan Park from Seoul National University, and David Kaplan from Tufts University (Somerville, MA) Their method is similar to what spiders do themselves: express recombinant silk proteins and then spin them into water-insoluble fibers.

For the successful expression of high-molecular-weight spider silk protein, Lee and his colleagues pieced together the silk gene from chemically synthesized oligonucleotides and then inserted it into the expression host--in this case, Escherichia coli, an industrially safe bacterium that is normally found in the human gut. Initially, the bacterium refused to produce silk protein because of the protein's unique characteristics, such as extremely large size, repetitive nature of the protein structure, and biased abundance of the amino acid glycine.

"To make E. coli synthesize this ultra-high-molecular-weight (as big as 285-kD) spider silk protein having a highly repetitive amino acid sequence, we helped E. coli overcome the difficulties by the system's metabolic engineering," Lee explains. His team boosted the pool of glycyl-tRNA, the major building block of spider silk protein synthesis. "We could obtain appreciable expression of the 285-kD spider silk protein, which is the largest recombinant silk protein ever produced in E. coli. That was really incredible," Xia adds.

But this was only the first step. The KAIST team performed high-cell-density cultures to mass produce the recombinant spider silk protein. Then, the team developed a simple, easy-to-scale-up purification process for the protein. The purified protein can be spun into silk fiber. To study the mechanical properties of the artificial spider silk, the researchers determined tenacity, elongation, and Young's modulus, the three critical mechanical parameters that represent a fiber's strength, extensibility, and stiffness. Importantly, the artificial fiber displayed tenacity of 508 MPa, elongation of 15%, and Young's modulus of 21 GPa, values that are comparable to those of native spider silk.

"We have offered an overall platform for mass production of native-like spider dragline silk," Lee concludes. "This platform would enable us to have broader industrial and biomedical applications for spider silk. Moreover, many other silk-like biomaterials such as elastin, collagen, byssus, resilin and other repetitive proteins have similar features to spider silk protein. Thus, our platform should also be useful for their efficient biobased production and applications."

For a small sampling of Medical Product Manufacturing News articles on the development of silk for medical device applications, see "A New Spin on Silk Could Advance Biomimetics Research," "Buzz Grows Around Insect-Inspired Silk," and "Mimicking Mother Nature."

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