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Tufts University team makes silk that forms patterns

15 Oct '19

4 min read

Pic: Tufts University

Scientists at Tufts University School of Engineering have made silk materials that can wrinkle into different patterns like words, textures and images as intricate as a QR code or a fingerprint. The patterns take about one second to form, are stable, but can be erased by flooding the surface of the silk with vapour, allowing one to restart the process.

In an article published in the Proceedings of the National Academy of Sciences, the team demonstrates examples of the silk wrinkle patterns, and envision a wide range of potential applications for optical electronic devices.

The smart textile takes advantage of the natural ability of silk fibre proteins – fibroin -- to undergo a change of conformation in response to external conditions, including exposure to water vapour, methanol vapour and UV radiation. Water and methanol vapour can soak into the fibres and interfere with hydrogen bond cross links in the silk fibroin, causing it to partially ‘unravel’ and release tension in the fibre.

Taking advantage of this property, the researchers fabricated a silk surface from dissolved fibroin by depositing it onto a thin plastic membrane (PDMS). After a cycle of heating and cooling, the silk surface of the silk/PDMS bilayer folds into nanotextured wrinkles due to the different mechanical properties of the layers.

Exposing any part of that wrinkled surface to water or methanol vapour causes the fibres to relax and the wrinkles to flatten. The smooth surface transmits more than 80 per cent of light, while the wrinkled surface only allows 20 per cent or less through, creating a visible contrast and the perception of a printed pattern. The surface can be selectively exposed to vapour using a patterned mask, resulting in a matched pattern in the textured silk.

Patterns may also be created by depositing water using inkjet printing. The resolution of this printing method is determined by the resolution of the mask itself, or the nozzle diameter of the inkjet printer.

Alternatively, use of UV creates a virtual mask, since UV exposed parts of the silk surface become less permeable to water or methanol and remain wrinkled when treated with vapour, while the parts not exposed to UV absorb the vapour and flatten out. The printed pattern reflects the pattern of UV light exposed to the silk surface.

After erasing a pattern with vapour, the textured silk can be regenerated with a cycle of heating and cooling. The authors demonstrated the ability to print patterns over at least 50 cycles, without any diminishment in contrast or resolution.

“We can print patterns of remarkably high resolution in the silk – and we even showed that we can pick up the moisture pattern left by a fingerprint,” said Yu Wang, post-doctoral fellow in the Tufts University School of Engineering, and first author of the study. “But beyond the novelty of reversible printing, there are many other functional applications that the silk patterning technology could provide.”

According to Wang, the list of potential applications includes materials with tunable optical properties, some of which may involve the use of dopants that allow the patterned fabric to absorb or emit different wavelengths of light and energy, or exhibit patterns only from specific angles; and materials that modulate their thermal properties, changing the amount of heat they let through. Due to the biocompatibility of the silk fibres, the micropatterning material could be used in biomedical applications.

The study also demonstrated how the patterns could be switched on and off at will by connecting the bilayer to a small electrical heating element, transitioning the silk between wrinkled and wrinkle-free states.

“Because of its versatility, and ease of manufacture, I think there may be many future applications that we and others will come up with that we have not even imagined yet,” said Fiorenzo Omenetto, corresponding author and the Frank C Doble Professor of Engineering at Tufts’ School of Engineering.

Fibre2Fashion News Desk (SV)