It’s just after sunset. The air is cooler. The sky is a mix of pink, purple and blue. Cicadas loudly stridulate. Between two widely spread boughs of a tall spruce tree, an orb-weaver spider spirals round and round, meticulously weaving her web. It’s hard to stop watching her work and marveling at the craftsmanship, the effort and the instinct of her kind. When she finishes, the web hangs like transparent lace.
These complexly crafted nets are a sight of wonder, beckoning us closer - but not too close, or at least that’s the case for most of us. However, there are some who dared to go a little further–study those forbidding fibers and its creepy creators just a little deeper.
The results of that bravery have paid off. Years of fascination with spider webs have inspired interesting tools, silk harvesting mechanisms, lore, even super heroes. We now know that the fiber is comparably as strong, if not stronger, than steel. It’s stretchy, biocompatible–far superior to silkworm silk. Unfortunately, harvesting this fiber in greater quantities has eluded us. Milking techniques have led to cannibalism, and mass production comes with great expense and challenges.
Recently, however, two scientists: Sarah Stellwagen of University of Maryland, Baltimore County and co-author Rebecca Renberg of the Army Research Laboratory turned their attention to spider glue. In June, 2019, these two researchers published the sequences of two glue genes of the orb-weaver spider.
While historic attention has been fixed on the durability, strength and versatility of the silk, the glue (which is composed of silk proteins that stays wet) also offers incredible value to spiders as well as society.
Stellwagen describes the up-close view of spider glue as beadlike, dotted along support silk. Behavioral properties of the glue will differ between different types of spiders, their needs and their local environment. Some glue is specialized for dryer locations, some are better suited for humid conditions. Glue differences can also depend on a spider species’ snaring tactics.
One of the more intriguing aspects of spider web glue is that it performs wet adhesion, the ability to affix to a material even in moist conditions. It performs well when wet and sometimes even performs better in humid situations. Such an adhesive, especially one that might be more biocompatible, could have future applications in the medical field as well as many other industries.
Other posed applications included using the glue on barn walls to protect livestock from insect bites or as an alternative to pesticides on crops, which could potentially reduce water and soil pollution.
One of the challenges Stellwagen and Renberg faced was the massive size of these genes. The genes of interest, aggregate spindroin 1 and 2 (AgSp1 and AgSp2) have coding regions that are more than 40 kb and 20 kb, respectively. While the size was one challenge, sequence repetition posed another obstacle for traditional next-generation sequencing.
In order to accomplish the task they used “error-prone long reads to scaffold for high accuracy short reads” by employing Illumina technology with long-read Oxford Nanopore technology to get an aggregate sequence. RNA-seq via short reads from Illumina corrected the error-prone technique of the genomic DNA long reads from Oxford Nanopore. The combined methods enabled researchers to get an accurate sequence (with the exception of introns). In short, they “sequenced, assembled and corrected two highly expressed spindroin genes…”
Now that the challenge of sequencing is out of the way, researchers interested in synthesizing the glue for innovative biological materials face the long road of developing refined techniques for mass production. This might be a little less challenging to achieve compared to spider silk due to the already wet nature of the glue. Spider silk, on the other hand, has been difficult to manufacture on large scales because the silk begins as a liquid material that solidifies through the spider’s spinning mechanisms.
A notable innovator trying to produce spider silk is Bolt Threads, a company using yeast and fermentation processes to manufacture the silk. One of their initial products was a spider silk tie, the first one taking approximately seven years to complete. The second tie, however, only took 60 days to produce.
Goats have been another medium for producing spider silk. These modified goats have silk genes incorporated in such a way that the goats make the silk protein in their milk. Researchers collect the milk and purify the silk proteins. The challenge with goats is the expense to feed them, the space to keep them and the general upkeep.
Finally, the Unites States Army has contracted a manufacturer to produce “Dragon Silk,” spider silk produced from modified silk worms.
Silk worms have been ideal silk producers for centuries because they are easy to maintain, produce a lot and can be kept in large volumes. Spiders, on the other hand, cannot be easily farmed because they are territorial in nature and resort to cannibalism when in confined spaces.
One of the benefits of Dragon Silk is that it is significantly more cost-effective to produce than other methods that are under development.
Silk worm cocoons are made of raw silk that is used for silk production
Artificial spider silk is another area of interest for researchers at the University of Cambridge. This imitation silk has similar strength, elasticity and energy absorption as spider silk. It is a hydrogel composed of 98% water and 2% silica and cellulose, which can ultimately be pulled and dried, forming a spider web fiber-like thread.
Humanity’s fascination with spider silk is an ancient one. Indigenous fishers from various islands have historically used the fiber for line, netting and lures. In the 18th century Bon de Saint-Hilaire made stockings and gloves using boiled cocoons. In the early 19th century, a Jesuit missionary by the name of Paul Camboue developed a mechanism for stabilizing, milking and spinning orb weaving spider silk. There is some criticism about Camboue’s work in that it exploited resources and labor on Madagascar.
Presently, it’s not hard to continue fantasizing and exploring all the possibilities of spider silk and spider silk glue. Our imagination and curiosity is what has driven us closer to these menacing creations, and has continued to drive us even when the research is painstaking. With continued passion and creativity, researchers will continue to see the fruits of their labor used in amazing ways.
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