Although spider webs can be torn apart with one well-aimed sweep of a broom, the silk that forms them is, by weight, one of the strongest materials found in nature. It’s also lightweight, biodegradable, sticky, and slightly stretchy—all properties that make it desirable for a variety of applications.
Humans have been collecting silk from silkworm colonies for thousands of years to create textiles. However, using the same strategy to harvest natural spider silk—which is stronger and lighter than that of silkworms—isn’t a practical option. Spiders can become territorial and even cannibalistic when kept in close proximity to each other, and the labor required to harvest their silk is too great to make the process commercially viable. Instead, scientists are studying the molecular basis for spider silk’s valuable properties with the hopes of eventually creating a commercially viable biomimetic synthetic silk fiber.
“One of the main obstacles to synthesizing true, biomimetic spider silks was, and still is, a lack of understanding about the natural material and processes by which it is created,” said Cameron Brown of the University of Oxford and an author of a review paper on synthetic silk in the January 14 issue of the Journal of Materials Research. “We are getting much better at making proteins similar to the natural proteins, yet our processing capabilities lag behind.”
The two main proteins believed to be responsible for silk production are large molecules called Spidroins, but over a hundred secondary proteins might help finesse the silk’s properties. Researchers have turned to a variety of organisms, from bacteria to goats, to recombinantly express these relevant proteins. None, however, has yielded synthetic fibers on par with natural ones in strength. Recombinant silk protein expression has suffered from low protein yield—making the process inefficient—and low molecular weight, weakening the resulting product. It seems that while spiders are well suited to efficiently express these large proteins critical to their survival, other species are not.
Scientists at Utah State University turned to silkworms as an alternative, engineering them to create a hybrid spider-silkworm protein. “This was a great idea, as most expression systems aren’t well suited to making proteins of the size of the main silk proteins,” said Federico Rosei, a researcher at INRS University in Quebec, Canada, and another contributor to the review.
Once the proteins are expressed, the resulting slurry must be spun into a thread. Spiders have specialized equipment for this task: glands in the abdomen release a molten protein mixture, which flows through ducts that reorder the proteins into a fiber before releasing it through spinnerets. For humans, it has proven to be more of a challenge. One promising solution uses microfluidics, where fiber assembly takes place at the interface of a fluid protein stream and a water-insoluble liquid like oil.
It’s too soon to call spider silk the next miracle material—many of the technologies being developed, though promising, are still in their nascent stages, and the exact properties of a given spider silk depend on the complex interplay between its molecular makeup and the fiber structure. Nevertheless, opportunities for its eventual use abound.
Randy Lewis, a Utah State University researcher who was not involved in the review article but has been studying spider silk for 25 years, points out that the potential for the material goes beyond the strong, lightweight fabrics one might expect. “We have found that we can make a variety of things other than fibers from spider silk,” he says, like adhesives, coatings, and highly absorbent sponges.
The authors of the review agree. “We think some of the most exciting applications aren’t the ones that follow the obvious properties of spider silk—applications in biophotonics and sensing, for example,” said Brown.
