A significant process for the artificial production of silk has been demonstrated by a Swedish-German team of researchers. These researchers used the intense X-rays from DESY's research light source PETRA III to observe how small protein pieces, called nanofibrils, lock together to develop a fiber.
The team discovered that the best fibers are not developed by the longest protein pieces. The strongest “silk” is instead won from protein nanofibrils with seemingly less quality. These details were reported in the Proceedings of the U.S. National Academy of Sciences by the team headed by Dr. Christofer Lendel and Dr. Fredrik Lundell from the Royal Institute of Technology (KTH) in Stockholm.
Silk is a material that has increasing use in a number of areas due to its many remarkable characteristics. It is stronger than some materials, despite being lightweight, and can be extremely elastic. Silk is currently harvested from farmed silkworms, which is quite costly.
Across the globe, many research teams are working on methods to artificially produce silk. Such artificial materials can also be modified to have new, tailor-made characteristics and can serve for applications like novel biosensors or self-dissolving wound dressings, for example.
Prof. Stephan Roth, DESY
However, imitating nature proved particularly hard as far as silk was concerned. The Swedish team concentrated on self-assembling materials.
That's a quite simple process. Some proteins assemble themselves into nanofibrils under the right conditions. A carrier fluid with these protein nanofibrils is then pumped through a small canal. Additional water enters perpendicular from the sides and squeezes the fibrils together until they stick together and form a fibre.
Dr. Fredrik Lundell, KTH
Lundell's team earlier used the latter process known as hydrodynamic focusing for producing artificial wood fibers from cellulose fibrils. “In fact, the process has several similarities with the way spiders produce their silk threads,” says Lendel.
In the recent study, the nanofibrils were developed by a protein obtained from cow's whey under the influence of acid and heat. The characteristics and shape of the fibrils depend on the protein concentration in the solution.
At less than 4%, straight, long and thick fibrils develop. These fibrils can be up to 2000 nm long and 4 to 7 nm thick. They remain shorter and thinner with an average length of only 40 nm and a thickness of 2 to 3 nm when the protein concentration is 6% or more in the initial solution. The fibrils also appear to be curved just like small worms and 15 to 25 times softer than straight, long fibrils.
However, in the lab, the curved and short fibrils produced much better fibers compared to the straight and long fibrils. DESY's bright X-ray light enabled the team to find out why: “The curved nanofibrils lock together much better than the straight ones. The X-ray diffraction patterns show that they largely keep their rather random orientation in the final fibre,“ says Roth, head of the beamline P03 at PETRA III where the experiments took place.
The strongest fibres form when a sufficient balance between ordered nanostructure and fibril entanglement is kept. Natural silk is an even more complex structure with evolutionary optimized proteins that assemble in a way with both, highly ordered regions - so-called beta-sheet - that give strength and regions with low order that give flexibility. However, the structures of the artificial and natural fibers are essentially different. In particular, the protein chains in natural silk have a larger number of intermolecular interactions that cross-link the proteins and result in a stronger fiber.
Dr. Christofer Lendel, KTH
From their experiments, the researchers acquired artificial silk fibers that were about 5 mm long and of medium quality. “We used the whey protein to understand the underlying principle in detail. The whole process can now be optimized to obtain fibres with better or new, tailor-made properties,” says Lendel. In this way, it is now possible to use the results obtained from the study to develop materials with novel features, for instance, artificial tissue for medical applications.