The whole story started with silk, not silk like spiders, but silk taken from an insect’s cocoon, which is boiled down to its basic protein building blocks called fibroin. The silk fibroin solution can be pushed through narrow-bore needles to form thin threads, and when exposed to solvents such as ethanol or acetone, it slowly begins to transform into a semi-solid hydrogel.
The problem is that this transformation usually takes hours. Spider silk hardens almost immediately upon leaving the glands, giving spiders the precise control that engineers have struggled to achieve.
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Then a small accident helped bring success. “I was working on a project to create an extremely strong adhesive using silk fibroin, and when I was cleaning my glassware with acetone, I noticed a web-like material forming on the bottom of the glass,” said Marco Lo Presti, research assistant professor at Tufts.
It turned out that dopamine, a key ingredient they were already using to make the adhesive work, dramatically sped up the freezing process. When dopamine is added to the solution, it draws water out of the silk protein, so the liquid fibroin does not require hours to set. Instead, once it meets the organic solvent, it turns into fibers in a matter of seconds.
From there, the team created a coaxial needle system where the fibroin-dopamine solution runs through the center while a layer of acetone flows around it. As the stream leaves the nozzle, the acetone rapidly solidifies and then evaporates into mid-air, allowing the fibers formed to stick to objects with which it makes contact. What emerges is a thread that can warp, stay in contact, and hold a surprising amount of weight even in the open air.
To boost performance, the team mixed the fibroin-dopamine solution with chitosan, a material derived from insect exoskeletons, increasing the tensile strength by up to 200 times. The borate buffer made the fibers approximately eighteen times more sticky. Depending on the hole of the needle, the diameter of the resulting fiber can be as thin as a hair or closer to half a millimeter.
In testing, the demonstration took on a playful form as the fibers picked up a cocoon, a steel bolt, a tube floating on water, a scalpel half buried in sand, and even a block of wood from about 12 centimeters away. Under various conditions, the fibers can lift objects weighing up to 80 times their own weight. For a jet of liquid silk that hardens in midair, the lifting force is remarkable.
Spiders don’t actually toss their silk into the air. They first make contact with a surface, attach a thread, then pull and arrange their webs with careful choreography. As Lo Presti explained, “Spiders can’t shoot their webs… We’re demonstrating a way to shoot fibers from a device, then follow them and pick up an object from a distance. Instead of presenting this work as a bio-inspired material, it’s actually a superhero-inspired material.”
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The natural spider silk in this study is still about a thousand times stronger than the man-made fiber, but this method opens the way for controlled shooting, instant setting, and stronger adhesion. With further innovations, it could become far more capable and find its place in many different technological applications.
“We can be inspired by nature. We can be inspired by comics and science fiction. In this case, we wanted to reverse engineer our silk material the way nature originally designed it, and comic book writers envisioned it,” said Fiorenzo Ommenato, the Frank C. Doble Professor of Engineering at Tufts University and director of SilkLab.
This research was published in advanced functional materials,
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