Web-slinging device shoots fibres that can pick up objects

Created at the Tufts University’s Silklab, the sticky fibres come from silk moth cocoons, which are boiled in solution and broken down into fibroin, which are their building block proteins. The team’s findings are detailed in Advanced Functional Materials.

The silk fibroin solution can be extruded through narrow bore needles to form a stream that solidifies into a fibre when exposed to air.

To date, Silklab has used silk fibroin to make glues that work underwater, printable sensors that can be applied to virtually any surface, edible coatings that can extend the shelf life of produce, a light-collecting material that could enhance the efficiency of solar cells, and more sustainable microchip manufacturing methods

Despite their progress with silk-based materials, the researchers had not been able to replicate the stiffness, elasticity, and adhesive properties of threads spun by spiders until a moment of serendipity.

“I was working on a project making extremely strong adhesives using silk fibroin, and while I was cleaning my glassware with acetone, I noticed a web-like material forming on the bottom of the glass,” Marco Lo Presti, a research assistant professor at Tufts, said in a statement.

The discovery is said to have overcome several engineering challenges in replicating spider threads. Silk fibroin solutions can slowly form a semi-solid hydrogel over a period of hours when exposed to organic solvents like ethanol or acetone, but the presence of dopamine, which is used in making the adhesives, allowed the solidification process to occur almost immediately. When the organic solvent wash was mixed in quickly, the silk solution rapidly created fibres with high tensile strength and stickiness.

To spin the fibres in air the researchers added dopamine to the silk fibroin solution, which appears to accelerate the transition from liquid to solid by pulling water away from the silk. When shot through a coaxial needle, a thin stream of the silk solution is surrounded by a layer of acetone which triggers the solidification. The acetone evaporates in mid-air, leaving a fibre attached to any object it contacted. The researchers enhanced the silk fibroin-dopamine solution with chitosan, which gave the fibres up to 200 times greater tensile strength, and borate buffer, which increased their adhesiveness about 18-fold.

The diameter of the fibres could be varied between that of a human hair to about half a millimetre, depending on the bore of the needle.

The device can shoot fibres that can pick up objects over 80 times their own weight under various conditions. The researchers demonstrated this by picking up a cocoon, a steel bolt, a laboratory tube floating on water, a scalpel partially buried in sand, and a wood block from a distance of about 12cm.

“If you look at nature, you will find that spiders cannot shoot their web,” said Lo Presti. “They usually spin the silk out of their gland, physically contact a surface, and draw out the lines to construct their webs. We are demonstrating a way to shoot a fibre from a device, then adhere to and pick up an object from a distance.”