New way to spin artificial silk

RESEARCHERS in Sweden and Germany have found a technique to produce a type of artificial silk from whey proteins using streams of water.

Silk is lightweight but incredibly strong and elastic, but at present it is harvested from farmed silkworms, which is costly and time-consuming. Many researchers are working to find a way to produce silk artificially. The team, led by Christofer Lendel and Fredrik Lundell at the Royal Institute of Technology (KTH) in Stockholm, Sweden, use perpendicular water streams in a process called hydrodynamic focussing, to force nanofibrils of whey proteins from cows’ milk to lock together. Lendel says the process is similar to that used by spiders.

The researchers first made nanofibrils by exposing the whey to heat and acid. If the protein concentration in the solution is below 4%, long, straight, thick fibrils form, which can measure up to 2,000 nm in length and 4–7 nm thick. At above 6% concentration, fibrils are just 2–3 nm thick and only 40 nm long, as well as being curved. A suspension of the nanofibrils is pumped through a small tube, while two more streams of water enter perpendicular to the flow, squeezing the proteins together and creating the fibres. The researchers found that it was, in fact, the short, thin, curved nanofibrils that created the strongest fibre.

To understand why, they turned to Stephan Roth at the Deutsches Elektronen-Synchrotron (DESY) in Germany. Roth is head of beamline P03 at DESY’s Petra III facility, a microfocus small- and wide-angle X-ray scattering beamline. Roth found that the curved nanofibrils can lock together better than the straight ones, improving the strength of the fibre. His images showed that the random orientation of the fibrils remains in the fibre.

“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 optimised 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 fibres 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 fibre,” said Lendel.

He adds that now they understand the process, they will be able to create fibres with better or tailor-made properties, for example for growing tissue for medical use, or in biosensors.

The paper is set to be published in Proceedings of the National Academy of Sciences later this week.